Hi Trev, you should be able to get it cooler than that.
Here's a pic of my experiment a few years ago, tho not for an astro related use(homebrew
).
It's a single 33W 40x40mm peltier and the cold side is at, from memory, about -10c.
Thats with a large heatsink on the cold side that can absorb a fair bit of ambient temp... tho ofcourse the ice would be insulating it. With a little finger it could go much lower.
Room temp was probably around 20ish(room rarely varies, hence used for homebrew) and the heatsink temp up around the 40 mark somewhere.
The trick is to keep the hot-side as cool as possible, and it's massively important here to reduce all thermal resistance, I don't mean the heatsink, it's only a small part of the issue and the easiest to deal with. The heatsink might feel cool but the hot-side of the TEC could be much hotter.
The max temp differential(hot to cold side) for a Peltier is approx 68c.
If your cold side is at 1c, and the heatsink is at say 30c, the differential is only 29c, where is the other 39degrees? In all the materials and joints.
Imagine every interface, and every material used, as an extra resistor in an electrical circuit... voltage loss here, voltage loss there, it all adds up. Same with TEC's, temperature loss here, temp loss there.
Even a tiny space of air(the best insulator) between two mating surfaces will cause problems, thats why it's recommended to use lithium heatsink transfer compound.
In the photo I have used an aluminium CPU heatsink, about 3"x3"x2" size that has a copper core running through it, a rod about 1" dia by 2" long, the 1" diameter was too small for direct contact to the TEC (40x40mm) so I had no choice but to use a plate between the two. This adds another material and another joint, both with their thermal resistance, each could easily add a few more degrees to the hotside temperature, which means a few more degrees to the cold side too.
Each interface had atomised-silver heat transfer compound to increase contact area(reduce air) and reduce thermal resistance.
Will be building a temp controled brew fridge soon, so will be doing this again with all copper and silver construction and fluid radiator systems instead of traditional heatsinks.
Want to get the fridge as efficient as possible and be able to get the brew to lagering temps (1 or 2 deg) and possibly go all solar/wind powered.