| DIY Cooling Modification |
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Schmidt Cassegrain Telescope (SCT)
Tutorial: How to install Peltier coolers on a telescope.
Introduction. Having a big telescope is all well and good when you first start amateur astronomy, however, when you get into imaging you soon learn that a commercial and off-the-shelf telescope only goes so far. There comes a time when you consider enhancing your telescope beyond its original capabilities. For some time now I have been dealing with focus issues, especially when trying to capture detail in planetary photography with a webcam. The problem arises from thermal differences and currents within the optical tube assembly. Unlike Newtonian reflectors, an SCT is a sealed unit thanks to having a glass corrector plate in the front aperture. This means it is a lot harder and slower for the mirror and thermal mass inside to cool down evenly with the outside ambient temperature. Differences in temperature make true and constant focus almost impossible, like looking at distant scenery over a hot tar road. The image shimmers and shifts constantly. Whilst browsing the internet and astronomy forums, I came across an innovative tutorial by Paul Haese from Southern Celestial Pole on Peltier cooling his SCT to fix this problem. Full credit must go to Paul and Anthony Wesley for their efforts and ingenuity, inspiration and encouragement to help myself and others attempt such an ambitious modification on an expensive and precision instrument. Their stunning photographs speak for themselves and I am indebted to them for inspiring me to perform my own modification and subsequent DIY project page. I would encourage anyone with a little bit of know-how and spirit of adventure to have a go at this fun, interesting and learning experience. *CAUTION* I, and those who have gone before me, accept absolutely NO responsibility for damage caused during the course of this modification. Un-qualified dismantling of a telescope automatically voids its warranty and is inherently risky. Attempting this modification using this and other tutorials as a guide is done so entirely at your own risk. *SAFETY* Metal shaping and working with electricity is inherently dangerous. Make sure you are conversant with all hand and power tools prior to attempting any activity involving these things. Materials. 3 x 80mm 12V, PC cooling fans 3 x Aluminium heat sinks 3 x 40mm 12V, PC cooling fans 3 x 12V, 51W/6A Peltier coolers (Jaycar) (Grab a spare too, they are cheap enough) Appropriate wire to carry current. Quantity of wire connectors Electrical tape Heat transfer compound Heat shrink 6 x M6 x 10mm Countersunk flat socket-head screws 6 x M4 x 12mm Pan Phillips-head set screws Soldering wire Masking tape Matt black spray paint Silicon 1 x inside/outside fridge thermometer 1 x DC 13.8V, 23A power supply (Dick Smith Electronics) or equivalent 2 x insulated banana clips – female 4 x insulated banana clips – male 1 x plastic shopping bag 2 x garbage bags or lint-free cloth bags (to keep optics clean) Tools.
Appropriate personal protective equipment Bench drill Centre punch Hammer M6 Tap and handle M4 Tap Assorted drill bits De-burring tool Hand drill Drill mounted sanding pad Marker pens and pencils Engineers ruler Screwdrivers Allen keys Wire cutters/strippers Soldering iron Caulking gun Multimeter Craft knife Step 1.
Prepare some areas to work in. Make sure the area around the telescope is clean and free from clutter. You don’t want to drop things and lose or damage them. Remove all peripherals from the telescope that may get in the way. Have some plastic zip-lock bags or glass jars handy for screws. Also have a couple of big bags to put the optics in to protect them from dust while you have them out. Make sure all the tools you need for disassembly are handy.
Step 2.
One thing I forgot to do and regretted it afterwards, was to make some index marks on the corrector plate before I removed it. This means I will have to spend a lot more time trying to get the scope properly collimated when I put it back together. Use tape or liquid paper to make a couple of marks so you can line it all back up exactly the same way when you re-assemble the telescope. I learned my lesson the hard way. Remove all the screws around the corrector plate, grasp the secondary assembly and pull it out, corrector plate and all. Place it in a bag to protect it and put it away somewhere safe. If you do this with the scope pointing down, you won’t get bits of paint and swarf off the screws on the mirror. Place all the screws in a container or plastic zip lock bag for safe keeping. Remember where everything goes. You can write on the bags if that helps.
