DESIGNING & CONSTRUCTING a
DOMED ASTRONOMICAL OBSERVATORY
C.J.R. Lord D.Phil.,B.Ed.,F.R.A.S.

INTRODUCTION:
The purpose of this article is to describe in detail how to design and build a domed observatory for a large amateur telescope. Of all the
different styles of astronomical observatory used by amateur astronomers, the domed observatory is the best. It affords a permanent shelter
from the elements, protects telescope and observer from wind and stray light whilst allowing access to a wide area of the heavens, and provides a
home for all the accoutrements of practical astronomy that otherwise clutter up the home.
I intend to establish some very relevant principles and fundamental rules to guide you in designing your own domed observatory. If you observe
these rules you will save yourself a lot of grief and wasted effort and you will be rewarded with success. Ignore them at your peril!
It should be made clear that designing and building a domed observatory is a lengthy, difficult business, not to be entertained lightly. But it is
thee most rewarding aspect of practical amateur astronomy, and a defining moment. If you are to succeed you will have to be prepared to
commit yourself to several month's (if not year's) hard work.


DESIGNING THE OBSERVATORY:
The first thing to consider is the size and shape of the proposed structure, and its location relative to the house. Before even considering a
permanent observatory, you should be aware that a large back garden with a good southerly aspect as well as access to the northern sky is very
important. The usual injunction is to locate the observatory as far from the house as possible so as to mitigate against the deleterious effects on
seeing caused by heat escaping through the roof and chimneys. However my recommendation is that you locate the observatory in your back
garden, at least 20 meters from the house, but most importantly, where you will be able to see the most sky. Try and avoid placing it alongside
property boundaries, hedges or rows of hedging trees, especially Leylandii or Lawsonia evergreen screens. You should also be prepared to consider
an elevated observing floor in order to improve your sky access. The only caveats to this advice are the avoidance of direct lines of sight to any
artificial lights, and local planning restrictions.


OBTAINING PLANNING CONSENT:
Before establishing the overall dimensions of your observatory there are a few things that need to be said apropos planning permission and
building regulations. The Town & Country Planning Act 1990 requires that all proposed permanent buildings erected by the owner of a private
dwelling, or on his behalf, be subject to prior notification and consent where necessary. The procedure is neither costly nor difficult and it is in
your own interests to observe these legal requirements. If your house is not listed, and your land is not designated as either an area of
outstanding natural beauty, an SSSI, or subject to a district bye-law, order or regulation other than section 57 of the 1990 Act, all you need to
do initially is advise the district council's planning department of the building you intend erecting, apply for an exemption certificate, and
provide a rough sketch showing its proposed location relative to the house and boundaries (SCALE 1:2500 or 1:1250) and its separation from the
rear elevation of the house, boundaries and road frontage (SCALE 1"100 or 1:250).
If the overall height exceeds 3 meters it will require full planning consent because under the new interpretation of the building regulations any
structure having a roof 3m or taller (as measured from the property frontage), that is other than flat, falls within the definition of the 1990 Act.
(Formerly a domed roof was classed as other than pitched and could be 4m tall before full consent was required). However unless your
observatory is to have a floor area over 30 sq. meters, it will be exempt from building regulations approval.

Before designing the observatory in any detail I strongly suggest you prepare drawings suitable for your planning application, because the
smallest useful size of dome (3 meters) is, under the new interpretations of the 1990 Act, going to require consent. These drawings and plans
should be in metric, and comprise:
a] Site Plans: scale 1:2500 or 1:1250, (use ordnance survey charts or site plan obtained from land registry), clearly showing the site and its
boundaries, edged in red. Any adjoining land owned or controlled by the applicant should be edged in blue.
b] Layout Plans: scale 1:500, showing existing land boundaries, and the existing and proposed layout; the position of all existing roads &c. and
the proposed observatory. Any existing trees or natural features, distinguishing those to be preserved, removed, planted &c. The proposed use of
the observatory, and existing buildings and land not built on. The position and width of any access roads/paths &c, indicating any alterations
to those existing.
c] Building Plans: scale not less than 1:100 (I recommend either 1:10 or 1:25) showing the materials to be used; the colour of the external walls
and dome; floor plan and a front and side elevation; the level of the observing floor relative to the level of the adjoining road frontage; the o'all
height, width and floor area - all in metric.
Your planning application must be accompanied by the application fee (£80 at time of writing), 6 copies of the application "Form 1", 6 copies of
each plan, and 2 copies of the appropriate land ownership certificate. You will also need to send a copy of the plans to the District Council
Building Control Services, who will issue an exemption certificate, except where the floor area exceeds 30 sq. meters.
The application procedure can take several months. If you live in a village or rural location the District Council will advise the Parish Council of
your proposals and they will circulate copies of your planned observatory to your immediate neighbours for comments and/or objections. You
should not be too concerned about any possible objections because, even though all manner of objections might be expressed, only objections
on certain grounds can be considered. The most immediate are the prominence of the observatory from your neighbour's purview, and its
appearance and colour.


