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Problems with calibration  

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(@tojuliin)
White Dwarf Customer
Joined: 10 months ago
Posts: 10
March 13, 2021 22:47  

Hello!

I'm totally beginner in astrophotography and I haven't experience of pre-processing tools. I have ASI294 MC (OAG & C8) and I tried to use calibration frames. I'm not sure do I have proper calibration frames (maybe not), but I wonder why I get very different results depending on light frames I use.

Raw light frame from NGC 3628:

1

And with calibration frames (ok result):

2

Raw light frame from M 44:

3

And with calibration frames (bad result):

4

I used totally same settings (gain, exposure time etc.) with NGC 3628 and M 44 photos and same calibration frames. I don't understand what happened. Can anybody help and tell how this works?


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(@wvreeven)
Quasar Admin
Joined: 3 years ago
Posts: 1226
March 14, 2021 01:48  

@tojuliin That image of M 44 indeed looks like there is an issue with the used calibration frames. Did you shoot all calibration frames and all lights in the same night? What calibration frames did you exactly use and what gain, offset, temperature and exposure time did you use?


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(@tojuliin)
White Dwarf Customer
Joined: 10 months ago
Posts: 10
March 14, 2021 09:51  

Yes, I took everything (light frames, dark frames, flat frames, dark flats, bias frames) in same night and about in same temperature (-7 celsius).

There were...

10 Darks (Exp-time 30s, Gain 121, Bin 2x2)
10 Flats (Exp-time 10s, Gain 121, Bin 2x2)
10 Dark flats (Exp-time 10s, Gain 121, Bin 2x2)
30 Bias (Exp-time 0.001s, Gain 121, Bin 2x2)
And Lighs (Exp-time 30s, Gain 121, Bin 2x2)

I have also re-opened Astro Pixel Processor and done everything from the zero position, but same results. Results differs lot depending on the light frame I use, but light frames are quite similar. I'm logical person and this is very strange for me, because light frames are similar. The most important difference in these examples I submitted was different histogram profile.

There were not over-illuminated stars in NGC 3628 image (now without DDP setting):

7

But there were over-illuminated stars in M 44 image (now without DDP setting):

6

Can this be a reason for this? I have very little understanding of process, how the program use example flat frames. Do this kind of things affect on results?

This post was modified 9 months ago 3 times by Tommi Liinalampi

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(@wvreeven)
Quasar Admin
Joined: 3 years ago
Posts: 1226
March 14, 2021 15:20  

@tojuliin Whether or not the lights have saturated pixels shouldn't affect how the calibration frames are applied. It is hard to say what causes the difference in the stack without being able to look at the calibration files themselves. What happens if you perform the integration without flats and dark flats but with darks and bias? And what happens if you perform the integration without darks but with dar flats, flats and bias?


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(@tojuliin)
White Dwarf Customer
Joined: 10 months ago
Posts: 10
March 14, 2021 21:43  

NGC3628: Result with dark and bias frames

9

NGC3628: And result with flat, dark flat ja bias frames

10

 

M44: Result with dark and bias frames

7

M44: And result with flat, dark flat ja bias frames

8

Can you make some conclusion from these?

If I am accurate bias + flat + dark flat frames were taken in -10° C and light frames + dark frames were taken in -7° C but is the system so sensitive that these kind of temperature differences will confuse the integration process? However, I managed to use calibration frames with NGC 3268 but not with M44 and these were taken in same temperature.

This post was modified 9 months ago by Tommi Liinalampi

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(@wvreeven)
Quasar Admin
Joined: 3 years ago
Posts: 1226
March 14, 2021 21:55  

@tojuliin The temperature difference should only affect the amount of dark noise which is very small between -7ºC and 10ºC. Would you mind uploading your files to

https://upload.astropixelprocessor.com/

using upload3 for both username and password? Please create a folder called tojuliin_flat_issue and put the files in there. Then we can have a look and try to help you discover where the issue lies.


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(@tojuliin)
White Dwarf Customer
Joined: 10 months ago
Posts: 10
March 15, 2021 16:23  

@wvreeven Now I uploaded the files. There are all master calibration files and one light frames per object in the folder. Please tell me, if you need more than these files. Thank you for the help!


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(@wvreeven)
Quasar Admin
Joined: 3 years ago
Posts: 1226
March 15, 2021 17:20  

@tojuliin Thanks very much. I’ll be able to have a look at them on Wednesday.


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(@wvreeven)
Quasar Admin
Joined: 3 years ago
Posts: 1226
March 17, 2021 01:31  

@tojuliin OK I had a look at your data. The histogram of the images of NGC 3628 and M 44 look very similar. In order to see the proper color image and the color histogram, go to tab 0 and select RGGB as pattern and make sure that force Bayer/X-Trans CFA is enabled. Then go to tab 1 and load the images.

Screenshot 2021 03 16 at 21.17.56
Screenshot 2021 03 16 at 21.18.15

As you can see, the green, red and blue channels have decreasing intensities which is to be expected with a one shot color camera like your ASI294 MC. However, the histogram and even the color of the (master) flat looks wrong:

Screenshot 2021 03 16 at 21.18.32

As you can see, the blue channel is brightest, then green and then red. I am not sure why this master flats works with the NGC 3628 image but I think that's coincidence. I would have expected a similar response to the master flat like the images of M 44.

What light source do you use for taking the flats? I suspect that you used a tablet or some similar light source with a lot of blue light. In any case, that's the reason why you get such a strange result on M 44 and I advice you to change the light source for your flats.


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(@tojuliin)
White Dwarf Customer
Joined: 10 months ago
Posts: 10
March 17, 2021 21:56  

@wvreeven

Many thanks for your analysis!

So if understood right the problem is the color of the master flat which isn't neutral or compatible with light frames. I took master flats using blue sky after the sunset but instead of white fabric (or t-shirt) I used white paper. Maybe that's why it was too blue.

But I have now the possible reason for strange results so it helps me to go forward. I already thought to buy a flatfield box so I do it now to help flat making process. Hope this helps!


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(@wvreeven)
Quasar Admin
Joined: 3 years ago
Posts: 1226
March 18, 2021 03:34  

@tojuliin One option is to point your telescope at a white wall (either inside or outside) or a white towel. When inside you can open or close doors and curtains to regulate the light intensity until you can take 1 to 5 sec flats. You'll still need to cover your telescope with a white t-shirt to ensure a diffuse enough light source.


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(@rawcoll)
Molecular Cloud Customer
Joined: 10 months ago
Posts: 3
March 18, 2021 16:48  

I have no personal experience with the ZWO ASI294MC-PRO, or expertise in this area, so sorry if what I want to add may be completely erroneous to your situation. I've been looking at various camera reviews myself, trying to decide which would be most suited for me, and I have come across quite a number of comments on Cloudy Nights about the difficulties of calibrating this camera. Eg https://www.cloudynights.com/topic/636301-asi294mc-calibration-%E2%80%93-testing-notes-thoughts-and-opinions/

The problem seems to particularly reveal itself with narrow band imaging, see https://www.cloudynights.com/topic/661685-word-of-warning-asi294mc-pro-and-opt-triad-and-nb/page-2

To be honest, I don't fully understand the issues, but if I understand the situation correctly, the cold finger of the cooler is a little smaller than the sensor, and this causes non-uniform temperature gradients in the sensor which result in substandard calibrations. Could the different temperatures at which you took the calibration files and lights be causing this? However, I can't quite understand why it only seems to affect the M44 image. Is it possible that the sensor temperature hadn't stabilized for one of the images? I hope that you get to the bottom of your problem.

Ian


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