Sunday, May 28, 2017
Issue #541: Taking Pictures with a C8
When I’m speaking about the history of Schmidt Cassegrains at star parties, club meetings, or cons, I often get puzzled looks and questions from new amateurs about one of the things I say: “One of the big reasons for the 8-inch SCTs becoming the most popular commercial telescope in the 70s was astrophotography.” What? Everybody knows SCTs aren’t good for taking long exposure deep sky pictures. For that you need a short focal length refractor, right?
Maybe and maybe not. Firstly, back in the 70s when the Schmidt Cassegrain began its rise to fame, the other common telescope alternatives for deep sky astrophotography were the cumbersome, shaky Newtonians practically everybody owned, and refractors with focal ratios of f/15 or more. Take it from someone who was there, it was a million times easier to take deep sky astrophotos with a C8 than one of those telescopes.
Also, while I won’t disagree that for beginners in astrophotography, a refractor of short focal length is easier to manage in the beginning, we don’t remain beginners forever. Eventually you may discover more focal length, aperture, and resolution than what your 80 – 100mm refractor offers can be a good thing. So what are the problems with using the average Celestron C8 or Meade 8-inch for deep sky imaging?
The first gremlin is simply all that focal length. With a C8, you start out with a native focal length of about 2000mm. That is what, more than anything else, makes long exposures tough with the telescope. At 2000mm, every tracking faux pas your mount commits will be exaggerated. Not as stable as it ought to be? A tiny gust of wind will ruin your picture by creating trailed stars no matter how well you guided. That may make anything but the shortest exposures problematical in autumn and winter when the winds are wont to blow.
Also, if you’re a plebe like me, you won’t be using a 10 thousand dollar mount for your telescope and will have to guide it. You’ll use a small auxiliary camera to keep the telescope precisely centered on the target despite the inevitable back and forth motion of periodic error caused by less than perfect gears. At 2000mm, you will have to guide precisely. How precise depends on the pixel size and sensor chip size of the imaging camera, but you can bet there won’t be much room for error.
Then there are the mirror flop blues. To focus, the primary mirror of a Celestron or Meade SCT slides up and down on the baffle tube that protrudes from the main mirror. The mechanical tolerances there are OK for visual use, but are loose enough that the mirror can move slightly when the attitude of the telescope changes significantly—as when crossing the Meridian. Result? Those darned trailed stars if you’re using a separate guide telescope for auto-guiding the mount. To the guide camera, everything looked fine, but the image moved in the main camera when the mirror flopped.
|An imaging rig back in the day!|
None of these things present insuperable difficulties, though. After all, me and my mates were using C8s to take good pictures—which I define as pictures that made us happy—thirty and forty years ago. We didn’t have electronic cameras, either. We manually guided our telescopes and usually exposed for a minimum of half an hour even on bright objects and with “fast” film in our SLRs. If we could get decent shots with a Schmidt Cassegrain then, certainly you can now.
Again, I don’t endorse a C8 or Meade 8 as your first astrographic telescope. Cut your teeth on the vaunted fast ED refractor—they are cheap now and come as close to being foolproof for deep sky imaging as you can get. But when you are ready to move up in focal length and aperture, however, begin collecting the astro-stuff you will need…
Get a Modern SCT
Get an Edge (Celestron) or an ACF (Meade). Their better field edge performance is a good thing, no doubt about that, especially if you also intend to use the scope visually. Admittedly, unless you are employing a camera with a full frame 35mm sensor, you won’t notice the difference in images, but you might as well invest for the future so that if/when you move to a bigger chip you’ll be ready.
The really big deal with modern SCTs for imagers is not necessarily the field edge, but that they have mirror locks. The Celestron Edges have them, and so do the 8-inch Meade ACF telescopes. These locks stabilize the primary mirror and prevent it from flopping if you are guiding with a separate guide scope.
Get a Focal Reducer
All the Meade and Celestron 8-inch SCTs come in at f/10, that 2000mm we talked about above. Not only does that many millimeters make guiding and tracking more difficult, it makes for longer exposures and can be a challenge for accurate goto pointing. The solution? If you get the Celestron, buy the Edge f/7 reducer. If a Meade, the standard Meade f/6.3 reducer corrector (the Celestron 6.3 works fine on Meade scopes too). The Meade and Celestron 6.3 reducers are reducer correctors, designed to flatten the field edge of non-ACF telescopes, but they work just fine with ACFs since most of their effect is to, yes, flatten the field rather than remove coma—which the ACFs’ optical system does itself.
|A 66mm f/7 makes a nice guider...|
How about other focal reducers? Like those from Optek? They can be a good choice if you’ve got a Meade scope, but some can’t be used visually. Those for the Edge scopes definitely can’t. Only the Celestron f/7 Edge reducer can be used for that. Since you’ll probably want to eyeball the heavens your Edge SCT once in a while, get a reducer that will work with an eyepiece.
Get a Good Enough Mount
This is the most important thing if you’re considering SCT astrophotography: how good is the mount’s tracking? Especially with a payload consisting of an 8-inch SCT, camera, and guide scope (which may be upwards of 30 pounds). It doesn’t matter if you image with a fork mount or a German equatorial—both have their pluses—it just matters that you get good tracking with a tricked out 8-inch SCT onboard.
Can you get by with the fork mount that came with your telescope? Maybe, if it’s of fairly recent vintage. Older forks can be a crapshoot. I once encountered a Meade LX200 GPS with 90” of periodic error (that’s a lot). Modern forks like the CPC Deluxe from Celestron and the fancy LX600 from Meade are certainly much better for imaging than the old ones. HOWEVER, thousands of good long exposure images have been taken with the minimalist AC driven fork mounts of the 70s and 80s. Use what you have, but a good mount makes things easier.
For most of us, a good mount is a GEM. A German equatorial has the advantage of allowing you to use a variety of scopes on the mount. You can do widefield with a refractor without the hassle of trying to piggyback it on a fork mount’s SCT OTA. One is also more portable than a fork mount, though an 8-inch fork SCT isn’t too much of a hassle for most of us to transport and set up.
How much should you spend on a mount? That’s up to you. Prices for GEMs usable for imaging with an 8-inch Schmidt Cassegrain range from about 800 dollars all the way up to 10 thousand dollars and more. Before spending oodles of cash, though, ask yourself how often you are really going to be able to or want to take pictures. For most of us that is maybe once or twice a month--IF the weather cooperates.
Me? Thanks to our stormy Gulf Coast, I rarely do astrophotography even once a month. For me, an inexpensive imported GEM is more realistic than a top of the line AP, Bisque, or 10Micron. Keep the sub-frame exposures down to 5-minutes for less and an Atlas or a CGEM can work very well with an 8-inch SCT. Given my usual conditions, it’s not like I’m going to be taking 12-hour exposure sequences anyhow.
Don't scrimp on the mount, though. While I’ve taken OK images with my C8 and a CG5 or AVX GEM, it was clear these mounts were at their limits with the telescope. And so are the other GEMs in this class up to and including the HEQ-5 (Sirius). For ease and reasonable consistency of results, consider the next step up, the EQ-6 (Atlas) or CGEM or CGX mounts. If your skies and your skills are better than mine, and you are less lazy than me, I wouldn’t criticize you for bumping the mount choice up to a Losmandy G11 (about 4K), but you don’t have to do that to shoot good deep sky astrophotos with a C8. An Atlas type mount will do it.
