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Old November 28th 05, 04:14 AM posted to sci.astro.amateur
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Default Why .avi format ?

Tom Rauschenbach:
From reading this newsgroup I understand that .avu format seems to be the
conventional file format for digital imaging. I usually associate that
format with motion pictures.


Davoud:
That's exactly it. People make motion pictures of solar-system objects,
then they use software such as Astro IIDC, Keith's Image Stacker,
Registax, etc. to select the best frames, which are then combined by
the software to produce a better image than could be obtained with a
single exposure.


Tom Rauschenbach:
That makes a great deal of sense to me. Especially in light of statements
that seem to imply that long exposures aren't really needed for CCD data
collection. Although I have to wonder why there aren't discussions
about readout noise/image. What are the costs and benefits of many short
exposure images over fewer long exposure images ? Of course tracking
issues are an obvious one.


Remember that when we are discussing webcams we are talking about
photographing bright solar-system objects only, not deep-sky objects.
Readout noise is negligible; the exposures are short. Webcam lunar and
planetary photography has little in common with deep-sky photography
using a dedicated CCD camera such as an SBIG or Starlight Express.

By the same token, long exposures don't enter into the equation, as the
objects we are photographing are bright. This technique -- combining
video frames -- is used because atmospheric turbulence is a severe
hindrance to planetary photography. Take a lot of pictures, though,
(4,000 maybe) and a certain number of them will exhibit much less
turbulence than the majority. These are the ones that are aligned and
combined to produce images such as these
http://www.buytelescopes.com/gallery/gallery.asp?c=17165 by Alan
Friedman. Mr. Friedman uses FireWire webcams and Astro IIDC running
under Mac OS X. (Note the exception to what I wrote above; the
referenced page includes a pretty remarkable photo of the M57 -- the
Ring Nebula -- made with a FireWire webcam.)

Davoud

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