It has been generally insisted so far in this book that observatories need to be big enough to accommodate, comfortably, all the equipment that is to be used, plus at least one observer. However, this need not be the case, if the observatory structure is only to be a shelter for the equipment when it is not in use, never needing to accommodate an observer. Run-off sheds are one example of this type of shelter, but there are others. Two of these will form our final two case-studies, exemplifying respectively the high-tech and the low-tech ends of the observing spectrum.
Richard Miles is a serious observer of comets, asteroids and variable stars, particularly interested in making photometric observations. He moved a few years ago to a thatched cottage in a village in north Dorset, in the rural south of England, in search of darker skies. In his previous location he had used a run-off roof observatory, but his wife did not want such a large construction here to detract from the beauty of the cottage garden (as she considered). The solution arrived at was a minimum-sized enclosure that only allows space for the telescopes and computers, with operations always being remotely controlled from the house (Fig. 9.57).
The telescope cover was made by Richard from 13 mm thick plywood, glued and screwed. It is in two sections. The lower section is 90 cm wide by 81 cm deep by 73 cm high (36 x 32 x 29 in.) . The base is made from two layers of 18 mm (0.7 in.) plywood bonded together, and coated with epoxy resin-based flooring compound on both sides to make a watertight seal. The walls have a 3 cm (1.2 in.) lip all round the base to allow ensure water runs off easily. The plywood base is mounted on a steel baseplate, which was made out of welded steel strips by the village blacksmith. The idea was to make it impossible for water to get into the enclosure from below, and also to minimise the incursion of small animals, especially woodlice, which are so prone to crawl through any available gap in an observatory. Hence this baseplate was mounted on four 20 mm (0.8 in.) bolts set in concrete (Fig. 9.58). The bolts are daubed with grease to dissuade climbing insects. They raise the baseplate 12 cm (4.8 in.) above the concrete. A pipe was set into the concrete, which acts as a conduit for two mains electricity cables and two ethernet (category 5e) cables, which run to the cottage.
The upper section of the enclosure is 102 cm wide by 94 cm deep by 78 cm high (41 x 38 x 31 in.). This upper section is attached to the lower via flanges on either side which are attached to cantilevered arms on the sides of the lower section by means of 15 mm (0.6 in.) steel bolts which, rotating in steel pipes, with large circular brass washers, act as hinges (Fig. 9.59). The flanges in the upper section are extended into arms, beyond the hinges, to which counterweight boxes are attached. The boxes extend from the hinges about 50 cm (20 in.), and are filled with cut sheet lead, creating a total counterweight of about 40 kg (88 lb.).
The total height of the enclosure is 153 cm (60 in.). Concrete slabs are laid round three sides of the concrete base. The paving on the fourth side is lower by about 12 cm (5 in.) and has a rubber mat positioned so that when the upper section of the enclosure rotates on its hinges, opening up the observatory, it comes to rest on the mat in a position that places it below the horizon as seen by the telescopes.
The upper section is lined with 25 mm (1 in.) thick expanded polyurethane sheets normally used for insulating cavity walls. These have aluminium foil on either side. Their purpose here is to minimise the condensation that collects on the internal surfaces of the enclosure when it is open at night. Several trays of granulated silica gel are left inside, after closing up, to absorb moisture. About every two weeks these trays are regenerated by leaving them in the "Aga" stove in the cottage for several hours. The quantity of gel used has the capacity to absorb 600 ml (one pint) of water.
The observatory is dedicated principally to photometry of asteroids and variable stars. The main telescope is a Celestron C-11 28 cm SCT, fitted with a 10 x 40 mm finder. Either side of it, mounted directly on to a wide saddle plate fitted to a Vixen Atlux mount, are two Takahashi FS-60C 60 mm (2.4 in.) aperture, f5.9 fluorite refractors. These two telescopes enable accurate photometry, as they are equipped with filters, one having a V-band filter, the other an I-band filter. These two filters are used to standardise brightness measurements - V stands for visual, and is centred on the green part of the spectrum, while the I is a near-infrared filter. The C-11 is used unfiltered, so as to maximise signal-to-noise ratio when imaging faint targets such as small, fast-moving near-Earth asteroids.
