Re: Astrophoto ATM scope

[Mel Bartels <mbartels@efn.org>]

Hi, Cathy,

Point sources such as stars depend solely on aperture for their
brightness.  This is because no amount of magnification will make them
be anything other than a point (within reason).  The mirror size gathers
the light and thereby sets the brightness.  A 12" mirror has 4x the area
of a 6" mirror, so stars will be 4x as bright.

Extended sources like nebula and galaxies work in a different manner. 
If you have two scopes, a 12" f/4 and a 12" f/8, the object will appear
to be twice as big in the f/8.  The object will therefore appear to be
1/4 the brightness because the area covered by the object is 4x as much
in the f/8.  Because the object is 1/4 as bright, it will take 4x longer
or more to photograph it.

Now to compare a 6" f/8 to a 12" f/4, the stars will be 4x as bright,
the image scale will be the same because both scopes have the same focal
length, but the extended objects at prime focus in the 12" will appear
to be 4x brighter because f/4 is 4x the brightness of f/8.

To compare a 12" f/4 to a 12" f/8, the stars are same brightness, the
image scale is twice as big in the f/8, and the extended objects are
photographically 4x fainter in the f/8.

To compare a 6" f/4 to a 12" f/4, the stars in the 12" are 4x brighter,
the image scale is twice as big in the 12", and the extended objects are
the same brightness.

It took me a while to figure this out, but that was many years ago!

A ccd covers a smaller area than 35mm film (unless you are rich and can
afford chips as big as 2kx4k or 2"x4", and you can edge butt these
together for even bigger sizes, but as they say, if you have to ask how
much they cost...), so a smaller but fast scope overall works best.  4"
f/4, 6" f/4, 8" f/4, and so on will work just great with ccds.  Friends
are getting great results with the Coulter 13" f/4.5 converted to a
drive scope.

The current practise is to take 3 images in each of the colors, then use
any one of many software programs to recombine the images, getting a
color image.  Poke around on some of the ccd webpages and you will see
incredible images!  Another issue is cost: a ccd will set you back more
than film and a camera will.

Does this help?

-- 
Clear skies, Mel Bartels    Programmer/Analyst, and amateur astronomer
atm, atm-digest list-owner  mailto:mbartels@efn.org
homepage: http://www.efn.org/~mbartels
Motorize A Dob: http://zebu.uoregon.edu/~mbartels/altaz/altaz.html