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Help with M31 post processing problem?

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26 replies to this topic

#1 Tailspin45

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Posted 27 September 2021 - 01:25 PM

This image is about 10 hours of total integration from two-minute subs, captured with an L-Pro filter, and callibrated with 60 flats, darks, and dark flats from an ASI294MC.

 

When I compare it to other images of M31 I don't see the red/yellow gradient slopping over the edge from the core or the dust lane leading to/from M110.

 

Can anyone suggest 1) the source of the problem, 2) how to avoid it, and 3) how to fix it using Astro Pixel Processor, StarTools, and Lightroom/Photoshop?

 

M31CN.jpg

 

 



#2 DubbelDerp

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Posted 27 September 2021 - 01:27 PM

Can you upload the raw stack to somewhere like Google drive? Kneejerk reaction is that perhaps the wrong bayer matrix was applied...



#3 jonnybravo0311

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Posted 27 September 2021 - 01:30 PM

You used a light pollution filter that cut out chunks of the visible spectrum, so the proper color data isn't there in the raw subs.

 

Not sure what scope you used, but if it's an achromat, that'll negatively impact the blues.

 

There have been issues reported with ZWO color cameras setting incorrect values for the white balance. Needs to be addressed in the drivers by using something like ASI Studio.

 

Would be helpful if you could upload the raw FITS file to some file sharing site like Google Drive or Dropbox so they can be evaluated in more detail.


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#4 Tailspin45

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Posted 27 September 2021 - 01:50 PM

Dropbox share here (162Mb FITS)



#5 DubbelDerp

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Posted 27 September 2021 - 02:06 PM

I'm assuming you didn't use flats? I suspect that your color is fine, but with the extreme vignetting in the data it's difficult to correct the uneven illumination with the size of the DSO. If you haven't taken flats yet, it may still be worth giving it a shot... just get your imaging train as close as possible to how it was when you shot M31, and hope there isn't too much dust moving around.



#6 Tailspin45

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Posted 27 September 2021 - 02:11 PM

I'm assuming you didn't use flats? I suspect that your color is fine, but with the extreme vignetting in the data it's difficult to correct the uneven illumination with the size of the DSO. If you haven't taken flats yet, it may still be worth giving it a shot... just get your imaging train as close as possible to how it was when you shot M31, and hope there isn't too much dust moving around.

I did use flats, but your comment makes me even more suspicious a calibration mistake is the underlying issue. 

 

I'll try running the lights through again, one session at a time, and make sure I apply the proper flats.

 

Will report back when I have new images.


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#7 Monkeybird747

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Posted 27 September 2021 - 02:45 PM

I did a super quick stretch in Pixinsight just to see if your color data is intact. I'm not seeing the crazy colors you have here when using a basic autostretch. Unfortunately I don't use the processing tools that you cite, so can't tell you how to fix it with those. All I can say is that the color data is there, and seems pretty normal to me. I did a quick background extraction, stretch, and saturation. There does seem to be something odd with the lower left portion of the galaxy. Again I think this could be from poor flat correction.

 

I do agree the flats have not corrected properly. You don't need 60 flats for a master flat frame. 15-20 is plenty.

 

M31x2 session 1 session 2
 
MB

 


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#8 jonnybravo0311

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Posted 27 September 2021 - 02:48 PM

If you could upload a single light, flat, dark and bias to that same file share location, folks could see if there's an issue with the calibration files. Also, here's about a 5 minute edit in PixInsight:

 

med_gallery_347158_14910_355556.jpg

 

There is some serious vignetting which makes me question the calibration process. Had to crop in a bunch to get clear of the stacking artifacts. As I mentioned in my first post, the colors are going to be wrong because you used the L-pro filter. Someone more talented than I am could probably figure some nice masks and curves to try and bring things back in line, if possible.

 

My suggestion would be to get that calibration process down so hopefully you can start with a better data set :).


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#9 Tailspin45

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Posted 27 September 2021 - 03:33 PM

Sample files here (51Mb .zip)

Single light sub frame, master dark, single flat sub, and master dark flat



#10 Tailspin45

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Posted 27 September 2021 - 04:33 PM

Here's the Sept 10 session by itself. Color looks better, the red is pretty much gone, but now the core has a bilious green tinge I don't like and the stars are really  bloated. Other than that, Mrs. Lincoln....

 

I like the vortex details in the core that I haven't seen before, but the outlying areas are way too dim. No idea what happened to the young blue stars that should decorate the near edge.

