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INDI Library v2.0.6 is Released (02 Feb 2024)

Bi-monthly release with minor bug fixes and improvements

What is a good HFR with autofocus?

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usually my autofocus results in a HFR of about 1.40.
Is this a reasonable value to image with or should it be better?

Camera ZWO ASI 294 MC pro, telescope Esprit 100ED.
3 years 9 months ago #55905

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Hi,

with the same camera, but with Celestron C9.25 with f/6.3 reducer (= 1457 mm effective focal length), on a good night I usually get HFR values around 2 pixels, from a non-saturated, medium-bright star. Depends on star brightness, exposure time during AF and very much on seeing conditions.

Translated into arcseconds: alpha = arctan(object size / focal length)

In my case object size is 18.5 microns (2 pixels radius = 4 pixels diameter = 4 x 4.63 microns), so my half-flux discs have a diameter of 2.6“.
In your case (100ED has FL = 550 mm, right?): Object size = 13 microns (2 x 1.4 x 4.63), so your half-flux star discs are 4.9“.

Normally, resolution is seeing-limited, 1“ would be excellent (mountains, no turbulences, dry air) and 4“ would be bad seeing. I would expect your discs can go down to 3“ or 2.5“ as well resulting in a HFR size of 1 pixel or slightly below that.

CS
3 years 9 months ago #55913

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