The details of Bob Garner's observatory will be found to contradict a great deal of the advice in the earlier chapters of this book. Bob has made the best of a remarkably unpromising location for astronomical observation, using much ingenuity, and frequently shoestring means. His observatory may well be the
1 Small Astronomical Observatories and More Small Astronomical Observatories, Patrick Moore (ed). The first book is on a CD ROM sold with the second book.
world's worst-sited, but he has managed to turn some disadvantages to advantages, and achieved remarkable results in the field of deep-sky CCD imaging -something one certainly would not expect from a location like his.
Figure 9.1 shows the road on which Bob's house lies. This is the A40, a principal arterial road that runs west out of London. It is brilliantly lit with high-pressure sodium vapour lamps on 15 m (50 ft) columns (much higher than the house roofs), and traffic thunders past day and night. The house is semi-detached, of typical British inter-war architecture, with an old garage to the side of it and a small, narrow garden behind it. Crucially, for astronomy, the house is on the south side of the road.
Bob first placed his telescope in his garage about 1975. At that time, he was using a 250 mm (10 in.) Fullerscopes reflector, on a Fullerscopes Mk IV mount (a good commercial telescope and mounting for the time), with setting circles, synchronous motors on both axes, and a 150 mm (6 in.) reflector guidescope, for film astrophotography. Bob found this setup was too heavy to move. He solved the problem by removing a panel of the slightly-sloping asbestos cement roof of his garage, and replacing it with a plastic panel that could be lifted off. By this means he found he could gain access to a limited part of the southern sky. The rest of the fabric of the garage blocked out the light from the streetlights to the north, and this "window" offered a view of the meridian area. The great storm of October 1987, which damaged or destroyed many small English observatories, blew this plastic panel away, and Bob created a more permanent solution, which has remained more less unchanged since.
Bob cut out, with a handsaw, most of one half of the asbestos cement garage roof, leaving in place only narrow sections at the side, in the half that had covered
the telescope.2 Below these side-sections, he installed aluminium rails on steel brackets jutting out from the walls, running the whole length of the garage. These rails are on an approximately flat plane, whereas the roof of the garage slopes gradually towards the south end. (With a structure that had an apex roof, such a modification would be far harder.) The missing roof section was replaced with plywood sheet mounted on a steel frame, fitted with small fixed castors which run on the aluminium rail. When the observatory is not in use, the moving roof panel is held vertically against the remaining fixed asbestos sections by three wooden blocks on either side (Fig. 9.2). These blocks are graded in length so as to hold the section up against the fixed edge sections, at the general angle of slope of the roof. Thus the roof continues to drain rainwater to the south. The moving section is also secured to the wall at the south end by strong springs.
When the observatory is opened, the blocks holding the roof section are removed, and it falls onto the rails, whence it can be slid northwards under the other part of the garage roof. This arrangement allows Bob to observe from 6° to 85° N in declination, with about 3 hours of RA available. Most of the rest is blocked by the walls and fixed roof of the garage, apart from a little extra horizon
2Note that cutting asbestos-containing material is today not recommended without protection against breathing in the dust, and that asbestos-containing material needs special disposal.
that can be gained by opening up the back door of the garage as well (Fig. 9.3). This might be thought far too restrictive, but if the rest of the garage were not there, the site would be flooded with streetlight. This arrangement creates a small window of darkness - the northern sky would be unusable in this location with any design of observatory. The way the telescope is placed in the observatory does not even allow an observer to get between it and the south wall, because observing is never carried out on that side. The telescope always remains on the east side of the mount, never being normalised (reversed). This preserves collimation and finder and guidescope alignments effectively.
The garage-observatory conversion probably cost less than £100 in 1987. At that time, Bob's interest lay in long-exposure imaging using hypersensitised, or "hypered" films, treated using forming gas, a mixture of hydrogen and nitrogen. He still has the gas cylinder nestling in a corner of the observatory, and the aluminium hypering chamber he made. There were many problems with this process. With the fickle British climate, a hypered film could be prepared and a few frames used, only for it not to be clear again for several weeks, after which time the hypering had worn off. Bob could observe little visually from his site (the naked-eye limiting magnitude is about 3.5), and at that time he was considering giving up astronomy. The CCD revolution came to his rescue.
