#41  
Old 10-11-2014, 06:43 PM
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Originally Posted by ZeroID View Post
Then the problem becomes how to keep dew\ice off the window ?
Youve just hit the brick wall I did . Ittl be interesting to see how you get round that little problem .
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  #42  
Old 11-11-2014, 11:54 AM
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Originally Posted by Bassnut View Post
Youve just hit the brick wall I did . Ittl be interesting to see how you get round that little problem .
You're no bl00dy help at all ... ..

Working on it.
Several ideas of creating a dry gas purged chamber in front with a clear optical window and using a smidgin of the warmed air off the radiator to blow across the external surface of the window to keep it from icing up.
Some silica in there might also help. The new setup has more room internally so I can hopefully fit a small bag or two inside.

I've also reduced the parasitic thermal loss with some insulation and reduced surface contact area at the junction of focusser and camera. The window will be ahead of that so it can be managed thermally seperate.
Fingers crossed ..

What do the commercial cooled cameras do to manage it ?
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  #43  
Old 11-11-2014, 12:27 PM
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Originally Posted by Peter Ward View Post
Just like a cold can of XXXX on a warm summer's day, too high a delta-T will always cause condensation (and eventually icing).

Pro cameras, such as SBIG, have sealed chambers with desiccating plugs, plus the chamber window is heated.

Further, the CCD chamber temperature is stabilised to 1/100th of a degree to allow accurate dark frame calibration.

Sure, lowering the camera temperature helps....but there are limits on unsealed systems.
Ooops, didn't see this first.
Thanks Peter. I think I can duplicate the sealed chamber and plug idea. I have enough hardware, now all I need is a 1.25" clear glass filter for the front window.
Heating... maybe airflow or maybe a resistive collar.

Here's a pic of the proposed setup. It's an unusual 2" adapter I acquired somewhere with a tapered collar, thus reduced contact surface. Insulators will go on the flat surface and I could wrap a resistive heater around the barrel and filter assembly (Couple of 1.25" EP extenders with a temporary yellow filter on front) that protudes forward of it into the focusser tube.
Or the warm air idea of course.
I may drill and tap another hole into the side of the adapter opposite the lock screw for a silica plug to screw into.

(Can you tell I'm having a quiet day at work ... )
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Old 12-11-2014, 12:06 PM
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OK, scrap the above configuration. Not enough internal space to get silica inside and there are some mechanical problems internally. I have a new idea that looks quite easy to implement and is mostly reversible should it not be as successful as I'm hoping. Details on that later.

But, heating the window is the next problem to be solved. In a simlar setup to the above, ie the window being forward of the adapter how much heat is required to maintain a temp sufficient to keep it from frosting up ? Pretty sure I can build a resistive ring that will give values in the half to quarter watt region but is this sufficient ?

I'm pushing the window out ahead of the camera to reduce thermal transfer back to the sensor etc. I could push the window back inside the adapter and use an airflow but that might introduce more moisture.
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Old 12-11-2014, 06:35 PM
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OK, here it is. A lucky combination of available parts.

First pic shows it in pieces. A 2" collar I had off an old focuser, the 1.25" ASI nosepiece with an extension on it and a 1.25" to 2" adapter. The collar perfectly fits the outer shoulder on the ASI 120. I will drill 4 x 5-6 mm holes in the nosepiece to open it to the silica.
Second pic, partially assembled with 3 silica gel packs inserted. The holes will expose the chamber to their effect.
Third Pic, the adapter seals off the silica collar and slides into the OTA focuser.

Now all I have to do is design a heater unit to wrap around the front filter glass and replace the yellow filter currently standing in with a clear glass unit. I'll also modify my neoprene outer cover to include the silica collar. Contact through that region is actually quite small, corner to edge so parasitic thermal loss should be manageable.

Looks quite cool too, just like a real one.
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  #46  
Old 12-11-2014, 07:29 PM
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And complete with TEC, radiator and fan system mounted, neoprene sleeved and yes, the holes are drilled in the nosepiece ... and I've blackened the edges. I've even sandwiched the neoprene between the adapter and the collar as extra insulation to minimise heat loss to the OTA. Only conduction path is up the thin walled nosepiece.

Window Heater is the last item to work on. I'm using a 2" extender on the scope to enable focus and it has a slotted top collar so I may be able to use that to get wiring inside to the heater.

The question now is how many clouds do you think this will bring on ?
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Old 16-11-2014, 01:14 PM
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And success !
Small piece of rubber tubing slotted into the radiator fins and directed at the front window through the slot in the 2" extender body.
Ambient temp at start was 22.2*. I got down to -1.8 at 47 mins and 12 volts. ( after 20 mins at 5 volts to see how it went ). Held temperature for an hour and 40 and no icing up or dew internally or on the front window and a clean good image of the houses on the ridge 5 km away.

Interestingly when I finally powered off the temp was back up to 11* within 4 minutes which just shows just how much heat transfer is going on. I'll improve on some of the insulation layers as well to minimise that.

I just need the clear filter window, some fresh dry silica bags and we're ready to have a go at some DSO imaging. If I can't get a clear glass window I'll grab a UV\IR filter instead. If I can get a 24* delta on a 10* night I'll be more than pleased but I reckon there is a bit more in it yet.

Here's a picture of it up on the scope.
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