It is no secret that many amateur telescopes take excessive time to equalize with the surrounding air, or cool down, so to speak. It is important to remove heat from components within the telescope to allow air in the optical path to stabilize. Even external parts, such as the mount, pier and surface area, will cause unwanted heat to enter into the telescope tube and spoil the stable air in the optical path. So, it is desirable to equalize the inside of the tube with the surrounding air as soon as possible.

In our atmosphere cooler air tends to drop towards the ground because it is denser, therefore heavier than warm air. When cool air descends, often down through a telescope tube, it mixes with rising warm air to cause turbulence in the optical path and form what some telescope designers refer to as "thermal cells." Because the density of air varies with temperature and the refractive index of air varies with its density, these "thermal cells" act like lenses floating around inside the tube. The result is similar to the effects of bad "astronomical seeing."

For serious planetary observers a portable telescope is usually not important. However, deep sky telescope user's desire portability and weight becomes an important design constraint. Another major concern is keeping the optics from losing collimation when moving the telescope around. The cost of materials is important when considering telescope design and while materials such as paper or fiberglass may seem to be cheaper than steel or aluminum this author found the opposite to be true.

This article will point out methods to reduce weight and annoying tube currents for both types of telescopes, and to reduce the overall cost of the telescope construction. However, a important rule to remember that we all subscribe to: the KISS rule -- K eep I t S imple, S tupid.

Do we want a material that stores heat? One that slowly releases heat into the optical path during observing periods or do we want a material that radiates heat into the optical path or out into space? Materials are described by their thermal properties as well as the their strength and rigidity. Many of the materials used for telescope construction are insulators, some radiators.

Heat waves cause turbulence within a telescope tube and interfere with the optical path. This causes the image to blur and move about. If this process is long lasting, for hours, then very little useful observing will take place until the tube stabilizes with outside air. Therefore, it stands to reason that the faster we rid the tube of heat, the quicker we can begin useful observations.

There are three processes of heat transfer at work within the components and materials of a telescope tube:

1) Conduction : When energy is transferred directly from a hot body to cooler body due to the temperature difference between them. As the telescope tube begins to cool when exposed to the night air, heat will transfer from some tube components to others, such as the walls of the tube to the secondary mirror support arms and then to the holder and mirror itself.

2) Convection : A typical example of convection is when hot air is moved from one part of a telescope tube to the other end by airflow. Because the telescope is usually pointed up, warm air rises towards the open top through the optical path. As heat enters the secondary system it rises in the optical path and this rising column of warm air will appear in the eyepiece similar to smoke rising from a fire.

3) Radiation : This is when energy (heat) transmission is through space from hotter body to a cooler body. The Sun radiates heat. Telescope materials and components radiate heat to other components from the upper side of tube outside into space (up). The lower side into the tube will radiate to the upper walls, and so on.

One must also be aware that various materials expand and contract at different rates. For instance, the thermal coefficient for aluminum is from 10 to 18 millionths of and inch per degree Fahrenheit. Fiberglass is can be from 3.3 to 8.2 millionths of an inch per degree Fahrenheit. If one uses a typical aluminum primary cell in an aluminum tube expansion and contraction will be very similar. However, an aluminum cell in a fiberglass tube will tend to loosen up as the temperature drops. Collimation problems may exist under these conditions.

Another reason some telescopes take longer to cool down is that telescope tubes are often made of the wrong material. Fiberglass, plastic, Formica, and other insulating materials store heat and are slow to radiate this energy to the outside air. Insulating materials, such as paper and fiberglass, tends to hold heat longer and radiated very slowly. Unfinished paper tubes are not much of a problem; however, when they are painted they soak up more paint that other materials and begin to store heat.

Fiberglass has two properties that deal with heat. First, it tends to hold heat longer than metallic materials and second it tends to pass more Infrared Radiation (Sunlight) though than metallic tubes. Sunlight will heat the interior of the telescope and that tends prolong cooling. Furthermore, unlike metal, insulating materials are usually porous, soak up paint, and may require several coats to produce a satisfactory finish. This adds to the weight. Remember that paint was invented to be an insulator.

