There has been a lot on collimating lately- I haven't seen the following
idea talked about - if someone has discussed it already, my apologies.

I think it should be possible to collimate with the CCD itself, dispensing
with the eyepiece. The basic idea is to take images out of focus enough
that the stars look like donuts, and adjust the collimating screws until the
donuts in the center of the CCD are of uniform brightness all around.
Of course the stars will move on the CCD as you adjust the screws- to get
around this, look at a nice star cluster (M35 or M44 or Pleiades maybe for
smaller scopes), so that there are lots of stars in the field.

I tried this the other night, but didn't have time to really do it right.
I first got a rough collimation with the eyepiece looking at an out of focus
star, then put on CCD and tweaked a little.
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Also, we just got a NFGs focuser. It really helps with focusing.

With the NGFs focuser, and initial collimation with the CCD, we got images
of 1.9 arcsec FWHM on 20 second long images. This is from the middle
of Norman OK at 363 meter elevation. (Before the collimation and before the
NGFs, our typical images were 3 to 3.5 arcsec FWHM). (Image FWHM measured
with IRAF, the same software I use on Keck CCD images).

If anyone tries or has tried collimating with the CCD, I would be curious
to know the results. I will try again to do this more carefully, but the
weather looks bad here for next 3 or 4 nites.
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My 2 cents worth on CCDs- Get a CCD with small pixels. You can always
bin to get bigger effective pixels, but you can't divide bigger pixels
into smaller ones! For example, we had been using our ST8 with 3x3 binning
(1.35 arcsec/pix with our 16inch f/10 system), but when I saw the good images
we were getting, I switched to 2x2 binning (0.90 arcsec/pix) to take
advantage of the good image quality.

dr bill (bill romanishin, u. of oklahoma physics and astronomy)

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