Author Topic: FWHM and Eccentricity, What Are Typical Values?  (Read 15188 times)

Offline james7

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FWHM and Eccentricity, What Are Typical Values?
« on: 2014 February 26 18:08:19 »
I've been using the SubframeSelector and FWHMEccentricity scripts to sort my images and I'm wondering what other users are seeing when they run these scripts. It's all very well to select frames based upon the capabilities of your own equipment and technique, but I'm curious about what values other users are getting. To make any comparison at all we'll need to know what your capture scale is (arc seconds per pixel), whether you are using a monochrome or full-color sensor (assumed Bayer pattern), and the aperture, field coverage (focal length and sensor size), and type of your telescope. Reports should also include the stage in image development where the images were measured. Typically, that might be immediately after calibration and debayering (if required), but before stretching or any other digital development.

My greatest concern (or interest) is in eccentricity values as I very seldom see results that are better than the 0.42 limit that is supposed to indicate round, undistorted stars. My FWHM values seem reasonable, but my typical median eccentricity values are always above 0.45 (values in the 0.5 to 0.6 range being quite common). When viewed visually at 1:1 scale, the center-field star shapes run from slightly elliptical to what I'd call very close to perfectly round. The odd thing is that even on very short exposures (say, one second) my center-field eccentricity values still don't reach that 0.42 limit. I recognize that some of the poorer median values may be caused by field curvature and edge aberrations, but I struggle to get values below 0.5 even in the center of my field (on multiple telescopes, not just from one sample).

As an example, here is the summary from my Astro-Tech AT72ED (a non-flat-field, 72mm aperture, f/6 doublet ED telescope mated to an APS-C Sony NEX-5N camera). The capture scale is 2.3 arc seconds per pixel with a 4.8 micron pixel size and the diagonal field of coverage was 3.7 degrees. This data shows a lot of field curvature (as would be expected), but even in the center of the field the eccentricity is between 0.6 and 0.55. Over this same center the FWHM ranges from 4 to 6 arc seconds. Here is the data from the full frame (includes the "bad" edges):

FWHM 4.76 arc seconds
Eccentricity 0.725
StarSupport 4168
FWHMMeanDev 1.54
EccentricityMeanDev 0.121

The sample was taken from a stack of ten images that were exposed for 10 seconds each, there was no image calibration, the ten images were debayered (VNG), registered, and integrated with no normalization or pixel rejection. The measurements were then taken on the linear image using the SubframeSelector script (with only one image, the stacked result as input).

Is anyone willing to put forth there own measurements or add some comments on their own use of these scripts.

Offline pfile

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Re: FWHM and Eccentricity, What Are Typical Values?
« Reply #1 on: 2014 February 27 10:07:23 »
i am seeing similar stuff with my TMB92SS, although i don't have proper (short) frames to test with. in a 1200s Ha exposure i see FWHM of 5.337 arcsec with a deviation 0.66 arcsec, and eccentricity of 0.54 with a deviation of 0.1227.

rob

Offline james7

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Re: FWHM and Eccentricity, What Are Typical Values?
« Reply #2 on: 2014 February 27 13:36:25 »
rob, thanks for the feedback. If your median eccentricity is running around 0.54 what do you see on your contour plots near the center of the field? I recently did some untracked and very short exposures (less than one second, ranging down to 1/30 second) and while I still have some analysis to complete it looks like those shots still have eccentricity values above the 0.42 limit that is supposed to indicate "round" stars. It could be astigmatism or some other optical trail of the telescope(s) or it might even be the camera (non-square pixels) but I'm beginning to wonder whether eccentricity values below 0.42 are even possible with my equipment.

Short, untracked exposures are kind of interesting because you can calculate the precise angular movement of the star based upon its declination. That then becomes the "error" and you can evaluate the images to see what is detectable (visually) and how that is measured by PixInsight. Given my camera's pixel size my general rule of thumb for untracked exposures is to use shutter speeds that are equal to or less than 100 / focal length in mm. Thus, for a 430mm focal length you get 100/430 or about 1/4 second. When I look at the images I find that to be a very stringent limit as I can usually go to twice that before I begin to see obvious elongation in the stars.

Offline Geoff

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Re: FWHM and Eccentricity, What Are Typical Values?
« Reply #3 on: 2014 February 27 14:16:32 »
I'm using a 12.5" CDK at 2540mm focal length at 1.2asp. I take 600sec subframes.  The following figures are from my last imaging session using subframe selector:
41 frames, mixed L, R, G, B straight out of camera (no calibration)
FWHM average: 2.48", SD 0.25"
Eccentricity mean: 0.47, SD 0.04

Upon calibration , registration and stacking I get
L:  FWHM 2.55", Eccentricity 0.30
R:  FWHM 2.56", Eccentricity 0.35
G:  FWHM 2.48", Eccentricity 0.40
B:  FWHM 2.58", Eccentricity 0.40

These are typical figures.  I usually keep an eye on my FWHM with CCDInspector while imaging.  I refocus when it starts to creep up near 3".  If that doesn't help, but it sticks between 3" and 4" I may continue with RGB, but not L and beyond 4" I stop imaging.
Geoff
« Last Edit: 2014 February 27 14:43:32 by Geoff »
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Offline james7

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Re: FWHM and Eccentricity, What Are Typical Values?
« Reply #4 on: 2014 February 27 14:51:22 »
Geoff, given your telescope's aperture it seems like your FWHM is probably limited only by your seeing conditions while with my AT72ED it's probably more of a limit from my scope (i.e. at 72mm of aperture the Airy disk is about 3.6 arc seconds to begin with and I'm getting a FWHM that is about one arc second larger than that). At prime focus and with my chosen camera I'm also working no where near the critical sampling that would be required to see the full resolving power of my scope (but I'd think that the same would be true for you since you're working with a sub-arc second Airy disk).

