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Roll-down roof: a new craze?

TEXAS TELESCOPE HUT. "For quite a while I had been looking at observatory ideas when I came across the one described by David Oesper and Andy Orngard in the May issue, page 90." So writes Ronald Rosenwald (Route 6, Box 87, Huntsville, TX 77340). In the third week of July he sent the photograph above, showing his own just-completed observatory of similar style!

"Rabbit's Run Observatory has turned out rather well. It is very functional, suits the site perfectly, and I no longer have to lug equipment in and out of the house. Since it blocks irritating headlights and security lights, I can see more in this setting than out in the open."

Balancing a scope. After your telescope has been carefully balanced on its axes, why does it sometimes become unbalanced when you swing to a different part of the sky? In a 14-page paper entitled How to Balance Your Telescope Properly, Ed Young discusses this enigma, noting that astrophotographers occasionally lose control of guiding part way through an hour-long exposure. They may correct the problem by grabbing frantically for something like a pair of binoculars to hang from the end of the tube or declination shaft, but the fix is only temporary.

"What's going on here? You are chasing this invisible gremlin called Center of Gravity," Young writes. With the help of a dozen diagrams and some high-school math, he shows how to calculate the location and size of the counterweight needed to balance a telescope when several accessories are attached at various points around the tube. Included is a brief description of how to cast a lead counterweight. To obtain a copy of the paper, send $6.95 to Ed Young, 3030 Oak Ridge Dr., Grand Prairie, TX 75051.

Polar alignment without Polaris. "October's Celestial Calendar (page 72) told how a knowledge of the sidereal time can help in planning an observing session. What was left unsaid is that sidereal time also lets you polar-align a telescope without reference to either Polaris in the Northern Hemisphere or Sigma Octantis in the Southern," comments Richard Bareford (11 Greenway Dr., Mechanicsburg, PA 17055).

"Level the telescope base and angle the polar shaft upward by an amount equal to your latitude. Then, point the telescope straight up toward zenith by laying a carpenter's level against the tube. This procedure automatically aims the instrument at the local meridian, and you can set the mount's right-ascension circle to read the current sidereal time.

"Now, slew the telescope until the pointers on the right-ascension and declination circles read the coordinates of any star or planet that is currently visible in the sky. Lock the clamps on both axes. Pick up the entire instrument -- telescope, mount, tripod, and all -- and reaim it in azimuth until the object appears centered in a low-power eyepiece. (Better yet, swivel the mount itself in azimuth, if possible.) You are now polar-aligned!

"This trick is occasionally mentioned in astronomy guidebooks, and Alphonse Pouplier touched on it in his article on page 76 of the August issue. But it isn't widely known, judging from the general inability of amateurs I saw at this summer's star parties to align their telescopes before the appearance of Polaris.

"The clear advantage is that you can set up your scope almost anywhere a patch of sky can be seen, not just in a place with a poleward view. A previously rejected balcony, parking lot, or forest clearing now becomes a potential observing site for your equatorially mounted telescope."

Newsletter on nothing. Now in its second year, the Bell Jar is a quarterly newsletter about a far-from-vacuous subject: vacuum techniques and related topics for the amateur investigator. Any amateur astronomer who has ever dreamed of aluminizing a mirror or hypering film with home-built equipment will find much of interest in its pages.

Recent issues have covered glass-to-metal seals, ways to split a glass tube longitudinally, and how to synthesize "buckyballs" (cagelike carbon molecules) that can be used in thin films having various magnetic and semiconducting properties. Other topics include the conversion of air-conditioner and refrigerator compressors for vacuum use.

"Access to moderately good vacuum apparatus opens up a whole new dimension to the amateur," notes the newsletter's editor, Steve Hansen, citing the many projects of this nature described in C. L. Stong's column in Scientific American in the 1960s.

Each issue has 16 to 20 pages. A yearly subscription to U.S. addresses costs $20. (Add $3 for Canada or Mexico, $9 elsewhere). Write to Hansen at 35 Windsor Dr., Amherst, NH 03031.
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Title Annotation:Optical Bemch Talk; roof for telescope hut
Publication:Sky & Telescope
Date:Dec 1, 1993
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