Re: [APML] focusing with CCD

[Stosh <stosh@ptd.net>]

Antonio,
I have tried this method making an adapter to use the ST4 as a focusing aid.  I
found it to be quite difficult.  Seeing had much to do with my problems.  Bad
seeing meant the ST4's readout wouldn't give consistent results.  Even when the
seeing was OK there was too large of a range that was at the "peak" of the ST4's
readout.  My estimate was that the ST-4 allowed a 3 times greater window of
"perfect" focus compared to knife edge.  I think the major problem is that all the
ST-4 gives you is a max pixel brightness.  I think if there were some software
that would instead combine the actual number of pixels lit and each corresponding
brightness and somehow put that into a quantitative measurement, there would be
more success.  I live in the mid atlantic region and the seeing is almost always
bad here.

Needless to say, I went back to knife edge focusing.  I now feel that large
distances between focusing star and subject are negligible compared to getting
consistent focus.  I use 3 adapters every time I take a shot.  First is ronchi
screen attached to an adapter.  Next is diagonal and eyepiece with same field as
camera.  The diagonal and eyepiece make it 50 times easier to frame because you
don't have to break your neck and you can see dimmer stars.  Finally, the last is
the camera adapter.

later,
Stosh

Antonio Agnesi wrote:
> 
> Hello,
> has anyone ever tried the ST4 or STV as a focusing aid? It is quite easy to
> make an adapter parfocal to the film plane, calibrate the exact parfocal
> length of the adapter against a knife-edge test on the photocamera and
> using the CCD for focus optimization. (for best results just stop-down the
> scope aperture to increase a little the field depth if the star size is
> smaller than the pixel size) This does not require to move the scope much
> farther from the target as when searching for a bright star to make
> knife-edge each time you change target.
> I would be curious to learn about similar tricks.
> Thanks and clear skies,
> 
> Antonio
> 
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