Cooling a 10" Newtonian
Anthony Wesley Cooling a Newt, part 1
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.
Cooling a Newt, part 2
4th April 2004 This section describes the electric heat pump that I have constructed to help in cooling the main mirror of my 10" newtonian. I had to find a way to attach a cooling unit to the mirror. The mirror cell (made from cast aluminium) is a very open design, with a 70mm diameter hole in the centre that goes all the way out the bottom of the cell. I decided to use this hole as the main thermal conduit to the mirror, using a cylinder of solid aluminium that would fit into this hole, and bolt onto an aluminium cold plate the same diameter as my mirror. The mirror sits on the cold plate (but is not firmly attached so as to avoid thermal stresses on the mirror).
Update The mirror no longer sits on the cold plate, but now sits on its flotation points again and these points in turn are attached to the cold plate. I grabbed some images of this system to make it a bit clearer...
The bits, spread out on my workbench. From left to right they are:
- The heatsinks and fans (top) and the peltier units and their aluminium plate (bottom).
- The 10" aluminium plate and solid aluminium cylinder with mounting threads.
- The mirror cell and mirror (upside down) showing the mounting hole in the centre.
The mirror cell back in the scope with the aluminium plate and cylinder fitted. The mirror sits on the plate with lots of thermal paste applied to help the transfer of heat from the mirror to the plate. More thermal paste was used between the plate and the central cylinder.
Update Due to concerns about inducing astigmatism with the cold plate directly touching the mirror, I have swapped the location of the cold plate and flotation points around so that the mirror now sits on its flotation points again, and these points in turn are anchored to the cold plate. That leaves a gap of about 3mm between the back of the mirror and the cold plate. I'm happy to say that after testing the new arrangem
ent I am still getting exc
ellent results, showing that the cold
plate doesnt have t
o actually touch the mirror t
o work.
Closeup of the cooling unit showing the two heatsinks (fans removed) bolted onto the hot sides of the two peltiers.
There is a 3mm thick layer of perspex between the heatsinks and the cold aluminium plate to thermally isolate them. This does not interfere with the operation of the cooler since the peltier units themselves are 4mm high. The cold side of each peltier is in thermal contact with the aluminium plate (more thermal paste) and the hot side is in contact with the heatsinks (more thermal paste).
The four holes in the middle of this unit line up with the four holes in the aluminium cylinder in the centre of the mirror cell (previous image).
The cooler bolted onto the mounting block. All that is left to do is re-attach the fans ( removed to gain access to the bolt holes in each heatsink) and apply some power.
The peltiers used here are
Marlow DT12-6. When attached to 12v the cooler draws 8 amps so you can forget about running a monster like this from a small plugpack :-)
For my initial tests and on-site test I used a heavy duty 12v power supply that can supply up to 25A, just to be on the safe side...
First Results
This unit was tested out in the field on the night of April 3. Unfortunately there was no opportunity to use the scope because persistent cloud rolled in just after sunset, but I was able to test the active cooler to see if it could cool the mirror faster than ambient air. Here is the graph from the temperature logger:
The relevant parts of this graph are:
- The RED is the mirror temperature.
- The GREEN is the air temp inside the tube (near the mirror).
- The PURPLE
is the external (ambient) air temp.
The cooler was switched on as soon as the scope was set up. Looking at the left hand side of the graph we see the mirror temp drop rapidly compared to the ambient temperature. If you compare this to the graphs given in part 1 of this project then it's clear that the active cooler is more than able to do its job.
The cooler was switched off when the mirror temp approached ambient (about 1840). This is when I discovered that my heavy-duty 12v power supply caused enough of a voltage spike to crash the temperature logger whenever it was turned on or off :-( (hence the gaps in the graph).
Just before 1930 I switched the cooler on again for another test. You can see that the mirror temp starts to plummet very quickly. Also note the effect on the internal tube temperature (green line) caused by unwanted warm air from the heatsink/fans leaking into the bottom of the tube. When the cooler is operating, enough warm air leaks into the tube to raise this temp by a degree or so.
While not serious, this intrusion of warm air into the tube is not wanted. I'll probably have to construct a shroud of some sort to stop this happening.
The rest of this graph shows me waiting patiently for the clouds to part... which they singularly fail to do. I don't bother reattaching the cooler later in the night... it's starting to look like no real astronomy will be done. By about 2am it's time to pack up and head home.
No webcam images, but at least the cooler is working :-) The next (and final) part of this project will hopefully show some improved images... weather permitting, of course :-)
Cooling a Newt, part 3
12th April 2004 Well, I've been using the cooler for just over 1 week, and it's now a permanent part of my scope. The improvement has been dramatic to say the least, and there is the unforseen bonus that I can now be setup and actually imaging only 1 or 2 hours after putting the scope out. I've owned this scope (and this mirror) for 14 years, and finally this puts an end to the frustration of waiting for the scope to cool down!
