A halfway-house to a roofed observatory is to have a permanently-fixed telescope pier and mounting outside, and a removable telescope, stored inside or in an outbuilding. Some people leave most of the telescope outside, removing the optics indoors after every session. This does not sound very convenient to me - it would require complete re-collimation every session, for a reflector.
However, there are distinct advantages to setting-up a permanent metal or concrete pier to carry the mounting, on proper foundations dug into the ground, and creating around that a comfortable observing surface, of which the best kind is a wooden deck. This might be regarded as the simplest and most economical type of observatory. The labour of carrying around the heaviest parts of the telescope, the mounting and counterweights, is removed, and there is no necessity for repeated polar alignment or levelling of the mount.
Observing on damp grass is no fun. A telescope is not completely stable on a soft surface, one's feet can get wet, small objects (set-screws holding eyepiece barrels or finders are particular culprits) fall off and get lost, perhaps for ever, and one can't kneel down. On the other hand, a concrete or slab surface is also poor. It will generally transmit too much vibration from roads, it will be very cold to stand on (concrete rapidly conducts heat away from the observer), and, worst of all, optical equipment dropped on it is not likely to fare well. If you are anything like me, you will drop eyepieces, prisms, filters and cameras with sad regularity.
A wooden deck supported on joists forms a good observing surface. Decking timbers are universally available from DIY outlets, and the construction involves only rudimentary carpentry skills of sawing and screwing. At this stage, I would suggest that anyone considering building an observatory of any kind, who does not have such skills and experience, and would like to acquire them, goes and looks at some good books on woodworking and other DIY topics, which abound in all bookstores, and possibly enrols for a course at their local adult education centre, if available. The knowledge acquired will certainly prove very useful in the long-term, even if they eventually decide to buy an observatory, or have one made for them. More will be said about construction methods in Chapter 5.
If it is on grass, the deck will need to be raised above the ground by bricks or concrete blocks to allow for ventilation. If on concrete or slabs already, the joists will be sufficient. Both proper decking and joist timber is pressure-treated and already contains preservative to protect it from rot, but this will only be effective for a short time. It must be treated with a wood preservative shortly after construction, and annually thereafter.
More will be said about telescope piers and foundations later. The point to note for the moment is that, with the possibility of leaving an equatorial mounting set up permanently, the polar alignment can be made perfect, and this is invaluable for all serious astronomical work, in imaging, photometry, astrometry and surveys. It also makes faint objects much easier to find, whether using traditional setting-circles, digital ones, or a GOTO system. Of course, this type of observatory requires that the telescope is easily removable from the mounting. This will generally be true with small to medium-sized telescopes on German equatorial mounts, but not necessarily true with fork mounts or other types. Of course, with large telescopes, carrying even the optical tube about becomes too difficult.
If the tube is stored in a heated house, the remarks made before about allowing it to thermally equilibrate with the outside air before observing apply. However, the telescope will be protected from the effects of damp, corrosion, insects and arachnids (spiders), which plague most observatory telescopes. The aluminium coatings on reflector mirrors will probably last much longer, and, of course, the most valuable parts of the telescope will be safer from theft. One well-known and highly-respected English observer kept his valuable refractor outside, but took the optics in after every session. Very sadly, the brass tube was stolen, and never recovered.
With the fixed mounting approach, the problem remains of how to protect the mounting from corrosion and other damage when it is kept outside. Flexible "scope-covers" are sold by some astronomical equipment suppliers, and these could be tied around the pier with rope, but I haven't tried them. A plastic car-cover is a possibility I have tried, but I found this tended to trap the dampness that accumulated during observing sessions, and promoted rust. A removable wooden box might be the best solution, purpose-built to fit over the mounting on top of the pier.
Such a semi-observatory can be supplied with mains electrical power, subject to the usual safeguards for outdoor electrical installations (contact a qualified electrician, and see the advice in Chapter 8 of this book), eliminating a major disadvantage of portable setups, the requirement for batteries or inconvenient, and possibly unsafe, mains extension cables. The problem with using batteries for astronomical equipment is that they will always fail at the crucial moment, just when conditions are perfect for obtaining that sensational observation or image. Even if regularly re-charged, they can lose power dramatically in low temperatures. Moreover, keeping them charged-up is a chore. Temporary mains extensions from another building are not to be recommended, as it will probably be necessary to split the power between different devices, for example, a telescope mount and a laptop computer, with a non-waterproof adaptor, that becomes exposed to damp. Such an arrangement poses a significant risk of fatal shock to the observer. Proper waterproof outdoor sockets, installed as part of an observatory project, additionally often become useful for operating gardening equipment.
