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.