Strange Gradient - Need Help Please!

Hi Dave,

I think Anthony may also be on to something when he suggests ice forming on the CCD optical window. I assume the camera is peltier cooled, in which case you might expect to see ice forming in the centre of the window - directly above the chip which is the centre of the cooling effect. It may be taking a while for the window to reach equilibrium... or you could even be getting ice forming on the surface of the CCD chip itself if there is *any* moisture in that chamber.

If this is the case then the only way to eliminate the problem would be to open the CCD chamber (unscrew the window) and place the camera in a warm to hot, dry atmosphere (your oven, perhaps) for a spell before sealing the chamber completely. You don't want the temperature too high when you seal the chamber again as the warm air will contract and you might possibly crack the window as a vacuum forms!

Phil.

PS - At what stage in a night's imaging does the gradient appear?
- as soon as you switch on the camera and it starts cooling...
- after the camera has been on (and cooling down) for some time...

I think Anthony may also be on to something when he suggests ice forming on the CCD optical window. I assume the camera is peltier cooled, in which case you might expect to see ice forming in the centre of the window - directly above the chip which is the centre of the cooling effect.

Phil I do occassionaly get that effect on the outside of the optical window. The inside is sealed with argon gas and is never a problem thankfully. H
However that is not the cause in this instance. I did examine the OW *very* carefully and even blasted it with the hair dryer just to be sure.

When this does happen it manifests as a darkening in the centre rather than a brightening. As you can imaging a circle of dew on the OW lets less light pass and darkens the centre. I am starting to suspect it was the flattener which was the problem. Further tests methinks.

Thanks for the feedback Phil. BTW are you away to IOM this Christmas?

Thanks for the feedback Phil. BTW are you away to IOM this Christmas?

No, I'm at home this year... and the ravening hoards are descending on me for Crimbo day to eat me out of house and home!

Me thinks a bite to eat and a glass of plonk are in order... just to limber up the system before the big day! When are Jed and Carol heading westwards? Could you be tempted south of the river for nibbles?

Phil.

Dave G,

In addition to Phil’s other solution, I remember having a similar problem with a haze effect persisting on images.

The CCD camera desicant was changed and further sky flat tests taken still showed the same problem. The problem I eventually discovered was that this was moisture on the CCD sensor due to a damaged seal.

The CCD chamber had to be baked, new process integration, and repurged. The chamber was baked to drive out moisture. A epoxy procedure was integrated to improve the seal on both the chamber and the chamber lid. The camera was repurged with argon and vacuum sealed. The camera was fully tested and cooled over 24 hours.

This does not mean that your camera has the same problem as above. This is to share with you one specific problem that had similar manifestations shown in your image.

Thanks Eamonn,

Yes I can see how this would cause that problem alright. A couple of months back I managed to drop the camera and dislodged the CCD from its socket. I had it sent back to the manufacturer who reseated the CCD, repurged the chamber and refilled the argon gas inside. I ran the camera indoors for a few hours and examined it with a magnifying glass and can see no evidence of any frosting over the CCD so I think I'm ok there.