The disadvantage with the semi-observatory is that it will, like the fully-portable setup, still most likely require a lot of journeys to be made to carry equipment in and out of the house or other storage shed, thus losing most of the setup timesaving advantages of a proper observatory. Some of these items are: eyepieces, diagonals, filters, cameras, computer, an observing chair, and observing table, power adapters, cables, and dew heaters. The list really goes on and on. The more complicated your astronomy becomes, the more inconvenient becomes the rebuilding of the system each session. Having all these things ready to hand, more or less in place, in a roofed building, is one of the main joys of owning a full observatory. Of course, some of them can be gathered together in a briefcase, but some cannot.
Run-off sheds are the next stage of sophistication of the amateur observatory. With the run-off shed, sometimes called a roll-off observatory, the telescope and its mounting are permanently assembled, but used in the open. A movable covering protects the equipment when not in use. This covering usually takes the form of a small shed-like structure, either in one or two sections, that moves on wheels constrained by rails or tracks.
The run-off shed preserves the completely open character of observing with a portable telescope. The whole sky can be seen (or as much of it as the local horizon allows), and many observers value this highly, and would not swap it for the environment of a dome, where only a narrow slit of sky is visible at any instant, despite the alluring comforts of the walled and roofed observatory. If a fireball streaks across the sky, making an unexpected display, or a grand aurora occurs, or a bright artificial satellite passes over, the observer in the dome may well miss all these unpredicted events, but they may be seen by the observer with the run-off shed. The run-off shed itself affords no protection from the wind, though windbreaks can be separately constructed, and it doesn't shield from local artificial light sources.
Equipment in a run-off shed suffers badly from the dew in damp climates, more so than portable equipment, which dries off after it is taken in. Run-off sheds tend to trap the dampness that accumulates during observing sessions, retaining it into the following day, unless considerable attention is given to ventilation. They suffer particularly from this as their volume tends to be quite small. Another factor contributing to the damp problem is that rainwater flows underneath the shed. I have found more problems with electronic equipment kept in a run-off shed than when housed otherwise, and perhaps the run-off shed is a less appropriate solution in these days of high-tech observing setups than it was in simpler times.
A problem with using a run-off for a "high-tech" set-up is that the electricity probably will have to be routed via the pier, and thus all electrical equipment will end up clustered round the pier. This can be a bit messy and "in the way" (Fig. 3.4). A fixed-wall observatory is more flexible with regard to wiring.
However, a run-off shed represents a big step-up in convenience from a portable telescope or fixed mounting arrangement, and is a type of true observatory. Run-offs can be made quite sophisticated. A deck can be constructed, similar to that described in the previous section, as a working surface (Fig. 3.4), and shelves and a desk can be installed in the moving shed. A desk in the shed would be useful for webcam imaging of the Sun or planets in daylight using a laptop, the shed shielding the computer display from the daylight glare. Items such as an observing chair can be stored in the shed and brought out for use.
There are several possible patterns of run-off shed (Fig. 3.5). A one-piece design is possible (A), with a hinged door or doors (two doors will save space). My run-off shed is to this pattern, and a complete description of this will be given in Chapter 9. The one-piece shed has doors that are opened; then the whole building, or box, is rolled clear. Mine is arranged so the doors can then be bolted in place so as not to flap about. This design has the disadvantage that it can be heavy to move. An advantage is that it has no roof joins which could leak. An alternative to using hinged doors is to have a removable side (B), but then one has to lift this clear to start with, and put it somewhere else, which also might be strenuous. Patrick Moore's famous run-off is of the two-piece variety (C). The sections of this design will be lighter than the one-piece, and hence easier to move, but there may be difficulty in waterproofing the join.
