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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