Fog on the CCD? Desiccant Plug ?

[rogerg (Roger)]

G'day all,

See attached image. Perhaps someone with a CCD camera (specifically SBIG perhaps) could confirm for me if this is fogging on the CCD glass screen requiring the Desiccant Plug to be "recharged" ?

It was only less than a year ago that I re-charded the plug and find it strange that it's already needing it again after only occasionally being below -10. Also when looking at the chip visually I can't see the fog, but that doesn't surprise me so much as it must be very light.

Thought I'd check before I bother doing the recharge.

The pattern of "fog" changes between exposures slightly.

(I'm talking aboutthe fog effect - not M95 sitting in the middle, I know what it is )

Roger.

[Dennis]

Hi Roger

I have an SBIG ST7E. In terms of condensation, when I have seen it on my system, it starts off as a general mist over the entire frame when I first switch on the temperature cooling. Then as the cooling takes effect, it begins to clear radially from the centre, like a ring or doughnut gradually expanding as the central hole expands and the rim spread to the corners, before finally disappearing after say, 20 or 30 minutes.

Baking the desiccant plug in the oven has always fixed this in my case.

I have also had something similar to your image after cleaning the glass window with 99% pure iso propyl alcohol. After re-cleaning with distilled water, that effect disappeared.

Good luck!

Dennis

[astroboy]

It looks like it to me .
Its easier to see if doing flat fields , little blotches everywhere ,also as you take exposures when the camera is warmer +0 ( flats ) everything looks fine and as the camera cools the problem starts.
Took me ages to work out this problem because Sbig think you should be able to see the fog on the CCD , I sure can't and they don't have a pic of how the images look with the problem.
Some people ( like me ) recharge every 6 months.

Zane

[Roger Davis]

Temperature doesn't create the fogging, humidity does. So it depends on how you store the camera. Put a few dessicant gel baggies in with it and it should help. Dessicant bags should be "de-humidified" every couple of months to maintain efficacy.

[rogerg (Roger)]

Originally Posted by Dennis

Hi Roger

...Then as the cooling takes effect, it begins to clear radially from the centre, like a ring or doughnut gradually expanding as the central hole expands and the rim spread to the corners, before finally disappearing after say, 20 or 30 minutes.

Dennis

This is exactly what I see - it clears after 20-30min as you said, in that radial process. It's great to have confirmation that this is exactly what I'm seeing, thanks.

Oh, and Roger (Davis) - thanks for the note on humidity, I hadn't realised that, thinking instead it was related to how much you "push" the cooling. Storing it with desiccant bags is something I hadn't thought of, any idea where you buy them from?

Roger.

[Roger Davis]

I have a box of the things at home, but you can get them from supermarkets. They sell them to place in the freezer with your meat to lower the humidity during storage.
Failing that a trip to your local photo shop may elicit some that normally get thrown out with the garbage.
You can also try the chemists as they place them into medicine bottles for the same reason.
Ohh and just use a good quality zip lock bag.

[astroboy]

Hi Roger and Roger
I don't get it , isn't the Sbig camera sealed .
Also I thought the colder something gets the more condenation you get.
maybe we get different problems here as I often cool the camera to -30 or more.

Zane

[Roger Davis]

Originally Posted by astroboy

Hi Roger and Roger
I don't get it , isn't the Sbig camera sealed .
Also I thought the colder something gets the more condenation you get.
maybe we get different problems here as I often cool the camera to -30 or more.

Zane

No, the only way you get condensation is if there is moisture in the system. Dry air means no condensation. That's why there are binoculars that are purged and filled with Nitrogen or Argon. They do not fog up. Mind you this is a bit of a gimmick as the atoms of those respective gases are so small that they will egress past most seals over the course of a year or two.
The SBIGs would have to be hermetically sealed in a dry air environment. If they fog up then the seals ain't workin'. So storing these cameras in a dry environment where a dessicant can keep them as dry as possible will help. Try a Tupperware container, they keep lettuce fresh for a week or more. So if it can hold the moisture in then it can keep the moisture out!!

[rogerg (Roger)]

Originally Posted by astroboy

Hi Roger and Roger
I don't get it , isn't the Sbig camera sealed .
Also I thought the colder something gets the more condenation you get.
maybe we get different problems here as I often cool the camera to -30 or more.

Zane

I figure it must be possible to remove all the moisture from the chip area at construction and keep it that way, but it's more cost effective to provide the Desiccant plug? My camera gets kept on the scope 50% of the year, so is exposed to the humidity variations of the observatory. I expect that doesn't help my situation.

Roger.