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Old 06-01-2022, 02:48 PM
TBA (Shaun)
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My darks aren't..... dark.

Hi All,

I am just starting out with astrophotography. I have been using my ASI294MC Pro and have been creating a dark library. However, my dark frames are more of a medium shade of grey and not the black with a mottling of noise that I was expecting.

I was wanting to know if this was normal with an OSC camera, or should I be seeing an almost black image? All of the examples of dark frames that I have seen online show an almost black image with a smattering of electronic noise and the odd mis-behaving pixel.

I have been taking the darks at night, with my photo kit connected to the OTA, sitting in a ventilated (cool) room. I am taking 100 darks for several times (30, 60, 90, 120, 180 seconds), with all of them running on the cooled camera at -10 deg C and a gain setting of 120. I am able to eliminate all light from the room, so there is no light leakage. I am using an ASIAIR Pro to take the pics and have set up a Autorun task to automate the process.

I've attached two pics - one is 60 seconds and the other is 90 seconds (the one with brighter amp glow is the 90 second one).

Am I doing something wrong, or do these look normal?

Thanks in advance for any assistance.
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  #2  
Old 06-01-2022, 03:49 PM
JA
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On the ZWO webpage for the 294MC there are some reference dark frames that you could consider comparisons with. The link to the dark frames (3 TIFF files) is about half way down the page in the Dark Current section ......

https://astronomy-imaging-camera.com...94mc-pro-color

I had a look at them and they are very different to yours. Their image titles suggest that they are taken at 100s and 300s at UG (Unity Gain) with one in HDR, whereas yours are at gain 120.

As a starting point, why don't you try to see if you can get the same results at Unity Gain with your camera and then probe further. Unless of course someone here has some higher gain darks you could compare your darks to.

Best
JA
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Old 06-01-2022, 04:23 PM
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Quote:
Originally Posted by JA View Post
On the ZWO webpage for the 294MC there are some reference dark frames that you could consider comparisons with. The link to the dark frames (3 TIFF files) is about half way down the page in the Dark Current section ......

https://astronomy-imaging-camera.com...94mc-pro-color

I had a look at them and they are very different to yours. Their image titles suggest that they are taken at 100s and 300s at UG (Unity Gain) with one in HDR, whereas yours are at gain 120.

As a starting point, why don't you try to see if you can get the same results at Unity Gain with your camera and then probe further. Unless of course someone here has some higher gain darks you could compare your darks to.

Best
JA
Unity gain for this camera is at 120
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Old 06-01-2022, 05:18 PM
JA
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Unity gain for this camera is at 120
And then there's that ....

Best
JA
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  #5  
Old 06-01-2022, 07:29 PM
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Hi Shaun.

Are the images you posted the jpegs off your ASIair ? If so then itís not unusual. The Ďair stretches the image to within an inch of its life when you take darks. If that is the case then the FITS will be fine to use. Iíve experienced the same thing.

Ryan
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Old 06-01-2022, 10:01 PM
TBA (Shaun)
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Hi Shaun.

Are the images you posted the jpegs off your ASIair ? If so then itís not unusual. The Ďair stretches the image to within an inch of its life when you take darks. If that is the case then the FITS will be fine to use. Iíve experienced the same thing.

Ryan
Hi Ryan,

Thanks for this - that was the answer I was hoping for I had seen the reference files that JA mentioned and I was worried that I had some sort of sensor issue.

I did notice that the histogram kept changing back to Auto resulting in the lighter frames and that when I pressed Reset, it would flick back to the black dark frame I was expecting - but then I also noticed that the thumbnails and JPEG exports to iPad from ASIAIR were still the stretched versions.... which had me worried again.

Great to hear you have encountered this same behavior and its something I don't need to worry about.... phew.

Thanks JA and Nikolas for helping also

Have a good one,

Shaun
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Old 06-01-2022, 10:29 PM
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+1. They shouldn't be dark. That's why we use them. And that's what darks look like when stretched

But just check that the dark looks like the one on ZWOs website that JA pointed out. I'm surprised about the amp glow though. I thought these cameras had circuitry to reduce that - maybe I'm wrong?
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Old 06-01-2022, 11:47 PM
TBA (Shaun)
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Originally Posted by ChrisV View Post
+1. They shouldn't be dark. That's why we use them. And that's what darks look like when stretched

But just check that the dark looks like the one on ZWOs website that JA pointed out. I'm surprised about the amp glow though. I thought these cameras had circuitry to reduce that - maybe I'm wrong?
Thanks for confirming that Chris.
I had seen those reference frames on the ZWO site and that's what freaked me out a bit - they are pitch black. As JA pointed out, very different to mine, but I didn't realize at the time that the ASIAir app was automatically stretching the images.

The amp glow is well documented for that camera - I knew about it before purchasing with the understanding that the artifact could be easily removed with dark frames (which is why I was getting nervous thinking that my dark frames were faulty).

