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Astro Projects

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All Sky Camera Mark 7


Gina

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This is my latest generation of all sky cameras and based on the ASI178MM followed by ASI185MC CMOS astro camera and a Fujinon fish-eye lens of 1.4mm focal length.  Although rated at f1.8, this lens lets a lot more light through than this would imply.  Image capture is provided by a Raspberry Pi 3 in conjunction with INDI software.  This is used with KStars/Ekos client software running on a Linux Mint desktop indoors.  Communication is via Wi-Fi.  The Mark 6 ASC has proved inadequate after being in use for some time. 

This blog will describe the problems of the Mark 6 and report my progress in developing this new version.

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Still clear (mostly).  Same settings.  Needs hot pixel removal. And I think it could also do with more cooling - temp 3.6°C.  Need to check what voltage the Peltier TEC is actually rated at - I thought it was 5v but now I'm not sure.

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Aha!  It's a 12v Peltier TEC but I don't know what power.  I also don't know it 12v is it's voltage rating or just the normal working voltage as many nominally 12v TECs are rated at 15v.  The only info I've found so far is from a post in my Mk 4 version around 18 months ago :-

Quote

Today I'm planning to set up cooling for my ASI178MM using the 12v Peltier TEC I bought recently.  I'll probably try running it from 5v as I don't want a lot of cooling.  For testing I have bench PSUs that will provide a variable voltage (or current).  I expect I shall want control over the cooling eventually and have set-point cooling control.  I have an Arduino sketch to do just that already.

In the next post I said :-

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For testing I connected the TEC wires to my smallest bench PSU - 0-5v or 0-15v @ up to 1A.  Increased voltage up to 5v while watching the current.  With no temperature difference the TEC drew 0.6A @ 5v but decreased to around 0.5A as the cooling started working and there was a temperature difference between the two sides.  It looks like the temperature difference achieved at this setting will be quite sufficient - it might want reducing in cold weather. 

It's still true that I probably don't need a lot of cooling but 5v doesn't seem enough.  I guess that answer is to use another buck converter to run the Peltier TEC above 5v but below the observatory main power supply of 13.8v.  I also have another possible use for an intermediate voltage and that's for powering the Arduino for my weather station wind sensors if I don't run it from a USB port on the RPi.  I have a spare buck converter so I can try this - probably tomorrow as I want to leave the ASC imaging for several hours tonight.

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Here's the latest image - same settings, camera temperature 3.5°C.  No dew heating.  North is towards brick chimney with cowl, at top of image..

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Gina

Posted (edited)

There is one other thing I could do to improve the cooling and that's to add thermal insulation around the camera (except where the focus motor is as there's no gap there).  This would have two benefits - stop heat from the TEC hot side getting to the camera and warming it and reduce the effect of the warm air rising from the hot side being cooled by the cold camera and providing more heat for dew heating.  It's possible that extra heat from the Peltier TEC could eliminate the need for a dew heater.  OTOH lagging the camera could cause it to overheat in daytime making Peltier TEC cooling necessary as the only form of camera cooling.

Edited by Gina
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Left the ASC running overnight though I see it clouded over in the early hours.  The camera was covered in a thick layer of ice and temperature was -1.8°C and I guess the ice provided some thermal insulation.  What is apparent is that the camera wants thermal insulation when Peltier TEC cooled - I shall look into that today.  It is not practical to seal the ASC to keep moisture out so I shall spray the electronics with ACF moisture repellent.

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Found some 5mm thick plastic foam sheet that had been used for packaging and cut it to size and fastened it to the camera.  Also, pieces between camera and heatsink.  Now testing...

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The 60mm acrylic dome has arrived.  I think it would be big enough but on carefully looking through it, i don't think the optical quality is good enough but I'll try it next time we get some clear skies or at least something other than the mist and totally bland grey sky we have at present.

