Getting hot under the collar

It seems I have another modification to make to the observatory, the weather in recent weeks has brought home the realisation that it needs some form of ventilation to remove the excess heat – sometimes reaching 42°C, this builds up during the day, meaning it takes extra time for the telescope to cool down to ambient temperature before imaging can begin. the reason is that a warm ( or recently, very warm ) heavy lump of glass, the main mirror – while cooling causes tube currents, the air inside the tube of the telescope is turbulent, like looking over a car roof on a hot day, this is called the Boundary Layer, it wobbles. This turbulence makes the light entering the tube to also wobble, blurring the image, when the mirror has reached ambient temperature this turbulence stops and imaging can begin, however the larger the mirror is, then the longer the cooling takes, and an 8 inch near 3.5Kg mirror takes a while to cool. The observatory is not airtight, there is some ventilation around the bottom of the roof as the roof is not fixed, it has to roll back and forth, but the air inside is essentially static and the roof itself is felted, this gets hot in the sun and radiates heat inside the observatory also.

tube currents
Tube Currents

I have then two problems, a hot roof when the sun is shining and static hot air inside the observatory. Painting the roof with silver heat reflecting roof paint will solve the first problem and installing a fan at both ends of the roof apex will solve the static air problem, one fan will draw in air from outside while the other extracts hot air.

I’ve ordered a 12v 10w solar panel and 2 x 120mm 12v DC fans of the type used in PC’s for cooling the cases, these should move 60 cubic feet of hot air per minute when the sun’s out, the best part though is this system will be automatic, when the sun shines the fans start, the solar panel also has a USB charge outlet that will come in handy, I have all the bits and bobs needed to install this when it arrives so it only cost £30 on Ebay, I have the silver roof paint already, time to get painting. 

Update

Finished painting the obsy roof and decided to do the south-facing side as this was obviously getting very hot too, it took about 2 hours to do – sweating profusely and gradually going snow blind, it is dazzling to look at. 

Observatory silver heat reflective paint
Observatory silver heat reflective paint

This made a noticeable difference once it was finished, will check the temperature tomorrow at about 1pm.

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