In the early nineties, amateurs started experimenting with the tiny CCD chips of that time, initially only for guiding, and then for imaging. In 1994, a book was
published by Richard Berry, Veikko Kanto, and John Munger called The CCD Camera Cookbook, describing how a cooled astronomical camera could be made using a Texas Instruments CCD chip, and a group of amateurs, including Bob, set out to build these cameras. Bob had no previous experience of electronics construction, yet he succeeded. Along the way he experimented with, and improved, the design of the Cookbook Camera in several ways. The camera used Peltier cooling, and water cooling to remove the heat transferred by the Peltier unit. The water was circulated using an aquarium pump. This arrangement achieved a cooling of 40°C below ambient temperature, better than most commercial units. The pipes for the water circulation at the telescope were awkward, however, and there was sometimes a problem of frost forming on the detector. The cost of making the Cookbook Camera was about £500, compared to a cost of £2,500 for the equivalent commercial camera of the day, the SBIG ST-6.
Since then, the cost of commercial CCD cameras has fallen considerably, whilst performance has improved drastically. Bob has recently been using the Starlight Xpress MX7C colour camera, and the Starlight Xpress MX7 monochrome camera with narrowband filters (which transmit the wavelengths of hydrogen alpha, oxygen III and sulphur II) mounted in a manual filter wheel to produce multi-spectral images of faint deep sky objects, such as the 14th magnitude planetary nebula PK164.8+31.1, known as the Headphones Nebula (Fig. 9.4). The cameras are used with a desktop computer, protected from dewing by a purpose-made cabinet on wheels (Fig. 9.5). This cabinet also incorporates, at the bottom, a home-made power supply, originally constructed to supply 12, 15 and 9.7 V to the Cookbook Camera. Other accessories Bob has constructed include a light-box for illuminating the telescope with diffuse light for the purpose of acquiring flat fields (which average out the response imperfections in CCDs). With the modern CCDs, Bob finds this is no longer necessary.
Figure 9.4. The Headphones Nebula, PK164.8+31.1, imaged by Bob using a 35 cm f4.6 reflector and a Starlight Xpress MX7 camera, total exposure 2.5 hours.
The original telescope was changed in 2001 for a 350 mm (14 in.) f4.6 optical tube assembly, by Orion Optics of Crewe. Bob adapted the Fullerscopes mount to the larger telescope with new cradles, and he also created a door in the Orion Optics tube so the mirror can be closely capped, and hence better protected (Fig. 9.6). This picture also shows the heavy guidescope, mounted opposite the declination axis so as to preserve balance. The declination axis is loaded with 60 Kg (130 lb.) of counterweights. In fact the guidescope is no longer used for guiding, as the Starlight Xpress cameras are capable of self-guiding (using the Star2000 interface) using drift-monitoring signals from the imaging chip.
In order to use this system, Bob had to have the drive system of the Fullerscope mount modified. The original synchronous motors were replaced with a stepper motor system, by AWR Technology of Deal, Kent. One of these motors in shown in Fig. 9.7. Note the substantial metalwork required to brace the motor for the task of moving the big telescope. This system not only allows the telescope to be guided by the CCD cameras, it incorporates a GOTO function, whereby the motors will automatically slew the telescope to RA and dec. coordinates keyed into a handset. Bob finds that the telescope slews to within 1 arc minute of the target reliably, which is good enough to put the target on a CCD chip without having to search for it visually (which would be very difficult with his skies). In fact, most of the objects he images, he never sees visually. Contributors to this GOTO accuracy are probably the limited range of movement used for the telescope, and the fact that it is never normalised.