OBTAINING PLANNING CONSENT (cont.):
Do not be tempted to proceed without planning consent because it is unlikely that retrospective permission will be granted without either a
summons &/or fine; &/or an order to modify/remove the structure. Once you have obtained permission you will be obliged to observe any
conditions imposed and commence work within a specified period (typically 5 years), but it does give you certain rights, e.g. freedom from
interference with your sky access, and from intrusive or objectionable garden; domestic or security lighting &c.
I recommend that once the application has been submitted you contact a senior councillor on your Parish or District Council, and keep abreast
of developments and preempt objections by submitting modified plans that meet, in part, any suggestions made by the Council, in response.
These actions will impose no additional costs and will help you get the District Council Planning Officer on side. Bear in mind that your
objective is a domed observatory with as much sky access as is feasible, and a dome finished in a heat reflective paint (either titanium white or
aluminium).


DESIGNING THE OBSERVATORY IN DETAIL:
When you have received planning consent it is appropriate to proceed to the detailed designs. The o'all height is determined by the maximum
height of the telescope above the observing floor, the height of the observing floor above the local ground datum (as distinct from the level of the
road frontage used in you planning application), and the diameter of the dome.
Begin by taking the height of the observing floor above the local ground datum. Tothis add the height of the pivot point of the telescope
mounting (the intersection of the RA/DEC or ALT/AZ axes), above the floor. This should be equal to the height of the dome rail above the floor
plus the radius of the telescope tube, so when the telescope is horizontal it just clears the dome rail.
You must then decide on the diameter of the dome. It should clear the front of the tube by at least 250mm, preferably 300mm, and in the case of
a German or English equatorial, allow for the offset of the tube from the centre line. To aid you in this decision, draw the outlines of the
telescope and mounting and locate the pivot point on the dome's centre line, and then draw a radius from the pivot point through the end of the
tube when directed to the zenith and the horizon. Ensure the lower mouth of the tube just clears the rail, and add 250mm to 300mm radial
clearance. This defines the dome diameter. Place the rail on the wall about 100mm within the dome skin, and add a 200mm skirt to the dome.
Make sure it clears the wall as well as the rail.
For example, if you refer to the diagrams  [elevation]  &  [section], you will see the elevations I submitted as part of my planning application.
Compare them with diagrams  [front elevation]  &  [side elevation]. Note that the detailed designs differ from, yet conform to, the agreed proposal.


DESIGNING THE DOME:
Once you have the size of dome needed to clear your telescope in all attitudes, you need to decide how and of what you are going to make it. The
dome must turn easily, so it mustn't weigh a lot, but it must be windproof. The shutter mustn't pop out of its track anditshould be as wide
as three times the telescope's aperture, so the dome must be sufficiently rigid not to flex when it has a wide slot cut out of it. These are
conflicting requirements because if we try to add sufficient strength to the dome structure to prevent it distorting when it has a wide shutter
opening, it will end up so heavy it will be a real struggle to turn round.
Timber or glass reinforced plastic ("fibreglass") seem easy materials to work with, but they lack rigidity and need a heavy support structure. They
also need regular maintenance. Steel is relatively inexpensive and has a high strength to weight ratio, but it is heavy and rusts. Corrosion
resistant steels are too expensive. Aluminium is light, corrosion resistant and readily formed, but it lacks rigidity. On the other hand, there are
copper-aluminium alloys (duralamin), that are both readily formed and have a higher strength to weight ratio than steel. Moreover they are, to
all intents and purposes, maintenance free andalso highly heat reflective.
All things considered, the ideal material; is "half-hard" aluminium which is a solution treated semi-hardened dural alloy. Weight for weight it is
about the same price as mild steel sheet.


