Temperature simulation of mirrors for daytime observation (Solar)

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allhoest
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Temperature simulation of mirrors for daytime observation (Solar)

Post by allhoest »

In observation, it is considered that it is necessary to minimize the temperature differences between the optics and the ambient air, in order to avoid the formation of instrumental turbulence.

In night time observation, it is well known that instruments must be taken out in advance, typically at nightfall.
In daytime observation, temperature changes also affect the performance of the optical system.
It is considered that the temperature difference between the optics and the environment must be less than 2 ° C, preferably less than 1 ° C.

A difference between the daytime and nighttime viewing regime is that during the day the sun shines towards the mirror, which depending on the type of used coating/substrate will absorb more or less energy.

Here are the results of a mirror temperature model, which incorporates 4 factors:
- Radiation: typically, the hot element (the atmosphere) emits towards the mirror (the opposite in the afternoon)
- Convection: on the surface of the mirror; there is a local exchange between the air and the mirror, linked to the temperature difference between the elements.
- Absorption in the mass: the mirror absorbs part of the incident energy, solar energy. Absorption depends on the coating/substrate
- The physical parameters of the mirror in order to determine its thermal energy absorption capacity

Here is the result for a 250mm diameter mirror, which has undergone an interference treatment and reflects part of the flux. The non-reflected flux is transmitted to the mirror.
The most important curve is the black one. It represents the difference in T° between the front surface of the mirror and the atmosphere.

All simulations start at 6:00 a.m., assumed time of sunrise.
The mirror is assumed to have been placed outside and is assumed to be in equilibrium with the ambience at 6:00 a.m., i.e. 0 °C in this simulation.
During the day, the sun shines more and more and the ambient temperature increases. The mirror heats up too. Their two values will reach a maximum around 12 p.m. / 2 p.m.

From daybreak, temperatures rise.
The ambient temperature rises faster than the temperature of the mirror.
Around 8 a.m., this difference is the most marked, with a value of nearly -1.5 °C (colder mirror).
The mirror heats up, and with the passing of the sun at the zenith at 12 o'clock, the gap is narrowed and then reversed.
The deviations are positive in the afternoon, but in solar observation, the best solar observation period is in the morning, when atmospheric turbulence is less.
A priori, this mirror can be used all day. But preferably from 10 a.m. until the end of the period of atmospheric stability, around 11 a.m.

http://www.presencenet.be/nucleus2.0/in ... imagetext=



Here is a simulation, considering that this mirror was stored in a room overnight.
The curves are very different, but a priori the mirror can be used from 7:30 am.
This is a somewhat unexpected result: starting a solar observation with a mirror coming out of the storage gives a longer observation range.

http://www.presencenet.be/nucleus2.0/in ... imagetext=



The 250 mm mirror used for the simulation above is quite thin, 25 mm thick.
The front, back and center of the mirror show a very small temperature difference.
Here is a simulation of a mirror 300 mm in diameter. This mirror measures 55 mm thick.
Its thermal inertia is much greater.
The mirror-ambient temperature differences are much greater, almost double.
The curves refer to the start of an observation at equilibrium.
In the typical solar observation period from 8 a.m. to 11 a.m., the difference in mirror-ambient T ° is too high.

http://www.presencenet.be/nucleus2.0/in ... imagetext=



Another simulation with a mirror taken out of a warmer room, shows that the mirror appears to be usable from 8:30 am.

http://www.presencenet.be/nucleus2.0/in ... imagetext=



This is all theoretical.
The simulations take into account the coefficients, and you have to choose them.
Then it has to stick with reality ...


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Re: Temperature simulation of mirrors for daytime observation (Solar)

Post by marktownley »

Interesting analysis. Thank you, much to think about...


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Re: Temperature simulation of mirrors for daytime observation (Solar)

Post by allhoest »

Additional information on the mirror model

The reference article for the development of the model is as follows.
Thermal characteristics of a classical solar telescope primary mirror
Ravinder K. Banyal, B. Ravindra

The model developed in one-dimensional mode, in the thickness of the mirror. The developed model was validated by comparing its results and those of Banyal.

For the irradiance curve, a priori Banyal used the latitude and longitude of the Indian Institute of Astrophysics at the equinox.
For the results presented by the model, the coordinates of Saint Véran on August 15 were used.
Image

Regarding the variation of ambient temperature during the day, the formulations of Banyal were used. The amplitude of variation during the day can be adapted, 15 ° C here.
Image

Regarding the absorption through the mass of the mirror, the base comes from the following article:
A Thermal Analysis of a 1.5 Meter f/5 Fused Silica Primary Lens For Solar Telescopes, Peter G. Nelson February 2007
Image


CS
Alex


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Re: Temperature simulation of mirrors for daytime observation (Solar)

Post by AndiesHandyHandies »

Hi

So we need to heat up a solar newtonian to expected ambient temperature in use before taking it out?

Having a look at my Hexapod 14" F4.2 mirror box later. F8 with a 1.9x Baader coma corrector.

Cheers. Andrew.


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Re: Temperature simulation of mirrors for daytime observation (Solar)

Post by marktownley »

AndiesHandyHandies wrote: Wed Aug 18, 2021 8:16 am So we need to heat up a solar newtonian to expected ambient temperature in use before taking it out?
Sounds like ambient heating is the way forward...


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