Run-off sheds generally run off on small wheels, which could be metal or plastic (but they cannot be swivel castors, which, undesirably for this application, spin on their axes). These wheels generally run on metal rails made from angle-iron. This is not all that easily available in the required lengths, and will need to be ordered from a metal fabrications supplier. The length needs to be at least twice the side-length of the shed, and probably a bit more for adequate working-room. A straight rail is, however, much easier to obtain than the curved rail required for a rotating dome. Martin Mobberley has used plastic rails successfully in one of his observatory designs, of which more anon. Another alternative to a metal rail might be a channel in concrete, cast in situ using wooden formers, which are removed when the concrete has set. I have not seen this put into practice. However, I came up with a design which avoids a rail or channel entirely, instead
Figure 3.4. With an observatory without fixed walls, wiring and electronics tend to get crowded round the pier.
running the shed on a constrained track on wooden decking. This works quite well. Martin Mobberley has exploited the variant of moving the telescope on rails, rather than the shed, and has successfully used plastic rails for this purpose. These examples will be described in more detail in Chapter 9. The most important points about rails are that they must be set, and remain, perfectly parallel, level and stable, but metal needs to be allowed scope to expand and contract with the changing temperature.
A run-off shed is not a very complicated construction, and it is a project that most people with some practical know-how could tackle. Alternatively, a carpenter or builder could easily construct it, though they would have to be well-briefed as to its operation and purposes. Run-off sheds are not made commercially as a regular product, so far as I am aware. The constructional problem of the run-off shed is that it has to be light enough to move by hand, but rigid enough to withstand the
Figure 3.5. Possible types of run-off shed.
force of moving it without distorting, and that it cannot be braced by a floor as can a normal shed (though the rails brace it slightly). It is further weakened by lacking one wall. If timber, therefore, it requires substantial internal bracing, which needs to be arranged not to restrict clearances too much. Probably the best construction for a run-off shed is a welded steel framework clad in plastic or wood. Olly Penrice has constructed two such sheds at his astronomy holiday centre, Les Granges, high in the mountains of Provence, France, starting with no knowledge of welding whatsoever. Most DIYers are happier working in wood, however, and most run-offs are timber, roofed with some waterproof material.
Run-off (or roll-off) roof observatories are the next stage in sophistication of the amateur observatory. The walls are fixed (or largely fixed), and the roof moves, probably in the horizontal plane, using wheels running on rails. The rails may be just below the roof level, or at ground level or an intermediate level. The roof can be in one part or two, but if two, again there will be an issue of waterproofing the join. The roof may be moved electrically, or using a mechanical winch, or just pushed. There is an almost unlimited number of slightly different possible variants of the run-off roof observatory.
Here we are dealing with a proper observatory building, and a rather bigger constructional project than the run-off shed, though not much more difficult in principle. The run-off roof observatory is a very popular form, and can be bought from specialist makers (Fig. 3.6), though most are home-built. They are always square or rectangular. The run-off roof observatory retains the all-sky view of the run-off shed, but gives a level of protection for the observer from wind and external light. Having permanent walls makes everything far more convenient. Accessories can be on shelves or in cupboards fitted to the walls, permanent wiring can be installed round the walls and under the floor, nothing apart from the roof need be moved when opening up and closing down, and, in essence, all the conveniences of a domed observatory are achievable, without the engineering challenge of creating a rotating dome and opening shutter. In addition, runoff roof observatories are thermally very good, a particularly significant factor for seeing-critical activities such as planetary imaging or double-star observing. Everything under the run-off roof, unlike equipment in a dome, rapidly falls to outside temperature as soon as the observatory is brought into operation. This advantage is shared, of course, by the run-off shed.
The principal problems with the run-off roof observatory are dew and wind. The former is the same as with the run-off shed, with, in a climate like the British, everything inside, typically, getting soaked over the course of a long, clear winter's night, and then all the dampness getting shut in during the day to cause corrosion and rot. Wind is particularly a problem with long telescopes, which are not well protected, as they will stick out above the level of the walls, and suffer a turning moment from the wind striking their upper end but not their lower end. Hence the run-off roof can lead to worse wind problems than having the same telescope in the open.
