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Really weird flats with ASI183MM

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#1 DrGomer

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Posted 16 May 2020 - 01:33 PM

Just got this camera and had first light last night.  Was processing my flats and noticed really weird checker pattern.

Gain111, 10 offset, -10C, 0.58sec, USB2 (shouldn't matter with ram buffer), ZWO usb cables, flat via LED light panel.

This shows up only in the flats, not the dark flats.  Never saw this with my ASI533.

Showing the a zoomed section of the stacked, calibrated flat because it shows the checker really clearly, but it is present in all filter channels, in the raw, uncalibrated frames. 
 

My first intuition is that these are discrete LEDs of the panel, but that doesn't make sense with the density of these and the panel appears to be an edge lit. Plus, with the telescope focused at infinity, even if discrete, they should not be this well resolved.

 

Thoughts?

Full frame showing preview area, and zoomed preview to show checker. 

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Edited by DrGomer, 16 May 2020 - 01:37 PM.


#2 DrGomer

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Posted 17 May 2020 - 04:02 PM

So, no one ever run into anything like this?



#3 stefannebula

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Posted 20 May 2020 - 10:05 PM

Hello, 

 

I observed the same thing with my ZWO ASI183MM-Pro at Gain150, 10 offset, -20C, 0.72sec, USB3 (see below)

 

I also thought it may have been my LED light panel but I saw the exact same thing when taking sky flats. 
I asked the same thing in this forum, I never got an explanation for the phenomenon but some other ASI183 users reported the same thing. 

It is apparently not an issue of the camera, and neither myself nor any other users have reported any impact on final calibrated images.

 

Sorry I have no explanation for the cause, I wish I did, but hopefully it's enough just knowing your camera isn't defective! 

 

Stefan

 

kPVn9O6.jpg



#4 imtl

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Posted 20 May 2020 - 10:10 PM

I don't have that with mine. You should maybe put a diffuser between the flat panel (or sky) and scope.


Edited by imtl, 20 May 2020 - 10:11 PM.


#5 stefannebula

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Posted 20 May 2020 - 10:20 PM

I don't have that with mine. You should maybe put a diffuser between the flat panel (or sky) and scope.

 

I used a t-shirt when taking shy flats and observed the exact same pattern. The pattern also remains the exact same when the camera is rotated and used with different filters, leading me to believe it is only produced by the sensor. Whether it is a fault in a batch of the ASI183's I'm unsure



#6 OldManSky

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Posted 20 May 2020 - 10:22 PM

So, no one ever run into anything like this?

Nope.  I have the same camera, flats look entirely as expected, with no pattern.  I do have a diffuser on the telescope, and use an iPad for my flat illumination source, but...yeah, never seen that.

Sorry.



#7 OldManSky

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Posted 20 May 2020 - 10:27 PM

If I can suggest something...can you lower the gain and do an exposure longer than 1 sec?

Just curious...



#8 stefannebula

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Posted 20 May 2020 - 10:29 PM

If I can suggest something...can you lower the gain and do an exposure longer than 1 sec?

Just curious...

Hi, would you mind providing one of your raw flat files so I can take a close look and compare with mine? smile.gif I also observe the same pattern


Edited by stefannebula, 20 May 2020 - 10:29 PM.


#9 imtl

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Posted 20 May 2020 - 11:26 PM

Do you see this in your master flat as well? How many flats do you stack per master?



#10 stefannebula

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Posted 20 May 2020 - 11:57 PM

I see it in my master flats even clearer. I stack at least 30 flats to make the master



#11 AtmosFearIC

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Posted 21 May 2020 - 12:10 AM

I’ve just recieved a QHY183M and notice the same pattern in my flats. I’ve so far only used it once and don’t some sky flats but it’s in both the Ha and OIII flats at 0.01s (taken in bright daylight). It doesn’t seem to have damaged the individual subs or caused any issues in the final integration.

#12 imtl

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Posted 21 May 2020 - 02:57 AM

Doing flats at 0.01s will show all kinds of weird artifacts. Flats should be a minimum of 0.2s with this camera I believe.


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#13 JukkaP

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Posted 21 May 2020 - 12:42 PM

I had the same pattern. The flats worked fairly well though. Did not see that pattern in my lights.

Something to do with short flat's. I wanted to use bias calibration to my flats so I took the flats with bright led panel.

#14 OldManSky

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Posted 21 May 2020 - 02:24 PM

Hi, would you mind providing one of your raw flat files so I can take a close look and compare with mine? smile.gif I also observe the same pattern

Sure.  Let me get one and find a place to put it...(I'll edit this when it's up).

 

Hopefully this works:

 

https://drive.google...IdpqEMSydTWjk5t

 

It's a flat for my HA filter.  RAW straight from the camera FITs file.


Edited by OldManSky, 21 May 2020 - 02:30 PM.


#15 kingjamez

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Posted 21 May 2020 - 07:21 PM

Any chance you are coming from a color camera? That pattern looks a lot like bayer artifacts. Like you are force displaying a bayer matrix over a camera with no bayer matrix.

#16 OldManSky

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Posted 21 May 2020 - 07:24 PM

Any chance you are coming from a color camera? That pattern looks a lot like bayer artifacts. Like you are force displaying a bayer matrix over a camera with no bayer matrix.

That was my first thought, too -- but it's an mm monochrome camera according to the OP.  


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#17 DrGomer

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Posted 21 May 2020 - 07:35 PM

So I could get the pattern to calibrate out now. Not sure why I couldn't earlier.  Anyways, I'm learning that the checker is PRNU, or pixel-response non-uniformity.  Basically meaning flats are required to remove this, in addition to dust motes and vignetting. 
And it's definitely a mono camera and I have my debayering off, but that'd be a good guess. 


Edited by DrGomer, 21 May 2020 - 07:35 PM.

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#18 kingjamez

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Posted 21 May 2020 - 07:35 PM

Yeah, I’m wondering if his software still thinks it’s a bayer matrix when it’s not.
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#19 DrGomer

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Posted 21 May 2020 - 07:47 PM

Not a debayering issue. it shows up in a simple file load of a raw fits file in pixinsight. Unless there is a setting I don't know about, PI doesn't autodebayer


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#20 kingjamez

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Posted 21 May 2020 - 07:50 PM

What software are you using for capture?

#21 DrGomer

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Posted 21 May 2020 - 08:06 PM

NINA.  It only saves the raw FIT files. debayering there is purely visual.  Edit, it also showed up in sharpcap. 


Edited by DrGomer, 21 May 2020 - 08:06 PM.


#22 stefannebula

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Posted 21 May 2020 - 11:17 PM

I acquire as raw FITS in SGP. SGP does not debayer even visually and the pattern is visible. Definitely not a debayering issue 




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