The design and construction of the observatory pier, dome, and control room for a small college observatory
This includes a suggested floor plan, elevation plan, control room location, traffic flow patterns, and
other factors. These criteria are discussed in respect to how they affect the efficiency of using the observatory
for student use, research use, and for public nights.
The required performance of the telescope, instruments, and related auxiliary equipment is considered.
Many observatories are
designed ignoring the actual use of the telescope.
With over 30 years of experience in small college observatory design and telescope manufacturing, we
will discuss telescope access, visitor flow, and optimal seeing conditions as well as considerations for
structural techniques, materials implementation and practical applications in the design process.
The observatory and telescope will be used for education, training of students to use research telescopes
and instruments, public outreach, and for public visitors.
GENERAL PIER CONSIDERATIONS
isolated concrete pier running all the way from a suitable footing below grade to the sole plate of the
telescope pedestal is the best solution. Isolation must be maintained.
This includes any conduits between the building and the pier.
The pier needs to be offset to the south the proper amount. Rarely will the pier be in the middle of the
height of the pier affects the convenience of using the telescope.
For enlarged versions, click on drawings or links below:
Pier Location or Placement
The pier is normally offset to the South of the dome center line (in the Northern hemisphere). The pier
needs to be centered East to West and the rotational alignment of the pier and the pier bolts MUST be TRUE
North-South (Celestial North).
The pier height is relative
to the dome horizon line and should be set to allow an unobstructed telescope horizon at 7 to 10 degrees
above the horizon.
In order to provide vibration
isolation, there should be adequate clearance allowed between the pier and the floor.
Also, provide vibration isolation between building machinery and the floor so as to minimize vibrations
induced into the building. Locate building machinery as far away from the pier as possible.
Provide separate foundations or footings for the pier and for the dome walls.
The telescope manufacturer must supply a “Pier and Dome Requirements” drawing showing the
pier offset and height relative to the dome horizon line and the recommended observing floor height.
NOTE: It is imperative that the institute’s person in charge of the new observatory check the
azimuth alignment before proceeding with project development.
It is more than likely that the building contractor will be unaware of the significance of this critical
Most often the pier is
made from reinforced concrete. A large footing is poured with a column coming up from the footing.
The upper part of the column can be hollowed out to reduce the moment of inertia and the thermal mass.
Steel piers can be made, but they tend to be much more expensive than concrete.
An offset can be built
into a steel pier to transition from a concrete footing or column to the sole plate for the telescope.
A pier running from a footing well below grade is much better than using the structure of the building.
In general, buildings constructed from concrete (slab on columns and beams) are much stiffer than steel
framed buildings and many observatories have been successfully built using the building structure rather
than a separate pier.
Many observatories have been less than successful when using the building structure of a steel building.
NOTE: The important
deflections are NOT the translations, but are the top end rotations of the pier and the torsion of the
This is because the telescope is looking at an object at a distance of infinity. If the telescope simply
translates, the image does not move in the field of view.
Any rotation does produce image motion. Motions as small as 0.1 arc second are detectable.
1. The Tip-Tilt and rotation in azimuth stiffness must be very high
a. For a load applied to the eyepiece, the stiffness needs to be 30 lbf per arc second or
b. Resulting torsional stiffness of the pier needs to be 30-lbf-ft per arc second (azimuth
2. Natural frequency
with telescope installed should be greater than 30 Hz.
3. The South bolt typically has a considerable upward directed force.
4. Concrete material should be used because it has some internal damping.
5. Adding damping is difficult due to the very small amplitudes involved.
Seismic vibrations will
couple into the telescope pier. Nothing can be done to mitigate these. Fortunately, seismic vibrations
are small and don't occur often enough to affect the telescope.
Building vibrations from Heating, Ventilating, Air Conditioning (HVAC) machinery are a major concern.
Most buildings support the HVAC machinery with vibration isolators, but then defeat the isolation by
using rigid electrical conduits.
Vibrations from the building
elevators may also couple into the pier. Even the dome rotation can couple into the telescope.
