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Old 02-07-2013, 10:12 AM
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LewisM
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FHWM expectations

When I image, I take my time to get focus SPOT ON, as near as I can tell - I do my focus by eyeball and FHWM readings alone. I do not have any automatic programs to do it - just a steady hand, a dual speed Vixen focuser and time (I typically spend 30 minutes refining a focus, allowing for seeing variations etc).

On average, my FHWM readings on a typical star (not overly bright, nor overly dim - mid range magnitude) will be between 1.09 to 1.3 - above that and I will keep going. I have experienced exceptional seeing with FHWM's sub-1.0.

In discussions with a few other imagers, I got the impression that "You cannot get FHWM less than 3 manually" and they relied on focusmax etc to achieve their small FHWM's, though even then, their values were consistent with mine. This perplexes me.

I use either Starlight Xpress' focusing routine, or run the focus routine in APT (works nicely!), with my SXVR-M25C. I do NOT use a Bahtinov - purely judging initially by eye for in and out of focus size, and then fine tuned with the fine focus knob watching the FHWM.

I guess if it works, don't rock the boat, but am I missing something here? I believe on the whole my imags to be well focused (USUALLY). I could maybe get beter using software to do the focus, but is 1.09 to 1.3 all that bad, considering a typical average plus seeing night?
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Old 02-07-2013, 10:33 AM
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rmuhlack (Richard)
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The difference between the numbers you see and those reported by others might come down to the units for FWHM.

I suspect that the anecdotal figures reported by other imagers are in arcsecs - this would certainly square with my understanding of typical seeing limitations etc

However, if the software doesn't take the focal length of the optical train into account, then its probably reporting FWHM in pixels. If the image scale of your scope and camera combo is say 2.5 arcsecs/pixel, and your software is reporting a FWHM of 1.3, then that is equivalent to a FWHM of 2.5x1.3 = 3.25 arcsecs, which would be consistent with the numbers reported by others.

As a further example, on my last imaging run with the VC200L with humidity close to 100% I was getting FWHM of about 4.0-4.2 however my image scale is ~0.9 arcsecs per pixel, so that gives me a FWHM of 3.6-3.8"

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Old 02-07-2013, 12:01 PM
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Thanks Richard - seems to make sense!
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Old 02-07-2013, 04:20 PM
rally
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Lewis,

If the reported FWHM is pixels, then by my calcs your image scale is 1.75 arc secs/pix
ie
Vixen FL102S focal length = 918mm
Starlight XPress SXV-M25C = 7.8uM pixels

So
FWHM 1.03 = 1.8 arc secs
FWHM 1.3 = 2.3 arc secs

This would tend to be around the local limits and is in any case the lower figure is basically at your telescopes resolving limit being 1.75.
So its hard to do better than that at image acquisition time.

You could possibly deconvolve during processing but since I think you are asking about your focus technique and the relative FWHM you are getting.
Its as good as your system can allow.

Rally
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Old 02-07-2013, 05:17 PM
Garbz (Chris)
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Sorry nothing constructive to add to your case, just that for future reference it's Full Width Half Medium, FWHM not FHWM.
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Old 02-07-2013, 06:10 PM
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Merlin66 (Ken)
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Chris,
Actually
Full width, Half Max (FWHM)
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Old 02-07-2013, 06:23 PM
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Yeah, I know, just a brain fart.
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Old 02-07-2013, 06:24 PM
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Quote:
Originally Posted by rally View Post
Lewis,

If the reported FWHM is pixels, then by my calcs your image scale is 1.75 arc secs/pix
ie
Vixen FL102S focal length = 918mm
Starlight XPress SXV-M25C = 7.8uM pixels

So
FWHM 1.03 = 1.8 arc secs
FWHM 1.3 = 2.3 arc secs

This would tend to be around the local limits and is in any case the lower figure is basically at your telescopes resolving limit being 1.75.
So its hard to do better than that at image acquisition time.

You could possibly deconvolve during processing but since I think you are asking about your focus technique and the relative FWHM you are getting.
Its as good as your system can allow.

Rally
Thanks Rally - MUCH appreciated!
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Old 02-07-2013, 06:25 PM
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LewisM
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Rally,

I concur - on average, I get 1.78 asec/pixel.
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Old 03-07-2013, 03:28 PM
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Geoff45 (Geoff)
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Quote:
Originally Posted by Garbz View Post
Sorry nothing constructive to add to your case, just that for future reference it's Full Width Half Medium, FWHM not FHWM.
Maximum not medium
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Old 03-07-2013, 07:43 PM
Garbz (Chris)
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Speaking of brain farts, that's the last time I correct someone
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