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PI - Dark acting weird - help my elephant please

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#1 mewmartigan

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Posted 11 September 2019 - 01:22 PM

Hi All,

 

I have been having this problem for a bit and every time I think I make forward progress I end up with the same error. When integrating the images in Pixinsight (manual or batch preprocessing), whichever reference image I choose it immediately fails with

 

 *** Error: C:/Users/Windrich/Documents/Astrophotography/TEST CASE/registered/L_2019-09-09_21-38-40_180s__0C_c_d_r.xisf: Zero or insignificant signal detected (empty image?)
[2019-09-11 18:07:55] <* failed *>
[2019-09-11 18:07:55]
[2019-09-11 18:07:55] ************************************************************
[2019-09-11 18:07:55] * End integration of light frames
[2019-09-11 18:07:55] ************************************************************

 

 

I have tried various things and if you look at the picture in the link you will see 4 images, each one has an STF applied:

 

top left - a single 180s subframe of the elephant trunk calibrated only with the master flat.

top right - the same subframe of the elephant trunk calibrated with the master flat and master dark.

bottom left - a single 180s subframe of the triffid taken with all the same equipment 3 days prior and calibrated with the same master flat and dark.

Why is the master dark frame messing up the elephant trunk and not any previous images?

The bottom right is an STF of the master dark.

 

 https://photos.app.g...V5YCVxp612f5N47

 

I already checked the FITS data and stuff and both the triffid frames and elephant frames have the same details as far as camera temp/gain/offset/binning/etc.

 

I got a recommendation in another thread about a pedestal value of 800 in calibration. I tried that and while each individual subframe looked a bit better when stretched, the integration still failed with the same error.

 

Any help is appreciated as I don't want to waste 4 and half hours of good(?) data.  I have provided a Google Drive link to a zip file with the master flat, master dark, 10 subframes and 1 subframe of the triffid just for comparison. No bias frames as the master flat is calibrated with dark flats.

 

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

 

Thanks!

 

EDIT: link to original thread where I though redoing the calibration frames fixed it.     https://www.cloudyni...on-integration/


Edited by mewmartigan, 11 September 2019 - 01:28 PM.


#2 jdupton

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Posted 11 September 2019 - 01:52 PM

Marcus,

 

   I see why you are having a problem but don't immediately see where it came from.

 

   The problem is due to severe clipping of your light frames when you calibrate them with the Master Dark you uploaded. Note that the Master Dark frame has a median ADU of 1918.933 while the one sample of a light frame (L_2019_09_09_21_58_08_180s__0C) I looked at has a median ADU value of only 1868.0 according to the Statistics process. This will result in significant clipping of your image data after calibration.

 

   Can you upload a sample single dark frame that went into the Master Dark frame? It might give some clues as to what is going on. I suspect I know what may be happening but need to see a single dark frame in order to rule out gain, offset, or temperature mismatches first.

 

   The PixInsight error is pointing you to missing data in the calibrated frames. A little more investigation should turn up when and where that happened.

 

 

John


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#3 mewmartigan

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Posted 11 September 2019 - 01:57 PM

John,

 

Thank you for the help. Here is a link to a single 180s, 0C dark frame.

 

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

 

I guess the ADU of the triffid frame was is higher which is why the dark seemed to behave properly?

EDIT:  yeah, the triffid subframe is up near 6112 ADU.

 

Thanks!


Edited by mewmartigan, 11 September 2019 - 02:03 PM.


#4 jdupton

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Posted 11 September 2019 - 02:22 PM

Marcus,

 

   Regarding your Trifid image, yes, it has a median ADU value of 6112 so calibration with the same master dark (at 1918.933) will not clip much data.

 

   I see nothing out of the ordinary about the single dark frame you just uploaded. However, the median value is still higher than that of the Elephant Truck image.

 

   I do not know what happened during the Elephant Truck imaging session that resulted in such a low median ADU value for those images. I have a wild idea but have never seen such a large discrepancy before.

 

   Let me ask a really weird, silly question. Did you, by any chance, initially take any images of any kind (Bias, Flat, Dark, Platesolve, Focus, etc.) with the camera before turning on the TEC on the night you ran the Elephant Truck session? I do see that the TEC was running at 0° C for all the files you uploaded but were there any images downloaded from the camera prior to the TEC being turned on and reaching the set-point temperature?

 

 

John



#5 mewmartigan

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Posted 11 September 2019 - 02:53 PM

John,

 

I turn on the cooler first thing but I did take some 5 second platesove images for alignment and centering. At least one of them is labeled _8C so it was still cooling down to zero and had not reached setpoint yet when the image was taken. 

 

I am not going to like where this is going am I? tongue2.gif 



#6 jdupton

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Posted 11 September 2019 - 03:40 PM

Marcus,

 

   I don't think the first image at 8° heading for a 0° C set-point could account for what is showing up. Had the first image been at ambient at >20° C, I might see that effect but this is even more extreme than that. As a precaution in the future, be very careful to not take any image until the TEC has reached the set-point temperature.

 

   Given that the Master Dark Frame and Master Flat Frame both look reasonable, the problem is with the Light Frames. They are much too dark. Assuming APT or the ASCOM driver didn't mess up and use a lower than expected Offset value, I cannot think of anything that would account for the difference between the Trifid sub a few days before and this set of Elephant Truck subs at a much lower median ADU value.