The steel telescope pier is not buried in the ground, it stands on feet on the concrete floor of the garage. Bob does get some interference from traffic
vibration due to heavy vehicles going over the flyovers on the road junction near his house, and this factor, along also with light pollution, tends to restrict the CCD exposures he can use to about 3 minutes. He achieves longer effective exposures by stacking and averaging many such "short" exposures using the Astroart software package.
This observatory is close to the dwelling-house, but in this case, this is not a great disadvantage as observing is only done in the direction away from the houses. An observatory further down the garden would have suffered from the incursion of streetlight reflected off the next row of houses.
Bob admits that his observatory is somewhat crude, but it works, and the results he achieves, remarkable for a highly light-polluted site adjacent to large roads in the middle of a city, are anything but crude. It was, he says, a temporary solution that became permanent. His maxim is "If it works, why change it?" -and this dictum has served him well.
Martin Mobberley's name is one of the best-known in UK amateur astronomy. He was a pioneer of the use of video cameras for imaging the moon, and is also very well-known for his superb images of comets. Two of his earlier observatories, a run-off shed for a 49 cm (19.3 in.) Newtonian, and a run-off shed for a 30 cm (12 in.) Schmidt-Cassegrain, have featured in the previous "Small Observatory" volumes in this series.
Martin greatly prefers run-off sheds to other observatory types, for the all-sky experience they offer, and their environmental non-intrusiveness and tendency to not provoke problems with neighbours. In 2002 Martin acquired a new telescope:
a Celestron 14 SCT on a Software Bisque Paramount ME mounting. This is one of the sturdiest and most sophisticated mountings manufactured in the amateur range today. Martin wanted a highly-reliable imaging platform, mainly for comets, but also for supernova imaging and possible searches. The Paramount ME is a robotic mount with a control system and software that allows it to track the motion of any celestial object, even a comet that moves rapidly with respect to the stars. It can also be programmed to undertake unattended survey astronomy, its slewing control being integrated with camera control through the Bisque software. With automated astronomy in view, Martin preferred to do without the complications that a dome, needing automatic rotation, would involve. Additionally, he did not want the horizon obstruction that is inevitable in a run-off roof design.
Hence he embarked, with his father, a DIY expert, on his fifth run-off shed project. (The two others were both sheds for a 36 cm (14 in.) reflector, the first of which rotted away after 18 years.) With all this experience, he had discovered which designs did and did not work for run-off sheds. His early sheds had been too heavy and flexed too much on pushing. Sometimes, the effort required to use them had proved a real disincentive to observing. He also found that, with wood and roofing felt construction, they needed considerable maintenance.
A possible solution to these problems was presented by a plastic shed spotted by his father in a DIY superstore catalogue. The external dimensions were 2 m long by 2 m wide by 1.45 m high (78 x 78 x 57 in.). It had double doors at one end that were 1.3 m wide by 1.9 m high (53 x 75 in.). This would be adequate for the C-14/Paramount combination if the shed were aligned with the door parallel to the tube, which is about 1 m (3 ft) long with a motor focus and CCD camera attached. From the outer edge of the tube to the end of the counterweight shaft is 1.4 m (57 in.), so it would fit in with the shaft horizontal. The shed cost £249.
It was found that an observatory could be created from this shed with a minimum of alteration to its basic design. The shed was purchased as a kit, flat-packed. Firstly, the plastic floor, of cross-braced construction on the underside, needed to be adapted. Martin wanted to retain as much of the floor as possible, to strengthen the shed and to keep dirt out, but, obviously, there needed to be a gap for the telescope pier. The floor came as two equal panels, one for the door end and one for the other end. The door-end panel was cut into three sections with cuts at right-angles to the door, and the middle section was discarded. The other panel then had a curved recess cut into it. Extra bolts were added to strengthen the joints between the floor sections, and four pulley-blocks were bolted to the underside of the floor, almost directly under the walls, to act as wheels (Fig. 9.8). The shed was then assembled, which only took a few hours.
