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 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. 

Sunday, March 19, 2017


Issue #535: The Final Piece of the Puzzle

In our pre-spring observing season drive to get novices (and maybe even a few not-so-novices) set up with a rig for deep sky imaging, we’ve addressed mounts, telescopes, and, last week, auto-guiding setups. This Sunday we’ll finish with suggestions for a low-cost camera. I’ve talked about imaging cameras with y’all fairly recently, but the difference is that this time I’ll try as hard as I can to keep the cost as low as possible.

So, you need a camera and a few accessories. Where do you start? The first question to answer is, “Do I want color?” While a monochrome CCD/CMOS astronomical camera can take color images by exposing successive frames through three or more colored filters, it’s not something you want to face when you are just getting off the ground in imaging. Unless you enter the ranks of the hard-core someday, you may never want to face it. In the beginning you will find just processing a “one-shot” color image enough of a challenge. Properly calibrating and combining three + separate frames into a color frame and then stacking and processing a bunch of those? Uh-uh.

So, it’s a color camera, a one-shot color camera, you want. How does one work? A color camera is different from a monochrome camera in that red, green, and blue color filters are built into the sensor chip. Software, either in the camera or in an image processing program, automatically combines the R, G, and B to produce a full color image. That is usually transparent to the user—with a digital single lens reflex (DSLR), anyway. You take a picture, you see a color image, end of story.

Some astrophotographers say a monochrome camera can produce visibly higher resolution images because it doesn’t waste pixels on the production of a color image. In truth, in the beginning at least, and especially on deep sky objects, you won’t notice any difference.

The next question is “CCD or CMOS?” That is not much of a question today. Unless you are interested in some special applications, mostly having to do with obtaining scientific data, there is no reason to choose a CCD chip over a CMOS chip. Today, the formerly preferred CCD has lost ground to CMOS sensors even for use in “astronomical” cameras. CMOS chips are now very sensitive and very low in noise. At any rate, almost all cameras in our price range, which I am topping out at 450 dollars, have CMOS chips, so the choice has already been made for you.

What a ZWO ASI120MC can shoot...
Next up, cooling. “Does a camera for taking long-exposure images need to have its sensor chilled to reduce thermal noise?” Today, probably not. With some camera/chip combos, an internal fan, at least, can be helpful to reduce the false stars of thermal noise, but the low-noise characteristics of today’s sensors usually means subtracting a dark frame is enough to deal with thermal noise.

And the Final Jeopardy Question… “Astro cam or DSLR?”  There are some interesting low cost astronomical cameras coming on line, like those from China’s ZWO, and I’ve actually taken credible deep sky image with one of their 1/3-inch cameras that cost a measly 200 dollars. However, I think for most of us a DSLR is just a much more sensible choice. A much more sensible choice.

Why is a DSLR better? There are several reasons, but there is one real big one:  when you’re not taking pictures of the night sky, you can be wowing everybody at your mother-in-law Margie’s birthday party with your snapshotting skills. There’s also that big elephant in the living room. Like many wannabe astrophotographers, a few nights wrestling with camera and scope may convince you you are actually more of a visual observer. If that be the case, you can still get years of use and enjoyment out of the DSLR, even if you never take another astrophoto with it.

Another big plus (for astro imaging) of the DSLR? Their relatively big chips. A less than 500 dollar camera will have an APS-C size chip. Lower cost astro-cams tend to have small chips that restrict your field of view, focal length for focal length, and also tend to make guiding more critical. 

Finally, while I control my DSLRs with a program running on a laptop (“tether them,” as we say in the photography business), which makes focusing and framing much easier, you don’t have to do that. You don’t have to have a computer out in the field when you are taking pictures. You can do just as we did in the SLR days:  telescope, mount, camera. You will, as in those SLR days, need a remote camera release (an intervalometer, preferably), but that is it.

OK, so which DSLR? The safe thing to say is still “Canon.” In some ways they still lead the pack in astrophotography. The Canons are remarkably low in noise over long exposures, and are easy to use in the field with a laptop if you choose to do that. Things are changing now, but until recently camera control software (like Nebulosity) was unheard of for other brands.

SCT Prime Focus Adapter
There’s also Canon’s longstanding involvement in our game. While Nikon and, now, Pentax are coming on strong for astrophotography, until the last couple of years only Canon acknowledged people were actually using their cameras for astronomical imaging and produced cameras with astronomy in mind.

Canon is a safe choice, in my opinion, but which one of their many DSLRs? If you are buying new and must keep the price tag low, the Rebel T6, which is available for about 450 dollars, is a remarkable value. Not only do you get a DSLR that will perform well for astro-imaging or anything else, you get a pretty good (zoom) kit lens for use in wide-field astrophotography or at Margie’s above mentioned b-day party.

