To fine tune the "Bahtinov obtained focus" of my telescope (or the "Canon Live view x 10 focus"), I start Backyard EOS, pick a visible star in BEOS Live View. Backyard EOS then gives me its FWHM value. I adjust focusing to minimize the BEOS calculated FWHM of the star.
How should I interpret this value ?
Intuitively, it seems to me that if I always use the same setup (same camera+ same scope), the FWHM of a fixed star is correlated to seeing. When I get 2.7 or 2.8, seeing is good. When it is 3.0 or worse 3.1, seeing is bad (always same star, same place). Correct ?
Also: is the FWHM dependent on the scope technology *newtonian, refractor, etc) ?
Also: for a family of identically architectured scopes but different focal lengths, would the FWHM be constant ? Eg, (assuming that the Esprit 120Ed and 150ED are architecturally identical inside), if I had 2 setups, next to each other, one with eg a Canon 6d and Esprit 120 and the other with a Canon 6d and Esprit 150, would the FWHM be the same
In short, what is FWHM dependent on ????
These are far from being critical questions. But after using the Esprit 80ED with a Canon T3i then 6D, I recently started using a Rokinon 135mm lens with my Canon 6D.
The FWHM is used to get was from 2.8 to 3.2 (different stars, same bad location). I am now going to a better location, darker site. I tried the Rokinon twice. In both cases, the FWHM of my chosen star for focusing was around 2.3 through 2.5. So that left me wondering: was the decrease due to the change in lenses or to the change of location.
Head scratching. Perhaps, certainly, useless but I have no idea how to tackle the problem. If you have a reference to recommend, that would be neat