The problem comes from supporting the telescope from the building structure and not an independent, isolated
Other Pier Considerations
Large diameter conduits
are usually run through the pier to provide cable runs. Optimally, the pier should provide multiple large
conduits for telescope and instrumentation wiring.
This would include at least 4 inch diameter conduits with outlets and inlets in convenient places.
It is not possible to have too many or too large of conduits for telescope instrument control cables.
Insulating the outside surfaces of the pier immediately below the telescope will help mitigate thermal
Conduits exiting the pier must be cut so there is an air gap so the conduits will not conduct vibrations
from the building into the pier.
OBSERVATORY CONSTRUCTION AND PLANNING
To achieve good seeing,
the observatory needs to be operated at the outside air ambient temperature.
This requires minimum heat generation, good ventilation, insulation, and low thermal mass construction.
It is important to plan thoroughly in achieving an optimal observatory.
Additionally, considerations for floor space planning, visitor access, handicapped access, visitor flow
and safety, lighting, power, future expansion of instrumentation, communications and maintenance must all
be adequately be addressed.
Observatory Floor Layout
The observatory should
be designed to be operated from an air conditioned control room. Auxiliary controls can be provided to
operate the telescope from the observing floor. Also, the observatory floor should be of low thermal mass.
The prime working space is the quadrant to the North of the telescope whereas East and West quadrants
are less used floor space.
The height of the observing floor relative to the telescope should be set for comfortable viewing. For
an observatory that is used for the public, this height is important. The proper value depends upon the
size and configuration of the telescope and the intended users (children would benefit from a higher floor
height, for example).
The observatory floor may require a hatch to allow lowering the primary mirror in its crate to a lower
level with access to a loading dock as the telescope mirrors will require periodic cleaning and re aluminizing.
Also, the floor may require a flush mounted lift table for handling large instruments and the primary
mirror and its cell.
Access to the Observing Floor
small and moderate size telescopes, the observing floor height will usually not allow a full size door
between the floor and the ring beam that supports the dome.
Entry to the dome using a full size door will be at a lower level than the observing floor requiring some
steps up to get to the floor.
These steps should be located in the South-West or the South-East. Usually the steps can’t be located
in the South because they would interfere with the pier. Sturdy handrails are needed at the stairwell.
Entry into a dome housing
a small to moderate telescope will require a small landing and then steps up to the observing floor.
Entry from the South is
preferred with the steps spiraling up along either the South Eastern or South Western quadrants of the
The upper end of the stairs will terminate near the West or East quadrant.
In either case, when entering the observing floor area, an air lock consisting of a short corridor with
two doors is essential.
The observatory must provide for easy access to move the primary mirror and cell into and out of the observatory.
This may require a hatch in the observing floor and a suitable hoist.
These are a few safety and practical suggestions for observatory floor access:
Do not use a short height door to enter the observatory.
Do not use a tight spiral staircase.
Do not use a stairway through a trapdoor or a hatch.
Incorporate a plan for an alternate exit to the roof or to the outdoors.
Insure that emergency exits are not blocked.
If the exit is to the roof, then roof safety measures must be taken.
All exits need appropriate lighting (downward directed, red, etc.).
observatories have a requirement for handicapped access to satisfy the Americans with Disabilities Act
Some observatories have wheelchair lifts that allow access to the observing floor.
Only a few observatories
actually meet the ADA requirements by providing handicapped access to the telescope eyepiece.
DFM Engineering, Inc.
offers an Articulated Relay Eyepiece model
It allows true handicapped access to the telescope.
A person seated in a wheel chair may simply pick up the eyepiece and bring it to their eye for convenient
viewing regardless of telescope position.
The images here show several wheelchair lifts that can be used at an observatory to provide access to
the observatory floor.
Dome Lighting, Power and Communications
White and red lights are needed throughout the observatory, stairways, and safety exit walkways.
White lights allow working on the telescope and instruments. Red lights are needed when operating the
telescope particularly during public nights.
All lights must be on dimmers. Provide many duplex power outlets on the dome walls for instruments and
auxiliary equipment using several separate circuits.
