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Re: downloading data



Richard writes,

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

No, I use the same flat field for quite a while.  If I do anything, clean
the lens etc., or if the conditions change, then I make a new flat field. 
The problem with trying to do them automatically is that one really needs
to study each flat field that is made.  Some clouds might come over in the
middle of a flat field operation and then it is junk.

I agree that it would be nice to study the flat fields over time.  I keep
all the old flat fields, so one could go to the directories where the flat
fields are saved, get a few and look to see how they change over time.  I
have looked at them (but not with any kind of measurement) and find that
things that happen every night are probably more disruptive than real flat
field changes.  But I don't really know this and would welcome a study.

Note that each camera moves about 30 degrees in declination as it scans
the sky.  I make the flat field for each camera using the whole
declination range. There are no doubt declination effects.  One might try
to make a flat field for each declination, but I am not sure that I could
work this into the present pipeline.

There is a big difference between taking one image the best you can, and
taking all the sky that passes the meridian every clear night.  Since I am
doing everything for operation of the system, compromises have to be made,
and I have made a bunch.  Mostly I work on things that are obvious
problems.  There seem to be plenty of these.  I would welcome anyone who
wants to study the data and find a better strategy.

I stand ready to send anyone data who wants to study something.

Tom Droege


> 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.
>
> Cheers,
> Richard Miles
>
>
>
>