Re: OPTICS FOR TASS: REFLECTOR OR REFRACTOR?

[hjohnson@pluto.njcc.com (Herbert R Johnson)]

On Wed, 07 Jan 1998 14:06:47 -0800, Cameron Tully-Smith <cameron@zippy.sonoma.edu> wrote:
*>
*>I'd be happy to take some star field shots when I get a clear night here.
*>The star field shots that I have already taken with the Mak are not 
*>in focus or were on a mount that doesn't track well (and wasn't guided).
*>I normally shoot on PPF...  If Tech Pan is a necessity, could we work
*>something out so that someone could send me a roll of hypered TP?

Hypering improves the linearity (dynamic range) and reduces reciprocity
(sic) failure. But the bottom line is not maximum exposure or dim objects,
but to test the optics. So, shoot something in the Milky Way so you can
see stars across the whole field. As you
mention tracking and guiding issues, a short exposure is preferable
anyway. I mentioned the Double Cluster: it's a pretty object, about
a degree wide, easily found and generally it's naked eye. The kinds
of thinks to look for in this image are distortions in the point images.
I suppose one could do some "photometery" but for an unfiltered exposure
it's an exercise rather than science. The blue-sky image would give
a better sense of vignetting (dropoff with radius). (At this point
I differ to my more knowledgable colleagues.)

*>If all this is too much trouble and what is needed is a fine-grained
*>shot, I have some unhypered Royal Gold 25 that I could use.

P-r-e-t-t-y slow film, but you get the idea that a fine grained film is
preferable. You should ask people who really do astrophotograpy as to good
choices: I mentioned the traditional one, and unhypered it's available
at any film store or shopping center, sometimes elsewhere. It works into
the IR as I recall, so you may get some interesting results. "Better the
devil you know", as they say, so use some judgement given the stated goal,
and perhaps RG 25 is a reasonable choice.

Herb Johnson

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Herbert R. Johnson                      voice/FAX 609-771-1503 day/nite
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