Step 3.
Undo all the screws holding the tube to the mirror back (Primary mirror housing). Turn the focus knob so that the mirror fully retracts inside the housing. This will give you access to the nuts. Make sure you hang onto the nut on the inside while you unscrew from the outside. Again, do this with the mirror pointing slightly downward so any loose grit falls away from it. Undo the four socket-head screws that secure the tube to the front of the fork rails. From this point you can wiggle the tube out and put it away somewhere. *Note* Later on you have a choice of either flocking the tube with some sort of light absorbing black fabric before you re-assemble it, or just put a dew shield on later. Either does a similar job when it comes to improved contrast in photographs. See Paul’s Southern Celestial Pole tutorial for flocking.
Step 4.
Remove the focus knob. Turn the telescope a little above horizontal to do this, as once you remove the focus knob, the mirror will slide up the baffle freely. Pull the rubber grip straight off the focuser. Undo the three long screws with a P2 tip screwdriver. Thinner screwdrivers may damage the head of the screw. This drops out the round black guard. Look inside the centre of the focus assembly and you will see another Phillips-head screw. Undo this screw and then unscrew the focuser all the way until it comes free of the threaded shaft that pushes and pulls on the mirror.
Step 5.
Remove the mirror and housing. Turn the telescope on its side and look at the baffle in the centre. About a third of the way down you will see a thin black ring that looks a bit like a rubber “O”-Ring. This is a steel cir-clip that comes away very easily when prised off with a fingernail. Once this retaining ring is removed, the mirror and its guide tube will slide straight up and off the baffle. Put it in a bag and somewhere safe immediately. Try to keep the grease on the baffle and in the guide tube clean. The mirror back or housing needs to be carefully unscrewed then levered out from between the fork rails. There should be enough flex between the fork arms and rails to allow the pins in the housing to come free. As a lot of this framework is cast aluminium, please exercise caution and don’t force anything. Take your time with this step.
Step 6.
Protect. Place a plastic bag over the baffle and tape it down. Flip the housing over and tape up the visual back. This will stop any foreign material getting inside the baffle and contaminating the grease on the baffle.
Step 7.
Planning, marking and preparing surfaces. Place all the heat sinks and thermometer on the rear of the housing where you want them to go. As you do, study them carefully and take time to think about it. Look at where your air-flow is going to go. Make sure that no holes you are to drill will conflict with raised sections inside the mirror housing. Mark out the areas with a black texta and proceed with removing the paint. I used a paint removing disk on a power drill. Your Peltiers will need close contact with the metal underneath to be effective.
 Step 8.
Preparing to mount the equipment. Measure mark and centre punch the holes to be drilled in the mirror housing that hold the fasteners for the heat sinks. Make sure you leave enough room between the holes to slide the Peltiers underneath (40mm). Countersink the holes on the inside to allow for your M6 x 10 flat socket-head set screws. Hold the heat sink in position, place a pencil through the hole you just drilled and mark the hole positions on the bottom of the heat sink. You can then drill and tap the holes in the heat sink with an M6 tap. Next, place the small fans on the inside as close as you can to be directly under the outside coolers. Mark two out of the four holes in the corners of these fans and drill and tap with an M4 tap. Pre-position everything where it goes, look for hole positions that will allow the wiring to go through the mirror housing using the neatest and shortest possible route. Mark, centre-punch and drill these holes large enough to allow all your wires to pass through smoothly. Make sure you clean up all edges of holes with a de-burring tool. Also, drill the two holes for the female insulated banana connectors to go through.
Step 9.
Touch-up painting. Place the peltiers in their positions, draw a line around them then tape the area with masking tape. Using a matt black touch-up paint, repaint the entire housing. When the paint is dry, use a craft knife to score around the tape then lift and remove it.
