Managing Telescope Temperatures: Importance of Thermal Optimization
Even perfectly aligned telescopic optics can produce blurry views if the temperature within the telescope tube and its components isn't stable. This often-overlooked factor in achieving clear telescope views is known as collimation, comprising both the alignment of the optics and the thermal equilibrium of the tube and its components.
When moving a telescope from a warm environment to a cold one, such as taking it outside on a chilly night from a heated home, the temperature difference may cause the metal of the tube to contract and the glass or mirror to change shape slightly. These changes can distort the image, resulting in a fuzzy or unfocused view.
Every telescope design presents a balance of trade-offs, with thermal equilibrium being one of them.
Inside Tube Air Currents
The differential cooling can cause what's known as "tube currents" - warm air rising and cooler air sinking inside the telescope tube, leading to turbulent air that blurs the telescope image.
The primary source of these currents is the telescope's primary mirror itself, which radiates heat directly into the optical path.
Refractor telescopes, with their thick front-located glass element, aren't generally affected by tube currents. However, they are a common issue with many Newtonian reflectors and pervasive with catadioptrics. In these designs, light travels down the tube and then back up again, encountering the turbulent air twice, making it an even more apparent and troublesome issue.
Air currents inside the tube manifest as wavy or swirling patterns at the eyepiece, resembling a view through moving water. The image appears wavy or fuzzy, with an inability to focus at times. This condition is a clear indication of thermal issues in the instrument, signaling the need to stabilize the temperature.
Uneven Cooling: Optical Aberration Culprit
Tube currents aren't the only thermal problem plaguing telescopes. When moving a telescope from a warm indoor setting to the cooler outdoors (or vice versa), its optics may not cool uniformly. This uneven cooling can induce optical aberrations, such as spherical aberration and astigmatism, which result in views lacking sharpness or clear points of focus.
Spherical Aberration
Spherical aberration during cooldown is primarily caused by the shape of most optics. In Newtonian reflectors, the dish-shaped primary mirror has a thicker edge than the center, resulting in the mirror's focal point changing more as one approaches the edge. Consequently, spherical aberration occurs.
Refractors and some catadioptrics also exhibit this issue due to their shaped lenses and multiple glass pieces. Some designs with conical or dome-shaped primary mirrors, like many catadioptrics, minimize this problem, but they often struggle with tube currents.
Large Newtonian telescopes with thin meniscus primary mirrors have unique properties that enable rapid cooldown times, even for mirrors as large as 20 to 40 inches in diameter.
Astigmatism
Astigmatism, characterized by blurry, oval-shaped stars or objectives with different points of focus during cooldown, is typically caused by poor annealing or strain in the telescope's optics. This condition is usually inherent in the glass itself, and there's little that can be done about it. In severe cases, it may be caused by a slight pinching of the primary mirror or lens. In such cases, loosening the retaining clips or ring slightly may help. Alternatively, simply waiting for the telescope's optics to equalize in temperature is often the best solution.
Factors Affecting Cooldown Time
The thickness of the primary optics in a telescope plays a crucial role in its cooldown time. Thicker mirrors and lenses retain more heat, translating to longer cooldown times. Additionally, the material of the mirror affects its rate of thermal expansion and contraction. Materials like ceramics or low-expansion borosilicate glasses (Pyrex) stabilize faster than BK7 or plate glass commonly used for mirrors. The expansion properties of crown glass, often used in refractors, are exceptionally poor, but this issue is usually mitigated by placing the most thermally problematic lens element at the front. Thin Maksutov corrector lenses, though lacking in thermal properties, are fortunately low-expansion.
While waiting for a telescope to reach thermal equilibrium, it is advisable to aim your telescope at less demanding objects in the sky, such as larger nebulae or star clusters. Using lower magnifications during this cooldown phase can ensure a more pleasing and less distorted view.
Fans and insulation can improve the cooldown of reflecting and catadioptric telescopes. Dedicated fans for Cassegrain telescopes and various dedicated fans for Newtonians are available. It is essential to use silicone or another rubbery, vibration-absorbent material to prevent the fans from jostling the telescope. Suspending the fans behind the scope using springs or wires can also help alleviate any impact.
Importance of Ventilation in Cooling Down Telescopes
Minute temperature changes in the shape of the mirror or lens due to thermal fluctuations can significantly impact image quality. Therefore, allowing hot air to ventilate out of the telescope is crucial to avoid trapping turbulent layers or inducing uneven cooling of the optics.
In the case of refractors, little can be done to improve ventilation. The dew shield is essential for blocking glare, and adding fans would only serve as dew magnets or cause more distortion than they remove. Very large refractors (10" and up) may suffer from thermal issues due to their enormous tubes, with some owners even resorting to cutting holes in the telescope for improved airflow.
For reflectors and catadioptrics, the design and construction of the mirror cell significantly contribute to the cooling efficiency, as well as alleviating tube currents. A well-designed mirror cell provides ample ventilation and facilitates even cooling, while a poorly ventilated one can trap heat. Most commercial telescopes, however, keep as little a gap as possible between the tube and mirror to minimize size and cost, often failing to address this issue.
