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Can someone help me interprete these FWHM/Eccentricity results?

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#1 RemcoNL

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Posted 08 August 2020 - 04:09 AM

Hi Everyone,

This week I've replaced my DSLR with my first astro camera (ZWO ASI533MC Pro).

Since I'm missing a ring of my field flattener I've been worrying about getting the correct back focus.
I've analysed an image from my DSLR and compared it with the initial results of my ASI533.

Obviously the FoV of the Nikon is much wider, but those results I can clearly see the circles that I'm expecting.
In the results of the ASI533 the shapes don't mean anything to me.
However the differences with the ASI are a lot smaller. 

Am I on the right track?

Any advice would be greatly appriciated.

Attached Thumbnails

  • results DSLR.png
  • results 533.png

Edited by RemcoNL, 08 August 2020 - 04:10 AM.


#2 james7ca

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Posted 08 August 2020 - 05:10 AM

Those results from the ASI533 look fairly good. However, both FWHM and eccentricity depend HIGHLY on the seeing conditions so it's hard to make a comparison just from a single trial. Also, you want to compare the FWHM in arc seconds, not pixels and the FWHMEccentricity script outputs only in pixels, so you need to do the conversion yourself using your image scale.

 

Generally speaking, an eccentricity of 0.42 and below is considered to produce "round" looking stars. But, that doesn't always hold and you can be either above or below that value and still have stars that look "good" or "bad" (it kind of depends upon how the stars are distorted and, of course, the individual observer). Also, the quality of your guiding/tracking can affect both FWHM and eccentricity, so it's not only an optical or camera issue. And like I said, the seeing conditions can both make and break these values. For example, it's possible to get very round looking stars during bad seeing, but your FWHM will likely be quite large (but, remember, you should be comparing using arc seconds, not just pixels).

 

Here is a thread that contains quite a few posts on FWHM and eccentricity with plots from PixInsight's FWHMEccentricity script:

 

  https://www.cloudyni...y/#entry6627920

 

When you get satisfied with your results I would appreciate it if you posted your plots and images and supporting data over in this same thread for others to reference.


Edited by james7ca, 08 August 2020 - 05:12 AM.


#3 RemcoNL

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Posted 08 August 2020 - 06:29 AM

Thank you, James.
The thread you've mentioned looks very interesting.

The image I've used for the ASI533 reference was definitely not representative but I figured any BF problems would show up non the less.

I'll take your invitation to learn more about the subject and will post my results in your thread when I have a more representative dataset.

Kind regards,
Remco
 




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