Temperature Changes and Optics

Jack Kramer

It's a common practice among amateur astronomers to set up their telescopes well in advance of when they intend to begin observing. This allows the optics to acclimate - to reach roughly the same temperature as the outside air. A temperature differential results from bringing your telescope to the outdoors from inside the house and from changes in the outside temperature that occur during the course of a night's observing. The degree to which your scope responds to temperature change depends on several factors.

A thermal boundary layer exists over an optic that is not in equilibrium with the air. The non-uniform change in the index of refraction of air above the mirror or lens can cause the air to act like a lens of random shape in the optical path. A small computer fan blowing over the face of a mirror generally helps to solve this problem. Sky & Telescope magazine had two articles on this topic: Sept 2000, page 125, Understanding Thermal Behavior of Newtonian Reflectors and Jan 2002, page 132, Thermal Management in Newtonian Reflectors.

A similar thermal layer may be created when you insert a warm eyepiece into a cold telescope. The eyepiece lens and metal housing give off heat for a short while, perhaps only about 10 minutes or so. Because an eyepiece is smaller than a primary mirror, the effects are not as great. A tipoff is that after you insert and focus the warm eyepiece, you find that the image is again out of focus, and you may have to re-focus several times.

In addition to the refractive problems with heated or cooled air, optician Al Misiuk pointed to the substrate itself as a culprit. ("Substrate" refers to the material out of which a mirror is made.) He said that with Pyrex, a 0.1oC difference in temperature from the inside of the glass to the outside of the glass results in 1/10 wave peak-to-valley distortion of the mirror. A two or three degree difference would make the mirror unusable at anything more than the lowest powers.

Low-expansion glass isn't the total answer either. According to Gary Seronik of S&T: "No matter what kind of expansion characteristics a piece of glass has, it still will have some mass and the capacity to store heat. Even though its optical figure may change very little because it is at a different temperature than the ambient air (the advantage of low-expansion glass), it's still going to perform poorly until it dumps off its heat."

 Different telescope designs respond in a unique manner depending on the thickness of the glass, the number and mass of optical elements, and air movement within the tube. Maksutov-Newtonian and Maksutov-Cassegrain designs take the longest to acclimate to the outside temperature due to the number of components and their thickness, especially the relatively massive Maksutov meniscus lens that resides between the closed tube and the outside air. Tube ventilating fans have become at least a partial solution to this problem. New Jersey amateur Vic Palmieri related his experience with a Maksutov-Cassegrain telescope: "I have also found cool down an issue with MCTs. The warm summer nights have not been a major problem but the cooler months are. This is especially true in the fall when a warm day gives way to an evening where temperatures drop all through the night. The MCT never catches up. MCTs are great compact scopes but seem temperature-sensitive. A slight temperature change creates image problems."

Schmidt corrector plates aren't as thick as Maksutov front elements, so SCTs and Schmidt-Newtonians settle down a bit faster, but they too take awhile to adjust. Simple Newtonians adjust more quickly, especially if the tube is well ventilated or entirely open. But a solid tube can cause unstable images if it acts as a chimney for air movement. Finally, refractors generally acclimate the fastest because the multi-element objective lens is the only large optic and is exposed more directly to the outside air. There's also less mass to adjust. Because of their cost and weight, refractor objectives tend to be smaller in diameter than the objective mirrors and corrector plates commonly used in reflecting scopes. By way of illustration, an Internet correspondent who has both a 5" Maksutov-Newtonian and a 4" refractor stated that the Mak-Newt takes about two hours to acclimate to winter night temperatures, while the refractor takes 20 to 30 minutes.

Well-designed lens cells and mirror mounts allow the glass to expand and contract without pinching the optics and causing further distortion. So to some extent, a telescope maker can somewhat affect the results from a purely mechanical standpoint.

To summarize, the answer to the initial temperature differential problem is to take your telescope outside to acclimate for a sufficient time before observing. Storing a telescope in an unheated garage or other enclosure is another solution. A cooling fan may also help and might be the only answer if your scope has problems acclimating as temperatures change during the course of the night. Also, the focuser drawtube (without an eyepiece) can be used as a vent to let the heated air inside the tube escape faster. Then too, you could just wait for a night when the outside and inside temperatures are the same (big grin).