[Aavso-photometry] SDSS074545

[arne]

Jim Roe wrote:
> Last night I stacked some images of SDSS074545 and detected it.  Using 
> MaximDL and Pinpoint I get the SNR at 5.4 and the magnitude at 19.292 in 
> V using the 158 star as reference (SNR = 39).  Pinpoint gives its 
> position at 07h45m32.0s and +45d38m29.3s which is right on the chart 
> value so I'm pretty sure its the right guy.  How should I report the 
> photometric uncertainty of this guy?

Yes, I'm still going through old email...
The usual way is to add the errors in quadrature:
total_error = sqrt (error_1**2 + error_2**2)

which means the total error will always be somewhat worse than
any individual error.  In the case mentioned above, the difference
in the errors is about 10x, so if you just reported the SNR=5.4 error,
you would be close (about 0.18mag).

However, remember that the actual error will always be worse than
just the pure Poisson error.  That does not take into account
measurement error (how well sky is determined, how well things
are centroided, pixelization, etc.), vignetting/flatfielding error,
etc.  So use these numbers as lower estimates to the true error.
Arne