 

Any suggestions for my next processing iteration? Or for next capture session, for that matter? Bortle 8, here by the way.

 

M31CN-2.jpg


Edited by Tailspin45, 27 September 2021 - 04:41 PM.


#11 bobzeq25

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Posted 27 September 2021 - 06:03 PM

Or for next capture session, for that matter? Bortle 8, here by the way.

 

attachicon.gifM31CN-2.jpg

Absolutely.  So-called "light pollution filters" don't magically filter out light pollution.  They whack out hunks of the spectrum, hoping that what they whack is mostly noise, what they pass is mostly signal.  That's a crude method.  It works OK on narrowband targets like emission nebulae.  On broadband targets like galaxies, not so much.

 

They distort color.

 

They reduce signal.

 

_Many_ experienced imagers don't use such filters on broadband targets like galaxies.  Instead, they do gradient reduction in processing. Not perfect, but preserves color and signal.   Most astro specific processing programs have suitable GR tools, I recommend Astro Pixel Processor.  It can do everything start to finish.  I recommend working on that before trying to use multiple programs.  They too make life difficult.

 

More total imaging time also helps.

 

Here's Andromeda from my Bortle 7 backyard.  No (not so) magic light pollution filters were used.  Spent some time getting the GR right in PixInsight.

 

Better version here.

 

https://www.astrobin.com/263253/J/

 

M31 SV70T small.jpg
 


Edited by bobzeq25, 27 September 2021 - 06:07 PM.

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#12 Tailspin45

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Posted 27 September 2021 - 07:52 PM

Better? I think I was being heavy handed.

 

M31CN v3.jpg


Edited by Tailspin45, 28 September 2021 - 07:04 AM.


#13 Mike in Rancho

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Posted 27 September 2021 - 10:28 PM

Tailspin,

 

The latest post is better, but, not really addressing the issue I think.  Just processing your way around it better than before.

 

The full stack you uploaded has major problems, and ST really does not like it.  Was this stacked in APP?  And using ST's stacking guidelines for APP?

 

Something seems to have gone quite wrong, but I don't know APP so can't help you there.  I did load your single sub files (though two were masters instead of the requested subs) into DSS, and calibration seemed reasonable.  The very corners still contained something like vignetting, though the appearance also made me wonder about hard physical vignetting.  What is the optical train setup used here, including size of filter?

 

I'm assuming this project was multi-night sessions?  And possibly with a fair amount of field shift between the nights?  Looking at the stacking artifacts in ST's AutoDev, I couldn't quite tell if these shifts were within session, between sessions, or both.  It made finding the correct cropping points rather difficult.  As you may know, Wipe has little tolerance for stacking artifacts.

 

I'm thinking that the multi-session stacking and calibration for your APP might need to be set up properly, and then re-stacked.  Did you take flats for each session (especially if the optical train changed or rotated at all between sessions)?



#14 Tailspin45

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Posted 28 September 2021 - 08:21 AM

Yes, stacked in APP, and yes using ST guidelines such as "remove light pollution" off in Calibrate, "neutralize background" off in Normalize, etc.

 

Optical train is Celestron 120, 0.6x reducer, spacers setup for 55mm back focus from the reducer as specified and illustrated by ZWO. The 1.25 L-Pro filter was mounted inside the ASI294MC as recommended by ZWO...in other words as close to the sensor as it can be.

 

The data I uploaded was for two sessions, each involving a meridian flip. Yes, because of the artifacts around the edge, I cropped the image before putting ST to work.

  • On August 27 I installed the reducer, successfully imaged IC 134, and created flats and dark flats the next morning before successful processing the results with ST.
  • On September 4th I imaged NGC 6960 with no change in the image train and produced a good dual band image with APP and ST.
  • On September 6th I imaged M31, again with no change in the image train.
  • On September 10th I again imaged M31, also with no change in the image train and created a new set of flats.

My last image, in post #12 above, was created with the new flats and just the Sept 10 lights. I'm terrible a keeping a useful log so I can't honestly say why I created the new flats—but it makes me suspicious that I changed something.

 

I suspect, thanks to your help, that the mess is the result of careless application of the new flats to both the new and old data.—that and over-enthusiastic post-processing.

 

I'm going to reprocess the Sept 6th data with the Aug 27 flats and then combine that result with the Sept 10 result that used the new flats and see what ST Compose produces. I'll post the result here


Edited by Tailspin45, 28 September 2021 - 01:51 PM.

