[Aavso-photometry] Camera with pixel square and rectangular

[Michael Newberry]

Laurent,

If the pixel is not square or the image is slightly trailed, then the star's 
isophotal shape is an ellipsie or at least approximately an ellipse. You 
might think of setting the measuring aperture so large that it "gets all the 
light" beyond the apparent edge of the star profile so that it does not 
matter that the aperture is circular and the star is elliptical. However, 
using such a large aperture is not usually the best way to do photometry 
unless the star has super-high signal to noise ratio (like 100's to 1) 
because adding the extra sky noise unnecessarily degrades the precision of 
the photometry (i.e., it increases the magnitude error, sigma(m)). Instead, 
aperture photoemtry should be done using an aperture that gets most, but not 
all the light of the star. Different photometrists have different rules of 
thumb for how large to set the star aperture so that it cuts into the star 
profile inside the apparent "edge". Even better, one can make theoretical 
models for the SNR (or magnitude uncertainty) as a function of radius and 
magnitude. HOWEVER, all stars on the image should be measured using the same 
aperture size unless you get into making aperture corrections---which is a 
whole 'nother barrel of snakes unto itself. I made SNR vs aperture models 
some 20 years ago back in grad school (holy cow has it been that long???) 
Anyway, back to getting aperture photometry out of an image having 
non-square pixels... there are two options:

1. Do the photometry using elliptical apertures which are elongated by the 
ratio of height/width (or vice versa, so that the ratio is > 1.0, then set 
the aperture ellipticity to this value oriented in the direction of the 
elongated star). This is the preferred option, since it does not require you 
to first re-sample the image to make the pixels square (see option 2).

2. Re-sample the image to square pixels, then you can use circular apertures 
if you want. The problem with this is that the image is resampled in order 
to make the pixels square, and that changes the noise structure of the 
image. Since the noise is reduced by re-sampling, the photometric error 
computed using image noise will be underestimated.

Michael Newberry

----- Original Message ----- 
From: "Laurent Corp" <laucorp at wanadoo.fr>
To: <aavso-photometry at mira.aavso.org>
Sent: Thursday, December 06, 2007 12:54 PM
Subject: [Aavso-photometry] Camera with pixel square and rectangular


> Hi all,
>
> Can you  say me if the result will be different if i make a measure of 
> photometry with a camera CCD with pixel square or pixel
> rectangular , and of course the same software of reduction ?
>
> Thanks for your response
>
> Laurent (Rodez - France)
>
>
> _______________________________________________
>
> Aavso-photometry mailing list
> Aavso-photometry at mira.aavso.org
> http://www.aavso.org/mailman/listinfo/aavso-photometry
>
>