[vsnet-chat 4911] Re: Telephoto + CCD survey cameras

[Brian Skiff <bas@lowell.edu>]

     Thanks to Akira Takao for the quick tests in morning twilight.  From
these it appears as a rough guide that one can expose at ~35 deg elongation
without much loss of limiting magnitude up to about 1 hour before sunrise,
which for mid-latitudes is close to the time of nautical twilight.  One can
also keep going for another 10 minutes (or start 10 minutes earlier in
the evening) with shorter exposures with a modest loss of magnitude limit.
This will be a useful starting point for further tests.  Takao's magnitude
limit figures suggest it could be very fruitful to conduct comet searches
close to the Sun with a similar set-up (perhaps 105 or 135mm telephoto to
cover more sky?).
     It is not surprising that the final image test only 4 minutes after
the second gave a bright sky background:  the brightness has begun to
increase exponentially by this time in the twilight sequence, so just a
few minutes time produces a large change in the twilight brightness!

\Brian

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