DOME MANUFACTURING TECHNIQUES:
Now let us consider manufacturing techniques. A truly hemispherical dome entails manufacturing "gores", sections of a sphere that look like the
outer surface of an orange segment. They are curved to the radius of the sphere in both horizontal and vertical directions, and they are
extremely awkward to make. They also need fastening to a rigid framework that must be made from either steel or dural angle, that requires
rolling to the correct radius.

What if instead of opting for a true hemisphere we opt instead for a faceted shape that closely approximates a hemisphere, and in order to save
time and expense, we make the facets with straight sides, and we abandon the idea of cladding a skeleton framework and fasten the facets to
one another to form a "monocoque" structure. Since the dome must have an opening which allows the tube to look vertically upwards, the
facets do not need to meet at the dome's apex, but a set distance down. That radial distance will correspond to half the width of the shutter.

The next consideration is the thickness of the dural sheeting and how much of it we will need to buy. 16SWG (1.6mm) is an easily worked sheet
thickness suitable for domes from 3m to 6m diameter. It comes in standard sheet sizes, 2.44m x 1.22m (8ft. x 4ft.), and the next task is to make
the biggest dome from the smallest number of sheets that makes use of the most sheet area, so we have the minimum wastage. For instance,
suppose you work out the facet sizes, and determine the minimum number of sheets you will need to make them. I guarantee you will have more
waste material than necessary. What you should do is then work out how big you can make the facets so that most of the sheets are used. You
can then work backwards and arrive at a slightly bigger dome diameter. Round this downto the nearest workable dimension (there's no point in
trying to build a 4,253m dome, but a 4.25m dome would be fine), and redesign your observatory based on that dome diameter, making sure you
keep within the o'all height specified in the planning consent.


DOME MANUFACTURING TECHNIQUES (cont.):
I designed my dome along these lines with the assistance of the late Alan Young, who ran a small fabrication workshop called Broadview
Engineering in East Sussex. Alan sadly died in January 1990 aged only 52 years, and he is sorely missed by all who knew him, if only because he
was a very practical man full of clever ideas. I settled on a "pseudo-gore" formed from 3 separate trapezoidal facets  [dome geometry]. Each facet has a
joggled flange top and bottom, and a return fold on both sides. Because the facets are flat they can be cut out of the stock sheet using a
workshop sheet metal guillotine, and the joggles and returns formed on a bench press and a brake fold. The three separate facet shapes are
drawn out using a template, and templates are also used to form the joggles and for drilling the rivet holes. They are then sealed andpop-
rivetted together along their joggles. They thus form a single "pseudo-gore" side panel, and when you fasten them side to side, they form a
"quasi-dome" with a hole at the top. This hole is the same diameter as the final shutter width.

To assemble all the side panels you need a jig. This takes the form of a vertical pole (a scaffold pole will do), with a tubular ring with spokes
welded at right angles to the top. (The finished fixture resembles a May pole with a cart wheel on top). Around the bottom of pole you must place
your base ring, which will become the bottom lip of the dome when all the side panels are riveted in situ. I used 50 x 50 x 6mm dural angle, cut
and formed into a 42 sided polygon. You can do this by making hacksaw cuts in the upper side of the angle, and heating the outer side with a
welding torch, just sufficient to soften it so you can bend it to the correct angle (171
º:26' in this case), and then welding the saw cuts together
once you've completed the ring. You must take care not to distort the ring when heating and welding it. It must lie flat and not become twisted
or buckled, and most importantly, it must be the same distance across flats all the way round. (A few millimetres leeway is tolerable - but no
more). You will need sufficient panels to go all the way round, save one, because of the gap you will leave for the shutter opening. I suggest you
make the last side panel section to suit, so as to accommodate accumulated build error as you go round the May pole. If you work accurately no
one will notice that one of the "pseudo-gores" is marginally wider or narrower than the rest  [pseudo-gore assy].