Just don’t want a Canon for whatever reason? The equivalent Nikon is the D3300, which is even less expensive than the Rebel. And it can perform very well for astronomical imaging. BUT… Computer control options for this camera are (very) limited—it is not supported by the major Nikon astrophotography program, BackyardNikon—so if you want to tether camera to computer, a Canon is a far better choice.

How about buying a used camera? Is that a good idea? That depends. A fairly recent camera or seldom used older camera can push prices even lower. A perfectly serviceable older Rebel, like a 450D, for example, goes for 150 or fewer dollars with a kit lens and a few accessories. Be careful here, though. While the Rebels, Canon’s introductory DSLRs, and Nikon’s comparable models are well-made, they are not professional grade cameras and won’t stand up to real abuse. So, when considering an inexpensive camera it’s best to limit yourself to one that’s for sale locally so you can examine it in person and make sure it’s fully functional.


Prime Focus Adapter

Prime focus adapter (1.25-inch)...
Once you’ve got a camera, of course you’ll need accessories. You always need accessories in astronomy, you know that!  First off, you will need a prime focus adapter in order to connect camera to telescope. “Which” depends on your scope style. SCT prime focus adapters screw onto the SCT’s rear port. Those for other telescope designs, like refractors, typically have 1.25-inch or 2-inch nosepieces and slide into the scope’s focuser. I like the 2-inch models, not because you have to worry about vignetting or something like that with an APS-C size sensor, but because they allow me to dispense with a 1.25 – 2-inch eyepiece adapter and seem to provide a more secure mounting arrangement.


You’ll also need a t-adapter for your camera, aka a “t-ring.” This is a, yes, ring shaped adapter with T-threads on one end to screw onto the prime focus adapter, and a lens mount for your particular camera on the other end. These two things in hand, you can remove the camera’s lens, mount the combo of T-ring/prime focus adapter in its place, and then mount the camera on your scope by inserting everything into the focuser or screwing the prime focus adapter onto the rear port of an SCT.


As you may know, DSLRs, most of them anyway, and certainly all the Canons, can’t expose for more than 30-seconds without the addition of a remote shutter release. Even if your camera could expose for longer without a remote, you’d still want one as it allows you to trip the shutter without bumping the scope and causing trailed stars.

An intervalometer is a remote shutter release, but it’s also much more. Not only will one of these (usually) wired controls allow you to trip the shutter from a distance and expose for as long as you like, it will allow you to shoot sequences of images. Say 30 3-minute exposures, which is exactly what we want to do. An intervalometer allows you to do many of the things a tethered computer would allow you to do, but without the computer. How much? A Vello is about 50 bucks and a genuine Canon is about three times that. Guess which one I’d choose?

Memory Card

If you’re not using a tethered PC, you’ll have to have a memory card, digital "film" on which to store your images. An SD card (used by almost all DSLRs, now) with at least 64gb capacity is my recommendation—you’d be surprised how much space an evening’s images can take up. Get a good, decently fast card. I like the Sandisk ones. About 40-bucks.


If you’re going to use a battery, make sure you keep an extra, or, better, two extras in your gadget bag. During long exposures, the camera is drawing current from the battery continuously, and you’re unlikely to get a full evening out of one cell, especially on cold nights. There are lots of third party batteries available, but I have had noticeably better performance out of genuine Canon, so that’s what I recommend here, the real deal, for a change.

Power Supply

Yes, batteries are a problem during astrophotography, so don’t use one, or use a real big one. Hop on over to Amazon and buy yourself either a 12vdc or 120vac power brick for your Canon (or whatever). I do most of my shooting at locations with mains power, so I prefer the AC option. The DC supplies have cigarette lighter plugs that will plug right into your jumpstart battery pack.

What do you plug one of these things into on the camera end? These power supplies have little plastic (wired) widgets that take the place of the normal battery in the battery compartment and supply power to the camera that way. I’ve found one of the inexpensive—less than 15-dollars—units on Amazon to work just fine, but Canon will sell you one for considerably more if you like.

Anything else? Well, a few things, maybe. If you are new to DSLR photography, you probably want a camera bag, a gadget bag, to keep camera and lenses and, well, gadgets, together. A nice piggyback bracket so you can mount DSLR and lens on your telescope tube is a nice addition and you may find you like doing wide-field shots from dark locations. A lenspen is good to keep your lens’ surface pristine. A broadband light pollution filter can be helpful if, like me, you do some of your imaging from an at least somewhat light-polluted backyard. And that is really more than enough to get you started.