Metals, such as steel and aluminum, radiate and loose heat faster.  Also, metal tubes are lighter that fiberglass and painted paper tubes as discussed later in this article.

A major aspect in stabilizing temperatures in a telescope is the color and thickness of the coating on the outside of the tube. A white or brightly colored tube may look very nice; however, heat (Infrared Radiation) is reflected into the black walls of the interior of the tube by bright commercially available coatings and causes the air to become unstable. If one remembers the theory of thermodynamics (the black body radiation part) then it would seem apparent that it is the best to paint the outside of a telescope tube black! Heat will tend to radiate from the black surface faster than from a white surface.  Heat will radiate from a black (high emissivity) surface faster than it will radiate from a shiny (low emissivity) surface.  That doesn't mean that a telescope painted black will always have significantly better radiation exchange with its environment than one painted white.  The amount of radiative energy (heat) exchanged between two surfaces depends on: the temperature difference of the two surfaces; the emissivity (blackness) of the surfaces averaged over wavelengths at the peak of the appropriate blackbody curve; and the relative angular extent (view factor) of the surfaces.

In visible light, black surfaces absorb and re-radiate heat much more efficiently than white or reflective surfaces.  Light from the sun --- with a 5000 K blackbody peak at a wavelength of 0.5 microns --- provides the standard reference against which we define colors. However, our common sense definitions of black and white may fail us at other wavelengths appropriate to different blackbody temperatures. Normal telescopes have temperatures around 260-300 K and thus have the peak of their blackbody radiated energy at wavelengths around 10 microns --- in the so-called ``thermal infrared''.  Essentially all painted surfaces appear ``black'' (high emissivity) at a wavelength of 10 microns, so any painted telescope tube will radiate effectively to its surroundings regardless of the color of the paint.  Typically, only metallic surfaces have low emissivity in the thermal infrared.

The nature of white is to reflect energy, and black absorbs energy. More energy (light and heat) is reflected from a shinny white surface than from a dull black one.  However, because paint is not a perfect reflector, Infrared Radiation (I.R.) will leak through and slowly heat the telescope even if it is housed in an observatory or covered with some material in the outdoors under the Sun.  At night, when the air is cooler, we want to remove the heat as fast as possible, so, if the outside of the tube is painted with I.R. reflecting white paint, where does this radiation go?  Yes, inside through the black painted inside of the tube!

It may be a good idea first to paint the inside the tube  (IR reflecting) titanium white then apply a light coat of flat black over the white paint.  The white paint would reflect I.R. to the outside of the tube and of course the black reduces light scattering inside.  Remember though, paint is an insulator and should be applied with the least amount of coats as possible on the outside of the tube. One might find it useful to black anodize a small aluminum telescope tube.   A tube made this way is black through and though. Given the need to channel heat from within the tube to the outside air a bare aluminum lightly coated with black paint may be the best solution.

For general use, the outside of the tube may be painted any color which suits the taste of the owner.  The painted surface will radiate to the sky and eventually cool below the surrounding air temperature.  Telescope tubes, and indeed the whole earth, radiate heat to the cold sky because the night sky has an effective temperature of around 235 K.  The sky would be much colder if it were not for the so-called “greenhouse gases” in the atmosphere which block many infrared wavelengths.  The effective temperature of the sky varies with location and season.  Deserts and high mountains tend to have colder skies because there is less water vapor to block the thermal infrared radiation.  External surfaces of telescopes or domes which cool below the ambient air temperature can have two unwanted effects.  They can collect dew which then drips on the optics, or they can create plumes of cold air (cooled by conduction from the surface) which negatively effect seeing in the same way that plumes of hot air do.  At least a little bit of cooling by radiation to the sky is desirable as it compensates some of the excess heat which may be stored in the mass of the telescope structure.

