Canon 550D cold finger

[MarkS]

Interesting idea.

However I don't think the sensor can be separated from the control board that sits immediately behind it - they form more or less a single unit.  In any case it's not something I would want to attempt because of the complexity and the risk of rendering it inoperable.

[julian]

I'm sure I used 2mm copper sheet, with heat transfer past between the copper and camera chip.
I also sealed the chip PCB with lacquer and went over it with a hot glue gun. I didn't lacquer connectors or push on chips, as the lacquer I used was very runny.
As of yet I have had no condensation problems with the camera.

[MarkS]

I'm sure I used 2mm copper sheet

The 450D you modified has a much bigger gap behind the sensor.
The gap has shrunk to almost nothing on these latest Canons.

[MarkS]

I've done some simple thermal conductivity calculations which indicate that to remove 1W of heat from the area behind the sensor (assuming it's unformly distributed) will require a 5C temperature gradient in the 0.3mm copper strip, which will then obviously result in a 5C temperature gradient across the sensor itself.

But I can't easily determine how much temperature reduction (below ambient) would be achieved by removing that quantity of heat, because there are too many unknowns - especially how much heat is generated by the sensor. and nerby circuitry.

I'll just have to experiment and see.

[mickw]

If you can operate the camera with the circuitry/sensor exposed, I've a laser thermometer you're welcome to borrow.
It might not be sensitive/accurate enough but should show any deviation.

[RobertM]

It should be easier than that, just read the EXIF data for the internal thermometer.  I think APT records that info.

[MarkS]

The temperature recorded in the EXIF is the temperature recorded at some place within the camera body (not entirely sure where).  It is not necessarily the same as the temperature of the sensor itself, which directly affects dark current.  I did once extract the EXIF temperatures as the sensor warmed up and they were not a good predictor of dark current (except for the first frame or two before the sensor began to heat up).

The best way to measure the thermal gradient across the sensor is to measure the noise at various positions in difference frame between 2 darks and remove the read noise component (using a formula very similar to Pythagoras's Theorem).  This has the advantage of directly measuring the quantity that will affect the signal to noise ratio in your final images.

[MarkS]

Here's the prototype cold finger created from 2mm and 0.3mm copper strips soldered together with plumbers solder:




Put into position in camera:




Screwed in place:




Camera reassembled.  The microphone socket had to be permanently removed to make room:




Peltier and heatsink added:




At room temperature of 20C, 1.5A at 12V was required to reduce the sensor temperature to 15C below ambient i.e. 5C
At this temperature I measured the thermal noise in a 5 minute dark frame to be 2.2 electrons i.e. lower than the read noise of 3 electrons. The darks revealed no obvious left/right thermal gradient.

Boosting the Peltier current to 2A reduced reduced the dark current by half again, giving a thermal noise of only 1.5e in a 5min exposure.

[julian]

Mark have you seen this post may help you get more cooling from the TEC .
Are you going to put the camera in a box as I had to due to condensation?
If you are, I would see if you can do it in a way that you can use a camera lens as well as a telescope as I didn't and wished I had, as I used a 2 " adapter due to the flash unit on top of the camera being in the way.

http://stargazerslounge.com/topic/146756-1100d-cold-finger-sensor-cooling-with-tec-and-water-cooling/

[MarkS]

Julian,

That's a very serious mod in the link you posted!  Not sure quite how practical it is though.

No, I have no intention of putting the camera in a box - I'd like to use the camera occasionally on the Hyperstar and for that I need the TEC and heatsink to be removable - that functionality has yet to be designed.  But I'm intending to replace the heatsink with a low profile copper one.

If condensation (on the imaging window) proves to be a problem I'll add a heating wire loop on the window.

Mark