You’ve now got all the pieces to the complicated astrophotography puzzle, but how the heck do you put them together? We’ll talk about that, about getting started with all this stuff, next week.

Addendum:  How good can a VX be?

Auto-guiding wise, that is. Some of you considering a Celestron Advanced VX mount (or the similar mounts on the market today) have expressed grave concern about my statement last week that 2” (arc seconds) of RMS guiding error is about what you should expect of this group without some fine-tuning (of PHD’s Brain Icon settings, I mean).

Anyhow, while 2” is perfectly suitable for some image scale/camera pixel combos, naturally it would be nice to do a bit better with this inexpensive and highly portable GEM. So, I set about the other night to see how much and how easily I could tweak the VX.

Surprise! I really didn’t have to do much tweaking at all to get this modest mount’s RMS guiding error down. I did do a decent polar alignment, and I did spend some time carefully balancing the scope (east heavy with a little declination bias as well). As for the settings, I backed off on a couple of them. Cutting aggressiveness in half and reducing hysteresis as well. Oh, and, conversely, I increased Max Duration both for RA and declination.

The result? Despite OK but hardly great seeing, my errors were immediately halved with me getting just under 1” of RMS error most of the time. Even when my target got low in the sky, and seeing began to deteriorate, the error was just over 1”, easily good enough to yield round stars with an 80mm f/6.9 despite the fairly small (1/2-inch) sensor of the camera I was testing.

While I warned you not to start chasing lower and lower numbers with these GP/CG5 clone mounts merely for the sake of lower numbers, given the small amount of effort involved in this substantial improvement, the few minutes I spent was well worth it.

The other take-aways? People naturally worry about their guide-software settings, but what makes one of the very largest differences? Seeing. Without good seeing you will not see great guiding, so don’t start messing with your settings on an unsteady night. Oh, and good polar alignment is important for good guiding as well. Having to continually chase alignment-caused drift just muddies the water and makes guiding more difficult to get right. Finally, with this class of mounts, correct balance is just as important as polar alignment and seeing. If you want 1” or less guiding errors, you’ll likely need to rebalance if you move to a radically different part of the sky—cross the Meridian, etc. 

Sunday, March 12, 2017


Issue #534: Getting Your PHD

PHD2, that is, as in America’s premier auto-guiding software. I have written about the program, originally done by software wizard Craig Stark and now carried on as an open-source project, a time or two before, but lots of people still have lots of questions about it. It’s rare that my virtual mailbag doesn’t contain a missive pleading for help with PHD.

Before offering some of that help, I suppose I should explain what PHD2 is for the uninitiated. You’re probably more knowledgeable than I was when I began astrophotography.  Unlike me, you know you can’t just point your telescope and camera at a deep sky object, open the shutter, and walk away. You have to guide. The gears in most mounts are not precise enough to allow the scope to track precisely enough over longer exposures to keep stars round without some intervention.

To keep stars round, you watch a “guide-star” either with the main scope or a small auxiliary telescope, a guide scope, keeping it precisely centered. Or a little camera does that watching for you. There are some mounts that will allow you to dispense with guiding for long exposures, but you are talking about mounts in the 10micron class, expensive, top-tier mounts. Proletarians like yours truly guide their mounts throughout long exposures.

How exactly do you do that guiding? Well, back in the day, you monitored a guide star in a crosshair eyepiece in  the guide scope or in an off-axis guider, and pushed buttons on a hand-paddle—what we called our non-computerized telescope mount hand controls—to keep the star centered. Naturally, when computers and CCD cameras came along, we were more than happy to pass the onerous task of guiding to them.

A guide camera is used to watch that guide star, but most guide cameras cannot guide the telescope mount without the help of a laptop computer and an auto-guiding program. That program is the brains of the outfit, and that is what PHD2 is, auto-guiding software.

If you need direction on getting PHD2 downloaded, installed, and initially configured, please see this (fairly) recent article. Today, we’re going to focus on what you need to do to get PHD2 performing by fine-tuning its default parameters. What you have to do to get those pesky stars round.

What does “PHD” stand for, anyway? It ain't “doctor of philosophy,” but instead, “push here dummy.” Mr. Stark’s original goal was to produce an auto-guiding program that was as simple as it could possibly be. One that would allow you to hook everything up, push one button and guide your way to round star heaven. That’s actually possible in some cases, but due to the nature of the beast, often not.

The Guiding Tab...
There are so many different possible configurations of telescope/guide scope/guide camera/main camera/telescope mount, etc., etc. that making a no-set-up auto-guide program is a near impossibility. Oh, if you stick to shorter focal lengths (500mm and down) on a decent  (VX and up) mount, and don’t insist on longer than 300-second sub-frames, it is possible all you will have to do is push that button and guide. Most of us will have to mess with PHD’s guiding parameters, which are accessed with the program’s famous brain icon. Before we attack that, though, a couple of preliminaries: “What is the best way to guide?” and “What is the best guide-scope to use?” 

