I'm a bit confused about the best way to set the aperture radius to do measures. In "The AAVSO
CCD Observing Manual" it is said that the best aperture is 3-4 times the FWHM for increasing precision. However, the standard automatic set in VPhot is 1.5 times the FWHM and is referred as the better radius. I'm been doing some test with different sets and cannot get a clear conclusion. Can anybody help me, please?.
And another question: my urban conditions keep me in a struggle with a very bad seeing and a high light pollution. My FWHM uses to be 4-5 arcsec at best focus (my OTA-sensor resolution is 1.03 arcsec/pixel). I know this is very high and I ought to bin to 2x2 to achieve less resolution, but considering my antiblooming sensor, wouldn't it cause a significant lost in accuracy?
Thanks in advance.
FWHM is a diameter. Therefore the "3-4 times the FWHM" in the AAVSO CCD Observing Manual" should be a diameter too. The "standard automatic set in VPhot is 1.5 times the FWHM and is referred as the better radius" seems to be a radius and would be very small for a diameter. If its a radius then the diameter is 2*1.5= 3.0 FWHM. So there is probably no conflikt here.
Many thanks for your answer, so it's a problem of radius-diameter interpretation. I must say that I considered this possibility but wanted to clear it.
I agree with 1.5*FWHM.
In very light-polluted skies be very careful not to use too large apertures, wich adds a sytematic error to the measure. I have experienced myself with this issue when I saw my data points brighter in magnitude than other simultaneous data from the other, and then I learned I was using a toolarge aperture wich added more light form background not corrected by sky aperure. When I recomputed photometry according to the 1.5*FWHM rule, my points were allright.
This effect is small in B, but it has to be taken in account in V, and specially in R and I.