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A 3-inch Gerrish polar telescope.

The outside temperature is 5 [degrees] below zero Fahrenheit. Inside it is 65 [degrees]. The sky is clear and moonless - a good night to use my new toy, a 3-inch wall-mounted telescope I designed and constructed mostly for nights just like this.

A pioneer indoor observer was Willard P. Gerrish, who set up a 12-inch refractor in a south-facing window at Harvard College Observatory early in this century (see the facing page). Years later, Oscar Knab mounted a 6-inch f/18 refractor in the wall of his house in Indiana. The University of Nebraska's 6-inch Minnich telescope is a recent variation on the theme (S&T: July 1970, page 46, and May 1996, page 75).

Some may scoff at a wall-mounted telescope as small as mine, given these elaborate forerunners. But 3 inches is a larger aperture than many amateurs use in winter. It does a nice job on bright open clusters and the Moon, and the optical system is compact enough to be lifted out of the wall receptacle when not in use.

The tube assembly contains a 3-inch f/8 Jaegers objective lens and a 2-inch focuser. These parts fit in a tube having an outside diameter of 3.4 inches. I also had an old Pacific Instruments clock drive with a 3.5-inch-diameter hole in the drive gear. By shimming the inside with two layers of Teflon, I was able to slide this gear smoothly over the tube assembly. I then made a clutch out of a 3-inch pipe-fitting connector of poly-vinyl chloride (PVC). Because the clock drive and clutch fit around the tube assembly, the tube assembly itself defines the polar axis.

I already had a 4 1/4-inch optical flat for reflecting starlight upward to the refractor's objective. By mounting this flat in a square box made from 1/8-inch Masonite and controlling its tilt with a 3-inch worm gear, I gained full adjustment for declination. Originally I wanted to do so without using a motor, but a direct linkage rod to a knob near the eyepiece would have been more difficult to contrive. Moreover, electric declination control makes it easier to seal the cold air outside.

The declination readout is a surplus 3-digit mechanical counter. Through a right-angle gear train this counter is joined to the declination worm. One worm turn changes the mirror's tilt by 3 3/4 [degrees] and shifts the optical axis 7 1/2 [degrees], while the display advances 10 counts. It was easy to draw up a conversion table so I always know exactly where the scope is pointing in declination.

Once the declination is set, I rotate the tube assembly by hand to sweep in right ascension for the object I want to view. Almost anything can be found with the help of coordinates read from a catalog or star chart. For the Moon the process is even simpler. Because the Moon always brightens the sky around it, it is found without tables by sweeping toward the brightest skyglow.

The south trunnion of the polar axis is a strip of 3/4-by-1/8-inch aluminum bent into a uniform circle 8 1/4 inches in diameter. Secured to the outside of the Masonite box, it rides on small ball-bearing assemblies that are permanently mounted inside the 10-inch PVC pipe that serves as a weatherproof housing for the entire telescope. The aluminum strip is not glued down, so it can easily be replaced if damaged. The top joint of the aluminum ring serves as a handle for the objective end of the scope when I install or carry it around.

The north polar bearing is a 3 1/2-inch-diameter hole in a piece of 1/4-inch plexiglass plate, shimmed inside with Teflon pads on which the aluminum tube assembly rides. This plate also acts as a weather seal and as a mounting plate for the right-ascension drive motor. I used plexiglass instead of wood so I can read the declination counter through it.

The lower end of the 10-inch PVC housing projects slightly outside. When the telescope is not being used and has been pulled indoors, the lower end of the housing is covered by a 1/4-inch-thick plexiglass cover that I can install or remove from the inside. Two spring-loaded clips secure it in place, and a small weep hole is drilled through the PVC under it to allow for ventilation and to prevent condensation. I chose plexiglass for this outside cover so I can see if a wasp nest has been built before opening it up. A plywood cover, lined on the outside with foam rubber, covers the inside end of the housing to make a weatherproof seal. I've used this setup for a year without any problems.

The end of the tube assembly must stick out of the PVC housing far enough for the entire optical flat to see straight up. But it does not need to go any farther than this, because the house itself will block any view to the north of overhead.


Half an hour before closing up for the night, I turn on the dew remover if it isn't already on. Then, just before pulling the telescope indoors, I install the optical flat's dust cover. This prevents dew from forming on the optics, which would occur immediately if the flat were left exposed to warm interior air.

When the telescope has been pulled from its wall housing and set down, the entire unit dews up anyway from the sudden temperature change. So I plug the dew remover back in and cover the unit with a cotton beach towel (not a waterproof tarp). The towel absorbs moisture and gives it a chance to evaporate.

For the same reason, I chose not to make the telescope unit completely airtight. Moisture inside must be allowed to escape. I just leave the dew remover plugged in all night and turn it off when I get out of bed the next morning.

Dew prevention is important, because dew will permanently damage optics if left unchecked. My antidew devices are simply "heat rope" cord obtained from American Science & Surplus (P.O. Box 48838, Niles, IL 60714; phone 708-475-8440). At the annual Astrofest convention I've seen similar material sold by the foot.

The dew remover for the optical flat has a resistance of about 50 ohms. It is set in a circle under the flat's center. That for the 3-inch objective has about a 160-ohm resistance. Wrapped around the cell, it is simply secured by a Velcro strap. Both are connected to a 12-volt plug-in-type transformer that also powers an interior red ceiling light (a car running lamp). Despite the fact that the walls of my observing room are white, it is so dark inside when the shades are drawn that I cannot change eyepieces without some kind of light.

My instrument is mounted directly in the wall, as the room was designed with this purpose in mind when we built the house. But anyone who has a south-facing window should be able to raise the sash and install a frame for carrying the scope instead of cutting a hole through the wall.

Since a Gerrish telescope has a fixed eyepiece position, it is important to determine experimentally how high off the floor the eyepiece should be for maximum comfort. Another important consideration is how close knees and toes will be to the wall. Both measurements can be made indoors, long before the telescope is finished, by setting an adjustable pipe at the local latitude angle and peering downward through it while seated comfortably.

For me the optimum eyepiece height turned out to be 42 inches. But I recently added an adjustable-height pneumatic stool so the instrument is also easy for others to use. Comfort is the basic purpose of a telescope like this, so keep comfort foremost in mind.


Where I live, wind usually interferes with a telescope used outdoors. But the Gerrish design is so solid that I'm hardly aware of any wind. Observing from within the warmth of the house is another obvious advantage. Further benefit accrues from being in a darkened room whose lighting you control.

For example, a very high degree of dark adaptation is possible. There is no extraneous light whatsoever to prevent the visual purple of the retina from achieving its full potential. Both eyes can remain wide open - no eye patch, no squinting - something not normally possible outdoors, even at a fairly dark site.

Nor is there any fear that night vision will be ruined by a passing car, or by a neighbor's yard light coming on when least expected. I have observed deep-sky objects that I didn't think were within range of a 3-inch telescope. NGC 2158, the companion cluster to M35 in Gemini, is quite visible even though many authorities call it difficult for a 4-inch.

One night last March, Comet Hyakutake actually looked better in the 3-inch than in a larger scope outdoors. Much of the reason must have to do with improved contrast. With this telescope the only light, anywhere around the observer's face, is that coming through the eyepiece itself.

JON NEHLS 7525 Pauling Rd. Monee, IL 60449
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Author:Nehls, Jon
Publication:Sky & Telescope
Date:Feb 1, 1997
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