Next you will need to fabricate an up stand for the shutter track. You could choose bi-parting shutters, like those on Mount Palomar
observatory, but I wouldn't recommend it, because although they are simpler to make, they catch the wind, and are easily popped out. An up
and over shutter is best, but because it must open beyond the dome apex, it must either be made in two separate sections (as in the William
Herschel & Keck telescope domes) or have a separate drop down flap at the bottom. (This option is easier because you only need a single shutter
track). You should form the track from "U" section, rolled to the desired radius, with the opening facing outwards  [shutter track section]. The roller
assemblies take the form of "spiders", and you need four assemblies (ie. 16 rollers) per track. The shutter must be made in three sections, and
fed into the track and riveted together in situ. Make sure it runs freely and smoothly before you assemble it. The bottom flap folds in half
outwards when you need to observe near the horizon. For most purposes you will not use it. In fact if you aren't bothered about observing at
zenith distances greater than 75
º you can dispense with it altogether.  [dome assy. elevation]  [dome plan]  [dome front elelevation].


DESIGNING & FABRICATING THE RAIL & ROLLER SYSTEM:
Next comes the dome rail and roller system. It is a simple matter to automate both dome rotation and shutter opening/closing. If considered at
the outset, it need not be the insuperable problem most advanced amateur astronomers make it out to be. If you want to be able to drive your
dome with a small, economical motor, it must turn freely and easily. The dome design described is light but exceedingly rigid, much more so
than a true hemisphere formed from a framework clad with gores. But if you push the dome round from the side it will not of its own volition
move in a circle. Every action has an equal and opposite reaction. You push tangentially to the dome so it reacts by trying to move sideways.
The only thing that forces it to go round is the roller system.

Now you could fasten the rail to the dome lip and place the rollers on the wall, but the rail needs to be heavy and rigid otherwise it will flex
under the weight of the dome, and the dome will jam. This makes the rail so heavy that the dome weight rises to the point where a considerable
effort is needed to push it round. Better to place the heavy rail on the wall so it is no longer a moving part, and place the rollers on the dome.

What section should the rail be? If it is rolled from strip it flexes easily and twists and buckles, so it needs lots of side stiffeners and support
cleats that interfere with the rollers. If it is angle or tee section it is expensive and difficult to roll into a true arc. What we want is a section
that is as stiff sideways as it is up and down, but can be easily rolled into an arc. There is a shape that fulfils all these criteria and it is the
tube. Choose a rail diameter about 200mm less than the across flats dimension of the dome, so as to provide clearance for the roller support
brackets and the roller standoff from the dome skin. I made my rail tube out of Schedule 10 47mm steel pipe (about 4mm wall thickness). I had
it rolled in five sections to a template, to a tolerance of ±3mm, and plug and butt welded on site into a 3.658m (12foot) centre line diameter. The
rail was located on wall stanchions using 100mm x 100mm x 6mm (4"x4"x1/4") welded steel seating plates. When bolted in situ the dome rail
must be round within ±6mm (1/4") and dead level all round. It is important to brace the sections with G clamps and scaffold poles and arc weld
in short radial sections, one section at a time, checking on the roundness as you proceed. If you weld one joint at a time the intense local
heating around the joint expands the tube and forces it out-of-round, and you will never get it back round afterwards; be warned. The butt welds
must be dressed flush all round. Keeper plates may be welded underneath, but they must clear the path of the rollers.

