Re: Dumb question

["Richard Miles" <rmiles@baa.u-net.com>]

Chris,

Good point.

On this subject, you may be interested to know I am putting together a
complex Excel spreadsheet (on behalf of the Variable Star Section of the
British Astronomical Association) designed for absolute (as well as
differential) photometry.  I keep coming up with the realisation that
aperture methodologies designed for differential work are not readily
transportable to absolute photometry especially where not all standard stars
or reference stars are on the same single CCD image.  My conclusion is that
it is actually necessary to utilise apertures that include about 99% or more
of the light flux  from each source.  The reason is that several things
conspire to change the PSF of images both across the image (e.g. coma and
other optical aberrations) but also from image to image (e.g. tracking and
differences in 'seeing' at different zenith distances, etc.).  I know that
this sacrifices some potential improvement in precision because of the
'over-large' area of sky background included in the software aperture
centered on the star or asteroid but it does this with a concomitant
improvement in photometric accuracy not achievable in any other way.  The
quest for accuracy in deriving magnitudes to a standard system is what
absolute photometry is all about.

Using 'cut-down' apertures or PSF fitting come into their own for
target objects that are relatively faint where the precision is especially
limited by photon statistics.  However, for relatively bright objects
(viz. filter photometry of objects having V<11 or 12 using a 100 mm diameter
collecting aperture), I intend to advise anyone wishing to use the
spreadsheet to reduce CCD images to make sure that they choose a photometric
aperture that includes 98-99 percent of the light flux from nominal point
sources not only for the entire area covered by the CCD image but also from
image to image used in the reduction.

Cheers,
Richard Miles

----- Original Message -----
From: Chris Lloyd <cl@ast.star.rl.ac.uk>
To: <tass@listserv.wwa.com>
Sent: Friday, December 13, 2002 11:25 AM
Subject: Dumb question


> 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
>
>
>
>