Pre-cooling - refrigerating an SCT in winter time?

[Omaroo (Chris Malikoff)]

Hi all

I was out tonight with the new baby - a C8 XLT for its first light (with me anyway), and noticed that it really took ages to come down to temp. Not as long as the 12", but a while nonetheless. It's still around 10 deg C outside at 9:30pm so it's not even cold yet. It'd brought it outside from the lounge room - where the heater was going to keep the house warm - at around 20 deg C. My guess is that the mirror temp was about the same at that point.

Without a well-placed temperature probe inside the SCT's mirror cell, it is going to be hard to set up a way to gauge ambient/mirror temperature differential, but I'm thinking of a new idea. Given that winter temps are likey to be around the -5 deg C mark (or colder) where we (Macarthur) view from, why couldn't I use my 12v compressor-driven Engel car fridge to pre-cool the whole unit to a measured -5 deg C (or whatever the forecast minimum was going to be) and keep it there until the outside temp was down to the same? I could then bring it out and we'd be at or about ambient. Driving to a viewing site I could keep the SCT in the fridge until I got there and then not have to wait until it cooled off naturally.

I know it initially sounds silly, but being nice and small I can mount the whole SCT, in the fridge, in an upright position - i.e. the corrector plate facing up. I can even manufacture a bracket to hold the scope in position by its dovetail so the fridge effectively becomes its carry box.

My question is this: what if I miscalculated the forecast temp at set up time and the mirror temp was below the ambient outside air temperature by a measurable margin when I set up? I can set the fridge/freezer to cool to at least -40 degrees below ambient, so there is a lot of scope in the temp range I can set. The seals are very good and condensation is nearly totally absent. It's a very dry cold.

Does anyone have any ideas on this?

Cheers
Chris

[mill (Martin)]

The only way to know is to try i say
I think it will work but making the temp lower than the ambient temp will make the ota dew up.
It will be better to have it say about 5 degrees Celcius higher than ambient, It will cool down faster than beeing 10 to 15 degrees Celcius warmer than ambient.
Nothing wrong with trying.

[Omaroo (Chris Malikoff)]

Good point.... but if I could use it to get down to within a couple of degrees then that'd still be useful for field trips. My wife hates being cold in the car.....

[h0ughy (David)]

bird and Davep had some items on mirror cooling some time back, have a search for peltier cooling and you will see some of the results they encountered, not quite what you are doing, but similar in it effect I suppose

[Omaroo (Chris Malikoff)]

Originally Posted by h0ughy

bird and Davep had some items on mirror cooling some time back, have a search for peltier cooling and you will see some of the results they encountered, not quite what you are doing, but similar in it effect I suppose

Yeah I saw those threads David - but I wouldn't need to use a peltier if I have a fridge.

I would place the scope in the fridge at lunch time and then it will have been there for a good 6-8 hours. I guess the secret would be to keep it sealed in the fridge until I had the temps balanced, and monitor the temp in there so that it didn't get below ambient - where it would otherwise dew up on exit from the fridge. Keeping the temp two degress above ambient would be ideal - then it wouldn't take long to reach equilibrium after that.

Cheers
Chris

[John K]

Interesting concept, and something I also considered before DP made a prototype peltier cooler for my 12.5". In my case I was simply going to remove the mirror assembly and place that in my home fridge.

As people have said, this is a bit of a trial and error situation so have a go and let us know how you go.

My advice would be to try and get a simple indoor/outside temperature sensor to know what the scope is doing relative to ambient as you try this set up. e.g. you might find that the scope needs to be say 2 degrees below ambient for xx hours to reach actual ambient etc.

Suggest that you have a hair drier on the ready to remove dew.

[rumples riot]

I know for a fact there is an aussie who has been doing this for ages and never had the OTA dew up. Asimov, was doing this all last year and it worked a treat. In fact I am sure that he had a thread here sometime ago. I know another site that he did the same.

If you are concerned about dew forming then try using the fridge and silica gel in the tube. This will dry the air out inside the tube. Besides if you leave the cap off the back the air will dry out from the fridge.

Active cooling is meeting a lot of resistence from the old school of thought. I come across it all the time, where people ask me about getting dew on objectives. However, the benefits outweigh the costs.

[rumples riot]

BTW my corrector often during winter dews from the inside. I simply use a hair dryer to heat it enough to remove the dew. Never any problem with dew then forming on the objective. I hardly ever use a dew heater on the corrector when I am imaging. It creates too many distortions in the image at 10,000 + millimeters. Old ways need to be reassessed.

I drilled a hole and have stuck a sensor on the back of my mirror in my C9.25. That sensor is attached to a temperature unit which gives both inside and outside temperature. I concur with John K's thoughts.