[Aavso-photometry] Ensemble photometry question

[arne]

Brad Walter wrote:
> A different but related topic
> I have been using a 106 mm aperture 530 mm F/L refractor to gather V
> band light curve data on exoplanets. My 250 mm aperture 3000 mm F/L
> scope doesn't have sufficient FOV to obtain suitable comp stars for
> these bright HD catalog objects. I don't seem to be able to get better
> than about .006 mag precision even though I have very good (several
> thousand) SNRs. I am defocusing to around 4-5 pixel FWHM. I average
> between 5 and 10 images depending on exposure duration and take readings
> from the resulting combined photos. I am using 25-30 each flats darks
> and flat darks. Exposure durations are constant during a session but
> range over 5 to 20 seconds depending on the brightness of the target and
> comp stars for the session. I am not auto guiding because I don't want
> the re-centering delay. Therefore there is slight but steady drift of
> the stars over a session. I suspect that the culprit is scintillation
> due to the seemingly random variation, but it may also be differences in
> pixel from movement of the stars over the CCD. Total movement over 4
> hours is around 20 pixels. Does anyone have any suggestions for
> improving the precision?
> 
The paper by Castellano et al. in the latest JAAVSO ("Detection of
Transits of Extrasolar Giant Planets With Inexpensive Telescopes and
CCDs", 2004, 33,1 and available on our web site) gives some details
about scintillation and how to work with it.  This is most likely your
biggest source of error.  BTW: there are many typos in that particular
paper; our apologies as this is not at our normal quality level.

If you have a photometric night, you can use your larger telescope
and just sit on the bright object, offsetting periodically to a comp
star; differential photometry in the FOV isn't always necessary.  There
are many ways of handling bright stars that have been suggested on this
list.  The one that might work best, but requires much more care, is the
"neutral density spot" to enable using fainter reference stars in the FOV.
This can sometimes be made by gluing a spot of exposed film on a glass
plate near the CCD.  There are some stars, such as TrES-1, where the
star is faint enough and has a good reference frame that photometry is
easy; and others that we haven't really covered yet where there is a
reasonably bright comp star in the FOV.  Sometimes you just have to wait
and not do every campaign that arises.

Since you are taking lots of images (~10second duration with transits
that take place over 3hrs), don't always get concerned about the precision
of an individual frame.  Least-squares analysis can find patterns hidden
by random noise pretty easily.  I strongly recommend using a photometric
filter for all transit studies.
Arne