The site for the new telescope was to be that of the old 36 cm (14 in.) reflector, and the pier constructed for that telescope was re-used. This is a huge construction, consisting of two 1 m long by 53 cm diameter (40 x 21 in.) interlocking concrete drainage pipes. One is below ground and one above, and they were filled with concrete and capped with a metal plate when the pier was constructed more than 25 years ago. The area around was paved with slabs at that time.
With such a light structure as the plastic shed, wind was always going to be a concern, as well as security. Two metal hoops on plates were bolted to the underside of the new shed floor at the west (non-door) end to attach to hooks bolted to the concrete slabs, to keep the shed stationary when not in use. To further prevent movement, holes were drilled through the lower flanges of the walls and into the slabs. Cane bolts (draw-bolts with a large range of movement) were bolted to the inside of the sides of the shed so that they passed through these holes and locked into the slabs when dropped. Additionally, a wooden collar was cut to fit round the west side of the pier, and screwed to it with wall plugs. This overlaps the plastic shed floor, slightly above it, when the observatory is in the closed position, and so prevents any vertical movement. After some use, Martin felt the observatory was still too light and potentially moveable in the vertical plane, and added some small concrete slabs inside to weigh it down.
The rails came from the run-off shed of the old 14 in. reflector. They were originally acquired from a scrap dealer, and are an inverted T shape. Only the area immediately around the shed consisted of flagstones, so the ends of rails had to be on the lawn. Narrow concrete slabs were inlaid into the lawn, and the rails were screwed to these with wall plugs, through the flange of the T-shape. Wooden buffers were added at the western end to prevent the shed being accidentally pushed off the rails. Initially, the rails were only screwed down with a few screws. Then the shed was experimentally run on the rails. The separation of the rails was fine-tuned until the shed moved effortlessly, and the rails were finally secured, as close to level and parallel as could be achieved.
An issue Martin had encountered with his previous run-offs had been the ingress of dirt and animal-life (he talks of the terrifying giant spiders of Suffolk) through the open bases of the sheds. In this case, the remaining plastic floor should act as a partial barrier to these, and this barrier was augmented by the installation of a section of hardboard to fill the gap on the east side of the plinth when the observatory is closed (Fig. 9.9). This seems to have deterred the spiders.
The Paramount ME mount was purchased with the heavy duty base-plate, which was pre-drilled, and the metal plate already on top of the pier was re-drilled and tapped to correspond to this, allowing the Paramount to be bolted down. The mount weighs 30 kg (66 lb.), and the telescope 18 kg (40 lb.), the total weight of 75 kg (165 lb.), including counterweights and other hardware, being less than half the weight of the earlier telescope, of similar aperture, on the plinth: a striking aspect of the changes in telescope technology over the intervening period. The observatory and telescope was entirely put up by the two-man team of Martin and his father.
Part of the idea of this observatory, as mentioned, was that it should be operable remotely from the house, once the run-off shed is off. Thus there was the issue of providing cabling between the telescope plinth and the house. The cables required were a PC USB port to CCD camera cable, a PC serial port to mount cable, and the focuser control cable, as well as the mains supply. These were run in a flexible conduit hose under the lawn. To provide a USB extension over the 30 m (100 ft) distance required, Icron USB extension boxes were used. One of these boxes resides at the observatory end, and one at the house end, and
they are connected with Category 5 cable. This is probably a neater and more reliable method than using daisy-chained USB repeater cables. The Icron USB Ranger box at the observatory end has four output ports, which would allow multiple devices to connect with the computer indoors, if required. This system has worked well, as has the communication with the Paramount over an RS-232 (serial) connection. The mount is controlled using Bisque's TheSky software.
Remote focusing of the Celestron is performed with a JMI motorised focuser. This can be controlled in two ways. Using another Category 5 cable and RS-232 connection, it can be controlled from a PC - this function is also included in the Bisque software. Alternatively, the JMI digital handset can be brought indoors, and will work over the Category 5 cable. I have commented earlier on the fact that RS-232 is an old standard, "going out" on modern computers, and that the astronomical manufacturers' insistence on sticking to it is increasingly requiring astronomers to use USB to RS-232 adaptors, which add another layer of potential software/hardware incompatibility.
