Dave Tylers A Priori Fibreglass Dome

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I have placed the words a priori in the title of this section, because this was the aspect of Dave's observatory that most impressed me on talking to him about it. It looks utterly professional, as if it was based on a detailed study of all the best dome-design precedents, amateur and professional. Yet Dave told me that when he designed it, he had not studied any other observatories, nor looked into the subject. He simply designed round the constructional techniques he knew he himself could use, and thought out the problem of the geometry and mechanism for himself, from scratch. The result is remarkably successful. Dave was able to do this because, as he says, engineering is in his blood. His father was an engineer, and Dave worked for most of his life in an engineering drawing office, latterly using CAD (computer-aided design) software. This experience gave him a semi-automatic feel for how engineering problems can be solved.

Dave's observatory stands at the end of a medium-sized garden, about 20 m (60 ft) from the house, in a large village in the Chiltern Hills, west of London. Dave is known as one of the finest planetary, lunar and solar imagers anywhere, and another of the finest, his good friend Damian Peach, lives just in the next village, in the valley below. This location is no rural idyll, however: the M40 motorway passes between these two villages, and only 200 m (600 ft) from Dave's observatory, and numerous other roads and railways follow the valley. The close proximity of two such successful imagers has caused much speculation as to "special atmospheric conditions" that might exist here, but this is unfounded. It is an ordinary English semi-urban environment with much the same conditions as the rest of Britain. What is extraordinary is the dedication of these two astronomers.

The observatory is 3.6 m (12 ft) in diameter externally. The dome consists of 10 fibreglass panels, each of which has two facets, so it is a 20-sided shape (Fig. 9.36). The drum-section is cylindrical, and consists of 8 curved fibreglass panels. The wall is only about 1.2 m (4 ft) high, and the door less than this, so, in common with most amateur domes, it is a bit of an exercise to get in. Once one is in, however, it is roomy. Dave did have a hardboard lining on the inside of the wall in the past, but recently he removed it to lay a new floor, and does not intend to put it back, as he likes the increased space.

The foundation for the dome was 10 cm (4 in.) of concrete, with a 1 m (3 ft) cube of concrete under the telescope pier. Joists were laid on the concrete, which in fact were old doorframes, and the new floor, consisting of waterproof plywood, further treated with Sadolin wood protector, is now laid on these.

The panels for the wall and dome were made using wood and hardboard forms. One form was used for all the dome panels. Each form was in two sections, to make it easy to separate it from the finished fibreglass. All the panels show a seam, where the two sections of the form separated. The forms were varnished and then waxed with slip wax. Fibreglass mats and resin were then built up on the forms to a thickness of 6 mm (0.25 in.). Silver pigment was put into the

Figure 9.36. Dave Tyler with his dome (and archery target behind).

fibreglass for the wall panels to make them opaque; the dome was left translucent white.

The observatory structure is unusual in that it essentially has no frame, either of wood or metal - one sign of Dave's independent thought. The wall panels were made with short side-faces, or flanges, at right angles to their surfaces, and these side-faces abut directly onto one another, and are bolted together, with metal plate reinforcements. At intervals, the wall panels have been strengthened by having vertical laths of wood incorporated into the fibreglass. The fibreglass has been folded over the laths, and they show as vertical ribs on the inside. The dome sections are likewise bolted to one another through their edge flanges, and have had waterproof tape (black in the photo) stuck over the joins on the outside.

The dome track consists of eight 16 gauge flat aluminium sheets bolted to the flanges on top of the wall sections. These were cut to the curves required using a band-saw (Dave has a well-equipped workshop, which has been crucial in fabricating components for his observatory and telescopes). Fixed castors bolted to the under-edges of the dome panels move on this track, and another set of castors, which retain the dome horizontally, are attached to the inside of the dome just above the lower edge, and bear on a sheet aluminium ring, again made from short sections, going round the top of the wall. It will be seen that no large metal components needed fabricating for this design: no forged circular rail, as is often considered essential for a dome, or similar. Dave has got round some of the most difficult issues of dome construction neatly and simply.

The dome has two shutters, which slide apart, moving on straight tracks at both ends, in the manner of the Mount Palomar observatory and others. They are not 100% waterproof: rain can be driven in by the wind from certain directions, hence the care taken to waterproof the new wooden floor. Creating completely waterproof dome shutters seems to be a very difficult, if not impossible, task. The shutter tracks are aluminium channel sections, held to the dome by solid alloy rods (Fig. 9.37). The rods have their ends angled to match the dome and track faces, and were tapped so as to allow securing to the dome and track. The shutters, made from fibreglass, move on PTFE rollers, which do not necessarily have to roll, as they are well-lubricated. The lower track lies below the bottom edge of the shutter, but the upper one passes through the shutter sides, near the apex of the dome, to securely retain the shutters.

