Re: [APML] focusing with CCD

[Antonio Agnesi <agnesi@ele.unipv.it>]

Thanks to Mark and Stosh who replied to my post.

Mark, the ST4 is really small and I cannot see how thermal fluctuations
might change things so much, anyway I will be aware of that.

Stosh, I understand your point, as I also live in a place with so-and-so
seeing.
Unfortunately the ST4 does not give a readout of the rms star size, which
should be the key parameter for accurate focus evaluation: I hoped that
setting AA = 3-5 with relatively short exposure might help to give more
consistent peak readings, but I have not tried it yet.

>My estimate was that the ST-4 allowed a 3 times greater window of
>"perfect" focus compared to knife edge.

Only for a trial, I am going to follow this procedure: put a graduate scale
on the focuser, read the positions corresponding to half-peak readout and
set the final focuser position as the average of the previous half-peak
positions. This is intrinsically more accurate than looking for the peak
value. This will be checked against classic knife-edge to see the real
accuracy. I know from an indoor laser test that this technique has enough
sensitivity to compete with knife-edge, but now (when it clears out) I have
to check it with star scintillation outdoor.
To reduce problems with scintillation, one may also stop down the scope
aperture to increase the star size on the CCD.
Knife-edge works great and will remain the preferred method for most of us.
I am only trying to see if we can take advantage of some powerful devices
we already use. I will let you know if I have success.

Thanks again for your insights.
Clear skies,


Antonio



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