[vsnet-chat 2092] Re: Plotting light curves

[aah@nofs.navy.mil]

Gianluca wrote:
>My CCD spends about 30 secs to download and save an image in full
>resolution (765*510 pixels) and about 7 secs to do the same in mid res
>(2*2 binning). On V603 Aql I was working in 2*2 binning, acquiring a new
>frame every 15 seconds. The software automatically saves the pictures
>and I can leave the instrument unattended. Unfortunately, to do so I
>have to limit the integration time to max 10 secs...
  Note that, for your case, you integrate for 10 seconds and then have
15 seconds of dead time.  That means you are collecting photons for only
40 percent of the time.  If you use full-frame mode, then you integrate
for 10 seconds and have 30 seconds of dead time for an even poorer
efficiency of 25 percent.  When you want to do high-speed photometry,
or are limited to short exposures due to the telescope mount, the goal
should be to find some method to increase the efficiency of data
collection.  There are two ways to do this:  improve your mount, either
with the PEC/autoguiding capabilities of the ST7 or with better gearing;
and decrease your dead time between exposures.  You have little control
over the readout rate of your CCD unless you built the camera yourself.
However, the exposure method can be changed to effectively give you a
more rapid readout.  Binning is one example.  Windowing the readout to
only include the variable and 1 or 2 comparisons is another.  Doing an
exposure, reading out n lines, and doing the next exposure is another;
it gives you a strip scan output with star images every n lines.  There
are several other techniques, but you get the idea: to maximize the
information content, increase your photon-collecting efficiency.  If
you can increase your 40 percent figure to 80 percent, you have effectively
increased your telescope aperture by sqrt(2) or 41 percent, and can
reach 0.7mag fainter in the same clock time!
Arne

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