Figure 9.37. The struts for the dome tracks at the top and bottom of the shutters.

There is nothing haphazard about Dave's constructions. The dome was carefully designed and drawn before the patterns were made, and Dave comments that after many years working in engineering drawing, it was only this dome design that caused him to understand the usefulness of radians (units of 57.29°) as angular measurement. Most of the dome components are a definite number of radians in their dimensions. He also comments that he never found the need to use radians again.

The observatory was constructed when Dave moved to this house, in 1977, and so has withstood 30 years of Chiltern hilltop weather, including the great storm of 1987, which damaged or destroyed many observatories in the south of England. The surface of some of the fibreglass is showing some slight crazing lines, in which algae are probably growing, and some of the galvanised fittings are slightly rusty, but the observatory shows few other signs of age.

Dave has had a number of telescopes in the observatory since it was built. He is perhaps best-known for the extraordinary images he has taken here of Mars, Jupiter and Saturn using Celestron C-11 and C-14 SCTs. The C-11 was, in fact, owned by Damian Peach previously, and has been called by others, jokingly, the WOMPOT - the WOrld's Most Powerful Optical Telescope, in consequence of the many wonderful images that have been taken with it. The idea that Damian and Dave's results have something to do with a specific telescope is, of course, another myth, like the special atmospheric conditions. Dave had much imaging experience behind him at this stage. In the early 70s, he was taking planetary images on film, using reflectors, from his garden in Slough. The observatory was constructed to house a large, skeleton-tube, double reflecting telescope of Dave's construction, consisting of two Newtonian systems - one having a 39 cm (15.5 in.) primary, the other a 21.6 cm (8.5 in.) primary made by George With in 1827. This latter mirror was one of the oldest surviving glass speculums anywhere. (Dave later exchanged it with a US astronomer for his C-14).

The large reflector housed in the observatory proved to be not too successful on planets. The mirror took a long time to cool down in an observatory in which the diurnal range in temperature can be large, and the skeleton-tube construction implied a long light-path through thermally unequilibrated observatory air. The smaller With reflector was better, and, with this, Dave started taking his first successful webcam images in 2004, after a period of visual observation using refractors. These instruments have now been displaced by the SCTs which he uses almost exclusively for planetary imaging. He sees these closed-tube instruments, with relatively fast-cooling optics, as being far better for high-resolution imaging, particularly in a domed observatory. To promote faster thermal equilibration of the observatory, he also now has fitted an extractor fan in the wall.

Recently, following a period during which none of the bright planets have been well-placed from here, Dave has become very interested in imaging the Sun, both in white light, and at the hydrogen alpha wavelength (655.3 nm). His instrument of choice for this is his huge 15 cm (6 in.) f15 refractor, and this is shown in the observatory in Fig. 9.38. However, Dave has engineered a system so that different large telescopes can be conveniently swapped on the German equatorial mounting. Each telescope has tube-rings attached to a base plate, fitted with two threaded studs at a separation of 26.7 cm (10.5 in.). These engage with holes in the saddle plate on the mounting. The tube-ring assembly is first attached to

Figure 9.38. The 6 in. f15 refractor in the dome.

the saddle plate without the telescope (after suitable counterweighting has been attached to the other side of the mount), the tube rings are held open by straps, and then the telescope is lowered in. For the C-14 and the big refractor, Dave says this is a ten-minute operation. Longitudinal balancing of the telescopes is also important, particularly in view of the heavy imaging equipment Dave uses. He has machined, in his workshop, an elegant arrangement consisting of a steel weight sliding on an aluminium bar, attached to the saddle plate (Fig. 9.39).

The 150 cm (6 in.) refractor was built by Dave using a doublet from Emerson Optical of London, a rack mount by Ronald Irving, of Teddington, Middlesex, and a 3 mm (0.12 in.) thick aluminium tube. This tube was too large to machine on his lathe, but he was able to work on it using a milling cutter attached to a power drill. Dave figured the objective himself, and determined the figure to be correct by star-testing it. (On other occasions he has had tested objectives using an artificial star created by light reflected off a ball-bearing placed as far away as possible.)