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#15 DubbelDerp

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Posted 28 September 2021 - 09:22 AM

In APP, when you re-integrate the data, try checking "enable MBB" and turn on local normalization correction to 1st degree, 3 iterations in the "integrate" tab. This should significantly clean up the integration artifacts.

 

When you applied the new flats to the last set of data, did you use the multi-session option for stacking the data?


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#16 Tailspin45

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Posted 28 September 2021 - 09:31 AM

I think I did use MBB, LNC and multi-session. Will check the log and/or the FITS header to make sure


Edited by Tailspin45, 28 September 2021 - 01:50 PM.


#17 Tailspin45

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Posted 28 September 2021 - 01:48 PM

I did use multi-session and MBB but only one iteration of 1st degree LNC

SIMPLE  =                    T / Java FITS: Sat Sep 11 06:21:55 PDT 2021
BITPIX  =                  -32 / bits per data value
NAXIS   =                    3 / number of axes
NAXIS1  =                 4577 / size of the n'th axis
NAXIS2  =                 3099 / size of the n'th axis
NAXIS3  =                    3 / size of the n'th axis
EXTEND  =                    T / Extensions are permitted
BSCALE  =                  1.0 / scale factor
BZERO   =                  0.0 / no offset
DATE    = '2021-09-11T16:53:21' / creation date of Integration
SOFTWARE= 'Astro Pixel Processor by Aries Productions' / software
VERSION = '1.083-beta1'        / Astro Pixel Processor version
INTEGRAT= 'Integration'        / integration of light frames
CFAIMAGE= 'no      '           / Color Filter Array pattern
NOTE-1  = 'INTEGRATION METADATA'
EXPTIME =              27600.0 / exposure time (s)
NUMFRAME=                  230 / number of frames used in this integration
BG-1    = ' 1.2949E-02'        / background estimate of channel 1
BG-2    = ' 2.2254E-02'        / background estimate of channel 2
BG-3    = ' 1.9794E-02'        / background estimate of channel 3
SCALE-1 = ' 3.6413E-04'        / dispersion of channel 1
SCALE-2 = ' 3.9422E-04'        / dispersion of channel 2
SCALE-3 = ' 3.7306E-04'        / dispersion of channel 3
NOISE-1 = ' 1.3325E-04'        / noise level of channel 1
NOISE-2 = ' 1.0788E-04'        / noise level of channel 2
NOISE-3 = ' 1.5588E-04'        / noise level of channel 3
SNR-1   = ' 5.6105E+00'        / Signal to Noise Ratio of channel 1
SNR-2   = ' 7.6125E+00'        / Signal to Noise Ratio of channel 2
SNR-3   = ' 4.3730E+00'        / Signal to Noise Ratio of channel 3
NOTE-2  = 'NR = Noise Reduction'
NOTE-3  = 'medNR = noise in median frame / noise in integration'
NOTE-4  = 'refNR = noise in reference frame / noise in integration'
NOTE-5  = 'ideal noise reduction = square root of number of frames'
NOTE-6  = 'the realized/ideal noise reduction ratio should approach 1 ideally'
NOTE-7  = 'the effective noise reduction has a correction for'
NOTE-8  = 'dispersion change between the frame and the integration'
NOTE-9  = 'because dispersion and noise are correlated'
medNR-1 = ' 4.1866E+00'        / median noise reduction, channel 1
medNR-2 = ' 7.3531E+00'        / median noise reduction, channel 2
medNR-3 = ' 4.4278E+00'        / median noise reduction, channel 3
refNR-1 = ' 4.2361E+00'        / reference noise reduction, channel 1
refNR-2 = ' 7.2734E+00'        / reference noise reduction, channel 2
refNR-3 = ' 4.3575E+00'        / reference noise reduction, channel 3
idNR-1  = ' 1.5166E+01'        / ideal noise reduction, channel 1
idNR-2  = ' 1.5166E+01'        / ideal noise reduction, channel 2
idNR-3  = ' 1.5166E+01'        / ideal noise reduction, channel 3
ratNR-1 = ' 2.7605E-01'        / realized/ideal noise reduction ratio, channel 1
ratNR-2 = ' 4.8485E-01'        / realized/ideal noise reduction ratio, channel 2
ratNR-3 = ' 2.9196E-01'        / realized/ideal noise reduction ratio, channel 3
medENR-1= ' 1.3748E+00'        / effective median noise reduction, channel 1
medENR-2= ' 2.3084E+00'        / effective median noise reduction, channel 2
medENR-3= ' 1.3243E+00'        / effective median noise reduction, channel 3
refENR-1= ' 1.3303E+00'        / effective reference noise reduction, channel 1
refENR-2= ' 2.0891E+00'        / effective reference noise reduction, channel 2
refENR-3= ' 1.1909E+00'        / effective reference noise reduction, channel 3
NORMMODE= 'regular '           / normalization mode
NORMMETH= 'multiply-scale'     / normalization method
NORMSCAL= 'BWMV    '           / normalization scale/dispersion calculation
NOTE-10 = 'REF-1 tag: used reference frame'
NOTE-11 = 'REF-2 tag: used reference frame'
REF-1   = 'Light_M31_120.0s_Bin1_gain120_20210911-015053_-10.0C_0110.fi'
REF-2   = 't       '
COMPMODE= 'full    '           / composition mode
REGMODE = 'normal  '           / registration mode
REGMODEL= 'projective'         / registration model
OPT-DC  = 'disabled'           / optical distortion correction
WEIGHTS = 'quality '           / integration weights
INT-METH= 'median  '           / integration method
OUTL-REJ= 'adaptive rejection' / outlier rejection filter
OUTL-LN = 'LN rejection'       / outlier rejection with local normalization
OUTL-DP = 'none    '           / outlier rejection diffraction protection
OUTL-KL =                  6.0 / outlier rejection kappa low
OUTL-KH =                  3.0 / outlier rejection kappa high
INT-MODE= 'Bayer/X-Trans drizzle' / integration mode
DRZ-DROP=                 1.00 / drizzle droplet size
DRZ-KERN= 'topHatKernel'       / drizzle kernel
INTSCALE=                  1.0 / integrate scale
NOTE-12 = 'PROJECT tag: projection type'
PROJECT = 'rectilinearProjection'
MBBLEND = 'MBB15 % '           / multi-band blending
LNC-DEG = '1st degree LNC'     / Local Normalization Correction degree
LNCRMS-1= ' 2.7399E-04'        / Local Normalization Correction RMS of channel 1
LNCRMS-2= ' 2.9722E-04'        / Local Normalization Correction RMS of channel 2
LNCRMS-3= ' 2.2357E-04'        / Local Normalization Correction RMS of channel 3
LNC-ITER=                    1 / Local Normalization Correction iterations
AD-PED  =                  0.0 / adaptive pedestal from data calibration
SESSNUM =                    2 / data combined from 2 sessions
SESS-1  = 'session 1'          / capture session
SESS-2  = 'session 2'          / capture session

