FHD v FWHM

[TrevorW]

OK in the light of a full moon in the burbs what type of figures should I expect, I' started using NINA in the last two months to control everything - figures under the full moon show FHD around 5-6 rarely under 5 however when i stack in DSS FWHM return between 8-10, the stars have to my eye have been in focus, I have yet to to see figures under 4 FHD or the magical 2-3


My camera is a QHY268C Gain set at 56 offset 30 3.76 um pixel size, Scope 10" RC at f/8


:question:

[Atmos]

The values you get are VERY conditional to what your imaging train is.
To talk about to different extremes we can talk first about the venerable FSQ-106 & 16803 combo. 9 micron pixels on a sharp 500mm telescope yielding 3.5"/pixel in focus in the centre of the image you'll see FHD around 1.5.

For a crazy comparison if you threw an ASI183/QHY183 on a Celestron C14; 2.4 micron pixels at 3911mm FL even under pretty darn good seeing of 1.5" you'll have a FHD around 12 but 20 wouldn't be unusual.

[JA]

As l understand it Deep Sky Stacker reports star sharpness as the FWHM Full Width Half Maximum in pixels. DSS does not have the ability nor the information about the focal length of the scope and imaging sensor size to help it determine the FWHM in say arcseconds used in some other software.

Nina, as I understand it (I don't use it) measures star sharpness using HFR Half Flux Radius, which given its name suggests it's computed slightly differently using integration. Its units may be in pixels (TBC)*, but given the difference in computational method, doesn't guarantee the star sharpness measures from DSS and Nina: FWHM and HFR respectively will be comparable.

Best
JA

* If Nina reports the sharpness in Arcseconds and that is what you are quoting then DSS and Nina will definitely not agree since DSS is reporting the sharpness as FWHM in pixels

[TrevorW]

Ok so with my setup it would seem that these figures are not to bad then :)