In the previous part of this series I talked about the problems with my heavy duty 25A power supply causing other computer gear to malfunction - this showed up as gaps in the temperature charts while I had to wait for it all to reboot. I've solved this problem by replacing the 25A supply with a normal 350W computer power supply. The new supply is much much lighter, and is rated at 12A on the +12V rail (more than I need to run the cooler, the scope and tube heater).
Consolidating these power supplies into one unit has made life a bit easier, and now I can switch the cooling on and off at will.
Results
Here is the temperature chart from last nights observing session:
The cooler was switched on from the outset - notice that the
MIRROR cooled just as fast as the
AMBIENT temperature was dropping, and in fact pulled ahead of the ambient from 19h30m. At about 9pm (2100) I switched off the cooler and started grabbing images. There's no doubt that the scope was very close to equilibrium as you can see from this processed Jupiter taken at that time:
Taken from my backyard in suburban Canberra, shooting over the neighbours roof with Jupiter at about 45 degrees above the horizon, this is about as good as the seeing will be. I'd have to head out of town to make much of an improvement over this.
After about half an hour the mirror had once again become warmer than the ambient air and so the cooler was switched back on again for a while. You can see that the mirror starts cooling straight away (at about 2135).
You can see brief periods in the
MIRROR line where the cooler was switched off so I could grab some images. These periods are at 2150, 2210 and 2220. Each of these periods shows up as a levelling of the mirror temperature.
The tube
INTERNAL temperature was all over the place - it was affected by waste heat from the cooler leaking back into the tube (that's why it suddenly spikes upward sometimes when the cooler is operating). I tried to plug up these leaks but as you can see from the graph I wasn't always successful.
Summary
Knowing the temperature of your scope and having a means to cool or heat as required opens up new opportunities for capturing high detail without being at the mercy of the weather or an overly thick Pyrex mirror!
The total parts and manufacturing cost for the cooler (2 peltier units, bits of waste aluminium and perspex, the cold plate and aluminium cylinder) for me was around AUS$300.00. I could have cut this in half had I chosen to do more of the work myself (but then I might not have ended up with a good result!).
The computer power supply to run it cost AUS$35.
All up, I am very happy with the new lease on life this gives me. I'll update this page with more images from tonight if the skies stay clear. I might even snap a picture of the beast with all its various cables. It's getting hard to find the scope under all the wires :-)
Cooling a Newt: Part 4
25th May 2005 Well, it's been over a year since I wrote the first 3 parts to this story, and it's time to add a f
inal chapter that summarises the dramatic improvements in imaging that I've found with this arrangement. I've spent plenty of time thinking about what I wanted to put into this
fourth part and so it will
probably turn out to be the longest chap
ter. I h
ope you don
't mind reading through it :-)
First though, a word of caution. The results that I am getting, and the requirement to build this peltier cooler come from the climate that I find myself living in. For those of you lucky enough to live in places that have mild climates with little day-to-night temperature swing then this will be of only mild interest. You probably find your scope cooled and ready to go after only a couple of hours.
For people like me who live in climates that have a regular 20C temperature swing from day to night, however, I hope you find this record of my cooling attempts to be a useful insight into the length that some people will go to get the most out of their scope, and maybe it will give you some insights into the importance of temperature control.
I have done a fair amount of imaging this year, starting in early January (mid summer for Canberra). In the previous 10 years or so that I've been living here I hardly bothered at all to try and image in summer cause the thermal gremlins would always kill me. The thermal profile is something like this: sunset temp = 28C, drops 2C per hour until overnight minimum of around 12C at about 3am. As I have said before, passive cooling just does not overcome this temperature range with my 10" 35mm thick mirror.
However, this year was the first summer that I was equipped with my peltier cooler so I was keen to give it a try. I still was only able to image from around 3am onwards when the air temperature had stabilised, but at least my aggressive cooling from sunset through until 3am meant that the mirror was already at ambient temp and ready to go.
Here's a short detour: I've put up a collection of images on my
Image Gallery that show the sort of results I could get by imaging in early morning with a scope that's in equilibrium. Compare that to the Jupiter image from Part 3 of this story, and you'll appreciate how much improvement is possible.
Peltier Cooler Mk 2
Most of the images in the gallery were taken using the peltier cooler as shown in Parts 1-3 of this story, but I have recently built a revised version that eliminates some of the usability problems with the original. The new version works just as well as the original but is much more convenient to transport, and solves the "heat leaking into the tube" problem of the earlier model.
Here are pictures of the new cooler. Click on them for a full-sized (large) image if you want to see a closeup:
Inside View of cooler Mk 2, showing the cold plate and circulation fans 
External View of cooler Mk 2, showing the hot side (heatsinks) with cooling fans and switches 
This is functionally almost identical to the old model, but there are a few important differences so I'll descibe the whole thing..