Now, what about the rollers? The dome tries to move sideways when it is pushed, so if the rollers have a flat cross section you will need both
horizontal and vertical rollers to ensure the dome goes round. The trouble with this two dimensional approach is that it ignores a very real
problem with any rail/roller system. Because the reaction of the roller on the rail due to the weight of the dome is not inelastic, the rollers
produce friction which causes rolling resistance. The accepted wisdom is that the rolling resistance is directly proportional to the load per roller,
and therefore the more rollers we use, the lower the load per roller(assuming all the rollers make equal contact), and consequently the
combined rolling resistance remains the same, regardless of their number. And adopting the ball race principle of the roller bearing, the more
rollers the better. Nothingcould be further from the truth. Rolling resistance is caused by elastic contact between rail and roller. The logic used
to advocate lots of rollers assumes the contact is inelastic; that is it treats the surfaces as infinitely hard. The contact area is not directly
proportional to load, and if you double the load you do not double the contact area, it only marginally increases. In fact the fewer rollers you
have the lower the combined rolling resistance. The minimum number of rollers you can possibly get away with is three. But, this assumes the
dome is perfectly rigid, which obviously it can't be. So for a dome diameter of 3m to 4m I suggest 6 rollers, and for a dome diameter between 4m
& 6m, 12 rollers. However, only half of them need be in permanent contact. The others in between can stand off by about 3mm (1/8"). They
merely provide sideways or lateral restraint, i.e. they prevent the dome "crabbing".


DESIGNING & FABRICATING THE RAIL & ROLLER SYSTEM (cont.):
Next we must consider the roller cross section and size. Small rollers suffer high axle loads, which in turn produces high frictional losses and
increased rolling resistance. As a rule of thumb, make the rolling contact line diameter about twice that of the rail diameter. We can also take
advantage of the dome panel design and fasten each roller off a square section tubular framework bolted to a lower panel  [dome drive assy.] 
Since the panel is inclined 13
º to the vertical, the axle of the roller will be declined 13º which will provide a semi-kinematic advantage in our bid to
encourage the dome to want to move in a circle rather than sideways. Because the rail has a tubular cross section, we can also choose a roller
section that runs on both the outside and the inside of the rail simultaneously, so we combine the actions of both horizontal and vertical
rollers in a single roller. To minimise the contact area the roller contact faces must be flat, not curved to match the rail section. Such a roller is
"Diablo" shaped.
I recommend you get your rollers turned out of Nylon 66, because it will not need lubricating, and in fact it both grips the railtube and scrubs it
clean in a pressure polishing action. The roller axles should be fitted with roller bearings, and cover plates which can be removed so the bearings
can be occasionally regreased.

I might also add a note about rail restraints, to prevent the dome becoming derailed in a high wind. A dome, properly designed and fabricated, is
a windproof structure. The effect of a high wind is to force it hard down on the rail. If the shutter is trapped by the track so it cannot pop out,
then the dome cannot be lifted off the rail. But it can be pushed off the rail, so it must be restrained by a series of brackets that run around the
inside edge. These do not have to lock beneath the rail. Even at low wind speeds where the airflow is lamina and suction is produced, there is
always a horizontal load sufficient to force the dome sideways. The dome will not be lifted upwards off the rail provided itremains windproof
 [rail restraints]. As the dome is pushed on any side, the rail restraints grab on the inside edge opposite and lock the dome to the rail. This design
was incorporated into my dome in January 1987. It withstood a force 9 gale later that month, a force 11 storm in March of that year, and the
great storm of October 15/16th. '87, when the wind reached storm force 12, and gusted to force 14. It was not otherwise held down. However a
conventional dome of true hemispheric shape, also built by Alan Young and placed on the east tower of Alexander Palace in January 1987, blew
off during the Great Storm, despite being tied down. My cleats had not been fitted! The trouble with tie downs is they work loose with
continuous buffeting, until the dome is derailed. Once that happens it is no longer windproof and lifts off due to the Bernoulli effect.


DOME AND SHUTTER DRIVES:
To motorise the dome all you need to do is fasten a drive belt gear and a torque limiter/scroll clutch to one of the contact rollers and drive it
with a right angle gear motor. A suitable drive rate would be one revolution every 4 minutes for a 4m dome, from which you can work out the
output speed of the gear motor. I also recommend you employ a single phase 250VAC capacitor start induction motor driven via an inverter and
a 12VDC gas recombination powerpack. All these components, when placed on a suitable support framework above the drive roller, force it down
on the railtube and cause it to maintain contact. The torque limiter on the drive belt gear prevents the motor stripping the worm drive in its
gearbox in the event of an overload or power surge on starting.
The most direct way to drive the shutter is by an endless cord system via a windlass (ref. fig. 11); limit switches control end of travel.
To prevent the dome drive roller riding outwards, a jockey roller is placed under the shutter transom so as to roll on a vertical axis around the
outside of the railtube. This means the drive roller mustbe positioned diametrically opposite the shutter opening, which is the most suitable
location for the shutter drive windlass in any case.


