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The Determination of Proper Observatory Dome Size

This worksheet will help you select the diameter and other dimensions of your HOME-DOME or PRO-DOME observatory, based on your telescope and the way you plan to use the dome. You can use this sheet in three ways:

    1. Use suggested Sizes. We've provided a table showing typical dome sizes for several common telescope types and sizes.

    2. Do-it-yourself. We give directions to estimate the size dome and walls you will need.

    3. Send it to us. Write down all the information requested in the tables below and then send the data to us for suggestions.

We emphasize that the design of your observatory must reflect your particular needs and values. The estimates on this sheet do NOT take into account special requirements for institutional users, including the need for handicap access ("The Americans with Disabilities Act") or the desirability of relatively high walls. If you have any doubts then please call or write to us for assistance in planning your observatory. And do not forget to download and read the Technical Innovations book At Home In a Dome (in Adobe "pdf" format), which will help you evaluate various observatory design issues.

The diameter and wall height of the observatory depend on the physical space needed by the telescope and the people using it. These include:

  • The height of the observatory wall should be low enough so that the telescope can see down to the horizon.
    It must be high enough for the telescope to swing up to zenith without touching the dome.

  • The dome diameter must be large enough so the telescope can point in all directions.

  • The dome diameter should allow at least two feet around the telescope for a single user, but more if several persons or substantial equipment will be present.

1. Suggested Sizes. This table shows several of the more common telescopes and mountings, and gives suggested dome and wall measurements for each. The two columns on the right give recommendations for two cases: (1), only one user most of the time and little need for equipment or work space, and (2), if groups use the dome, or if there is need for work tables, storage, or equipment space. In either case, the wall heights will be the same.

Telescope Type Mounting Typical Total
Wall Height
Of Extra
Suggested Dome Diameter
One User Multi User
Schmidt-Cass up to 10 in. or Refractor less than 36 in. long Yoke or German 3-4 ft. 2-3 6 ft. 10 ft.
up to 16 in.
Yoke or German 3-4 ft. 2-3 10 ft. 15 ft.
Newtonian up to 14 in. German 3-4 ft. 2-3 10 ft. 15 ft.
Newtonian over 14 in. German 4 ft. 3-4 15 ft. 15 ft.
Yoke 1-2 ft. 0-1 dome dia. = 2 x tube length
Refractor tube
length up to 5 ft.
German 5-6 ft. 4-5 10 ft. 15 ft.

2. Do-It -Yourself. Using the telescope cartoons at the bottom of this page, identify the type of mounting that fits your situation. If you can set up your telescope and mounting, then measure the dimensions listed below. If you cannot set up the scope, then use drawings from the manufacturer or photos to estimate the dimensions.

The measurements you will need are:

D Optical Diameter of the Telescope (objective or mirror diameter)

L Physical length of the telescope, excluding the eyepiece holder

W Height above the floor of the lower edge of the scope when the telescope is horizontal

H Maximum height above the floor when the telescope is vertical

The total wall height will be equal to W if you wish to see down to the horizon. Because the PRO-DOME and HOME-DOME models include a one foot high base ring, then the additional wall needed will be W minus one foot.

The radius of the minimum size dome necessary to accommodate your telescope will be H minus W. Multiply by two for the minimum diameter.

The actual size of your observatory must allow for people as well as the telescope. The observer and visitors must be able to move around without bumping the telescope, and space is needed for a storage cabinet, electronics, table, etc. If the telescope is a Newtonian, space is needed near the "top" end for the person's head at the eyepiece, and floor space is likely needed for a ladder of some type. If the observatory will usually have only one person inside, add three to four feet to the minimum dome diameter needed for the telescope. This provides 1-1/2 to 2 feet for your body on any side of the scope. With less space than that, you increase the risk of bumping the telescope while moving around, and you will have little space for working. If the observatory will frequently contain groups of people, you will need even more added diameter (more so in cold weather).

Your final dome diameter equals the minimum diameter plus allowance for people.

Example. Assume the minimum dome diameter for your telescope is five feet. If several people will frequently be present in the dome, you might allow three feet on all sides of the telescope. Three feet on each side adds six feet to the diameter. Thus, the total diameter needed is five plus six or eleven feet. This shows that a ten foot diameter dome will be a little on the tight side, while a fifteen foot diameter will provide more than sufficient space. The choice between the two will then be a matter of your own judgment.

3. Send it to us.We can advise you on dome size if you copy the table below, fill it out, and send it to us. We will review the information, and make recommendations for your observatory. If your telescope, mounting, or use is relatively unusual, or if the telescope will be a tight "squeeze" into the dome, we may need to talk to you in more detail, and may carry out additional analysis to help you in your decision.
Brand of Telescope and Model
Type (Newtonian, Refractor,
SCT, Dobson, etc.

Physical Tube Diameter
Type of Mount
Number of persons usually present
Types of Use (visual astronomy, photos,
CCD, groups, research, etc.

Amount of extra equipment present

Three most common consumer telescope mountings (18,273 bytes)
Above: The three most common designs of consumer telescope mountings. (18,273 bytes)


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