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The case for an observatory...dome that is.

(the advantages of observatories and domes in particular)

by Doug Kniffen

geodesic domeEvery amateur astronomer should have some sort of observatory. The reason is time. Most people are so busy that it isn't possible to find much more than an hour to do some observing. Even in an urban area an observatory, with all necessary equipment ready for use, will provide new opportunity to observe.

There are two primary advantages to a dome compared to other types of personal observatories; environmental moderation and, consequently, improved deep sky observing ability. The later benefit is the most important. This particular dome advantage is realized through greater dark adaptation. While obvious to the urban amateur, few observers are aware that even the darkest rural sky will limit the degree of your low light perceptive ability. The stars themselves will prevent full dark adaptation. In a rural location, a dome will facilitate observing extended objects a full magnitude fainter than possible with the same instrument used in a run-off roof type structure. The dark adaptation benefit would be greater at an urban site.

A well designed dome will both moderate the wind and reduce nocturnal dampness. Not all domes adequately accomplish these two tasks. Since I have not had the opportunity to observe within one of the newer plastic domes, some of the succeeding comments may not be relevant when applied to this type of material.

If thermal equilibrium, with ambient conditions, is achieved then metal domes can become uncomfortably damp and cold, especially on nights with little or no wind. Despite the maintenance and longevity considerations, wood is arguably a superior choice of material for amateur domes. The personal comfort factor may be the most important, but the drastically lower cost of material and requisite tools should not be easily dismissed. My whole observatory (pictured) project cost less than $2000 dollars over the winter of 1992/93.

Most small observatory buildings, not just domes, are not well designed regarding ventilation. If inadequate airflow is a problem, both temperature and humidity will become higher than the outside air during the day. In addition to the potential problems of perpetually damp equipment, it may take many hours to cool off the building at night. Such local disturbances to seeing don't impair visual observation very much, but they may become important if imaging celestial objects. My dome was deliberately designed to provide adequate ventilation. Only rarely does the temperature difference exceed 4 degrees and, despite wood construction, the relative humidity difference doesn't vary by more 6% between the inside and outside of the building. Several times per year (very hot very humid days with absolutely no wind) I use electric fans to help cool the building at night.

With adequate ventilation, the building will be drafty, but wind is moderated. In my observatory, any breeze less than 5 mph will pass through relatively unimpeded. When outside wind speed exceeds 50 mph the draft inside is strong enough (10 to 12 mph) to blow papers off the desk and across the floor (the dome may also turn in the wind above 50mph).

Finally, local non-thermal turbulence can be caused by wind velocity. With an obstruction to airflow, such as a wall or dome, there is always a change in velocity relative to unobstructed air flowing nearby. This change will create swirls, eddies and shear layers that distort light passing through them. The higher the difference in flow velocity across a given area, the greater the distortion. A dome will cause less turbulence than a wall (run-off roof) at most wind speeds. There does exist a difference between domes at relatively high wind speeds. Informal experiments with scale models demonstrated the advantages of using a first level geodesically expanded dodecahedron. A conventional smooth dome will generate strong shear down wind due to the even acceleration around the dome. My particular dome was designed with a simplified geodesic expansion method that I developed (I removed 95% of the math, and 99% of the confusion compared to the original geodesic design method). An expanded dodecahedron offers an improvement over a conventional dome regarding turbulence caused by the building. This shape of the dome is round enough that it doesn't act like a sail, and pointed enough that it doesn't create as much lift as a smoother dome.

Doug Kniffen

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