A telephone with sufficient cord to reach to the telescope is needed to allow communications when working
on the telescope or instruments. Provide wiring or fiber for high speed data communications (e.g.. CAT5e
or Cat6 cables).
Ventilation and Thermal Control
Minimum heat generation
is provided by moving as much of the electronics and people out of the dome as possible and placing these
items in the control room.
A telescope can be made that dissipates less than 20 watts while a person at rest dissipates about 150
to 200 watts, so the primary heat source during observations can be the observers.
At a major observatory, equipment heat generation can be the largest source of heat.
In some climates, air
conditioning and/or dehumidifying of the observatory can be beneficial. To improve the seeing, many major
observatories air condition the telescope primary mirror because the mirror has a large thermal mass.
The optimal temperature of the telescope and the inside of the observatory should be the expected night
time seeing temperature.
Powered or forced air
ventilation should be provided. The amount of airflow should be equal or greater than 3 telescope and observatory
masses per hour.
The telescope can weigh 4000 pounds (20,000 Newtons) and a concrete floor can weigh twice this amount.
The building walls and other structure can also be very massive.
This is why the observatory should be made with metal sided, steel type construction and the floor should
be wood or aluminum. Concrete/brick construction should be avoided.
With a low mass construction,
the total mass can still be 5 tons requiring about 7,000 cfm (cubic feet/minute) of air flow.
A typical 20-inch window box fan flows over 4000 cfm with no restriction, so the ventilation can be provided
by a relatively modest fan or fans. Wind produces excellent ventilation if entry and exit areas are correct.
The flow should suck air in through the dome slit and exit near the base of the observatory walls-preferably
down wind and across the floor.
The air should be discharged
down wind and in a broad, diffused manner. This usually requires different fans, so the observer can choose
which way to diffuse the air.
Locate the building vents and heat discharges as far away from the observatory as possible. They should
be down wind (for prevailing winds, anyway).
All vents should be as diffused as possible. The building vents could be located to the North as this
part of the sky is not a prime observing area.
Infiltration is usually
not a problem with a modern domed observatory. However, the dome must be air locked so when the door to
the dome is opened, there is minimal air exchange between any conditioned building space (including the
control room) and the observatory dome space.
It is essential to prevent warm building air from entering the observatory and damaging the seeing.
The East, West, and South walls of the dome should be insulated. The dome walls should be insulated on
the inside so they won' become a major heat source during the night.
While the dome should be insulated, it typically has low thermal mass and a lot of surface area, so its
time constant is short.
Shelter Comparisons (Dome or Roll-Off Shelter)
Inevitably, a dome is
the better choice between the two options both in cost and weather protection criteria.
Observatory “roll-off-roofs” typically leak when it rains. This causes an obvious need for
a floor drain and possible dehumidifier considerations.
The roll off roof may provide better seeing because of better ventilation. This is its only advantage.
In the college environment, the stray light in a roll off shelter will be a greater problem than for a
dome. This type of shelter provides much less screening from stray light than the dome option.
A roll off shelter is thought to cost less than a dome. This is almost always NOT TRUE.
There are always other factors to consider when making this type of cost comparison.
Maintenance of the telescope is complicated because the telescope is usually stored nearly horizontal.
The telescope must be moved to the storage position in order to close the shelter.
The shelter still needs a control room.
Personnel safety is not as good as a dome.
Each roll off shelter is a “one of a kind”, so inherently has many bugs and inconsistencies.
Observatory domes have been developed over many years and have been engineered to accommodate the multiple
considerations necessary in order to alleviate viewing distractions.
An observatory dome, as opposed to a roll-off shelter, enables the viewer to eliminate the focus on the
observatory shelter and instead focus on the observing.
Therefore, a dome is recommended as the preferred shelter option and as big as you can afford.
Control Room Location
The control room should
be located within 125 cable feet of the telescope pier. The minimum recommended size of the control room
is 100 square feet. 200 square feet or larger is preferred as the control room will need to accommodated
multiple desks similar to a multi user computer lab atmosphere.