A mirror cell usually benefits from a fan (or two, or three) at the back if there is an exposed portion of the mirror or at least a hole. For telescopes up to 14", an 80mm fan is sufficient; a 120mm fan is suitable for larger scopes. Try to center the fan(s) on the mirror.
In some thin-walled tubes or many truss telescope mirror boxes, a boundary layer of stagnant air can form around the mirror. This layer acts as an insulating blanket, slowing the mirror's cooldown. Sideways fans have been found effective in disrupting these layers, ensuring uniform cooling, though a vent must be added on the opposite side to let the air blow out.
Ed Allen has developed an excellent forced-air ventilation system for larger reflecting telescopes, which has been implemented by another telescope maker, Howard Banich. This system is overkill for most people but may be ideal for preserving the fragile and short-lived spray silver coatings on telescopes like Howard's 30".
Insulating The Telescope
In cases where a telescope seems to resist stable, even cooling, it might be beneficial to insulate the tube to restrict ventilation and keep the telescope indoors or in a garage if it's cold and dry. By maintaining the telescope in a "pre-cooled" state, one might mitigate some of the more common thermal issues.
Reflective insulation wrapped around the telescope or used in tropical environments can have a similar stabilizing effect while helping to ward off dew.
Dew Effects in Optics
Rapidly cooling a telescope to prevent thermal issues may lead to condensation. If your tube has dew or frost forming on it, great! This typically means that you don't have much to worry about in terms of tube currents. However, it's also a warning sign that there is potential for dew and frost to form on the optics.
Condensation on the optics can not only obscure the view but can also be damaging. In humid regions, it may seem counterintuitive, but it is often wise not to let the telescope cool down completely. The reason is straightforward: when the room temperature falls below the outdoor dew point, the telescope becomes a magnet for condensation. This moisture doesn't just obscure the optics; it can be long-term damaging.
Over time, I've come to rely on electric dew heaters for my eyepieces, even in drier climates. My own breath and eyeballs eventually fog them up over extended observing sessions, or the weather simply ends up more humid than I expect.
Dew shields are crucial for catadioptrics in any climate, both for condensation prevention and to act as a light shade and prevent unnecessary touching or dust/pollen from getting on the corrector. Dew heaters for your secondary mirror and heated dew shields are also available if needed.
Special Considerations for Catadioptrics
Catadioptric telescopes often have limited ventilation at either end due to their closed tube designs, which can significantly slow the cooldown process and intensify the effects of internal tube currents. Different catadioptrics are affected variously:
Schmidt-Cassegrains (SCTs)
Schmidt-Cassegrains have a closed design, with their Schmidt corrector plates being relatively thin, allowing for faster thermal acclimation. Unfortunately, the limited ventilation means they're not entirely immune to thermal issues.
The thin correctors in SCTs are made of plate or crown glass, which expands significantly with temperature. Subjecting it to a huge temperature change in a short span of time can shift the telescope's collimation, as the tube and, more importantly, the metal or plastic secondary mirror holder attached to the center of the corrector are made out of metal, which moves at a different rate. Theoretically, a large enough temperature shift in a short time could even crack the corrector.
Some larger Meade SCTs have vents at the back, as do Celestron's EdgeHD telescopes, but it's very hard to add ventilation or fans to most of these telescopes without a lot of drilling and tapping of the rear cell.
Maksutov-Cassegrains (MCTs)
Maksutov-Cassegrains are particularly tricky in terms of thermals. Their defining Maksutov lens is thick, retaining heat much longer than thinner optics. Combine this with a thick primary mirror and almost no ventilation, and we have what I'd term a "thermal nightmare." This extended cooldown time can be especially challenging for impromptu stargazing sessions.
Rutten-Maksutov or "Rumak" telescopes with a separate secondary mirror face an even greater challenge since the secondary is now a separate piece of glass with a metal holder inside the corrector.
From my own experiences, it's basically impossible to cool Maksutovs over 5" without adding fans or wrapping the telescope tube in reflective insulation.
References
[1] "The Physics and Maintenance of Newtonian-type telescopes." Edward, Michael. https://www.mendor mirror.com/Maintenance/Recommendations.htm
[2] "iOptron 12" Photron Truss Tube Ritchey-Chretien Telescope." iOptron. https://ioptri.com/products/12-photron-truss-tube-ritchey-chretien-telescope
- To obtain clear telescope views, it's essential to balance optics alignment and maintain thermal equilibrium within the telescope tube.
- Moving a telescope from a warm environment to a cold one, such as outside on a chilly night, can cause differences in temperature that lead to distorted images due to tube currents.
- Refractor telescopes are less likely to be affected by tube currents, while these issues are common in many Newtonian reflectors and catadioptrics.
- In some reflecting and catadioptric telescopes, uneven cooling can induce optical aberrations like spherical aberration and astigmatism, affecting image sharpness.
- A well-designed mirror cell for reflectors and catadioptrics can provide ample ventilation, facilitate even cooling, and alleviate tube currents, while a poorly ventilated mirror cell can trap heat.
