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Old 21-06-2011, 12:03 PM
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Paul Haese
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What does frosting look like?

Recently my QSI has taken to producing the following images. The first one is a large circular spot (in this case on a flat image). It then takes anywhere up to 60-80 images to reduce the spot to nothing (see the later image). Once gone it performs flawlessly all night. Is this frosting or something else? It looks like moisture to me but when I look into the sensor window I cannot see anything. Any thoughts?
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Old 21-06-2011, 12:29 PM
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lepton3 (Ivan)
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Paul,

Could it be moisture (condensation)?

I get a similar artifact on my Atik camera when I start the cooler, although it is at one corner, not in the middle. It seems to be worse in summer, and takes about 10 minutes to clear completely. As you say, it does not reappear all session after that.

I've considered storing the camera in a box with dessicant, but it hasn't been bad enough to make me do it.

-Ivan
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Old 21-06-2011, 12:47 PM
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Hi Paul

All the times I have suffered from frosting it has always appeared on the edges of the sensor. The resulting images all look like cockroaches have been eating away at the edges. In short, nothing like the pictures you've submitted here, perhaps that is only idiosyncratic of the SBIG design however. This effect never decreased either, the colder and longer the camera was on for the worse it got.
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Old 21-06-2011, 01:08 PM
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multiweb (Marc)
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It's a bit of dew on the glass. This happens if the camera goes down in temperature very quickly from ambient. Moist doesn't help of course but if you can crank the cooling in stages it won't happen. Frost or ice is usually on the sensor and creeps in from the side coming from the peltier. This happens if the sensor surfaces is dirty/contaminated with grease or finger prints, or wet from a previous session to start with.
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Old 21-06-2011, 05:40 PM
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Yep, dew/frost on the front glass. As Marc says, frost on the chip shows as neatly defined "fingers" etc on the edges first. One way is to turn the camera on without cooling and let the camera body warm up.
Or try a heated strap around the nose-piece.
Gary
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Old 21-06-2011, 09:13 PM
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Probably moisture freezing on a small dust mote as the camera initially cools. It then sublimes off as the cold finger begins to work. It may be time to remove the small panel and recharge the silica gel.
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Old 22-06-2011, 07:45 PM
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Very close Gary and Allan. Dew forming on the window on the outside as Kevin from QSI seemed to think, when he wrote this yesterday.

"What we've seen when the dew point is very close to the ambient temperature (high relative humidity) is that radiative cooling from the sensor can temporarily lower the temperature of the window below the dew point. The camera body is a big heat sink designed to gently warm the window with heat from the TECs. In a short amount of time, perhaps 30 minutes, the window will be warmed above ambient temperature and therefore the dew point."

His suggestion is to start cooling about 1/2 an hour or so before imaging and slightly have the set point higher for a bit then go down to the desired temp.

Dew at present is at its best (meaning full on) so this is the reason for the problem. So I don't need to do the desicant just yet as the argon gas should last another year or so yet.
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Old 22-06-2011, 07:58 PM
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I always try and get the QHY9 to about -1 to 1 degrees as soon as possible, then leave it there as long as I can while finalising set up, drift aligning, locating object etc. Once I'm getting closer with object framing, guidestar selection and tracking set, etc whack it all the way down to -25 or whatever the target temp is. I believe its good practice to warm up slowly too where possible. The only few times I've ever had any frosting issues I had usually had the camera open AND slammed it straight down to -25 without a pause. QSI guys are going to know best about your beast though of course.
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Old 22-06-2011, 08:16 PM
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philiphart (Phil Hart)
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i had something similar on my QHY9 before Theo installed the heater elements to warm the front window (now standard). mine was not as sharp edged but was in the centre of the window.
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Old 22-06-2011, 08:34 PM
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So at least it seems there is a solution. Must be more dew this year than in previous years. Not seen this before in the last two years. Having a heater might be useful to use the camera as soon as possible.
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Old 22-06-2011, 08:53 PM
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It sounds like the QSI is similar to the sbig in that the sensor and chamber window are embedded in the heatsink. Sbig suggested running the camera flat out at 100% cooling to warm the heatsink, then they came up with that add on heater element to warm it up.
Here's a foggy M83
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Old 22-06-2011, 09:01 PM
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Certainly the current heater solution on the QHY9 is great. I ran my camera without the outer window for the first 6 months while sourcing suitable adaptors for the filter wheel and imaging train (tight on the Newt so every mm counts), and only once or twice suffered any minor frosting even during nights with heavy dewfall. Currently doing the Peter 4059 and Gama "thing" and running with argon inside the camera shell. Hoping will save infrequent need to slight dust or frost marks from outside edges once or twice a year.
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Old 26-06-2011, 10:37 PM
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Kevin was right. I just started the cooling 35 minutes before I started imaging. No problem this time. No need for a heater.
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