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.

Comments:
Very good point that the mounting of the Polemaster is not critical. It does not have to be aligned with anything. It just has to be able to see Polaris and has to rotate with the telescope -- which it can't help doing! So long as it's not loose, it's perfect.
 
I don't have a guide scope as I have always done OAG guiding. The SharpCap site specs a FOV of 1.5-2° (and a rough alignment within 5°). If you are like me and don't have a guide scope make sure to check the dimensions of your imaging setup as your gear may already meet these requirements. My ASI1600MM-C and FLT98 w/ 0.8 flattner will give me a FOV of about 1.5x2° so I should not need to purchase a guide scope.
 
ROD for president ... such an inspiration ... ur blog is one stop guide to everything astro for me ... live long n happy Rod
 
Thanks Rod for the excellent review of the SharpCap polar alignment tool. I'm interested in your experience with the up/down/left/right adjustment - they are *supposed* to be how you would adjust the pointing of the mount if you are standing behind the mount looking north, but they are obviously not working out that way. If you'd be interested in discussing this further then you can get in touch via https://forums.sharpcap.co.uk/.

 
Great entry Uncle Rod. I will definitely try this before dropping 300 simolians....so I can drop it on a different toy.

I wonder if your direction reversal "issue" was due to the fact that you were rotated 90 degrees? Just a thought.
 
Thanks for the article. I just purchased the Celestron Advanced VX 8" telescope. I am looking for an easier way to do polar alignment.
 
Hi Jack: I would doubt that, since that's part of the normal procedure. In any case, I communicated with Robin yesterday, and he's not sure why some people are experiencing this. Doesn't make for any problems, anyway...as long as you properly position the star in target, all is well. :)
 
Rod, you failed to mention the simplicity of the iOptron ZEQ25 polar alignment tool. I get very precise polar alignments and excellent dead nuts goto's with my ZEQ25 mount. While I have had issue with the Nexstar mounts alignments until I started using a 12mm crosshair illuminated EP to do my goto alignments, I never had a problem with my ZEQ25 mount. The handy polar alignment tool that comes with the GotoNova hand controller always gets me right on the money with my polar alignments. I am sure the CEM60 has the same polar alignment tool as well. The less complicated the better!
 
Hi David:

That was intentional. While I know the iOptron polar align app has a good reputation, I am reluctant to comment on gear I haven't used, and I don't own an iOptron mount. Also, I note that more than a few of the folks I know who do have iOptron mounts are going to Polemaster. I have little doubt, however, that the iOptron polar scope and app are more than sufficient for quite a few people. Finally, my issue with ASPA has never been that I've had trouble doing it (or the goto alignment), just that it takes a while. StarSense and Sharpcap have cut that time by at least 1/3. :)
 
Rod,

I'm one of the guys who recently messaged you about trying SharpCap PA. I'm so glad you did and that your results mirror mine. Great blog post!

Randy Roy
 
You mention that SharpCap is free in your post. It looks like the developer took the free publicity and ran with it. There now appears to be a cost. Charging for software is obviously acceptable but it may be worthwhile to make a change in your post regarding the cost of SharpCap. What really bothered me was the fact it's not a one time purchase. It's subscription and not just an update subscription. It appears the functions cease to operate upon the expiry of the yearly licence. This is not software I'm going to be considering based on the current type of subscription model. If it was to continue to function at the expiry of the yearly licence, and an amount had to be paid for further updates I may reconsider. Thanks for the post!
 
Hi Randy:

I understand Robin is keeping the freeware version of Sharpcap available, and that the new subscription version will have many additional features.
 
Also worth pointing out, it can be done with just a DSLR, a piece of tape and whiteout. Get the CP and RA projection in the FOV, rotate at high speed around RA taking a pic at high ISO, and mark the center of the circular star trails with a dot of whiteout on a piece of tape on the DSLR's LCD. Then adjust Alt/Az to move the CP to the RA projection by taking 5 second pictures iteratively. You need to study the asterisms near the CP to do this, with Stellarium studying the positions right before doing this it's not too hard. You can physically tell how far off you are from the pictures. It works great and you don't have to mess with a computer.
 
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