[vsnet-chat 4903] Telephoto + CCD survey cameras

[Brian Skiff <bas@lowell.edu>]

     I was interested to see in the report on V657 Cas (on vsnet-id and other
lists) that Akira Takao is using a CCD + telephoto combination.  I have thought
this set-up would be a powerful one for surveying, far preferable to using
an ordinary telescope.
     I was wondering if Takao or another person using such a set-up has 
experimented with taking images into fairly bright twilight, particularly
close to the Sun (say 20 to 30 degrees elongation).  Exposures would of course
necessarily be short and the limiting magnitude would be bright.  I am
wondering about the possibility of finding relatively bright comets (perhaps
mag. 6) close to the Sun.  The low-elongation region at less than 90 degrees
elongation is now the only area where northern-hemisphere amateurs will find
comets, since all the rest of the sky is covered several times per month to
faint limits by the large asteroid/comet survey "machines", one of which I
help operate.  Some of us push down to 60 degrees alongation, but the coverage
there is not complete or consistent.
     Whether it's comets or first/last observations of variables for the 
season, does anyone have some advice to offer?  How deep into twilight is it
productive to go, what elongation range, what magnitude limits?  Does it help
to use a red filter?

     Just as a reminder, it will be very helpful in interpreting data from such
instruments if a standard photometric passband can be employed in such work.
If a Cousins R filter is not available, then at least blocking the far-red
light will avoid problems with extremely red objects such as V657 Cas.  One
could make an ersatz R filter by using both an IR-blocking filter together with
something like a Wratten 24 (or equivalent).  Yes, you lose perhaps half a
magnitude in your threshold, but nearly any professional astronomer would
prefer to sacrifice some raw efficiency for accuracy in photometry.  Using a
filter also makes data from different instruments nearly directly comparable.

\Brian

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