THE DOMEWALL:
The height of the wall largely depends on the height of the pivot point above the observing floor. In the case of a refractor or a Cassegrain you
can make the wall 2.2m (7 feet) high and employ a full height door. In the case of a Newtonian I recommend you use the bottom half of a stable
door. I have seen removable rail sections, and access through the shutter, but neither are ideal. The only other alternative is a much higher wall
permitting a full height door beneath the floor, and an internal stairway affording access via a floor hatch.
In the case of Brayebrook Observatory I dug down to increase the clearance beneath the floor joists to about2m. Access to the under-floor
space is via a floor hatch and extending aluminium loft ladder. The observing floor can be kept fairly free of obstructions because all the
accessories and other observing paraphinalia are stored beneath.
The wall can be brick or galvanised steel or dural sheeting. I don't recommend timber. It has too low a thermal conductivity, and so retains its
heat. It is important that the inside and outside air temperatures remain as close as feasible. This is not easy on a hot summer's day when there
is little air movement, so I suggest either a cavity brick wall, or a part brick, part dural or "zintec" clad wall. If the cladding is dry lined with
"Daler" board, it will act as an insulator, and provide a surface onto which you can pin charts, posters, &c.
A dampcourse will be needed between the second and third course of brickwork, above the rainfall splashback. Use either tar paper or
"Synthaprufe". The concrete floor pad should also be damp proofed, and screeded and sealed to prevent dusting. I recommend an epoxy
industrial floor paint.
Finally,bear in mind that the power supplies and any other electronic control cables, telephone or modem cables will have to be brought in
through a below ground, water tight, services duct, beneath the floor, and that all junction boxes, power points and control boxes must be
accessible via a floor hatch or brought up to the pier if it stands above the observing floor.

PIER AND FLOOR:
The next consideration is the pier and floor. An air space under the floor prevents the joists rotting. Even if you place your observing floor
immediately above the ground datum you should allow at least 450mm clearance below the joists for free air circulation, and fit air bricks.
The pier must have a separate footing and not touch the floor in any way. (Do not be tempted to infill the gap with an expansion joint of either
expanded polystyrene or compressed paper, such as "flexel", it will still transmit vibration). I constructed my pier from concrete blocks, 450mm x
225mm x 100mm (18" x 9" x4"), infilled with kiln dried sand and capped with engineers bricks. The equatorial is located on a slate bedplate (the
telescope will be described in a later article), but yours will require whatever bolting fixture is necessary. The important thing to check as you
proceed is the meridian alignment, which I will describe later.
Floor joists (195mm x 47mm or 8" x2" for a 4m span) can be suspended off the brickwork using joist hangers, but not the pier. Either a separate
"bund" wall is needed surrounding the pier or individual brick support piers. The pitch of the joists should be about 450mm (18") to 600mm
(24"), with noggins every meter (40").


THE FOUNDATIONS:
The concrete footings for the wall should go down at least 225mm (9"), and the floor pad should be at least 110mm (41/2"). If it is reinforced
with wire mesh, so much the better.

CONSTRUCTION OF THE BUILDING:
I am assuming you have already fabricated the dome. This can be done throughout the inclement winter months when you are obliged to stick to
the confines of your workshop/garage. It makes life easier if you do not complete the dome assembly, but leave it in two halves, and the shutter
sections. These can then be manhandled onto the rail once the rest of building is finished, and assembled in situ.
I don't recommend site work during the winter months, December thru' March, not unless you yearn for an endurance medal. This sort of work
is best done when it is dry underfoot and warm, let us say, April thru' October/November.

