The Telescope in the Blaber Observatory

 

The first part of the observatory to be built was a permanent pier. This was made at a local metalshop from 8" x 0.25" diameter pipe, with 12" x 0.375" round end plates welded onto it. I designed it to be 42" tall, so that the range of the eyepiece would be within the range of the seating height adjustment of my chair. It also allowed the scope to peer over the wall to the north and see the north star (not critical but helpful for alignment). The metal shop primed it yellow, and had to keep their dog away from it because it apparently wanted to urinate on it.

I dug a 4' deep x 2' diameter hole and poured in a bunch of concrete and six threaded rods with which to attach the base of the pier. Pretty darn solid. It is isolated from the floor, and so walking around or closing the door does not disturb the telescope.

The telescope is a 12" diameter Meade LX-200. It came with a tripod, as if to suggest that it was portable! Ha! What a cruel joke. This thing weighs about 80 pounds and the intense discomfort of lugging it out into the back yard, and back again whenever I wanted to look at the stars was a main impetus for building the observatory.

 

The telescope is mounted on an equatorial wedge. This is aligned with the axis of earth (i.e. "polar aligned") and precise rotation around this axis (i.e. the "clock drive") allows the telescope to maintain position of a celestial object as the earth rotates. The telescope also has a CCD camera that can be used to either take small black and white photographs, or it can be used to control fine adjustments of the tracking of the scope to keep an object centered during long exposure photographs (i.e. "autoguiding").

 

I have spent a considerable amount of time in getting a precise polar alignment of the telescope. At the moment, it can track an object for 15 minutes with essentially no north or south deviation. Deviation in the east and west direction is related not to the alignment, but rather to errors in the clock drive mechanism. Tiny imperfections in the gears can result in positional shifts in the east/west direction. The telescope has the ability to be record the periodic error inherent in the clock drive and to compensate for it. I have spent a lot of time adjusting these corrections, and currently can take so-called "unguided" images of about 4 minutes with minimal error. Using the CCD as an autoguider allows you to take essentially unlimited exposures.

A certain amount of time is required before taking pictures to make sure the telescope is aligned, the optics are aligned (i.e. "collimated") and the image is in focus. Sometimes this process takes 1-2 hours. One difficulty is focusing and the effects of atmospheric disturbance. I usually focus by taking images of a star with the CCD camera and determining the value of the brightest pixel (correct focus results in a smaller and brighter image). However, movement of the atmosphere distorts the image and you have to average over several images to be sure the scope is focused. You also have to wait for the temperature of the air in the telescope to come to equilibrium (currents within the telescope tube will distort the image. As you might have surmised, just getting ready to take a photo takes a lot of time. However, photographs are much more sensitve than your eye, so you can "see" much more when you take a long exposure photograph.

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2001 Michael Blaber