Sunday, January 27, 2013

 

The Polar Alignment Party


This Sunday morning, muchachos, you Dob and alt-azimuth mount users can sit back, have a cuppa Joe, and be glad you don’t have to join our little party. If you are an equatorial mount user, though, especially a new equatorial mount user, you’d better listen up, since the dreaded polar alignment will be a fact of life for you to some extent even if all you want to do is a little casual visual observing.

First things first, I reckon. If you read my epistle on German equatorial mounts not too long back, you know an equatorial mount is needed to track the stars without field rotation. Oh, you can use gadgets like field de-rotators to make alt-az mounts accurately track the course of the stars across the sky, but such things are expensive, finicky, and have yet to catch on with amateurs for those reasons. That means most of us doing imaging or other things that depend on accurate tracking use an equatorial mount of some kind. For an EQ mount to be able to track satisfactorily, it must be set up correctly. It must have its right ascension (east-west) axis aimed at the celestial pole.

“The whosits of the whatsits?” If you are a novice, or are just new to the world of equatorial mounts, here’s the straight poop. The stars move across the sky because the earth is rotating. To counter that rotation, an equatorial mount must have its right ascension axis lined up with the axis of the earth. Where the heck is that? As I tell my freshmen astronomy students, the easiest way to understand how the sky works is to exercise Suspension of Disbelief. For a minute, let’s consider that old Ptolemy and his bubbas were right: the sky is a huge crystalline globe surrounding the Earth.

The lines on the globe of the Earth are all repeated on this sky globe. If you extend the equator of the Earth onto the sky globe it becomes the celestial equator. Lines of latitude become lines of declination. Lines of longitude become lines of right ascension. AND…if you extend the axis of the earth into the sky globe, it becomes the Celestial Pole, "hitting" the sky globe at a spot near Polaris (North Celestial Pole) or Sigma Octanis (South Celestial Pole).

While it’s the Earth that is rotating, from our perspective it appears the sky globe is what’s turning, and that it is rotating around the Celestial Poles. That being the case, if we could line up a mount so one of its axes is parallel to the Celestial Pole’s axis, and drove it at a speed matching the sky globe’s rotation speed (23-hours, 56-minutes, 4-seconds per revolution), an object would seem to stand still in the telescope.

Part of the equation for good tracking is the motors, gears, and computers, that drive the right ascension axis, but aligning the mount with the Earth’s rotation axis by pointing that R.A. axis at the North Celestial Pole is just as important or moreso. If you are “off,” the scope will not track precisely, no matter how good the gears, with objects drifting in declination (mostly). OK, so how do you do this “polar alignment,” and how cotton picking accurate do you need to be? Glad you asked.

Roughing-in

No matter how accurate polar alignment needs to be, start by roughing it in. You can do that one of two ways. If you are antsy to get started and it’s still daylight and you don’t have a prayer of seeing the pole star (or it is behind a tree), you can rely on a compass and on the “latitude” scale on your mount. Begin with the mount/scope plunked down roughly facing north (or south for you Southern Hemisphere boys and girls) and the tube of the scope at 90-degrees declination. If you’re using a fork mount scope, the tube will be parallel to the arms of the fork. If you’ve got a GEM, the tube will be parallel to the mount’s R.A. housing.

All set, loosen whatever you’ve got to loosen to move the RA axis in altitude (tilt). If you are using a wedge-mounted fork SCT, that usually involves several bolts. If the wedge doesn’t have fine latitude adjusters, hold onto the fork so it does not flop down when you loosen those bolts. If you have a GEM, what you usually do is loosen one bolt, the one on the “north” (if you are in the Northern Hemisphere) side of the mount. That will allow you to turn the “south” bolt to raise/lower the R.A. housing to point at the pole.

When you can adjust the mount in altitude, observe the scale that almost every GEM and wedge has, and tilt the RA axis up or down until the pointer is on your latitude.  When you are done, lock the altitude adjustment.

Now it’s time to align in azimuth. If you have a wedge-mounted SCT, you either loosen a single central knob on the threaded rod that fastens the wedge to the tripod (Meade) or the bolts that hold wedge on tripod (Celestron). Get the compass out and observe where north (or south) is. Stand back from the scope far enough so the needle is not affected by the mount’s mass of metal, and eyeball the fork or R.A. housing’s position with regard to north/south. Need to go a little left? Move it left a little in azimuth. A little right? Do that.

