Chris, I hope that the fan doesn't suck dust in, and blow its way around to the chip!!!
Also, I thought an aluminium case would have been better. Aluminium gets cold and would keep the insides even cooler.
But your work is looking good so far
Ken - there'll be a grommet between the circumferential edge of the optical surface of the CCD array and the inside surface of the box. This should, in theory, prevent both dust and condensation-derived moisture from going anywhere near that surface. I'll be installing an external 1.25" IR filter on the Mogg - so that will prevent ingress via that direction too.
The plastic case will be fine I think, as heat dissipation is very direct - from the rear of the CCD to the finger billet to the peltier to the heatsink to the external fan. There shouldn't be any heat transfer to the case as such because nothing within the thermo-dynamic path touches the case at all. The whole billet/peltier/heatsink and fan assembly protrudes through the hle in the case and isn't directly mounted or touching at any point.
Thanks so much, everyone, for your kind words of encouragement thus far!
I continue-on with what I though was going to be the hardest to get right - the mounting of all the cooling hardware. I don't have access to milling machines, lathes or fancy drill presses. All I use are electric hand-held tools, a hacksaw and a flat file.
I've used 4mm aluminium strip sheet to fabricate the bottom mounting plate, CCD tunnel as well as the top finger plate that presses against the peltier. The peltier sits on top of the finger and is clamped down snugly by the heatsink - which I've drilled and tapped so that the four screws from underneath can screw up through the set of plates and into the heatsink. The bottom plate has a countersunk (45 degree-bevelled) hole in it through which the CCD pokes. I've painted the whole plate matte black so that there is nowhere for stray light to bounce around in. The plate sits on the four threaded sleeves in the bottom case of the jiffy box.
With the 30w peltier running, the first test proved that the heatsink got very warm without the fan and stayed cooler with the fan running. The cold finger and CCD tunnel was VERY cold.... and without humidity in the air it remained dry. To the touch it filt like it was almost about to freeze. Yay!
I bought two peltier devices - one 30w and another rated at 60w. I'll do some experimentation in realistic conditions when I've finished this project to determine which I'll stay with. I can swap them over fairly quickly, so it's almost easy enough to do in the field.
Next - the wiring-up. I'll get that done over the next couple of days - stay tuned
I hope to have the wiring done over the next day or three - depending on the rain outside......
After that, I'll test it in normal exposure mode to make sure that it's all still working as it was designed to. Given that it still works I'll then get one of the guys in our club to do an accurate polar alignment on their scope and I'll try some long exposure tests with and without the peltier running. Just the fact that the CCD array is in contact (via thermal paste) with the aluminium plates and the tunnel walls it should make quite a difference already.
I've added a new layer of 2mm cork between the tunnel edges and the front mounting plate. The CCD pokes through a hole on the cork. I reckon that this will at least shield the front plate, and therefore the CCD face from the cold. I'll investigate covering the rest somehow soon. I've just screwed it all together for good, and so have applied the thermal grease to all plate surfaces as well as the back & sides of the CCD at this point (now that I know it all fits together.
The back and sides of the CCD are still in direct contact with the aluminium "finger" above it. The front plate is in front of the CCD, so I've isolated that plate with the cork. The little piece of cork that you see over the ribbon cable is just a small piece I've used at either end of the tunnel to clamp the wires down.
Pic 1: 2mm cork gasket between the tunnel fingers and the front plate to hopefully stop the front surface of the CCD from getting too cold, and to keep the front plate from generating it's share of condensation.
Pic 2: The front plate painted black, and now you can see the CCD through the aperture.
Hi Chris,
I have noticed that the cold finger is not exposed very much at all...
While this could be OK as far as thermal conductivity and efficiency are concerned, you may run into a trouble with ice accumulation on the CCD window (which would otherwise deposit on the cold finger, being cooler and if ti was exposed more).
Have you planned to desiccate the interior of your camera and how?
Maybe to hermetically close it after flushing the interior with dry nitrogen gas?
Hi Chris,
I have noticed that the cold finger is not exposed very much at all...
While this could be OK as far as thermal conductivity and efficiency are concerned, you may run into a trouble with ice accumulation on the CCD window (which would otherwise deposit on the cold finger, being cooler and if ti was exposed more).
Have you planned to desiccate the interior of your camera and how?
Maybe to hermetically close it after flushing the interior with dry nitrogen gas?
Bojan - The CCD window through the front plate is now totally isolated from the cold finger by the cork. This should, in my mind anyway, prevent the front plate from gathering much ice, as such. The box is open oround the heatsink - to the air, so will be impossible to hermetically seal and fill with nitrogen.
Can I ask what you meant by " I have noticed that the cold finger is not exposed very much at all..."? What would you do?
Bojan - The CCD window through the front plate is now totally isolated from the cold finger by the cork. This should, in my mind anyway, prevent the front plate from gathering much ice, as such. The box is open oround the heatsink - to the air, so will be impossible to hermetically seal and fill with nitrogen.
Can I ask what you meant by " I have noticed that the cold finger is not exposed very much at all..."? What would you do?
Cheers
Chris
What I meant is, cold finger is not present in the cavity where CCD is.
You may have a situation where the CCD is the coldest part in a cavity, and then you may have undesirable condensation and ice accumulation on the CCD window, if that cavity is not sealed and filled with dry gas..
In other (classical, like Coockbook camera) designs, people deliberately left cold finger uncovered, and as this is the coldest part, the ice accumulates on it and once all moisture is removed from the (sealed) cavity this way. CCD stays free of ice.
In my own design (I placed a picture in another thread some time ago), I followed that principle.
You can find it by searching "Home made CCD astro-camera - my version" and in attached picture. (no it is still not finished.. but it will be shortly. However, it saw a first light ).
Anyway, you will see how it goes.. it may not be the problem after all
Wow. My eyes are aching after doing this latest wiring. They weren't kidding when they said "small". The D16510 is a TINY surface-mount chip that I decided to left the legs on where needed, and soldered to both the lifted pins as well as the pads underneath rather than tracing the pads out to a via point.
The use of some old wire-wrap wire was fantastic as it is just so much thinner than a ribbon cable strand.
I've connected power and, after mounting the board properly, I've also connected the CCD back up again. Fiddly, fiddly, fiddly!
Next I have to complete the piggy back board, cut it out from the rest of the protoboard and wire it up...
It may just make first (or is that second) light tomorrow evening.
Sorry folks - I haven't finished the last stage yet - I've been horribly busy at work and have just come back from leave as well. I hope to finish it next week and fire it up to see what happens.
*hint*
(Read: if I spend any more tme on my hobbies I won't have one).