I am frequently asked by newbies how they should guide. Should they use an ST-4 connection, a direct connection from a camera to a mount’s auto-guide port, or should they guide through the hand control’s serial port?  I asked myself that very thing years ago when I first essayed auto-guiding.

Some people think serial port guiding, particularly “pulse guiding,” a feature of some ASCOM telescope drivers, is better since each guide message going to the mount contains not just the direction the telescope needs to move, but also for how long. With ST-4 guiding, once the software decides the mount needs to move, it will cause the camera to close an electronic “switch” to move the mount. When the move is done, the switch is opened. With pulse guiding, there is no (possible) time-lag resulting from ST-4 mode guiding having to send an additional command to open the switch. On the other hand, ST-4 fans say that since no back and forth computer talking is needed with ST-4 mode guiding, it must be inherently more responsive.

The ground truth? With my mounts/scopes/guide-cams, there was absolutely no difference in accuracy between the two methods. The pluses for each have more to do with convenience. If you are controlling your mount with a computer, why not pulse guide? If you are using EQMOD in particular, that seems a natural—everything, goto commands and guide commands, is routed to the mount over a single cable. On the other hand, while ST-4 guiding requires an additional cable run from camera to mount, there’s no fooling around with serial connections and USB to serial adapters, which is a good thing. I normally do ST-4 for that reason.

Calculating cal step size...
The other question concerns the guide-scope or lack thereof. What sort of a guide-scope should you use? In my opinion, the answer is one with a focal length of about 400 -500mm. That provides a fairly wide field for small guide-cam sensor chips, but also has enough image scale for precision guiding. The venerable Short Tube 80mm is a good choice as long as you can lock the focuser down firmly and mount the whole thing securely to prevent image-destroying flexure.

Me? I use a short focal length 50mm finder-guider. One of these will work up to about 1200 – 1300mm of imaging scope focal length, and is small, light, and easy to mount firmly. For anyone who doesn’t top 1000mm of imaging scope focal length, a finder-guider is a natural. Having that wide field is often a blessing when it comes to choosing guide stars.

There’s always the option of doing without a guide scope, too. Using an off-axis guider (OAG) which intercepts a small amount of the light coming out of the main scope for guiding. Obviously, since you are guiding through the main scope, there is no flexure to worry about. If you are running an imaging telescope at over 1500mm of focal length, you may find an OAG is your only workable option. The downside? You only have access to stars at the edge of the main scope’s field, and for that reason it can be quite difficult to find a good guide star. Luckily for me, a long time OAG hater, I rarely image at a focal length long enough to require one.

One final thing to discuss before we do “brain surgery.” How good does your guiding have to be? How much error is acceptable? The answer is, “that depends.” At 1000mm or less with an APS-C sized camera sensor chip, an RMS error of around 2” or so is good enough. Stars will be round and small enough to please. You can even get OK (if sometimes not perfect) stars at that error level to about 1500mm of focal length.

It’s a good thing this degree of error is acceptable at the focal lengths I use, since the plebian mounts I have in my inventory, GP clones like we discussed last week, and the EQ-6 and CGEM mounts a step above them, will deliver 2” of RMS error with fair ease. Getting guiding much tighter than that with these sorts of mounts isn’t always easy and will often take considerable experimentation.

Alright, click PHD2’s brain icon and let’s get started entering some guide parameter values in place of the defaults, parameters than will bring us round stars (we hope). With the brain window displayed, skip its first two tabs, “Global” and “Camera,” since I’m assuming you’ve gone through them in the initial program setup. Which brings us to…

Guiding Tab

The Algorithm Tab...
The first entry here is “Search Region.” This is the size of the tracking box PHD2 draws around a star. Normally you should leave this at the default value. If you have so much drift between guide exposures that the box needs to be larger, you aren’t going to get anywhere with guiding anyway. The accompanying “Star Mass Detection” has to do with PHD2 monitoring the star’s brightness as compared to the sky background. Leave this as is as well. Likewise, leave the tolerance setting for Star Mass Detection alone.

The next part of the window is quite important, “Calibration.” Enter the focal length of your guide scope (you should already have entered the size of the guide-cam’s pixels in the “Camera” tab), push the button labeled “Calculate,” and PHD2 will figure out how long guide pulse duration should be during calibration. The main concern here? If you have a short focal length guide scope like I do, you need to enter a much higher calibration step size than the default. I have a value of 1350 here. Given the short focal length of my 50mm finder-guider, I need that large a setting. Otherwise, calibration would take all freaking night to complete. Leave the other stuff here alone.