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One of the NEMA 11 motors has arrived and quite a bit smaller than the NEMA 14 and looks good for the cover/dome motor.  Mind you, I am wondering if a removable dome is worth the effort.  If the moon is up the stars disappear anyway so reflections don't really matter.

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This project is taking too long so I think I'll go back to a fixed dome.  I've been getting to darn clever for my own good!! :D

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I put the camera outdoors last night but there was a very heavy dew and the lens misted up almost immediately so I brought it back in.  It definitely needs a dew heater as I suspected.  Next job will be to finish adding the components for that to the HAT and fitting the resistor string that produces the heat.

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Designed and now printing a new top for the casing to take a 4" dome (of which I have two).  Dimensions of top 35mm high, 96mm inside dome.  Dome flange 9mm.

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Edited by Gina
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I have a 3" dome on order on a slow boat from China and might make a new casing top when that arrives.  I think a smaller dome should be easier to heat to prevent dew.

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ASC outside on the guttering, no dome.  Exposure 30s, gain 200, gamma 10, temperature 9.1°C.  Ambient temperature below freezing.

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Making the parts to hold the dome.  A seal underneath and just inside together with a ring and skirt to hold it down onto the casing top.  First pic is a cross-section of the dome, seal (red), casing top (green) and clamping ring (yellow).  Then the seal and clamping ring.

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I've set up a buck converter for 9v output and will be adding this to power the Peltier TEC for camera cooling.  Then there's the dew heater to set up and the circuitry on the HAT to finish.  Also, sometime I have to sort out the Wi-Fi which isn't working.

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Been testing and 9v on the Peltier TEC doesn't seem to cool as well as 5v :icon_scratch:  Tried with the dome on and with it off and little difference.  Some way of controlling the voltage would be good.  No, not PWM, TECs don't like it.  Maybe I could take a PWM output, smooth it and apply to an analogue voltage dropper.  I'll have to measure the current at 9v and see how much power dissipation there would be in a voltage regulator.  No, don't like that - too complicated.  I think bench testing would be adequate so I could run the rig from two bench supplies.  ie. a separate one for the Peltier TEC.

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Been trying the camera cooling with various voltages on the Peltier TEC but the thermal lag makes it difficult.  Also, the exposure makes a very big difference - it's running imaging that generates the heat in the camera.  One thing that has become clear is that higher voltage means that there is more heat to get rid of from the hot side and the plain aluminium pipe isn't shedding heat all that well - it needs a better heat dissipater.  Maybe some aluminium or copper fins arranged around the pipe just below the ASC.

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The 3" dome has arrived from China (earlier than expected) and looks a much better option that the 4" ones.  Several advantages.  Here is a new design for the casing top to take this dome.

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I'm back on this project and it seems I have a couple of things to sort out before the unit can be set up outdoors. 

I've decided to separate the ASC from the weather station and put them on separate masts.  As far as the ASC is concerned, this means simply removing the arm that took the weather station wind measuring parts - the ASC casing needs no other modification.

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This seems to be the main problem that needs to be resolved.

On 14/01/2018 at 10:56, Gina said:

Been trying the camera cooling with various voltages on the Peltier TEC but the thermal lag makes it difficult.  Also, the exposure makes a very big difference - it's running imaging that generates the heat in the camera.  One thing that has become clear is that higher voltage means that there is more heat to get rid of from the hot side and the plain aluminium pipe isn't shedding heat all that well - it needs a better heat dissipater.  Maybe some aluminium or copper fins arranged around the pipe just below the ASC.

There are several ways of getting rid of heat from fins relying on convection through air blown over the fins to water cooling.  The problem with using a fan is that this is out in the open and subject to damp or even horizontal rain.  The fan would have to be water resistant.  A large finned heatsink might be sufficient.  Water cooling is something I have some experience of and it works well but the problem is the height of the ASC - the water reservoir needs to be at the highest point.  I think this puts water cooling out of court.

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