Frosting on camera sensor window - Forums

I am facing issues with frosting on ASI294MC Pro camera sensor window. Have confirmed that it is indeed frosting by recreating it on flat frames where I cool it rapidly to -10deg C.
I live in an area of high humidity (>80% is normal in 4-5 months). In summer, the outside temperature is as high as 40 deg C, so there is no way the frosting can happen on outside window. I have admitted to be constrained to image at 1 deg C to avoid frosting for now.

I have tried all of the following methods, but it does not just go away.
- Opened the camera window to take out dessicant tablets, heat them in microwave and put them back in
- Blow the hot air of a hair dryer into the camera for 30 mins
- Cooling in slow steps of 10 deg to 5 deg to 0 deg to -5 deg. The moment I hit 0 or lower, the frost shows up no matter how slow i cool it.

Wanted to check on other forum member's experience to see if this is common with only the ASI294MC Pro, or I have a very defective model, or the humidity is constraining me and I have to accept the fact that I cannot image below 1 deg C?

Attaching the master output for flat frames (high and low rejection frmo pixinsight) that clearly shows frosting.

Well, that somehow indicates that the sensor chamber is poorly sealed, so whatever you do, you have a humid atmosphere there. (the same as outside). Try to make sure that it is properly sealed . Recirculate your tablets (or better get new ones) and try again. It can take hours before tablets would effectively dry out the chamber. I do not know whether 294 has built in heater, make sure that it is on . If there is no built in heater ZWO and some others make special tinny heaters that you could eventually use.

It is not the window, it is the sensor and the fault is entirely on ZWO for such a poorly designed chamber. Spare yourself frustration with the ineffective tablets be either new or recycled ( I tried both to no avail). The only way to al least temporarily improve is to use a very large, very tight rubber band around the chamber being sure to cover the flange between window and chamber once you brought the whole camera in a very dry enclosure and be left there so as to leave as little humidity inside the sensor chamber. Use new tablets. Seal the camera with the elastic band or alternative use electrical tape of the thick large variety, something that doesn't yield or relax when subject to hot to cold cycling. The scope here is to seal the chamber starting from a very dry situation and avoid letting any humidity in.

I used to have a 294mc pro as well that gave me issues if cooling it down to fast as well. I changed my desiccant tablets regularly, maybe 2 a year. In all honesty to me this don’t look like what I had experienced with mine, but maybe yours just looks different then mine, don’t know.
What I did that really helped me was add a dew heater around the body of the camera where the sensor is and cooled it down slowly. I always ran the external dew heater and never had problems after that. Even got to the point where I was only changing out tables 1 time a year, and btw I always bought new ones. I know you shouldn’t have to but I just fount it to work better for me. Also when you do change them put them in when they are warm, also found that to help quickly when you button up the camera.
ZWO does make an external heater unit that you tape on but I really didn’t like it and found my dew heater strap to work much better.

At this time, I am really frustrated with the frosting issue. Thinking of upgrading to a mono camera at the same time - but want to be careful of models that can give similar problems.

I will try to explore the dew heater option before thinking of an upgrade (or getting away from effect at a high cost!)

Will keep you posted of progress with other attempts to seal the chamber.

I use a 294mc, and I do get the occasional frozen dew at the beginning of a session - not at all different from what you are reporting.
But it's typically gone for me in 10-15 minutes, 30 top. I made a habit to start cooling the camera about one hour before I can start shooting.

I use a 294mc, and I do get the occasional frozen dew at the beginning of a session - not at all different from what you are reporting.
But it's typically gone for me in 10-15 minutes, 30 top. I made a habit to start cooling the camera about one hour before I can start shooting.

I will try doing this slower. I set my camera at 10 deg when setting the right. Takes about 8-10 mins for polar alignment during which I drop it to 5. Another 10 mins to plan for the session during which I drop to 1 or 0 deg. I have been doing in steps of 10 mins.

When you say "it is gone in 10-15 mins", can you please elaborate it? Do you slowly cool it to -5 or -10, leave it for some time till it frosts and Defrosts? Also, can you please tell more about the steps and time for each step - may be my cooling is not slow enough?

I had similar issues with this camera, when it was new, during about a year it worked fine, no problem, but from that moment, frosting was always there. New dessecant and recicling them …not a great improvement. I have to wait for 20 minutes or so, to get rid of it, but I don´t believe is a way a product should be. I will follow A. Taselli suggestion, I got a friend with a laboratory equipment for humidity extraction, so I will let the camera there for a long period and then seal the union with something to prevent humidity entering again. The other possibility is to get a new one, but not a ZWO.