Most GEMs have a pair of bolts that push against a post on the tripod. Screw in the bolt that moves the rig in the desired direction while unscrewing the opposite number. A wedge is either just manhandled left or right (sometimes, like the old Celestron Celestar, by nudging the tripod) or with an azimuth adjuster that pushes the wedge right or left.

After that? That's it. Rough polar alignment is done. How rough is it? You might get lucky and be dead-on, but likely not. What you will usually get is “good enough for visual.” You will have to mash a north or south button or turn a slow motion control once in a while to keep an object centered as it drifts in declination due to the less than perfect polar alignment, but it will not be too bad. If it is too bad, you can wait for dark and do a better rough alignment.

A better rough alignment requires one thing: that you be able to see the pole star. In the north, that’s easy unless there are obstructions. Polaris is magnitude 2 and stands out like a sore thumb as darkness falls.

In the south, the pole star, Sigma Octanis (or “Octantis” if you prefer), is not as hard to find as reputed. It is at magnitude 5.5 or so and is visible in a finder or even in a polar alignment scope from all but the worst locations. Not sure which star is Sigma? It forms a little parallelogram with Tau, Upsilon, and Chi Octanis.  Wait for sufficient darkness before looking for it, of course, and have a finder chart at your side or a laptop running Cartes du Ciel or another planetarium program.

What do you do with the pole star? You center it in the telescope’s finder (or in the polar alignment borescope in a GEM). If your GEM does not have a polar scope in its RA housing, just center Polaris in the hollow bore. Unless your eyes and skies are pretty dern good, you will have to use the scope’s main finder for Sigma.

What does this better rough-in get you? Tracking most assuredly good enough for most visual applications. It will not be perfect, since Polaris is about ¾-degree from the true North Celestial Pole, and Sigma, which occasionally goes by the name “Polaris Australis,” is about 1-degree from the South Pole.

While the two rough methods are good enough for many purposes, if you are imaging or doing very high power visual observing you will have to do better. Where does that better start? If you have a GEM, it starts with a GEM mount’s polar borescope.

More Better Gooder

A GEM’s polar finder is a simple device; it is a little refractor inserted through the hollow R.A. axis of the mount. It will have a special reticle that may have pictures of constellation stick figures etched on it, or just a crosshair and a small circle where Polaris goes. Most polar scopes have reticles that also accommodate Southern Hemisphere observers.
Rough-in the telescope as above, and remove the fore and aft lens-caps from the mount’s R.A. housing. You will have to turn the telescope (or just the mount’s saddle if the scope ain’t on it yet) until it is perpendicular to the R.A. axis of the mount in order to open up a hole in the declination shaft for the polar scope to look through.

How do you do the actual alignment? There are several possibilities. You can use the etched constellation figures, if any. Undo the R.A. lock and rotate the mount in right ascension until the constellations in the polar scope roughly match their positions in the sky. Note that the constellation etchings (usually Ursa Major and Cassiopeia in the Northern Hemisphere) will not be able to be overlaid on the real constellations, which will be too large to fit in the view of the polar scope.

Just eyeball the real things in the sky and match their rotational orientation as well as you can. When done, lock the R.A. axis, and use the mount’s altitude and azimuth adjustments to put the pole star in the circle on the reticle where it is supposed to go. Be sure to tighten down the mount’s altitude and azimuth so you don’t lose your nice alignment.

The “match the constellations” method is better than just centering Polaris, but it’s not overly accurate, and if you are going to the trouble of using a polar scope, you should aim for "much better than roughing in." You can get that by using the special scales found on most polar scopes. The manual that came with the GEM will have instructions, which will include directions for making corrections depending on a site's position in its time zone and stuff like that. Stuff that is confusing for beginners and everybody else. Luckily, there is a simpler way. 

