Re: Dumb question

[Tom Droege <tdroege2@earthlink.net>]

Not a dumb question but one that needs to be continually studied as we 
solve other problems.  First, I think TN70 is pretty obsolete.  I recall 
that this work was done before we turned the last lens around.  It had been 
installed backwards at the factory.  This greatly reduced the coma.  I no 
longer can "see" coma in the corners of the image.  Perhaps someone can 
study this with one of the current data sets.

I have tried different apertures on the theory that we were losing light 
from the brighter stars.  I found that increasing the aperture did not 
change the error for the brighter stars but did increase the noise for the 
fainter stars.  This work eventually led to the discovery of the current 
flat field problem which completely masks any other problem.  When this is 
solved, then we need to go back and do aperture studies.  I think Michael 
R. made a pretty good choice in putting a 4 pixel radius aperture as the 
default in the pipeline.

Tom Droege

At 11:25 AM 12/13/02 +0000, you wrote:
>Forgive me if this is a dumb question, but I suppose the aperture used for
>the photometry is large enough to encompass the whole image at the centre
>and edges of the field. I was prompted to ask by the comments in a recent
>posting about the shape of the images at the edge of the field. Having
>looked at TN70 I can see where the figure of 7 pixels comes from but
>depending on how the aperture is centred it may be possible to lose the
>faint extremities. I wondered if any experiments had been done to compare
>photometry across the field using different aperture sizes. If not then
>perhaps DS24 is where to do it.
>
>Apologies for any dumbness...
>
>Chris