Insulating a newt tube.

[doug mc]

I am thinking of insulating my 8inch dob ota. I found a material at Bunnings that is used as a car carpet. It is like a black felt with a firmer side that would go against the floor. I have used it as a dew shield. You can roll it and glue it into a tube for that purpose. It is light and would insulate quite well. A good idea?

[billdan]

Hi Doug,

Have a look at this thread on CN about this subject, the consensus is it works great for sealed tubes like SCT's or Mak's but not so good for Newts.

They found the reflectix insulation the best for stopping the OTA from cooling down too quick.

https://www.cloudynights.com/topic/605123-insulation-jacket-for-mak/

Picture attached from Richard O'Neill

Cheers
Bill

[Wavytone]

Sure it will work... so will a roll of bubble wrap, as this relies on the insulating properties of 6mm of air trapped in the bubbles.

I've come to the conclusion that either:

a) the OTA and optics should cool down to ambient as fast as possible (minutes), as is possible with an open tube Newtonian, Cassegrain or RC; or

b) if the OTA is sealed (SCT or Maksutov) insulate the OTA so that heat is lost so slowly internal tube currents are negligible.

Personal observation of the tube currents in my maks suggest:

1. the internal tube current is caused by the internal baffle being at a different temperature than the air inside the OTA, noting the central baffle is aluminium, and

2. Although the baffle tube is attached firmly to the backplate of the scope, it is also supporting the primary mirror - a large thermal mass - sufficient to keep pouring heat into the baffle tube while the primary mirror is above equilibrium.

3. The rate of cooling of the baffle tube (and primary) is primarily driven by the temperature the scope is stored at minus the external temperature of the OTA, divided by the thermal resistance outside to inside the OTA.

4. The thermal resistance is easily increased by adding insulation, as you have done.

5. Alternatively the temperature difference can be reduced by pre-chilling the OTA - 2 hours in my car (driving to the mountains) with the aircon set to icebox mode will bring it down to 12 degrees by the time I arrive. Mind you, I'm wearing a jumper, beanie and gloves.

[OICURMT]

I'm not certain it would work for a Newtonian, as one end is open to the sky and thermal convention would be a problem as cold air "slides" down the tube regardless of the insulation.

In a cat, the corrector traps the air and therefore micro-convention occurs within the ota itself, with heat being radiated out the corrector and radially out the tube itself. Insulation can slow the radiation from the ota walls and since the tube is enclosed, currents are minimized.

[Baza]

If you're concerned about cooling a newt, you could consider
http://www.deepskywatch.com/Articles/rear-fan-newtonian.html
http://www.deepskywatch.com/Articles/rear-fan-newtonian.html
http://www.cruxis.com/scope/mirrorcooling.htm

[doug mc]

Thanks everyone. I think I will leave it as it is.