Edited by Tailspin45, 28 September 2021 - 01:50 PM.


#18 DubbelDerp

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Posted 28 September 2021 - 02:09 PM

After looking at the single light frame and flat, dark flat, and master dark, I don't think it will make a difference. As other posters mentioned, I think the real problem is with the flat field correction. Even after calibrating with the flat, you're still left with some significant vignetting. If you can get your flat correction working better, your color processing will be a lot easier.

 

Although the vignetting in the light is pretty severe, there is some data in the dark corners, which makes me think that it should calibrate with proper flats. There is some correction being done by the flat you uploaded, but it's not fixing all of it. 


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#19 Tailspin45

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Posted 28 September 2021 - 02:28 PM

I'm going start fresh with M31 next time we have clear skies and will take a new set of flats and dark flats.

 

As long as I've had this (used) scope I've felt like it has an extraordinary amount of vignetting.

 

If someone tried some ill-advised tweaking of the glass could they do something that caused vignetting?


Edited by Tailspin45, 28 September 2021 - 02:59 PM.


#20 DubbelDerp

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Posted 28 September 2021 - 02:38 PM

Not sure, but you might be able to troubleshoot during the daytime. I've done daytime shots of the moon with my Meade LX200 and it shows plenty of vingetting that can be fixed with flats. Take some images of a far-off feature on the horizon, and you should still see the same vignetting that you do in your m31 image. Take some sky flats, and some dark flats, load them into APP and see how they calibrate. Then maybe swap out some things in the imaging train. Take off the reducer, take another few shots, maybe try without the filter. If the scope without the reducer or filter still shows this level of vignetting that can't be fixed with flats, then you know the problem is with the scope not having a large enough image circle for your camera sensor.



#21 Tailspin45

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Posted 28 September 2021 - 02:59 PM

Logical diagnostic ideas, thanks!