Get a Sufficient Guide Scope
Today’s sensitive, high resolution guide cameras don’t require the crazy long focal length guide-scopes we used in the day of manual guiding. Still, you need a guide scope (a refractor or a reflector that does not use a moving primary mirror to focus) with enough resolution so the guide camera can “see” small errors when imaging with an SCT.
I am lazy and get along with one of those 50mm finder-guide-scopes that are so popular now, but I suggest a minimum of 400mm of focal length for the guide telescope when doing C8 astrophotography. A Short Tube 80 or similar will do as long as you can lock the focuser down securely. And you have a sturdy mounting for the 80. That is incredibly important when imaging at these focal lengths, since the smallest amount of flexure in the guide scope rings will show up as trailed stars in the main scope’s images.
Get an Off-axis Guider
Well, maybe. I suggest you try a guide scope first and only if you find you just cannot get the gremlins out of your guiding setup no matter how you tighten things down or tweak the Brain settings in PHD2, should you consider an off-axis guider.
An “OAG” allows you to both guide and image through the main scope. One contains a little “pickoff” prism that diverts a small amount of the light at the edge of the telescope’s field to the guide camera. Since it is seeing the same images as the main scope, problems like flexure and mirror flop instantly disappear.
Unfortunately, there’s a price to be paid. The OAG will only pick up stars around the periphery of the telescope’s field. There may be few of the them, and their shapes may be distorted if you are using an older “standard” SCT whose field edge is not perfect. In this day of sensitive guide cameras, the problem of finding a suitable guide star is not as bad as it used to be, but it can still be difficult. I used an OAG all through the film days, but never found it to be a pleasant experience.
Get a Good Polar Alignment
Declination drift due to poor polar alignment just makes the task of guiding more difficult. Strive to get within a couple of minutes of the celestial pole if possible. That used to be tough, but innovations like the Polemaster polar alignment camera, and the polar alignment routine in Sharpcap (which uses the guide scope and camera to do the alignment) have made it positively easy.
Tips for Getting it all to Work
|C8 Edge plus Atlas EQ-6: not quite perfect but mine...|
With a sub-Losmandy mount, a Chinese GEM up to and including the iOptrons, be scrupulous about balance. That means balancing the mount so it is slightly east-heavy. Of course, you will likely have to rebalance if you move far from your initial target. That is not a big problem for most of us, since we’ll usually only image one or two objects a night and it’s easy enough to pick two subjects in roughly the same part of the sky. “East heavy” can make a big difference in how an imported mount performs, since it ensures the R.A. gears are always properly engaged.
Keep Subs Short, but…
With a C8 riding on an AVX or similar mount, you may find it to your advantage to keep individual exposures short. To pehaps a minute or two. If you have a bad spot on your gears, just throw out that sub-exposure and be on to the next one. Over an exposure of 5 – 10-minutes, there’s a lot that can go wrong with a light mount’s tracking ruining that whole, long shot.
Do remember, though, that sub-exposures have to be long enough to capture desired detail. While stacking subframes will make a shot less noisy and smoother, no detail not visible in a single sub-frame will show up in the final, stacked, photograph.
Keep Working with PHD Settings
I didn’t for the longest time and am now sorry I didn’t. The settings I had were good enough for the APO refractors I usually use for imaging these days, yielding RMS guide errors of 2” or a bit more on my AVX and CGEM. Couple that with my laissez faire approach to polar alignment, and most of my shots with a C8 (reduced) didn’t have perfectly round stars if I zoomed in enough in Photoshop or whatever.
Eventually, I decided I needed to do something about my guiding, since I wanted to begin imaging with the Edge C8 again once in a while. I read up on the PHD2 Brain settings and devoted one entire evening to tinkering with them. In just that one night my RMS guide errors went from 2” to 3” to a bit more than 1” at best, and under 2” at worst. That, coupled with Sharpcap polar alignment, has meant that for me imaging with the C8 is easier than it ever has been.
Shoot Appropriate Targets
If a target, a medium-small galaxy or globular cluster, perhaps, cries out of an 8-inch SCT, by all means use one as the imaging scope. If it doesn’t? Use a nice 3 – 4 – 5-inch ED refractor instead. Why make things hard on yourself for no good reason? In addition to less focal length, a refractor in this range will be lighter than the SCT, and an inexpensive GEM mount will always track better with a lighter load.
And that is that. Don’t be afraid to try long exposure deep sky astrophotography with an 8-inch SCT, no matter what you may have read on the darned Cloudy Nights BBS. A little experience and you may find it’s not as difficult as you'd been led to assume, and that the focal length and aperture of your friendly, neighborhood C8 or M8 brings a new dimension to your astrophotography.
Sunday, May 07, 2017
Issue 540: Get Thee to a Dark Site II
Last ish we got you a dark site, or at least gave a few pointers as to how you and your fellow astronomy club members could find and keep one. This time, we’re going to talk about using that site.
And you know you want to use it. Sure, in this day of electronic cameras and computer processing you can take pretty good pictures from the backyard, but you’ll always get better results under dark skies. Visual observer? As I said last week, the galaxies of spring cry out for the darkest skies possible.
There is no doubt about one thing, spring weather in the USA, and especially east of the Mississippi, can be capricious. Before talking about what you bring to your club’s observing field and what you do there, maybe we should discuss “whether.”
Obviously, if you’ve got beautiful blue skies and the forecast is for more, a dark site trip is a natural. But what if the sky is unsettled and the weather forecasts ambiguous? Back in the day, back when I was more sanguine about hard-core observing, back when I lived downtown and couldn’t observe anything from my backyard, I had a rule, “If it ain’t raining, head to the dark site.”
That stood me in good stead for years, and resulted in me seeing far more than I would have if I’d let a few clouds scare me off. Most of the time—though certainly not all of the time—I at least saw something at the club site in return for an hour’s journey into the west. I would sometimes wimp-out observing plan and gear-wise, though. If the sky really did look doubtful, I’d tend to change my plans from “astrophotography” to “visual,” and the telescope from my C11 to my 5-inch ETX Maksutov Cassegrain, Charity Hope Valentine. Anyway, I always found that even if I was mostly skunked, I had a better out on the observing field than I would have had sitting at home watching television.
Let’s say, you’ve got a night that looks to be uncompromisingly good, though. What do you load into your vehicle? The simple answer is “everything you need, nothing you don’t.” Certainly you want all the gear you require to allow you to execute whatever your observing plan is, but there are things you’d take to a multi-night star party that you will likely want to leave at home for a club site run. Remember, you’re going to have to pack all that stuff back into your car at the end of the evening and possibly unload it at home.
What NOT to Bring to a Dark Site…
You may actually need an observing table depending on your vehicle and what you are doing, but maybe you can back off from a big camp table to a TV tray. If I am doing visual observing, a table just large enough for an eyepiece box and maybe a star atlas is more than enough. Imaging? I’ll need something to put the laptop computer on, but not anything more than that.
When I switched vehicles from a sedan (a Camry) to a truck/SUV (4Runner), I eliminated observing tables altogether, operating out of the back of the 4Runner, tailgating it as it were, which is the best of all worlds—I even have AC power available there from the truck’s built in inverter and auxiliary battery.