The final part of the window contains things you don’t have to worry about in the beginning. Well, except for one thing. Make sure “Enable Guide Output” is checked, otherwise PHD2 will not issue guide commands to the mount. It will be like that goober in the TV commercial, “I’m not a dentist; I’m a DENTAL MONITOR.”

Algorithms Tab

Here’s where we get down to the nitty gritty, the place where you can change the settings that really and truly affect guiding. You’ll see that the window is divided in two, with one area for right ascension and one for declination. Let’s begin with RA.

The first thing to set is Hysteresis. PHD2 is pretty smart; it can remember what the last RA correction was like and use that information in formulating the next correction. The number here is a percentage. It is how much the remembered previous correction affects the next one. At 40%, the next RA correction will be 40% based on the magnitude of the previous correction, and 60% on the star movement PHD2 is seeing at the moment.

Guiding Assistant...
What should you set it at? More Hysteresis yields smoother guiding. Too much, however, and a sudden guide star movement will not be adequately compensated for. I have my value at 40%, which seems OK.

Coupled with Hysteresis is “Aggressiveness.” That setting is how much (as a percentage) of the calculated necessary movement PHD2 actually sends to the mount. The reason for this is to decrease the chance of the mount overshooting the star, going back the other way on the next guide command, and overshooting in that direction too, “ping-ponging.” Normal settings rage from about 70% to 100%. I am set at 85%.

Next is “Minimum Move.” This is the amount the star is allowed to drift without PHD2 issuing a guide command. The reason for this is to reduce unneeded guiding corrections caused by non-tracking related star motions due to seeing or other momentary events like mount vibration, wind, etc. The default is .15 and that’s where I’ve left it.

Max RA duration, the last setting on the RA side, is similar to the above in that it’s meant to smooth out guiding, to prevent herky-jerky guiding. This figure is in milliseconds, and limits the duration of the RA guide command. I’ve settled on a value of 1200 for RA through trial and error. I am thinking that is low, however, and might try a higher value next time out. 

Now for the declination side of the house…

First up is “Resist Switch,”  which means PHD2 tries to avoid reversing the guide direction in declination. That is always a good thing, since in many cases issuing a guide command in dec to go back the other way will be a problem. Star movement in declination opposite the constant slow (you hope) drift caused by polar alignment errors is usually caused by seeing, vibration, mount flexure, wind, etc., and as with RA, we want to avoid issuing guide commands for these things. Most of all, many mounts have considerable backlash in declination, which would create a considerable time lag between command and movement if the mount reversed direction in dec.

Also on the declination agenda are aggressiveness, minimum move, and backlash compensation settings. I have the first two at the same value I have for RA. The backlash compensation option determines whether PHD2 will use a backlash compensation value it has computed if a declination correction opposite the previous one needs to be issued. I have this off, since I don’t seem to be having any major dec problems.

Max Dec Duration has the same purpose as in RA, to smooth guiding. I have my value set a little higher here than I do in RA, 1500, but it could probably be higher still.

Finally, there is “Dec Mode.” Normally this is set to “Auto,” which tells PHD that the occasional declination reverse guide command (caused by whatever) is permissible. Why would you want to disallow this by selecting “North” or “South”? If your mount has really bad declination backlash, trying to make a “reverse” correction may cause serious problems—the cure may be worse than the disease. I am set to “Auto.”

And that is it, folks. The other Brain tabs cover use of adaptive optics guiders and are of little interest to most of us.

Getting round stars with an import mount is fairly easy at 900mm...
How do you fine tune your mount if these values don’t work for your particular setup? Trial and error, which was what I did to arrive at the numbers I’ve given here. There is one alternative, though, PHD2’s “Guiding Assistant.” Theoretically, invoking this tool should allow the program to decide what your guiding values should be. When the procedure has completed its work, it will make suggestions, which you can implement or ignore at your discretion.

Alas, when I tried Guiding Assistant some time back, one night at the 2015 Peach State Star Gaze, the figures PHD2 came up with seemed to make my guiding worse rather than better. However, that was over a year ago, so the Assistant may have been improved by now. If you invoke it and use the suggestions, make sure you’ve written down your old numbers so you can get back to the way things were if Guiding Assistant doesn’t work for you.

I hope all this stuff didn’t put you off too much. Again, with a halfway decent mount and a reasonable focal length, you might not have to do much with anything beyond basic setup other than just setting your calibration step parameter. And remember, if your stars are round your stars are round. Don’t start chasing lower and lower error values just for the sake of lower values, “The Only Enemy of Good Enough is More Better.”

