Help needed for SBIG fogging issues.

[[1ponders]]

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

[[1ponders]]

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. :mad2: not happy Jan.

[h0ughy]

paul at least I know my cooled DSLR is a sealed unit:whistle: - 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:shrug:? could always toss it:D??

[[1ponders]]

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.

[[1ponders]]

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.

[[1ponders]]

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.


:shrug:

[h0ughy]

http://en.wikipedia.org/wiki/Dew_point

[h0ughy]

http://www.natmus.dk/cons/tp/atmcalc/atmocalc.htm

[h0ughy]

and this http://www.decatur.de/javascript/dew/about.html

basically it also related to air pressure too

[gbeal]

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

[Robby]

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

[Dennis]

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

[[1ponders]]

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.

[jase]

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.

[[1ponders]]

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

[[1ponders]]

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 :eyepop:

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 :mad2: 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. :shrug: I know flats will take care of it but it is frustrating.

[gbeal]

Thats not a mote, thats a worm!!
Get a new ring, and get the vacuum cleaner out.

[[1ponders]]

Well that aint actually the mote Gary that is the pinched section of the O ring :lol: (I dunno, country kiwis :lol: :poke: ) 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

[Terry B]

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?

[[1ponders]]

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.

[[1ponders]]

Here is an image of what happens when I turn the camera off and let it get back to ambient and then turn it back on to -15C. Just a bit of frosting :rolleyes: and look at the size of that dust mote. Compare with the second one that had been brought down slowly and allowed to slowly sublimate over about 1/2 to 3/4 of an hour

[[1ponders]]

Emailed SBIG last night, received and email this morning, a new ring is on the way. Pretty good service, wot!

BTW those darker marks across the first image, they are there in exactly the same position each time the glass frosts up. they are clearly visible when I look at the chip when frosted. There must be some sort of contaminant on the inside of the glass that the frost collects on.

[Dennis]

Hah - call that an ST7 Flat??? This is a real Flat, circa 2003.:lol::lol:

Cheers

Dennis

[[1ponders]]

:eek:

How did you clean that?????

[Dennis]

Hi Paul

I made my own swab based on this DSLR cleaning how to (http://www.cleaningdigitalcameras.com/howto.html)and used PEC pads and Eclipse cleaning fluid. Worked a charm.

Cheers

Dennis

[[1ponders]]

I gather they were on the outside of the glass and not on the inside then? I'm wondering where mine is as it is so big. It must be a fair way from the chip surface.

I remember someone posting info somewhere here on how to work out where the dust is based on it's size but I can't find it now.

[[1ponders]]

I was interested in seeing how long it would take for a frosted chip to clear through sublimation so I did a little experiment and here are the results.

I left the camera off for half an hour to reach room temperature and then turned it on and set the temp to -15deg C. Naturally the chip frosted over pretty much immediately. I then left the camera at -15 and took a series of images until it was clear again. It took 40 min to sublimate clear which is handy to know. Anyway here is the animation.

[Dennis]

Hi Paul

To remove the dust motes, I removed the large, circular cover plate (including O-ring) and cleaned the glass surface on top of the ccd chip itself, as cleaning the glass window on the cover plate did not remove the marks.

Cheers

Dennis

[[1ponders]]

I'd say with the size of my spot that my dust mote is on the underside of the glass coverplate. I'll give it a bit of a clean once the new O ring turns up.

[[1ponders]]

Problem looks like it's fixed. :prey: I'll still have to recharge the desiccant plug again, but fingers crossed.

The first shot is 1 min @ 100% cooling immediately after it stabilized at -14°C. No frosting. There is only an outer ring of frosting present. The second shot is from 5 min later and the frost has retreated considerably in that time.

Well on the way to being a very happy chappy. :D Once the new seal gets here I'll bake the desiccant again, while that is happening I'll replace the seal and dust the inner surface of the faceplate glass window (might replace the seal at work in the airconditioning ;) ).

[gbeal]

Don't be a whimp Paul, like Dennis I have had the cover off the front on one of my SBIGs, it wasn't rocket science even for a Queenslander. As you say, great time to do it as well, when replacing the seal, just don't sneeze.

[[1ponders]]

I'll bet the hands were shakin' just a bit though Gary :lol:

I've had the cover off already to reseat the seal, but I must admit the decision to remove the face plate wasn't made lightly that's for sure with warranty and everything.

[gbeal]

Shakin'? Nope, a few red wines fixed that, LOL. Really though, it wasn't all that bad. I wouldn't put up with dust just becasue of the fear of removing it though.

[[1ponders]]

I won't have any fears using the fantastic Arctic Butterfly for dust removal. Works a treat on all my other cameras. ;)

[[1ponders]]

Ok problem solved (I hope permanently).

The first photo was taken on the night of the 22nd before I rebaked the desiccant. A straight drop from ambient to -14 deg in one go. You can clearly see frost around the extremities of the image. The second photo was taken this afternoon with the sweat running off me, very muggy. Ambient at about 25 deg. I dropped the camera temp straight down as far and as fast as it would go, down to just below -10 deg (mine must cool more than the stated 30 deg which is fortunate :) )

As you can see there is only the slightest trace of frosting in the corners (you may need to flick between the two images). There is no way I could have done this before. When the new seal comes from SBIG I'll just keep it in reserve. If this one is working it will be handy to have an unblemished spare ;)


Added an animation

[Dennis]

Glad to see your problem with the failing O ring will soon be fixed Paul.

Cheers

Dennis

[jase]

Ditto

Pleased to hear you're back on track Paul.

[[1ponders]]

Thanks guys. I've actually gotten a lot out of the experience. I certainly know my way around the CCDSoft Setup dialogu box now and feel much more comfortable with using the camera. Definately a case of "Every frosty cloud has a silver lining" :D