Re: An easier Pasillx N & S Polar Alignment method

Chuck Hancock

Hey Joe,
The concept you should know here is that the OTA doesn't need to be orthogonal. You only need the finder scope orthogonal to the mount (not the OTA) in order to polar align the mount's RA axis.

Suppose for a moment that your scope OTA, finder scope, and mount are all three orthogonal and all three perfectly polar aligned. So your tracking is "dead nuts". Add to that a camera that you want to photograph any object in the sky. You can mount the camera to the scope, piggyback on the mounting rings and point it in ANY direction in the sky. As long as the scope is tracking, the camera will track as well, as it too is rotating about the pole (and the RA axis). It points at an independent target, but still benefits from the tracking of the scope. The fact that it is on a different target makes no difference at all. Try setting up your mount and do it yourself to prove it. It really helps to see it on the mount, even if you just set up indoors.

Go back now to the situation that was described in the procedure to kick off this branch of the discussion ("Howard and myself..."). The finder scope helps gets the mount aligned. The OTA is a little off. No big deal. Go to the target object, center the object, then do a realign (or not). Start your camera exposure, and the mount and OTA will track just right even thought the finder is not pointing at exactly the same spot as the scope. The only time the orthogonality of the OTA will make a difference here is when your mount flips to the other side of the sky, that is, when you goto a different object. Then the pointing will be off by twice the orthogonality error. In the case of the OTA, just a half a degree (2x 15 arc minutes). In the case of our imaginary piggyback camera, who knows where!

Best regards,
Chuck Hancock

Joseph Zeglinski wrote:

Hi Rick,

In the Orthogonally section of the latest GTO Keypad manual, it states that:
"especially in GEM mounts, a non-orthogonal mount will cause errors in ANY routine that uses the scope for polar alignment (e.g. N Polar Calibrate routine) ..."

So, I assume this is true for all other routines, including Quick Drift, which does not make it a secondary issue of concern, and certainly necessary. The routine to square it up is essentially identical to the one describing Roland's GTO Quick Star Drift Method using Meridian Delay - except rather than adjusting mount's azimuth adjuster , you use metal shims to prop up the OTA.

The only mention of using the finder is if the mount alignment (or I suppose orthogonally) is way off, and the reference star is out of the field. In such case, the manual states using the finder, for the first iteration, before trying the OTA again.

As far a I can tell, getting the OTA orthogonal is the first concern (assuming it is carrying a finder aligned with its optical axis already). Once the OTA/finder are trued up, you should align the mount.

However, I must say, there seems to be a "chicken and egg" quandary here.
For the Quick Drift Method, you need a properly aligned mount to run the orthogonally test, BUT, the OTA has to be orthogonal to begin with, in order to align the mount.

Based on the east-west drift during that procedure, how do you know whether the drift is caused by a misaligned mount, or a non-orthogonal OTA?

I suppose, you could just assume the OTA is aligned, and IF you can't align the mount after many iterations, ONLY then assume the OTA is non-orthogonal, and repeat the tests, BUT this time use "shims" under the OTA rings, rather than the AZ adjuster.
Does this make sense? I don't recall it being stated this way in the manual.

As for the finder in Roland initial discussion, if it is physically part of the mount, then you still have to run the two procedures for the finder of mount alignment - followed by (if needed), finder orthogonally shimming (i.e screw adjustment).
You can't do the same if the finder is on the OTA, since you could have both the finder and the OTA individually non-orthogonal. That is why it seems best to align the finder with the OTA using a special eyepiece - perhaps in the daylight, on a distant object.

Yikes !

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