Installing Peltier Cooling in a C14

In early 2006 I met Anthony Wesley at Snake Valley in Victoria. Amongst other things he and I discussed at length the idea of using peltiers to cool the primary mirror of reflecting telescopes. I watched a demonstration over two nights and I was totally convinced that not only was he onto something, but that he would gain fantastic images as a result of using the cooling system. He was using the peltier system on a Newtonian telescope. The trouble was that I had a C9.25 at the time. I had no idea how to make this system work on an SCT. I did know that the C9.25 would not readily make itself adaptable due to the size of the mirror back. There was just not enough room on the back to install a peltier and use my Moonlite focuser. The remote motor arm would make contact with the type of peltier cooler I had in mind.

Early this year I decided to upgrade from the C9.25 to a C14. The sheer size of this scope allowed me to install two peltier coolers on the mirror back and this article tells how I went about doing that.

Disassembly

The first thing that you must do is remove the retaining ring that holds the corrector plate in position. There are 8 screws that simply unscrew and allow you to remove the ring. I found that if you just tighten one of the screws a little first and then unscrew it; this will give you an index to how tight the screws should be once you reassemble. They are quite tight, but it pays to know that they are not too tight.

The image below shows the C14 with the retaining ring removed.

In the above image you can see the registration marks that I made once I lifted the retaining ring off. I was careful not to move the corrector at all while lifting the ring off. I then made 4 lots of registration marks with paper tabs. Then I also used white out (liquid paper) to make more registration marks that would be totally permanent. Once again I made 4 marks in this fashion.

Additionally it should be noted that Celestron has installed tiny grub screws into the corrector assembly to maintain position of the corrector. In previous versions of the OTA paper shims were employed to serve the purpose. I found two of the grub screws were screwed into position against the corrector and the other two were loose in their threads. Later when I reinstalled the corrector I ensured that all the grub screws were tightened up into position against the corrector.

Next, grab the secondary assembly and lift the corrector straight out. Immediately place it into a new garbage bag sitting on the actual secondary assembly. That way no dust can get onto the secondary. I found it useful to find a place where the corrector would not get knocked over so I put it into the storage box.

Once this is taken care of, lay the scope on its side and start taking out the retaining screws which hold the tube onto the mirror back. First though, ensure the primary is retracted to its rear most limit. Turn the focus knob and watch the mirror retract. Once this is done slide your hand slowly down the tube and stop once you get to one of the nuts that holds a screw. With your other hand use a short Phillips head screw driver to unscrew the bold from the nut. Repeat this procedure until all the screws are removed.

Now gently return the OTA to its upright position but do this by lifting the mirror back. Now slide the OTA tube out of the mirror back. I found that the tube is a very snug fit and requires a fair bit of twisting to remove it. Also you need to have registration marks here too, so that the tube goes back in the same orientation.

The image below shows the mirror back with mirror and baffle.

Once the tube is removed you can think about removing the mirror. To do this turn the mirror back and face it towards you. Pull the rubber hand grip of the focuser off the focusing knob. Then undo the screws that retain the whole assembly. Remember which order the assembly fits together. Place all the parts into a snap lock bag to prevent loosing any later. Now turn the mirror back around and remove the rubber retaining ring located on the baffle. It is a small rubber ring and looks like an O-Ring. Now if you push on the threaded bolt poking out of the rear of the mirror back (you exposed it by removing the focuser assembly) the mirror will slide towards you. Keep pushing this until the rear edge of the mirror is exposed. Now all you need to do is pull the mirror straight off the baffle. Upon doing this immediately turn the mirror towards the ground. This will prevent dust and flakes falling onto it while you are placing it in a safe place. Watch out for the grease on the baffle too. Do not get this grease on the mirror! Place the mirror assembly face down and resting on the baffle. (The baffle is comprised of two parts; one part is connected to the mirror and the other part is connected the mirror back). I also found that with the C14 mirror I could carry it around by hanging onto the mirror carriage that enables the mirror to be locked down during transport.

The image below shows the exposed mirror back where the peltier system is going to be installed (Well the internal fans and wiring for the system).

