Somebody on drift polar alignment!!

Hello guys,

I hope somebody can answer this: I've been trying to perform a drift alignment and I thought I got it although the altitude alignment (the one close to the eastern horizon) was not perfect (clouds rolled in) since the star showed a small drift in the 5 min. tracking at 225X (a little more than half radius drift). I remembered that the "5 minute" rule was to keep the star without any N-S drift for at leat 5 minutes to achieve exposure times in the order of the 2,5 hrs. without any field rotation.
However, I assumed that I did not need that long and so I was content of the little drift registered.

I assumed also that that, once well aligned, my scope would have tracked any star in the sky keeping it in the center of the eyepiece even at 225X for a long period of time (with small errors due to the intrinsic mount ability to track).

My problem is that, when I do this and center a star, this seems to move slowly but noticeably to the east (always at 225X): CAN ANYBODY TELL ME WHAT THIS MEANS IN TERM OF ALIGNMENT?
Is it a consequence of the fact that I did not have a 5 minute alignment on the altitude adjustment? The movement was very very small though and for the most part of the 5 minutes almost undetectable. I'm confused.

Sorry for the long post but I didn't know how to put it in a shorter way.

Thanks for any help.

Ciao
Luca

Any movement in RA, E or W, is due to inaccuracies in the RA drive, either motor or gears and not due to alignment. It sounds like your polar alignment is fine. Does your drive have provision for varying the drive speed? Does it have PEC?

Walt

Luca,

You did not say, but I assume before you went to the eastern horizon, you did do an alignment of a star at zenith at about zero dec. This is the first step.

Then after you finish an eastern horizon star alignment, you have to go back and do another "zenith-0 dec" alignment, and then another eastern just to be sure. Resetting the azimuth, in short, means you have to go back and reset the altitude. It is an iterative process.

Also, you say "The movement was very very small though and for the most part of the 5 minutes almost undetectable" makes it sound like periodic error. In short, it is stable for some time, and then it moves. Periodic error acts just like this. It is not in the alignment, but in the drive mechanisms.

Alex

>>My problem is that, when I do this and center a star, this seems to move slowly but noticeably to the east (always at 225X): >>

You do not say what kind of telescope you have, but it is well known among us imagers that SCTs, especially in the larger sizes will have enough differential flexure in their optical tubes so as to produce a slow trailing, even when the mount is tracking perfectly and is perfectly polar aligned.

The evidence that this was happening was discovered by people who had small light weight guiding refractors mounted side by side with the heavier SCT tube assemblies and discovered that a star would stay perfectly still on the crosshairs of the smaller scope, while it moved in the large one.

The problem with Cassegrains is that they don't have the mechanical rigidity of the optical compnents to stay perfectly aligned with their mountings at different orientations in the sky.

Roland Christen

Luca,

As others have stated, the issue is not apparently with polar alignment, but rather with motion around the RA axis or flexure of the optical tube. One possibility might be that your scope isn't balanced properly and the clutch is slipping slightly. Try achieving a better balance for the particular area of the sky that you are aiming at and perhaps locking down the RA clutch a bit more.

Another possibility is periodic error, which would be seen as a slow drift either E or W and then back again the other way; this should happen in a repeatable fashion according to the rotation of the worm (PEC takes advantage of this repeatable (periodic) nature to make corrections to minimize it). If you are only seeing movement in one direction though, then it sounds like PE is not the issue. Watch the star's movement with an illuminated reticle eyepiece (if you have one) over the entire period interval of the worm (usually 4-8 minutes) to see if there is a pattern.

Also, if you are using a battery and the voltage starts to drop, your drive may start to slow down. I have noticed this on one of my mounts; pics come out with trailed stars (not from field rotation). Try a different power source and see if that cures the problem.

Best of luck!
Tim

Check your tracking rate. Frequently, when autoguiding, it is advisable to slow the tracking rate to les that 100%, that way the autoguider is always correcting in one direction reducing the effects of backlash. It almost sounds that, somehow, your tracking rate got set to be less than 100%. Since you are not autoguiding, check to see if your telescope had a configurable tracking rate and ensure it is set at 100%.

Also, let us know what type and brand of telescope you are using.

Good luck and let us know what the solution is.

Derrick.

Thank you very much guys. However, I forgot to mention my equipment:
Nexstar 11 GPS on an APT wedge on permanent pier (still setting it up, as you may guess).

I did both the alignment, i.e. the "meridian close to equator star" and the "low in the eastern horizon" (actually west, since I'm blocked to the east, but I just reversed the "too high/too low" instructions) and checked for declination drift. So the procedure is complete.

Thanks for all the useful hints (weight balance; worm gear problems; PE; etc.) I will check them all and let you know on this same thread if and how I solved the problem. I want to calculate the FOV to clock the movement, that seemed to be only to the east.

However, now that I have learned it is a fairly easy procedure and I can't wait to see its results: It is much scarier on paper than in actual practice.
Needless to say, as soon as I was finished last night, clouds rolled in and today it looks grim (windy and cloudy). Patience is key, but I'm lousy at it!

Ciao and thanks again for your help.

Luca

Hello again guys,
as promised I give you an update on the result of my polar alignment.

I went back and refined the process bearing in mind your suggestions.
The result is that now I can track a star (meridian/equator) at 560X for more than 12 minutes without any declination drift at all.

Only still a little RA drift but, I believe it is negligible since, after the 12 minutes, the star had moved from the center of the reticle only about a diameter to the west and, at 560X, I believe that is acceptable for my purpose.

I wish to thank you all again for your help and support.

Ciao Luca

Special thanks to Paolo who explained it to me in Italian and with an easy example.