The simpler way uses the right ascension setting circle and Polaris’ (or Sigma’s) “Local Hour Angle.” Hour Angle is a sort of time that indicates the position of the pole star with regard to the true pole. To use the LHA method, rotate the mount in R.A. until the circle (or other marker) on the polar scope reticle where Polaris goes is on the bottom. If the reticle has a crosshair above the circle, you can get it exact by centering the pole star in the crosshairs and using the altitude adjuster to run it up and down a crosshair till it stays precisely on the line. When the marker for the pole star is at the exact bottom, lock the mount in R.A. and set the R.A. setting circle to “0” hours.

Now, find the current hour angle of Polaris. How do you do that? If you are using a Synta SynScan mount (like the Orion Atlas EQ-6), its hand controller will display LHA after you enter time and position. If you are not? Some planetarium programs, including ever'body's fave, Cartes du Ciel, show LHA (click on Polaris and bring up the information window for it). So do Polar Alignment helper programs (more on them in a minute). There are even iPhone programs that display Hour Angle.

When you have Local Hour Angle, undo the R.A. lock and rotate the R.A. axis of the mount until the reading on the right ascension setting circle (usually the upper scale in the Northern Hemisphere) is the same as the current LHA. If LHA is 14:30, move the mount till the R.A. under the pointer is 14 hours 30 minutes. Lock the R.A. axis again and use the altitude and azimuth adjusters to move Polaris to the spot where it goes on the reticle. When done carefully, the LHA method will produce an alignment good enough for at least 5-minute exposures at medium focal lengths.

Potential gotchas? Be sure the polar scope is correctly aligned with the mount (see the manual, natch). Newbies: some of y’all assume you turn the polar scope in its housing till the reticle is properly positioned. Nope. Like I been saying, you turn the whole shebang by undoing the mount R.A. lock and rotating in right ascension.

I like the LHA method of polar alignment a lot, but there is another way to use a polar scope that is also quite accurate. That is the “Kochab’s Clock” procedure of my old buddy Doc Clay Sherrod. If you have a GEM (and, natch, live in the Northern Hemisphere), this is an easy way to get the Polaris marker in your polar scope properly positioned. With the mount pointed north and ready to go, find the bright 2.08 magnitude star Kochab, Beta Ursae Monoris. Undo the R.A. lock, and rotate until the star is “on” an imaginary line drawn from the declination counterweight bar (or on the top edge of the bar itself). Lock R.A. Move the mount in altitude and azimuth until Polaris is in the correct spot. For full instructions, see this nice page.

“All that’s cool for GEMs, Unk, but I got me a Meade 2080 SCT that ain’t got no polar finder.” A few companies used to sell add-on polar scopes for forks, but all of them appear to have discontinued these alignment tools, including the last holdout, Jim’s Mobile. Quite a few SCTs from the 1990s and before had polar alignment reticles built into their normal finder scopes, which can be used with polar alignment helpers to produce OK alignments.
But if the fork has no polar finder, Doc Clay has a somewhat more complicated but very accurate Kochab’s Clock alignment for fork users that can be found here.  It can potentially be used with any SCT finder, even one without a polar reticle. While this take on the Kochab Clock works very well, I would guess that today most fork users, those with computer go-to mounts, anyway, are using the hand control to produce a good polar alignment.

Hand Paddle Polar Alignment

Need to kick it up at least part of a notch in accuracy and a big notch in “easy”? Some mount makers, most notably Synta/Celestron and Losmandy, have been incorporating polar alignment routines into their hand controls for a while. It works like this:  you do a go-to alignment after doing a “rough in” polar alignment as above, and tell the mount you want to polar align. It has you select and go-to a star. The mount then points at where the star should be if polar alignment is perfect. All you’ve got to do then is center up the star using the mount’s altitude and azimuth adjusters.

The good? This alignment method is quite accurate, at least as accurate as a polar scope alignment, and often somewhat better. There’s no kneeling down on the cold hard ground to peer through that consarned polar scope. And, to top it all off, go-to computer alignment is as useable with a fork-wedge scope as with a GEM.

The bad? While you have a lot of alignment choices other than Polaris or Sigma, which is a god-send for folks with a blocked northern/southern sky, you can’t really, as Celestron suggests with the name for their polar alignment process, “AllStar,” use any star. To begin with, they recommend against Polaris, and while you can use any other star, theoretically, when you get away from the area near the intersection of the Celestial Equator and the Local Meridian, it becomes a bear to center a star with the alt-az controls. All in all, though, automated polar alignment for go-to mounts is a big advance.

