"Observatory Design"

Dr. Frank Melsheimer, DFM Engineering, Inc. Longmont, Colorado, USA

See Observatory Design Services Here

I.  Introduction

II.  General Pier Considerations
  a. Location or Placement
  b. Construction
  c. Structure
  d. Vibrations
  e. Other Considerations

III.   Observatory Construction / Planning
  a. Observatory Floor Layout
  b. Access to the Observing Floor
  c. Handicapped Access
  d. Lighting, Power, Communications
  e. Ventilation / Thermal Control
  f. Shelter Comparisons (Dome vs. Roll-Off)

IV.  Control Room
  a. Location
  b. Access
  c. Features

V.  Telescope Choice
  a. Required Performance Features
  b. Instruments

VI.  Appendix
  a. Definition of Terms


The design and construction of the observatory pier, dome, and control room for a small college observatory is discussed.

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.


An 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 dome.

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 alignment.

Pier Construction

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.

Pier Structure

NOTE: The important deflections are NOT the translations, but are the top end rotations of the pier and the torsion of the pier.

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 stiffer.

  b. Resulting torsional stiffness of the pier needs to be 30-lbf-ft per arc second (azimuth stiffness).

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.

Pier Vibrations

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 pier.

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 mass concerns.

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.


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

For 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 dome walls.

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.).

Handicapped Access

Many observatories have a requirement for handicapped access to satisfy the Americans with Disabilities Act (ADA).

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 ARE-125™. 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.

For example:

  • 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, etc.

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 testing purposes.

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 system.

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.

< p>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

    • Needs to Point very well - 30 arc seconds RMS or better

    • Needs to be very stiff so the observers don't move the telescope and image around when looking through the telescope

  • 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 access

  • Efficient GO TO so minimum time is spent finding the next object

Necessary Instruments:


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 feet away.

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 Area Network.

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 or unattended.