A domed observatory is the most suitable type for solar observing, since the dome blocks out most of the direct sunlight. Solar observing and imaging can be very awkward in the open or in a run-off. There is too much glare, and using a computer monitor is difficult. This can be partly remedied by some box arrangement to cover the observer's head, together with the monitor. Dave does in fact make use of a cardboard box as well, as his fibreglass dome is quite bright inside in daylight.

Hydrogen alpha imaging is made possible by a Daystar etalon-type filter which fits into the drawtube. This requires a power supply, as it contains a heater to

Figure 9.39. Adjustable counterweight and bar on the saddle plate.

maintain the filter at one precise temperature, at which it passes the H alpha line. It also needs to receive light from the objective in a cone at f30 or greater in order to optimise the selectivity of the filter for the H alpha waveband, and minimise the transmission of heat. Hence it is used on the f15 refractor with a 2x Barlow. The Daystar filter also has to be used with an energy rejection filter (ERF) mounted over the objective. Dave has discovered that these Schott glass ERFs transmit a significant amount of infra-red radiation, and that, for best results, an IR blocking filter is required in front of the Daystar as well. The final element in the imaging chain is a Lumenera LU75 USB 2 camera. This set-up produces superlative H alpha images (Fig. 9.40). Dave comments that a part of this performance must be due to the exceptionally great un-amplified focal length of the big refractor. There are few, if any, other observers using an f15 instrument for this type of imaging. The short, large-aperture refractors favoured by many observers today tend to suffer from spherochromaticism, which is significant spherical aberration at extreme ends of the spectrum, which can influence monochromatic as well as colour images. Of course, the use of such a large refractor is only practical in an observatory that is, by amateur standards, large.

Dave also carries out white-light imaging of solar features with the refractor. For this, he has constructed his own Herschel-type solar wedge. This sends most of the sunlight out of the back of a prism, away from the camera, and it is used in conjunction with various filters. It provides higher resolution than reflective-film methods of reducing the solar energy, such as the commonly-used Mylar sheets.

The telescope mounting currently in use in the observatory is a large German equatorial that Dave has built. It is simple, but extremely stable. The rectangular

Figure 9.40. A magnificent solar prominence imaged by Dave in hydrogen alpha light with the 6 in. refractor.

concrete pier is topped with a steel rectangular section (the height was increased with the change from Newtonian telescopes to refractors and SCTs), and bolted to this is a slab of synthetic resin-bonded paper, a hard, incompressible material not unlike thick Formica. The mounting base-plate is bolted to this. The main sections of the mounting were made from castings by Ronald Irving, which Dave machined. There is only a synchronous motor drive on the RA axis. Declination adjustment is by nudging.

The friction in the dec. movement is adjustable using screws which adjust the pressure on a slip-clutch arrangement concealed within the aluminium dec. setting circle. The friction in the RA movement is fixed. The RA axis turns on ball races, and a PTFE (Teflon) pad below the worm wheel ensures the friction below the wheel is less than that above, so the telescope is driven, but can easily be slewed manually.

Dave cut the RA gear himself by the method of "hobbing" on a lathe. This involves using a rotating tap, mounted on the lathe in a special attachment, to cut the edge of a rotating disk of metal. It is not easy to produce the correct number of teeth on a gear wheel by this method, but it is possible. Dave also engraved the RA and dec. circles himself using a dividing engine, and stamped the numbers on them. The RA circle is fitted with blocks front and back engraved with lines (Fig. 9.41). When these correspond, the telescope is on the meridian, and the local sidereal time is the RA of the meridian. This method of calibrating RA is used by Dave to find planets in daylight. Being entirely an observer of the Sun, Moon, and bright planets, he sees no need for GOTO facilities or digital setting circles.

The RA drive rate is only alterable in a limited way. To slow it, Dave switches it off, and to speed it up, he switches from mains current to the slightly higher frequency AC derived from an oscillator and step-up transformer. This crude system he finds sufficient for imaging the Sun, Moon and planets. If he were

Figure 9.41. Front and back of the RA assembly.

designing the mounting again, however, he thinks he would include a dec. drive. This cannot easily be incorporated into the mount now, but it would be useful to him for the purpose of producing image mosaics of the surface of the Sun and Moon. The dec. nudging method is a little crude for this. A very valuable feature of the mount, however, is the way the telescope can be turned most of the way round in RA without colliding with the pier, so minimising the need to reverse on the meridian (normalise). In this it differs from most modern GEMs. This is particularly valuable in the practice of high-resolution imaging, for preserving collimation.