Sunday, March 05, 2017


Issue #533: A New Way to Polar Align

If you are using an equatorial mount, fork or German equatorial, for imaging, that mount has to be accurately polar aligned. The right ascension axis has to be pointed precisely at the North Celestial Pole or South Celestial Pole. If it’s not, longer exposures will suffer from a phenomenon called “field rotation,” which makes stars trail no matter how accurate the guiding. Heretofore, there were basically two ways to polar align a mount, the easy way or the hard way.

The easy ways? One was to use a polar borescope on a GEM. Once you figure out how to set it up, a polar finder can yield alignments ranging from excellent to usable depending on the borescope’s manufacturer and your expertise in using it. For many folks kneeling on the ground to peer through that dim little telescope in quest of a sometimes-rough polar alignment (unless you have a Takahashi mount and its excellent polar finder) is a bummer. Also, no truly accurate polar finder has ever been produced for fork mount telescopes, though some people, like the late Roger Tuthill, have tried.

Another fairly easy polar alignment method is “Kochab’s Clock.” That involves lining up the RA axis with the help of one of Ursa Minor’s stars. Kochab’s can potentially yield a good alignment  if done with care, but in most cases, not a sub 5’ – 10’ alignment.

Finally, there is the sure thing, a declination drift alignment, which, unfortunately, most of us don’t consider overly easy. Or at least not overly quick. You observe a pair of stars near the Celestial equator, and watch their drift in declination (through the main scope) as the telescope tracks, adjusting the mount’s altitude and azimuth controls until there is no significant north/south drift of either star over at least five minutes of time.

“Drifting” is not hard once you get the hang of it, but it does take time, and you have to be able to acquire suitable stars, one near the intersection of the Celestial Equator and the Local Meridian, and one near the Celestial Equator and about 15 - 20 degrees off the eastern or western horizon. That’s not always possible at every observing site.

And there things remained for years. In the 1990s, I used a fork mounted SCT, and did a two-step polar alignment. First, I’d rough it in using a 50mm finder scope with a polar alignment reticle. That was, as above, not a recipe for a good alignment on a fork mount scope, but it at least got me in the neighborhood. Then, I’d go on to drift, which took about half an hour or so once I gained some experience. I never liked drifting, though, and for that reason I usually quit before my polar alignment was quite good enough for the long exposures required in the film astrophotography days.

Typical polar borescope finder reticle
Flash forward ten years or so to the coming of the computerized GEM mounts like the Celestron Advanced GT CG5. One of the big breakthroughs with the CG5 (and also a few other brands) was an automated polar alignment routine. With the CG5, you did a three-star (no 2+4 in the firmware’s early days) goto alignment. You then requested “Polar Alignment.” The mount would then point at Polaris, and over the course of a couple of steps would slew away from the star. You’d then re-center Polaris using the altitude and azimuth adjusters. While you would be centering Polaris in the eyepiece, what you’d really be doing would be offsetting the RA axis to place it on the true Celestial Pole about ¾ of a degree away from Polaris (the routine also worked in the Southern Hemisphere).

This procedure didn’t produce a great polar alignment, but it was a little better than what I could do with the CG5’s (pitiful) polar borescope, and it was definitely quicker. It was sufficient for the short exposures at short focal lengths I was doing with my Meade DSI CCD camera at the time.

Then came Celestron’s new polar alignment routine, AllStar Polar Alignment, ASPA, in late 2008. This alignment procedure was different mainly in that it allowed you to supposedly use any star (other than Polaris) for polar alignment. We eventually found out a good ASPA star was not really any star, but a star due south and on or lower than the Celestial Equator. Get a good star, do two iterations of ASPA, and you’d have a close enough polar alignment for most imaging tasks.

While AllStar was not inherently more accurate than the old Polaris system, it was coupled with the new and much more accurate 2+4 goto alignment in Celestron’s updated firmware. With these types of polar alignment routines, the better the goto alignment, the better the resulting polar alignment. How accurate was/is ASPA? You’ll wind up about 10’ away from the pole or a little better, usually, with one iteration.

The downside? If you wanted better than that 10’ or thereabouts, you needed to do two ASPAs.   That could be a bummer since you’d normally want to do a new goto alignment after each ASPA (or at least “replace” the last goto alignment star). If you chose to do a new ASPA after each iteration, by the time all was said and done you’d have centered as many as 18 stars for goto alignment. The automated StarSense alignment camera made doing two ASPAs a little more palatable, but you’d still be spending around twenty minutes doing goto and alignments.    
Nothing changed for nearly another decade, till the enterprising Chinese CCD camera maker, QHY, came up with a new idea, which they called “Polemaster.” I was skeptical at first. A tiny camera not much different from my QHY-5LII guide camera save for the addition of some wide-field optics would be mounted in place of the mount’s polar borescope on the forward end of the RA housing. You would point the RA axis roughly toward the pole, toward Polaris, and the cam would plate solve the star field and tell you how to move the mount for precise polar alignment. That seemed like a pretty tall order to me.
Polemaster camera...

