Most telescopes need to be cooled down to the ambient environmental temperature before they will perform their best optically.

When a scope is brought from inside where it is warm, to outside where it is cooler, it will release heat through convection between the solid mirror and the air, trying to reach equilibrium.

When this heat is released, a boundary layer forms at the surface of the mirror. This boundary layer is usually turbulent. As light passes through air of different temperatures in the boundary layer, it is refracted. It is the temperature differences and the turbulence that causes poor "mirror" seeing right at the boundary layer. Light in a reflector has to pass through this boundary layer twice, once before it hits the mirror and once after it is reflected, making the problem worse.

The mirror seeing will not improve until the temperature of the mirror is the same as that of the air and the boundary layer goes away. In practice, when the temperature is within a degree or two, the effects of the boundary layer are greatly reduced, and mirror seeing will improve.

Another important consideration is that the ambient temperature usually continues to fall during the night, so the mirror is playing catch-up all night long. Very large mirrors may never reach thermal equilibrium unless they are actively cooled (see cooling aids below).

The primary mirror in Newtonians, Schmidt-Cassegrains and other reflector-type telescopes store the most heat because of their thickness and mass. The larger and thicker the primary mirror, the more mass it has and the longer it will take to cool down. Schmidt-Cassegrains are particularly problematic because the primary mirror is in a closed tube, which slows down cooling and prevents the easy use of a fan to help with cooling.

Refractors are much less problematic. The objective in a refractor is usually smaller than in a reflector so it does not have as much mass. Since the objective is at the sky end of the telescope, it can release heat to the open sky more quickly. The light also doesn't have to pass twice through any turbulence in the boundary layer. Refractors usually cool down much more quickly than reflectors and catadioptrics. It may still take 30 minutes for a 4 or 5 inch refractor to cool down, and up to an hour for a 6 inch scope.

The bottom line is, if you want to do high-resolution planetary imaging with a moderately sized 6 to 16 inch mirror telescope, you are going to have to deal with thermal cool-down problems.

Bryan Greer, in his seminal article "Understanding Thermal Behavior in Newtonian Reflectors" in the September 2000 issue of Sky & Telescope magazine, shows that no mirror, regardless of glass type, will perform adequately until this boundary layer goes away. He says that it took almost two and a half hours for a 6 inch mirror to reach thermal equilibrium from an initial temperature difference of 25 degrees C. This is not an unusual amount of temperature difference when a scope is taken from inside a heated house to outside on a winter night.

The amount of time it will take for a telescope to cool down will vary with the size and thickness of the primary mirror, the initial temperature difference, and the use of any additional cooling methods, such as using a fan to blow air onto the mirror.

Note that it is not only the initial temperature difference that must be overcome, but also any additional difference caused by ambient temperatures falling further through the night.

An indoor-outdoor thermometer can be used to monitor the temperature differential between the mirror and ambient air. Attach the outdoor probe to the back of the mirror, put a small piece of cork over it to insulate it, and tape it down. If you have more than a couple of degrees difference in temperature, you can expect to have some mirror seeing issues.

Store your scope outside or at least in an unheated garage to minimize the initial temperature difference. If you must cover your scope while camping or at a star party, use an aluminized mylar blanket to keep it cool during the day when the Sun may be shining.

Cooling Aids

Using a fan to blow air onto the back of the primary mirror can be a great help in cooling down a telescope. The air flow helps with the heat transfer, speeding up the cooling.

Many Newtonians come with cooling fans, and the open rear end of a Newtonian's tube makes it easy to hook up a small 12-volt computer fan if you don't already have one. Keep the fan running all night if the temperatures are falling.

Make sure to check the image at high magnification with the fan on and off to be sure the fan is not introducing vibrations that will degrade the image quality. If you are seeing vibrations, then turn the fan off while you are shooting a planetary video. Turn it back on again in between videos while you are waiting for the seeing to improve.

