Resolved Artefact in Flats

I have an SBIG 16803 attached to a C14 Edge HD.

This morning I went to do Flats. I have a large Lightbox which I place on the ground alongside the pier and I arrange the OTA to be in a vertical position just above it to take Flats. This has been working well for me.

This morning I was quite perturbed to see a prominent shadow in two lobes on the first image with the Luminance filter. I switched to the Red filter and it was still there suggesting that the object was on the camera side. At first I thought perhaps that a large piece of lint or something had fallen onto the sensor. However, a friend to whom I showed the image to suggested it is condensation or frosting. Although I have never seen this before, we have had two weeks of incessant rain and I am pretty sure the environment in the observatory is very humid, if not saturated.



I have had the camera for over two years now and I haven't, at this stage, regenerated the desiccant plug.
In setting up the imaging session, with SGPro it followed the normal start up procedure which is to get to my target temperature of -20C over a 5 minute timeframe.

I think the most likely explanation is that the internals of the camera have equilibrated with the relatively humid, albeit cool, interior of the observatory (5C to 15C). When I started the cooling, the protection window in front of the sensor will have cooled, due to convective heat transfer across the air gap. Given the humidity of the air, the dew point was probably not much below the ambient temperature and when the temperature of the protection window fell below that condensation occurred. Do you think this a reasonable explanation?

I removed the camera and carefully placed a silica gel sachet in the tubular adaptor in from the the camera. I have left the camera sealed inside a plastic bag for several hours. This should dry it out. I will reattach the camera and repeat the exercise. Hopefully the shadow will be gone. I realise that my Flats will be compromised, so I may need to do more work in processing, or it maybe that my previous set of Flats will suffice. It may be that the condensation will have affected the position of any dust motes for example.

• Do you concur with my explanation of the problem and if not, what do you think the issue may be?
• If condensation on the outside of the protection window is the issue, then I need to prevent the camera's internals from becoming humid during rainy conditions. I have in the past, covered the entire scope with a plastic cover and then left 2 x 1kg bags of silica gel atop the pier to keep the OTA and camera dry. Does this seem a reasonable course of action for enduring bad weather? If not what other precautions do I need to take?
• Do you recommend I regenerate the desiccant plug?

I will hopefully be able to report soon as to the success or otherwise of my attempt to dehumidify the camera.

Thanks,

Niall MacNeill,
Wattle Flat,
NSW,
Australia

 

As a follow up, after using the desiccant to dry out the camera internals, the artefact was gone when I repeated the Flats. Also CCDOps tells me the window heater is activated.

 

Thanks Colin. Have you seen this issue before?

 

Or better yet in the desert at a high altitude.

I have a flattener that has a port for dry nitrogen purge from the old days of hypered Tech Pan.