[vsnet-chat 1741] Re: BVRI Photometry

[aah@nofs.navy.mil]

Just a couple more questions for Stan, plus a comment.
Does the ST6B have an actual electromechanical shutter,
or is it using the frame-store of the TC241?  There is
a simple method to check the shutter accuracy by using
constant-voltage LEDs which I discuss in the CCD book.

I'd suggest using an off-center aperture mask to remove
the secondary mirror obscuration.

The best system for what you are doing (looking at bright
variables where there are no nearby comparison stars) is
to use a wide-field telescope, something like the TASS
Mark IV.  Then you *do* have comparison stars in the
same field.  Otherwise, you have to use only photometric
nights (which are often hard to predict).  Plus, if you
are only looking at comparison stars every 30 minutes or
so, this is much too long between measures even for PEP work.
Where exactly are you located?  I thought you originally
stated Auckland (my memory must be failing!).

Another technique for comparison stars, which will save
you some time, is to take *two* exposures of the same
field with the same filter, back-to-back.  Make the second
exposure much longer than the first, and use the fainter
stars for your comparisons.  Then, at least, you have
the same airmass and are much closer in time to the
variable's measure.  You could even sandwich the variable
measure inbetween two deeper exposures to monitor transparency
variations.

The R&I flatfields should look pretty much the same (except
for dust, of course).  The B flat especially will look
quite different because of the different QE in the blue.
That is why your R&I data turn out more consistent.

A 30 arcsec aperture is *really* big for CCD work.  The
usual rule-of-thumb is 4 to 5 times the fwhm seeing disk.
If your seeing is 2arcsec, then you should be using an
8-10 arcsec aperture.  The larger aperture will include
neighboring stars, more sky noise and lower the accuracy
of your results.  You don't have to include exactly 98 percent
of all of the flux, it just has to be the same percentage
of flux from image to image.

You indicate that binning your CCD doubles the number of
stars you can observe.  I'd think that there is considerable
dead time just in rotating the filter wheel and moving to
a new position, such that the 25 second vs. 6 second read
time shouldn't be a major difference in your efficiency.
Using full resolution will permit longer exposures before
saturating the ADC.

Arne

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp