Re: Photometry

["Gary W. Billings" <obs681@telusplanet.net>]

Tom, et al.,

Inspired very much by TASS Mk III and IV, ASAS, ROTSE etc., I've been
experimenting with a 135 mm F/2 lens (at F/2.4) on an ST-7E (13.8"/pixel).
My analysis (using snr.xls from Doug George at www.cyanogen.com), suggests
that in the part of parameter space we are searching   ;-)   viz the focal
lengths, and pixels sizes we are talking about, the crucial parameter is
plate scale.  At 13.8" per pixel, even the smallest photometry aperture is
larger than is needed to actually measure the brightness of the star, hence
there is a lot more sky (=noise!) in the measurement aperture.  In other
words, my stars can probably be focused to 4" FWHM, but my measurement
aperture cannot possibly be smaller than 13.8".  My observing location is
very bright (within a city of nearly 1 million people).

With the 135 mm + ST-7, I too am getting about +/- 0.03  mag (1 sigma) at
V=11.5  (same as you found with Mk III)

Recently I've been experimenting with 480 mm focal length (same camera, with
9 micron pixels), and find the same noise level is now at V>13.  This is the
kind of change that was predicted using the spreadsheet.  The aperture of my
480 mm scope is a little bit larger (85 vs 56 mm), but what led me to trying
the 480 mm scope was the platescale considerations.  (Mathematically)
increasing the aperture of the 135 mm scope to 85 mm does not predict the
S/N gain that I achieved with the 480 mm scope...  though obviously the 480
mm FL gives me smaller FOV.

Of course I'm very interested in hearing what others think about this.


Gary Billings



>
> I am left with two possible conclusions.   Some of you may think of
others.
>
> 1) This is what can be done in a suburban location.
>
> 2) Michael has somehow written into both his codes a floor of 0.03 mag.
>
> I can even rule out 2).  It is possible to improve the noise floor by
> tracking one field on a good night.  So it can't be the software.
>
> I conclude that 0.03 mag is the practical limit of what we can do here in
> Batavia, IL.  Andrew might differ.  We shall see what can be achieved.  I
> note that the quoted ASAS [Acta. Astr., 50, 177] results taken in a good
> location with inferior optics and a wider 8 degree field are similar.  I
> have tried many things (i.e different reference catalogs) to improve this
> error floor.   Nothing seems to make much difference.
>
> Tom Droege
>
>
>
>
>