[Aavso-photometry] Re: [AAVSO-DIS] Complete analysis of CCD accuracy for Var Her 04.

[Michael Koppelman]

Arne could state it for sure, but in the past I have gathered from him 
something like the following: for CCD observers who use standard 
filters and have comp stars that are relatively close to the same color 
(and I presume using the same sequence) we should expect something 
around 0.05 or less in scatter. This also assumes good S/N (which may 
be one issue with Var Her 04).

Possible causes of scatter greater than this:

1. Different charts.
2. Different comp stars (of possibly varying colors)
3. Ensemble vs. single comp star photometry.
4. Airmass, transparency and other observing conditions.
5. Quantum efficiency difference in CCD chips.
6. Difference in the filter bandpasses of different manufacturers.

This is not including outright incorrect processes with respect to CCD 
reduction, photometry or things like typos.

I routinely mix my observations with other CCD observers and when we 
use the same comp star magnitudes, they usually match to about 0.02.

Cheers,
Michael

On Jul 31, 2004, at 3:26 PM, Michael Linnolt wrote:

> Another consideration, of course, is are we interested in relative or
> absolute photometry? There's no question the data submitted to AAVSO is
> more accurate for the former than the latter, as superhumps, etc. can
> easily be resolved without a proper zeropoint. (Yet, there are a number
> of observers whose random error is on the order of the size of the
> superhumps!) But I think absolute photometry is one of the goals of 
> CCD,
> and if observers cannot achieve proper zeropoints in their submissions,
> that is a significant issue.