Anyway, I think I am all good now - thanks again
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Old 07-01-2022, 10:02 PM
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They look like pretty typical darks for the ASI294. I had one before I went to an ASI2600 and the darks form it looked the same as that, and the ZWO web page for the ASI294 showed the same. A master dark made from a good number of darks should subtract out the starburst from light frames nicely.

This is a 300 second master dark from my ASI294.
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Old 07-01-2022, 11:51 PM
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Hi All,

.
.

I have been taking the darks at night, with my photo kit connected to the OTA, sitting in a ventilated (cool) room. I am taking 100 darks for several times (30, 60, 90, 120, 180 seconds), with all of them running on the cooled camera at -10 deg C and a gain setting of 120. I am able to eliminate all light from the room, so there is no light leakage. I am using an ASIAIR Pro to take the pics and have set up a Autorun task to automate the process.

.
.
I probably should not ask the obvious, but you don't mention if the lens cap is on...
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Old 08-01-2022, 06:41 PM
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That's a nasty artefact in the dark. Hope its consistent, otherwise would be hard to compensate. I had a look at the dark on the zwo website and its flat over the whole image when highly stretched.

Quote:
Originally Posted by The_bluester View Post
They look like pretty typical darks for the ASI294. I had one before I went to an ASI2600 and the darks form it looked the same as that, and the ZWO web page for the ASI294 showed the same. A master dark made from a good number of darks should subtract out the starburst from light frames nicely.

This is a 300 second master dark from my ASI294.
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Old 08-01-2022, 06:41 PM
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He, he!

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I probably should not ask the obvious, but you don't mention if the lens cap is on...
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Old 08-01-2022, 09:36 PM
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That's a nasty artefact in the dark. Hope its consistent, otherwise would be hard to compensate. I had a look at the dark on the zwo website and its flat over the whole image when highly stretched.
It was not an artefact, just a characteristic of the IMX294 sensor. Where are you finding darks on the ZWO website, I had a search but did not turn them up. They used to be on the ASI294MC pro product page but the seem to have removed them, the one on the page for the 294MC Pro looked just like mine (Mine is smoother as I think the one on the page was a single frame but I made my masters out of at least 50 darks)

I am pretty sure I saw an identical looking flat on the 294MM page when they were first released too. It calibrated out nicely on my 294 so I was not bothered by it.
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Old 08-01-2022, 09:49 PM
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Sorry, artefact is a bad word. I did mean the amp glow characteristics of that camera

Quote:
Originally Posted by The_bluester View Post
It was not an artefact, just a characteristic of the IMX294 sensor. Where are you finding darks on the ZWO website, I had a search but did not turn them up. They used to be on the ASI294MC pro product page but the seem to have removed them, the one on the page for the 294MC Pro looked just like mine (Mine is smoother as I think the one on the page was a single frame but I made my masters out of at least 50 darks)

I am pretty sure I saw an identical looking flat on the 294MM page when they were first released too. It calibrated out nicely on my 294 so I was not bothered by it.
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Old 09-01-2022, 04:16 AM
TBA (Shaun)
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Originally Posted by The_bluester View Post
They look like pretty typical darks for the ASI294. I had one before I went to an ASI2600 and the darks form it looked the same as that, and the ZWO web page for the ASI294 showed the same. A master dark made from a good number of darks should subtract out the starburst from light frames nicely.

This is a 300 second master dark from my ASI294.
Hi Paul,

Thanks for that info and including your master dark - doing my 300s tonight and looking much the same.
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Old 09-01-2022, 04:26 AM
TBA (Shaun)
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I probably should not ask the obvious, but you don't mention if the lens cap is on...
Hi Zuts, yes lens cap on (though as mentioned, the room is completely dark).

I actually did some test shots with the lights on, and even with the lens cap on, I could see a streak across the frame, so I knew a little light was getting into the image train somehow (Iím guessing via OTA). Thatís why I ensured I took the images at night in a completely dark room (I even covered all the power LEDs)
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Old 18-01-2022, 10:27 AM
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Hi Shaun,

I just recently created a dark library with my newly purchased asi294mc. I also have amp glow and greyish background when stretched - completely normal. My darks were taken in pitch black conditions with lens cap on and look similar to yours. Individual darks have those random horizontal lines, but these should disappear when integrating a large number of darks (I use 50).

Below is an integration of fifty 5-minute darks at 121 gain and 30 offset at -10 degrees (I use 121 gain rather than 120 as some people have complained that the high dynamic range doesn't always kick in at 120 - not sure if that is true, but 121 is the safest option.) You can see that the horizontal bands should completely disappear over a large integration, but the amp glow and much of the noise should remain.

Regards,
Stťphane
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Old 20-01-2022, 12:26 PM
TBA (Shaun)
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Hi Stephane,

Thanks for that info and the example. Thanks also for tip on having gain at 121 - I have not heard that before but will read up on it now.
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Old 20-01-2022, 12:52 PM
Stephane
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No problem. Some interesting discussions on gain 121 below. Iím personally not convinced there is any benefit, but definitely no harm.

Regards,
Stťphane

https://www.cloudynights.com/topic/7...1-vs-120-gain/
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