That's impressive Peter. I might try the cooler box before I install a cold finger. Do you have any desicant or dew prevention measures inside the box? Condensation mangement is a major issue with the cold finger and likely requires a low pass filter heater.
Hi Glen.
No I don't. 2 reasons.
1. I have a small hole in each of the lower back corners to drain off any condensation that might pool inside the box;
2. With the operating camera sensor running at 10C above chamber temperature, I figure I am unlikely to experience "on-the-chip" condensation if I maintain a temp range of 5-10 inside the box. Hope I'm right about this.
I base this on the formula:
Td = T - ((100 - RH)/5.)
where Td is dew point temperature (in degrees Celsius), T is observed temperature (in degrees Celsius), and RH is relative humidity (in percent). Apparently this relationship is fairly accurate for relative humidity values above 50%
On that basis, the dew point last night when relative humidity was 55-60% i.e.:
Did some work on my cooler box and its working. I am still using the Chinese TEC that I bought off ebay and it seems like it's going to be ok. The box is aluminium but I decided to make the TEC end of 1.2mm copper plate (that I had for the cold finger project). The plate is attached by bolts and the sides where it joins the aluminium are bedded in Arctic Silver thermal paste. The TEC is directly through bolted to the copper plate, again with Arctic Silver paste, and I mounted the cold side fan and sink to the other side of the copper plate. Progress photos attached.
Tested the box today, and despite some power plug problems (I was powering it off a dew heater controller via an RCA cable - which was thin, heated up, and the controller power plug kept pushing out of the battery box (spring loaded); despite the power problems I manage to get the internal air temp down to 7.3C, while outside air temp was 23.9C, so Delta T of 16.6 C.
My new dedicated power supply should arrive tomorrow and I will try again. I believe a delta T of around 16C is near the average for these boxes. The sensor was hanging in mid air inside the box so it's an air temp. I am using a digitial temperature controller, the OPEGG WTS-3A016.
I need to address the box insultation gaps, particularly around the TEC end. The insulation is just 20mm closed cell foam that came as packing for one of my scopes so I should be able to do better.
Is there a cooler box delta T record for the single TEC versions made here? Would be nice to know the limit.
s I manage to get the internal air temp down to 7.3C, while outside air temp was 23.9C, so Delta T of 16.6 C.
That's about as good as these things get I think. If your camera has a temp sensor on or near the chip, it will be an eye-opener to see how a few long exposures rapidly push that temperature up. With my 40D it runs up to about 16C from a starting point of about 7C - much the same results as yours. But that is still about 16C cooler than it would otherwise have been.
I did a full test with the camera in the box, and with the new power supply (mains to 12V 6 amps) and can report a little better results:
With an ambient of 27C I was able to get the air inside the box down to 9.8C which is a delta T of 17.2C and believe that's as good as it's going to get. It took an hour to get down to 10.8C then another 30 minutes to get down to 9.8C. The camera was not powered up and I imagine if I was imaging that the delta T would be significantly less. Photo is attached.
I am concerned about the T adaptor opening and even with foam insulation tape around it the metal is exposed to the air. If it's fitted in the focuser tube that just adds another 'cool sink' to the equation. Additionally, the left side of the camera is very close to the internal cold side fan and this has to hinder air flow. I have been considering taking the plastic side piece off the camera (not just the connector door) allowing air to flow into the camera through that opening, it would be blowing directly on the camera frame metal and the sensor shield. If I were to open up the other side with some holes it might be possible to get flow through the sensor area (I don't use the battery when the adaptor is in there so the holes could be free flow, and could also remove the battery compartment door). Desiccant would be a must.
However, this is a fight for diminishing returns and I might switch over to working on my cold finger now that I have a working box as a baseline.
Parasitic heat loss up the T Adapter can be considerable as I found. The whole focussing tube and lower section of the Lunt got quite cold. I swapped to another 2" adapter which had minimal suface contact and made some plastic isolation shields to reduce losses.
The one time I had the battery door open on the 450D it would not power up. Is there an interlock involved ?
You're right about diminishing returns on this design and also the lack of controllability at the sensor. I note from earlier posts that the metal bodied cameras seem to be more compatible with this design as it conducts better but I'm not quite ready to lash out the extra $$$ just to validate this idea.
Ta Glen ( in both cases )
Buttons I can fix, I'll chk out Bintel as well for the PS. Easier than changing batteries all the time although they seem to last a night for me.
Has anyone considered modifying the airflow through the heat exchanger. Theoretically, when air is expanded into a plenum it cools. If airflow inside the cooler box is ducted through a narrow duct (between the vanes of a heat sink at the point of maximum cooling) and then expanded into the camera box would there be a further and measurable drop in temperature.
Rather than locate the fan over the heatsink (exchanger) place it to one side and cover the top of the heat sink with a plate to form a series of parallel ducts and channel the flow through the heatsink, expanding it into the cooler box.
Has anyone considered modifying the airflow through the heat exchanger. Theoretically, when air is expanded into a plenum it cools. If airflow inside the cooler box is ducted through a narrow duct (between the vanes of a heat sink at the point of maximum cooling) and then expanded into the camera box would there be a further and measurable drop in temperature.
Rather than locate the fan over the heatsink (exchanger) place it to one side and cover the top of the heat sink with a plate to form a series of parallel ducts and channel the flow through the heatsink, expanding it into the cooler box.
I think you'd be needing a lot more compression than can be acheived within a small closed cycle enclosure. Internal circulating fans as used so far have been tiny CPU units just designed to shift the enclosed dry air (hopefully) to cool the camera by moving heat to the TEC plate for removal to the external heat radiator. Also need to be as vibration free as possible.
We've been able to get quite usable temp drops (16-20*) within the box. The real issue is to get and maintain that at the sensor when the camera body is designed to protect it so well. Professionally modded cameras are in some cases being reduced to skeletons in order to achieve this I believe.
Ta Glen ( in both cases )
Buttons I can fix, I'll chk out Bintel as well for the PS. Easier than changing batteries all the time although they seem to last a night for me.
Bintel want $149 AU$ (- 10% but + postage and Exchange rate to NZ) for a 12VDC to Canon adapter !
I need an 240VAC to Canon power supply. I can make one for $0. Already have 2 x 240VAC to 7.5VDC adapters. I'll just rip open an old NP5 battery case to make a connector unit. That will sort the button issue as well.
Thanks Glen - looks like I've got the one Canon model that isn't covered by that kit (1100D). It's also a bit on the pricey side..I don't need the mains supply, just 12v -> Canon. The hunt continues
You can pick up 240VAC or 12VDC power supplies from local electrical outlets with adjustable output voltages. I have a couple with a small slide switch on the side that work from 3 v to 12 with 6(?) steps in between.
The Camera will accept any voltage between 7 to 8.5v, it has some power management inside. I just happen to have a 7.5VDC supply that came off a small network hub I think. Very fortuitous indeed.
And I've got 2 Canon batteries, one of which is slightly swollen, not a good sign so guess which one is going to be ......
(1100D). It's also a bit on the pricey side..I don't need the mains supply, just 12v -> Canon. The hunt continues 
