The light is on with cooling

[netwolf]

Thanks all, I thought it might be something like that.
So my next question is. Can we start cooling indoors during the day to keep the mirror at ambient? Pre-cooling as such? would this help?

Regards

[asimov (John)]

My idea was to keep the whole scope at at predetermined temp. The hardest part is coming to a conclusion on what temp to have it at. If it was kept at slightly below your *average minimum* temp for your area, that means the scope would have to 'warm up' to the ambient temp. I'm not sure what the consequences of that would be though..

[bird (Anthony Wesley)]

In summer I've tried to do that as best I can. but I don't have enough aircon available to really keep the scope cool, or pre-cool it enough to make any practical difference. In wintertime the day-to-night difference around here is about 20C, makes the job a bit harder still.

cheers, Bird

[RAJAH235]

FWIW. Oatley Electronics also supply the peltiers & the control unit for them at reasonable cost.... L.

[GrahamL]

I freeze some water in a 10 inch pastry dish and slip it under my 10 " dob
(upright position ).. a while before viewing .. It "seems" to cool down quicker
but to be honest I really don't know .. I was just going to do this until I got hold of some more rechargeables to run the fan thats on it.

[IanW]

Originally Posted by netwolf

Thanks all, I thought it might be something like that.
So my next question is. Can we start cooling indoors during the day to keep the mirror at ambient? Pre-cooling as such? would this help?

Regards

You sure can pre-cool and it helps a lot. Most of the major observatories run airconditioning all day to keep the optics cool. Stand inside the AAT dome and it's quite chilly.

Large observatories (and many smaller ones) also have massive air circulation (blowers / extractors) to further cool the systems down around sunset.

Originally Posted by asimov

My idea was to keep the whole scope at at predetermined temp. The hardest part is coming to a conclusion on what temp to have it at. If it was kept at slightly below your *average minimum* temp for your area, that means the scope would have to 'warm up' to the ambient temp. I'm not sure what the consequences of that would be though..

You'll find the average temperatures by state and month online or you can purchase more accurate data for a fairly hefty fee from the Met Bureau.

Warming up will cause exactly the same issues as cooling down, ie: tube currents (eddy currents) and mirror distortion if using pyrex or any of the normal glasses used in mirrors (float, plate, white, BK7 etc). If the mirror is made from Zerodur, Duradur(sp), cervit or Aerosital then there will be no change in the mirrors shape during heating or cooling.

[GTB_an_Owl (Geoff)]

anyone ever tried water cooling the mirror?

[IanW]

Originally Posted by bird

As for the reason that the temp rises when the cooling is turned off, there are really two heat sources to blame - one is the core of the mirror as has already been pointed out, and the other one is the hot side of the peltiers (the heatsinks). As soon as you turn off the power the "heat pump" effect stops, and heat can flow back into the system from the hot side.

The fans that are normally used to blow the cold air off the coldplate and onto the mirror then turn into fans that are blowing warm air as the heat from the hot side gets back into the system via the peltiers.

cheers, Bird

Hi Bird,

Just a thought.

Have you considered going the chilled liquid route for cooling the mirror? I used such a method to cool a Medium Format "cold" camera over 20 years ago.

It's a lot messier to construct but does have the advantage that the heatsinking and radiator for the hot side of the system can be kept well away from the telescope.

On the bonus side, you could use a 2-3 stage Peltier pump.

Cheers
Ian

[bird (Anthony Wesley)]

Originally Posted by IanW

Hi Bird,

Just a thought.

Have you considered going the chilled liquid route for cooling the mirror? I used such a method to cool a Medium Format "cold" camera over 20 years ago.

On the bonus side, you could use a 2-3 stage Peltier pump.

Cheers
Ian

I've thought about it, mainly because I could separate the cooling stage (refrigerator) from the main body of the scope and save some weight. I would probably use a jacket that enclosed the rear and side of the mirror with a gasket of some sort to seal around the edge near the front surface.

Wasn't sure what coolant I would try, some light oil or water + antifreeze. Need to be sure that it doesn't affect the mirror chemically.

The down side is that it is not as portable as the current setup, and anything with liquids is messy...

cheers, Bird

[GTB_an_Owl (Geoff)]

simplicity gentlemen!
items -
1 pump (maybe a reservoir)
1 length of plastic pipe

fill pipe with radiator coolant (so it don't freeze)

2 wraps of the pipe around the edge of the mirror then bring it out and two or three wraps on the outside of the tube and connected to the pump

theory -
external tube picks up ambient temperature of air/scope surface and transfers it to the mirror

comments?