How would the alignment of the Polemaster camera affect the resulting polar alignment? How would you mount the cam if your RA axis didn’t have provision for a polar borescope? Or you didn’t want to remove or block the polar finder? Even if everything was perfect, how precise an alignment could a small-chip camera like the Polemaster produce?

When I had the chance to see the Polemaster in action at the Maine Astronomy Retreat last summer thanks to my friend Bruce Berger, all my doubts were dispelled. The camera was completely sufficient unto its task, producing more than enough stars in short exposures to allow it to do its job. The real key, however, was the software. Once I had a good understanding of the process, it was obvious what you had to do to move the mount’s RA axis to the pole. Not just obvious, but quick. If you are in a hurry, you could probably the entire Polemaster polar alignment in five minutes.

Further, I later learned the mounting of the camera was not critical. As long as it is attached to the mount somehow, someway in reasonably secure fashion it will work. I’ve seen people use it successfully, for example, just by duct-taping it to the mount head. Alignment is also not an issue. The camera does not, repeat, does not have to be finely aligned with the right ascension axis.

Watching Bruce polar align his CEM 60 quickly and precisely, I decided this was just the solution I had been looking for. Well, it would have been save for one thing:  the price. While the Polemaster is not overly expensive, about $300.00 with an adapter for one telescope mount, that was more than I wanted to pay given that ASPA was working pretty well for my purposes, with its main problem being it was time-consuming and occasionally annoying.

Annoying? Yes. There’s a bug in the Celestron StarSense firmware that sometimes causes the auto-align process to fail after the ASPA (and StarSense requires you to do another goto alignment after ASPA). It’s not a big deal to turn the mount off, reset it to home position, and start another StarSense align from scratch, but it is annoying.   

Oh, and I would have liked a little better accuracy than what ASPA produces, especially after only one iteration. For my (mainly) short focal length, short sub imaging, I can get away with less than perfect polar alignment, but it would still be nice to have the option of being able to expose longer thanks to a better polar alignment.

Initially, I was hoping QHY might have pity on us and sell their software separately. I figured my QHY-5LII would work just fine for polar alignment in conjunction with my wide-field 50mm finder-guider. Alas, they have not seen fit to do so; the software will only work with the Polemaster cam. So, I continued ASPAing it. What else could I do?

Then one day a couple of weeks back, I began to hear about Sharpcap’s polar alignment tool. I was well aware of Sharpcap itself, Robin Glover’s fantastic camera control program. Despite its somewhat nondescript and generic name, Sharpcap is a well-respected piece of astronomy software. It began as a tool for planetary imagers using webcams and webcam-like cameras, but has evolved into a program that can do long exposure deep sky work easily and well. Sharpcap is compatible with just about any camera out there as long as there is an ASCOM driver for it. Best part? Sharpcap is free.

Screen 1
Anyhow, I was told the latest release of the program, version 2.9, included a polar alignment routine similar in concept to that used on the Polemaster. A visit to the Sharpcap website revealed I had everything I needed to give this Polar Alignment Tool a try:  a compatible camera (the QHY-5LII is supported natively by Sharpcap), and that short 50mm guide scope. All I needed was one of those increasingly rare clear nights to give it a try. I read over the instructions a time or two in preparation, but, frankly, there isn't much to the procedure once the camera is connected to Sharpcap. Press an onscreen button a few times, move the mount once, and adjust the polar alignment with the mount’s altitude and azimuth adjusters.

That nice night finally came, and saw me setting up my AVX mount and Celestron Edge 800 SCT in the backyard. Why the AVX? It’s light and I am lazy, as I admitted not long ago. The SCT? I figured the scope’s long focal length would serve to reveal how good Sharpcap’s polar alignment results are. Further, I needed to take a few Moon pictures for a magazine article I am writing, and 4000mm (with a 2x Barlow) is just right for high resolution lunar vistas.

I put the telescope in normal “home” position, that is, pointed north with the counterweight “down.” The QHY was inserted into the guide scope and connected to the computer, which I positioned (temporarily) right next to the scope so I could adjust while watching the indications on Sharpcap’s screens.

