[Aavso-photometry] Ensemble photometry question

[Radu Corlan]

On Thu, Jan 12, 2006 at 03:18:10AM -0500, Robert J. Modic wrote:
> Thanks Pertti and Arne.  Let me clarify what I'm trying to 
> do.  I already know the errors of each comp star (taken from 
> one of Arne's .dat files for example).  I'm just trying to 
> calculate the portion of the error due to the ensemble of 
> comp stars.  Adding all the errors in quadrature results in 
> an error that is too large.  I then tried dividing the previous 
> result by the square root of number of stars used.  This gives 
> an error that is very close to a simple average of all the 
> comp star errors.  Since the error should be reduced by the 
> square root of the number of comp stars, the result of this 
> second method still seemed too large.  The only way I can 
> calculate an error that looks right is to add all the errors in 
> quadrature and then divide this by the number of comp stars
> (not the square root of the number).

Bob,

If you use no weighting, you should divide by the quare root of the 
number of stars. you will get a large errors, and that is because the 
larger error of the faint stars will dominate the quadrature sum. so 
this works well mainly with comp stars that have similar errors.

if you use weights, you should include the same weights that are used 
for the zeropoint calculation into the error estimate. an example of 
how this is done may be found in the appendix to the manual of my gcx 
program (http://gcx.sourceforge.net/html/node11.html). 

Radu

> 
> I read the Honeycutt paper on ensemble photometry.  The
> methods mentioned in that paper are meant for a non-homogenous
> set of comp stars and seem too complicated for the more
> simple type of homogenous ensemble photometry that I'm trying
> to do.
> 
> Again, I'm just trying to find a simple way to calculate the 
> component of the error due to the comp stars (the zero point
> error).  This will then be added in quadrature to the other
> errors (measurement, transformation, etc.) to get my total
> error for each magnitude measurement.
> 
> Bob
> 
> 
> 
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-- 
Radu Corlan      

You pay now, or you pay later, but you always pay the entropy tax.