#1  
Old 18-04-2010, 09:37 PM
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Is this frosting?

Calling all CCD gurus - SBIG gurus even better

Been having fun with the new ST8300M. Have noticed recently some severe vignetting - or so I thought.

Working on some basic photometry this evening - I found that my images developed what appeared to be snow all over the image - except around the centre of the shots. So I inspected the corrector plate and sure enough, I had a fair amount of dew on the corrector plate - mainly inside the scope (I have a schmidt-Newtonian).

So out comes the trusty hairdryer and I clear the dew.

Next shot - same problem?!

I check the corrector plate - clear

I check the primary and secondary mirror - clear

I pull out the MPCC - clear.

Now what?

I take out the MPCC, adjust teh focus - same problem.

So it must be on the sensor itself.

So now what do I do? I had expected that the the camera would be clear of this sort of thing after only a few months of use.

Here are a couple of shots to give you an idea - it was a dewy night and I was running the cam at -10 C as usual.

The attached don't give you a clear picture - if I stretch the images, its an impenetrable fog. You can tell the difference between the shots as the clarity of the image has dropped off between them - count the stars at teh centre of the picture and you can see the difference. There's about 90 minutes between photos.

Open to suggestions

Pete
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  #2  
Old 19-04-2010, 05:05 AM
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[1ponders] (Paul)
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Yep that's frosting.
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Old 19-04-2010, 05:16 AM
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Definitely frosting
You will need to bake the dessicant plug ( I assume these cameras have one?)
frank
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Old 19-04-2010, 06:04 AM
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Thanks gents - I suspected as much

dunno how to do it - the manual only gives information on how to replace the dessicant material.

Pete
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Old 19-04-2010, 07:55 AM
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Have sent an email to SBIG and the local distributer - will let you know how this goes.

I have seen a similar issue raised by one of our Brisbane-based colleagues on Cloudy Nights. Have posted there too.

Do you think I need to run the camera for half an hour before setting the temp to -10?

Pete
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Old 19-04-2010, 08:17 AM
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Omaroo (Chris Malikoff)
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Pete - with my ST-8 I found that if I started the camera and ran it at ambient for 10 or 15 minutes, and then, in 5 degree steps 5 minutes apart made it down to zero and then into negative temps I rarely frosted. Reaching your goal temperature over time is the key in most cases - the more humid/dewey it is, moreso.

The dessicant plugs are easy to bake. Remove the plug in the camera and immediately replace it with the dummy (I presume you still get one these days). Place the plug on a ceramic plate (removing the o-ring first!!!!) in the oven when the oven has reached 200C. Leave it there for 4 hours and turn the oven off. Let the plug cool while still in the oven, as it's a dry atmosphere in there.

When cooled, swap it with the dummy and away you go.

There are dessicant kits available for most of these cameras that include cartridges of replacement dessicant so that you don't have to go baking cakes every few months.

One thing I've noticed of my QHY-9 is the good performance of the heated front window - I've not suffered frosting at all, which is fantastic and one thing I wish that my SBIG had.

Last edited by Omaroo; 19-04-2010 at 08:42 AM.
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Old 19-04-2010, 09:45 AM
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Thanks Chris

I'll give the stepped approach a try tonight (clouds permitting). It was particularly dewey last night so it wouldn't have helped.

The camera didn't ship with a dummy dessicant plug - the manual simply says that SBIG will supply a replacement dessicant free on request. Looks like I'll need to make a request

Pete
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Old 19-04-2010, 11:57 AM
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I've now learnt its most likely dew on the chamber window (outside) rather than frosting on the sensor (inside).

If thats right (which I expect it is), then dropping the temp slowly and allowing the heating of the outer chamber to do its work on the chamber window as Chris suggests should help.

Thanks everyone for the tips - will let you know how it all goes tonight.

Pete
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Old 19-04-2010, 04:06 PM
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Even after changing the large seal around the camera housing and the desiccant plug seal I still get frosting if I drop the temp too quickly. Its a right royal pita. Even following the procedure that Chris described, I can still get frosting. It a real nuisance at times.
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Old 19-04-2010, 07:33 PM
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Na this is frosting when imaging and your coffee cup freezes over
Leon
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Old 20-04-2010, 06:15 AM
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HAHAHAHAAH


Quote:
Originally Posted by leon View Post
Na this is frosting when imaging and your coffee cup freezes over
Leon
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Old 20-04-2010, 06:44 AM
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Originally Posted by pvelez View Post
So it must be on the sensor itself.Open to suggestions
That's a bit of dew. Most likely on the glass surface just before your sensor. I'd assume those camera would have a dessicant chamber of some sort. Bake it and you're good to go.
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Old 20-04-2010, 07:17 AM
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I ran the camera at -10C for about an hour last night and it wouldn't shift - dropping it to -18C (the lowest it would go) didn't shift it but bringing the temp back up to around -1C did. It took a while though.

I also had an email from Michael at SBIG overnight who suggested I check where the problem appeared to be - inside or outside the chamber window. He also said that they are looking at a hardware fix - I suspect the chamber window heater is struggling.

I did try and drop it slowly as Chris suggested - no joy there.

I hope that if it continues, SBIG do come up with a fix.

Its odd that I only struck this issue this week - we've had a few warmer (and more humid) nights earlier in the year without any problems.

Will keep an eye on it over the next few nights - it should come right in time for the fuller moon and cloudy weather forecast for Sydney at the weekend

Pete
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Old 20-04-2010, 08:14 AM
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[QUOTE=Omaroo;582953]Pete - with my ST-8 I found that if I started the camera and ran it at ambient for 10 or 15 minutes, and then, in 5 degree steps 5 minutes apart made it down to zero and then into negative temps I rarely frosted. Reaching your goal temperature over time is the key in most cases - the more humid/dewey it is, moreso.

snip
[QUOTE]

This is curious. With my ST10XME I just turn it on and push it straight to -25 as fast as possible. I do use a water cooler and this may help.
I never get any frost.The camera is permanently on the scope.
I will get dew on the secondary if I don't use a heater but never anywhere else.
This is in cold Armidale.
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Old 21-04-2010, 09:03 PM
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Ok - for those of you still interested...

I ran the camera at -10C for about 30 minutes before slipping it into the OTA - no dew.

It is a lot less dewey than the other night but still - no dramas at all.

V V odd.

Thanks for all the posts on this.

Pete
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Old 22-04-2010, 04:11 PM
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I don't know if this could help but QHY through Gama produce a heater ring for just this problem that has a T-thread it fits in front of the nose piece on the QHY camera so it may suite you as well
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Old 22-04-2010, 04:46 PM
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I don't know if this could help but QHY through Gama produce a heater ring for just this problem that has a T-thread it fits in front of the nose piece on the QHY camera so it may suite you as well
I have one. It has male T-thread one side, female the other, and pushes the CCD back 7mm.
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Old 22-04-2010, 05:03 PM
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No good for the sbig, that ring warms a filter window and the sbig doesn't have one. Even if it did the body isn't sealed and air comes in via the back which has slots in it. The sbig fix will be a sensor chamber heater coil for sure and it's going to have to be damn flat to fit under that huge shutter.
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