The Importance of Polar Alignment

 One of the major innovations in amateur astronomy since I was a kid is the regular use of polar alignment scopes. They existed, but were beyond my budget and patience as a fifteen-year-old. Now they are standard features on a lot of mounts, including my Kenko Sky Memo S.

So, how good does your polar alignment have to be for astrophotography?

Let's say you're off by one degree, toward R.A. 0, for simplicity. That means that if you're aiming at something at the equator (where the effect will be the worst), your aim will be a degree low at R.A. 0 and a degree high at R.A. 12h. The maximum slew rate will be when you're looking at R.A. 6h or 18h.

How fast is that slew rate? Let's see...that up and down cycle will be two pi per 24 hours...max slope of a sine wave is 1, but gotta match those units...one radian per 3.82 hours...so that would be a max rate of one degree per 3.82 hours. That's 15.7 arcminutes or 942 arcseconds per hour. About one arcsecond of drift every four seconds.

Is that a lot? Well, my professional astronomer friends tell me the seeing at sea level is about 1 arcsecond, at best. It's also about the diffraction limit for a 100mm objective, roughly. So worst case, this would limit our exposure time to four seconds, if our alignment is off by a full degree.

But whether we are working near that limit depends on focal length and camera resolution, too. My Orion photo I just posted was about 3k*5k pixels in the original, covering about 5x7 degrees (before I cropped), shot at 145mm f.l.  I calculated that one pixel in that image is about 6 arcseconds.  (All of this is very back-of-the-envelope, so call it 4 to 8 if you're picky.) So, a 1 degree misalignment would drift by 1 pixel in 24 seconds (well, 20-30, give or take). Since I was stacking 30-second exposures, I only needed to be within one degree!

Very roughly, eyeballing my raw images, over about 25 minutes I drifted by about 90 pixels, about two pixels per shot. A little more than I'd like, but with the stacking I did, not noticeable. In fact, zoomed all the way in, the best stars don't really show any distortion in that direction.

So, I thought I had the polar alignment really nailed during that shooting session, but I might have been off by a full degree. (As it happens, I had to allow rotation as well as translation in the alignment of my stack, which may be due to polar misalignment.)

So I'm in pretty good shape for 30sec exposures with the Canon 70-200mm zoom lens. But if I'm going to shoot long exposures with the new C90 Mak, f.l. 1250 and f/14, I'm going to have to do a lot better than one degree. (I'm also going to need a lower-vibration mount.)

Of course, serious astrophotographers shooting at long focal lengths usually guide their scopes, for exactly this reason, using a longer-f.l. scope and a lighted reticle eyepiece to keep the imaging rig pointed right. It's pretty arduous work. But technology is making this one easier, too, if you've got the budget!