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Old 17-10-2007, 09:42 PM
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[1ponders] (Paul)
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Help needed for SBIG fogging issues.

I've had some problems with my new ST-7XME fogging up. This has been a problem from the first time I used it. The suggestion put to me on the forum here was that my desiccant needed recharging. I felt that this was strange as this is a brand new camera and I would have thought that they would have come out with a charged desicant plug.

To cut this short, I charged the plug tonight, 4 hours at 180 C, reinstalled the plug, waited an hour and a half for it to soak up any humidity in the chamber and cranked up the camera. I set the temp to -15 and let it sit for half an hour. Sure enough the glass screen fogged over.

How long should I need to wait to expect the desiccant to work. The manual says "Expect the camera to take an hour or two to reach the frost free state". They don't actually say if the camera needs to be running or not, though I am assuming that the desiccant is like Silica Gel and just absorbs moisture from the enclosed environment.


signed
Frustr8ed
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Old 17-10-2007, 09:45 PM
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[1ponders] (Paul)
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Ok while I was typing the above I thought I'd see what would happen after leaving the camera unconnected but with the fan running for the last 15 min.

Initial temp 26 at camera connection, screen crystal clear. Temp drops down to 4, check screen....fogged. not happy Jan.
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Old 17-10-2007, 10:06 PM
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h0ughy (David)
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paul at least I know my cooled DSLR is a sealed unit - any moisture is external and the on board heater takes care of dewing. how much humidity do you have, maybe there is a theoretical and a practical limit for the unit humidity wise? could always toss it??
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Old 17-10-2007, 10:15 PM
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[1ponders] (Paul)
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4 degrees C just doesn't cut it with me Dave. I could accept it at -20 or so, but when it starts to fog up before fog would normally appear outside my bedroom window I start to get stroppy.
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Old 17-10-2007, 10:17 PM
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[1ponders] (Paul)
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It has been running for more than half an hour now and the imaging glass is just a grey smear across the chip. I can see the chip but now I know what they mean when they ask you if you want frosted glass for the shower screen in the bathroom.

BTW the guide chip is as clear as a bell.
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Old 17-10-2007, 10:33 PM
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Center of the chip is just starting to defrost at 5 after dropping it to 0 first for a few minutes.

Update: @ 10 there is a circular spot about 2/3 the size of the chip clear now. It is quite strange, the frost clears in a perfect circle but not quite centered on the chip.

Update 2: current ambients
Temp: 19.3
Dew point: 13.3
RH: 65%
Camera temp: 15 Chip itself is clear but a ring of frost can still be seen on the glass window.



Last edited by [1ponders]; 17-10-2007 at 10:48 PM.
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Old 17-10-2007, 10:53 PM
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Quote:
Originally Posted by [1ponders] View Post
4 degrees C just doesn't cut it with me Dave. I could accept it at -20 or so, but when it starts to fog up before fog would normally appear outside my bedroom window I start to get stroppy.
http://en.wikipedia.org/wiki/Dew_point
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Old 17-10-2007, 10:55 PM
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http://www.natmus.dk/cons/tp/atmcalc/atmocalc.htm
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Old 17-10-2007, 10:59 PM
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and this http://www.decatur.de/javascript/dew/about.html

basically it also related to air pressure too
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  #10  
Old 18-10-2007, 05:08 AM
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I had this problem Paul. Baked mine (with the scones like I said) and slipped it back in. Fogged again. Give it a day or so to "soak" up the moisture. Mine cleared after about a day I think. I also don't think the camera needs to be "running" for the dessicant to work?
Gary
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Old 18-10-2007, 05:14 AM
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No, the dessicant is a passive device. The camera does not need to be running. Give it a day, then do another bake, as if for some reason there is a lot of moisture in then the desicant could need 2 or 3 cycles to suck it all up.
Cheers
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Old 18-10-2007, 05:31 AM
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Hi Paul

After a desiccant bake out, my ST7 sometimes starts off with a fogged image which then gradually clears from the centre over say, 30 to 60 minutes.

