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FWHM units?

Posted: Thu Feb 28, 2019 1:39 am
by cshine
I understand what the FWHM measurement is but I'm curious as to the unit of measurement.. my best guess is pixels but I was unable to confirm that in the docs or old forum posts. If it's pixel-based then it would be a sensor-dependent score.


Re: FWHM units?

Posted: Thu Feb 28, 2019 2:28 am
by oopfan

It is in pixels.

You can convert FWHM in pixels to FWHM in arc seconds with this formula:
FWHM_arcsec = FWHM_pixels x ( pixel_size_um / focal_length_mm) x 206.3

And the other way around:
FWHM_pixels = FWHM_arcsec x (focal_length_mm / pixel_size_um) / 206.3

My camera:
pixel_size_um = 2.9

My telescope:
focal_length_mm = 416

Average seeing:
FWHM_arcsec = 3.0

FWHM_pixels = 3.0 x (416 / 2.9) / 206.3 = 2.09 pixels

It is a generally accepted principle that a star's disc should cover a 2 to 3-pixel square area to optimize signal-to-noise. If fewer pixels then the camera is under-sampling. If more pixels then the camera is over-sampling. Here is a handy calculator to determine the suitability of your camera and telescope:

Over-sampling is not harmful but it spreads light over more pixels so longer exposures may be required. Under-sampling can be particularly harmful to one-shot color cameras. Color reproduction is affected leading to "green stars". A common technique to remedy it is to employ "drizzle" when stacking.


Re: FWHM units?

Posted: Thu Feb 28, 2019 4:55 am
by cshine
Thanks oopfan, very helpful!

I was using the FWHM filtering feature the other night and had to set the limit to 8, average FWHM score was around 7.5. So it sounds like I'm over-sampling and I should have switched to 2x2 binning at that point? That would get me back to ~3pixels of FWHM

My pixel size is 4.63 (IMX294) and effective fl is 1280mm, so using the image scale formula I'd have 7.5 x (4.63 / 1280) x 206.3 => 5.6 FHWM_arcsec .. which is bad.. worse than I thought!

I realize I'm straying from my own topic here a little but is there any way to determine how much of that 5.6" is down to seeing and how much is an issue on my end (optics, focus etc)?


Re: FWHM units?

Posted: Thu Feb 28, 2019 12:10 pm
by oopfan
Hi cshine,

I had to edit my reply after noticing that the CCD Suitability Calculator explicitly uses FWHM_arcsec in the "Seeing" dropdown box.

I've only Live Stacked once in my life so I can't say with certainty if the 'FWHM Score' is the average or maximum FWHM_pixels or some other function. For it to be FWHM_arcsec then it would need to know your telescope's focal length. I don't remember having to enter that in. So I am fairly certain that it is in pixels.

So your FWHM score was 7.5? Hmm, is that consistent night after night? It is either due to poor seeing conditions or focus. Of course it could be optics but check the least costly things first. How do you focus? I tried the various methods but ended up doing it manually by zooming in 300-400% and tweaking the fine adjustment knob until minimum disc size achieved.

Another possibility is poor tracking/guiding. Do you have a FITS file that you can share?