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ASI1600MM-Pro Cooling, Dark Frames

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#1 MountainAir

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Posted 02 June 2020 - 04:28 PM

I have a strange issue with my ASI1600MM-Pro, though maybe it's imaginary.

 

I normally only cool to 0°, so I have some TEC capacity left when I'm at locations with a high overnight temperature.  Recently I decided to grab new darks at -15°C for those cooler nights, but I noticed this pattern in the stretched images (see attached, upper left).  Is this just a very slight gradient that the auto-stretching highlights out, is it "AMP" glow despite the cooling, or is this something else?  

 

I'm hoping it's not the sensor de-laminating from the backplane....

 

Notes:

  • All the software I use employs the ASI-prescribed cooling rate in the driver, plus I've rarely cooled below 0°C.  This makes me doubtful that this might be thermal shock.
  • I don't normally slowly warm the sensor, because it's usually only 20°C below ambient.  I hear arguments both ways, but again I doubt this is thermal shock.
  • Frames are being captured with the scope in the house, shaded, with 72°F temperature @42% humidity (according to UPBv2).
  • The scope cover is on, and I don't believe this is caused by light leakage.
  • If I increase the temp above 0, it seems to go be significantly reduced in intensity.
  • This doesn't seem to happen with my other cooled cameras (or is just too subtle to see).  However, my other cameras cannot get quite as cool as the ASI1600.
  • This pattern doesn't really match the AMP glow pattern noted here, but I'm not sure if there's a difference between the ASI1600 mentioned in the article and the ASI1600MM-Pro I have.

Any ideas?  Thanks in advance!

Attached Thumbnails

  • Dark.jpg

Edited by MountainAir, 02 June 2020 - 04:32 PM.


#2 barrabclaw

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Posted 02 June 2020 - 04:48 PM

My first thought was that it was light leakage. I know you said you don't think that's it. But, I saw some funny dark behavior with my setup once (same camera). I had the scope's cover on, but there was a little leakage either through the focuser drawtube or from behind the primary mirror where there is a cutout for a fan. When I put it in the basement at night, my darks looked good.

 

You say it is reduced at higher temperatures. But that could be the auto-stretch not stretching as much due to the increased noise in the dark image. So, the artifact in the corner appears to be less, but isn't really.

 

Just speculation and I could be wrong. But try getting the whole scope or even just the camera itself somewhere really dark. If you already tried this, ignore me.


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#3 klaussius

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Posted 02 June 2020 - 04:50 PM

That circular shape is way too reminiscent of vignetting to be a coincidence.

 

Are you absolutely sure it's not light leakage?

 

I discovered light leakage in my setup by inspecting heavily stretched darks just like your example image, only the pattern was different (brighter in the center). Maybe yours comes from the sides, maybe the filter wheel (assuming you're using one).

 

Try this: take a few darks the way you normally do, and then repeat the procedure after covering your whole setup in something that blocks stray light. If you can turn off lights and everything all the better.

 

Compare both darks, preferrably by subtracing one from the other and then stretching heavily. Even if you didn't perfectly block all light in the second run, the difference should show as a residual. Then you can repeat the procedure covering different parts of your setup selectively to try and find where the light is coming from.


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#4 MountainAir

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Posted 02 June 2020 - 05:01 PM

I dropped a blanket over the whole setup, and sure enough -- it changed.  Good call, folks.  I had considered this early on, but I've never had light leakage issues before, and I'm taking these darks in a fairly dark corner of the house out of direct light.

 

I'm checking over the whole setup with a fine-toothed comb (and a bright flashlight) to find out where the light is coming from...


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#5 MountainAir

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Posted 02 June 2020 - 05:33 PM

I was easily able to find the source of the light leakage:  The vents at the rear of the Celestron EdgeHD OTA.  Covering each vent with a dry washcloth while I took a 60-second sub seems to have almost entirely resolved the issue (the remainder is very faint and seems to match the amp glow in the ZWO document above).

 

The vents on the Edge HD are great (I actually wished they placed them at the front and back of the OTA, for better convective cooling), but now I know that I can't take darks without covering them.  But this raises more concerns:

  • This now seems like it may affect imaging when I am pointing the OTA at the sky, but the local street light is shining on the back of the OTA.  Honey, I need an observatory!
  • To a lesser extent this may also affect those of you with observatories, which may have low levels of interior lighting and/or lots of little LED power/status lights from other equipment.  Those conical & dome LEDs can be quite bright when viewed dead-on.
  • I wonder if this is a concern for those of us who use red light headlamps.  I frequently check my scope for cable snags, bugs, hour-angle position, etc.  It's probably a far more subtle effect than daylight or a flashlight, but I'll have to test this in the near future.