 

I can try different cameras, too, such as the ASI120MM guidescope and a cheap Svbony SV305 with a 6.5mm IMX290MC sensor (compared to 23.21 for the ASI294MC)

 

And I have an old QHY5-II that I bought used but it had spots on the sensor that moved! It was infested with tiny spiders and there was some nesting goop on the sensor I couldn't remove. But I can still use it for testing.



#22 Mike in Rancho

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Posted 28 September 2021 - 03:17 PM

Even if the vignetting (gradient style) is pretty severe, the flat correction should handle it.

 

I might be out of ideas though if you did indeed properly calibrate the sessions with their respective flats, and there was no change of the optical train in between your sessions.  Not meaning different parts, but that you didn't take it apart and put it back together each time.  It really did seem like the flats weren't properly aligning with what they were correcting, leading to that overly bright and squared-off mush across the center of the final stack.

 

Hmmm....confused1.gif

 

The other reason I asked about the optical train was the seeming physical vignetting (i.e. the light cone is hard blocked) in the very corners.  And with your answer of 1.25", there might be something to it?  If JonnyBravo can chime in again, I believe he had issues with 1.25 sized filters and a 294, albeit the mono version.  Of course there's likely more to it, meaning the field created by your particular telescope and reducer, but I assume both 294 color and mono might be the same size sensor and perhaps same distance?  Anyway, might be something to look into, and could explain your far corners being clipped off.



#23 Tailspin45

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Posted 28 September 2021 - 03:30 PM

I've wondered about the filter size issue to the extent that I looked rather carefully at where it is located and how close it is to edge of the sensor when installed the way ZWO says to do it.

 

Which sounds like I'm confident that's not the problem, but I'm definitely not.

 

Still, why would ZWO sell 1.25 filter wheels and show them attached to the 294 if it caused vignetting? I may be wrong, but I have the sense they're better than that.

 

Oh, what am I saying?! Take the dam(n) filter off and see if it's still a problem, then we'll know. Duh.


Edited by Tailspin45, 28 September 2021 - 03:31 PM.


#24 bobzeq25

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Posted 28 September 2021 - 04:06 PM

I've wondered about the filter size issue to the extent that I looked rather carefully at where it is located and how close it is to edge of the sensor when installed the way ZWO says to do it.

 

Which sounds like I'm confident that's not the problem, but I'm definitely not.

 

Still, why would ZWO sell 1.25 filter wheels and show them attached to the 294 if it caused vignetting? I may be wrong, but I have the sense they're better than that.

 

Oh, what am I saying?! Take the dam(n) filter off and see if it's still a problem, then we'll know. Duh.

It's not the CLS vignetting that's the main thing.  It's the fact that it rips out most of the spectrum.  A very crude device.  The LEnhance is much more sophisticated.

 

https://www.hnsky.org/CLS_filter3.jpg

 

4/3 sensors like the 294 and 1.25 filters definitely don't go together. 

 

https://astronomy.to...ccd_filter_size

 

People sell all kinds of funny combinations.  Celestron markets (marketed?) the C11/AVX, as a DSO imaging setup.

 

Thing about DSO AP.  It's very unintuitive, your intuition about what could be the source of problems will often send you astray.  The antidote is actual knowledge.  In this case color issues are the CLS spectral bandpass, vignetting is using a 1.25 filter with a 4/3 chip.  These are not close calls.  <smile>


Edited by bobzeq25, 28 September 2021 - 04:10 PM.


#25 Tailspin45

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Posted 28 September 2021 - 05:38 PM

Well, this is confusing.

 

It's not the CLS vignetting that's the main thing.  It's the fact that it rips out most of the spectrum.  A very crude device.  The LEnhance is much more sophisticated.

I'm was using an L-Pro not a CLS. The L-Pro is much more selective than the CLS, which as you say, rips out big hunks of the spectrum. But so does the L-eNhance, it's a dual bandpass filter, after all, albeit more precise. Seems like the much broader bandpass of the L-Pro would be better for imaging galaxies, no?

 

4/3 sensors like the 294 and 1.25 filters definitely don't go together. 

 

https://astronomy.to...ccd_filter_size

This confused me too because the calculator recommend a filter no smaller than 25mm, which is a quarter inch smaller than a 1.25" filter. I used that calculator before I bought the camera, in fact.


Edited by Tailspin45, 28 September 2021 - 05:44 PM.



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