Hi everyone, as I start to try to tune up my autoguiding I'm trying to figure out what a "good" FWHM value would be for my setup.
I'm shooting with a Canon T5i on a William Optics 66SD which yields 2.29"/pixel. With 120 second subs of the elephant trunk last night I was getting between 3.83 - 4.00 FWHM values recorded in DSS (in pixels I believe). Conditions were pretty decent last night, and these values seem pretty typical of my setup so far.
That doesn't seem good. The stars are nice and round, and I've been able to product images with decent detail, but I do wonder how good I might be able to get if I tune things up a bit.
So, anyone have thoughts on what a good goal would be?
If you are using an unmodified DSLR, then my guess is the filter stack over the sensor is probably one of the key reasons your FWHMs are large. Most terrestrial cameras have a low pass filter over the sensor, and its express purpose is to blur high frequency data to avoid issues with aliasing (notably, the moire artifact that can occur with repeating patterns in subjects.) Even if your seeing was good, the low pass filter alone could still be causing your FWHMs to bloat to ~3-4 pixels...would depend on how strong the filter is in the T5i.
DSLRs are also color cameras (with a very few exceptions), which means they must be demosaiced to produce a full color image. That also introduces some blur.
Two other key factors will affect your FWHMs. The size of your scope...smaller apertures will increase the amount of diffraction, and that can account for 3-4" (not pixels) of FWHM in and of itself. The final factor is focus. At ideal focus over 70% of the energy of the star will be in the central peak, but as you lose focus a lot of that energy will be thrown out into the halo of the star rather quickly.
It is most likely that your seeing is not 9" (that would be atrocious, and you would most definitely know if you looked at the stars in the sky yourself), probably not even 3" if you felt the night was good. I would say the top two factors are the low pass filter and focus, with diffraction coming in a close second. Seeing unless the skies looked visibly bad, could be the least significant factor here (3" seeing is pretty bad, 4" is terrible...I myself will stop imaging if seeing gets to 4", and I often toss subs where seeing is over 3".)