This is a maybe/maybe not thing. Even if you are doing imaging, you may be able to eliminate the laptop. Using a standalone auto-guider and a digital single lens reflex (saving images on the camera’s memory card) can allow that. By saying “ixnay” to the laptop, you can also leave one large battery at home (a laptop’s internal battery will rarely last an entire observing run), the above mentioned table, cables, mouse, mousepad, etc., etc., etc. Yes, it’s nice to have a computerized star atlas like Stellarium, but in the interests of simplicity, sometimes I don’t mind getting reacquainted with Sky Atlas 2000 or Uranometria. Just can't go back to that? SkySafari running on a tablet is a good compromise.
Stuff you always bring and never use
You tend to throw a pair of binoculars in the car, but never/rarely use them? Leave them at home. The same goes for stuff like extra flashlights, a second box of eyepieces, radios, ice-chests, etc. All that junk is nice at a big star party, but you are not going to be at the dark site long enough to feel the need for this stuff.
A telescope that is just too much
|Small can be beautiful...|
We all want to maximize our observing experience, but if a telescope is so large and/or complex, that by the time you get it assembled and working it’s time to go home, leave it at home. Give me a freaking C8 for dark site use not a C11 or (horrors) C14. When all your buddies are packed up and ready to hit the road and you still don’t have the scope off its mount, I think you’ll begin to believe that sacrificing some aperture and/or features might not be such a bad idea.
Things that will annoy your fellow observers and possibly the landowner
Radios blasting your particular preference in music and green laser pointers that make the sky look like something out of Return of the Jedi have no more place at the club dark site than they do at a big, organized star party.
What to Bring
Sure, you know to pack the telescope, but make sure you pack all of it. One night, one cloudy night, when it wasn’t raining, nevertheless, I headed to the dark site with my C11. As soon as I arrived onsite, almost magically the clouds began to scurry off and I began assembling my big scope. Yes, as above, it was really too much for a short dark site run, but I was younger, stronger, and dumber then.
I had just got the NexStar 11 GPS on her tripod when I had a vision. Of the telescope’s hand control sitting on the dining room table of Chaos Manor South. And that was just where I’d left it. What to do? There wasn’t anything to do. I packed up and went home. I was just thankful I wasn’t at a star party 400 miles away.
In the interests of this sort of thing not happening to you, it’s a good idea to have a checklist. If you know an item is necessary, put it on the list and don’t check it off till it is packed in the car.
|A telescope that doesn't need batteries can be nice sometimes...|
Some lucky folks have AC power available at the club site, but that is rare. Be prepared to operate off batteries all night. So, ensure your batteries are fully charged beforehand. Don’t just assume they are. What sort of batteries? I favor the ubiquitous 17ah jump start battery packs. Not only do they have enough juice to power most scopes and accessories all night, they usually have built in lights which are handy when you are packing up at the end of the evening. Yes, don’t take too much stuff, but don’t scrimp on batteries. I always take one for the (goto) scope, one for the dew heaters, and one for the laptop.
At home, in my backyard, I can often get by without a dew heater system on my SCTs and refractors. My house and neighboring houses and trees shield much of the heat sucking sky from the view of my scope, acting as giant dew shields. On an open field out in the country? Uh-uh. Even if your area is drier than my Gulf Coast stomping grounds, you’ll need something to keep dew off.
Dew prevention is a subject for an entire article, but I can offer some basic guidance here: use heater strips on objective or corrector. If all you have is a dew-zapper gun—a 12 vdc hair drier cum window defroster—you will soon lose the battle against dew. A zapper can be sufficient for the secondary mirror of a Newtonian reflector, however.
You’d think this would be something I’d tell you to leave at home, but it isn’t. Even for a relatively short visual observing run, being comfortable means you will see a lot more. Bring the chair along.
Use that checklist to make sure you bring the vitals: eyepieces, star diagonals, star charts (or a smart phone or tablet), red flashlights, etc. Don’t overdo. I restrict myself to one eyepiece case and one accessory box (a large Plano tacklebox).
|"If it ain't raining." Sometimes you eat the bear, and sometimes...|
Whether “just” a can of Deep Woods Off, or a Thermacell, don’t even think about heading to the dark site without bug zappers except in the very depths of winter.
“But Uncle Rod, it’s only gonna get down to the lower 70s.” Bring a coat or sweater or sweatshirt anyway. You will never be colder than when standing nearly stock still at a telescope under an open sky. Let it get to the mid-60s and you will begin to shiver and will throw in the towel unless you are prepared.
Don’t just always bring your cell phone with you as we insisted last time, make sure it is fully charged before leaving home. Taking a DC charger to the site might not be a bad idea either.
A Few Amenities
You won’t be out there that long, so don’t pack too much additional stuff, but certainly a few bottles of water and maybe even a couple of snack items is “reasonable.”
It’s your dark site, set up anywhere you like, right? Sure. But some places are better than others. If there’s been a recent rain, you’ll be better off on your field's high ground if it has any. You probably don’t want to be on a slope, however; telescopes are happiest on level ground. One other thing? Togetherness is fine. You want to be close to your buddies so you can share observations, chat, etc. However, if you’ve only got a few people at the site there’s no need to set up 3-feet from the next scope. Spread out and give each other some room.
You observe the way your normally observe at home or at a star party. What I’m really talking about here is observing rules. Your club probably needs to come up with a few. You want to prohibit white light and probably green lasers. But you don’t want to keep adding so many rules that people feel stifled. And be aware that at a club site with two or three people on the field, all those beloved rules formulated at that marathon club business meeting are likely gonna be observed in casual fashion at best.
For example? You wouldn’t dare fire up your vehicle and drive off a star party field at midnight, but at the old dark site with a few people around? It’s likely to be, “Had a great night Wilbur! See you next time, Hiram! Gotta head on home.” If everybody’s observing visually they can shield their eyes while you motor off. If someone’s in the middle of an astrographic exposure, wait until they are done. Just use common sense—and the same goes regarding any rules you and your mates think up.
The Moon is rising, or it’s just late, or it’s just you and your friends’ usual turns-into-a-pumpkin time. What about tear-down of the equipment? If, as above, you have to leave earlier than your pals, you’ll need to pack by red light (one of those red LED head-lights on a head-band is good if you keep it pointed at the ground).
|You'll soon tire of using a dew-zapper gun...|
Anyway, when the time comes, disassemble your scope, taking care that everything gets back into the vehicle. I tend to be a little less than scrupulous about putting every widget back in its proper place in the cases; I just shovel it all into the car so as not to delay my friends, and worry about sorting everything out the next morning (I can leave all my stuff in my car overnight thanks to a safe and secure neighborhood).
Once everything is back in the car, go over your area carefully with a white light to make sure nothing got dropped and that you’re not leaving any trash behind. Help your fellows out with the same thing. Then—well, you’re off for home (or perhaps your favorite late night bar out in the boondocks).
You’re off if everybody is ready to go, that is. One rule our club has made and that we observe scrupulously is, “Nobody leaves till everybody leaves.” Obviously that doesn’t apply to someone who has to go early for whatever reason. It just means those of us left at the end of the night linger on till the last person has their gear packed. That’s good for security’s sake, but even if, like us, you have a very safe dark site it’s still a good rule to live by. What if the last person standing has trouble getting their vehicle started?
Then, just say your goodbyes, “Great time, y’all! See you next time.” If you did everything right, and your club has the “right” site, believe me, you’ll hardly be able to wait for next time.