 

   The only recovery I can suggest is to manually calibrate these subs with your existing Master Dark and Master Flat frames. When you set up the ImageCalibration process, enter a hefty Pedestal. The maximum value that PixInsight will allow you to add is 1000 ADU. I don't think that will be enough to prevent significant clipping but is worth a try. Try calibrating and integrating a subset of 25 subs and see if you get something usable. If so, do the whole session's worth.

 

   I am sort of out of ideas regarding how to recover the session data. You could try using and ImageContainer and PixelMath to add about 2500 to 3000 ADU to the light subs and then run through ImageCalibration without a Pedestal but I would expect that to really mess up the Flat Frame calibration. It would be possible to recover the data from the session but would involve quite a bit of manual manipulation of the data (light subs).

 

   I don't use APT, but if it creates session logs, it might be worth pouring through them and comparing to the session on the Trifid to see if anything jumps out as being different. Something odd happened during your Elephant Trunk session. If it were my data, I would look deeper for a cause for the very low median ADU in the light subs. Not knowing the cause, it could happen again.

 

   I always make it a point to look at the median ADU values of all the frame types at the beginning of a session. I take a sample Bias, Dark, Flat, and random light frame and check median ADU values. They should order themselves from lowest to highest as Bias -> Dark -> Light -> Flat. Since I also use Library Darks, I check against those also. If something doesn't look right, I will power down the TEC, disconnect the camera from the software, unplug the camera USB, and restart with the camera connection from scratch.

 

   Sorry I could not be of any help.

 

 

John


Edited by jdupton, 11 September 2019 - 03:44 PM.


#7 dhaval

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Posted 11 September 2019 - 03:58 PM

Correct me if I am wrong, but shouldn't the median value of darks for the ZWO1600 be close to 100-200 ADU (its been a while since I used the 1600 that I had, so I could be wrong)? But if that is where your darks ADU needs to be, I would imagine that there is some form of light leak in your darks and that will most definitely cause all sorts of issues with calibration - with this scenario being classic.

 

CS!



#8 jdupton

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Posted 11 September 2019 - 04:10 PM

Dhaval,

 

   Marcus uses an ASI294MC-Pro, the same camera I have.

 

   He is running Unity Gain = 120 and an Offset = 30. I run at Gain = 200, Offset = 30. Marcus's Master Dark Frame looks about right for this camera at 1918.933 and compares pretty well to what I see at the same gain and exposure. My Library Master Dark for Gain 120 Offset 30, and exposure of 180 seconds is 1919.960 so we are in close agreement. 

 

   It is only his light subs for this one session that are bafflingly low.

 

 

John


Edited by jdupton, 11 September 2019 - 04:33 PM.

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#9 mewmartigan

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Posted 11 September 2019 - 04:22 PM

John,

 

I can't thank you enough. At least I have an answer now since I have been wracking my brain for the past 2 days. If the data can't be recovered so be it, but at least I know to immediately check the ADU value of the first few frames. I have never had something like this happen before.

 

Since I thought the problem was calibration files, I took the new flats and darks today. So since those seem okay I guess it isn't anything wrong with the camera or cabling. I think APT has a log but you have to manually choose to export it, which I did not. The camera cable is USB3 all the way from camera to powered hub to computer. 

 

Definitely something too keep an eye on. Thank you again, at least I have an answer even if it means the data in those subframes is lost.



#10 dhaval

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Posted 12 September 2019 - 07:59 AM

Dhaval,

 

   Marcus uses an ASI294MC-Pro, the same camera I have.

 

   He is running Unity Gain = 120 and an Offset = 30. I run at Gain = 200, Offset = 30. Marcus's Master Dark Frame looks about right for this camera at 1918.933 and compares pretty well to what I see at the same gain and exposure. My Library Master Dark for Gain 120 Offset 30, and exposure of 180 seconds is 1919.960 so we are in close agreement. 

 

   It is only his light subs for this one session that are bafflingly low.

 

 

John

Thanks John. Not sure why I believed the OP was using a 1600...

 

In any case, I do believe that there has to be some form of light leak with regards to darks. I would like to know what process did the OP employ to take darks. I am guessing the CMOS does not have a mechanical shutter, in which case, the OP would have had to cover the camera, scope and everything else to ensure no light leak. 

 

CS! 



#11 pbkoden

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Posted 12 September 2019 - 11:21 AM

I've had this problem and found a solution that works for me. I found that using the dark optimization option, with the optimization threshold set to 1 will prevent this from happening.

 

I never looked at my darks vs. lights ADU, but will have to see if my data also shows that error. It DOES only seem to affect narrowband data though, which would point to the same cause.



#12 mewmartigan

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Posted 13 September 2019 - 06:36 AM

Thanks John. Not sure why I believed the OP was using a 1600...

 

In any case, I do believe that there has to be some form of light leak with regards to darks. I would like to know what process did the OP employ to take darks. I am guessing the CMOS does not have a mechanical shutter, in which case, the OP would have had to cover the camera, scope and everything else to ensure no light leak. 

 

CS! 

Dhaval, I took darks in my house. Set it up in a dark room and then tossed a blanket over the whole thing. I did not cover the cooling fan on the camera though. 
It sounds like my darks are right in line with John's though.

 

I've had this problem and found a solution that works for me. I found that using the dark optimization option, with the optimization threshold set to 1 will prevent this from happening.

 

I never looked at my darks vs. lights ADU, but will have to see if my data also shows that error. It DOES only seem to affect narrowband data though, which would point to the same cause.

Thanks, I will give this a try. I did a stack without the darks and it came out okay...probably not nearly as good as it should have been with proper ADU values.




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