Re: (ATM) digitizing images

[wjlee@itd.ssb.com (William Lee)]

> 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
> 
> 

Good point.  You also got me thinking, how does field curvature affect
CCD imaging.  I mean, with film you can (sort of) curve it with a holder.
Or is that not a problem with a 600x400 (or whatever) resolution CCD?
(That is, if the edge is out of focus, does it not matter because the
pixel of the CCD is so large?)

-- Bill