HFD vs FWHM for Focusing

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nexusjeep
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HFD vs FWHM for Focusing

Post by nexusjeep » Sat Sep 28, 2019 12:26 pm

Hi Robin,
I have been doing some reading which is always a dangerous starting point :D , in regard to focusing methods and from what I have read HFD gives a more consistent value when accessing the star during focusing as it is less affected by the conditions between the optics and the star. With the FWHM measurement in SharpCap last night I was seeing a .4 - .5 change in the FWHM reading between successive exposures and was wondering on your opinion and whether it could be an option to use HFD as a choice over FWHM in the focusing tools.

Cheers
Nick

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Re: HFD vs FWHM for Focusing

Post by admin » Sun Sep 29, 2019 1:29 pm

Hi Nick,

To be honest I don't think it would make a great deal of difference. The reason I'm saying this is because of the way that SharpCap measures the FWHM score. There are actually two different ways that this is performed, but both of them estimate the FWHM by measuring area rather than length, making them somewhat similar to the measurement of the HFD score which would also measure the area that contains half the flux and then estimate a circle diameter from the measured area.

The single star FWHM focus score use the slightly more sophisticated technique which models the light distribution as a 2D Gaussian curve and from that estimates the FWHM. In other places the code counts pixels that are brighter than the half maximum of the star to get an area measurement and then converts that to a diameter of equivalent area circle. In the latter case the code is usually averaging over many stars which should lead to a more stable value, although it may be advantageous for a future version to interpolate the star brightness profile onto a finer grid than the pixel grid when calculating this measurement.

Hope this makes sense, Robin

nexusjeep
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Location: Gloucestershire

Re: HFD vs FWHM for Focusing

Post by nexusjeep » Sun Sep 29, 2019 2:43 pm

Hi Robin,
Yes it makes sense I usually use the single star option when using the FWHM as I find the multi star suddenly reports 20 even if the preceding frame was looking reasonable. If I am having fluctuation issues I tend to use the bahtinov mask and visually access the crossing point by eye otherwise I end up chasing the value nearest to zero. It may improve if I used longer exposures to assess the focus as this would average the variation over a longer period of time so possibly give a smoother output. My usual focusing exposure is 1s - 2s so perhaps need to look at extending this.

Cheers
Nick

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Re: HFD vs FWHM for Focusing

Post by admin » Mon Sep 30, 2019 6:35 pm

Hi,

Longer exposures would probably give less noisy results because you would have less noise in the images themselves. Obviously a noisy image makes it hard to properly measure the star width.

The value of 20 that you see for the multi-star option is generated when not enough stars are detected to give a meaningful average. If you are seeing that frequently then you should adjust your star detection settings to try to increase the number of stars being picked up by the measurement routine. Giving a high value of 24 not enough stars found ensures that the autofocus scanning routines don't get suckered in to areas in which stars aren't detected at all thinking they are the best focus areas.

Cheers, Robin

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