Re: ATM CCD's and mirror F/#

[akelly@ghgcorp.com (Albert F. Kelly)]

>Not having any experience with ccd's could someone explain why celestron 
>has come out with a seemingly very fast something that works at 
>f/1.95,and I think they hope to have a mirror that fast aswell for ccd work.
>Do you really need or get better images with these faster compensators - 
>or whatever they are?

Ric,
       "Fast" optical systems, which are usually considered to be those at
f5 or lower focal ratios, allow deep-sky objects to be effectively imaged
with shorter exposure times than "slower" optical systems.  This does not
necessarily mean better images, but it does cater to the high sensitivity to
light quanta of CCD chips.  In this era of instant gratification and
real-time or near-real-time image display, it is rewarding and useful to
many amateurs to be able to take short exposures of deep-sky objects (from a
few seconds up to a few minutes) and get a decent image.  Such efficient use
of short-term photons also means that tracking and guiding requirements are
reduced.
        Of course, the really good images, both B&W and color/tricolor,
still result from sufficient exposure time with good optics and tracking
under a dark sky.

Al Kelly