DFM Engineering, Inc.
1035 Delaware Ave. Unit D
Longmont, CO 80501
Phone: 303-678-8143
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"Observatory Design"

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

Update issued: 03/2017
Permission granted for distribution only IF:
Title and Author's name remain and it is distributed at NO COST.

(These notes are for the Northern hemisphere. The Southern hemisphere will be slightly different.)


The meanings of the words “local” and “remote” have changed during the past few decades.

These words and several others are defined below in Definition of Terms:

1. Pier Placement

2. Pier Structure

3. Observatory Floor

4. Entry to dome

5. Dome Lighting Power, and Communications

6. Dome Alternate Exit

7. Dome Ventilation

8. Control Room

9. Communications

10. Dome vs. Roll Off Shelter

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. Unlicensed wireless (2.4 GHz.) Ethernet is now available that allows 11 megabits per second data transmission over a maximum distance of 20 miles line of sight.

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. The distance is indeterminate, the user may be anywhere in the world as long as they have access to the Internet.

Remote Observing
: Observing from a location other than the control room or the observing floor. Also see Far Remote and Internet Access. This phrase has two distinct operational modes defined below.

Unattended Remote Observing and 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, 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.


GENERAL NOTES: By Dr. Frank Melsheimer

1. Pier Placement

  • a. Offset pier to South of dome center line in the northern hemisphere. The offset depends upon the telescope and the latitude.

  • b. Pier needs to be centered East-West.

  • c. Azimuth alignment of pier and pier bolts MUST be TRUE North-South (Celestial North).

  • d. Pier Height relative to dome horizon line to set unobstructed telescope horizon at 7 to 10 degrees above the horizon. wiring-at least 4-inch diameter with outlets and inlets in convenient places. Pull boxes are needed where ever there are 90-degree direction changes in the conduit.

  • f. Clearance between pier and floor for vibration isolation.

  • g. Locate building machinery as far away as possible.

  • h. Provide vibration isolation between building machinery and the floor to minimize vibrations induced into the building.

  • i. Provide separate foundations or footings for the pier and for the dome walls.

  • j. When designing hand rails and placing equipment such as a wheel chair lift, remember that the telescope rotates so will require more clearance to the East and West.

2. 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.

  • a. The Tip-Tilt and rotation in azimuth stiffness must be very high.

    • 1. For a load applied to the eyepiece, the stiffness needs to be 30 lbf per arc second or stiffer.

    • 2. Resulting torsional stiffness of the pier needs to be 400 in-lbf per arc second (azimuth stiffness).

  • b. Natural frequency with telescope installed should be greater than 15 Hz.

  • c. South bolt typically has a considerable upward directed force.

  • d. Concrete material should be used because it has some internal damping.

  • e. Adding damping is difficult due to the very small amplitudes involved.

3. Observatory Floor

  • a. The prime working space is the quadrant to the North of the telescope.

  • b. East and West quadrants are less used floor space.

  • c. 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).

  • d. 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.

  • e. May require a flush mounted lift table for handling large instruments and the primary mirror and its cell.

  • f. Should be of low thermal mass. (Not concrete)

  • g. A drain in the floor is needed.

4. Entry to Dome

  • a. 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. Remember that the telescope rotates so will require more clearance to the East and West. The total height of the steps will be about 18-inches to 35-inches.

  • b. Do not use a short door to enter the observatory.

  • c. Do not use a tight spiral staircase.

  • d. Do not use a stairway through a trapdoor or a hatch.

  • e. Double doors opening to the outside will usually suffice to allow moving the primary mirror and its crate into and out of the observatory for mirror service.

5. Dome Lighting, Power, and Communications

  • a. White and red lights are needed. White lights to allow working on the telescope and instruments. Red lights are needed when operating the telescope particularly during public nights.

  • b. All lights must be on dimmers. LED lamps are recommended.

  • c. Provide many duplex power outlets on the dome walls using several separate circuits.

  • d. A telephone with sufficient cord to reach to the telescope is needed to allow communications when working on the telescope or instruments.

  • e. Provide wiring or fiber for high speed data communications such as Ethernet with access in dome area and control room.

  • f. Stairwells need light switches on both ends with dimmers.

  • g. All building AC power wiring must be of three (3) wires. Using conduit in place of the ground wire (the green wire) is insufficient. The power should be "clean" computer grade.