Many observatories place the control room to the north of the telescope and have a window looking into
the observing area. Typically, the windows are covered with an opaque material and are seldom used.
Another option is to have an inexpensive closed circuit TV looking at the telescope with a display. This
is less expensive than the window approach. The control room may be located at any convenient location
and is often located one floor below the telescope.
Control Room Access
It is nice to have a door going outside from the control room to where the sky may be checked for clouds,
The control room must
be air locked so when the door to the dome is opened, there is minimal air exchange between the control
room and the dome. This provides greater thermal control in the observatory dome in an effort to keep the
observatory dome at seeing temperature.
Access to the control room should allow for heavy and bulky instruments to be brought into the room for
Control Room Features
Large conduits should be available between the control room and the telescope. These should be 4 inches
in diameter or larger. The conduits should be sealed off with foam rubber to prevent air flow from the
control room to the dome.
A telephone with sufficient cord to reach anywhere in the control room is needed to allow communications
when using the telescope or instruments.
There will be at least
4 PC type computers with displays in the control room. Wiring or fiber for high speed data communications
should be provided as well as a duplex outlet (15 amps) on a separate circuit for the telescope control
Many power outlets along the wall need to be provided.
The control room environment needs to be air conditioned to an office environment and provide ample desk
top area with cable pass through holes similar to a multi user computer laboratory.
It needs to be equipped with white and red lights - all on dimmers with special consideration to additional
lighting needed to illuminate computer keyboards. It is suggested that a low power overhead track lighting
(with dimmers) would offer sufficient keyboard lighting.
Too often a telescope
is chosen with insufficient performance. The telescope pointing, stiffness, and access should be considered.
The telescope should be the best the institute can afford to provide that maximum usage when the weather
conditions permit observing.
The observers should not have to fight the telescope, but the telescope should be easy to use and very
reliable so the observers don't waste the few hours they are in the observatory.
An equatorial fork mount Cassegrain telescope provides easy access to the eyepiece and instruments with
minimum eyepiece sweep and minimum need for observing ladders.
Required Telescope Performance Features
Telescope Required Performance
Floor height set relative to the telescope for maximum convenience
An Articulated Relay Eyepiece provides maximum convenience for visual use of the telescope and handicapped
Efficient GO TO so minimum time is spent finding the next object
The meanings of the words “local” and “remote”
have changed during the past few decades.
These words and several others are defined below:
Definition of terms:
Local: Local control means occurring from the control room. Most modern observatories are now operated
from a control room (or warm room) not located in the same space as the telescope and instruments. The
telescope and instruments are controlled by the operator interfacing to a computer.
Remote: Remote control means operating away from the control room. It may mean operating from the
observatory floor, which is often done for public nights, for example. Remote may also mean operating or
observing using dedicated cables from a distance such as from a planetarium hundreds or a few thousand
Far Remote: The proposed meaning of this phrase is to indicate operating or observing from a distance
where dedicated cables are not used. For example, the observatory could be controlled over a campus Local
Internet Access: The proposed meaning of this phrase is to indicate operating or observing from a far
distance where the communications between the observatory and the user is performed over the internet.
Remote Observing: This phrase indicates observing from a location other than the control room or the
observing floor. Also see Internet Access. This phrase has two distinct operational modes defined below.
Unattended Remote Observing - Attended Remote Observing: Remote observing places considerable demands
upon the hardware. If all of these requirements are totally automated, the observing may be performed without
human intervention (Unattended). If some of these requirements are performed by an attendant, and some
are automated, the observing is combined (Attended).
Robotic Telescope: A robotic telescope accepts commands from another controller. Most modern professional
and many amateur telescopes may be considered to be robotic.
Robotic Observing: This phrase usually means that the telescope and its instruments are being commanded
to perform routine observations that have been preprogrammed. Such observations may be performed attended
For additional information, please see the following links:
Engineering Articles for the Optimal Telescope
How to Buy a Telescope
Internet Telescope Performance Requirements
Comparing Telescope Drive Technologies
US Naval Observatory 1.3M Telescope