A final noteworthy point about Martin's C-14 setup is that he found a cheap way around the "dew-shield problem". This particularly afflicts catadioptric telescopes. Refractors, even cheap ones, are invariably supplied with reasonable dew-shields which limit condensation on the objective. Catadioptrics, even the most expensive ones, never are. This is totally incomprehensible, as it makes them unusable "out of the box" pretty much in any climate other than a desert one. In any normal location, dew will quickly form on the front corrector plate at night. It is rather as if automobile manufactures did not supply windscreen wipers as standard. Maybe SCTs are all manufactured by people living in deserts - but I don't think so.
Anyway, Martin's anti-dew system consists of a dew heater strip that wraps round the corrector end of the tube, a dew heater controller made by Astro Engineering, which takes a 12 V supply and modulates it to provide the minimum necessary power to the dew-strip, and a dew shield which was made from a flexible plastic offcut, spotted in a DIY warehouse, and acquired for a nominal sum. This offcut was rolled into a cylinder and reinforced with metal hose-clamps round the circumference, held together with nuts and bolts. These needed fine adjustment to get the diameter of the cylinder just right to be a friction fit on the C-14. The inside of the dew shield was then coated with matt black paint to reduce reflections. This saved the US$150 to $300 (plus UK import taxes) that a ready-made rigid dew shield for this telescope would have cost.
Since 2002, the C-14/Paramount/plastic shed observatory has been in frequent and successful use, principally for imaging comets and galaxies containing recent supernovae, with an SBIG ST9XE camera. However, more, recently, Martin has become very interested in planetary imaging using webcams, following the pioneering work on this by observers such as Don Parker and Damian Peach. They were using simple, cheap, non-astronomical cameras connected to computers, in conjunction with very long effective focal length telescopes, usually long-focus Newtonians and SCTs fitted with Barlow lenses, and image registering and stacking software, to produce remarkably detailed planetary images, surpassing anything that had been produced before using earth-bound telescopes. Many amateurs like Martin, who had previously tried to image planets using film, but become frustrated due to the limitations imposed by seeing, became enthused to try again using this new technology, and the discipline of planetary webcam imaging was born.3
The principal discovery made by these observers was that the traditional limitation of the resolution of large telescopes to perhaps the resolution of only a 15 cm (6 in.) telescope, on most nights, due to seeing, was overcome by the process of stacking and averaging very large numbers (thousands) of images taken in rapid succession, and that large amateur telescopes could produce images by this technique that equalled or even exceeded their classical theoretical resolution limit (the Airy Limit), if two conditions were met. These were that the surface accuracy of the mirror had to be excellent, of the order of one-eighth wavelength peak-to-valley, and that the collimation had to be very precise.
Though he already possessed a considerable stable of telescopes, Martin decided to buy one specifically for planetary imaging. His criteria were high optical precision, a fairly long focal length consistent with manageability and ease of use, a system that would cool down quickly to eliminate tube-currents and thermal distortion of the optics, and one that would retain its collimation well. It had been found that, although the high surface-brightness planets, Venus, Mars and Jupiter (plus the Sun and the Moon) could be effectively webcam-imaged using telescopes in the 15-20 cm (6-8 in.) bracket, a larger aperture was required for Saturn because of its low surface brightness. Consequently, Martin selected an Orion Optics of Crewe 25 cm (10 in.) f6.3 Newtonian on a Vixen Sphinx German equatorial mount for his planetary-imaging telescope.
Already having two full size run-off sheds in his garden, Martin decided that something low-profile would be in order to house this new addition. He reflected that he did not want his garden to look like a collection of chemical toilets. A paved area by the house was a possible observing site, with a good horizon from south-east to south-west. The obstruction of other directions by the house did not matter for planetary work. He decided to try the unorthodox system of having the shelter fixed, and the telescope moving on rails. This allowed a saving on materials: the house wall formed the back of the shelter, the patio formed the floor, and the shelter needed only two shallow side-walls, a sloping roof, and two wide doors (Fig. 9.10).