For telescope tubes or domes which are exposed to direct sunlight during the day, TiO2 (titanium dioxide) white paint is a very effective coating.  This special paint does the best job of reflecting away solar radiation during the day.  TiO2 white paint appears quite ``black'' at thermal infrared wavelengths so it radiates well to the sky at night.  About 20 years ago, astronomers with white domes began to notice excess seeing caused by air cooled by being in contact with the radiatively cooled dome surface.  Measurements of the inside of my 12.5” f/30 and f/7 telescopes have shown that a TiO2 painted surface may be as much as 6 K below the surrounding air temperature. To bring the tube back into equilibrium with the nighttime air, we need a coating which has low emissivity at thermal infrared wavelengths.  An example of such a coating is the shiny aluminum tape which is seen on many newer telescopes and domes. Note that the shiny aluminum surface actually gets hotter than the white surface in the daytime because it cannot easily radiate away the solar heat that it does absorb.  These telescopes compensate for this effect by having very lightweight domes which don't hold much heat after the sun goes down.

Telescope tubes made from insulating materials are heavier because they are intrinsically weaker and have to be thicker than metal to provide the necessary strength to keep optical components in place. Aluminum is nearly twice as stiff and nearly three times as strong as fiberglass.   Steel is approximately three times stronger than aluminum, but nearly three times heavier.  However, aluminum is more rigid than steel -- an important ingredient to maintain the shape of the telescope tube.

Heavier may not be better. A lightweight tube assembly mounted onto a heavy, stable mount has certain advantages. First, a lighter tube tends to vibrate at a higher frequency than heavier tubes. Usually, higher frequency oscillations result in lower amplitudes; that is, the telescope will not oscillate with large swinging motions. A heavy tube tends to oscillate at lower frequencies but will take longer to dampen out. We must reach a compromise and select the right materials that offer high frequency oscillations if disturbed and will expel heat as fast as possible. When selecting materials we must ask some important questions about the conditions we live in and what we expect from our instrument. What material is best suited for telescope tubes?

Several quantities will be necessary to calculate how rigid our telescope tube will be. Three important quantities to measure the stiffness, weight, and modulus of elasticity (E) resistance to bending are listed in table I.

Table I. Stiffness, density, and modulus of elasticity (E) of the three tube materials listed.

Figure 1. Simple illustration of the two elements to consider when selecting shaft sizes, linear deflection and tortional deflection

At this point we will need to know the weight for each material per inch, so we first find the square area of the tube per inch from: p (r ²   OD -   r ² ID) , where OD is tube outside diameter and ID is the tube inside diameter and p is 3.14159. We then multiply that times the length of the tube and the weight can be found my multiplying the total area of the tube by the density of the tube material.  We may wish to use an actual finished and successful telescope tube. Using this author’s 12.5-inch f/7.04 Newtonian telescope. The tube is 15" OD x 14.875" ID x 92" length (0.0625" thick).  In the case of a 0.0625” thick, 92” long aluminum tube and the density from Table I, we find that:

A fiberglass tube with the same dimensions would weigh:  2.93297 x 92” x 0.07 lb/in ³   = 18.9 lbs.

However, in order for a fiberglass tube to be as strong as aluminum one the wall thickness will have to be 2.86 times thicker than aluminum (Ea / Ef = 104 / 53 = 2.86). Then a fiberglass tube that is 2.86 times thicker or 0.17875-inch thick would weigh:

Therefore, the front tube section weighs  2.93297 x 50" x 0.097 lb/in ³ = 14.2 lbs. for aluminum, and 8.33452 x 50" x 0.070 lb/in ³ = 29.2 lbs. for fiberglass. We can see that a fiberglass tube would weigh -- twice as much – as an aluminum tube.

When designing a Newtonian telescope one must be aware that the tube should be long enough to block direct light from entering the focal plane through the focuser.  This can happen if the focuser is mounted too close to the end of the tube, so make sure the focuser hole is far enough from the tube entrance so direct light does not enter the focuser.  This is not a big problem for focusers with a high profile and long draw tube; however, low profile focusers may be short enough allow direct light from the tube entrance to enter into the focuser tube.