A telescope is at its best optically when its mirror is at the same temperature as the night air.
There are two aspects to consider regarding thermal properties;
1. Expansion/contraction of the mirror material.
2. Mirage effects caused by a thin layer of warm air next to the mirror (called the thermal boundary layer).
Pyrex minimises distortions as the mirror changes temperature. Many years ago when mirrors were made from plate glass, the amount they expanded or contracted was far greater and presented a problem. Modern Pyrex mirrors virtually eliminate this problem.
The material a mirror is made from is less important for smaller apertures because the contraction during cooling is correspondingly less, while mirrors of 10 inches diameter and larger can suffer noticeable effects on the image depending on what they are made of, so the material used for the mirror body becomes a more important consideration. This is why telescope manufacturers researched better materials to use for the mirrors. For smaller mirror sizes Pyrex is just a "nice to have" and you would need a very keen eye to spot thermal distortion effects.
Thermal Boundary Layer
More importantly however, when the optical face of the primary mirror is even 1-2 ºC (2-3 ºF) or more above the ambient air temperature, then observable image degradation will exist.
Just the same as mirage effects you can see on the surface of a car on a hot day, the light entering the telescope will be deformed as it passes through temperature gradients in a thin layer near the face of the Primary mirror. This is called the thermal boundary layer. For most locations on Earth, changes in night-time air temperature occur quite rapidly, and even a small Newtonian's primary mirror will not track the falling nighttime temperature closely enough unless fans are used to speed up the cooling process.
The Orion XT scopes have threaded fixings available on the rear of the mirror mounting plate to allow a simple modification; the addition of a standard computer cooling fan.
When powered by a 12volt PSU or battery this blows air onto the back face of the mirror, round the sides and upwards through the scope tube. Even a small fan greatly reduces the time required to cool the substantial mirror, and ensures it cools evenly throughout. The fan also helps move cool air into, and warm air inside, upwards and out of the tube, breaking the thermal boundary layer and resulting in a more stable column of air inside the tube.
If you keep your scope in the warmth of your home, it is recommended to put your scope outside 1 or 2 hours before you start viewing! With a fan this can be reduced to 30 minutes or so. Personally I just get started viewing, and with the fan running I know that the image will improve during the course of the evening, and generally I let it run all night. The fan I installed is very smooth running and does not introduce any micro-vibrations that might blur the view.
Even after the telescope has been cooling outside for some time, and the mirror begins the night viewing at
exactly the same temperature as the air, you'll soon have a boundary
layer develop since the air temperature will fall faster than the
mirror's temperature can. So any help you can provide to remove temperature differences or break the boundary layer will reduce distortions in the image. [Note that adding more fans does not necessarily provide significant improvement over a single slow-running fan - read Bryan Greer's article for more info].
Blowing Hot & Cold
While on the subject of thermal effects I will add another couple of points.
Thermal differences moving the air in and above the tube can account for a great deal of distortion in any telescope. Of course one curse that affects how well you can observe celestial objects is the "seeing" (this is described later), but since this occurs at very high altitude above the earth in the jet-stream, there is not a lot you can do about it!
However there are other things nearer to home that you can control.
One very cold still night I was looking at a bright star, and out of interest I de-focused the image as far as it would go until the star was a huge white blur. Then I put my gloved hand right in front of the scope tube. I was amazed at just how much the warmth of my hand was causing convection currents in the still night air even with the glove on. Then I noticed that the same effect could still be seen, even caused simply by my warm breath rising up in front of the scope.
While this may not noticeably affect points of light such as stars as much, it is certainly more apparent when viewing larger objects like the moon or blankets. You might bear this in mind when using your telescope on particularly cold nights. Control your breathing (but don't pass out!). Even simply exhaling away from the front of the scope can help prevent thermal distortions.
Of course warm moist air from breathing can also cause dew to settle out or even freeze on the cold surfaces of eyepieces and the finder scope. Here it is possible to make use of a dew cap to keep your breath away from finder scopes, red-dot finders, etc.
Eyepieces likewise suffer on very cold nights from any moisture evaporating from your eye. Dry your eyes if they "tear up". Keep eyepieces warm in your (dry) pockets rather than in racks or bags when not using them, and swap to another eyepiece if the one in use dews up. Allow moist warm air from your eye to move away from the eyepiece by folding down the eyecup which would otherwise trap the moist air.
Article: Using Fans with Newtonian Telescopes by Bryan Greer.
Other Topics in this XT10 Review: