Thermal imbalance is a major performace killer of medium- to large- size newtonians. There are three parts of the thermal system that must be at the same temperature - the primary mirror, the inside of the tube, and the ambient (external) air. Unless all three of these parts are within a small fraction of a degree of each other then you can forget about crisp, high resolution views of planets.
There is a common piece of lore among home astronomers - place the telescope outside for at least an hour before you use it to allow everything to come to thermal equilibrium. This is a nice idea, but I have repeatedly found that my 10" newtonian telescope refuses to come to equilibrium in this time. On many occasions I have waited around and eventually given up at about 2am and pack it all away without having looked at anything because the scope simply *did not* come to equilibrium, despite having plenty of time (7 or 8 hours)!
Being curious about this, I recently built a 4-way digital temperature recorder (shameless plug - www.picaxe.co.uk. If you're in Australia then the importer for these excellent microcontrollers is Micro-Zed Computers. My project is non-standard, but is just a simple variation using the 28X starter pack.).
My logger sends temperature readings back to a PC that stores and graphs the temperature changes over an evening with a resolution of 1 second. I put one temp sensor on the mirror (on the back, of course) that was insulated against everything but the mirror temperature, another sensor inside the tube not far from the mirror, and a third sensor outside the tube in the ambient air.
The graphs from this experiment clearly show why the scope never makes it to equilibrium. Here is a typical set of data:
The three channels of temperature data correspond to MIRROR, INSIDE TUBE and AMBIENT.
This data was captured from my backyard in suburban Canberra on a clear, still evening in March. The temperature at sunset was about 17 degrees C, as you can see at the start of the graph. The telescope had been stored inside, so the mirror temp started out a few degrees higher.
The temperature drops smoothly for a few hours - notice that the ambient and internal temperatures track very closely for the first part of the graph, and also notice that the mirror can't lose heat fast enough to ever catch up with the falling air temperature.
The sudden and chaotic temperature swings that start at about 10.30pm result from the air falling below dewpoint and starting to shed its load of water onto the telescope and surrounds. When this happens, the air temp inside the tube quickly drops about 1 degree below ambient, elevating the temperature difference between the inside air and the mirror to more than 2 degrees. The temperature seems to be finally levelling off at about 2am when I had to pack it all up, having finally reached something close to the predicted low of 7 degrees for that night. Even at this stage - 7 hours after setting up - the mirror is still about 2 degrees warmer than the surrounding air. Visual checks at regular intervals while this data was being acquired showed very poor seeing conditions, as you would expect with such a large thermal imbalance.
Now, lets contrast this with a graph from the night before:
Here we see something quite different - by about 9.30pm (2130) the mirror had come into thermal balance with the surrounding air, promising very good viewing. The only problem was that the sky was heavy with low cloud for the whole evening, making any viewing impossible!
In fact, in something of a catch-22 situation, the reason that the mirror came into equilibrium so quickly was caused by the low cloud - notice that the ambient temperature stabilised at about 16 degrees. The presence of the low cloud kept the ambient temperature high, but prevented any useful astronomy. There was a brief thinning in the cloud at about 10.30pm, showing up clearly on the temperature chart as a sudden drop of about 0.5 degree C. If the cloud had cleared away then I would have had a time window of only about half an hour before the dropping temperature puts the system too far out of balance to be functional.
The data suggests that on clear nights I will have to wait until 3 or 4 am to get everything into equilibrium. This is not acceptable :-)
Clearly something has to be done. My current plan is to construct and attach an electric heat pump to the back of the mirror to assist in pumping heat out of it faster than it can manage by itself. Hopefully that will steepen the cooling curve of the mirror enough for it to catch up to the ambient temperature at a more sensible time.
In the past I have tried adding extra cooling using fans on the end of the tube (I have no temperature graphs of this), but still the mirror cannot lose heat fast enough without active help. I've ordered a Peltier cooling unit (Thermo-Electric Cooler, or TEC) which should be able to do some serious chilling around the mirror. I expect that I'll have to cool very aggressively to drop the mirror temperature and then switch off the cooler and wait for everything to warm up to ambient.
To be continued...
Anthony Wesley