How much quality? (was: Re: (ATM) digitizing images)

[Chuck Grant <grant@aretha.llnl.gov>]

> > I see many people here and other places debate over wheter they will use 
> > thier telescopes for photo or CCD immaging.  I also have my wn B&W 
> > darkroom which I have been using for 15 years, and i see lots of 
>information,
> > much of it in error.  
> > 
> > If you are about to grind a mirror, and you do not know whehter or not 
> > you will use it for visual, photo or CCD work, here are a few pointers to 
> > consider before grinding:
> > 
> > 1) CCD vs Photo - thewhole argument is a uselss waste of time.  In the 
> > "real world", IE magazine & desktop publishing, the two technologies are 
> > so amalgamated with eath other on a dialy basis it is oftn hard to tell 
> > where one starts and the otehr stops.
> > 
> > 2) Visual and Photo use - grind your mirror to 1/4, even 1/2 wave, and be 
> > happy at a fast f ratio of f6.  In some cases, it might be even better to 
> > have a faster f ratio if you only plan occasional photography.
> > 
> > 3) CCD work - grind down to at least 1/4, even btter if you can.  and go 
> > with a longer f ratio.  I ahve a friend who built a classical casselgran 
> > (spelling?) 8 inch f 20.  Wonderfull telescope.  Tolerance levels for CCD 
> > work are much higher than visual and photographic work, partly because 
> > you are working with a much smaller and more highly magnified field of view.
> > 
> > Anyhow, this may or may not have much to do with the topics discussed 
> > here, but I thought I should jump in before the discussion got to far in 
> > any particualr direction.
> > Joe
> > 
> > 
> 

Yes much of the infomation is in error.  For example, visual work at
high powers is much more demanding of an optical system than typical
prime focus photographic use.  Why?  Because film grains have a minimum
size which is usually much larger than the diffraction limit.
Sharpening the focus smaller than the film grains does not make
resulting image any better.  On the other hand visual use at
magnifications where diffraction rings are visible is quite common
requiring very good optics.  On the third had, planatary photography at
magnifications much higher than that at prime focus is very demanding of
the optics.  This high-power-visual/prime-focus-photographic difference
is more pronounced as the focal ratio gets smaller.  I know it is not
optimum to use a low focal ratio scope for high power observing, but it
can be done effectively with good optics.

The way amateurs use CCDs typically has the pixel size much larger than
the diffraction spot.  In this case, high magnification visual use is
more demanding of the optics than CCD use.  Maximum resolution CCD
imaging occurs at the Nyquist limit of two pixels per diffraction limit
(linear dimensions).  CCDs are as demanding as visual use only as you
approach this limit.  Since the field of view is so small at that
resolution, most amateur CCD use is at lower magnifications. (Planatary
CCD use is a notable exception.)

[A smaller field of view actually makes many things much easier as far
as optics goes.  Your friend can use a classical Cassegrain design with
a CCD only because it is at f/20 and has a small field of view.
Classical Cassegrains have far too much off-axis abberations to make
very good wide angle scopes. The small field of view and long focal
ratio hide the flaws of the classical Cassegrain design.  Any design
would work great at f/20 with a small field of view.]

So the level of optical perfection required depends entirely on the kind
of visual observing, the kind of CCD observing, and the kind of
photographic observing.  It is entirely meaningless to say something
like "CCD use is more demanding than visual use."

I fail to see how desktop publishing has any significance to telescope
building.  I would think that "the real world" might be professional
observatories, where it is all CCD and no photography.  The only real
advantage photography has in professional astronomy is the ability to
use a large detector (i.e. a large piece of film).  For the amateur
without a computer, lower cost is also an advantage for photography.
But you get what you pay for (and this message is free:-).


Chuck