What if you’ve got a go-to, but it ain’t a Celestron or a Losmandy and doesn’t have a polar alignment feature? All is not lost. You can use another accurate computer method that will work with any mount that has a “sync” function in its hand control. This process, “Iterative” alignment, is not as quick as AllStar but is at least as accurate. Its only potential drawback is that you must be able to see Polaris or Sigma.

It seems a little complicated the first time out, but once you’ve done "Iterative" a couple of times, you'll find it is easier than drifting, more accurate than a polar scope, and good enough for all but the very longest imaging integrations. To begin, rough in the polar alignment as always, go-to align the scope (some folks advise a 1-star alignment), and go-to a star. The only criteria for star selection is that it be at least 90-degrees from the pole star in declination.

When you’ve picked and done a go-to to the star, center it up with the hand control and SYNC it. Then, go-to Polaris (or Sigma). Don’t sync here; use the alt-az adjusters to move the star HALF the distance to the center of the eyepiece. Next, go-to the alignment star again, center it with the HC buttons and sync, go back to the pole star and move it half the distance to the center once more. Keep going back and forth till Polaris (Sigma) is dead center after a go-to. When it is, you are done and have a very good alignment.

You are one of my brothers or sisters who take loooong exposures? You, like Rod, stack your “subframes” into a final image, but unlike Unk’s 2 – 5-minute “subs,” yours are half an hour long or longer? If that is you, you need an exact polar alignment to prevent field rotation over lengthy exposures. Yep, you gotta drift align, as in “use the declination drift method of polar alignment.”

The Best

Some folks, including Unk, who don’t have observatories and must polar align every time, don’t have the patience to do a drift, but in truth it ain’t that bad. After you’ve done it a couple of times, you can probably be up and aligned in 30-minutes, and it gives you something to do when it’s still too light to start taking pictures. What do you need to do a drift? Just an illuminated crosshair reticle eyepiece and a clear view to the south and east (or west). You will want a magnification of at least 150x or so, but there is no need to go to crazy high powers. What kind of mount will work for a drift alignment? Any kind of equatorial. A fork’s wedge must have fine adjusters, but that is the only caveat.

OK, let’s drift. At least rough-in the polar alignment; the closer you get to the pole, the shorter your drift time will be. Pick a star that’s near the Local Meridian and about 10-degrees from the Celestial Equator. Center that star in the reticle and rotate the eyepiece (and position the diagonal and yourself as required) until moving the mount’s declination adjustment drives the star up and down a crosshair. Re-center the star and you are ready to go.

Let's drift:  watch the star. If it moves up (north), adjust the mount alt-az control that will move it right. If it goes down (south), move it left. After a move, center the star again and watch for more drift. How much should you move the star? I generally move it about the distance it has drifted. Keep doing this until there is no drift for 5-minutes. You may see east-west movement due to your mount’s periodic error, of course. Ignore it or guide it out, your choice.

OK, on to star two. You can use a star on either the east or west horizon. Whichever you choose, set the eyepiece/diagonal up so the star moves up or down a crosshair in declination as before. For a star ten degrees above the east horizon, if it drifts up (north), move it down. If it goes down (south), move it up. The instructions are slightly different for a star in the west.  If that star drifts up, adjust altitude to move the star up. If the star drifts down, adjust altitude to move it down. As with the Meridian star, keep going till there is no drift for five minutes.

What then? You are done and have a polar alignment good enough for long exposure work. Do note that some people suggest checking the Meridian star again after doing the east or west star, but Unk is usually way too impatient for that.

Drift alignment works just as well in the Southern Hemisphere as it does in the Northern Hemisphere, but the instructions are a little different. See this excellent article for details. Which also has considerably more detailed Northern Hemisphere instructions than the above.

