Mirror Temps?

[Dirk]

This might be a silly question! Why is the temp of the mirror in telescopes so critical? Dose it have to be at the same temp as the outside air or can it be kept at a constant temp warm with a heater?
:bowdown: :einstein:

[vespine]

I'm just a begginer too but I believe it is because if your mirror is warmer then the air around it, air currents will rise (warm air rises) from the mirror and causes turbulance which effects the light travelling through the scope.
So yes, the aim is to cool the mirror down to as close to ambient temperature as possible, not to just keep it 'steady'. Warming the mirror is probably the opposite of what you want to do. A fan not only cools the mirror down faster but it would also blow away the warm air that forms around the mirror before it has a chance to accumulate too much.

[rmcpb]

Good answer Vespine, it shows that we are really sharing knowledge through this forum :)

Basically a warmer mirror causes thermal layers in the air near the mirror and, therefore, thermal currents up the tube. These act a bit like looking over a hot, tarred road in summer where the shimmer is evident. The currents and layers in the telescope are much more subtle than those experienced on a hot road but then we magnify it several hundred times!!

By getting the mirror the same temperature as the ambient air these thermals are removed and that is where

leaving your scope out to naturally cool for a period before observing,
fitting a fan to increase the rate of heat loss and,
even the fitting of peltiers (sp?) to force the cooling of the mirror in a short period for high quality imaging all come in.

Heating the surrounding air to the temperature of the mirror would work but is an unworkable option.

[davidpretorius]

spot on vespine!.

for normal viewing, you might get away with a 1-2 degree difference, but for imaging at 1000x - 1600x, then you must have your difference at 1/2 degree.

Leaving your scope out for a few hours is a good start, but if the temps keep dropping, then the mirror has to catch up.rough rule of thumb my 10" cools at 2 degrees an hour unassisted.

take out a thermometer and just watch the ambient drop and how quickly it drops, when it settles, allow time for mirror to catch up and bob's your aunty.

I actively cool mine based upon birds method http://www.acquerra.com.au/astro/equipment/13/ and http://www.acquerra.com.au/astro/cooling/4/

Here is mine http://www.iceinspace.com.au/forum/showthread.php?t=8805

I get at least a 5 degree an hour loss of heat from the mirror this way.

I pull it out at sundown, plug it in and then come out to a settled temp scope

[IanW]

There are two reasons why a mirror should be brought to very close to ambient (outside) temperature. As the other posters have pointed out thermal currents in the optical tube (aka tube currents) cause image abberations, however there's a secondary and just as important reason that is overlooked by many amateur astronomers.

The secondary reason is to cool the glass down so it stops expanding and contracting due to the influences of temperature changes. Mirrors and lenses expand when it's hot and contract as the temperature cools. The amount of expansion and contraction varies with the matieral lenses and mirrors are made of. The rate of expansion per °C is known as the thermal co-efficient of expansion. Plate glass for example expands many times more than Pyrex which in turn is worse than Zerodur/Duradur or Aerosital which have co-efficients that are for all purposes essentially zero for most optical purposes.

The typical change in a mirrors 'shape' may be only a a few millions of an inch from hot to cold, yet it is sufficient to wreak havoc on the image. How the mirror or lens is held, it's thickness and the type of optical design all have bearing on the way the optical element behaves while cooling down.

Finally optical surfaces may need to be heated to prevent dew formation, it's typical to aim for a temperature of about 1°C above the dew point as this will prevent dew forming. Such a small temperature rise generally won't change the optics sufficiently nor induce tube currents in a well designed tube.

[Satchmo]

Keep in mind too that it is only an internal heat differntial in the glass that warps a mirrors figure. If the temp is even through , a mirror will perform identically at +20 or - 20 degrees C. Zero expansion materials like Zerodur have poor emmisivity, so while the mirror may not bend with a differntial temp, it will still be shedding image destroying currents onto the light path for much longer than a Pyrex mirror. I doubt there is any practical advantage in it except for very large mirrors. Optics are best to be kept thin if from Pyrex , at 40mm thick at 20" diameter and below is optimal.

Mark

[IanW]

Hi Mark,

I agree in general with what you say, however I'm not so sure about the cool down times as from memory Zerodur has a higher Thermal Conductivity level than Pyrex 740. When I get home in a week or so I'll look them up and see. The major issue however is still tube currents as generally a mirror will reach thermal equilibrium long before the tube does.

I really don't have much knowledge about thin mirrors over 16" as I use full 1:6 ratio blanks for mirrors over 16" diameter and never pyrex which while it's a killer material for smaller mirrors does have sag issues with larger sizes.

[Satchmo]

Hi Ian



Zerodur is a little stiffer than Pyrex but nothing a slight increase in thickness of Borosilicate can't replicate .

Emmisivity and conductivity are not the same thing. Pyrex is about 33% more cunductive but Zerodur is worse in the emmisivity value, so although it will be slower to take up heat , it is worse at shedding it.

Most altaz scope use 10: 1 or less these days , particularly as it is easy to computer optimise a mirror cell. A 20" X 1.6" mirror has a PV error of about lambda/60 surface flexure on an 18 point cell, and will yield a textbook diffraction pattern under good seeing at all but near the horizon. Most of the old scopes with thick mirrors have serious thermal equalisation problems. Web-back or honeycomb blanks are more common in the Equatorial mounted observatory scopes, but they are very expensive. A 24" Hextek blank runs at $25 K USD still cheaper than a Zerodur blank.

Mark

[rmcpb]

Can you two blokes ease up on us mere mortals? If I'm not careful I may learn something here!!

Great discussion.

[davidpretorius]

i thought it was quite straight forward rob, the Zerodur eyepiece is a little clearer than Pyrex toucam, but nothing a slight increase in thickness of Borosilicate can't replicate for the dob base.

see i understand :screwy:

yes it is great to have such wealths of knowledge floating around!

keep at it guys and please don't give up on the rest of us asking stupid questions