[Aavso-photometry] Camera with pixel square and rectangular

[Wolfgang Renz]

Hi Laurent

The results might differ very slightly depending on the hardware (x
to y pixel size ratio), the choosen software and the actual choice
of parameters of the aperture photometry. But the slightly different
results one gets by choosing different large star apertures should
be about in the same range.

When using a large circular star aperture (that does include the
by far majority of the star light) on non-square-resized images
taken with non-square CCD pixel chips, some additional unne-
cessary background will be included. This should not influence
the results much, but might influence the noise and SNR a bit.

When using a (too) small circular star aperture (that doesn't include
the by far majority of the star light) on non-square-resized images
taken with non-square CCD pixel chips, not the same percentage
of star light will be included in x and y direction. But as this is done
systematically it shouldn't throw you off the track much either.
If your setup shows optical aberrations that influences the star sizes,
a too small star aperture isn't recommendable anyway.

You could square-resize your images before using circular aper-
tures. But this implies an interpolation that will cause some loss
of information:
E.g. you will loose the ability to check if a pixel has exceeded the
linear range, as the counts of a single pixel might get distributed
over two pixel with undersampled images.
E.g. you will lower the noise a bit as every interpolation is also a
smoothing. Using a different interpolation algorithm than mono-
linear (which will preseve the total ADU count but will widen the
PSF of a star by up to 1 pixel) is dangerous as minor artifacts
might get overemphasized with nearly all more complex ones.

But you could also use aperture photometry software (like e.g.
Mira) that supports elliptical apertures.

My recommendation would be:
- Use elliptical apertures if available.
- Don't square-resize images taken with non-square CCD pixel
  chips for aperture photometry.
- Use not too small star apertures with aperture photometry.

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



----- Original Message ----- 
From: "Laurent Corp" <laucorp at wanadoo.fr>
To: <aavso-photometry at mira.aavso.org>
Sent: Thursday, December 06, 2007 8:54 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)