Sunday, April 30, 2017
Issue #539: Get Thee to the Dark Site Part I
Spring is here, and with it maybe some clear skies that will encourage you to get out to your club or personal dark site (we’ve had very little rain here, but almost constant clouds). Yes, I constantly preach the worth of the good, old backyard as a spot for deep sky observing, but this is spring, and spring cries out for dark skies.
While some of the bright objects of winter are still on display, if you’re like me you’re focused on the “new” stuff now, the spring wonders on the rise. And what is spring all about deep-sky-wise? Galaxies. The mind-blowing Realm of the Galaxies that stretches from northernmost Coma through southernmost Virgo is back.
Alas, no variety of deep sky object is more harmed by the average suburban sky's light pollution than galaxies . Yes, diffuse nebulae can be tough from the backyard too, but at least a light pollution filter can help some with them. Unfortunately, there is no such thing as a “galaxy filter,” though a few rascals have sold mild LPR filters labeled as such a time of two over the years. The only way to see the marvels of spring as they should be seen is to get to a dark(er) site.
“Dark site? What dark site?” How do you find one if you don’t have one? Lots of factors can influence that process. Are you a club member or a lone wolf astronomer? Are you out in the semi-hinterlands where better skies are a short distance from your domicile, or are you stuck in an urban megalopolis? I can give you at least one unvarying piece of advice to begin, however: if you are indeed a lone wolf, join a club. It’s much easier to secure a dark observing location as a group.
“But I’m not really a joiner, Uncle Rod.” That can be fine if you’ve got a close friend with a piece of land out in the dark and that friend is amenable to letting you use it for observing on a regular basis. If you don’t know anybody like that, though, watcha gonna do?
Forget parks, state and national. Most will insist you buy a camping permit and stay overnight if you intend to be onsite past dusk. Even if you are amenable to that (not me; 3 a.m. is my absolute witching hour), few parks have anything that will serve as an observing field. If there is a suitable open space—and there won’t always be one—it will likely be festooned with streetlights. That’s just the way it is—east of the Mississippi anyhow.
As a member of a club, the possibilities increase exponentially. Even if the club you join doesn’t already have a dark observing location—and it likely will—there will be enough people, even in a smaller club, to guarantee a much larger circle possibilities, a larger group of friends and friends of friends with property out in the dark.
If you get down to “friend of a friend of a friend,” in a dark site quest, you’re much more likely to receive permission to use the property if you approach the person as an organized group rather than an individual (“What kind of a nut are you wanting to come on my land at night?”). And a doubtful land owner can sometimes be swayed with the promise of a modest yearly or monthly check from the club.
Let’s say your club doesn’t currently have a dark site, but sure does want one. What sort of spot do you look for? If you’re bereft of anything but the backyard, any dark location will seem like heaven, but, nevertheless, some places are better than others…
If a site is farther than about 60-miles from the club’s home base, it won’t get used much. As you may have heard tell, it’s often hard to get club members out for observing anyway. My invariable experience is that you can expect maybe 5% of the membership to show up on any given dark of the Moon weekend.
Place your site farther from home than those 60-miles and a maximum 1.5 – 2-hr drive and you probably won't even get the five percenters. You’ll find yourself alone most nights, and you’ll probably stop going frequently yourself after some of the initial fun wears off. 60-miles, however, can work, and is about the distance you need to get from medium-sized and larger cities before sky conditions begin to improve dramatically. See this light pollution map for guidance on how far you need to drive from your particular town (and which direction you need to drive in) for good observing.
If you live in a city that’s got a population of less than 250,000 or so, you can fudge on the 60-miles. My own site is about 30-miles to the west of Mobile, Alabama. Yes, there’s a significant light dome to the east, but that short drive ensures our site gets used frequently. There’s often no more than 2 – 3 observers on site on a clear night, but there is always at least that many folks on the field on any nice evening.
Yeah, I know you often can’t afford to be too choosy, but if there are alternatives there are some things it is best to avoid, starting with bad access roads. Yes, the site is nice and dark, but if getting to it requires traversing a rutted dirt track best suited for 4-wheel drive vehicles, and which is a swamp for weeks after a rainstorm, pass the place by if at all possible. Likewise eliminate a site where any part of the drive is difficult, not just to include the final access road. Paved highways leading to the site and gas-stations and/or convenience stores along the route are a practical must.
Before settling on a dark site choice, a few final checks are mandatory. First of all, get out there with a group from the club (whether your dark site committee or just an interested group of observers) and give the place a try. Try to hit the field on an average, not outstanding, night to get an idea what you should expect most of the time. What to look for? What’s the zenith limiting magnitude? Can you see all the stars of the little dipper (if you can, this will likely be a profitable location). Are there any light domes in addition to the one in the direction of the city? If so, is there still enough good sky for productive observing?
How about the field itself? Is it cut regularly? Can it be? This is very important. The site might be OK in the winter, but in the summer with grass three feet high, what are you going to do? Forget leveling your tripod; how are you going to avoid stepping on Mr. Snake? If the property’s owner doesn’t cut it, you’ll either need to induce him to do so with a financial donation or arrange to get it done yourselves if you are leasing the site, either formally or informally.
Are there any/many ambient lights? You may be surprised at how many land owners have multiple security lights. Frankly, due to the growth of the meth trade, the country ain’t what the country used to be. How many lights are there? Can you live with them? If not—especially if you are formally leasing the land—look into providing the offending lights with full cut-off fixtures (with the permission of the owner, of course).
Finally, how are the bugs? Almost any open field anywhere is going to have some bugs at sundown spring – fall. But are the skeeters, midges, no-see-ums, and blackflies worse than normal and can they be dealt with with Off and/or a Thermacell? Often, really bad bug problems can be traced to a nearby farm pond. Before seriously considering a site, check Google Earth to see if there is a stagnant body of water nearby.
We’re conditioned to think “country safe, city scary.” In recent times, however, thanks to the above-mentioned drug explosion in the country, there has been a reversal. There are certainly some unsavory goings-on out in the boondocks these days. How do you pick a safe dark site?
The worst scenario is a piece of land in full view of a frequently traveled road with ungated access and no homeowner/farmhouse nearby. Before passing up on a site like this, check to see what the crime scene is like in the area (if there’s a newspaper covering the county, you can get crime reports there). Talking to people familiar with the area can be highly illuminating.
If you have no other choice than a dubious site and think it’s worth the possible risk, go ahead, but I suggest making it a rule that “nobody observes alone.” Actually, that’s a good maxim even at a secure site. If your vehicle decides it doesn’t want to start a two in the morning, you’ll be glad to have a buddy or two to lend a hand.
Which brings us to the eternal question, “Should you go armed?” I carried a handgun with me to the dark site a few times a couple of decades ago when I was observing alone, but gave that up. I found that if I were so worried that I thought I’d need firepower with me, I’d be too nervous to observe anyway. I just couldn't concentrate on what I was seeing in the eyepiece. After a few minutes I'd begin thinking every snapping twig represented the approach of a psycho killer. When that train of thought began, I learned it was time to just throw in the towel. Far better than a weapon, I found? A couple of fellow observers. Even with just one other person with me, the place went from scary to friendly and familiar.
