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Condition of ASI1600MM Dark Subs

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#1 BobinBend

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Posted 31 July 2021 - 11:19 AM

I need some help understanding how dark subs are suppose to look. I just bought an ASI1600MM Pro camera and connected it to my computer to take multiple Dark (60) subs at 180sec and 300sec exposures. The camera is NOT mounted on my SCT but just sitting on the table with a cap over the lens in a dark room. When stretched both exposures showed light areas at the corners, which surprised me. My old OSC Canon 60D dark subs seemed to be completely black even when stretched. 

 

When I calibrated the 180sec subs and stretched one of them, the image corners seemed to be even lighter. Is this normal? I'm pretty sure these darks were generated in complete darkness. I'm not that familiar with the ASI1600MM.

 

Attached is a dark sub and the same dark sub calibrated.

 

BTW - My Flat subs seem to have the same issue with lighted corners, but probably caused by my SCT optics and F6.3 reducer.

 

Thanks for any help on this issue.

Bob

 

  

Attached Thumbnails

  • darks2.jpeg


#2 dghent

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Posted 31 July 2021 - 11:28 AM

The 1600MM's Panasonic sensor has a distinctive amp glow pattern, and your 1600MM dark frame example exhibits this pattern. It's normal and looks like what I'd expect. Amp glow naturally gets more pronounced with longer exposures.



#3 BobinBend

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Posted 31 July 2021 - 12:03 PM

The 1600MM's Panasonic sensor has a distinctive amp glow pattern, and your 1600MM dark frame example exhibits this pattern. It's normal and looks like what I'd expect. Amp glow naturally gets more pronounced with longer exposures.

dghent,

Thanks for your input. It was very helpful. One more question; I've attached a flat sub (on the left) and a flat master (40 subs) on the right. Both are stretched to bring out the dark edges. Is this normal as well or should my calib/integrated master be one solid gray?

 

BTW Thanks for your contribution to NINA. What a great tool!!

Bob

Attached Thumbnails

  • dark5.jpeg


#4 dswtan

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Posted 31 July 2021 - 02:24 PM

Let me save our NINA developer the time in replying so he can focus on NINA for us. :-)

OP (BobinBend): yes, your flats are totally normal.

 

Think about it -- if they were completely even, that would indicate something went wrong, or you have the most perfect optical system in the universe (the former is more likely!).

 

If a flat is totally even, it means it would have no net effect on your images, so there would be no point using such flats. You *want* to have uneven flats (though a sequence of them should all be alike of course). The unevenness is showing you exactly the aberrations in your optical train that they are going to take out of your images. That's the point of them.


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#5 BobinBend

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Posted 31 July 2021 - 03:00 PM

Dswtan,

Your answer about my flats sound very logical to me. The reason for these questions was that someone in another thread viewed my integrated image of the Elephant Nebula and thought there was a problem in the calibration of my flats because of the light corners in my nebula image. But the problem may be with the calibration of my light subs. Would you agree with that assessment?

Thanks again for your help, I really appreciate it.

Bob



#6 idclimber

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Posted 31 July 2021 - 04:57 PM

Dswtan,

Your answer about my flats sound very logical to me. The reason for these questions was that someone in another thread viewed my integrated image of the Elephant Nebula and thought there was a problem in the calibration of my flats because of the light corners in my nebula image. But the problem may be with the calibration of my light subs. Would you agree with that assessment?

Thanks again for your help, I really appreciate it.

Bob

I am very familiar with the 1600mm. Both images look pretty normal to me. Did you also create dark flats? This sensor really is better with them instead of using bias and that flow. 



#7 BobinBend

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Posted 31 July 2021 - 05:07 PM

I am very familiar with the 1600mm. Both images look pretty normal to me. Did you also create dark flats? This sensor really is better with them instead of using bias and that flow. 

Thanks Dave. I never have used dark flats. Are they used in place of the Bias master?

 

Can you refer me to a thread or video link that talks about creating them?

 

Thanks, Bob



#8 idclimber

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Posted 31 July 2021 - 05:16 PM

The 1600mm does not like bias frames. They are inconsistent. Instead you create dark flats. 

 

Dark flats are exactly the same exposure as your flats. This is a ton easier if you also have a flat panel and keep the exposures consistent. It allows a library of these. Used 0.25, 0.5 and 1 second on my flats and dark flats.

 

The calibration is done differently even on the creation of the Master Darks. Here is the workflow in a diagram that I have borrowed from the DeepSkyStacker website. 

 

 

 

 

Attached Thumbnails

  • Calibration_Alternate2.jpg


#9 idclimber

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Posted 31 July 2021 - 05:22 PM

I do my calibration manually, I just have the process icons saved in PI to make it easier. 

 

Dark flats are just longer exposure bias frames that are matched in exposure to the Flats. If you notice you only stack the Darks without calibrating them into Master Darks

 

The Dark Flats are also stacked into a Master Dark Flat per your exposure. 

 

Then you calibrate your Flats with these Master Darks and Stack to create them 



#10 dswtan

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Posted 31 July 2021 - 05:24 PM

The reason for these questions was that someone in another thread viewed my integrated image of the Elephant Nebula and thought there was a problem in the calibration of my flats because of the light corners in my nebula image. But the problem may be with the calibration of my light subs. Would you agree with that assessment?

There's not enough information here to tell, but you should definitely follow Dave's advice and do Flat Darks/Dark Flats (same thing IMO, religious war on this) not Bias on the 1600, so try that next. Where's the Elephant post BTW?



