Re: Dumb question

[Chris Lloyd <cl@astro1.bnsc.rl.ac.uk>]

Tom,

Yes, aperture photometry is always a compromise between including as much
of the profile as possible while not compromising the signal to noise
ratio. The profile will invariably be truncated somewhere, it's just a
question of where to do it.

Chris

On Fri, 13 Dec 2002, Tom Droege wrote:

> 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
>
>