The defocused star method is a very nice addition! Thanks. I do have a couple of questions. The other night I was testing it out. I got down to 1-3 arc sec with the single star and worked it until the arrow kept shifting around all over the place like it says to do and then you know you're done. However, when I went back to the multi-star, I was back up to 7-9 arc sec and then got all anxious about it and started to mess with it again. Am I just 'chasing my tail' in this instance? If you do a single star collimation and get good results, should you even check the multi-star collimation or should you just call it a 'seeing problem'?
Also I was wondering about the centering technique with the single star. Do you recenter the star after every correction?
Thanks again,
Dave

Hi Dave,

Overall, I trust the defocused star collimator more than the multi-star, for the following reasons:

1. It's not dependent on having hundreds of stars in the FOV
2. It's not as affected by tracking errors due to a very short exposure
3. It's less sensitive to seeing variations

Where the multi-star collimation tool can help is if you don't have enough back-focus to defocus the star sufficiently, when using a system where focusing can affect collimation (such as some mass-produced SCT's with movable mirrors), or if you want a quick collimation check without defocusing the system.

With a 1-3 arcsec error with the defocused star collimator, I wouldn't try to fine-tune it any further than that with the multi-star one.

Regards,

-Paul

quote:

Originally posted by dave:
The defocused star method is a very nice addition! Thanks. I do have a couple of questions. The other night I was testing it out. I got down to 1-3 arc sec with the single star and worked it until the arrow kept shifting around all over the place like it says to do and then you know you're done. However, when I went back to the multi-star, I was back up to 7-9 arc sec and then got all anxious about it and started to mess with it again. Am I just 'chasing my tail' in this instance? If you do a single star collimation and get good results, should you even check the multi-star collimation or should you just call it a 'seeing problem'?
Also I was wondering about the centering technique with the single star. Do you recenter the star after every correction?
Thanks again,
Dave

quote:

Also I was wondering about the centering technique with the single star. Do you recenter the star after every correction?

That depends on your optical system. If the system is fairly flat over the FOV, it will tolerate much more decentering than something like an uncorrected SCT.

You can figure out how much decentering your system allows by taking a reading at the center, then shifting the star away from center in some large increments. By taking these readings you can see how much the decentering will affect the collimation error value and direction.

I find that I can move the star about 10-15% of the FOV away from center without affecting the error in any serious way. This usually translates into re-centering after every 2 corrections or so.

I am working on an automatic recentering of the star, but this has been on hold due to lack of clear nights to do the testing! :-(

Regards,

-Paul

Thanks Paul. I find the defocused method a huge plus in addition to the very short exposure, it is easy to determine which screw is doing what! Thanks again,
Dave

Paul,

I would really like the automatic re-centering option!

I have to calibrate drives just to re-center the star for collimation purposes.

Hi Kurt,

I hear you! I want it working for myself, too

But, even with the recentering option, you'll need to do a calibration step -- CCDI will need to determine your camera orientation.

Regards,

-Paul

Paul - great idea,

I'll try it tonight (weather dependent) to see what it thinks of collimation.

Last night CCDI v2 reported 0" in both X and Y axes - but 7% tilt! (Turns out my OAG had shifted 2mm at the top due to weight I guess!)

I was puzzled what was going on - CCDI really helped diahgnose my problem here!

I also found cleaning my SCT's front corrector plate the secondary mirror was touch loose! So I hand tightened it, and whilst I was at it I changed the original 3 collimation bolts for Bob's Knobs. I did the rough collimation with a 8mm eye piece on a moderately heavily unfocused bright star to get reasonable collimation - then focused it alot finer to see the stary disk to do finer focusing (seeing wasn't too great thanks to bush fire burn offs and gusting high winds).

So tonight I really want to put CCDI through its paces to see just how well I can tune my optics.

Clear skies,
a delighted Matt