Re: aperture photometry and sky variations

[Arne Henden <aah@nofs.navy.mil>]

There is no perfect solution when clouds are involved.  The
best one can do is select the ensemble to be close to the variable
and to surround it spatially.  You then compute the mean deviation
within the ensemble; that tells you how photometric the conditions
are and gives you an idea as to whether the variable's measure is
likely to be correct.  This is far better than the standard variable-comp
single-star solution.
   Bottom line is that clouds or any other kind of local disturbance
cannot be removed perfectly.  High quality photometry requires high-quality
skies.  On many occasions, the simultaneity of CCD multiobject photometry
will correct for the majority of problems, but the amount of correction
will depend on the actual conditions.  I've done one percent photometry
with 5 magnitudes of cloud extinction; I've also had 0.2mag errors under
better conditions on other nights.  An equally large problem is that
clouds decrease the overall signal/noise, so you have to boost the
exposure time to compensate.  This is rarely possible with survey-type
observations, so cloudy conditions will usually result in the discarding
of data, even if good photometry might have been possible.
Arne

Tom Droege wrote:
> 
> The reason is that I worry that the situation is not so simple.  If all 
> the photons from the star make it to the image, plus a contribution from 
> the cloud, then the scheme is OK.  However, in the likely case that some 
> of the photons from the star get dispersed in the cloud, then it will be 
> measured low compared to a star of the same brightness sitting in a 
> clear patch.  I think the ensemble photometry will be confused if an 
> image contains stars in clear patches and stars peeking through clouds.  
> Comment Michael?  Others?