ESTABLISHING THE MERIDIAN:
You need first of all to decide precisely where the telescope pivot point is going to go, and to establish a north/south and east/west line
intersecting at that point. The north/south or meridian line, is the first objective. I have witnessed all manner of oddball methods, ranging from
a compass and Ordnance Survey map allowing for magnetic deviation correction, to sighting Polaris at upper or lower culmination. They are all
a ridiculous nonsense. The method I am about to describe will suffice, and prove the least troublesome and the most accurate. All you need is a
large sheet of thick brown paper and some paper weights, a tripod and a plumb line and bob, a black felt marker pen, a watch set to GMT, an
Astronomical Almanac, a meter rule, and pegs and string. You must also have your longitude to the nearest arc minute which may be obtained
from the 20000 series Ordnance Survey.
Having levelled the site, lay the sheet ofbrown paper as close to the north-south line as you can estimate, and hold it down with the weights.
Place the tripod across the south end and suspend the plumb bob from the head so it can swing freely, with the bob just clearing the paper.
Mark the bob point when it has settled.
Calculate the time of local noon from the Almanac. To do this look up the time of solar transit at Greenwich. Correct for your longitude.
Remember if you are east of Greenwich local noon occurs earlier, and if you are west ofGreenwich it occurs later. Convert your longitude from
arc to time (based on 1o equals 4 minutes time & 1' arc equals 4 seconds time) and add it to the Greenwich time of solar transit if west or
subtract it if east. This is the time the Sun will be exactly due south at your site. Set your watch by the speaking clock or the MSF time signals
and correct for BST. (Remember if you do this in summer the clocks are set an hour ahead of Greenwich). It is imperative you make this
calculation without any mistakes, especially the longitude correction. I have seen professional observatories with equatorials skewed on their
piers due to stupid mistakes made by so called experts. Take care, and get someone to check your sums.
About an hour before local noon begin marking off the end of the plumb line's shadow on the brown paper using the felt marker pen and noting
the time. Do this at regular intervals (say 12 minutes), and mark local noon as well if the Sun isn't clouded over at that precise moment. If it is,
carry on until about an hour after local noon, and then interpolate the shadow marks. The best way of doing this is by drawing a straight line
from the first to the last mark and measuring the intervals of each shadow mark along the line from the first, and listing the distance against
the time interval, #anchor722328(ref: Establishing Meridian List).


ESTABLISHING THE MERIDIAN (cont.):
Once you have established the noon shadow mark, draw a line from the bob point to the noon mark and extend it across the paper, both north
and south. Then peg out a long line so the string runs contiguous with the line on the paper, several meters beyond each end. This is your local
meridian. Do not move it - it is sacrosanct!
Set out a similar length of line at right angles to your meridian line, intersecting the meridian line at the bob point. To make certain it is a true
right angle, make a line of 12 equal knotted lengths, and peg it out as a 3,4,5 triangle, and use the 4 knot length as a reference baseline. As a
final check strike off equal offsets along the meridian and east-west lines from the bob point, and then measure their separations. They should
be equal (lets says within ±10mm).
Having established the meridian you can then peg out the foundation footings for the wall and the pier. The wall may either be round or
polygonal. I don't recommend a square or rectangular wall with the dome sat on the flat roof because it is almost impossible to keep it
watertight. You will have problems with the dome skirt and the flatroof, and the heat radiated from the roofing felt will cause local air
currents.

THE FOOTINGS:
You will need to fabricate a shuttering box for the pier footing, and the wall footing if you intend to have a sunken floor base as I have. This
shuttering can either be sacrificial, or it can be left in situ, in which case it will have to be sealed with a bitumastic paint such as
"Synthaprufe". The finished internal wall can be dry lined or clad afterwards, once the floor has been built and the dome is in place.
The concrete mix is important. I recommend a BS5328 C20P mix or C25P. You can make this by weight batching in the ratio 1:2:4, cement,
aggregate, ballast. As a rough guide, 25kg of cement should be mixed with 65 litres of ballast and 40 litres of aggregate. For example a 1 cubic
meter batch will require 300kg cement, 490kg aggregate and 800kg ballast. When calculating the total volume of concrete required, please note
that the cement does not add to the batch volume of the mix.

