Re: [APML] Converting CCD Workflow to Film?

[Wei-Hao Wang <whwang@gmail.com>]

Hi Wade,

In terms of making both stars and comet untrailed, the technique you
mentioned should work well on film images.  In terms of bringing up faint
deep-sky objects, perhaps it will still help, but not that much.

Suppose the total exposure time is 30 minutes and we cut this 30 min
into 3min x 10.  On linear devices like CCDs, as long as the readout
noise is sufficiently low and the images are processed properly, the
combination of 3min x 10 will be very close to (if not identical to) a single
exposure of 30 minutes.  This is not the case for film.  At the beginning
of each exposure, the incident photons are all wasted.  Until a minimum
exposure is reached, photons don't create latent image at all.  This makes
short exposures extremely inefficient in creating latent image.  For this
reason, combining 3min x 10 film exposures will not give you an image
as deep as a single 30 min film exposure.

Nevertheless, if this is the only way to achieve the result you want, you don't
have other choices.  You just need to take many many short exposures
and hope the combined image will be deep enough.

If I were going to do this, I will track the stars and do 4 minute exposures
on FSQ 106.  Comet is not a point source so it allows longer exposure
times than stars.  4 minutes on FSQ106 is about 0.05mm of moving, which
can be hardly noticed.  In terms of recording faint objects, any increase in
individual exposure time will greatly help.

Cheers,

Wei-Hao

-- 
________________________________________________________________
Wei-Hao Wang  :)

Institute for Astronomy at University of Hawaii

Address:                       
2680 Woodlawn Drive         Personal Website:
Honolulu, HI 96822             http://www.ifa.hawaii.edu/~wang
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