It might be thought that the walls would raise the horizon to a high altitude, and they always do limit the horizon somewhat. The situation must be considered in each local case, and carefully optimised. Very common is the idea of having an upper section of the south wall folding down after the roof is moved off, so allowing a clear southern horizon. Most commonly also, the roof rolls to the north, but this is not always possible. In designing my run-off roof observatory, I decided not to have a folding wall, for the sake of simplicity, speed of operation, and structural strength. Instead, I made the walls low, so that an average-height person cannot stand inside with the roof on, but the telescope can still be stowed away, almost horizontally, and on the meridian. The way the walls were arranged with respect to other local obstructions (trees and buildings) means that very little usable sky is obstructed by the walls, or open roof, in this case. But in any case, it is not really an issue to be worried about too much, as very little useful telescopic observation can be undertaken at an altitude of less than 25°.
For my first observatory, which I built at the age of 17, I created an almost cubic structure that had a two-leafed roof that opened on hinges, like the petals
of a flower (some historic shots featured in Fig. 3.7). Since this observatory was only 1.5 m (5 ft) square (far too small), the roof sections were not too heavy to be opened in this manner. However, it never worked well. The join proved impossible to waterproof satisfactorily (as can be seen, the rainwater flowed down the slope into it). In any case, I am sure such a design would not be practical if made to a sensible size; the roof would be too difficult to open. Since then I have been rather against two-piece roofs and two-piece run-off sheds. My second observatory was the run-off roof shed I use today, described in Chapter 9, with a one-piece roof. It never leaks at all, unlike most amateur domes.
Figure 3.8 shows some of the possible variants of the run-off roof idea. In (A) we see a pent roof shed which rolls to the south along the line of the apex,
taking with it the eaves sections. In (B) we see a design where the roof slides northwards and perpendicular to the apex, and then the top of the south wall folds down. In (C) we see a flatter, pent-roof shed, where the whole upper section of the walls moves with the roof, to leave very low walls. Case (D) is the slightly unusual design of my run-off roof, which moves perpendicular to the slope of the roof, on rails of different heights. The wheels operate on a flat plane, as with the other designs. It is also possible to have a pent roof which slides in its own sloping plane (i.e. not on the flat), if it is operated by a winch mechanism (E). The diagrams show one-piece roofs, but two-piece roofs, sliding in opposite directions, are also possible. However, this is usually not necessary in terms of ease of operation. The roof can be made quite light, so even a large run-off shed
can normally have its roof moved manually in one piece, if it is well enough made and maintained, thus avoiding a leaky join.
Types (A), (D) and (E) are the strongest constructions, being uninterrupted boxes, while (B) and (C) do a better job of clearing useful horizons. Type (C) is like a run-off shed elevated into the air slightly. This is going to be the hardest type to move manually. The diagrams are by no means exhaustive of the designs that could be, or have been, used for run-off roof observatories. The choice will be made on the basis of local circumstances and the instruments to be used, and there is huge scope for invention.
The popularity of run-off roof observatories is down to their relative simplicity of construction and engineering compared with domes. They supply a proper, fixed observatory without the requirements for fabricating curved components, and solving the very difficult challenge of making a dome slit that opens easily, is large enough, is waterproof, and doesn't fatally weaken the dome. They are thus the natural choice of the dedicated amateur who can make things, but not excessively complicated things, who doesn't want to spend an enormous amount of money, and who wants an easy-to-use solution that will last. The visibility of the whole sky, and ease of quickly re-pointing the telescope to a completely different part of it, is another attraction. The total space taken up by a run-off roof is large, since the space must be available for the roof to slide untrammelled, but this space can be dually-used. In my case, it is also used as a tomato-growing sun terrace.
Run-off roof observatories are, by and large, made of wood, though it would be possible to build the walls in brick or concrete if one were sure one would never be leaving that location again. Timber walls have an advantage of retaining little heat, adding to the good thermal properties of this type. A metal framework clad in something else is also possible, and some constructors have adapted the plastic storage sheds that are on the market. Adaptations of mass-produced sheds will be dealt with further in the next chapter. The rails are usually iron or steel, as in the run-off shed type, but I have successfully used wooden rails for my run-off roof, combined with rubber-tyred wheels. The supports for the rails away from the shed can be brick columns, as I have used, concrete fence-posts, metal posts, or wooden posts bedded in concrete (a wooden post in the ground risks rot), it doesn't much matter, so long as the rail is held rigidly, without flexure, in use.
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