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More Thoughts About Observatories

These thoughts contain a summary of some of the topics that need to be considered when building a personal observatory.  They represent those things that came to mind while I was considering a new observatory and include some of the ideas and concerns expressed by other member on the MAPUG-Astronomy list.  All conclusions presented here are my own.  For some more detailed discussion of thermal effects see this link (Observatory Thermal Discussion)   For conciseness, some of the following arguments have been shortened.  For more detailed discussion of some of these issues see (Observatory Design Discussion Addenda)   There is also a section on Control Room concerns. (Control Rooms)

Note: this discussion is about basic considerations for an observatory and does not include detailed construction topics. Such topics will be added in addenda to this discussion from time to time. There discussions are at (Observatory Construction Details)  Much detailed information on the design of buildings and piers is also available on the web in the Mapug-Astronomy Topical Archives at:  <www.mapug-astronomy.net> (This is Ed Stewart's fine Mapug-Astronomy site.)

Considerations for the Design of an Observatory

Climate:  The type of observatory required is highly dependent upon the climate.  This is an obvious fact.  However, in any climate, the purpose of the observatory is to protect the equipment, allow the equipment to be used easily and to provide an environment which provides for the comfort of the observer as much as possible.  If the climate is harsh, the observatory must be waterproof, snow proof and free of leaks which might allow damage to the astronomical instruments.  At the same time, the structure should be designed in such a way so it can be brought into use in a short time.

Creature Comforts:  A major element of the observatory design should be the comfort of the observer.  For normal visual work, it is necessary for the observer to have direct access to the telescope eyepiece and thus the observer will be at the same ambient temperature as the telescope. However, protection from wind and radiation to the open sky can give the observer some comfort from the worst of the elements in cold viewing conditions. There are a number of "very local" accessories that can help greatly to keep the observer more comfortable.  Among these are, a heating pad for the seat or placed on one's lap. a heating pad onto which the eyepieces are placed so they stay a bit warmer and will not be so ready to accumulate dew when they are used. When instruments are used with the telescope in such a way that an observer does not have to be continually in attendance, it is desirable to have a two or more part structure.  One part can then provide the best conditions for the telescope and its auxiliary equipment and another part can provide for the comfort of the observers/operators. The observer/control area can easily have several functions as well.

Types of Telescopes:  The observatory must be designed to be large enough so that the telescope or telescopes can be used easily and so that auxiliary equipment is conveniently located, accessible and protected.  Within reason, if the observatory is large enough, it can accommodate a variety of telescopes.  Remember, of course, that refractors on GEMs take a considerably larger area than fork mounted Cassegrains because of their design and mode of use.

Types of Observing:  Direct visual observing is interesting for the novice and should be made possible with the observatory set up. However, more and more the purpose of the serious amateur is photography, imaging, photometry, spectrometry and other activities which require instrumentation at the telescope other than the human eye.  If one type of use dominates, t is desirable to optimize the observatory for that use even if it makes a more minor use a bit less convenient.

Automation:  Automated, robotic and even remote control and observing are becoming more and more common.  Thus the observatory should be designed so that it can accommodate these function or can be modified easily to accommodate them.  The telescope for remote or robotic use will of course have to have its own computer control. The housing for the telescope will, in the most extreme case of remote use, have to be designed in such a way that the building can be opened, set up for use and put to rest at the end of a session without local human intervention.  For the intermediate case, of automated use, but with a local attendant, the building can be much less automated.

Observatory Quality Factors

Space:  There might need to be enough space for just one observer and possibly for several observers who take turns at the eyepiece of the instrument.  This is necessary only if the telescope is to be used for visual observing and for demonstration of viewing to a group. For the more advanced user, the desirability of having multiple person access to the instrument becomes less important when quality personal observing is the primary concern.   In this case, principle attention has to be paid to proper space for use of the instrument and its accessories by one individual.

Sky View:  There must obviously be a satisfactory view of the sky for the instrument.  There are two major types of sky view provided by observatories.  One is the view provided by a traditional domed building with a shutter opening for the instrument.  The other is the so called roll open building which is in most cases provided by a roll off roof.  The domed building has some disadvantages of cost and requires some level of automation for either remote or local robotic use.  The roll off roof has the advantage of relatively low cost and full exposure of the instrument to the sky.  These factors are discussed n more detail below.

Thermal Considerations:  Thermal considerations are very important since they affect the comfort of the observer, the behavior of the telescope and instruments as well as seeing factors such as dewing and disturbances in the visual air path. There are competing considerations regarding thermal factors.  In the one case, it can be argued that the telescope and its associated equipment will come to thermal equilibrium with its surroundings more quickly with a fully open roof.  Contrarily, the domed building needs more time to come to thermal equilibrium and there is a greater possibility of air disturbances do to mixing of ambient outside and internal air at the shutter which might spoil seeing.

