Regulated peltier cooling mod for Canon DSLR

rmuhlack (Richard) · #1 16-12-2014, 07:20 PM

Earlier this year I set about to install a regulated cooling mod for each of my two astro imaging camera: a Canon 1000D and a Canon 450D.

I decided to go with a cold finger rather than a cold box as I was intenting to use an OAG, which due to the clearances required would have been incompatible with the cold box designs I had seen. The cold finger was made out of 1mm thick copper sheet, which was cut to size with a hacksaw. The cold finger is inserted into the gap behind the CMOS sensor, as shown.

rmuhlack (Richard) · #2 16-12-2014, 07:23 PM

A short length of nichrome wire was placed around the telescope side of the CMOS to act as a dew heater to help control condensation during operation. The cold finger and CMOS sensor were then placed back into the camera.

Regulated temperature control is achieved by means of a simple thermostat controller, sourced from eBay. The thermostat control board is shown in the small grey plastic box to the left of the camera. The peltier is also shown in the top left corner of the green cutting board.

rmuhlack (Richard) · #3 16-12-2014, 07:28 PM

so far so good - it still boots up!

rmuhlack (Richard) · #4 16-12-2014, 07:34 PM

the cold side of the peltier is then placed against the copper cold finger. on the other side of the peltier a CPU heat sink and fan is placed. Thermal paste is smeared on both sides of the peltier to ensure effective thermal heat transfer.A piece of perspex is used to secure the CPU heat sink against the peltier and cold finger.

The heat sink chosen was a Noctua NH-L9i. This is a fairly light weight heat sink/fan combo, but which still has capacity to dissipate both the cooling load, as well as the power required for operation (most of which is also lost as heat). The heat sink must have sufficient capacity to dissipate both of these loads in order to keep the hot side close to ambient and therefore provide efficient cooling on the cold side.

rmuhlack (Richard) · #5 16-12-2014, 07:40 PM

The finished mod with the heat sink and jiffy box housing the thermostat. The thermostat included a small temperature sensor, which was placed inside the camera next to the CMOS sensor.

The thermostat has been wired up such that the fan operates continuously to dissipate heat. if the temperature as measured by the sensor is above the set point the peltier kicks in and starts cooling the cold finger. once the cold finger comes down to temperature the thermostat turns the peltier off. From memory the controller has a hysteresis of about 0.1 C. I also wired in a small LED in parallel with the peltier, to indicate when the thermostat had turned on the peltier. Foam with an adhesive backing was also placed on the perspex and the exposed parts of the cold finger to improve cooling efficiency by limiting cooling losses.

With the peltier on flat out I can achieve a delta T (ambient to sensor) of about 30C.

rmuhlack (Richard) · #6 16-12-2014, 07:48 PM

The modded Canon 1000D shown with my Vixen VC200L

rmuhlack (Richard) · #7 16-12-2014, 07:51 PM

The modded 450D is shown here on my 130mm f5 newtonian.

Both cameras feature a second L-shaped mending plate which supports the weight of the cold finger and heat sink via the tripod socket, as shown

rmuhlack (Richard) · #8 16-12-2014, 07:56 PM

The camera peltier cooler plugs into a constant 12V socket on my 1000 oaks dew heater controller (powered by this 12v power supply), whilst the camera's nichrome dew heater simply plugs into one of the four dew heater control sockets.

Unfortunately the dew heater that I fitted to the CMOS sensor is not super effective at controlling dew or ice when operating at a temperature set point significantly below the ambient dew point. To compensate, I settled on a regulated temperature of 10 C, as this is typically above the ambient dew point but still much lower than the operating temperature of the CMOS during long exposure imaging.

rmuhlack (Richard) · #9 16-12-2014, 08:01 PM

Of course the proof of the pudding is in the eating, so here are some examples of recent deep sky images with these camera.

1. Orion widefield
2. M42
3. NGC1365
4. NGC1097 (inverted image to show jets)
5. Corona Australis

rcheshire (Rowland) · #10 16-12-2014, 08:10 PM

Well done Richard - a looks good. Dew heating these systems is a bit of a problem. Because I use a lens only I can get away with a little brute force, but two heaters are better. One around the draw tube and the other the objective. Not sure if that will help in your case?

rmuhlack (Richard) · #11 16-12-2014, 08:18 PM

Thanks Rowland, your considered feedback is always much appreciated

I could look at wrapping a dew heater around the focuser draw tube. However to be honest i wonder if there is much to gain in terms of tangible camera performance. As you have previously noted, there isn't a great deal of dark noise going on at 10C with either of these cameras, and regulation means that my calibration frames are nicely matched with my lights with respect to temperature (which as you know is a significant part of the battle with DSLR imaging).

rcheshire (Rowland) · #12 17-12-2014, 06:46 AM

Ah! I should have read your post with more care. My sensor dew heater regularly operates dew free down to -5C. It's a slightly different design. Resistance ~7ohms and 5v seems to do the job, providing the edge of the wire is line of sight to the sensor. However, I think a certain amount of conduction is at play through the plastic as well. It's quite aggressive and diminishes the total cooling effect by 4C or so. Still adequate to get down to -5 most nights.

rmuhlack (Richard) · #13 17-12-2014, 07:11 AM

Quote:

Originally Posted by rcheshire

It's quite aggressive and diminishes the total cooling effect by 4C or so. Still adequate to get down to -5 most nights.

this may be the key to my dew issues - i suspect now that my dew heater is simply not getting hot enough. perhaps I should swap out the in-line resister in the dew circuit for something with slightly lower resistance...

might need to do some bench testing to ensure that I dont get it red hot though

rcheshire (Rowland) · #14 17-12-2014, 04:57 PM

I was concerned about overheating too. ~7-8ohm at 4.5 - 5v is safe.

benzomobile (Beppe) · #15 22-12-2014, 11:16 PM

Hello Richard,
I have modified my Canon 1100D using a copper cold finger, a 45W TEC and a squared ring-shaped NiCr wire, about 35 ohms.

https://sites.google.com/site/beppel...al-dew-control

I filled with small granules of silica gel the tiny groove inside the plastic frame, empted after the lpf#2 filter removal.
This has resulted in the reduction of the relative humidity inside the sealed chamber I made between the filter and the sensor surface.
A NiCr resistive wire (384 ohm/m) was stacked on the frame, at a small distance from the filter edge. All is 12V powered in series with a resistor of 120 ohms.
Therefore, the wire dissipates about 1/5W (2.7V x 78 mA).
The developed power heats the lpf#1 filter only (I left it in its place), while no significant calor is transmitted to the air inside the chamber.
I noticed through various tests that the power dissipated by the NiCr wire should not be too high. In this case, in fact, the temperature inside the chamber will increase too much, causing the formation of some (few) filaments of frost on the sensor.
With a power of about 0.20-0.25W, I have completely solved the condensation issue, even at a sensor temperature of more than -10 ° C.
Note that in this manner the sensor surface is not directly heated, therefore the sensor total cooling is not diminished by an excessive power of the dew heater.
The current flowing inside the TEC is efficiently controlled by my homebuild PWM & low pass filtered controller. This is not a standard thermostat that turns on and off the cell, but it's exactly a power supply that varies the voltage just in continuous, being driven - in real time with very small hysteresis - by the variation of the NTC resistance.

Giuseppe
Como, Italy