Im very new. Took my first pic just trying to determine my seeing. My question is that my stars FWHM vary greatly throughout my frame. It is my understanding that it should be constant. What am I not getting here?
The FWHM will bounce around mainly due to atmospheric dispersion. The atmosphere isn’t perfectly still so your star size will vary because of it. If you are measuring the FWHM, take several measurements and average.
Just to clarify: are you talking about a single image, where stars in that image have different fwhm, or are you talking about multiple images of the same field, where from image to image, the fwhm varies?
unfortunately, the forum won't permit file attachments. Sigh.
If you are dealing with a single image, then here are some suggestions. FWHM for a perfectly focussed image on a collimated telescope with long enough exposures to remove scintillation, and with perfect optics, will have consistent fwhm everywhere, for well exposed, isolated stars. Lots of "ifs" there. Consistent means there will be some scatter, but it will be more like 5.01, 5.10, 4.95, etc. values, and not 5 to 25. This variation is just random noise - the calculation of fwhm isn't terribly precise.
So if you have a wide range, then something else is happening. You may have coma, so the image shapes near the edge will be very different than in the middle. You may not be using well-exposed stars. Your stars may be contaminated from close neighbors. Your stars may not be stars at all, but instead faint background galaxies. 5 pixels to 25 pixels fwhm is a huge range, and way oversampled, so I'm betting you have some basic problem. Probably your best path forward is to place an image on DropBox or a google drive and share it, so that others can look at it.
Hi Arne...Just realized you answered my question regarding linearity test as well...Thank so much for your help, sorry to be a pain lol
But what you said regarding my FWHM makes sense to me. I surely had a focus issue this night and some other issues as well. Im also using a full frame on an 8in SCT so major vignetting and such. I will take a more calibrated image and re test this once the clouds go away. Thank you again!