Auto box position in Refocus using star FWHM

[ChrisR Oz]

Hi Robin,

I tend to just use a manual step-In followed by scan out in the FWHM focus tool, rather than auto refocus, because of the large pop-up and need to confirm selection area/box/ROI.

However, as I am focussing on a single strong star, the centering of the selection box/ROI could be done automatically. How about an option in the FWHM focus tool to set the centre of the ROI automatically? Then refocus would not need to pop-up options, confirmations, etc. it would just go ahead and work.

Cheers, Chris.

[admin]

Hi Chris,

it would certainly be possible to write code to find/use the brightest star in the image, but I do wonder if the single star focus tool gets much use? I personally rarely use it, preferring to use the multi-star option, which I feel is less sensitive to random fluctuations in the focus measurements as it averages a number of stars.

cheers,

Robin

[ChrisR Oz]

Hi Robin,

I did start by using the Multi-star focus tool. However, as I reported at the time, if the selection of stars changes during the focus scan (e.g. weaker ones drop out when off focus) then the average FWHM that is being used jumps around and disrupts the focus curve. Switching to the FWHM focus tool on a bright star gives a nice smooth curve. I believe there is some systematic error in your FWHM as a function of star intensity (at least back then, may have been v3), so that weaker stars score a narrower FWHM, if I recall.

Hence, my interest in making the FWHM focus tool work more simply on a bright star.
Cheers, Chris.

[admin]

Hi Chris,

thanks for explaining.

There is an issue that saturated stars may give incorrect FWHM, since the peak value as measured from the image will be the saturation level of the image, rather than the (presumably brighter) true intensity of that part of the star. This causes the half maximum (HM) part of the measurement to be under-estimated, meaning a wider FWHM is calculated. This is supposed to be dealt with by ignoring stars at or close to saturation when using the multi-star FWHM tool (the level that is used for the cut-off can be adjusted).

cheers,

Robin

[ChrisR Oz]

Thanks Robin. I guess I should give “multi star” mode another go. Have these controls/exclusions changed with v4.1 ?
Cheers, Chris.

[admin]

Hi,

yes, there have been quite a few improvements between 4.0 and 4.1 in the multi-star FWHM, including better handling of stars that are a long way out of focus, better handling of 'donut' stars, etc. Worth a try.

cheers,

Robin