All three telescopes are equipped with Starlight Xpress SXV-H9 cameras, each connected to a laptop computer in the enclosure via a USB 2 interface. The field of view of the cameras is 11.4 x 8.4 in. for the SCT and 87 x 54 in. for the refractors. The C-11 and the V-filter refractor are also equipped with Robo-Focus focus motors controlled via the serial ports of the PCs. These PCs can themselves be controlled from another computer indoors, via the ethernet link. Each telescope has heaters fitted, not only around the front element to prevent dewing, but also around the adapter between each CCD camera and telescope, so as to prevent condensation or frost forming internally, close to the camera. This can otherwise be a problem with Peltier-cooled cameras. The cameras are turned on about 15 minutes after the heaters are switched on to prevent frost forming on any optical surfaces. The main telescope also has a flexible dewshield which is slid back over the tube before the observatory is closed up.
The Vixen Atlux mounting is on a pedestal base, and this stands on the base of the lower section of the enclosure. The final adjustment of the polar alignment, to within a few arcminutes of the celestial pole, was achieved by adjusting the four nuts supporting the entire observatory. The counterweight shaft can be retracted by unlocking a clamp on the axis and sliding it inwards: this allows the enclosure to be smaller than it otherwise could be. The mount is normally controlled remotely over the network, from a desktop computer in the study of the cottage running Windows 98SE and the planetarium program Guide 8.
For unfiltered photometry of asteroids and other objects such as gamma-ray bursters, all three cameras are run simultaneously. The wide fields of the small telescopes, about 1.5 square degrees, usually contain stars of accurately-known magnitudes, and these can be used to calibrate comparison stars within the field of view of the main telescope. Since stars are imaged using two filters, the difference between the two magnitudes obtained (Mv-Mt) provides a measure of the colour of the comparison stars, and allows unsuitable ones, such as very red stars, to be weeded out. The system can be used to carry out absolute photometry, by measuring atmospheric extinction to a high precision, and it can be used to determine the magnitudes of the stars in a comparison sequence to a lower limit of Mv=13.
Speed-of-use is one of the chief features of Richard's observatory. It can be opened up in a few seconds, similarly closed again if rain threatens. Once the heaters and cameras have stabilized, it takes about five minutes to power-up the computers in the observatory and calibrate the mount. After opening up, the trays of silica gel are brought in and covered up. At the end of the session, Richard normally comes out again to cap all three telescopes in order to take "dark frames" with all three cameras (these are used to help even out the variations in response across the camera detectors in the image analysis). Images taken with the cameras are stored in the outdoor computers during observing, but can be downloaded indoors at any time, to be stored on an external hard drive in the study. At the end of an observing run, the outdoor computers are turned off remotely, two of them being placed in "hibernate" mode in order to make starting-up quicker on the next occasion. The silica gel is returned, the cameras and Robo-Focus devices are switched off, and, from indoors, the electricity supply to the whole observatory is shut down.
Other uses of Richard's set-up have been to image the bright Comet McNaught of early 2007, Venus, and bright stars in daylight, using one of the refractors, with the objective capped with a very dark neutral filter to cut down transmitted light by 99.999%, or 12.5 magnitudes.
Apart from the computer technology, the main factor that makes possible a very compact observatory enclosure such as Richard's is the use of modern compact telescopes. A hinged box of this type would not be very practical with a traditional long-focus reflector or refractor. It is also a solution suited to particular types of observational work, but not others. For example, my experience has convinced me that remote-control high-resolution imaging of solar system objects is not a practical proposition, because the amount of sensitive adjustment of the instruments required every time makes it, ultimately, an unavoidably "hands-on" operation, which does entail being in the observatory, getting cold. This will continue to be the case until rather more sophisticated telescopes than are currently usual for amateur use become the norm. Richard admits that he rarely collimates his telescopes, as it usually makes little difference for photometry. The computerised mini-observatory is a clear sub-genre of the species now, that is growing in popularity.
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