Re: Long focus telescopes

From: Eric (ekfuller_at_telus.net)
Date: 12/05/04 

Date: Sun, 05 Dec 2004 00:00:00 GMT

Ed Majden wrote:
> Tim:
> True, long focal lengths are very useful for planetary imaging. What I
> was referring to, most professional telescopes were often long focus
> Cassegrains. F-20 and longer in some cases. The DAO 72 inch Cass focus is
> f-18 but most often today it is used at the f-5 Newt focus with a CCD.
> Ed
>

It certainly depends on the use of the telescope. If you want to do
wide-field surveys, that's one thing. If you want to do high resolution,
that's another. Take the time to decide what you want to do.

I like solar/lunar/planetary imaging myself. I don't have adaptive
optics, but the next best thing is stacking short exposures (lucky
imaging). Then you need image processing. Just aligning images doesn't
get you to the limit of your telescope, you need to over-sample the PSF.
Let's say a minimum of 2 pixels across the PSF. If my pixels are 5.6 um
across, then my PSF must be at least 11.2 um. If I'm looking at RGB, the
blue light is the worst case, so 400 nm wavelength. I need a focal ratio
of at least f/28 in order to sample the PSF properly. If my pixels are
larger, I need a greater focal ratio.

If you look at the pro telescopes, they go even further to improve the
sampling of the PSF. A well-sampled PSF makes the processing more stable
(LR, Max Ent, Blind Deconv). So you sacrifice S/N for resolution, what
else is new ? Get a larger mirror. Even the pro's have aperture fever :)

Eric.

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