[Quark (Trevor)]

Hi All,
When researching the subject of Peltier cooling for the primary mirror on my 16” F4.5 Newt, I found the extensive information on this subject on Bird’s website extremely helpful and I thank him for his help.
Each telescope represents a different challenge, as far as Peltier cooling goes. I was fortunate that my original design for my 27 point Primary mirror cell was extremely open and robust.
Following experimentation with a cooling fan only, I found that I could keep my Primary within 1.5 to 2 degrees C of ambient. Thus I have set out to build a Peltier cooling system that will provide that extra 2 degrees of cooling.
I sourced 3 Peltier’s from Jaycar, 40mm x 40mm, each rated at 6 amps at 15 VDC.
I sourced two triangular pieces of 5mm thick aluminium for a cold plate and hot plate but as the work on this project progressed I gave away the idea of using a large hot plate with heat sinks on the hot plate.
Instead I purchased a large, heavily fined heat sink that was in two halves. When assembled it is rectangular 240mm long with a cross section of 80mm x 80mm. The heat sink fining is internal and the heat sink is designed to have a fan mounted either end, one blowing and one sucking to get rid of the heat.
I went this way as I wanted to confine the heat generated by the hot side of the Peltier’s to a relatively small area but with enough forced draft air flow to expel the heat as forcefully as possible away from the telescope.
I replaced my original cooling fan with 3 cooling fans; they are installed in line, directly above where the Peltier’s are in contact with the cold plate. Between the cold plate and heat sink is a sheet of heavy white Perspex that I have used for my light box (another continuing project). The Perspex is slightly thinner than the Peltier’s and is bolted to the aluminium cold plate. The Peltier’s are located in cut outs in the Perspex and are in direct contact with both the cold plate and heat sink. To help with heat transfer these contact surfaces are coated with Thermal heat sink paste.
The aluminium cold plate is screwed to the primary mirror cell with 3 off 1” by 3/8” setscrews. The large Perspex disk is really just a spacer and is 4” larger in diameter than is my telescope tube. The side of this disk facing the back of my primary mirror is painted flat black. The cooling fans have a skirt that extends down to within 6mm of the cold plate; this was done to make sure that the air flow has to be from across the face of the coldest part of the cold plate.
The fans are powered by a regulated 4 amp 12 VDC power supply and I run them continuously. The power supply for the Peltier’s is capable of 25 amps at 15 VDC and the voltage is adjustable from a low of 6 VDC to 15 VDC.
Initially I connected my Peltier’s in series but found that they did not deliver the amount of cooling that I required. They are now connected in parallel and I can adjust the voltage to provide the required output. When the temperature drops to within .5 of a degree C I wind the voltage right back to 6 VDC which pulls about 8 amps. I have the Peltier power supply on my computer table and can switch it on or off as required.
I used it on my scope for the first time last night and it easily controlled the temperature.
I was a bit worried about hot air bleeding back in past the large Perspex disk and fitted temporary shrouds around it, however as the ambient temperature dropped I found that my cooling fans worked much more efficiently without the shroud and I could detect no heat gain from the exhaust coming from the heat sink. The heat sink is installed such that the air flow through it is directed up at an angle away from the scope.
This is early days in the development of this system; however the first test on the scope, was most successful.
The attached images illustrate what I have done.
Image 1 The aluminium cold plate attached to the Perspex disk
Image 2 As previous
Image 3 The cold plate with cooling fans position as they would be in the scope.
Image 4 The heat sink, Peltier’s are installed in cut outs in the Perspex spacer disk ( forgot to take a photo of the Peltiers)
Image 5 Heat sink inlet fan
Image 6 Heat sink outlet fan
Image 7 heat sink
I will post more photos in this thread
Sincere thanks to Bird
Regards
Trevor