First task was getting an image, a focused image. That was easy enough to do (well, after I remembered to remove the lenscap from the guide scope). Once I was close to focus, the sensitive QHY was producing more than enough stars to meet Sharpcap’s requirements in a mere 1.5 seconds of exposure. To work, the program needs 15 stars within 5-degrees of the pole, and according to the information on the first polar alignment screen, I was getting more than twice that many despite a crescent Moon and the usual backyard light pollution.

Ready to go, I clicked Sharpcap’s Tools menu and selected “Polar Align.” I was then presented with Screen 1, shown here. Stars marked in yellow are the ones Sharpcap is using for plate solving the star field (figuring out which star is which). I didn’t worry about that, just let the program think for a little while as the frames rolled in. Shortly, the “Next” button was enabled, meaning I was ready for step 2.

After pressing “Next,” screen 2 was presented and I was instructed to rotate the mount 90-degrees in right ascension. I did, so, moving the mount roughly 90-degrees to the east. Sharpcap then studied a few more frames in order to determine where the Celestial Pole was and what I needed to do to aim the mount there. Once it knew these things, the Next button was enabled again.

Screen 2
After pressing Next for a final time, a star was highlighted in yellow and there was a yellow arrow connecting it to a circle, my target . The task was to move the mount in altitude and azimuth so as to position the star in the little circle, not unlike what you do with a polar borescope (by the way, you don't need to return the mount to home position before adjusting; leave it rotated 90-degrees). As you move in the proper direction, the yellow arrow gets shorter and shorter and eventually disappears. It is then replaced with a pair of brackets around the target to allow fine tuning. As you center the star in the target circle, the brackets will move closer and closer together.

How easy was this to do? Quite easy AFTER I understood exactly how to do it. In the beginning, I was fairly far from the pole, with the arrow extending off screen. I’d been told that at this stage it was best to adjust while watching the error numbers Sharpcap displays instead of worrying about the arrow. These numbers (degrees, minutes, and seconds) indicate how far you are from the pole. They aren’t labeled as altitude and azimuth; instead they read “Up/Down” and “Left/ Right.” Sounded easy to me. I’d adjust the mount’s altitude until the Up/Down number got smaller, and the azimuth till the Left/Right went down. Alas, that didn’t work at all.

It turned out there was a catch, and until I understood what it was, I was all at sea. Up/Down does NOT mean the mount’s altitude, and Left/Right does NOT equal azimuth. Instead, these error numbers relate to directions onscreen (that's what I thought, anyway; see the addendum at the end of the article). At first I was mightily confused by the fact that moving in azimuth changed the Up/Down distance instead of Left/Right, and vice versa. As soon as the light went on in my head, that moving the mount’s altitude control changed the Left/Right distance, and adjusting azimuth affected “Up/Down,” the rest was duck soup.

In just a minute or two, I had the program indicating my distance from the pole as under a minute in both directions, which was where I left things. If your mount has precision altitude and azimuth adjusters, you can get the distance lower, but the AVX’s controls, while OK, are not exactly precise.

How long does a Sharpcap polar alignment require? Next time out, I doubt the procedure will take any longer than the few minutes required by Polemaster. Most of my time was, as above, spent scratching my head wondering why adjusting altitude moved the darned Left/Right numbers.

Screen 3
The accuracy? Some night soon, I need to fire up PHD2 and find out exactly how good Sharpcap’s polar alignment is. I know one limitation is that I am a little close to the equator at 30-degrees north, and that since the program does not currently take refraction into account there will be a limit to how close it will get me. However, I will tell you it looked darned close on this first night given the declination drift (or lack of it) of the Moon and stars at f/20. It was obvious the alignment was at the very least as good as two iterations of ASPA, and likely better.

Ground truth? I doubt I’ll use ASPA anymore. Now that I understand Sharpcap’s procedure, its Polar Align Tool is just easier and, I believe, more accurate. Sure, to do it you have to have the guide scope and guide camera mounted on the telescope, but if you are after a precise polar alignment you likely will be imaging and will want to guide with that guide cam and scope anyway.

So, friends, why not bop on over to the Sharpcap website, download the program and give it a try? Don’t cost nuttin’, and its polar alignment feature is only one of the many good things this wonderful program offers. At the very least, it’s made me stop wishing I had a Polemaster, and has allowed me to keep 300 George Washingtons in my hot little hands. 

Addendum:  Just heard from Robin (see the comments) concerning the "direction" issue that I and some other people are having. He says that moving the polar axis up or down should indeed affect the up/down numbers. At any rate, the program works great despite the direction reversal, and what's important is to shorten that arrow, which I found easy to do once, as above, I understood what was happening.

This page is powered by Blogger. Isn't yours?

stats counter Website Hit Counters