Re: [APML]: Using 8" SCT as telephoto

[Chuck Vaughn <aa6g@aa6g.org>]

>In addition to using my recently purchased Celestron 8" (orange
>tube) scope for astrophotography, I want to use it during the day
>as a telephoto too.

>I'm curious if any of you have
>experience doing this and if there are any tips on doing so - what
>I'm getting at here is that maybe if you were to use a fast film,
>there might be enough latitude to "stop down" the "lense" and gain
>sharpness as a result.

One problem with using a C8 in the daytime is that it is hard to focus.
You cannot use the split image screen. At least with the OM-1, the aerial
imaging screen is required but there's still a little subjectivity thrown
in. Jerry Lodriguss might want to comment about this but I don't think it
would be a good lens to use for sporting events where the focus has to be
changed rapidly and often. I think it would be better for still photography.

I took a few photos through a C8 in the daytime years ago and I found them
not to be very sharp. The seeing in the daytime across land tends to be 
quite bad.

Another problem that affects sharpness is the camera shutter. Most camera
shutters will have some impact on the sharpness for short exposures (<1 sec).
I ran an experiment once with my OM-1 body mounted on my C14 with my GEM
which weighs a couple hundred pounds. I looked at the moon and snapped the
shutter. There was obvious vibration. Short of sitting it on a rock, shutter
vibration will be a problem.

Another problem with the C8 is the very curved focal plane. Even if you
compromise the focus the entire field will not be sharp. Stopping down the
C8 will increase the depth of focus but I don't think you could make it
slow enough to eliminate the effects of the curved field.

Having said all this, go ahead and take some daytime shots with it and see
what you get. I'll bet you'll be less than pleased.

Chuck  <aa6g@aa6g.org>