I’ve redone my coefficients three times due to small changes in my “imaging train”. The last time was in 2023. Since then, doing stellar photometry, I’ve become more precise with my focus and the resulting FWHM - usually attaining a value of 2-3 (which corresponds to arc-seconds in my system). In looking at the FWHM in the M67 images used for the 2023 coefficients, I see they were in the 4-5 range. Here’s the question, should I calculate new coefficients using new M67 images (with a smaller FWHM)? Would it produce more accurate results?
Lane:
As long as you were selecting your apertures to match the seeing (FWHM) at the time of measurement, your results (coefficients) should be similar.
It has been 2 years. Always good to check occasionally to confirm. I suspect they will be?
Ken
2 Likes
Lane, hello.
I try to update my coefficients twice a year.
Cheers,
Enrique
(BETB)
Greetings,
Again there are many factors involved… Depending on your image scale and if using a crowded field with many stars, some stars may be overlapping their light on your sensor and when you are using aperture photometry the photometry can be influenced by close adjacent stars putting unwanted light into either the sky-ring or the sky-ring and aperture-ring of the adjacent star star you are trying to measure.
Proper photometic methodology says you should always use the same aperture when processing a single image. It is easy to decide what aperture to use by generating what is called a “growth curve” for a well behaved star of similar magnitude to your variable. A “growth curve” is a plot of aperture vs instrumental magnitude. Pick the aperture to use near the knee of the curve.
It certainly doesn’t hurt to do anther calibration now and then! More accurate? Maybe… did you make an error the first time? Statistics say the numbers you get in a new calibration should be similar but not exactly the same.
Jim (DEY)