[Aavso-photometry] Sources of accuracy and precision in photometric measurements

[Justin Pryzby]

On Thu, Dec 15, 2005 at 12:10:07AM +0200, Radu Corlan wrote:
> > There are 4 main areas which affect doing accurate, high-precision 
> > photometry:
> > 
> > 1) the sky background estimate
> > 2) the partial pixel algorithm
> > 3) the centroiding algorithm
> > 4) the random error estimates (sigma of magnitude) must be valid
> 
> Michael,
> 
> These are certainly important areas, and neglecting any one of them 
> can introduce unanted errors. I've added a few comments below, in the 
> intention of showing that unforunately there are no "fast" solutions.

> > 2. Partial Pixels:

> Ah, but apportioning the flux of a pixel to an aperture based on the 
> fraction of the area inside the aperture is only "exact" if the pixels 
> were uniformly sensitive _and_ uniformaly illuminated. In any practical 
> case (particularly with smallish apertures), especially the second 
> hypothesis is far from being true. So in this case we are left with 
> determining whether the error introduced by using some approximation of 
> true circular or elliptical apertures is significant compared to error 
> introduced by sampling. My tests have shown me that there is a 
> detectable improvment in using "true" apertures vs whole pixels, but do 
> detectable improvment over the simplified "irregular polygon" algorithm 
> of iraf phot. The author of phot also seem to be holding the same view. 
> I have to say however that in these days of plentyful computing 
> resources, there is not much justification in rejection an even 
> marginally superior algorithm on the basis of complexity alone.
Hello Radu, from the VS front tonight,

For those interested, this is ./noao/digiphot/apphot/phot/apmeasure.x

                    fctn = max (0.0, min (1.0, aperts[k] - r))
                    sums[k] = sums[k] + fctn * pixval
                    areas[k] = areas[k] + fctn

For inner pixels, not on the boundary, fctn=1; for outside
pixels, fctn=0; for border pixels it is between 0 and 1 .. an
approximation as noted.

Justin