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What is a good FWHM with my equipment?


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I was wondering how my focusing and guiding is working and wanted to know if I´m within reasonable range of what I can acchieve.

My camera is a Canon 550D, pixel size 4.3 micrometers

Scope is a Skywatcher 150-PDS, 750 mm FL.

When I analysed one of my subframes (300 second exposure) in Pixinsight using the FWHMEccentricity script I get the following values:

Median FWHM: 2.892 px
Median eccentricity: 0.4329
MAD FWHM: 0.3114 px
MAD eccentricity: 0.1106
MAD residual: 1662 DN
Star support: 10059

This was one of the better subs that night. If I check the stacked image (38 frames) I get Median FWHM of about 3.26.

If I´m not mistaking my resolution is 1.18 arcsecs/pixel so that would mean a FWHM of 3.41 arcsecs. Is that OK considering my gear? What would be the best one could hope for?

Attached is a 100% crop from one of the better subs that night. It was quite windy so I´m glad they turned out so well. Only had to discard 10 or so from the entire night. My wind shield seems to work :)


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I don't think you can do anything other than note your best values and try to replicate them. I'm interested in FWHM at the focusing stage where, using 3 second subs, I might get 0.8 on one rig and maybe 1.1 on the other, but we all depend on the night's seeing in the end.


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Hi Martin

Not sure where you got 3.41 from, but  Nyquest sampling states a best resolution of 2x arc sec per pixel, so you should be able to achieve 2.4 arc sec/pixel when your local seeing allows. In my area of East England where there is fairly laminar airflow (no mountains!), seeing is in the region of 1.5 to 2.5 arc secs per pixel. For small focal lengths, then small pixel camera's are the way to go.


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