The mount was accurately aligned, on first setting-up, using a theodolite and solar transit to establish the meridian in the observatory, shown by a mark on the wall, and no latitude for azimuthal variation was built in, other than the potential play on the bolts holding the mount down. Latitude adjustment is by means of a single M20 bolt, and dowels have been inserted into the vertical part of the mount base to hold it for more accurate adjustment of the latitude setting (Fig. 9.42).

Dave likes making his own equipment, and is one of a rare breed now, though he is not sentimental about it: he recognises that times have changed, and that many things can be far better and more cheaply mass-manufactured today than ever they could be before, and that this naturally has limited what it is sensible to try to do oneself. He saw the first BBC Sky at Night broadcast in 1957, and, soon after, read a book on telescope making. He subsequently ground various mirrors, re-figured lenses, and made optical test equipment including a Foucault tester. This work culminated in the construction of the 39 cm (15.5 in.) reflector and a 21 cm (8.25 in.) folded refractor, but he realised that the latter was an outdated, unnecessary idea when he compared it to the similar-aperture catadioptric telescopes coming onto the market in the late 1970s, which could provide similar performance in a far smaller and lighter package.

In 2004 he started to use the Philips ToUcam colour webcam. Initial experiments, using it afocally with refractors, were not a great success because of the lack of colour correction of the doublet objectives across the range of sensitivity of the CCD. This prompted a return to the With reflector. At the same time, Dave learned, with Damian's help, how to use Cor Berrevoets' Registax image stacking software, and Adobe Photoshop, to process images. This set-up was succeeded by the Atik Instruments ATK 1HS monochrome webcam, used with a C-11 and

Figure 9.42. Base of the mount.

filters, to build up a more detailed colour image than could be created with the "one shot" ToUcam, by imaging in each colour in quick succession, stacking and sharpening the monochrome images in Registax, and then combining them and adding a luminance layer in Photoshop.

The Atik USB 1 camera was succeeded by the Lumenera LU 075 USB 2 camera, which offers faster frame rates without data compression. It will run at up to 100 frames per second (fps) on reduced-size fields, or 30 fps on the whole 640 by 480 pixel field. The primary advantage of the faster frame rates is that they allow the collection of more data in the short period available before a planet has rotated too much. The faster frame rates, however, can only be used on objects that are sufficiently bright - Saturn, for example, has a low surface brightness, and is only imaged at 15 fps. Dave usually uses the Lumenera camera with the C-14 for planetary imaging. The success of the C-14 here is probably not wholly down to the increased resolving-power of a larger aperture. It is rather that the large light-gathering power of this telescope allows a very large image scale to be used while keeping the brightness, and signal-to-noise ratio, high. The processing software is able to much better perform its tasks of aligning images, and sorting them in quality order, if the image is kept bright and the signal-to-noise ratio high. Additionally, more data can be collected from a brighter image, because the camera can be run faster. Thus, larger apertures have a triple or quadruple benefit in webcam imaging.

In 2005 and again in Summer 2006, Dave and Damian both took their C-14s, Celestron CGE mounts, Lumenera cameras, computers and other equipment to Barbados to capture the southerly-declination oppositions of Jupiter in those years, which could not be well seen from the UK because of the low altitude. They also took many images of the other planets while they were there. The seeing conditions on Barbados proved to be generally superb, and an amazing body of work was built-up, particularly covering the development of atmospheric features on Jupiter. One result of this work (co-ordinated with observations by others in different parts of the globe by John Rogers of the BAA) was a new determination of the rotation period of the Great Red Spot, which had previously only been accurately measured from images sent back by the Voyager probes.

Dave's excellently-equipped workshop is key to his endeavours. He built this himself, utilizing some metal doors and windows that were being thrown out by a neighbour. It is an L-shaped shed to the west and north of his observatory, containing a lathe, drilling machines, a milling machine, mechanical saws and metal cutters, grinders, and much else, including an impressive selection of other out-of use telescopes and telescope parts. Dave can make pretty much anything in metal, or other materials, provided it is not too big. Among the smaller items of his making that I saw is an adaptor, ingeniously baffled by having a sharp screw-thread cut into it internally, which makes the un-painted, un-anodized metal virtually non-reflective from the point of view of the optical path.

Dave has put his workshop skills to non-astronomical uses, such as the making of cross-bows for archery. This has been his other long-standing enthusiasm, and he has been British field archery champion with the crossbow and recurve bow. One can see the connection between the accuracy required in this sport and his dedication to precision in manufacturing astronomical equipment and acquiring world-beating images with it. His attitude is that once he tries his hand at something, he has to do it as well as it can possibly be done, and it is this that drives him to push back the boundaries of amateur high-resolution astronomical imaging.

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