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Gradiants Again...



Let Me Start Over,

        Looking at the background image plots using the SExtractor program I
see two different types of gradiants in the TASS images I have been taking
here in Dayton:

        (1). Changes due to being in a suburban location with city lights
being slowly turned off over the course of the time period of say 10:00 -
2:30 AM. (This produces a changing gradient from left-to-right in each
image, which is considerably differnet from one image to another). 

        (2). Changes is sky background due to changes in Declination , this
also apperas to change from frame-to-frame (image to image). It is easy to
see once you look at the background plots mentioned above.These change from
top-to-bottom in each image and *are* considerably different in a sequence
of images taken over the course of an evening. 

        I am sure that these effects are also present in the images that
Mike G. and Tom have been taking. These effects would (probably) have an
effect on the zero-point of out photometirc measurement since they change
from image-to-images over the course of a night. 

        If we all were in a dark-site such as Arne's location (where such
gradients stay relativley constant over the course of a night) then they
would be calibrated out of setting the "zero point" for our photometric
calculations as he has mentioned before in a pervious posting.

        If Arne could explain how these effects mentioned above (1&2) would
not effect our photometric accuracy (trying to get to the ~1% level) using a
wide-field setup from light-polluted suburban locations then I will quite
being concerned about them....

Glenn G. 
Glenn Gombert <gleng@infinet.com>