# FWHM BackyardEOS

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I was messing around with the frame and focus last night for the first time and i noticed the FWHM which i presume was a focus score based on how good my focus was (Total guess work here ), i managed to get down to 95 but no better, am i doing it right or can someone please explain the frame and focus to me so i know what i am trying to achieve

Cheers

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jasperuk,

It’s more than just a score… it’s the width of a star at half its height.

Full Width Half Maximum is the width of a star's image at half its peak. Focus is achieved when you get the lowest value for the same star over time, indicating a tighter star. BackyardEOS implements FWHM by calculating the standard deviation (the square root of the variance) of all pixel values in a very small selected area. Read more here (albeit a bit technical) Full width at half maximum - Wikipedia, the free encyclopedia. Look at the graphic in the upper right corner of the wiki web page, a picture is worth a thousand words.

You will get a different value for different stars so the point here is not to compare FWHM reading with readings from other users and/or stars. Once you have located your star and placed it in the smaller zoom box in the lower right corner in BackyardEOS, the data of the star is analyzed, a bell curve is calculated, and it’s width at half its height is the FWHM feedback returned by BackyardEOS.

Hope this helps,

Guylain (Creator of BackyardEOS)

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• 10 months later...

"You will get a different value for different stars so the point here is not to compare FWHM reading with readings from other users and/or stars. Once you have located your star and placed it in the smaller zoom box in the lower right corner in BackyardEOS, the data of the star is analyzed, a bell curve is calculated, and it’s width at half its height is the FWHM feedback returned by BackyardEOS."

Thanks!! I just started playing around with BYEOS and I haven't been able to find anyone to explain it as clearly as that.

Thank you!

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