When using the focusing tools like FWHM or Multi-Star FWHM, do the scores signify current resolution in say, arcseconds? Or are they just a calculated score and lower is better? I did read the manual but didn't see anything about what the scores mean other than lower is better.
I ask because I am using a C8 with a 0.63 focal reducer to a ASI 813mc (2.4 micros per site) which works out to 0.39 arcseconds per site. I struggle to get a score below 4.5 using FWHM or multi-star FWHM. Can I do better or is that it? I do have a ZWO EAF that I have just started using but even then I can't get below 4.5.
Just curious.
question about focusing scores
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Re: question about focusing scores
Hi,
the number is an approximate measure of the star width, in pixels. If you know the pixel scale of your camera then you can multiply that by the number from the focus scope to get the approximate width in arc seconds. Even then, it's not really a measure of seeing as the diffraction limit from your aperture and any effects from interventing optics/collimation also impact on the score.
Taking your C8 score, you end up with a star width of about 1.75 arc seconds, which is quite a bit more than the theoretical diffraction limited FWHM of about 0.5 arc seconds for your aperture. That means that you are picking up a fair bit of extra width from seeing, optical aberrations, collimation, scattering and possibly other issues.
cheers,
Robin
the number is an approximate measure of the star width, in pixels. If you know the pixel scale of your camera then you can multiply that by the number from the focus scope to get the approximate width in arc seconds. Even then, it's not really a measure of seeing as the diffraction limit from your aperture and any effects from interventing optics/collimation also impact on the score.
Taking your C8 score, you end up with a star width of about 1.75 arc seconds, which is quite a bit more than the theoretical diffraction limited FWHM of about 0.5 arc seconds for your aperture. That means that you are picking up a fair bit of extra width from seeing, optical aberrations, collimation, scattering and possibly other issues.
cheers,
Robin
Re: question about focusing scores
Thanks Robin. Helpful to know that. Seeing wasn't great that night.