[Aavso-photometry] Exposures for automated AAVSO observing

[arne]

Richard Campbell wrote:
> Hi Brian,
> I run partially unattended and my strategy is to take a series of 15 to 25 
> 30 second images for most stars. That gives me the option
> of median combining in groups of 3 or 5 , averaging or summing images , 
> depending on their brightness or conditions. I can image
> unguided with those short exposures, needs only 1 set of darks and it gives 
> a  pretty good range of magnitude coverage. The drawback
> is processing takes longer.
> 
> Clear skies,
> Dick Campbell CMP
> 
> 
> 
> --------------------------------------------------
> From: "R. Brian Potter" <potterrb at comcast.net>
> Sent: Sunday, July 27, 2008 1:36 PM
> To: <aavso-photometry at mira.aavso.org>
> Subject: [Aavso-photometry] Exposures for automated AAVSO observing
> 
>> Hi All,
>>
>> I have been experimenting with CCD Autopilot for automating my observing
>> runs.  The program works fantastic for automating tasks like taking darks,
>> biases, and dawn flats. It is built primarily for astrophotographers,
>> however, and requires that exposure times be specified for each target in
>> your observing run.  This is difficult for VSOing, as the optimal exposure
>> time varies with airmass and with the current magnitude of the target.
>>
>> Is there a way to calculate reasonable exposure times for variable stars 
>> in
>> advance, perhaps based on images of a standard field and recent 
>> observations
>> of the target for a given filter?  I am working primarily with Mira-type
>> variable stars, where I will be hopping from field to field and taking a
>> number of images of each in one or more filters.  I suppose this could 
>> also
>> apply to light curve photometry of faster varying stars, if the potential
>> range of the light curve were reasonably known.
>>
>> As a professional with a day job in another field, saving sleep time is
>> quite an important problem for me at the moment...any thoughts would be 
>> most
>> appreciated!
>>
The approach Dick indicates is probably the easiest to follow.
I'd set the base exposure to not saturate the variable or comps when
the variable is at its brightest, and set the number of exposures to
properly measure the variable at its faintest.  This will mean that you
take lots of extra exposures during maximum and just the right number
at minimum, so the net effect is that you will waste some time.

With Miras, and with V filters, you don't have a lot of choice unless you
have smart software.  For example, you could take a test exposure, do the
photometry in real-time, and take a second exposure based on the results
of that test.  We always did that at USNO-Flagstaff to make sure the
target star always had sufficient signal/noise - there, we took 30sec exposures
and used that to set the final exposure time of up to 90 minutes.  The other
option is to somehow use the current brightness as obtained through
quicklook to set the exposure for the upcoming night.  Miras
at V can often have ranges exceeding 5 magnitudes, so setting exposure times
and picking a good set of comparison stars takes work.  Then you have to
consider seeing and sky transparency, which can have an even larger effect.
Expect to lose some data when working automated.  Other classes of stars,
such as eclipsing binaries or cepheids, are more predictable and usually
lower amplitude so that a single set of exposure times will work.  Walter
McDonald has a nice system for looking at CVs, where if the star is detected
in outburst in real time, his software can switch to time-series mode to
follow the outburst.

For setting the exposure if you know the current brightness, there are
several online exposure calculators that can be used, and I've also seen
a couple of spreadsheets that do it empirically (use the exposure times
needed to properly expose a star of a known magnitude for your telescope
to set a baseline, and then use the input brightness to return the new
exposure time).
Arne