BUILDING THE WALL:
A circular wall is constructed using a central vertical pole and a radial arm to position each brick. The trouble with a circular wall is fitting
things to the inside, and making the door frame. I made my wall 12 sided, roughly 0.9m (37") per side, and 3.66m (12 feet) across corners. If you
do the same or similar, ensure the width across corners is less than the across flats dimension of the dome ring (not rail), otherwise the dome
will foul the wall. (Bear in mind the narrowest part of the dome is the inside across flats dimension of the dome ring and the widest part of the
wall is across corners).
Pug or mortar for the brick and blockwork is a 1:3 or 1:4 mix by volume of cement and washed sand. You may improve the plasticity of the mix
by adding a flow agent or plasticiser, such as "feb-mix". Brickwork can be waterproofed using a "water-glass" liquid sealant such as "Aquaseal".
Water that runs off the dome will fall from the lip of the skirt. The skirt should stand off the wall by about 100mm (4"), and the rainfall runoff
should be able to soak away, so I strongly recommend you extend the wall footing only about 50mm (2") beyond the outer wall perimeter, and
that you strip any turf from the site and cover it with aggregate and crushed limestone. Limestone has a high visual and infrared albedo, and
does not absorb solar radiation well, so it does not cause local thermals at night, as does tarmac or concrete. Grass is best of all, but it is
difficult to maintain right up to the dome wall, so a perimeter paving of crushed limestone about a meter wide is needed at least.

BUILDING THE PIER:
The pier itself needs to dampen vibrations induced by moving the telescope. If it is made of blockwork, fill it with kiln dried sand. If it is a steel
tube, fill it with either dried sand or concrete. Cap the pier either with engineer's bricks or a steel plate.

INSTALLING THE FLOOR:
The flooring may be either tongued and grooved boards, or MDF flooring. I used the latter because it is easy to cut and fit and doesn't have the
irritating habit of creaking. I then tiled it with "Parkiflex" wood floor tiles. If you do the same take the tiles out of their sealed packs and stack
them in a cool dry place so the air can circulate around each panel. Leave them for a month or so, until they have absorbed the moisture in the
air. Wooden floor tiles are kiln dried and if laid straight from their vacuum sealed packs, will absorb moisture from the floor boarding and the
atmosphere, expand and push upwards and shatter! When left to acclimate and then laid the resulting flooring looks very professional. If
rushed, a more amateurish bodge job you couldn't wish to find! Finish the floor once the dome is in place.


INSTALLING THE DOME:
The railtube must be fitted to the wall, and levelled. It is advisable to place theequatorial mounting on the pier before placing dome on the rail.
For either or all of these operations you will need many able hands, or a mini-crane, and/or an engine hoist and scaffold.
The telescope can either come in through the door and be stood on chocks on the observing floor (before it is tiled and varnished), or through
the dome slit.
Once the power services are fitted and the AC circuits checked, the dome rotation and shutter opening/closing can be tested, after which you
will be able to turn your attentions to the mounting's alignment (to be described in a future article). The dome lighting should include a
white/red light system with dimmers, and a bright white light system for when you need to work on the telescope or dome controls at night.

FINISHING OFF:
If your observing floor is raised off the ground you may also need to make some steps to gain safe access to the door. All the electrical wiring for
the dome lighting can be run along the floor joists. Fit duck boarding to the basement floor pad, even though it is "tanked". You can make this
out of old palettes. Anything you store down there will be raised off the concrete and will stay dry. You should treat the joists with an anti-
fungicide, such as "Cuprinol", and check them periodically, and also ensure the air bricks aren't blocked.
You must either leave the dome unpainted, or if you intend to paint it white, first apply a chromate free etching primer (e.g. Crown 5SU 245 -
use thinners 4DE/000), and a heat reflective titanium oxide based paint(e.g. Crown Moisture Cure Polyurethane4ZL102 u'coat & 4ZW102
gloss - use MCP thinners 4DO/000). It is no use painting the railtube because the Nylon rollers will scrub it off. However you can apply a zinc
based etching primer, or a phosphating solution, which will prevent rusting. The alternative is to use an aluminium alloy railtube.
The final task will be linking the dome motor's controls to the telescope's, so that as the telescope tracks the stars the dome moves with it. I
shall describe this in a separate article.
If you require any advice on any aspects of dome design and construction, please feel free to ask, and if you would like to see what I have done,
it is always possible to arrange a visit.

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