QHY294C Cooler Performance

I log lots of information about my astrophotography system for every photo taken. I do this so I can go back at some later date and study what happened and learn what works and what doesn’t work.

I have been using my QHY294C camera for 2 years now. In 2020 I used a cooler setpoint of 0 Celsius. I had no idea what to expect from the cooler and I certainly didn’t want to wreck it by driving it too hard. The QHYCCD folks talk about the possibility of a thermal shock that can occur if the cooler is turned on fully for too long or if it’s shut-off suddenly.

The cooler is controller via a PWM power output which can be varied from 0 to 255. I have the ability to measure what the PWM values correspond to in Amperes. I estimate that 255 = 2.4 Amps. I use a Celestron 5 Amp @ 12VDC power supply to run all of my system. That includes the Camera, Mount, 2 Dew Heaters, 7-port USB Hub and a couple of devices like Arduinos. I have barely seen the current transiently reach 3 Amps. During Imaging the system typically draws 2 Amps. The Celestron CGEM mount seems to draw roughly 0.6 Amps.

I spent a lot of time programming a cooler control strategy into my control software. One of the first issues I tackled was the fact that QHYCCD programmed their driver to immediately hammer the camera temperature down to 0 Celsius when it is first connects to a computer via USB. The Cooler power rises up to full power within a second or two and stays there as the camera temperature falls towards zero.

The programming I settled on was to determine the camera temperature when my software first connects to the camera. This temperature is then used as the Cooler setpoint. This does not avoid the full power step that initially occurs. The Camera driver is first to connect and causes the full power step. Immediately after this the driver allows my program to connect and change the Cooler setpoint. The net result is that the full power step lasts for 2 to 3 seconds and then falls back to zero. I have posted to QHYCCD about this but they were not interested in changing it. I concluded that QHYCCD believes that most owners want the camera to immediate start cooling at zero Celsius.

My programming takes care of ramping the camera temperature up or down as requested by the user. The ramp rate is set at 0.05 C/sec. This is probably too slow and I suspect I will increase this rate. Typically this ramp rate can bring the camera temperature down to zero Celsius in 5 minutes…which is acceptable considering all the other tasks I need to do when starting up. The other concern here is that this ramp rate does cause the camera to draw 20 to 40% more current than it needs during steady state.

The Cooler is controller via a PWM power output which can be varied from 0 to 255. I have the ability to measure what the PWM values correspond to in Amperes. I estimate that 255 = 2.4 Amps. I collected data from 2020 and 2021 for the camera cooler.

In 2020 I used a Cooler setpoint of 0 Celsius. These reading are the orange colored dots. QHYCCD states that the cooler is capable of 35 Celsius of cooling. This means it should be able to maintain zero Celsius up to an air temperature of 35 Celsius. Fortunately, I rarely see 35 Celsius in Ontario Canada. We certainly do see cooler temperatures and in November of 2020 I was able to do some imaging but by this point I had realized that the Cooler was barely being used below 15 Celsius. Because of this I set the Cooler to -10 Celsius for those very cool November evenings.

In 2021 I used a Cooler setpoint of -10 Celsius. These are the blue dots. With the cooler operating at -10C, it should be able to maintain the camera temperature up to an air temperature of 25 Celsius. There are a handful of data points above 24 Celsius for a good reason.

June 6th, 2021 was a hot evening. The air temperature started off at 23 Celsius and climbed during the evening. As the temperature rose I could see that the PWM output and the system current were climbing. This is unusual because during most nights these signals slowly fall as the air cools. It became clear to me that the PWM output was rising a little too fast for my tastes. When it rose to 135 I decided to switch the Cooler setpoint to 0 Celsius for the remainder of the evening. This is the only night where this happened…so far.

I was able to run the cooler at -10 Celsius with an air temperature of 23 degrees. That represents 33 Celsius of cooler. I suspect I will continue to run the cooler at -10 Celsius. Perhaps I may try running the cooler at -15 Celsius on cooler evenings. The extra 5 degrees Celsius will reduce the DARK current noise by almost 50% which is tempting. For some of those cooler nights I will be at my DARK site which is adjacent to Lake Huron…so high humidity is a given. The QHY294C is equipped with an objective glass heater but I always worry about issues with frost on this glass or moisture on the imaging chip. It has not happen yet…and I want to keep it that way.


BTW the spell checker says that the correct spelling is Celsius. On my graphs I used Celcius and my original draft used Celcius. I checked the web and 61% of Canadians spell the word as Celcius even though the man’s name is Celsius.

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