22" binocular roll-away enclosure

The 22" binocular and its enclosure at the Comanche Springs Astronomy Campus in Crowell, Texas, part of the 3 Rivers Foundation¹.

The amateur astronomer waiting for the night skies is Tony Buckley from Sydney, Australia.

A small footprint enclosure for a large binocular

Many amateur astronomers live under night skies bathed in light pollution and have to take their telescopes to remote rural locations for decent observing. For those fortunate enough to live away from city lights, keeping the telescope in an outside enclosure is an option.

Enclosures — usually called observatories by ATMs¹⁰  — come in all shapes and sizes. I built two roll-away enclosures for the 22" binoculars, mine and another for the 3RF (shown above). In Texas, the binocular is used for star parties, and the enclosure can be withdrawn to provide room for dozens of attendees (photo above).
 

Requirements for the 22" enclosures include:

1. Withstand wind gusts 50 to 75 mph
2. No obstruction for horizon-to-horizon viewing
3. No hindrance to telescope assembly or disassembly
4. Protect a 10-foot tall telescope
5. Telescope and ladder footprint with a 12' radius measured from the azimuth axis
6. Maintain or improve upon ambient temperatures

Design

The best enclosure is one that disappears when using the telescope. This not only provides open, unobstructed space around the telescope, but promotes natural ventilation and telescope cooling.

Modern professional observatory enclosures use louvers, front and rear doors, sliding dome halves, etc. in order to equalize the internal and external air temperatures. The old idea that it was best to enclose the telescope tightly — except for a narrow opening to protect it from turbulence within — has been abandoned. 

An extreme example of open enclosure design is found at the Air Force's Starfire Optical Range (SOR) at Kirtland Air Force Base in New Mexico². Besides performing pioneering work in adaptive optics, SOR built a unique enclosure for its 3.5 meter satellite tracking telescope.  Its roof retracts and its three-tiered wall collapses around the base, completely exposing the telescope for rapid satellite tracking. This design minimizes thermally-induced turbulence and provides complete ambient air circulation, improving image quality. 

Backyard enclosures built by amateurs usually adhere to a roll-off roof design³. Although easily built using conventional wood frame construction techniques, I rejected the roll-off roof approach because its supporting walls would have blocked views of lower-elevation objects, and because its large footprint wouldn't fit on my observing deck.

These considerations led to the roll-away enclosure design, one that is permanent, weatherproof and completely removable^4,5. Tracks for a roll-away structure were precluded because of its deck location; it had to roll on wheels.

Construction

The problem to solve with a roll-away enclosure is how to make it sit flush on the ground when around the telescope, yet provide a way to raise it up on wheels so it will roll. I did this using an adjustable-height floor lock⁶, replacing its flat foot with a caster. Floor locks are normally used with mobile heavy equipment tables on uneven floors; they prevent table movement.

The enclosure's 3' x 5' x 10' structure consists of an aluminum frame with fittings made from 80/20 Inc.'s modular framing systems⁷. This "industrial erector set" manufacturer has a refined product line that makes custom design a simple, straightforward process. 80/20 precisely cut the T-slot aluminum frames to my specifications; all I had to do was bolt the enclosure together.

For the walls and door, the enclosure's skin is cut from ¼" Alucobond⁸, which consists of two sheets of smooth .02" aluminum bonded to a polyethylene core. This laminate is manufactured primarily for architectural use as siding. The surface is pre-painted white with highly durable Kynar coil coating.

The telescope stands on a concrete pier, topped with an aluminum plate that is flush with the deck. The pier is isolated from the deck so vibrations aren't transmitted to the telescope.

The pier is about six feet long and is embedded in a large concrete block below ground. It was poured using a Sonotube⁹ at the time the deck extension was built. Sonotubes, once sold exclusively to ATMs for their Dobsonian telescope tubes, are now found mostly around construction sites and in homemade subwoofers.

Cooling and heating

There is a small plenum below the deck level that surrounds the pier. A 75' underground, 6" PVC cooling tube terminates in the plenum and supplies air to the enclosure. The other end of the cooling tube surfaces in the shade of a pine tree down the hill.

When the enclosure is in place, it forms a relatively air-tight seal with the plenum. A small 100 CFM electric fan inside the plenum is mounted over the end of the cooling tube. The fan is controlled by a switch in the house. The box is vented at the top. When the fan is running, air is drawn through the cooling tube, forced up through the enclosure and out the vent. The cooling system keeps the air inside the enclosure close to the the ambient air temperature. According to a recording thermometer, the maximum temperature inside the enclosure during the summer was around 102°.

During the winter I attach a small gooseneck lamp with a 100-watt bulb to one of the telescope's struts, about a half-foot above the mirror. This raises the temperature of the mirror above the dew point and warms the enclosure slightly.

Winds in Texas often exceed 50 MPH, so the enclosure is secured by both guy wires and bolts that screw into the pad. The guy wires are made from stainless steel cables, attached to the top of the enclosure on all four sides. The enclosure is bolted to nuts embedded in the pad (or deck) through floor mount base plates attached to the enclosure (shown at  right in raised position).  

Internet links

1. The 3RF's mission is to promote science interest and literacy primarily through astronomy

2. SOR's mission is to develop and demonstrate optical wavefront control technologies

3. Jay Ballauer has written an excellent article on his enclosure's roll-off roof construction

4. PIER-TECH Inc. sells roll-away enclosures (see Tele-Enclosure Roll-Away model)

5. Phil Harrington built a roll-away enclosure on his deck

6. Vestil Manufacturing Company, model FL-ADJ-46

7. 80/20 Inc. sells frames, connectors, hinges, plates; almost anything you could conceivably need for framing construction

8. Alucobond is manufactured by Alcan Composites

9. See Sonotube for some really long telescope tubes

10. ob·ser·va·to·ry (əb-zûr′əvə-tôr´ē, -tōr´ē) noun, a location or an institution equipped for making astronomical or terrestrial observations