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
Further images of my Peltier cooler
Image 8 Primary mirror cell and remote temperature sensor heavily insulated onto back of mirror.
Image 9 Primary mirror cell with cooling fans installed.
Image 10 Peltier system fully installed
Image 11 Power supply for fans, 4 amp 12 VDC. Power supply for Peltier’s 25 amp 15 VDC adjustable.
Well done trevor. It's good to see more people having a go at this sort of ATM gear.
Closing off the tube is good for a couple of reasons - it stops (or slows) air movement through the tube which gives steadier images, and also lest the cooling do its thing more efficiently.
Peltiers are especially nice, as they follow a constant delta from the ambient temperature - ie if the air drops a few degrees suddenly then the cold side will follow.
Well done trevor. It's good to see more people having a go at this sort of ATM gear.
Closing off the tube is good for a couple of reasons - it stops (or slows) air movement through the tube which gives steadier images, and also lest the cooling do its thing more efficiently.
Peltiers are especially nice, as they follow a constant delta from the ambient temperature - ie if the air drops a few degrees suddenly then the cold side will follow.
cheers, Bird
Bird, I am glad that you have become more active on IIS with Saturn and the storm data. Your work has inspired me to get better. Also the sharing of your experience via your website has been most helpful. I look forward to more collaboration with you in the future.
Obviously my Peltier cooling system will further evolve over time as I get more familiar with it.
Quote:
Originally Posted by h0ughy
very impressive Trevor - you have put a lot of thought and effort into this
Thanks David, I never do anything in half measures.
well done on joining the peltier club Trevor. The applications for these units is just about every telescope design. Many are still unconvinced of the merits but in time will be swayed by the body of evidence.
I look at my birds nest of a cooling system and compare to yours, pauls and birds and I have to laugh
It really is worth the effort. If you can get the mirror to +/- .5 degrees of ambient and the seeing is great, then you will have a GREAT night of imaging. Simple as that.
Agree with the others, Trevor. You've done a great mod there and it should serve you well into the winter months. Just remember to let the air settle for 15-30 minutes after switching the cooling off (and before you start imaging).
At the moment, I use a power-point timer to turn the unit on and off. Eventually you might want to get tricky like Anthony, and have the temperature sensors feed back into a computer to control the switching on and off of the unit, so you can leave it running all night and know that when you get up at 4am the mirror is at ambient.
Once I left mine on too long and when I got up, the mirror was all dewed up from cooling to 2 deg below ambient.
I had to get the hairdryer out to get rid of the dew/fogging up until the mirror warmed up a bit
Agree with the others, Trevor. You've done a great mod there and it should serve you well into the winter months. Just remember to let the air settle for 15-30 minutes after switching the cooling off (and before you start imaging).
At the moment, I use a power-point timer to turn the unit on and off. Eventually you might want to get tricky like Anthony, and have the temperature sensors feed back into a computer to control the switching on and off of the unit, so you can leave it running all night and know that when you get up at 4am the mirror is at ambient.
Once I left mine on too long and when I got up, the mirror was all dewed up from cooling to 2 deg below ambient.
I had to get the hairdryer out to get rid of the dew/fogging up until the mirror warmed up a bit
Thanks Mike, at the moment I can adjust the voltage to the Peltier's and switch the power supply on and off from my computer table beside the scope. I have found that, due to the thermal mass of the aluminium cold plate, even after switching off the Peltier power supply all together, the mirror temp continues to drop. Last night after switching off the Peltier's and then the cooling fans in stages, the mirror remained at or below ambient for about an hour. There are still some further trials that I want to perform and log the temp data to try and understand precisely what is happening and what procedure works best.
Quote:
Originally Posted by davidpretorius
THat is a top piece of work.
I look at my birds nest of a cooling system and compare to yours, pauls and birds and I have to laugh
It really is worth the effort. If you can get the mirror to +/- .5 degrees of ambient and the seeing is great, then you will have a GREAT night of imaging. Simple as that.
Well done again.
Thanks David, the initial results seem very promising. I am able to bring the mirror down to ambient surprisingly quickly and ounce it is down to .5 of a degree C below ambient, due to the thermal mass of a 16" primary, the mirror temp remains stable for a considerable time, at least it does in the mild conditions that I have had for my trials thus far.
Quote:
Originally Posted by Paul Haese
well done on joining the peltier club Trevor. The applications for these units is just about every telescope design. Many are still unconvinced of the merits but in time will be swayed by the body of evidence.
Well done.
Thanks Paul, I suppose this is just the next step as an amateur planetary imager to strive for better results. I really do enjoy the ATM side of amateur astronomy and I have designed and built most of my equipment. In the past I have been stuck in that rut of spending the majority of my time building stuff instead of using it but fiddling with this cooling system and comparing results with various settings is most satisfying. It definitely has made a difference to my imaging.
