Thanks Bill. That helps a lot. Since I do not have any guiding system at
all right now, I guess it's pretty stupid of me to consider anything like a
20-30 min exposure. The comet would show the same apparent motion as the
starfield plus it's own.
Tom C.
----- Original Message -----
From: "Peifer, William [OCDUS]" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Friday, June 29, 2001 6:39 AM
Subject: RE: Comet Picture Question
> Tom C. (aimcompute) inquired thus about stacked astro-exposures:
> > The picture credits at the above link says the image is a combination
> > of 4 exposures totaling 20 minutes.
>
> > I was wondering why, but came up with the reciprocity failure issue as
> > I was writing this. Is that the reason? Does four 5-minute exposures
> > combined equal a better photo than one 5-minute exposure?. Is the
> > gathered light cumulative somehow? What is the reason for using this
> > technique?
>
>
> Hi Tom,
>
> Part of the reason this technique is useful is to overcome guiding errors.
> Depending on how sophisticated the telescope or camera guiding system is,
> you can accumulate a certain amount of error over a period of a few
minutes.
> Even if your guiding system is exquisitely accurate for tracking apparent
> motion of stars, it will still have some inherent error tracking comets
> (especially ones that are close to the Earth), since comets -- indeed,
even
> the Moon -- have a different apparent motion across the sky than the
> background starfield does.
>
> Another reason this is often done is to improve signal-to-noise ratio in
the
> final image. A single twenty minute exposure is going to be degraded to
> some extent by twenty minutes of atmospheric turbulence, scattered light
> from cars passing in the distance, etc. By shortening the duration of
each
> exposure, you get less of this noise. The four stacked negatives still
have
> the same signal intensity, since the total amount of "signal" from the
comet
> will be the same. However, each of the four negatives will have less
> "noise" than a single 20-minute exposure, and the noise in these four
> shorter exposures will tend to cancel out somewhat.
>
> There are quite likely some practical reasons as well. These breaks
between
> short exposures might also give you time to clean accumulated dew from the
> lenses, get a cup of coffee to warm your hands, or go in once in a while
to
> coax your wife outside to the telescope. And if you blow one exposure --
> perhaps the neighbor next door decides to power up the flood lights around
> his hot tub -- you've still got another three good ones. A single long
> exposure could be ruined in less than a millisecond.
>
> Not sure how much the reciprocity issue comes into play with modern
> emulsions, but I know that was a big issue with older emulsions, and it
may
> be important in this case depending on the particular emulsion the
> photographer used.
>
> Hope this helps.
>
> Bill Peifer
> Rochester, NY
>
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