Atik 383L Dark Frame Question

[herrman]

Hi,
I'd be grateful for some advice/comments on the attached dark frame, which is a 300 sec exposure at -30 C. I was surprised to see that the image displays a large glowing band across it (looks a bit like the milky way!). Is this usual with this camera, and if not what might be causing the glow.
Any thoughts on this will be gratefully received.
Many thanks, allthe best, Herrman

[NickK]

Hmm looks like some light getting in somewhere. Even stretched darks I have don't look like that!

Was this shot during the day or night?

[joelshort]

That's weird.  It does look like light is getting in somewhere.  Are you using a manual filter wheel?  Are you sure you had the scope covered when taking darks?  This also looks like when I had some frost forming on the CCD window when the desiccant began to run out, but of course that should affect darks since it supposed to be, well, DARK.
joel

[herrman]

Hi,
Firstly, many thanks to you both for your responses- much appreciated!
Yes, I do use a filter wheel, and I took the dark frames with the camera/filter wheel assembly attached to the scope. It's possible that the corrector plate dust cover wasn't fully on as  sometimes I leave an air gap to make sure the plate dries-off after being outside. I understood that this camera has a mechanical shutter, so I'd assumed that it wouldn't matter if the camera remained connected to a non-light-tight optical assembly. I guess this assumption was wrong?
I have problems with condensation and frost in the past, I did change the desiccant a couple of months ago, when the problem was more noticeable, the camera is under 18 months old, and I'm surprised that I've need to change the desiccant at all.
Again, thanks to you both for your help; I'll try re-taking the darks with the camera off the scope and its dust cap fitted- I'll let you know if that solves the problem. In the meantime I'd be interested to hear if anyone has thoughts on what's causing the frosting problem.
All the best, Herrman

[joelshort]

The frosting problem was just a possible cause that I mentioned.  If you changed the desiccant 2 months ago then you shouldn't be getting condensation.  Have you observed frost since changing the desiccant?  If so then I would contact Atik support.

[herrman]

Hi Joelshort,
Many thanks for your reply. The problem with frosting has improved since I changed the desicccant-but it's not totally gone away; so I think a word with Atik would be in order.
Many thanks.
All the best, Herrman

[niteman1946]

Hi Herrman,

It's not clear to me when you shot your darks. If done at night and with the scope covered, the light should not have been a problem. But likely this was a light leak.

I find moisture and ice more problematic. I've recharged (or replaced) twice since receiving my camera in September.  I suspect humidity (camera resides on the scope in the observatory) and relatively long exposures overwhelm the desiccant.  And per their instructions, it does take a full day before a recharge can wipe out the moisture.

Good luck,

Mark

[herrman]

Hi Mark,
Many thanks for your reply.
The darks were taken with the camera still attached to the filter wheel and telescope; so you're quite right that the problem was caused by a light leak. I'd assumed up to now that the camera's shutter would adequately shield the sensor from light- but I now realise this assumption was wrong! I've re-taken the darks with the camera's dust cap in place, and the frames are much improved. 
Up 'til now I've been storing the camera in my 'observatory' (shed!); I reasoned that storing the camera in a warm house and then taking it out into the cold was more likely to cause condensation problems than leaving it outside permanently, but as you say, perhaps this results in the desiccant becoming saturated. Difficult to know what to do for the best!
Thanks again for your response.
All the best, Herrman