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Gina

All Sky Camera Revisited

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With rain all day, I have been able to assess the effectiveness of the dew heater.  With the failure of the power MOSFET that was driving the dew heater and the latter connected to the TEC, the dew heater has been running off 5v.  This has proved adequate for dry conditions so far but there is now some misting of the dome and it wants full power.  It would seem that 3 power levels might be appropriate for the dew heater viz.  OFF - 5v - 13.8v.  This would require changes to the circuitry and to the Astroberry Board coding to provide the control.  ATM the only way to get full dew heating is to set the cooling to HIGH.

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With dew not clearing from the dome and the need to fix the dew heater control I have brought the ASC indoors.

The moisture is on the inside of the dome and yet the colour-change desiccant is still showing orange meaning DRY!!  Well, the dome certainly isn't dry on the inside!!  So I haven't cured the problem of ingress of moisture into the casing!

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Looks like I need a complete redesign of the ASC.  I'll list the requirements and see if anything comes to mind.

  1. Capable of airtight seal
  2. Transparent casing or part of to show colour-change desiccant to confirm dry inside
  3. Big enough to take camera with lens and thermal lagging plus the electronics
  4. Cooling for the Peltier TEC
  5. Ability to take apart for maintenance yet retain airtight seal
  6. WiFi transparent casing

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The experience of the present design shows that the camera doesn't need cooling as much as I thought so the super efficient cooling is OTT.

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I'm thinking a totally different approach is indicated.  The idea of a sealed and dry environment for the cold camera was fine as long as a sealed casing was practical.  Now I think it isn't and I've had a really good try but not succeeded.  Also, the camera I'm using now doesn't need as low a temperature to get rid of noise and hot pixels. 

Edited by Gina

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I've taken the ASC apart and a few drops of water came out of the camera.  Fortunately, any condensation or ice forms on the case of the camera and not on the PCB or components.  The sensor will always be warmer than the case.

It's clear that if a perfectly sealed enclosure is not possible, there has to be provision for melting ice to drain out of the camera away from any electronics.  Condensation of moisture on the camera case will counteract the cooling slightly and there is always going to be some dampness in the air except in very dry weather.  If the camera case if cooled below freezing, ice will continuously form on the inside and any outside surface not lagged.  It may be sufficient to cool the case to just above freezing and provide a pipe to carry condensate away.

Condensate wants keeping away from the hot side of the TEC or it will evaporate and fog up the inside of the dome and also condense on anything cool.

Edited by Gina

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I think somehow I need to keep the air round the camera separate from the air under the dome.  Maybe take incoming air and warm it up with the heatsink that's on the hot side of the TEC preferably taken from inside the observatory where it's likely to be drier (warm room perhaps).

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I think I would like to use the same mast pipe as it provides a good way of mounting the ASC and simpler than attaching it to the roll off roof.  Makes it easier to take off for maintenance too.  The pipe was used to carry the power cable and water cooling pipes - now it could carry air from the observatory as well as the power cable.  For the air supply I would use a 3D printed adapter to enclose the pipe and provide a side-entry.

I have a 40mm x 40mm x 20mm heatsink that would fit on the hot side of the Peltier TEC.  The incoming air could blow through this and be ducted up and into the dome.

1514801423_Screenshotfrom2020-06-0420-25-49.png.5d2132ac96f344f6976ca03b482aeb49.png

 

Edited by Gina

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I shall have to set up a test rig to check that this heatsink is suitable.

Edited by Gina

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That's the air intake but I need to sort out how and where it escapes too without allowing wet to get in if we get horizontal rain!

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I'm thinking that heatsink may be too small and I have a bigger one which may be more suitable.

Firming up on the design, relatively dry air from inside the observatory could be blown up the hollow mast and into the ASC casing, firstly cooling the electronics then through the fins of a heatsink on the hot side of the Peltier TEC.  From there warmed air could be ducted up to the dome and down the other side and out down an air duct to the bottom of the casing and out to the atmosphere.  As before, the camera would be lagged to help with the cooling.  A pipe could be used to connect the camera enclosure to the relatively dry observatory and carry any condensate away from the camera.

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I think I'm changing my mind on blowing warm air from the heatsink up to the dome.  Even relatively dry air from the obsy is going to have a certain amount of dampness in it (particularly in wet weather) and it would be better if the air in the dome was as dry as absolutely possible.  I could still use air cooling of the heatsink but vent the warmed air straight out and just use a small positive pressure of dry air in the camera and dome area.

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Now I'm wondering if piping air from inside the observatory up to the ASC is actually more complicated than water cooling.

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Screenshot of camera, lens, TEC and 60mm heatsink crudely modelled.

297919165_Screenshotfrom2020-06-0522-06-58.png.f6a1a1ab76d2ad6d8ab83bbbe3a25be7.png

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