For now, I need to re-take all my darks... and explain to my wife why there's an OTA and RPi sitting in a dark bathroom!

 

Thank you both for the quick insight!


Edited by MountainAir, 02 June 2020 - 05:33 PM.


#6 Peregrinatum

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Posted 02 June 2020 - 05:40 PM

You can take darks with the camera by itself and the sensor cover on.  No worries about light leakage this way.

 

Thanks for mentioning the vents!  I hadn't thought of that, so when I take flats inside I will make sure to cover them up, although with flats it might not make much difference.


Edited by Peregrinatum, 02 June 2020 - 05:41 PM.

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#7 klaussius

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Posted 02 June 2020 - 05:47 PM

I was easily able to find the source of the light leakage:  The vents at the rear of the Celestron EdgeHD OTA.  Covering each vent with a dry washcloth while I took a 60-second sub seems to have almost entirely resolved the issue (the remainder is very faint and seems to match the amp glow in the ZWO document above).

 

The vents on the Edge HD are great (I actually wished they placed them at the front and back of the OTA, for better convective cooling), but now I know that I can't take darks without covering them.  But this raises more concerns:

  • This now seems like it may affect imaging when I am pointing the OTA at the sky, but the local street light is shining on the back of the OTA.  Honey, I need an observatory!
  • To a lesser extent this may also affect those of you with observatories, which may have low levels of interior lighting and/or lots of little LED power/status lights from other equipment.  Those conical & dome LEDs can be quite bright when viewed dead-on.
  • I wonder if this is a concern for those of us who use red light headlamps.  I frequently check my scope for cable snags, bugs, hour-angle position, etc.  It's probably a far more subtle effect than daylight or a flashlight, but I'll have to test this in the near future.

For now, I need to re-take all my darks... and explain to my wife why there's an OTA and RPi sitting in a dark bathroom!

 

Thank you both for the quick insight!

It will (affect imaging). It did in my case. In fact, I've been battling with that leakage for a long time without realizing, thinking it was moon-caused gradients or stuff like that. Nope. It was that leak doing funny things to all my calibration frames.

 

If you cover the vents, you'll be better off covering them during imaging as well. I built a cover that blocks light but not airflow for my OTA to fix a similar issue. Haven't had a chance to put it to the test yet, but I have high hopes. The difference it makes on darks is dramatic, and that same light leakage is sure to be there on each sub. Worse, it varies depending on where in the sky the OTA is pointing to, which means it doesn't fully calibrate out.



#8 Stelios

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Posted 02 June 2020 - 06:38 PM

Curious, why are you taking darks with your telescope when you have a cooled camera?
 

Fastest way is to use a fridge at home, put the cover on, and go to sleep. Come morning, all the darks are done at your desired temp (or temps) and you are set for a year or more.

 

Oops, just saw Peregrinatum beat me to it. Bears repeating anyway.



#9 Michael Covington

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Posted 02 June 2020 - 06:45 PM

Innnnnteresting!  I have been tracing down some unusual light leaks or something with my C8 EdgeHD.  I'm going to try covering the vents!


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#10 MountainAir

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Posted 02 June 2020 - 07:31 PM

You can take darks with the camera by itself and the sensor cover on.  No worries about light leakage this way.

 

 

Curious, why are you taking darks with your telescope when you have a cooled camera?
 

Fastest way is to use a fridge at home, put the cover on, and go to sleep. Come morning, all the darks are done at your desired temp (or temps) and you are set for a year or more.

The easiest answer is I didn't really expect it to be a problem with the lens cover on!

 

I know I can take my darks with the camera off the OTA and the sensor cover on, but every time I take my imaging train apart I get dust or metallic thread filings (gasp!) on something.  It's my curse, much like getting bike chain grease on my shorts minutes after retrieving them from the laundry.  That one infernal scratch I had on an old filter was from thread filings that wouldn't blow off.  Glad it was a cheapie.



#11 Michael Covington

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Posted 02 June 2020 - 08:52 PM

The other thing to watch out for is that your sensor may be able to see near-infrared light that passes right through black plastic lens covers.


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#12 cuivienor

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Posted 02 June 2020 - 09:08 PM

Ha! I was just investigating such light leaks in my setup: https://youtu.be/5ogIaGn67bQ

 

For me it was a newt, and light leaks came from the primary mirror and the focuser. Fixed now - I'm exactly the same, worried about how moonlight or LP or neighbor's lights (I'm in Tokyo) could affect the image. Now I have some peace of mind. Plus I don't like removing cameras from my equipment to take darks - because then I need to think about flats again. So I take my darks on cloudy nights, with my light leaks fixed, and my TG365 cover on.

 

Interesting on the vents! I had an EdgeHD800, and had never thought about checking those for light leaks! Good for you on finding the source!




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