Dave Curtis wrote:

> The new ST-9E has 20x20micron pixels which maybe more suited at f10
> than say 9 micron pixels (without binning).
> Would using a f6.3 focal reducer (75 inch focal length system) with
> 20mu pixels be acceptable with average seeing.

This all comes from a pervasive myth that needs to be wiped out.
A 20u pixel camera at 2m FL (~12" f/6.3) will be horrendously
undersampled unless you have 7 or 8 arc-second seeing. A 9u
pixel at this FL is perfect for fair to average seeing (2" - 3").

At f/10 the 9u pixel is perfect for slightly better than average
(2") seeing. Unless the seeing drops below 4" the 9u is going
to still give you a better image.

Remember, the _minimal_ sampling should be pixel FOV = seeing/2.
The _optimal_ sampling is closer to pixel FOV = seeing/3.

This is all assuming that you are trying to get good images,
which your post implies.

This "large pixels" cr*p bascially came from an article in
CCD Astronomy several years ago. The author really pushed
undersampling (basicaly FOV = seeing) as being great _for_the_
beginner_ as it covers up errors. Which it does. But it
also means that (1) you'll be severly hindered in improving
your skills as you'll never see your errors, and (2) when you
finally do improve your skills to where you want to take good
images you have to go out and buy a new camera.

Now, not only is this very bad advice, people read the article
up to the point where it says "for the beginner" and ignore that
point.

The ST-9 is a camera for the longer FLs (in excess of 3m FL).
The C-14s, 16" Meades, 18" and 20" RCs and the like. Buying it
for 3m FL is marginal. Buying it for 2m is a big mistake.

Assuming one is trying to get good images.

Gene Horr


A focal reducer at f3.3 would need
> a smaller pixel camera - is this correct?
>
> Thanks
> Dave Curtis
> Dunedin, New Zealand

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