  • h. The building AC power wiring must include a heavy ground cable connected to a robust earth connection.

6. Dome Alternate Exit (To roof or outside)

  • a. Must not be blocked.

  • b. If to roof, then roof safety is needed.

  • c. Needs appropriate lighting (downward directed, red, etc.).

7. Dome Ventilation

  • a. Powered ventilation of the dome is required. A rough rule of thumb is three telescope masses of air per hour at an excellent site. More would be needed at a site where the night time temperature varies considerably from the day time temperature. For example, a 2000 lbf. telescope will need at least 900 CFM (cubic feet per minute) of ventilation air and twice as much would be better. The observatory thermal mass usually is much more than the telescope thermal mass, so much more ventilation flow is needed.

  • b. Suck air through the dome shutter across the dome floor.

  • c. Discharge air down wind. Usually requires different fans so the observer can choose which way to discharge.

  • d. Discharge the air in a broad, diffuse manner.

  • e. The dome must be air locked so when the door to the dome is opened, there conditioned space and the dome. This is very important.

  • f. For best dome "seeing", the dome should be insulated and air-conditioned to the expected night time temperature.

  • g. It may be necessary to dehumidify the dome during day time.

  • h. Locate building vents and heat discharges as far away from the (for the 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-wind permitting.

8. Control Room (Warm Room)

  • a. Minimum size is 100 square feet with 200 square feet better.

  • b. Large conduits between the control room and the telescope. 4-inch diameter or larger. Seal off conduits with foam rubber to prevent air flow from control room to dome.

  • c. A telephone with sufficient cord to reach anywhere in the control room is needed to allow communications when using the telescope or instruments.

  • d. Provide wiring or fiber for high speed data communications such as Ethernet.

  • e. Provide lots of desk top area with cable pass through holes similar to a multi user computer laboratory.

  • f. 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.

  • g. Needs white and red lights-all on dimmers.

  • h. Needs air conditioning to office environment. There will probably be 4 PC type computers with displays in the control room.

  • i. Provide a duplex outlet (15 amps) on a separate circuit for the telescope control system. Provide many power outlets along the walls with "computer grade" power.

  • j. Provide some form of lighting to illuminate computer keyboards. Perhaps low power (with dimmers) overhead track lighting would work. Illuminated keyboards are available for PCs.

  • k. Control room should be within 125 cable feet of the top of the telescope pier. 100 cable feet is better.

  • l. Access to the control room should allow heavy and bulky instruments to be brought into the room for testing purposes.

  • m. Can have a double paned window looking into the dome area with a pull down shade. Most observatories end up with the shade permanently closed.

  • n. Can have an inexpensive closed circuit TV looking at the telescope with a display. This is less expensive than the window approach.

  • o. It is nice to have a door going outside from the control room so the sky may be checked for clouds, etc. This door can aid ventilating the dome.

9. Communications

  • a. Ethernet appears to be the current Local Area Network (LAN) of choice for observatories. Instruments are being supplied that allow direct connection to ethernet.

  • b. The Observatory should be designed with ethernet cable (CAT-5e) runs between all conceivable points within the building.

  • c. Wireless ethernet 802.11b is an economical choice between the campus and the observatory. This can provide 11 mega bits per second (about 1 mega byte/sec) data transfer in both directions. Line of sight is required completely clear from even trees. Typically a 50-ft diameter cylinder of clearance is needed with the cylinder tapering down in diameter near the ends of the transmission path. The distance for one observatory we know about is about 35 kilometers with no intermediate relays.

10. Dome vs. Roll Off Shelter

  • a. The roll off shelter may provide better seeing because of better ventilation. This is its only advantage.

  • b. The roll off shelter is thought to cost less than a dome, but almost always this is not true.

  • c. The roll off shelter provides much less screening from stray light.

  • d. The roll off shelter almost always leaks when it rains. The floor needs to have a drain.

  • e. Maintenance of the telescope is complicated because the telescope is usually stored nearly horizontal.

  • f. You still need a control room.

  • g. Personnel safety is not as good as a dome.

  • h. The telescope must be moved to its storage position in order to close the shelter.

  • This is more difficult during a power outage.

  • i. The roll off shelter costs more than a dome (deliberately repeated).


For a detailed study of Observatory Design , please see:

Observatory Design by Dr. Frank Melsheimer


For additional,related information, please see the following articles:

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