Step 10.
Securing the equipment. Squeeze some thermal past out on the exposed aluminium on the mirror housing. Quickly test the Peltiers by connecting them one at a time to a power supply for a couple of seconds. Holding them in your fingers will very quickly tell you which side is hot and cold. Mark the cold side with a pencil. Place the Peltiers in position (cold side to the aluminium housing) and pass their wires through the holes to the inside. Squeeze more paste on top of the Peltiers and place the heat sinks on top. Screw in the M6 set screws from underneath. Tighten very carefully as the Peltiers are very brittle ceramic. If you hear a crack, there is a good chance you have broken it. Remove it and use your spare. Screw on your fans to the heat sinks and thread all their wires through their holes inside the housing. Carefully strip the plastic off the thermometer sensor and thread it through one of the holes too. Stick some Velcro on the mirror housing and position the outside sensor on it.
Step 11.
Wiring. I am not the best at this but here goes. Make sure you do your homework on voltage, current (amps), and appropriate thicknesses of wire to carry the current needed and power supplies! Electricity is dangerous and can cause injury, loss of life and fires, not to mention damage to your precious telescope and anything near it. Try to use good connectors and avoid joining wire by twisting wires together as this introduces unnecessary resistance and often twitchy connections. If you have the patience, use heat-shrink on all the wiring harnesses to make it neater. For improved connection, use a soldering iron and solder to “Tin” all the exposed ends of wires to be joined. In the end, what you want is all the positive wires feeding into one line, all the negative wires into another and connected to the inside of the female insulated banana connectors. The wires have to be the right length and neat enough that they do not interfere with the traverse of the mirror inside the housing. Try to keep everything low-profile. One method suggested by Paul Haese was to use aquarium grade clear silicone to glue down the wiring as it will not affect the mirror coatings. Before you do that, bench test the electrics and make sure everything is running and the Peltiers are actually cooling the metal housing.
Step 12.
Fixing the sensor to the mirror. Secure the mirror upside down in a vice, holding by the baffle. To protect the surface of the mirror and guide tube, use a big roll of tape over the guide tube as it fits neatly and keeps the mirror form touching anything. Pad the jaws of the vice and be careful not to over-tighten the vice or you will crush the guide tube beyond repair. Next, slide the assembled housing and baffle into and over the mirror, lining up the focus rod with the focus knob opening. Place something underneath the tube (see the picture below) that will keep the housing raised enough to allow you to work on the back of the mirror. Find the sensor dangling down and tape it down to the mirror at its thickest part. Cover this with silicon and let it set overnight. If you have room you can place a silicon-filled bottle cap over it instead. The silicon ensures the sensor will measure the mirror glass temperature, rather than the air temperature inside the telescope.
Last step!
Re-assembly. Make sure you bench test again before you put everything back together! Every time you pull a telescope apart, you risk damage, so you want to avoid any unnecessary disassembly. *Afternote* A word of warning! Make sure you do not overtighten the screws in the corrector-plate retaining ring. I tightened mine up in freezing winter. Weeks later, during the first warm days of spring, thermal expansion caused my corrector to crack under pressure of one of the screws. The corrector costs $2300.00 to replace and is matched with the primary and secondary mirrors. Break one, replace all. Total: $6500.00 Hard lesson.
Put everything back in the reverse order. Ensure you put all the right screws in the right holes. Now your telescope is a little heavier on the back end, you will need to adjust your counterweights if you have them.
Congratulations. You have just completed the very risky, but potentially very worthwhile task of cooling your telescope. I hope you found this tutorial and photographs helpful in the planning and execution of your mod and pray your resulting photographs improve substantially. I would like to thank Paul and Anthony from Southern Celestial Pole again for development of this technique and all their support and encouragement. God Bless! Baz. A.S.I.G.N. Observatory |
