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Question on FWHMExcentricity script in Pix :-?

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11 replies to this topic

#1 Mert

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Posted 21 May 2019 - 11:01 AM

Hi all,

 

I am using an ASI1600MCC, OSC camera and when I use the

script on a raw undebayered image, FWHM is aprox. 5 pixels

equatng to about 2".

When I use the debayered image and take the luminance of it

and then apply the script I get 1.5 pixels for the FWHM of that

particular frame.

How should I use this script??

 

Thanks for any feedback,

Mert


Edited by Mert, 21 May 2019 - 11:02 AM.


#2 bobzeq25

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Posted 21 May 2019 - 11:33 AM

Hi all,

 

I am using an ASI1600MCC, OSC camera and when I use the

script on a raw undebayered image, FWHM is aprox. 5 pixels

equatng to about 2".

When I use the debayered image and take the luminance of it

and then apply the script I get 1.5 pixels for the FWHM of that

particular frame.

How should I use this script??

 

Thanks for any feedback,

Mert

That's exactly what I'd expect.  The 4:1 (close enough) ratio that you got and the four component RGGB matrix are not a coincidence.

 

Use it however you want.  I tend to use it on undebayered subs for the purposes of evaluating my data gathering that night, and sorting out good subs from bad (using the implementation in SubframeSelector).   I sometimes run it on the final image, just for my amusement.


Edited by bobzeq25, 21 May 2019 - 11:35 AM.


#3 Mert

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Posted 21 May 2019 - 12:15 PM

Thanks Bob, that is exactly how I wanted to use it and for my understanding

this is quit strange.

How come when imaging at 0.46"/pixel using the debayered image and then

extract the luminance of it, the script gives me 1.5 pixels which would

be a FWHM of 0.69", whereas with the raw frame with 5 pixels that is 2.3"

more or less the seeing I have here in town.

Seems strange to me!!

Right now I am processing an integration from yesterday night, selecting

frames under 3" FWHM and <0.5 excentricity, result is quit nice taking

into account that it was taken from my balcony with nice LP from the

city closeby. ( only 24 x 120 seconds on M51 )

The accumulated FWHM on the integration is 2.6" and an excentricity of

the stars of aprox. 0.36


Edited by Mert, 21 May 2019 - 12:16 PM.


#4 Jon Rista

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Posted 21 May 2019 - 01:11 PM

Debayering shouldn't be changing the FWHM. You should have the same number of pixels in a debayered image as in the raw image, it is just that in the debayered image the discrete color channels of the raw image have been interpolated to produce full color information at each and every pixel. Normal interpolation shares input pixel info across many output pixels. You would only have 1/4 the image dimensions if you used super-pixel debayering. 

 

Sfsi2PT.gif

 

An FWHM of 0.7" is ... well, it would be as good as 10-meter professional telescopes under EXCELLENT skies... I don't know of any amateur who has seeing so good that they could resolve 0.7" FWHM stars... So yes, something seems to be awry. 

 

Now, BAYERED data may be artificially inflating the FWHM of stars a bit, I guess it would depend on how well sampled or oversampled you are. You mention an image scale of 0.49"/px, so I would have to imagine you are pretty darn well sampled, so I don't think that the bayered data would be skewing the FWHM much. If your skies are ~2", then you should be getting around 2" FWHM with both the bayered and debayered data with the oversampled data you have.



#5 Mert

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Posted 21 May 2019 - 01:56 PM

Thanks John for your great explanation!

I don't know what I'm doing wrong but anyway

the results are getting better now that I'm close

to get the collimation more or less good!

I am imaging with my RC8 at F8 from the balcony

so that is not optimal.

Also my guiding is still a bit rough ( RA oscilation

of aprox. 0.4 with the curves at +- 1" )



#6 Jon Rista

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Posted 21 May 2019 - 02:07 PM

Thanks John for your great explanation!

I don't know what I'm doing wrong but anyway

the results are getting better now that I'm close

to get the collimation more or less good!

I am imaging with my RC8 at F8 from the balcony

so that is not optimal.

Also my guiding is still a bit rough ( RA oscilation

of aprox. 0.4 with the curves at +- 1" )

Yeah, with those conditions, 0.7" FWHMs would be sublime...! :p 

 

Can you share some of your data? 



#7 Mert

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Posted 21 May 2019 - 02:28 PM

Of course, where can I put it, or better: can I do a Wetransfer?

That way I can put Mdark, Mflat and lights?

I don't have much space on my website anymore, so...?

What options are there to share??

Whoops, maybe Google Drive? I'll read now how to use that.

 

Ok, done, will be Googledrive.

What do you like me to upload John?


Edited by Mert, 21 May 2019 - 02:33 PM.


#8 bobzeq25

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Posted 21 May 2019 - 03:20 PM

Thanks Bob, that is exactly how I wanted to use it and for my understanding

this is quit strange.

How come when imaging at 0.46"/pixel using the debayered image and then

extract the luminance of it, the script gives me 1.5 pixels which would

be a FWHM of 0.69", whereas with the raw frame with 5 pixels that is 2.3"

more or less the seeing I have here in town.

Seems strange to me!!

Right now I am processing an integration from yesterday night, selecting

frames under 3" FWHM and <0.5 excentricity, result is quit nice taking

into account that it was taken from my balcony with nice LP from the

city closeby. ( only 24 x 120 seconds on M51 )

The accumulated FWHM on the integration is 2.6" and an excentricity of

the stars of aprox. 0.36

You're using this exactly the way I would.

 

I just don't worry about it as much.  <grin>



#9 Mert

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Posted 21 May 2019 - 04:19 PM

Yeah, with those conditions, 0.7" FWHMs would be sublime...! tongue2.gif

 

Can you share some of your data? 

Here you go ( hope it works )

 

https://drive.google...tYr?usp=sharing



#10 james7ca

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Posted 21 May 2019 - 10:48 PM

In addition to what Jon has said, when using a OSC camera I would not "trust" the output from the FWHMEccentricity script until AFTER calibration and debayering. In particular, I could imagine that the eccentricity measurements could be wildly inaccurate if run on the non-debayered data.

 

That said, if you get use to evaluating your results using the non-debayered data then there could still be some value in making comparisons between different sessions, since in this case you're just looking for relative differences.  However, for accurate results I'd suggest doing the debayer before you run the script.



#11 Mert

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Posted 22 May 2019 - 12:45 AM

Thanks James, I will evaluate my frames to see what I'm missing here!!



#12 Mert

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Posted 23 May 2019 - 11:34 AM

Here you go ( hope it works )

 

https://drive.google...tYr?usp=sharing

Hey John,

 

Have you been able to see something strange in these lights??

Thanks for any help on this!




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