Did you re-fit the rubber O-ring?

Cheers

Dennis
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Old 18-10-2007, 05:56 AM
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Thanks guys. I'll give it 24 hours and then try again. If still the same problem I'll rebake.

Yep I put the rubber O ring bake in Dennis, and it isn't pinched. I even swapped it over for the spare one to see if that made any difference.
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Old 18-10-2007, 07:09 AM
jase (Jason)
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This is really surprising for a brand new camera to frost so early. This usually occurs after a year or two. I have not needed to recharge the desiccant on the STL11k yet and it sits out in the observatory attached to the telescope 24/7 so it is subjected to varying temperatures. The fan is always on, but not actively cooled until I wish to use it.

I have heard of the ST series have problems like this, though few and far between. It was something to do with the sealing of the CCD chamber. I understand that your location is probably quite humid, however I still wouldn't consider this normal operation. As the others have said, recharge the dessicant again if you still don't have much luck. You should insert the plug no later than 20-30 mins after baking. Failure to do so can result in drawing too much moisture from the air. If you've still got problems after that, I'd be contacting SBIG. There isn't a camera manufacturer in the astro industry with better customer service than these guys.
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Old 18-10-2007, 09:42 AM
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Thanks Jase, I'll give the desiccant another cook tonight and if that doesn't work I'll contact SBIG. I've just joined the Yahoo group so I'll post there when accepted and see what they say.

Cheers
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Old 19-10-2007, 06:13 PM
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OK I think I've solved the problem. After two bakeouts it was still frosting. On the SBIG yahoo group it was suggested to check the faceplate screws. So it did and they were fine. Then I remembered what you wrote Jase about problems with the sealing of the CCD and I remembered that there was another O ring (big one) under the face plate. So I removed the face plate and the O ring fell out with two big creases in it

When the plate had been put on originally the ring must have been dislodged and part of it was protruding inside the housing so when the plate went on it pinched the ring in two spots where it crossed the O ring groove. That meant I had about a 2 cm gap where there was no O ring sealing the chamber I've reseated the O ring but only time will tell if I need a new one. I will be contacting SBIG though.

The bugger is now that I have a huge big mote on the inside of the glass cover that I didn't notice the first 2 times time I used the camera. I'm wondering if it may have been sucked in during the drawing in of outside air. I know flats will take care of it but it is frustrating.
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Old 19-10-2007, 07:07 PM
gbeal
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Thats not a mote, thats a worm!!
Get a new ring, and get the vacuum cleaner out.
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  #18  
Old 19-10-2007, 07:47 PM
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Well that aint actually the mote Gary that is the pinched section of the O ring (I dunno, country kiwis ) If you look closely you can see the two pinched sections and how the sectionn between doesn't follow the line of the ring. The section between was protruding inside the chamber.

Here is an example of the frosting after it has started to sublimate and you can see the dust mote near the center top of the image
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Last edited by [1ponders]; 20-10-2007 at 07:28 AM.
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Old 19-10-2007, 07:50 PM
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My home made genesis CCD isn't perfectly sealed and after about 2 months I need to recharge my desiccant. It is not the same as the SBIG but a similar scenario. After heating the desicant and resealing the camera I leave it for about an hour and can then turn it on with no frosting. The SBIG desiccant cannister is smaller than my sachets but I still think that it should take at the most only 2 hours to dry out enough now that you have fixed the leak (hopefully).
I can speed up the process if I purge the camera with dry gas. I use to use evaporating liquid nitrogen but don't have access to that anymore so I have to wait for the hour. Maybe this is an option for you?
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Old 19-10-2007, 07:57 PM
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[1ponders] (Paul)
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I'd love to be able to seal the chamber with nitrogen but unfortunately not on option for me. Hopefully the pinching won't interfer with the seal to much. I've emailed SBIG anyway with images of the seal so with a bit of luck they will send one out my way and I'll have no more problems. I do think though that I'll need to bake the desiccant again seeing as the chamber was being "re-moisturized" everytime I turned it on.
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