(ATM) digitizing images

["Joseph O'Neil" <joneil@suspects.com>]

I hope this is not off topic too much.

Some pople have been talking about digitizing images & pictures, whether
it be for posting on a web site or for storing astro photos.  I just
pulled a few atm files form soem web sites, and I can tell you it is very
nice to have an accompanying gif or tif to actually "see' what I am
supposed to be doing.

However, there is a caution to all of this.  I ahve a flatbed B&W 
scanner I use for desktop publishing.  If you are scanning in line art
or B&W drawings, you can usually scan in at 600 dpi no problem.
However, if you are scanning in an actuall photgraph, well, look at
these figures I got from a recent exercise:

5" x 7" B&W photo

@100 dpi  -   250 K
@150 dpi      700 K
@300 dpi      8000 K

figures are approxamate.  My math is not that good, but basically, the 
higher the resolution, it basically becomes an expotential rising scale.
Another problem is that 75 or 100 dpi looks real good on a computer
screen, but it takes a resolution of about 600 dpi to reproduce what I 
would call "photo quality" on a GOOD laser printer.

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