Always bring a cell phone on observing expeditions. Not necessarily because it will be handy in case of trouble with bad guys, but in case somebody has car trouble that can’t be resolved and needs a tow. Or, worse, someone has a medical emergency. A cell is worth ten times its weight in Walther PPKs.
Maintenance of the Site
Often, if you are formally leasing a piece of land you’ll be expected to take care of its upkeep. Not just to include the above-mentioned grass cutting, but care of the access road. Members’ cars put some deep ruts in it during the damp spring season? It will be up to y’all to get them filled in. It’s best to have a standing club “Dark Site Committee” as a vehicle to get things like this taken care of and paid for.
You’re not leasing a piece of land, just using it thanks to the kindness of the owner? Don’t wear out your welcome. Even if you’re not obligated to get those ruts filled in, do it anyway (or get a check to the owner). As for the site itself, make sure than when the group leaves it is as much as possible in the same condition as when you arrived. No trash, no cigarette butts, etc. If the owner’s home is nearby, keep the hee-hawing down in the middle of the night. Yeah, know that meteor was pretty, but don’t holler “GOOD ONE!” at the top of your lungs at two a.m.
Yes, it’s OK to invite a prospective club member to the dark site, but… Make it clear that that is a one-time good deal and that regular access to the club dark site requires a paid membership (and possibly an additional dark site fee to cover site maintenance). If you don’t, the word will eventually get out, and you’ll have people you don’t know and don’t know anything about showing up at your observing field.
Enjoy observing from a safe, secure, and dark location! How do you best do that? That’s a story for next time.
Sunday, April 09, 2017
Issue #538: Is This the End?
Certainly not. The Blog from Chaos Manor South has been rolling along for ten years, and I have every intention of continuing it into the foreseeable future. B-U-T. For a number of reasons, I’m backing away from weekly updates—at least for now.
The Astro Blog actually began considerably more than ten years ago, well before it came to Blogger.com. It was first offered on AOL’s long-gone dark ages blog service, blogger.aol.com. What was it like then? It was short and it came out irregularly. I published when I wanted to, whether that was a week after the previous entry or a month.
It’s been obvious to me for some time that the Blog can’t continue as is on a weekly basis. So, what should I do? Shorten it or publish less frequently?
I’ve demurred on “shorter.” One of the joys of doing a blog is that, unlike in a magazine article, I can stretch out. If a subject needs 3,000 words, it can have 3,000 words. That leaves “less frequently.” How less frequently will it be? I’m considering every other week as a reasonable schedule. But don’t hold me to that. It might be “once a month.” On the other hand, if there’s something I really need to talk over with you, only a week might elapse between entries.
Is this the way things are going to be from here on out, then? Not necessarily. When the current semester ends and I have a whole summer of semi-idleness before me, we could get back to once-a-week. Don’t count on that, but I suppose it could happen. How will you know when the Blog is updated? Follow me on Facebook and/or Twitter.
So, what’s an Uncle Rod True Believer to do on Sunday mornings? If you are new to the flock, there are months and months and months of archived articles for you to peruse. I like to think most of them are as useful and as much fun as they were when they were first published.
Do you find this disturbing? If you do, THANK YOU. It’s been my pleasure to bring this blog to you almost every Sunday year after year, and I am constantly amazed and gratified at how much of a looked-forward-to routine it’s become for many of you. That is reward enough for the labor that has gone into this and is THE reason I intend to continue the Little Old Blog from Possum Swamp.
Sunday, April 02, 2017
Issue #537: The Novice Files: Star Charts Part I
When we last left off with the Novice Files, we’d talked about stars, constellations, and catalogs of stars and deep space objects. This time we’ll be dipping a toe into the somewhat deep water of star charts, “sky maps.” Like that guy standing on the corner in Hollywood hawking his wares says, “You can’t find your way to the stars’ homes without a map.”
Basic Star Charts
You’ve got to have star charts if you’re going to learn how to use star charts. If you want to begin cheaply, I suggest these maps from Sky & Telescope, which cost just a couple of dollars, an equatorial star chart (SC001) and a north circumpolar star chart, SC002 (they have a south circumpolar chart, too). These simple paper maps have been around for decades, will teach you a lot about both the sky and star charts, and will remain useful as long as you do astronomy.
Let’s look at the equatorial star chart first. As the name implies, this chart is centered on the Celestial Equator, the imaginary line in the sky that divides the sky globe into Northern and Southern Celestial Hemispheres. The Equator on this map is the horizontal, triple, hash-marked line that divides the chart in two. Everything above the line is the Northern Celestial Hemisphere, and everything below is the Southern Celestial Hemisphere.
Intersecting the Celestial Equator at two points is a curving, sine-wave-like line. That is the Ecliptic, the apparent path of the Sun through the sky. Why is it curved? As we learned previously, due to the tilt of Earth’s axis the path of the Sun moves north and south in the sky over the course of the year. When the Sun’s path is as far to the North as it goes, we have summer in the Northern Hemisphere. When it is as far to the south as possible, it’s Winter (and summer in the Southern Hemisphere).
You’ll further note that the ecliptic is marked with dates. Those dates represent the position of the Sun at noon on that date with relation to the background stars. On June 1, for example, you’ll find the Sun in the midst of the stars of the constellation Taurus. Finally, the places where the Ecliptic intersects the Equator are the Equinoxes, the Autumnal and Vernal Equinoxes.
|A portion of the equatorial chart...|
What was the first thing that probably caught your attention on the chart? The stars and constellations. The stars are represented by dots of varying sizes. The bigger the dot, the brighter the star. The range of stars shown on this simple map goes from -2 at the bright end to 6 on the dim end. Actually, while there are a few stars down to magnitude 6 shown, most are left off of this large-scale chart. A magnitude 6 star is the dimmest star most people can see with their naked eye from a reasonably dark site.
How does stellar magnitude work? It’s a logarithmic scale. A magnitude 1 star is 2.5 times dimmer than a magnitude 0 star, and a magnitude 2 star is 2.5 times dimmer than a magnitude 1 star. There are objects, like the planet Venus, the Sun, and the Moon that are brighter than magnitude 0, so there are negative magnitude values as well. Something with a magnitude of -1 is 2.5 times brighter than something that shines at magnitude 0.
You’ll notice that every star on the chart is identified, either by its proper name if it has one, a Greek “Bayer” letter, or a Flamsteed Number, all of which we went over in the last edition of the Files. You’ll also see there are a few deep sky objects scattered amongst the stars, but just a few; mostly the brightest Messier objects. There’s a key at the top of the chart next to the magnitude scale that identifies deep sky object symbols, allowing you to tell if an object is a nebula, galaxy, or star cluster.
But how do you find things on the map? The same way you do on a terrestrial map, using latitude and longitude. As we learned previously, celestial latitude is declination, and celestial longitude is right ascension. The right ascension scales run across the top and bottom of the chart, showing distances east and west of the Vernal Equinox (located at 0h right ascension), while the declination scales are, naturally, on the right and left, since declination is position north and south of the Celestial Equator.
There are two ways to use the declination and right ascension scales. You can, most of all, use them to locate objects. If you have the right ascension and declination of Sirius, the Dog Star, for example (from a catalog or from a Google search, perhaps), you can easily find the star on the chart.
