Norm's beautiful observing site in Maryland, USA, is shown in Fig. 9.43. This rather open location has the disadvantage of being windy, and, in the winter, temperatures can fall very low - to below -18°C, zero degrees Fahrenheit. For these reasons, Norm opted for a domed observatory, and purchased the 10 ft ProDome from Technical Innovations, also based in Maryland. The ProDome is constructed from four fibreglass quadrant sections, and has an "up and over" shutter.
The ProDome design offers a solution to the common problem with amateur-sized domes that the door is too low to get through comfortably. The dome does not have a continuous base ring. The base ring is interrupted at the wide shutter, which is 1 m (3 ft) wide, and the dome support ring, on top of the dome wall, is continued in a short section forming the top of the low door. Thus, when the door and shutter are opened and coincident, the dome can be entered though
the combination of the two, without having to stoop. When the door is shut and the shutter is open, the dome can be rotated all the way round on its track.
A disadvantage of the wide shutter is that it does allow condensation in the dome on still nights. Norm has countered this with the installation of a dehumidifier (Fig. 9.44). This has a temperature sensor, and is set to run only if the temperature is above 2°C (35° Fahrenheit). The prevailing wind is from the west, and the exposure is also greatest in this direction, with few trees. There are some trees to the north, but south and east are almost tree-free. Consequently, Norman prefers to observe to the south and south-east, so that the dome blocks out most of the wind.
The main telescope is currently a Takahashi Mewlon 25 cm (10 in.) Dall-Kirkham Cassegrain, used largely for visual observing. This is shown in Fig. 9.45 on an Astro-Physics 900GTO mount, mounted by way of tube-rings and a Robin Casady dovetail system. A Takahashi 10 cm (4 in.) refractor is also mounted to the tube rings, by way of a Losmandy dovetail and mounting ring system, opposite to the declination axis. This secondary instrument has since been replaced with a larger one, and the mounting has been upgraded to the larger Astro-Physics 1200 model. Two 8 kg (18 lb.) and one 4 kg (9 lb.) counterweights are used to
balance these instruments. This combination of weights allows the flexibility to add heavy accessories like a binoviewer and compensate with a small adjustment, or to change a whole OTA, and make a large one.
For about a year Norm used a Meade LX200 40 cm (16 in.) SCT in this observatory, on the AP900 mount. This was a successful combination, but he found the telescope a bit too large for the observatory, and ultimately went over to the smaller Dall-Kirkham, which he finds to be a superb instrument for visual planetary observation, though he misses the light-grasp of the big SCT for seeing faint objects. The smaller telescope does leave enough room for the observer in the dome, however, which is rather important for visual observing. (Note how much larger a 10 ft dome is compared to the 7 ft domes offered by some companies: it has twice the floor area.)
The mounting is bolted to a 20 cm (8 in.) diameter steel pier that extends through the wooden floor, and this is bolted to a 3,500 kg (3.5 ton) concrete block 1.2 m (4 ft) square on its top surface, that goes 1.8 m (6 ft) into the ground. The deck was engineered slightly higher than expected when the pier was ordered, so the pier ended up being 7.5 cm (3 in.) too short. Norm raised the mounting to the required height using four short galvanised steel pipe sections (nipples). Bolts run down from the mount base plate, through the pipe sections, to holes tapped in the pier top. Since the pipe sections were pre-cut, they are the same length. This arrangement is very stable (Fig. 9.46).
The ProDome is made by Technical Innovations to be mounted on their 30 cm (1 ft) high fibreglass wall rings, which can be stacked and bolted together to achieve any required height. The dome, alternatively, can be mounted on any other shape and size of building. Norm mounted his on three wall rings. The top ring has a flange at its upper edge which goes over the dome base ring on the inside of the observatory, making it impossible for the dome to lift off. The rubber rollers on which the dome rotates bear on the dome base ring through gaps in the underside of this flange. The rollers and flange can be seen in Fig. 9.44. Norm's observatory overall is about 2.4 m (8 ft) high from the deck. It is raised
Telescope pier, mount controller and power supplies.
Telescope pier, mount controller and power supplies.
above the ground on the sloping site by a deck which varies between 20 cm and 60 cm (8 and 24 in.) in height above the ground. To access the square platform from the low-ground side, Norm constructed a short flight of steps and an elegant balustrade (Fig. 9.47).
A significant feature of this observatory is the lightning-protection system that Norm has built in. On such an open site as this, lightning is a very real risk. The 6 m (20 ft) lightning conductor can be seen in Fig. 9.48. However, specific to fibreglass domes in open locations like this, there is an additional risk that one might not guess - that of electrical discharge from the dome to the sky (reverse lightning). This phenomenon was recognised after several fibreglass observatories in the USA were struck. After some research, it was discovered that the problem was the same as one that had previously been encountered with fibreglass gliders. As the gliders moved through the air, they could build up a static charge resulting in eventual discharge to the clouds. This problem was solved by glider manufacturers by building in a fine copper mesh between the glass layers, cross-connected during assembly, and routed to static dissipators on the wing tips and vertical stabiliser. Fibreglass observatory domes experience the same static build-up when wind blows continually over them under certain atmospheric conditions. The dome manufacturers, however, have been reluctant to admit there is a problem here, and, unlike the glider makers, have not changed their methods of manufacture.
It is therefore necessary to ground a fibreglass observatory in a open, windy situation like this effectively. Norm has painted the inside of the dome and the wall with a dark blue, electrically-conducting latex paint. The conductivity of this paint is quite small, but it is enough to dissipate static before it becomes a major problem. The dome wall is connected to ground, across the wooden platform, by a thick braided cable, cross-connecting to the lightning conductor with a heavy connecting clamp (Fig. 9.49). The problem remains getting a connection between the dome and dome wall, which are separated by insulating rubber rollers. Norm has solved this problem with the white brackets attached to the base of the dome inside, one of which is seen in Fig. 9.44. When the dome is closed, the copper loops below the brackets contact with aluminium plates on the top of the dome wall flange. Thus, when it is closed, the whole dome is grounded. The dome is never in open configuration when there is bad weather.
Norm is largely a visual observer, producing fine drawings of the planets, such as those of Mars shown in Fig. 9.50. The other "personal" features of the dome shown in the pictures mainly add to the comfort of the visual observer. The seat
Figure 9.50. A couple of Norm's drawings of Mars.
is an old one, to which Norm stapled a piece of lamb's wool as a cover. This prevents dew forming on the seat, and him from slipping off. A plastic screw and nut holder attached to the inside of the dome is used to keep filters handy. The white dome lights on the wall are in fact red, with a dimmer to produce a very low level of lighting if required. There is also a white rope light all round the inside of the wall-top flange, to provide light for working inside the dome (natural light has been excluded by the anti-static paint). There are two separate 12 V power supplies on a shelf at the base of the pier, one for the mounting, and one for dew heaters. This ensures that the mounting does not experience fluctuations in voltage when dew heaters are turned on and off. An electrical conduit runs round the inside of the dome wall to supply further power points. One other small feature Norm has added, to combat the disorientation often experienced in a dome, is that he has painted the letters N, S, E and W on the wall inside.
The observatory has a name, as all observatories should, and a dedication, both of which are engraved on a copper plaque on the balustrade outside. The name is Norlin Observatory, a combination of the names of Norm and his wife Linda, and a tribute to her understanding of his regularly coming in at 3.30 in the morning. The dedication is "To the Fires of the Night", a quotation from the writings of Percival Lowell, who got it in turn from Christiaan Huygens. The true astronomer, Norm considers, is, indeed, one who is "captivated by the Fires of the Night".
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