I use a 294mc, and I do get the occasional frozen dew at the beginning of a session - not at all different from what you are reporting.
But it's typically gone for me in 10-15 minutes, 30 top. I made a habit to start cooling the camera about one hour before I can start shooting.

I will try doing this slower. I set my camera at 10 deg when setting the right. Takes about 8-10 mins for polar alignment during which I drop it to 5. Another 10 mins to plan for the session during which I drop to 1 or 0 deg. I have been doing in steps of 10 mins.

When you say "it is gone in 10-15 mins", can you please elaborate it? Do you slowly cool it to -5 or -10, leave it for some time till it frosts and Defrosts? Also, can you please tell more about the steps and time for each step - may be my cooling is not slow enough?

When I started cooling with my 294/071 I would start the 1st cooling to 10º then move to 5º and then from there I would move more slowly in steps of 1-2º until I was down to where I wanted to be.
Now that I have the 2600mc pro cameras I no longer worry to much about it. I run my heater pretty regularly and without issues now.
Some of the older cameras are more finicky thats for sure and from experience the 294 is one of those cameras. Once you get the quarks figured out it performs well.

I just disassembled the camera optics window to see where the frosting indeed happens. It is not on the outside or inside of glass window. The frosting is happening on the actual sensor itself. Wiping the glass window revealed no moisture, where as the frosting was visible on the sensor.

I have again dried the dessicant tablets and put them back, tightly screwing the optics window. Not very hopeful of results - I have a feeling the moisture seal is broken from the other side too (i havent tampered the other side at all) because this issue has been persistent ever since i got the camera a year ago.

I was hoping if the frosting happened on outside glass window, then I could get an anti-dew heater or dew strip installed on the camera. But this will evidently not serve the purpose.

I think I am destined to imge at 1-2 deg C only. Sub-zero temperatures and low noise is an illusion that I cannot chase likely.

I just disassembled the camera optics window to see where the frosting indeed happens. It is not on the outside or inside of glass window. The frosting is happening on the actual sensor itself. Wiping the glass window revealed no moisture, where as the frosting was visible on the sensor.

I have again dried the dessicant tablets and put them back, tightly screwing the optics window. Not very hopeful of results - I have a feeling the moisture seal is broken from the other side too (i havent tampered the other side at all) because this issue has been persistent ever since i got the camera a year ago.

I was hoping if the frosting happened on outside glass window, then I could get an anti-dew heater or dew strip installed on the camera. But this will evidently not serve the purpose.

I think I am destined to imge at 1-2 deg C only. Sub-zero temperatures and low noise is an illusion that I cannot chase likely.

Give an external dew heater around the body where the sensor is a try. Run it at full tilt and slowly cool down your camera in smaller incremental steps and see if that helps you.

The other option is this: https://agenaastro.com/zwo-anti-dew-heater-strip.html?srsltid=AfmBOoor7tckYAJHS3TeagIlbC6yzpQlvhwIb8mmykDZTeg-Uegwkm4s

but I didn’t like it. It works but just looks bad and I didn’t like the appearance of it.

When you say "it is gone in 10-15 mins", can you please elaborate it? Do you slowly cool it to -5 or -10, leave it for some time till it frosts and Defrosts? Also, can you please tell more about the steps and time for each step - may be my cooling is not slow enough?

I don't cool my camera particularly slowly; it gets from ambient temperature to 0C in about 10 minutes.
If I take flats at that point (as soon as it reaches 0C), in most scenarios (unless the weather is very dry) I do see the same kind of frost you are observing; it's also very evident in the histogram, showing up as several sparse spikes to the right of the typical "active" area.
If I keep taking occasional flats now (every 5 minutes or so), I can see the frost area getting visibly smaller and smaller; eventually, as I mentioned, it just disappears. I suppose that the heat generated by the sensor itself eventually takes care of it.
Hope this helps.

The ability of air to hold water is it's Humidity Capacity. Cold air can hold much less water than warm air. Air at the temperature inside my kitchen freezer at 18F (-8C) can hold 0.022 grams per cubic meter, whereas heated air at 160F (71C) can hold 2.1 grams per cubic meter: about 100 times more.