Thanks for your notes. Would it be possible for you to post one of the contour maps showing your performance. It would be interesting just to see something that was that good.
« Last Edit: 2014 February 27 15:15:34 by james7 »

Offline Phil Leigh

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Re: FWHM and Eccentricity, What Are Typical Values?
« Reply #5 on: 2014 February 28 01:07:16 »
James - don't know if this is useful or not... Skywatcher Equinox 80+Televue TRF2008 reducer (so 400mm FL, 80mm aperture)  300s exposure

Offline james7

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Re: FWHM and Eccentricity, What Are Typical Values?
« Reply #6 on: 2014 February 28 01:24:59 »
Phil thanks. What kind of camera (DSLR or dedicated CCD) and what is your image scale in arc secs per pixel? Based upon your eccentricity values it looks like the TRF2008 works fairly well on the Skywatcher (although when not knowing the size of your sensor I can't calculate your field of coverage, but it looks fairly wide).

Offline Phil Leigh

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Re: FWHM and Eccentricity, What Are Typical Values?
« Reply #7 on: 2014 February 28 01:31:48 »
James,
The camera is a CentralDS600 (cooled DSLR). Sensor specs attached...

2.22 arcseconds/pixel
Regards
Phil

Offline james7

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Re: FWHM and Eccentricity, What Are Typical Values?
« Reply #8 on: 2014 February 28 02:32:46 »
Okay, then your median FWHM seems to be about 6.6 arc seconds and your (uncropped) field coverage is about 3 degrees 11 minutes by 2 degrees 8 minutes.

Offline Phil Leigh

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Re: FWHM and Eccentricity, What Are Typical Values?
« Reply #9 on: 2014 February 28 03:56:38 »
Okay, then your median FWHM seems to be about 6.6 arc seconds and your (uncropped) field coverage is about 3 degrees 11 minutes by 2 degrees 8 minutes.

Correct. Seeing was not good when this image was taken :-(
On a good night the FWHM will be ~4.5 arc seconds

Offline jerryyyyy

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Re: FWHM and Eccentricity, What Are Typical Values?
« Reply #10 on: 2014 March 01 20:53:40 »
If you are interested, I have a new Takahashi 180ED that I have been collimating.  The data are out here:

http://www.astrobin.com/full/78328/F/

I am going to get some more as soon as the clouds clear here. 
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Offline james7

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Re: FWHM and Eccentricity, What Are Typical Values?
« Reply #11 on: 2014 March 19 21:42:54 »
If you are interested, I have a new Takahashi 180ED that I have been collimating.  The data are out here:

http://www.astrobin.com/full/78328/F/

I am going to get some more as soon as the clouds clear here.
Jerry, did you finish your work on the collimation? I note that the link you provided showed eccentricity values around 0.5 which is above (i.e. worse) that the 0.42 that I've seen recommended for round-looking stars. It's very rare that I get much below 0.5 with any of my scopes (all fairly fast refractors). I've never been able to determine whether that last little bit to get to 0.42 and below has to happen through guiding or better optics (or focus, etc.).

Since we're now imaging at 5 micron pixels and below I'm beginning to wonder whether it is even possible to routinely achieve eccentricity values at or below 0.42 (uniformly across a wide field). Geoff posted some really good numbers, but I suspect that is for a fairly narrow field of view (i.e. probably not much more than one degree on a side).

Offline mschuster

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Re: FWHM and Eccentricity, What Are Typical Values?
« Reply #12 on: 2014 March 19 23:19:14 »
My portable FSQ setup runs at 4.2"/px, a 2 degree field, a mono sensor, and 40 minute subs. Median eccentricities are almost always below 0.4 and FWHMs are almost always near 1 pixel with temperature compensation, which reduces FWHM a lot. For collimation checks, defocused star wavefront analysis also is possible.

Cheers,
Mike

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Offline james7

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Re: FWHM and Eccentricity, What Are Typical Values?
« Reply #13 on: 2014 March 20 02:40:12 »
Mike, I'm using an APS-C camera with a Bayer pattern so I can't really compare my resolution directly to yours, but I typically image at anywhere from 1.5"/px to 2.4"/px (mostly 1.5 to 2.0"/px) and I often have a FWHM between 4 and 5 arc seconds. However, my eccentricity values are seldom better than 0.5 which is something that I'm trying to improve. I don't think my non-round stars are solely related to guiding/tracking, because even when I do unguided subs in the one second to ten second range it's difficult for me to crack 0.45 even in the best parts of my field. This happens with three different scopes where the only commonality is the camera (I have two Celestron GEMs, an AVX and a CGEM). Frankly, I'm beginning to question my technique (or lack thereof) and that's why I was curious about what other PixInsight users were measuring.

Offline jkmorse

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Re: FWHM and Eccentricity, What Are Typical Values?
« Reply #14 on: 2014 March 20 03:14:03 »
Geoff,

What settings to you use for your auto focus routine for the CDK and are you using PWI or some other software for focusing?  I image with a CDK12.5 as well, but your FWHM numbers are coming in substantially better than mine (mine push against the 4" mark) so I would appreciate understanding your method.  CCD is an F16M and I use 600 sec iterations as well.  My eccentricity is fine, typically in the 3.5 range, so its just the focus that's the issue.  Could just be my skies, but I am curious.

Thanks,

Jim
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