Re: Photometry

[Tom Droege <tdroege2@earthlink.net>]

Doug and all,

I had best let Michael reply to this.  I am looking at what comes out of 
the end of the pipeline.  I believe this is "reduced to the standard 
system".  Our reference catalogs are "standard system" reference catalogs 
and I have been suspicious that they were part of the error from the 
start.  As you realize, there is a problem getting I data for such 
catalogs.  It has to be derived from other data, a problematic process.  It 
also includes the error caused by measuring a star in different positions 
in the field which in turn includes the error from using a different set of 
reference catalog stars.  It is what you get in a practical way.

If one puts a star in a fixed position in the frame and tracks it many 
nights, then one can get below 0.01 mag.  This is not practical for this 
survey.

Tom Droege

At 11:37 AM 6/7/03 -0400, Doug Welch wrote:

>Hi Tom,
>
>Perhaps you explained this in an earlier message, but is this an internal
>error comparison? That is, is it the RMS scatter of the same stars on the
>*instrumental* system? It is easy to introduce an additional 0.02 or 0.03
>mags from using the (typically good but not perfect) standard systems
>available.
>
>Cheers,
>Doug
>
>On Sat, 7 Jun 2003, Tom Droege wrote:
>
> > I have been reading the Mark III paper (PASP, 112:397-408) in preparation
> > for writing one on the Mark IV.
> >
> > I quote from the Mark III paper:
> >
> > "We find the standard deviation from the mean has a minimum of about 0.03
> > mag for bright stars, increasing to more than 0.2 mag for stars near the
> > limits of detecton."
> >
> > I could say the same thing for the Mark IV data.
> >
> > First, there are differences between the two systems:
> >
> > 1) The Mark III was a drift scan system, the Mark IV operates with a stare
> > mode drive.
> >
> > 2) The Mark III used camera lenses with a large variation over the
> > field.  The Mark IV uses lenses of our own design with an order of
> > magnitude less variation.
> >
> > 3) The Mark III used a drift scan effective exposure of 471 seconds, most
> > of the present Mark IV data was taken with 90 second exposure.
> >
> > 4) The Mark III software used PSF photometry.  The Mark IV uses aperture
> > photometry.
> >
> > 5)The Mark IVs are usually operated 5 C or so cooler than the Mark IIIs.
> >
> > 6) The Mark IV uses the CCD442A operated in MPP mode.  The Mark IIIs used
> > the KAF0400 operating in a somewhat different mode.  i.e. different gate
> > structure.
> >
> > 7) The Mark IV uses 16 bit ADCs, the Mark III used 12 bit ADCs.
> >
> > With all these differences, we seem to get the same result.  There are 
> some
> > common features:
> >
> > 1) All the data was taken in suburban locations.
> >
> > 2) The Mark IV and Mark III have a similar field size, 4 and 3 degrees.
> >
> > 3) Most of the Mark IV data and all the Mark III data was taken near the
> > equator.
> >
> > 4) Both software implementations were written by the same person.
> >
> > To investigate common #3, I have just looked at the sigma vs mag plots for
> > data taken at different declinations.  This is now possible with the TOM2
> > and TOM3 data.  I find that there is again no obvious difference.  The 
> Mark
> > III statement could well be applied.
> >
> > 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
> >