Re: Photometry

[Doug Welch <welch@physics.mcmaster.ca>]

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
>