16803 frosting up no matter what I do

No. The O-ring for the STX desiccant plug is a BUNA-N size -011, McMaster 9557K463

 

Mark, I can't really tell - we don't see frost tendrils. so I'm going to ask @Doug and @Bill to chime in.

A few thoughts, and other folks will hopefully have some suggestions:
- Pull the camera off, bring it indoors, let it adjust to ambient - maybe bag it before bringing it in. Could take an hour or two.
(eg you don't want condensation all over it - if you have dew outside, the humidity must be pretty high).
- Put it on the workbench/kitchen table.
- Prepare it to be used indoors. eg connect to your laptop, plug it in.
- Orient the camera so you can look down the nose toward the sensor.
- Fire up MaxIm or CCDOPs (or whatever you are using).
- Connect and take a 10 second image through the luminance filter.
- Turn on the cooler - set point should be about -25C or so.
Let it get cold.
Take another shot.
Are you seeing frost or moisture forming inside?

I'm inclined to think that the moisture is on the OUTSIDE of the chamber window, or on the filter.
If you are fighting dew with heaters, I'd call it a night.

What's your relative humidity when you took this sample shot?

Why not cool the camera more aggressively? 5C per minute is not unreasonable, so you should be able to get it down to -30C from ambient of 15C in just 9 minutes.
Is the Window Heater turned on?

 

If the chamber has moisture, the chip will often frost over before the inside of the chamber window because the CCD ceramic package is the coldest part, being in contact with the TEC cooling stack.

If the chamber is dry, you won't get frost, so you should be able to cool it to maximum delta T without babying it.

Hmmm... then I think it's OUTSIDE the chamber.

If moisture is inside the chamber, and the components (chip, cold finger, TEC) get below 4C, signs of condensation or frost will start to form inside. I'd expect it in the first 30 minutes.

If it is happening toward the end of the session, ambient is probably nearest the dew point, and so the condensation is forming OUTSIDE the chamber as the cooling outside air contacts the colder surface of the camera. Are you getting moisture on the camera body or the filter wheel or telescop?

So then the exterior front face of the chamber window, either side of a filter, or back of the flattener are likely suspects then.

The thought is Process of Elimination. We're trying to figure out if the chamber seal is bad (or dessicant o-ring is worn out).

If there is moisture is in the chamber, the moisture will condense and show up on the sensor or inside of the chamber window when the cooler gets the temperature low inside the chamber. That's regardless of the ambient temperature and humidity.

If we don't get any formation, then it's EXTERNAL - and so you've done a fine job of baking the desiccant and drying out the chamber. (attaboy).

If INTERNAL - then the camera probably needs to go to @Bill to have the seal replaced, and/or purchase a replacement desiccant plug.

If the filter wheel body is getting condensation (or filters are), maybe wrap a couple dew straps around it.

BTW usually frost forms fractal tendrils, and I don't seem any in the earlier samples. It looks like it is fogging up near the centre first.

Anyway, whatever's going on, bringing it inside and dialing up the cooler should show you in a few minutes what is going on.

I'll take a look at the flats. Thanks for sending them over.

 

It does rather sound like it's on the window. The window will be chilled slightly by having the cold CCD underneath it, and in humid conditions that can get you below the dew point.

Do you have the window heater turned on?

 

Yes, there is a heater. In MaxIm DL this is where you would look:

https://cdn.diffractionlimited.com/help/maximdl/HIDD_SETUPST7.htm

CCDOPS has a similar option.

It is possible that the filters or flattener is fogging, but it's more likely to be the chamber window because it's slightly cooled by the sensor underneath it (via convection).

The purpose of the heater isn't to actually warm up the window; it's to create a bubble of slightly warm air in the front of the camera. That prevents dew from forming.

I don't know about you... but it's really humid here in the winter! I'd turn it on if your humidity is above maybe 70-80%.

 

You need to be connected to the camera in order to see the available options for that model.

Connect to the camera, then go to the Expose tab and select Options menu, Camera Settings.

See https://cdn.diffractionlimited.com/help/maximdl/HIDD_EXPOSE.htm#bc-1

 

Is it on the outside or inside of the window?

 

Probably there is some contamination on the window that is acting as a condensation nucleus.
Window heat is the option.
Cleaning may help.
You could also bag the whole camera and add a bunch of desiccant packs (silica gel would do) to the sealed bag, to get any remnants out.
But the first humid night, you may be back in the same situation.

 

The heater should fix that. It should stay on if you don't power down the camera when switching to another application.

Maybe check that the heater is plugged in. If you pop the lid off the camera, and orient it with the connector panel facing you, the heater will be glued onto the far side of the chamber. There will be some wires running to a small connector on the circuit card. Make sure that's plugged in and the heater isn't damaged. Power up the camera and verify that the heater gets warm when it is turned on.

The purpose of the heater isn't to warm up the chamber walls. It's supposed to create a small "hot spot" of warm air in the front of the camera. The idea is to keep the air in front of the window above the dew point.

 

Where are you located, what's the temp, humidity, and dewpoint when this happened?
Can you confirm the window heat is on when you are outside?