First, locate Sirius’ right ascension, 6h 45m, on the scale at the top or bottom of the chart (each little tic is 5’). Place an index finger on that. Now find -16-degrees on the right or left dec scales (each tic is one degree). As you’ll recall, a minus declination is a south declination, so you’ll be on the part of the scale below the Celestial Equator. Place your other index finger on -16-degrees. Now, run your two fingers down and across. Where they meet will be, approximately anyway, the location of Sirius.
|The circumpolar chart...|
The other way to use the scales is to use them to find the declination and right ascension of an object. We see where Sirius is, but what are its coordinates? Place an index finger on Sirius, go straight up or down to the right ascension scale, and you’ll have its R.A. Move your other finger straight left or right to the declination numbers, and you’ll have its dec.
In addition to stars and a few deep sky objects, the chart shows the constellations, the “stick figure” star patterns we introduced a few weeks back. The Sky & Telescope charts use a set of stick figure designs sometimes referred to as “traditional” that are in my opinion the clearest and most easily remembered shapes for the star figures. One question I’m occasionally asked is “What is the right ascension and declination of a constellation?” Since the star patterns cover a fairly large area of the sky, the way you do that is either to use a point in the middle of the stick figure, or to use the constellation’s brightest star as your reference point.
The Circumpolar Chart
While using the declination scale on the equatorial chart, you may have noticed it stops at 60-degrees and -60-degrees, the chart is cut off to the north and south. Why is that? Think back to elementary school. Likely there was a map of the world on the wall, probably a Mercator map, a map using the Mercator projection system. What else might you remember? What I remember is that, weirdly, Greenland was bigger than South America on the map, something I knew wasn’t true.
“Spreading out” the curved surface of the earth onto a flat plane causes distortion north and south. That makes the smallish Greenland huge. If the equatorial star chart continued above 60-degrees north or south, there’d be this same sort of distortion—the far northern and southern constellations would be badly misshapen. The mapmakers here decided to avoid that by placing those constellations on a separate chart, which shows the last 30-degrees before you get to the pole.
Everything in the circumpolar (“around the pole”) chart is the same as on the equatorial chart with two exceptions. The right ascension scale goes around the outer circumference of the chart circle, and the declination scale cuts the map in half. To find right ascension of, say, the bright star at the end of the Big Dipper’s handle, Alkaid, move straight down from it to the periphery of the chart circle, landing on 14h 10m (approximately). Declination is slightly harder to find, but not much. In addition to the declination scale that cuts the circle in half, there are additional unnumbered scales with tick marks. By referencing the one closest to Alkaid, I see it’s one tick down from a major line of declination (each tick is one degree). Referring the labeled scale, I determine the star is at just a bit over 49-degrees north.
There’s one other interesting feature on the circumpolar chart, a big, dashed circle labeled “orbit of the precession of the pole.” What’s that? Well, have you ever played with a child’s top? What happens when it begins to run down? It begins to wobble. The same thing is happening with the Earth. Don’t worry; it isn’t going to fall over, but it is wobbling. Imagine placing a laser beam at the north pole. As the Earth wobbles, the laser will scribe a circle on the sky globe. The point where the laser beam touches the sky globe is, or course, the position of the North Celestial Pole. Precession, the wobble, is slow and it would take 25,765 years for the Celestial Pole to move around the circle one time.
Because of Precession, as the long years roll by, the North Celestial pole moves among the stars. Looking at the circle on the chart, in the distant past, in the days of the ancient Egyptians, the pole was closest to the bright star Thuban in Draco. At that time Thuban was the North Star. In the distant future, the pole will be nearest Vega, and it will be a brilliant pole star. Naturally, the same thing is happening with the South Celestial Pole, and our colleagues to the south will eventually get a good pole star (their current one is relatively dim). Since the pole, 90-degrees declination, is moving against the background stars, the coordinate system and the Equinoxes are being dragged along with it. That’s why star atlases are often identified as “Epoch 2000” or similar. That means that the coordinates in the charts were in the places shown with reference to the stars in the year 2000.
There’s another result of this slow movement over the centuries: it’s put astrology’s Sun signs off by one constellation. Find your birthday on the ecliptic and you may be surprised your “sign” is totally different from what’s given in the newspaper horoscope. According to the astrologers, I am a Cancer, but looking at the ecliptic on the chart shows that on my birthday, July 17, the Sun is actually closer to Gemini. The astrologers set up their Sun Signs many a long year ago and never bothered to change them despite Precession throwing everything increasingly out of whack. Oh, and as you'll see if you look along the path of the ecliptic on the equatorial chart, the band of constellations that lie along it, the Zodiac, includes our old friend Ophiuchus, which the astrologers somehow overlooked.
So, you can find stuff on the equatorial chart now. But it would also be nice to know what was where in the sky for any given time. At first, it’s not immediately obvious how to do that with a chart like this that shows the whole (equatorial) sky, but it’s really simple.
Want to know what’s overhead? Find today’s date on the ecliptic. The constellations that lie long the line of right ascension that passes through that point on the ecliptic are those that are overhead at noon. Unfortunately, it’s not too helpful to know which constellations are overhead at noon. Midnight would be better. That’s easy to do, though. Say the right ascension line overhead at noon is 21h. Count 12 hours of RA to the east (left). That line, 9h, and the stars and constellations along it will be overhead at midnight. If you're interested in what's up at 11 p.m. go 13h to the left, and so on.
Monthly Star Charts
Determining what is "culminating" (straight overhead) for a given time is easy enough with the above system, but it’s not overly convenient. It would be nice to have a chart that shows how the evening sky looks at a given time of year, maybe for the current month. You can get that easily. Sky & Telescope includes an excellent monthly sky chart in each issue. One won’t lead you to tons of deep sky objects, but the brighter ones are marked, and the monthly chart is wonderful when you are just learning the constellations. The Sky & Telescope "annual," Skywatch, features 12-months of these charts under one cover.
What would be better still? Something, some sort of sky map, that would tell you exactly how the sky looks right now, at the current time and date. Certainly, there are plenty of computerized star charts, “planetarium programs,” that will do that, but we’re not quite ready for them. Instead, let’s begin with a simple analog computer.
You know what an analog computer is, right? Like a slide rule (blank looks from youngsters). A planisphere is a special sort of analog computer that can show how the sky looks for any time or date. It’s very simple—no batteries, no lights, no screen, just a couple of pieces of paper or plastic—but this device has helped generations of amateur astronomers.
A “sky wheel,” as some people call planispheres, is as above in two parts: a round wheel on which the sky is printed, and a stationary piece with a window. Around the sky wheel’s periphery are dates, and around the stationary piece are times. Line up the current time (or the time you are interested in) with the date and the planisphere will show the way the sky is laid out at that time/date.
It gets better. Set the planisphere for the current date/time, go outside, hold it over your head with the arrow or letter on the stationary part that indicates “north” pointed north, so that the west side of the planisphere lines up with actual western horizon and east and south line up with actual eastern and southern horizons, and you’ll be looking at a chart that not only shows what’s where in the sky, but which corresponds to actual directions in the sky.
Using the planisphere to show how the sky looks at a given date and time is one way to use it. There’s another way, though. You can use the planisphere to tell you when some event will occur. When will Orion rise on a given date? Turn the sky wheel so Orion is just above the eastern horizon. Then find the date you are interested in. The time Orion will be in that position will be opposite that date.You can also find the date when Orion will be rising at, say, 10 p.m. Locate 10 p.m. and read the date opposite it. Simple, neat, elegant.