The only concern with this plan was how the telescope would fare if it was moved on rails. Would the polar alignment remain accurate, and would colli-mation be unaffected? The experience has shown these concerns to be unfounded, though this system would not be advisable if the telescope were to be used for long-exposure imaging of faint objects, when very precise polar alignment would be more of an issue. Planetary imaging runs last only a few minutes, typically (longer would be pointless because of the rotation of the planets), and even a rough polar alignment will hold them on the detector for this period.
The shelter was made 196 x 122 cm (78 x 48 in.), with a roof sloping from 130 cm (52 in.) height at the house to 117 cm (47 in.) at the doors. The basic framework was formed by 2 x 1 in. timber battens, screwed to the brick wall and to the paving slabs using wall-plugs. The walls and roof were formed of sheets
3For more on this subject, see the Lunar and Planetary Webcam User's Guide by Martin Mobberley, in this series (Springer, 2006).
of 9 mm (0.35 in.) thick plywood. These were simply screwed to the battens, and further battens were screwed on along the tops of the side panels, and joining the outer top ends of the side panels to support the roof above the door. The roof panel was covered with roofing felt, and the timbers were painted with green preserving wood-stain. The timbers in contact with the patio probably run some risk of rotting due to being in contact with a potentially saturated surface. In retrospect it might have been better to separate these from the floor with a damp course. The doors to the shed were made of more plywood, ledged and braced with timber. (Personally, I am not totally convinced about the use of plywood outdoors. Even the "external" grade tends to warp and delaminate with exposure. If the preservation is really diligent, I suppose it might last.)
Unusually low-profile rails were conceived to fit under the opening doors of the shed. These consisted of strips of white, flexible, plastic electrical conduit, having a channel-section shape and a total thickness of only 6 mm (0.25 in.). These were bought from a hardware chain, and were laid as a double thickness on the patio slabs, and screwed down using wall-plugs. The gaps between the patio slabs were first filled with concrete to smooth the telescope's path. Three rails were laid, for the running of fixed castors attached to the three feet of the telescope's steel pedestal. There is an end stop on the middle rail, but not on the others. This system provides a wide, unrestricted operating space around the telescope, with little possibility of tripping over the very low-profile rails.
The provision of mains electrical power to the facility, frequently a headache with constructions distant from the dwelling-house, was easy in this case. A hole was drilled in the house wall for the passage of a mains cable, and a waterproof double socket was installed inside the shelter just under the roof (Fig. 9.11). The overall cost of the shelter Martin estimates as under £150.
There were a few other refinements. Martin set up an artificial star (an Astro Engineering Picostar) on a pole at the end of the garden, 30 m away, for colli-mating the telescope. When seeing is too bad to collimate on a real star, Martin points the telescope at this device, which consists of a light shining through a 50 micron diameter fibre, and checks the collimation. The drawback of this method is that it is always possible, with any reflector or catadioptric scope, for the collimation to change when the scope is raised from the horizontal plane and directed to a planet. Martin also taped a thermometer probe to the side of his telescope mirror, and another to the outside of the tube, to monitor the difference in temperature between the mirror and the air. He reports that really good results are obtained only when the two are within a fraction of a degree Celsius of one another.
The only other addition to the telescope has been a JMI motorised focuser, essential for really precise on-screen focusing during imaging, when, using an effective focal length of 7.5 m (300 in.) and an imaging chip 4.5 mm (0.2 in.) across, touching the telescope at all would probably move the image off the chip.
Having used some very large (by amateur standards) telescopes in the past, Martin has come to believe that user-friendliness and manageability are more important in getting observational results than telescope size. The biggest telescope and best-equipped observatory will achieve nothing if it is so daunting and complicated to use that the owner can never summon up the determination. This is the philosophy behind this minimalist observatory, which can be brought into action in seconds to take advantage of any fleeting observing opportunity.
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