What's that? I see a raised hand in back:  "Unk, how come you ain't said nothing about leveling the scope's tripod before polar alignment?" Well, there's a reason for that, Skeezix—it ain't necessary. The level or lack thereof of the tripod will have exactly NO effect on the quality of a polar alignment. Having the tripod reasonably level, can make it a little easier to do a drift alignment; it may make doing a check of the second star unnecessary since there will be no "interaction" between altitude and azimuth moves, but that is it. Don't waste a lot of time leveling.

PC Help

This being the computer age, the age of PCs on the observing field, there are, as you might expect, computer programs that help with polar alignment. These can be as simple as polar align helper programs that show where the pole star should go on a polar scope reticle and give LHA like my fave from the good folk at Takahashi (this page can be saved and run on your computer without an Internet connection, by the way). But some go a lot further, basically automating the polar alignment process.

There are methods for using CCD imaging programs to aid in drift polar alignment, or, hell, even using PHD Guiding to get you on the pole. But there is also software expressly designed for polar alignment. There are quite a few polar alignment softs out there now, including WCS, PoleAlignMax, and EQ Align. The most developed program of this type, though, is probably Matthias Garzarolli’s AlignMaster. What it does is kinda like AllStar and kinda like Iterative. It slews to two stars, calculates misalignment data, re-slews to the second star, and has you center that using the alt-az adjusters. Unlike some similar utilities, you don’t need a camera on the scope for alignment, just a reticle eyepiece.

I tried AlignMaster with my Atlas mount, and was quite impressed. It seemed to yield very good polar alignments, certainly good enough for my modest requirements, will work on any mount that is ASCOM compatible, is available in a free 30-day evaluation version, and costs a modest 20 bucks. If all my mounts did not have built-in polar alignment routines (thanks to the latest SynScan firmware upgrade I now have AllStar for my Atlas mount), you can bet I’d have AlignMaster on my PC.

And there you have it, polar alignment. Yeah, it sounds like a pain, but it ain’t really. Remember, for visual work a quick roughing-in is all you need, and even a drift polar alignment should be duck soup after a few outings. And maybe it will be for you. I wish I’d been able to convince myself of that back in the day. My album of film deep sky shots would probably feature a lot more with round stars if I had. But I just didn’t like to drift then, and I don't like to drift now. Thank god for today’s shorter exposures AlignMaster, AllStar, Iterative, and Kochab’s Clock is all I can say, muchachos.

Well Deserved Plug Department:  You've probably bought something or other from JMI, Jim's Mobile Industries, over the years. How could you not? They make everything from simple counterweights to gargantuan telescopes. They were good way back when, and they are good now, campers.

As y'all know, the JMI Motofocus, which I use on both the C8 and C11, is indispensable when your old Unk is going after the dim stuff. So you can imagine how upset he was when the C8's Motofocus installation developed a problem after years of faithful service.

To make a long story short, JMI helped me, helped me right away, and made Unk both happy and relieved. The Motofocus is a must-have for chasing those pea-picking Arp galaxies as I am doing now. Couldn't ask for more than the excellent support I received. People say they are worried there aren't many U.S. gear manufacturers left, and wonder what to do about that. Well, I'll tell ya, JMI is still around.  Support them as they have supported us over the years.

Next Time: Tick Tock Clock...

Comments:
Uncle Rod,
Is there a reason why the Kolchab clock method should not be used on a GEM (say a CG5)?

Also, will it help with the two plus four alignment and tracking accuracy if one first centers the scope on Polaris instead of doing the rough align using compass and latitude dial?

Thanks,
Don Horne
 
There's no reason you can't use Kochab's clock with a CG5, but the AllStar is probably easier and as accurate.

Bottom line on the CG5? Go-to accuracy is relatively immune to polar alignment being off. A buddy of mine "polar aligned" on Kochab one night by mistake and his go-tos were still good (his tracking was pretty bad as you can imagine). A decent polar alignment first can help the initial alignment stars come a little closer to the finder's center, but that is it.
 
"Bottom line on the CG5? Go-to accuracy is relatively immune to polar alignment being off"

What about the other way, since we always do the polar routine after star align. You'd assume that if Gotos are OK but not dead center, the polar routine can't be more pecise than the Gotos?

 
If your go-to alignment is poor, polar alignment--polar alignment using the AllStar rouine--will be affected, yes.
 
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