The reason for me taking on the challenge of attaching a cooler to my Meade DSI Pro is simple. The camera is a bit noisy. I’m not taking anything away from the Meade DSI Pro, it is a very sensitive camera and I am ninety-nine percent satisfied with how it performs. I simply began to notice that during imaging sessions, if I waited to long between taking darks and taking light images, new hot pixels would appear and cause problems in my captures. I tried using a fan to cool the camera, but this did not sufficiently correct the problem. I then decided to go with my next option, Peltier cooling of the camera.
The Meade DSI Pro has a large heat sink with a cold finger on the inside which makes direct contact with the ICX254AL CCD chip. I figured this was a good thing as it shouldn’t be too difficult to attach a cooler to the heat sink, and have the chip be cooled by the cooler via the cold finger. The only problem is that the heat sink on the DSI is rounded and not flat. I thought about just using putty to fill the empty space around the heat sink prongs, but I wanted something more efficient than this. I decided to use my Dremel tool and grind off a total of 42 heat sink prongs to allow the Peltier to lay flat on the surface of the case. This is a very intrusive approach and might not be for everyone. I know, because I had a very hard time convincing myself to doing it. It won’t hurt the camera in any way though, so long as you have taken the heat sink off the camera first! Nevertheless, proceed at your own risk.
I used a 40mmX40mm 4mm thick Peltier which I purchased from QualityKits. A Peltier this size fits nicely in an opening similar to the one I created on the DSI heat sink. I then took a CPU heat sink and fan from an old Celeron computer and placed it on top of the DSI heat sink to see how well it fit. I had to slightly grind a few rows of prongs to allow for the CPU heat sink to fit snugly over the Peltier. But once done I had a very good contact between the DSI case, Peltier, and CPU heat sink. I used a silicone based heat sink paste between all the surfaces. I also purchased a small kitchen temperature probe. I took it apart and placed the sensor on the actual cold finger of the DSI heat sink. I then ran the small wires out between the two halves of the cases ,and placed the LCD temperature gage on the side of the DSI. Placing the temperature sensor on the cold finger should yield very accurate temperature readings.
Another important part of this design is the power supply. Powering the Peltier cooler is straight forward. A good 12 Volt DC source which is able to supply a few amps should be sufficient. A variable power supply is even better. In my case I am using a homemade variable power supply with a 3 Amp transformer. I added a 4 Ohm 70Watt power resistor on the positive side between the power supply and the Peltier to limit the current as well as to try to supply the Peltier with a constant current source. I’m currently able to drive the cooler at 8 Volts and 2 Amps.
Well, what has all of this achieved? I’m happy to say promising results! I tested the camera without cooling and with cooling. Here are the results. When the DSI Pro was imaging without any cooling, it’s internal temperature increased from an ambient 77 degrees Fahrenheit to about 84 degrees Fahrenheit. That’s a good five degrees above ambient. I then turned the cooler on at 8Volts/2Amps. At this power level I was able to drop the temperature down from 84.6 degrees Fahrenheit to 54.2 degrees Fahrenheit. This was a 30 degree drop in temperature achieved in about thirty minutes of operation, and translates into a 16 degree Celsius drop in temperature. So far the maximum I have been able to lower the temperature down to has been 52.3 degrees Fahrenheit. I took 60 second dark frames both at the 84 degree mark and at the 54 degree mark for comparison. Here are the results:
Five combined 60 second darks taken at 84 degrees Fahrenheit.
Five combined 60 second darks taken at 54 degrees Fahrenheit.
There is a definite improvement between the first and second dark images. There are still hot pixels on the second dark, but they are now much more manageable. I could potentially drop the temperature even lower with more current and a better heat sink for the Peltier, but I’m already getting close to the dew point down here in Florida during the summer months. The dew point and consequent condensation of the imaging chip is a real limiting facture to how low you can go unless you take more elaborate actions like enclosing the CCD chip in an air tight chamber with all the humidity pumped out of the chamber. This is far beyond what I’m willing to do, and at any rate, I’m totally satisfied with the results I have achieved thus far.
BS2e Peltier Cooler Controller
After some time of using the Peltier cooler on the Meade DSI cameras I came to realize that a controller was in order. The problem I was seeing was two fold. Firstly, I was having to adjust the power to the cooler way to much in an attempt to keep the temperature above dew point during imaging sessions. Secondly, the variations in temperature generated by me fiddling with the power supply were causing problems with my dark frames. There were hot pixels which I could not correct by dark frame subtraction because of the wild variations in temperature.
I decided to build a microcontroller based around the BS2e by Parallax to solve the problem. I chose this microcontroller because I had one laying around doing nothing. The Basic Stamp line by Parallax is for the beginner in the world of microcontrollers. The language has some limitations that can make life a bit difficult at times. Nevertheless, the BS2 is a capable microcontroller.
I used the Parallax Super Carrier Board for the build. This board provides a nice area for soldering additional components. On the prototyping section of the board I installed an SHT11 temperature/humidity sensor, an IRL3103 MOSFET switch circuit for turning the cooler on and off and an RC circuit used by the AD592 temperature probe which is attached to the cold finger inside the DSI camera. I added an LCD screen to the board to be able to quickly get readings while outside. From within the house I use the serial console to read the values. The board is powered by a 12 volt power supply.
The most difficult part of this project was coding the Basic Stamp due to it’s math limitations. The dew point portion of the code was particularly difficult and I couldn’t have done it without help from this site. Here is the .BS2 cooler controller source code for anyone wanting to give this a try. And here it is in plain text for a quick look over.
There are a million different ways to build one of these peltier cooler controllers starting with what microcontroller one uses to hardware to software. I decided to post my experience with building one in the hopes that it helps point someone in the right direction. This project took me a number of weeks to build and there were quite a few instances when I thought I would never be able to complete it successfully. But the satisfaction I now get every night I use the telescope knowing that I don’t have to worry about the CCD fogging up as well as just seeing the temperature track the due point so effortlessly makes it all worthwhile.
Well, I hope the information here will come in handy and if you have any questions just send me an email.