Installing the Peltiers

Now that you have the mirror back empty you need to cover the baffle and preserve the grease that you have. I got a plastic carry bag (the kind you get from supermarkets with your groceries) and put it over the baffle to cover it up. I then packed tissue paper into the hollow where the baffle meets the mirror back and placed further tape over the entire area. Doing this will prevent swarf getting onto the baffle and into the grease.

The image below shows what you should aim for.

Once this is completed you can commence drilling the holes you need to support the peltiers. The peltiers I used came out of two small car fridges that run on 12 volt supply. I simply gutted the units and retrieved the peltier units.

Before screwing the peltiers into place you need to do two things. The first is removing the paint from the region where the aluminium plate will come into direct contact with the mirror back. I used a paint stripper to do this. Make sure you remove the all stripper afterwards. (The paint if not removed will hinder the ability of the peltiers to cool the mirror back.) Then once this is done apply heat paste directly onto the area where the paint has been stripped. Use a lot of heat paste. It will aid in the cooling of the mirror back. The image below shows the peltiers installed onto the mirror back. You can only install two peltiers on the back of a C14 with the type of peltier cooler I was using. You need the remaining space for your wiring ports and temperature sensor unit. Alternatively you could use peltiers and heat sinks from an alternative source which are smaller and then you can install a third peltier. This will increase the efficiency of the cooling and make for a faster cooling.

After the peltiers are installed you next need to install the internal fans. The aim here is to install them as close to location of the peltiers. It will be more effective to have the fans in that location than any other point. The heat exchange will be greatest at this point and once again adds efficiency. Also note that the fans must blow air onto the mirror back. The natural assumption is that the fans are better served by pulling the cold air off the mirror back and onto the mirror itself. This assumption is incorrect in that having the fans in the reverse position will rapidly cool the air in the tube, which will cool the mirror more evenly. The image below shows the size of the fans (40mm) and locations.

Now that you have all the components screwed in place it is time to connect all the wiring together. You have to join all the positive wires together and all of the negative wires together. If you want you can isolate the fan wires from the peltier wiring. This gives you the option to run the fans without the peltiers. I have found that this is not necessary; however you may find it more ideal for your circumstances. Use banana plug connectors both male and female. The female part gets attached to the mirror back and the male part is used on the power supply end. This gives you the freedom to simply unplug the power supply and start imaging. Also this means that the mount does not have to also support the weight of the power supply even when it is switched off.

Most of the wiring will need tinning to create the lengths needed. Using heat shrink tubing to exposed wires will prevent other cables shorting out. Getting a short once the unit scope is back together is not an option. As an adjunct to this I also wrapped all the cables in electrical insulation tape. The image below shows all the wiring completed.

Now that the wiring is completed you need to do a bench test. First test all the circuits with a voltage meter. Ensure all the wires have the same continuity. Then; with a 12 volt supply (such as a computer power supply) test to see if the unit works correctly. Run the peltiers for 20 minutes, in that time the mirror back should get cold and the internal fans should blow air onto the mirror back. If everything works as it should, then it is time to apply silicon to glue down the wiring. I used aquarium grade clear silicone as it will not affect the mirror coatings.

Now it is time to install the temperature sensor. I obtained a unit that is slimline in nature and gives an internal reading and an external reading. You will need to be able to determine that the temperature of the mirror is equal to the outside ambient. The image below shows the unit that I purchased. It is about 4 inches long and 0.5 an inch wide.

The cable and sensor are over 3 metres long when you buy the unit. This needs to be shortened somewhat. I initially made the cable 1.2metres long. You need it this long for reassembly of the OTA. You will need to also ensure that the polarity remains the same. If you split the cable length-wise and then cut one side and then tie a knot into each side you will have maintained polarity. When you cut the other part of the cable just leave it without any knots.

Now grab the plastic top off a coke bottle and fill it with silicone. Pull the paper off the sensor and stick it directly onto the middle of the rear of the primary mirror. Application to this region will give you a mean average of the temperature that the mirror is undergoing during cooling. (Another option is to have one sensor on the mirror's edge and one near the centre of the mirror. Doing this will give you a more comprehensive view of the cooling process.) Then with the coke bottle top turn it over and place it directly on top of the sensor. This will ensure you have thoroughly isolated the sensor so that it gives a correct reading later. You don't want it to give you the temperature of the air inside the OTA; you want it to give you the temperature of the mirror. Let the sensor dry for a day so that it does no leach or drop off the mirror.