This unit is designed to fit over the mirror end of my tube. The three evenly spaced large holes in the cooler are for my collimation adjusting bolts on the mirror cell. They have to pass through the cooler so I can make collimation adjustments with the unit attached.
The
circulation fans visible on the first image are small computer cpu fans that just happen to be the same size as the hole in the centre of my mirror cell. When the cooler is attached these fans sit inside that hole, pulling air off the cold plate and blowing it onto the back of the mirror. I have since added a black rubber shroud around these fans to make sure that air must be pulled from near the cold plate. The fans are powered in parallel from the 12v power supply that runs to the peltiers.
Notice the circular perspex disk (clear plastic). This is 3mm thick and separates the cold side from the hot side. This disk is large enough to close off the mirror end of my tube, and overlaps by about 1cm all the way around. The overlap prevents any hot air coming off those two heatsinks on the outside of the unit from finding its way into the tube.
Sharp eyed readers will recognise the aluminium disk used in this cooler as the same disk that was used in the previous model. I didn't see any reason to have a new one made, so I re-used it. Don't be confused by the four holes visible in it's centre - they were used in the old cooler but are not needed in this version.
The aluminium disk is firmly attached to the larger perspex disk using silicone sealant, the same variety normally used to seal bathrooms or gutters. I found it was important to seal around the edge of the aluminium disk to stop water condensing between it and the perspex when the plate gets very cold.
The perspex disk has two rectangular pieces removed for the peltier units to sit in. The peltiers make contact with the aluminium plate on their cold side, and the large heatsinks on their hot side. I used nylon bolts to anchor the heatsinks in place so that the bolts would not act as conduits for heat from one side to the other. You can see the heads of these bolts (four on each peltier) in the first image above.
I re-used the peltier units and heatsinks from the earlier cooler, but added two switches that can be seen in the second image. These switches let me turn the power on or off to each peltier unit. This gives me more versatility, since I can now operate in three modes - no peltiers, 1 peltier or 2 peltiers. The circulation fans are always running so the "no peltiers" mode is useful when I just want to circulate the air around the mirror without extra cooling.
Pretty Graphs
As you might excpect I have many more temperature graphs now than I did a year ago :-)
Temperature data for Feb 22 - 23 2005, Canberra Australia
An image of Jupiter taken at 4:02am on this night..
NOTE Due to the 1 hour difference introduced by daylight savings in summer, this graph is 1 hour out. 4:02am actually shows up on this graph as 3:02am.
The scope was setup pretty much on sunset (8:30pm, shows up on the graph as 21:30, again out by an hour due to daylight savings) and the cooling turned on. You can see that the mirror (
back sensor and
side sensor) starts out at about 22C and cools quickly when the peltiers are running.
The
air sensor and
tube sensor show how the ambient temperature changes during the evening as everything cools down.
Notice also that the mirror temp starts to oscillate once the mirror reaches the same temp as ambient. I have a small laptop that is watching these temperature readings, and it turns power off to the peltier automatically when the mirror reaches the target temperature. If, as we see here, the air temp keeps dropping, then it will turn the power back on again a short time later. It averages together the readings from the two mirror sensors (
side and
back) so that I have some idea of how the mirror is reacting as a whole.
The (
back mirror sensor shows a temperature rise of around 0.5C when the cooling is turned off, I am guessing that t
his is due to the thermal gradient inside the glass mirror slowly evening out. The inside of the mirror will be a bit warmer than the edge at first, but over th space of about 30 mins this gradient disappears and the mirror as a whole
reaches equilirum.
On the graph we can see that the coolin
g was turned off
at a
bout 3:30am (marked as 2:30am on th
e graph), so by 4am the mirror was dead on ambient temp, and some very nice images were grabbed.
Temperature data for Feb 17 - 18 2005, Rockhampton Australia
This temperature chart was made from Rockhampton, up in sunny Queensland where the day to night temperature swings are much lower, and the climate is generally much nicer. As you can see the overnight minimum was around 19C, very pleasant. I was there on holidays for two weeks, so I took the scope and gear with me to have some Quality Time with Jupiter :-)
The very high
ambient reading at the start of the graph was because that sensor was in direct sunlight for a while, before sunset.
Again, notice that the cooling is able to very rapidly bring the mirror temp under control, and by about 7pm (19:00 on the graph) the system is in housekeeping mode, just kicking in every now and then to make a small adjustment.
The temperature spike at about 22:40 was caused by operator error, I accidentally unplugged the sensors while reorienting the scope to point at Jupiter.
Here are two images from that night: Jupiter at 10:33pm localtime (1233UTC)
Jupiter at 10:35pm localtime (1235UTC)
I am now very happy with my cooling system and the results it produces. I can highly reccommend something like this for people who live in regions with large day-to-night temperature swings if you want to get the most out of your scopes.
Anthony Wesley
