Johannesburg Centre, Astronomical Society of Southern Africa

A unique little Observatory

You may be forgiven for thinking that this is a toolshed or, judging from the pipe sticking out the bottom, some kind of toilet. But no, it is an amateur(ish) observatory.
The reason why it sits at a jaunty angle on it’s base is because it rotates; the entire structure runs on castors in a circular track in the floor.
One could intuitively grasp that such a structure is probably not that complicated, and indeed it is not.

It is at least a lot simpler than trying to get a rotating roof to stay on fixed sides, which would really be going professional. But why go to the trouble? I had some good reasons for going this route, as opposed to the more "traditional" roll-off roof type structure.

Standing in the observatory does not give one the grandiose feeling of solitude and "oneness" with the telescope and the clear open sky above stretching from horizon to horizon that I experienced in Dave Gordon’s roll-off-roof observatory at a star party a little while ago. Indeed, it is somewhat pokey inside; and one has to exercise caution moving about inside in the dark, lest you receive a sharp rap on the head or kneecap from a mount counterweight. But it was not designed for human occupation. It houses a 10inch Newtonian on a German equatorial mount, and a CCD.

There-in lies the key for my choice of a rotating dome. CCD’s tend to be small in size, unless one has plenty of bucks, and therefore span a rather small section of sky(typically 10 to 30 arc minutes). This, coupled with the fact that one can achieve image resolution of a few arc seconds, means that you don’t need the wind pushing the open end of a Newtonian around while trying to record fine detail during an exposure of anything up to an hour. Hence the desirability of a narrow slit, and subsequently a kind of a dome to hold the slit! There are other advantages; stray light from neighboring security lights or your own kitchen becomes much less of a factor. And it also has a small footprint, which can be an advantage on small properties.

The way I use the observatory is as follows; I decide on an object to image. I go into the observatory and manually slew the telescope to the desired starfield using my setting circles, rotate the dome so that the slit is above the business end of the telescope, and center on the object, or the section of the starfield where I know the object is, with a 26mm eyepiece. The scope tracks automatically. I then replace the eyepiece with the CCD, switch off any lights that may have been on, close the door of the observatory behind me, and cross over some lawn to the toolshed where my computer is installed. There I work under pleasant lighting, coffee close by, and control all further telescope and CCD functions from the computer. I like it.

The open slit is visible here. The slit is wide enough so that the scope can track for about an hour, which means that there is no need for automatic rotation of the dome. Although I sometimes have to nudge the dome a little from outside. The pipe is obviously used for power and signal cabling between the observatory and the computer in the shed.

Construction

This is a view of the scope on it’s mount behind the open door. I decided to use my portable pier on legs in a tripod fashion here, because I often strip the scope down for trips to dark sites, and I was probably too lazy to make a solid pier in concrete. But it works well enough. The tripod legs extend through holes in the floor and rest on a concrete plinth below the floor. That means that I can walk around inside or rotate the dome without disturbing the stability of the scope.

Cables come up through the center of the floor next to the pier, and go direct to the electronics.

Four fairly heavy duty castors are mounted in the middle of each side of the structure. They run in a shallow track cut into the pine floor. This was easy to make; I simply mounted a router at the end of a strip of plank, and held the other end of the plank in the center of the floor with a single nail.

The thing worked like a drawing compass, with the router

switched on, I rotated the router held in place by the plank round and round in the floor, cutting the track exactly where I wanted it. I had to move the router in a little along the plank for subsequent grooves, thereby making the track to the desired width.

You might notice from the top pictures that the sides seem to be assembled from 2 sections. I used 2.44m x 1.22m x3.2mm (standard size) exterior plywood in two sections, giving me a square dome measuring 2.44m x 2.44m. All plywood sheeting was joined in the middle and corners with 38mm pine brandering and plenty of chipboard screws with an electric screwdriver. Believe me, you don’t want to assemble the entire structure with a manual screwdriver. The corners and joins were also clad with narrow wooden strips to prevent rainwater from entering the joins.

Because the structure is square, I also had to reinforce the corners inside with triangular wood sections (done at the base) for rigidity required for rotation. The roof provides rigidity at the top.

This last picture shows one of these triangular corner sections, a little work table mounted in the corner, one of the pier legs, a castor mounted in the middle of one of the sides, and some skirting at the bottom made from canvas, that drags across the floor when the dome is rotated, to prevent dust blowing in from outside. The interior sides and roof were left untreated. The floor, however, was painted top and bottom with polyester resin for waterproofing.

The sides were varnished using probably one of the best and most expensive treatment varnishes, called "Rubbel", available from Barney’s Paint outlets. I still have to varnish once a year. The roof was clad with fiberglass medium weight cloth and polyester resin, and painted white with Plascon Glatex 8 enamel. Maybe I should have fibreglassed the whole thing, that would have circumvented yearly maintenance of the sides; next time.

By the way, I can dissemble the dome and floor and take it with me, should I ever decide to move to a darker (future retirement) location.

Bert van Winsen
(011) 315 0565
winsen@iafrica.com

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