4 replies to this topic

[ngc2218]

In an attempt to model the thermal behaviour of my scope with the purpose of temperature compensation - focusing, I started recording the tube temperature and ambient temperature.

 

I have a Optec Leo focuser with an external temperature probe, attached to the tube and covered with an insulator pad provided by Optec.

The ambient weather data was gathered with a Lunatico weather station.

 

I collected  temperature readings every minute during a night of imaging, approx. 940 data points. The night was pretty good with no clouds and low wind.

I expected the telescope to equilibrate with the ambient but this wasn't the case.

To my surprise the scope started cooling and went below the ambient temperature with a maximum gradient of 4.3 degrees Celsius.

After an inquiry on physics-StackExchange forum, it seems that metals like aluminium equilibrate with the upper sky temperature, or in case of no atmosphere, they radiate to the cosmos. Also, even though aluminium has a low emissivity coefficient, if it is painted, this could drastically change the thermal behaviour, by increasing the emissivity coefficient.  

 

To verify this, we can compute the heat transfer using the Stefan-Boltzmann equation in 2 situations. The first one assumes equilibrium with the ambient, the second one assumes equilibrium with the sky/cosmos. The absoulute value is not very important for this testing, what matters is the sign of the result. If the sign is negative the scope looses heat, and vice-versa.

 

Luckly the Lunatico weather station, also measures the sky temperature, so it was a simple matter of computing the Stefan-Boltzmann equation at each data point. The result had a negative sign throughout the night, indicating the scope was continuously loosing heat, until I closed the observatory roof. Only then the scope started to warm up.

 

Below is the main graph and the table data.

 

 

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Edited by ngc2218, 12 October 2023 - 11:27 AM.

[Kitfox]

Not sure where you are, but here in the Southeastern US, my optical tubes get below the dewpoint within an hour.  Unheated parts are wet before the grass is...I find them to be pretty efficient radiators 

[Jeff B]

Very "cool" info.

 

You can also cross plot actual focus position with time, tube & ambient temperatures.  Remember the objective focal length changes as it cools down.

 

Jeff

[ngc2218]

Very "cool" info.

 

You can also cross plot actual focus position with time, tube & ambient temperatures.  Remember the objective focal length changes as it cools down.

 

Jeff

For now I've plotted tube temp vs focus position with 2 different filters. The correlation for the linear regression model is pretty sweet at around 96%, with a p-value (statistical significance) < 0.001 (excellent). Given the large gradient between tube and atmosphere I won't expect the focus position to be highly correlated with the ambient T.

Also it could be nice to model the objective temperature variation, but I wont stick a probe on a 8 inch apo lens, too risky!

It crossed my mind to isolate the tube with refletix ....  if the tube temp will stay constant, then the focus position will change mostly due to  objective cooling, changing its refractive index. Worth a try, but from the statistics it seems 96% of focus variation is explained by the tube, the rest should be the change in refractive index.

[ris242]

 Unheated parts are wet before the grass is...I find them to be pretty efficient radiators 

my reference is not the grass........its the wife's car parked outside. If the car is not dewing up, I don't need to start up the heaters........or even get them out if its a short session.