Quote:
Originally Posted by rsbfoto
Hi Ian,
I appreciate upir concerns bnut the building is not concrete. Is is build with brick which has a completely different mass as concrete.
Here you can see the whole building progress
http://rainerehlert.com/OR14html/
I painted it white in order to have as low as possible heating.
The cold room has 6 ventilation holes in the bottom each 6" diameter and the lower part of the cold room ege. where you can see both piers does not get any Sun in Winter when we have here the best onserving season.
Each pier has a mass of 6,000kg and I guess that is also a good source of lower temperature against the ambient. I have in each pier 2 holes into which I will put a temperature sensor in order to find out how the temperature of the piers evolve with the time.
Yes I did take your concerns into account as there is another astronomer here in Mexico who build his Observatory in a similar way and he has not reported any trouble with the heat generated by his walls and yes he has concrete block walls ando no vents like I did.His Observatory is sitting on top of a water reservoir.
I did take into consideration what you wrote and I said to myself. Better to use in such environment my telescopes as often as I can instead of waiting and having then collecting dust and waiting to go out maybe 2 -3 times a year to a dark site.
I am setting up the equipment and I will see what comes out. As last remedy I can specialize in Moon and Planet imaging which is a lot fo fun too ...
I am working also at the moment to be able to control the observatory remote from where I live.
Anyway I appreciate you having mentioned that posible risk I have and yes as I said I am aware of the possibilities ...
The best telescope one has is the one you use most often. |
If the walls are made of a homogenous material , you need only know the outside and inside temperatures at the surface and the history of the air temperature inside and outside to get a picture of the rate at which the structure will equilibrate. This can all readily be determined marthematically and modelled.
The thermal mass of the walls , suspended floor and massive pedestals will all be significant.
There is a good reason why professional observatories use materials and structures that are of low thermal mass.
The hugely massive piers will heat up slowly in summer and hold most of that heat at night and will radiate and conduct heat to air , .... bad for steady seeing above in the observatory. Gets hot in Mexico I hear , not good. Built now , so maybe you should consider encasing the piers in a low thermal inertia insulating material , this way it wont matter much what the thermal mass of the solid structural piers is, the surface of the insulation will equilibrate quickly and will prevent thermal cycling of the pier and reduce the bouyancy effect that degrades seeing. My reasoning is based on many years as pyromet process engineer and my accumulated experience in thermal engineering.
Thermal properties that might be helpful are :
Bricks have a thermal conductivity k = 0.6-0.7 W/mK
Light concrete = 0.12-0.23 W/mK
aerated concrete = 0.12-0.23 W/mK
Foam glass = 0.04 W/mK
Rock wool = 0.04 W/mK
PVC-foam = 0.035 W/mK
Cavity wall isolation = 0.05 W/mK
cf
Air = 0.023 W/mK
Ideally you structure needs to approach the bulk k value for air to be a good as an observatory , for best possible seeing and imaging.
From Perry's Chem Engr's HB.
The k value will influence how long it takes for the floor and walls to heat up and cool down
The thermal resistance to heat diffusion is R = l/ k , l is the thickness in m.
and U = 1/ (Rso + ∑Rn + Rsi) ,
where Rso is external surface R
Rsi is internal surface R
Rn are other layers' R if different to Rso and Rsi
U is overall thermal conductance from the outside to inside covering all modes of heat transfer.
Qc = U * A * (T outside – T inside) is overall heat absorbed or released.
and the rate of heat transfer can be calculated in 1 dimension using the url here :
http://hyperphysics.phy-astr.gsu.edu.../heatcond.html
The bouyancy of the warmed air that will flow over the walls is more difficult to calculate , but doable , need to calculate the Prandtl No and the
and the Grashof No to do the Free Convection Calcs. See
http://rpaulsingh.com/teaching/Lectu...er_handout.pdf.
And the example calcs in
http://www.ercoftac.nl/database/test09/test09.html will give the picture of how the bouyancy effect of the warmed air that will effect seeing inside the observatory can be assessed.
Hope that all helped.
I have other problems to think about and they concern the installation of my Astronomy equipment