Schmidt-Cassegrains and Maksutovs are more difficult to cool because the primary mirror is in a closed tube. A simple aid in cooling these scopes is to blow air from a large box fax onto the scope. The heat transfers from the mirror to the air inside the tube and ultimately to the walls of the scope tube and corrector plate where it is released. Blowing air on the outside of the tube can help speed up this process, although not as well as a fan blowing directly onto the primary in an open tube.

There are also devices that you can make or buy that can be inserted into the focuser of traditional Schmidt-Cassegrains to force cooler air up into the tube, such as the Lymax cooler. This will force warmer air out of the tube, helping to cool the primary.

If you are adventurous, you can also cut holes into the back of your SCT and attach fans there that blow air directly onto the primary. Note that is will void your warranty however.

The new Celestron Edge series of Schmidt-Cassegrain telescopes present additional challenges in cooling. They have an optical assembly inside the baffle tube which prevents blowing air up inside the focuser. Celestron has added two small vents that are supposed to aid in cooling the primary, but in my experience with a C11 Edge, these vents are not very effective. On some nights it has taken 3-4 hours for the scope to cool down.

Starizona makes a Cool Edge cooler for Celestron Edge SCT telescopes. It is a fan assembly that takes the place of the secondary mirror, which can be removed in the Edge series.

These coolers do work for SCTs, but introduce additional problems. The first is that if you are working in a humid environment, you are going to be blowing cool moist air into a closed tube where it can't escape. If the primary, or corrector plate, cools down to the dew point, you can get moisture condensing inside the tube on the primary mirror and on the back of the corrector plate.

You can clear the moisture off the back of the corrector plate with a traditional anti-dewer that is used to keep dew off the front of the corrector plate. Clearing any moisture that condenses on the primary inside of a closed tube is, however, a nightmare.

Another problem with the Cool Edge specifically is that when you remove the secondary to replace it with the Cool Edge, you will trash your collimation. Normally you want to tweak the collimation anyway before high-resolution planetary imaging, but you will usually be close to start with. After you take the secondary out and put it back in, you are usually back at square one, and the collimation may not even be remotely close to start with. We will discuss collimation in detail in the next section.

Dew

Once your scope cools down to ambient temperature, it is possible that heat will continue to radiate away to a clear sky, and the optics may drop below the dew point, and dew may form on them.

This is more of a problem for refractors and SCTs where the objective or corrector plate is at the top end of the telescope tube. Because they are not shielded from most of the sky, as is the mirror down in a tube, they can be very prone to dewing.

Refractors normally come with a dewcap which will slow dewing, but on some humid nights dew may form anyway. The best way to deal with dew is to prevent it from forming in the first place. When the telescope objective nears the dewpoint, a gentle, small amount of heat should be applied to it with an anti-dewer, such as a Kendrick or Dew-Not.

Schmidt-Cassegrains usually do not come with a dew cap. Dewing of the corrector plate can be a real problem because corrector plates are very thin and easily radiate their heat away. A dew cap can be made or purchased and is highly recommended. In especially humid environments an additional heating element anti-dewer may also be needed. Astro-Zap makes an SCT dew cap that incorporates a 12-volt anti-dewer into the design.

Newtonian primary mirrors generally don't dew up because they are shielded from most of the sky by the tube, and because they frequently don't reach thermal stabilization with the ambient temperature.

Newtonian secondary mirrors, on the other hand, can sometimes dew up more easily in humid environments because they are up at the top end of the scope tube exposed to the sky where heat can radiate away, and because secondaries don't have a lot of mass to store heat to start with.

Note that if dew is a problem, applying heat to remedy it can introduce its own set of thermal problems. Applying too much heat can generate boundary layers that affect the seeing in a telescope.

Some people wait for dew to form then use a hair dryer to remove it. This is not a good practice. It is better to use an anti-dewer to prevent it from forming in the first place. It should be used at the lowest possible setting to keep the temperature just slightly above the dewpoint.