While you are waiting for the silicone to cure, it is now time to install the flocking onto the inside of the tube. I used an adhesive backed black felt that I purchased from a local haberdashery. It has never dropped any lint onto the mirror from the moment I re-assembled the scope. Take your time with the installation; don't overlap the felt as it may lift in the future. Make sure you have good contact over the entire felt. I cut the felt into strips (around 4-5 inches wide) and then installed it piece by piece. Cutting it into smaller strips makes it more manageable especially if you have to cut around any objects. The image below shows you what an un-flocked scope looks like.

And; the image below shows what a flocked scope looks like.

You can see that the reflected light is lessened by the flocking. Although some people say that this flocking makes little difference to the overall performance of an SCT because of the length of the central baffle. Technically this could be correct, although I have found from empirical evidence that Jupiter now shows more contrast in the bands (in fact colour is now easily seen, which was not evident before) and images of the moon are less overwhelmed and give superb contrast. I would say in my estimation that this is about a 15% increase in contrast over an un-flocked tube.

So, once the flocking is completed and the silicone is cured it is time to reassemble the telescope. The assembly is the reverse of the disassembly. Things to keep in mind are the screws and nuts to fix the tube to the mirror back. I found that gently sliding your fingers down the tube with a nut in between index and middle fingers works well. Be vigilant in doing this as you can easily touch the mirror. One little smudge is nothing but several smudges will annoy you.

Another important assembly point is that of the focuser. I found it easier is I pulled the mirror all the way down and then tighten the screws that hold the mirror for transport. This gave me perfect alignment of the focuser assembly. Additionally, make sure you fully torque that screw that goes in the centre of the focusing knob. It will be holding your mirror when you have it inverted at some point in the future.

When you have finally got the OTA assembled there are two more jobs to do. The cable that you pulled out of the mirror back that is connected the mirror sensor now has to be joined to the temperature monitor by tinning and covered by heat shrink. I ran the mirror all the way forward and then cut the cable off at 3 inches hanging out of the mirror back. This will allow a bit of slack in the cable and prevent the assembly from fouling. Make sure also to maintain polarity of the cable.

The other job that needs doing is installing insulation to the mirror back. You will need to do this so that the cooling system can work quickly and efficiently. I used adhesive foam strips for this. It is commonly used in the partition and ceiling part of the building industry. The image below shows the strips stuck into position. I used two layers to aid in insulating the mirror back.

Final Analysis

Finally, I want to show you what sort of images can be gained from this modification. The two images below were taken after the modification on nights of good seeing (7/10). It was well worth the effort and I encourage all to undertake this mod. Your images will improve by controlling the mirror's release of core heat and that will give you more time to image earlier in the evening. I have cooled my mirror recently from 39 degrees Celsius to 9.1 degrees Celsius in less than 2 hours. That would have been impossible without active cooling. It would have taken all night for the mirror to reach ambient.

Now, the first image is of Jupiter, it was taken on the 3 September 2007. Lots of fine detail can be seen throughout the image. It was taken 1 hour after sunset with the temperature dropping rapidly from 18 degrees to 7 degrees. I started cooling one and a half hours before reaching ambient. The mirror temperature at the commencement of cooling was 23 degrees. When I stopped cooling, the mirror was at ambient and the temperature was 7.3 degrees. There was no visible evidence of tube currents after allowing the mirror to settle for 15 minutes. The image was taken at a focal length of 12,500mm. Jupiter by this stage was just past the zenith and no where near as close as it was at opposition. Opposition was in June and so an image of this calibre was not expected this late in the season. If nothing else the scope has been reassembled correctly!

The next image is that of the crater Moretus located near the southern polar region of the Moon. The image is taken at 12,100mm focal length. It was taken 2 hours after sunset. Cooling was stopped 35 minutes before the image was taken. Fine detail can be seen in the smaller craters and it seems that small craters can be seen everywhere.

So I thank you for reading this and if you have any questions relating to the modification you can contact me at http://southcelestialpole.org.au . I am more than happy to help you with any questions that you may have.

by Paul Haese