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Old 23-03-2021, 12:43 PM
RugbyRene (Rene)
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Weird mottling on my images

Hi all,

Seeking some advice. I have an ASI294MC-Pro which has been showing some weird mottling in images. I've attached an example. This is a 2sec exposure taken at -20c. If you look closely you can see an area of mottling, what almost looks like really bad chrominance noise. I suspected it was dew forming on the sensor or protective glass so I followed the instructions on the ZWO website for cleaning the sensor and drying the desiccant tabs. I put it all back together, took the attached shot but it seems the dew or whatever it is still there. When the sensor is warm I don't see it.

Any ideas?
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Old 23-03-2021, 12:48 PM
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Atmos (Colin)
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Frost!
My ASI094 does the same thing but it passes after a while. I start cooling the camera really early on in the night and after an hour it’s all gone.
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Old 23-03-2021, 12:53 PM
RugbyRene (Rene)
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Quote:
Originally Posted by Atmos View Post
Frost!
My ASI094 does the same thing but it passes after a while. I start cooling the camera really early on in the night and after an hour itís all gone.
Yeah frost not dew. It never did it when the camera was new. Only in the last 12mths or so. I've left the camera running and am taking exposures every 10-15 mins. I can see it shrinking. Very annoying.
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Old 23-03-2021, 03:29 PM
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The_bluester (Paul)
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Yep, frost inside the chamber. My ASI294 developed it after about 12 months use.

I opened it up and dried the desiccant tablets and it cleared after about a week. I got another twelve months and it has started up again. This time I need new desiccant as two of the tablets crumbled when I tried to remove them. I redried the remaining pair and it is improved but not quite right. New ones in the post to me now.

I think it is inevitable, the seal is an O ring (Much more than that would not be practicable at a consumer price) and I have long wondered if cooling the cam would cause a low enough pressure in the chamber to draw air past the O ring.
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Old 23-03-2021, 08:04 PM
RugbyRene (Rene)
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Quote:
Originally Posted by The_bluester View Post
Yep, frost inside the chamber. My ASI294 developed it after about 12 months use.

I opened it up and dried the desiccant tablets and it cleared after about a week. I got another twelve months and it has started up again. This time I need new desiccant as two of the tablets crumbled when I tried to remove them. I redried the remaining pair and it is improved but not quite right. New ones in the post to me now.

I think it is inevitable, the seal is an O ring (Much more than that would not be practicable at a consumer price) and I have long wondered if cooling the cam would cause a low enough pressure in the chamber to draw air past the O ring.
What you described sounds like what mine has been doing. 12 months it worked fine, I could cool it down to -15 no worries, then it started to frost up. I dried the tablets and I can get to -5 with no frost. When I push it to -10 I start to see the frost build up.

Would putting a few desiccant pouches in a bag and wrapping the camera up help remove some of the moisture?
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Old 23-03-2021, 08:36 PM
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Nikolas (Nik)
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At the end of each session after the camera has been switched off a while I place the camera pouch with a desiccant bag over the camera while on the scope and store it like that. Haven had frosting issues.
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Old 24-03-2021, 06:57 AM
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The_bluester (Paul)
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Rene, give it a week, it took at least that long for the tablets to suck up the moisture in mine the first time I dried it.

I tend to leave my setup mounted with caps off (I aim manually at the ground and leave caps off to avoid trapping moisture against the objective) until all sign of moisture is gone before I pack it away, but I normally bring the scope and cam in complete. I don't remove the camera between sessions at home to avoid introducing more dust bunnies.

Last edited by The_bluester; 24-03-2021 at 12:43 PM.
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Old 24-03-2021, 10:44 AM
RugbyRene (Rene)
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Quote:
Originally Posted by Nikolas View Post
At the end of each session after the camera has been switched off a while I place the camera pouch with a desiccant bag over the camera while on the scope and store it like that. Haven had frosting issues.
Yeah I'm going to do that.
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