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--- In C14@y..., Scott Evans <sevans@i...> wrote:

Richard,

Your comments are interesting because I had always heard that the

focal

ratio would be the primary factor in determining exposure time

regardless

of focal length. Shouldn't the f/5 fsq be a shorter exposure than

the f/11

c-14?

BTW with the fastar lens installed I have an 735mm focal length 14"

scope. ;-)

Scott

First of all, I am talking specifically about CCD imaging.

The effective focal length of the telescope coupled with the physical
size of your ccd chip's imaging area (when used in the "normal" way,
ie prime focus) determine the field of view of the imager. I say
effective focal length to take into account the possible use of a
focal length reducer or extender.

This is independent of focal ratio.

In the case of the C14, it has a very long focal length and therefore
cannot provide me the wide field of view that I can get from the FSQ.
On the other hand, because it is 14" in diameter, it gathers 1225%
more light than does a 4" scope. So if somehow you could give the two
scopes the same FOCAL LENGTH, therefore getting exactly the same
field of view in either scope, you'd find the C14 to be a LOT FASTER
to expose.

What I was noting as I was beginning my quest into widefield imaging
is how much longer the exposures take, simply because the only way I
have to get a short focal length is from a small aperture telescope
and they sure don't gather light as quickly as that C14 does.....

I understand the attraction of the FASTAR adaptor now, for widefield
work. Too bad that it is not recommended for any of the popular high
resolution cameras such as the SBIG 8 and 10 series and the FLI
Maxcam and Dream Machine series or the Starlight Express HX and MX
series. All the cameras weigh too much and offer too much of an
increase in the central obstruction.

So you can get and ST237 CCD camera and FASTAR and shoot at lower
resolution fast or you can use a newer generation camera and shoot
higher resolution slowly in a refractor. Funny thing is that other
than download time, there's nothing about high resolution from an
image capture perspective that is any slower than low resolution. But
because the mechanical designs of the cameras are so big and heavy,
the interesting cameras cannot be used with the most interesting
telescope configuration option that Celestron sells for the C14.




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