Temperature comparison

[neilallison]

Hi guys

I am going to fit some temperature sensors to my scope so I can monitor the temperature of the primary mirror against the surrounding temperature (probably haven't worded that very well though !!)
I am going to fix one sensor to the back (or side) of the primary mirror.
Where would you guys suggest I fix the 2nd sensor - Inside the tube or will anywhere outside be sufficient?

Thanks
Neil

[clive milne]

Neil,
The reference temperature probe should not be attached to any solid object. For the most accurate ambient air temperature you would house it in an insulated tunnel and draw a continuous stream of air over it using a cpu fan for example. The best location for it should be determined by experiment. After using a thermal infra red imaging camera to analyse the cooling properties of large newtonians I came to the realisation that there is even a temperature gradient from the top to the bottom of the telescope. The top cage is likely to be one or two degrees cooler than the mirror box (at any given time) and often radiates enough heat to be somewhat below ambient temperature (this is why you get condensation on dewy nights)
The best way to avoid dew and thermal plumes is simply to blow air across your optics at 2.5m/s (Ref; Wilson, ESO)

[neilallison]

Thanks for the reply Clive.

I currently have a "cap" (for want of a better description) over the end of the tube with a fan installed which draws air down the tube and over the primary mirror.
Would it be sufficient to have the sensor in the airflow of that fan to get the reference temperature or do you think that maybe it may be a little inaccurate due to the fact that the air has already gone over the primary and removed a little heat from it?

[clive milne]

The easiest way to determine that is to use a differential temperature meter and see if there is a temperature gradient between different locations. The calculations required to simulate it are not that difficult but the situation you describe includes a lot of variables that need to be determined and measured... best just to measure what you are looking for directly.