Wessex AS - Durlston

New Astronomical Observatory

With the cooperation of the Country Park, the Wessex Astronomical Society began planning the astronomical observatory at Durlston in mid 2007. Sufficient funding was raised through a grant from Awards-For-All of the National Lottery, through donations from the members of the Society and by a payment from Dorset County Council. Planning consent was granted at the end of June 2008 and work on the site started immediately

Aspects of Design of the Observatory

This section describes those aspects of the design that may be most useful to others who may be undertaking a similar project.

Initial Considerations From an early stage the general size and appearance of the observatory was determined on the one hand by the expected amount of funding that could be raised and on the other by the location i.e. close to the Visitor Centre of the Country Park and in an area with numerous designations - AONB, SSSI, Special Area of Conservation, National Nature Reserve and within the Jurassic Coast World Heritage Site. The best form of structure from the astronomy point of view would be a dome and the limited availability of domes available commercially quickly led us to choose the Pulsar Observatories 2.6m fibreglass dome. Pulsar will supply this as a complete observatory, with a 2.6m diameter wall section, but this would give inadequate space for a work area. The chosen structure was therefore a recangular building of about 3m x 4m with the dome mounted into the roof.

Special Design Aspects

Thermal – During observations the inside temperature and that of the telescope should as close as practicable to the outside temperature. Otherwise warmer air will funnel out through the slit and cause turbulence in the air immediately in front of the telescope. The major factors working against this are (1) solar heating of the walls which may store their heat for hours and (2) solar heat on the dome causing excessive heating of the inside air and hence of the telescope, inside walls etc. Clear skies that are good for observing are generally preceded by a sunny day!
Stability – Vibration and other movements are the enemy of clear visual observation and high quality imaging. The angular stability of the telescope needs to be better than 0.5 arc seconds (close to one ten-thousandth of a degree). For this the pier carrying the telescope needs to be heavy, rigid and mounted on a heavy base structurally isolated from the floor.
Dampness – This can be a problem in observatories since condensation will cause deterioration of equipment, mould on interior surfaces etc. The inside surface of the dome could be particularly susceptible.
Doorway(s) – The need for the telescope height to be such that when pointing horizontally its view is clear of the bottom of the slit, together with the need to reduce the extent to which observers have to stand on something to reach the eyepiece, forces a compromise with the height of the roof and hence of the doorway. Because of the low doorway the distance through the door to where a person can stand upright, i.e. within the dome, is kept as short as possible to avoid it being more like a tunnel than a doorway .

The floor of the doorway and its width should permit wheelchair access (for people who find it difficult to bend and walk). Disabled people who are unable to stand would be unable to use this telescope but may nevertheless wish to enter.

Resultant design choices

Walls - meeting the thermal and low moisture requirements pointed to a cavity structure. Using a double wall of 100mm blocks would give an undesirable "tunnel effect" at the doorway. We therefore chose a single 150mm block construction with foil-backed plasterboard lining on 50mm batons. Ample ventilation of the cavity is provided by 18 air bricks. The outside is cement rendered and painted white.
Roof - The roof was constructed of 100 x 50mm joists faced with 12mm ply. After the dome collar was mounted the roof was covered with three layers of felt by an expert roofer, the felt being taken 80mm up the sides of the collar. The joists were kept clear of the doorways.
Door height/telescope eyepiece height tradoff - For this explanation I revert to imperial units. The dome collar as supplied by Pulsar Observatories has a wall height of 15". There is a further 15" from the dome track to the bottom of the slit. For a 14" aperture telescope the eyepiece needs to be 7" above the bottom of the slit when pointing to the horizon with no aperture blockage. If the depth of the roof is 4" then from the top of the doorway to the eyepiece will be 3' 5". For a doorway height of 4' 4" the eyepiece would be almost 8' from the floor when observing at low angles. Since this doorway height was considered the minimum acceptable for access by the general public the following adjustments were made
      - Aperture blockage of 50% accepted when pointing at the horizon
      - Roof joists kept clear of doorways
      - Dome collar height reduced by cutting the sections in half and fitting the lower sections inside the upper ones, bolting them together with stainless steel screws.
The result was the height of the telescope declination axis being about 72" above the floor level. Taken with the flexibility in eyepiece level provided by the diagonal this has meant that most adults can observe at most telescope positions while standing on the floor. A small stepladder is used when this is not possible.
Pier and pier base - The base for the pier, a 700mm rough cube of concrete, was cast independently of the 100mm deep base of the observatory. It has reinforcing bars up into the pier itself which is a 450mm diameter cylinder of concrete. A test of two people jumping on the floor showed no transmission of vibration to the telescope mount. When the floor slab of the observatory was cast a cable channel from the wall to the pier was created.
Reason for two doors - The decision to have two doors was based on the use of the observatory for public events. Firstly, it was thought some people might find it claustrophobic in the dark and with only one door that one has to stoop to get through. Also, when trying to get significant numbers of people through the observatory two doors would enable a circulating flow. Experience in practice has proved the usefulness of having the two doors although the second door is not used for many of the events and was probably not essential.
Internal furnishings - A large piece of used carpet became available to cover the floor and to wrap round the pier column. This provides not only a good apearance and warmth but also gives a softer landing to any pieces of equipment dropped. A small office desk, with shelves constructed on it provides adequate desk and cupboard space for two computers and for eyepieces, CCD camera and other equipment. The desk is located in the space not under the dome. There are several chairs and stools and there is sufficient space to store these outside the dome area when more than about 6 people are expected. The most that can be comfortably accommodated for observing in addition to the operator is about nine.
Lighting - We were fortunate that one of our members was able to supply and fit a tailor-made LED lighting system consisting of both red and white LED strips which are mounted under the rim of the dome around the complete circumference. The LEDs themseves cannot be seen for eye level above about 1.4m. The controller allows selection of any desired combination of the colours and to vary the brightness to any level from maximum to off.