A second part of the thermal equilibrium equation is what happens after some time has passed.  The totally open structure exposes the instrument to the sky and by radiation the instrument and everything else in the building cools to a temperature lower than the ambient air and collects dew. The amount of dewing depends upon the dew point relative to the ambient temperature upon wind conditions.  A still air condition allows dewing to build up to a situation where condensate actually runs and form pools of water.  A significant wind will dissipate light dew but the wind in turn can shake the telescope so as to spoil viewing as well.

In the case of the domed building, the telescope radiates to the dome which is very nearly at ambient temperature.  The temperature of the dome will be at actual ambient temperature or possibly slightly cooler since it too radiates to the sky.  However, the dew in this case forms on the dome and tends to keep the dome at the dew point which is usually not that much different from ambient when dew is a problem and certainly not near sky temperature.  Telescopes, instruments and other items and surfaces within the dome almost never collect condensation.  With domed observatories, it is very useful to have an exhaust fan near the base of the domed structure to force the temperature of the dome and inside air to be near the ambient temperature as quickly as possible.  This exhaust fan will also remove the heat do to the observers in the building. This heat can raise the internal temperature of the building enough to cause deterioration of viewing do to heat current eddies through the shutter opening.

Versatility:  The design of the observatory must accommodate the telescope installed but should also allow for installation of a different or larger instrument and accessories that might be needed in the future. The totally open design is more versatile in that almost any type of instrument can be set up as long as space and headroom allow.  In fact, if there is enough floor space, several instruments can be operational at the same time.  The dome limits the multiple instrument option unless the instruments can be mounted on the same pointing platform and see through the shutter opening.  With the dome, some added thought has to go into the placement of the pier and the instruments on it.

Conclusions:  These conclusions are based on at the design of a building in Wisconsin which is in a cold/hot, dry/wet, calm/windy location.   The telescope is) will consist of several instruments mounted on a single pier/pointing platform. The use will be almost entirely for photography/imaging and possibly other instrumented use such as variable star measurement or solar studies. Creature comforts are essential since the principle observer/operator is too old to want to get his eyelashes frozen to the eyepiece.  The telescope/instruments will be automated, but run locally (not run remotely) at this time.

The building will be a two part structure 11 by 22 feet in size with a partition separating the control room from the telescope room. The dome will be a Pro Dome 10' in diameter with full automation. (I would really like an Ash dome but they are just a bit too expensive for me) The control room will be both heated and air-conditioned for the comfort of the operators.  There will be a single pier with a large pointing platform that will take a variety of instruments.  The most likely candidates at this time are a 14" Celestron Faststar,  Meade 10" CAS, Celestron C-5 and several large APO photographic lenses. (I have all but the 14" which looks very good to me because of its versatility)

The decision to have two rooms, a control room and observatory room, was not difficult. The control room can be held at a temperature suited for computer equipment on a long term basis and brought quickly to a comfortable temperature for the operators.  The control room will be well insulated and connected to the observing room by a common insulated wall and thermal window.  An insulated pocket door will connect the two rooms.

The decision to go with a conventional domed structure was the most difficult and deserves some explanation.  After reading the many opinions recently posted on Mapug-Astronomy and after considerable other reading and calculation of various factors, I came to the conclusion that the conventional dome is the best for the following reasons:

  1. Provides the best protection against weather conditions. i.e. can be well sealed against blowing snow and heavy rain as well as high winds and dust.
  2. Provides best protection for the observer from radiation to the night sky.  This is particularly important in winter when skies are often very clear in Wisconsin.
  3. Provides the best conditions for eliminating dewing on the telescope and all of the other instruments in the observatory. An open venue allows for condensate to form on everything in the observatory.  Cases, shelves, anything on the shelves, instruments, telescopes even canvas chairs and tables in an open venue get wet.  The dome prevents all of these problems.
  4. Has no disadvantage to viewing do to air mixing if the right sort of forced ventilation (exhaust fan) is provided.  This requires the shutter to be opened sometime before use and significant forced ventilation to be employed to bring everything to ambient and hold it at ambient.
  5. Provides the best protection against wind for both the operator and the telescope.  Even a slight wind can cause considerable operator discomfort.  And as we well know, wind spoils imaging very easily. An additional wind screen can be easily installed in the shutter opening. It is even possible to consider adding a Lanphier style shutter.
  6. Can be easily fully automated with commercially available equipment, especially from Home Dome.

I  welcome any discussion and arguments or counter arguments anyone has to offer.  Special thanks to John Menke of Technical Innovations for discussion and input to this analysis.
 

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