Freezers are frosty because water is released from the humid room temperature air that gets cooled. Your camera is acting just the same way.

Using your heat gun to warm the air and camera and then sealing it while it is warm risks trapping more, not less, water in the camera.

Hot air only feels dry by virtue of its higher humidity capacity. It is pulling moisture from the cooler air around it, including moisture from the surface of your skin, hence you sense it to be dryer but it is becoming wetter in the process, and it must release this water as it cools down inside your camera.

Try opening your camera and putting it in your freezer. Cool it down, and later remove it while quickly installing the just-microwaved dessicants and sealing it while still very cold.

I don't have your specific camera model but live in a humid locale. On my larger sensor cameras I add 2-3 extra 0.25 gram dessicant packs along with the factory tablets. Rapidly cooling the sensor can cause condensation on the sensor as you describe, even with the extra desiccants. Allowing the sensor to heat up and cool back down a couple times drives that moisture off the sensor and into the dessicants and it's good for the rest of the evening.

Try new tabs with the combination of opening the camera housing and putting it in a large ziploc bag overnight with some desiccant bags to absorb residual moisture, then assemble with new tabs.

When you say "it is gone in 10-15 mins", can you please elaborate it? Do you slowly cool it to -5 or -10, leave it for some time till it frosts and Defrosts? Also, can you please tell more about the steps and time for each step - may be my cooling is not slow enough?

I don't cool my camera particularly slowly; it gets from ambient temperature to 0C in about 10 minutes.
If I take flats at that point (as soon as it reaches 0C), in most scenarios (unless the weather is very dry) I do see the same kind of frost you are observing; it's also very evident in the histogram, showing up as several sparse spikes to the right of the typical "active" area.
If I keep taking occasional flats now (every 5 minutes or so), I can see the frost area getting visibly smaller and smaller; eventually, as I mentioned, it just disappears. I suppose that the heat generated by the sensor itself eventually takes care of it.
Hope this helps.

Tried this out - I set the camera at -10deg C and let it cool at its pace. Frosted/Dewed up on sensor window as expected around 0 deg. But when I left it sufficiently long, the size of the central blotch started reducing. It did really got reduced to about one-fourth its size after 45 minutes, but no more reduction hece after! I have left it running (been 60 minutes now) to see if it makes any difference in couple hours.

I will try attaching a dew heater strip around the sensor window, as well as placing the camera in a bag full of dehydrant and try again - will keep posted once I am able to buy and test it out. More than 60 minutes of time to remove the frost will be a bit too much, I could live with 20-30 mins!

As you've identified the root cause being you cooling it too fast, have you considered cooling it slowly? The 071MC will frost if you cool it too quickly as well. So cooling it slowly is the way to go. Set your rig(s) up a couple of hours before imaging. Try cooling it in 3 degree f steps with a pause of 90 seconds between steps. Also be sure the dew heater is always on as well. Cheers

Try to run a sequence of short expositions (1 or 2 seconds) all the time during the cooling cycle. that would keep the sensor worm and maybe it helps preventing instant freezing

Monty Chandler:
As you've identified the root cause being you cooling it too fast, have you considered cooling it slowly? The 071MC will frost if you cool it too quickly as well. So cooling it slowly is the way to go. Set your rig(s) up a couple of hours before imaging. Try cooling it in 3 degree f steps with a pause of 90 seconds between steps. Also be sure the dew heater is always on as well. Cheers

I've had that same problem with my 071 until finally cracking it open recently, and letting it dry out, then replacing the tabs. (and to remove a few pesky dust motes on the inside of the window that had to go lol)

It was a gradually getting worse issue, that finally had to be dealt with. I guess it all depends on the mileage of patience with handling a lengthy cool down sequence every night.

@Jim Lindelien comment here:
Allowing the sensor to heat up and cool back down a couple times drives that moisture off the sensor and into the dessicants and it's good for the rest of the evening.

Is exactly what I would do also in very bad instances. Once I got the camera down to the temp I wanted I would back it off until the frost/dew was visually off the sensor by looking at the individual subs (via SharpCap). Once it was gone I would start from that point again and then cool the sensor down in 1-2º increments until it was cooled to where I wanted it with know visible signs of moisture issues.
Yes I know this is a pain and time consuming but this is what I had to do with both my 294 & 071mc pros like I mentioned above.