[GTB_an_Owl (Geoff)]

Mmmmmmmm!
vibration on mirror maybe from the pump circulation?
then again, when it reached temperature you could turn it off

[IanW]

Originally Posted by GTB_an_Owl

simplicity gentlemen!
items -
1 pump (maybe a reservoir)
1 length of plastic pipe

fill pipe with radiator coolant (so it don't freeze)

2 wraps of the pipe around the edge of the mirror then bring it out and two or three wraps on the outside of the tube and connected to the pump

theory -
external tube picks up ambient temperature of air/scope surface and transfers it to the mirror

comments?

Insufficient cooling! Ideally you need to climb the temperature differential between the anticipated observing temperature and the current temperature plus a couple of degrees. It requires a lot more cooling than you'd think.

Re: follow up post on vibration.

As long as you don't have air bubbles in the system and have the pump off the telescope then it's vibrationless. It's a technique that's been used for years in professional observatories for cooling camera systems both, film (Cold Camera) and CCD (cryogenic cooling). A few commercial CCD camera in the early days of CCD imaging for amateurs used fluid cooling systems, eg: Cookbook CCD camera by R. Berry.

[IanW]

Originally Posted by bird

I've thought about it, mainly because I could separate the cooling stage (refrigerator) from the main body of the scope and save some weight. I would probably use a jacket that enclosed the rear and side of the mirror with a gasket of some sort to seal around the edge near the front surface.

Wasn't sure what coolant I would try, some light oil or water + antifreeze. Need to be sure that it doesn't affect the mirror chemically.

The down side is that it is not as portable as the current setup, and anything with liquids is messy...

cheers, Bird

Hi Bird,

Correct me if I'm wrong, but from the look of the scope it's using an alloy/metal tube? Have you thought about wacking a Peltier cooler on the tube?

A few thoughts based on the cooling systems I used on the camera.

The use of formed copper or stainless steel tubing (thin wall) works extremely well. A coil that spirals round the back of the tube beind the mirror cell (like your current cooling plate) and then around the edge of the mirror. Gasket using thin neoprene and use swaged connectors.

For the cooling pump, a double or triple peltier pump running on to a copper (best) or aluminium cylinder with copper pipe coiled around it (10~20 turns works well), lag well with insulation. A windscreen wiper or heavy duty fishtank pump with good quality silicon tube would be ideal for the interconnections between the refrigeration unit and cooling coil. One thing that is needed is a trap/reservoir to remove air from the lines, this only needs to be a 1L bottle with an inlet and outlet pipe brought the top lid with the pipes immersed at unequal lengths in to the coolant. The outlet pipe should be at a lower level.

For coolant, I used a silicon based industrial coolant which is probably overkill, something like Dynalene HC50 or possibly Paratherm HC etc would do the trick. Avoiding water based products unless they have a good corrosion inhibitor in them for obvious reasons!

Of course the ideal way to dispose of the internal temperature gradient of a mirror would be to fit coolant lines inside the mirror, something that would not be impossble to do with a mirror blank and some of the current hole drilling technologies.

Cheers,
Ian

[iceman (Mike)]

Excellent discussion going here, it's definitely going to help me when I start on this peltier cooling route.

I'll be starting the project soon and will seek advice in the next few days.

I've got a few months before Saturn is back high enough, so I can get it right over the next few months and practise on the moon.

[bird (Anthony Wesley)]

Ian, I've thought about these possible ways to fit water cooling, but I would have to find that it provides a substantial improvement over the simple peltier-based cooling before I'd want to try it, based on the KISS principle (Keep It Simple, Stupid!).

My rough and ready peltier rig that I have now seems to give a suitable amount of cooling, enough that I'd probably want to try to improve on it rather than switch to a different design when I want more cooling.

But I do think about water cooling from time to time. Maybe one day I'll build a test rig and see how it checks out.

cheers, Bird

[Lester]

Hi everyone,

interesting disscussion. I use Ice packs held around the rear of the OTA of my 14" SCT with ocy straps. Tried a peltier but it needed a lot more than 1 for this sized scope. Five ice packs brings the OTA down to below ambient in 2 hours. I don't get the freezing conditions that some of you chaps get. Usually only gets down to 5 degrees minimum.