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Re: downloading data

Michael R. wrote:

>      b) instrument errors -- we know that the Mark IV units
>             suffered from a variation in sensitivity across
>             the (very wide 4x4-degree) field, which was
>             not perfectly corrected in the flatfielding.
>             This is independent of the stellar brightness
>             and aperture details.

>  The information needed to evaluate the effect of large-scale
> "flatfield" errors has not been dragged out of the data itself,
> except for a very small first step described in Tech Note 97.

My understanding is that flatfields are derived more or less each night.
Has anyone studied the 'apparent' variation in the flatfield night-to-night?

The reason for the question is:

1. Analysing / plotting the flatfield over several months will give a 
measure of the longer-term drift in this correction.

2.  More importantly the statistical variability in the flatfield (at 
various positions on the frame) relative to the mean drift will be a measure 
of the accuracy of each flatfield.  I suspect that most of this variability 
(night-to-night) is caused by systematic bias and is not based on true 
variation in the flatfield.

3.  From 2., if it is concluded that there is indeed a significant 
night-to-night statistical variability in the flatfield, it would be 
possible to average flatfields over many nights (provided no change is made 
to the physical setup) and reanalyse the data retrospectively thereby 
getting a significant improvement in photometric accuracy.

Richard Miles