[ATM] Quirky idea

[mikell at optonline.net (Michael Lindner)]

On Wednesday 04 February 2004 08:47 am, Grey Coyote wrote:
> Call me Rob, please. ;O)

Howdy, Rob!

> I may actually be using the wrong terminology here.
> What I am using for magnification is focal length of
> the telescope, in my case 750mm divided by the
> aperature of the eyepiece, say 12 mm.

You are confusing aperture (diameter of the opening) and focal length 
(distance from optics to focal point) here. A 12mm eyepiece has a focal 
length of 12mm, not an aperture of 12mm.

I think you're also confusing image scale and magnification. Magnification is 
the ratio of angular size of and object (e.g. at 50x magnification, the 1/2 
degree moon would appear to span 25 degrees of the view). When imaging on 
film or CCD, there is no magnification - you're talking about image scale. 
For instance, in your 750mm scope, the 1/2 degree moon would form an image 
about 6.5mm across  (if I did the math right).

Try reading "Telescope Optics" from Edmund Scientific, or another introductory 
book on optics. I think you'll not only find some interesting ideas, you'll 
get the terminology to communicate them.

> > I have a spreadsheet a friend got off the 
> web that shows optimal viewing for large nebulas is
> 48x.

These charts refer to angular size for visual observing. They have no 
relationship to CCD imaging.

> My idea is if one could fashion an array of ccd chips
> say 2 across and 2 deep.

That's what was done on the Hubble space telescope, so the idea has merit. It 
increases the field of view over a smaller CCD chip. However, there are many 
technical problems, like getting the area of the chips contiguous, so you 
don't have a "window pane" effect on the image, and perfectly aligning the 
chips (to within a couple of microns) so you don't get image skew.

If you are designing a multi billion $$ space instrument where observing time 
costs more than manufacturing, it makes sense. For amateurs it is much 
cheaper and easier to either just use a larger chip, or to just take a series 
of astrophotos of overlapping regions and combine them in software (which 
still isn't easy, but at least it doesn't cost $$$).

Clear skies.
-- 
Michael Lindner
http://www.starastronomy.org *** http://home.att.net/~mikel
http://www.atmsite.org *** http://www.atmlist.net