A planisphere is very useful no matter what your level of experience in astronomy, and I always keep one in my accessory box. Where do you get a star wheel? Sky & Telescope sells a nice one. One of the better planispheres I’ve used over the years is made by David Chandler. You can even find them in book stores, including a cool oversize model by my friend David Levy (of Shoemaker – Levy fame, natch).
Are there any gotchas to planispheres? Only a few. In the spring most of us have to move our clocks forward for daylight savings time. While we can move the hands of the clock to suit ourselves, however, we can’t grab the sky and move it forward. That means planispheres always work on standard time. If DST is in effect and you want to know how the sky will look at 9 p.m., you must set the planisphere for 8 p.m.
Looking at the Sky & Telescope planispheres on the webpage, you’ll notice star wheels are sold for specific latitude ranges, 30N, 40N, etc. They are tailored so their northern and southern horizons are at the proper (approximate) declinations. In truth, even if all you can find is a 40-degree one and you live at 30-degrees, or vice versa, that “wrong” planisphere will still be quite usable.
Finally, because the sky on a planisphere is just a map printed on paper or plastic, it can’t depict the planets, which move among the "fixed" stars. That is where computer planetariums come in. And that subject, getting started with computerized charts, will be up next for the vaunted Novice Files. Till then…
Sunday, March 26, 2017
Issue #536: Deep Sky Imaging in Seven Easy (Sorta) Steps
|All those cables!|
We’ve spent the last several weeks setting you up with a telescope, mount, camera, and guide system. Now it is finally time to get outside with all that gear (assuming you, unlike me, have clear skies) and use it to capture the deep sky wonders of Spring.
Step One: Set Up
Naturally you’ve got to assemble the telescope and mount. But where should you assemble them? Unless your backyard is really, really horrible light pollution wise, I strongly council you to use the good, old back forty the first couple of times you work with the new gear. You’ll be dealing with a bunch of unfamiliar equipment and some complex tasks, and it’s always easier to do that at home where you can run inside for a look at a manual (or for a quick drink if you get really stressed) under white light rather than squinting at the instructions with a red flashlight at a dark site.
Anyhow, set up tripod and mount and level them. How precise does level need to be for a German equatorial? Technically, you don’t have to level the mount at all. All it needs is to be level enough so that it doesn’t tip over. In some cases, being close to level can make polar alignment easier, however. Level won’t affect polar alignment, period, but being near level means the mount’s altitude and azimuth adjusters won’t interact. When you move in altitude, it doesn’t also affect azimuth, and vice-versa, making it quicker to dial in alignment.
Next, attach the counterweight(s) to the declination shaft. Always mount the weights first, followed by the telescope. You will be mighty unhappy if you do the reverse, your R.A. lock isn’t secure, and the telescope slams into the tripod. When tearing down at the end of the run, reverse that. Remove the telescope first, then the weights. Once the scope is safely on the mount, install everything you’ll be placing on the tube: guide-scope and camera, imaging camera, finders, etc.
Balance is very important if an inexpensive GEM is to track well, so spend enough time with that to get it exactly right. First, decide which side of the Meridian you’ll be imaging on, east or west, and balance accordingly. You’ll always want to be slightly east heavy. If you are going to be imaging on the West side of the Meridian, you should be “scope heavy.” On the east side? “Counterweight heavy.”
|Polar alignment with Sharpcap...|
Balance in R.A. first. Point the scope north, lock the declination lock and, with the counterweight down and halfway up the shaft (if you don’t know approximately where it should be), undo the R.A. lock at least partway and move the mount in R.A. so the counterweight bar is level. Do not let go of the scope. Now, still without completely letting go, allow the scope or weight to rise or sink. When you’ve determined which way the weight needs to go on the shaft, up or down, return the mount to counterweight down position, lock the R.A. lock and move the counterweight as required. Return the mount to counterweight bar level, and see if balance is perfect. Keep on going with this procedure until it is.
Now for the East heavy bit. When you are in perfect balance, move the weight about ½-inch up the shaft if you are imaging to the west, and ½-inch down the shaft if you are imaging to the east. That should be more than enough to ensure the R.A. gears remain constantly meshed in the interests of good tracking.
For declination balance, return the mount to the counterweight bar level position, lock the R.A. lock and, holding on to the telescope, release the declination lock. When you know which way the scope needs to go in the mount saddle, forward or back, return to the counterweight down position, move the scope (carefully) as required return to the counterweight bar level position, and check. Keep going till the scope is balanced in declination.
What if your mount, like many in this class, is a little stiff on the declination axis and is somewhat difficult to balance? Don’t worry about it too much. Your mount is not tracking in declination, and if you’ve done polar alignment well, PHD shouldn’t have to issue many guide corrections on that axis. Good R.A. balance is far more important; declination balance can be “approximate” without hurting anything.
Step Two: Polar Alignment
If you are using a polar alignment borescope, Polemaster, Sharpcap, or the Kochab’s Clock method, do polar alignment now. None of these methods require the scope to be powered up and tracking, so it’s convenient to do the polar alignment before the mount is all festooned with cables and hand controls (I use Sharpcap these days). Take your time and do as good a polar alignment as you possibly can; you’ll thank me later.
Step Three: Hooking Up
|Connecting to the mount...|
Plug in all the cables and the telescope’s hand control. You’ll have at least four and maybe five cords to deal with: Power cable, serial cable for scope control, imaging camera USB connection, guide camera USB connection, and an ST-4 cable if you’re going to guide through the mount’s auto-guide port. Try to do a neat job with the cords, arranging them so they are not prone to snagging on the mount or tripod—which will ruin your guiding. Don’t forget to attach dew heater strips, dew heater controller, and dew shield if you need them in your environment.
Step Four: Goto Alignment
Take care of goto alignment now. Do whatever procedure is required to get the mount going to its gotos. How exactly do you line up the alignment stars, though? You can choose one of two methods. You can either remove the camera from the telescope and temporarily replace it with diagonal and eyepiece, or you can use the camera to do the alignment. In the beginning, it may be easier to remove the camera and do the goto alignment with an eyepiece.
If you do use the camera, you’ll, of course, need to turn it on (and the laptop and its software if you are tethering to a PC), and center the alignment stars on the camera’s display or the laptop screen. Since alignment stars are bright, one will allow you to get rough focus, too.
If you do use the camera, you’ll, of course, need to turn it on (and the laptop and its software if you are tethering to a PC), and center the alignment stars on the camera’s display or the laptop screen. Since alignment stars are bright, one will allow you to get rough focus, too.
If you choose to use Celestron’s AllStar Polar Alignment, which is built into the hand control firmware, take care of that once goto alignment is done—ASPA requires the goto alignment be accomplished first. If you do a declination drift polar alignment (horrors), now is also the time to do that, since having the telescope tracking during the procedure makes things much easier and is practically required.
Step Five: Focus
|The Double Cluster is an easy and pretty target...|
If you goto aligned using the DSLR, you’ve got focus roughed in, and can now do a fine focus procedure. If you used an eyepiece instead of the camera, however, get rough focus with the imaging camera at this time. If the last alignment star was a good, bright one, stay on it and use it to focus.
