Photometry

[Tom Droege <tdroege2@earthlink.net>]

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