[Aavso-photometry] Bright star wide field CCD photometry

[Richard Miles]

> Has anybody ever tried to do photometry from images taken with
> short FL camera lenses ?
> Say 50 mm FL or even down to 8 mm (= real all-sky photometry ;-) ) ?

==> Yes - I have also done photometry using a digital video camera but I 
have not done very wide-field work, which is what you're interested in (the 
flats could be a stumbling block!)

>
> Has anybody ever dared to try high precesion photometry on bright
> variables like gamma Cas, beta Lyr, alpha Ori, beta Per, alpha Sco,
> delta Sco, alpha Tau, ... using a CCD ?

==> Yes, recently I made observations of Gam Cas (V=1.9) in V and I band but 
have switched to some fainter variables of this class.  I use 60mm aperture, 
FOV = 1.0 x 1.5 deg) and defocus.  Going even brighter is mainly a question 
of defocussing even more since the alternative is to keep reducing the 
exposure time but then you lose out to scintillation.  I prefer to stop at 
about 0.2 sec exposures (with an SXV-H9 and V filter).  I usually take 
30-100 images to derive a single magnitude result accurate to 0.01 or 0.02 
mag.  You can really only do these bright stars on clear nights where the 
transparency is relatively stable as you sometimes also have to take a 
series with longer exposure times (say 5-10 sec) to accurately measure 
comparison stars in the same field (or separately, see below) if these are 
relatively faint.  These exposures are usually bracketted by observations of 
the bright variable before and after the comparisons.

> The AAVSO PEP charts use comp and check stars that are often
> several degrees away and several mags fainter than the variable.
> To get such a wide field it would require a FL of ~ 100 mm or less
> for the ST-8 or ST-10  or  ~ 50 mm or less for the ST-7.

==> No problem - again given it is a reasonably steady, clear night just 
move the scope and take images of the comparison(s) then move back to the 
variable.  This in fact was the way that all photometry was done during the 
pre-CCD era (excluding photographic photometry).  I like to use Hipparcos 
stars as references for V magnitude.  They are usually good to +/-0.005 mag 
in V provided they are no redder than B-V = +1.1, but don't assume any 
particular Hipparcos star is constant, you should check these for 
variability.

Taking CCD frames of a single reference star seems to be against the mindset 
of the majority of CCD observers.  I do this for extinction stars at high 
and low altitude with great success.  Also for asteroids, lots of people do 
differential photometry but rarely do people move the telescope to image a 
reference star a couple of degrees off the asteroid's path.

Richard Miles