#11 BobinBend

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Posted 31 July 2021 - 05:42 PM

Dave, I will take your suggestion and look into using Dark Flats. Thanks for your explanations and recommendations. 

 

Dswtan, Here's the image that started this discussion. It's a integrated Ha image (about 5hrs) on my 8" SCT and ASI1600MM Pro. Someone had mentioned that the corners of my image had not been calibrated correctly and this led to a discussion about my flats not being calibrated correctly.

 

I'm still not certain why my integrated Ha image has light corners. If my darks and flats look normal then does the problem stem from calibration of my lights?

 

Thanks, Bob

Attached Thumbnails

  • Elephant.jpeg

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#12 idclimber

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Posted 31 July 2021 - 05:53 PM

That is absolutely a calibration problem. Mostly likely in your flats. This workflow will fit this. I am in the middle of calibrating my last weeks worth of subs right now. When I am done I happy to email you a PI icon file of my calibration workflow with that camera. If you think that would help PM your email address. 

 

What is the duration of your flats? What gain and offset and temperature are you using? Calibration works best when they all match. 

 

 

Dave



#13 BobinBend

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Posted 31 July 2021 - 06:38 PM

Dave, That would be great!

 

My email is: bmimiaga@gmail.com

 

ASI1600MM Gain = 139, Offset =21   These are the same for all my NB filters.

 

My NB flat durations vary from 1.45sec to 5.2sec.  I used NINA's FlatWizard to create these flats using the sky light since I don't have a flat light. So my flat durations vary from night to night. Is that a problem?

 

I would love to receive your PI workflow Icon.  Thanks again for your help,

Bob



#14 idclimber

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Posted 31 July 2021 - 08:07 PM

Email sent with the process Icons. Earlier this year I switched to the newer ASI2600mm. It works with the bias frames quite well and as such I have modified my calibration to using those frames instead. 

 

The main advantage the bias workflow has is that the exposure time for the bias is either very short or zero. You don't have to match the exposure time. I am assuming you are doing sky flats?? How consistent are the times?

 

With Dark Flats you are supposed to use the same times. This is trivially easy with a flat panel that you can adjust brightness. This is a bit harder with sky flats as on my program (voyager) it varies the times on each exposure. I do not know how NINA does it. 

 

I think I would try doing a Master Dark Flat at intervals every 0.25 seconds. They are pretty easy to take then stack to created masters. Then all you would need to do is match them as close as you can to your flats. 



#15 BobinBend

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Posted 01 August 2021 - 08:37 PM

Hi Dave,

I was able to use your process workflow to create my DarkFlat Master. But I'm not sure where I introduce the DarkFlat Master. Is it used during the Lights calibration or Integration processes? Also I don't see a DarkFlat input in either the Image Calibration or Image Integration processes.

 

I'll post my results after I get your recommendations.

Thanks,

Bob



#16 BobinBend

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Posted 01 August 2021 - 08:43 PM

Dave,

One other thing. I noticed in your ASI1600MM Workflow that the Image Calibration process had the Enable CFA checked. Is this necessary for a Mono Camera?

Thanks,

Bob



#17 idclimber

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Posted 01 August 2021 - 09:13 PM

It helps if you use the diagram to follow the steps. you might also benefit looking at the Bias diagram on DeepSkyStacker website where I got the diagram I posted above. I would provide a link, but for some reason that does not work with their site. Bias is labeled Alternate 1 and dark flat is Alternate 2. 

 

The master dark flat (MDF) is simply an integration of the dark flat (DF)frames. I would suggest doing 20 for each exposure length that most closely matches your Flats. This MDF is simply subtracted from each of the Flats using the calibration routine. This removes both the bias and the dark noise present in that calibration frame. 

 

The easiest way to calibrate the flats is to drag those 20+ frames into the target frames window. Then you can see the bias is selected but blank. Just enter the MDF where you would enter the bias frame. Then select the output folder and execute. You will get copies of your flats that have the MDF removed that can be stacked in the next operation. That integration will give you your master flats for each filter. 

 

As I also pointed out you need to redo your Master Darks (MD) as well. They change from having bias removed before stacking to just stacking them without any calibration. We need the bias to remain in the MD so it is removed when we calibrate the light frames in the next step. 

 

I don't think the CFA will harm anything, it should be switched off though. 



#18 idclimber

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Posted 01 August 2021 - 09:22 PM

When you calibrate the light frames you enter your new MD in the normal spot for that calibration frame. You also do the same for the Master Flat. No Bias is selected when calibrating the lights. 

 

I also gave you an icon for using a pedestal of 100 on the calibration of the lights. This keeps any of the data going negative and is recommended. 



#19 BobinBend

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Posted 02 August 2021 - 06:37 PM

Hi Dave,

I finally had the chance to create my Dark Flats and recalibrated and integrated my Ha Flats and Light Ha Images. It turned out much better (see attached) with no vignetting. I tried several methods but what worked for me was substituting the Bias master with the Dark Flats master. Using WBPP I couldn't find a way to add a Pedestal value but the results were good anyway. 

Thanks for your help,

Bob

 

Attached Thumbnails

  • dark5.jpeg

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#20 BobinBend

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Posted 03 August 2021 - 12:55 AM

I just wanted to show my completed image of the Elephant Nebula IC1396. Thanks again to everyone who provided their help over the last week. I learned quite a lot!

Bob

Attached Thumbnails

  • Elephant_Nebula_IC1396_TN.jpeg

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