To get in the focus ballpark, adjust the focuser on the telescope (I am a big fan of remote moto-focus for imaging) until the bright star is as small as you can make it and dimmer field stars begin to appear and sharpen. Exposure? I like one to two seconds; that allows me to see results quickly after tweaking focus. Set camera ISO as high as needed to get a good image of the stars. If you are way out of focus, you may need to max ISO out and increase exposure time till you detect the big round globe of a star (in a refractor). Once it is closer to focus, back off on ISO and exposure for a less overexposed star image.
When the field stars are as small as possible by eye, tweak focus with a fine-focus method of choice, which may be a Bahtinov focus mask, or a focusing routine built into imaging software (like Nebulosity) if you are tethering the camera.
Step Six: Acquire Target and Compose Shot
Rough focus done, I interface my planetarium program (Stellarium these says) to the GEM. I start Stellarium (or whatever), and connect it to the telescope mount, that is. How? I invariably use the ASCOM telescope driver system, even if the program on the laptop, like Stellarium, has built-in telescope drivers. Why? Because ASCOM includes a little onscreen telescope HC that allows me to move the mount (at different rates) from the computer. That means I don’t have to get up and walk out to the scope and HC, press a direction button to center the object, walk back to the computer, etc.
|A Bahtinov Mask makes fine focusing easy...|
Alright, time to get on our first subject. What should that be? Even if you are at least somewhat experienced in deep sky imaging, begin with something easy with this new rig. This time of year, perhaps a nice winter open cluster over in the west like M35 or M37 or the Double Cluster. One important consideration given the economical mounts we’re using? Stay away from the Meridian. These GEMs just don’t track well in that area. Don’t image an object that will come within 10-degrees of the Meridian before your sequence is done, and don’t begin imaging an object until it is at least 10-degrees past the Meridian.
Once the scope goto stops, take an exposure long enough to reveal your target to see how the composition of the shot looks. If the subject is not centered, or just not framed the way I want it, I use the ASCOM HC to fix that. I keep the exposures short enough to make framing easy, maybe referencing a bright star in the frame if the object doesn’t quite show up in 1 – 3-second shots.
What if the target object is not in the field of the camera at all when the goto slew is done? That’s not much of a concern these days for most mounts, but if you have a problem, a quick solution is to slew to a nearby bright star, center it with the aid of your finder and “sync” on it. You should then be able to slew back to the target and have it in the frame. Oh, before you do that, be sure it really isn’t in the frame. If the target is a dimmer one, increase exposure and ISO and see if it appears.
When the subject is properly centered fire up PHD2 and get auto-guiding going. The main gotchas there? Make sure the guide scope is well-focused and that the guide star you’ve chosen is neither too dim nor too bright (saturated). When you put the cursor on a star, PHD2 will tell you all about that. Some imagers believe the guide star should be slightly out of focus for best guiding, but I’ve found I get better results from sharp stars.
When the mount is guiding, I go back to the imaging camera and do a test exposure. How long should that test subframe be? That depends on the sky and the subject. If I’m in the backyard, going much beyond a minute causes the background sky to brighten up so much that processing can be difficult later. At my dark site, I’ll expose for 2 – 5-minutes. Exposure also depends on the subject. An open cluster like M37 will be just fine in 30-second – 1-minute subs. The Horsehead Nebula will not be.
One important thing to remember is that while you’ll be stacking many shorter sub-frames into a finished exposure, you still have to have each individual exposure long enough to pick up all the detail you need. Stacking subframes will make the final result smoother and less noisy, but will not show any detail not present in the individual subframes. Longer subframes are always better.
|Get your guiding going on...|
What should camera ISO (nee “ASA”) be set to? Normally, use as low a value as you can to capture the detail you want while keeping noise down. The higher the ISO, the noisier the image will be (and the brighter a light polluted background). I rarely go above ISO 1600, and try not to exceed 800 in the backyard if possible. Naturally, ISO and exposure time interact. In general, I’ve found it better to go with a lower ISO and a longer exposure when possible.
Also examine the test exposure for signs of star trailing. Assuming PHD is not going wacky on you, you’ve got a good polar alignment, and the seeing is OK, that should not be a problem at the 400 – 600mm focal lengths we’re using. If the stars don’t look right, go back to PHD and make sure it’s still guiding well (bring up the graph as shown in the image here). If it isn’t, you’ll have to troubleshoot.
If the stars are eggs or worse, first make sure the values you’ve entered for the guiding parameters are close to those we outlined here. One variable that can change from night to night is the guide camera’s exposure. While mounts in this class tend to do best with 1 – 1.5-second guide exposures, if seeing is not good a somewhat longer one can improve guiding. There’ll be less tendency for PHD to try to guide on movement caused by seeing.
Step Seven: Expose
Time to do what we came for, take an exposure sequence. Set the laptop program or the intervalometer to take a number of subframes at the exposure value you determined was best. How many? As many as possible. Even on an easy object like M37, more subs always make for a better looking finished picture. I generally aim for 20 - 30.
Before beginning the sequence, though, let’s put the dark frame question to bed. Since a DSLR’s sensor chip is not cooled, dark frames are mandatory to eliminate the false stars of thermal noise. There are two ways to subtract dark frames from subs, manually or automatically.
If you go manual, finish the imaging run and then, just before packing up for the night, cover the telescope objective and shoot subframes equal in number to the maximum number of lights you’ve taken in a sequence. For example, if you did one 20 and one 30-subframe sequence, take 30-darks. If you used different exposures on different sequences, you’ll have to do more than one sequence of darks—dark frame exposure values need to be the same as their corresponding lights. The darks will be subtracted from the light frames during image processing. This way of working is certainly acceptable, but, in addition to being labor intensive, it isn’t as effective as it could be in my opinion.
Me, I go automatic on dark frames for a couple of reasons. Not only do I not have to worry about messing with darks at the end of the evening or during processing, I think automatic darks are more effective.
How do you do auto dark frames? DSLRs allow you to select a mode called “long exposure noise reduction” (or a similarly named menu item). Engage that, and the camera will take a dark frame after each exposure and automatically subtract it. Yes, that means an imaging run will take twice as long as it otherwise would—30-minutes of subs will take an hour to complete—but I think the results are just better.
Why would the results be improved by taking a dark after each light? Because the temperature of the DSLR’s sensor chip will vary throughout the night. Ambient temperature will drop throughout the evening, and, as an exposure sequence goes on, the internal temperature of the camera due to its electronics will tend to rise. To be most effective, a dark should be taken as soon after its matching light subframe as possible, so the temperature it was exposed at is close to that of the light frame.
OK, set that computer or intervalometer for the number of exposures at the required exposure value, push the “go” button and…and…wander around and do something else while the exposure sequence completes. I usually just go inside and watch TV. If I’m at the dark site, I’ll cadge looks through my buddies’ telescopes. I’ll come back periodically and see how things are going—especially how PHD is guiding—but I rarely encounter problems unless clouds have moved in and my guide star has been lost, temporarily or permanently.
Once the sequence is finished, it’s time to go on to the next target. How many targets should you do? That’s for you to decide, but I tend to believe fewer targets, maybe just one or two per evening, and more subframes (and maybe longer exposures) is the way to go.
Done, I’ll pack everything up and head for home if I’m at the dark site, or, if I’m in my secure backyard, I’ll just cover the refractor and GEM with my Telegizmos cover and only take the computer inside—which is a much more pleasant way to end an evening under the stars than having to disassemble everything and carry it back into the house when I’m tired.
And next? Next is processing the images, but that is a story for some other Sunday.