I still stand by my by my original suggestion by replacing the desiccant tables with NEW ones and then running a dew heater around the body of the camera at the high setting where the sensor located. I think if you do that it will help you. It may not fix the whole problem but I think it will help significantly.

Good luck!

No success with trying to get the frost away. I have tried and exhausted all of the below:
1. Drying the dessicants and putting them back
2. Replacing the dessicant
3. Sealing the chamber in a non-humid and very dry room
4. Placing the camera in a bag of silica gel and dessicants for ~3-4 days at a stretch
5. Adding a ZWO dew heater strip - it seems like a coil that sticks on the metallic window of camera, but I dont feel it generates sufficient heat based on how it feels when I touch it.
6. Very gradual cooling in steps of 2 deg, and about 5 mins apart
7. Repeated cooling to -10 deg and turning off cooler

The size of the frost does decrease a bit, especially in #6 and &7 but the complete thing just doesnt go away. The frosts are big enough, and they arent like they could be removed if they show up in the flats as well. I dont think calibration with frosted flats will remove them!

So it is back to imaging at 2 deg C. Very disappointing!

Any other help or leads will be greatly appreciated! The camera is out of warranty, so replacing or repairing is not a question. It was actually the same way in the first year of purchase too - just that I was a bit busy and couldnt get to explore different techniques to try and get the frost away.

I think this is on ZWO. You have a product that is designed to be cooled down to -20C or whatever and it should be able to do that without frosting over despite environmental constraints. If you have to tackle this on your own, have you considered doing a dry nitrogen purge? You might need some help with that but if the problem is moisture trapped in the camber then purging with a dry gas for a certain period of time and then sealing the chamber should be almost a lifetime solution, if all your seals are good.

Just as an aside because someone did mention chamber design. I believe some older CCD cambers where designed with a cold finger. The idea was that if you couldn't seal the camber with dry air, you could at least provide a surface that was colder than the sensor where any moisture present would condense first, keeping the sensor and sensor window clear. I'm sure there's a whole school of design on this kind of problem.

Tony Gondola:
I think this is on ZWO. You have a product that is designed to be cooled down to -20C or whatever and it should be able to do that without frosting over despite environmental constraints. If you have to tackle this on your own, have you considered doing a dry nitrogen purge? You might need some help with that but if the problem is moisture trapped in the camber then purging with a dry gas for a certain period of time and then sealing the chamber should be almost a lifetime solution, if all your seals are good.

Just as an aside because someone did mention chamber design. I believe some older CCD cambers where designed with a cold finger. The idea was that if you couldn't seal the camber with dry air, you could at least provide a surface that was colder than the sensor where any moisture present would condense first, keeping the sensor and sensor window clear. I'm sure there's a whole school of design on this kind of problem.

You are absolutely right, it is on ZWO. I have a similar experience with this camera, 3 changes of dessicant pills and very little improvement. The last time I did try a dry enviroment, using a laboratory dry chamber with RH control, to less than 5% RH. The only thing I did not try was to change the O'ring, but it should not be this way. Maybe there is a leak somewhere else, I don't know. I have the possibility of doing even dry Ar purge but, if I add the time and cost...better to get a new camera of another brand...

Indeed, I have also given hopes at this time. Working with temperatures above frosting for now and planning to upgrade later when budgets and capabilities permit!

Hello, I am in Thailand. Temperature is 30-35°C and humidity is 60%-85% at night.

I have been using ASI294MC Pro and ASI294MM Pro for about 3 years. I have the same problem, no matter I change the desiccant or put it in the microwave, it doesn't work for me.

My method is, I use SharpCap to reduce the temperature from the ambient temperature to 0°C with a cooling rate of 1°C/minute. It works well for me. (After shooting for about 30 minutes, I lowered the temperature to -5°C.)

If I do this and there is still frosting on the sensor window, I will increase the temperature to 10°C and cooling it to 0°C again.

To be fair to ZWO they "kind of guarantee"a DeltaT of 35 degrees C which means that if your outside temperature (dry!) is more than that do not expect it can cool the thing to below zero. If the outside temperature is, say 25 degrees C @ 85% RH your dew point is 22.3 degrees C, so condensation will occur from any temperature below that. The main point to take away here is that as long as your dark library is up to date there isn't much to gain going below zero except that the thermal noise will go up (a bit) but in a gentle fashion (in fact gentler than some more modern sensors). Also, the dark current is pretty small so you don't loose much with exposures up to 300s (and depending on your targets, up to 600s).