DFM Engineering, Inc.
1035 Delaware Ave. Unit D
Longmont, CO 80501
Phone: 303-678-8143
Fax: 303-772-9411

 
 
 
College of Southern Idaho Centennial Observatory
AN UPDATE
 
 

The DFM Engineering, Inc. CCT-24 telescope was installed in April, 2004 at the Herrett Center for Arts and Science located on the north edge of the College of Southern Idaho (CSI) campus. Now, almost two years later, this report looks at the design of the observatory and also is an update describing the telescope activities since the installation. We review the things that were done correctly and those items that could have been done better.

The observatory director, Mr. Chris Anderson, and Dr. Frank Melsheimer, DFM Engineering founder, spent considerable time working with the observatory architects and engineers to produce an observatory that was free from building vibrations and to control the thermal degradation of the images (the dome 'seeing'). In a typical situation, architects, engineers, and general contractors have little to no experience designing and building an observatory. This was the case at CSI.

The telescope is located on the roof of a two story building with an observing deck for small telescopes located to the south of the telescope. This configuration is very common among college campus observatories.

There are several important design criteria for the telescope installation. These include:

VIBRATION:
The telescope vibration environment is a critical criteria. We have seen roof top observatories where the image quality and usefulness of the observatory are severely limited by vibration of the telescope pier. At CSI the telescope is supported on a 22-ft tall concrete pier with a rectangular cross section of 40-inches by 60-inches. The pier passes through the floors and ties into a 9-ft diameter footing. The footing goes down to the basalt bedrock and is completely isolated from the building footings. A gap is maintained between the pier and each floor that the pier passes through and no building components of any sort are attached to the pier. Between the outside diameter of the footing and the back filled soil, the footing is covered with a low density material to minimize the transmission of horizontal vibration waves. The change in density reflects the vibration wave rather than transmitting the wave into the footing. The top surface of the footing is also covered with the low density material to decouple any vertical or horizontal vibrations which could be transmitted from the first floor into the footing.

We have performed optical tests where we have hit the pier very hard and observed the resulting image motion. When very high loads are applied, we can just detect image motion and the image motion damps out in a small fraction of a second. In normal use, one does not see any image motion coming from the building, the dome rotation, or the nearby elevator. The performance of the pier was definitely worth the extra expense and long arguments with the architects, engineers, and the general contractor.

THERMAL IMAGE DEGRADATION:
The observatory thermal mass is a critical consideration since the building mass will slowly heat up during the day and release that heat during the night. The released heat causes a change in the optical index of refraction of the air within the observatory which causes image degradation. A low thermal mass will minimize the heat storage and reduce the time required for the observatory to quickly approach the outside ambient temperature. The thermal mass includes the actual mass of the materials, their specific heat, surface area, and may be reduced by insulating the surfaces of the materials. The time constant can be considerably reduced by ventilation.

Wind can provide natural ventilation if the building is designed with louvers in the proper places. However, even at the best sites, sufficient powered ventilation must be provided. At several observatories, refrigeration of the telescope and the observatory is provided to provide highly controlled thermal conditions.

The CSI observatory was designed with low thermal mass as a prime consideration. The walls of the circular observatory building including the steel dome support ring beam were fabricated using typical steel building construction. Square steel tubing was used for the columns and the siding was corrugated aluminum matching the appearance of the dome.

The observing floor is another area where low thermal mass is essential. The CSI observatory building does contain a concrete floor, but the observing floor is raised about 30-inches above the concrete floor forming a crawl space. To minimize the thermal mass, the observing floor is aluminum plate. The concrete floor underneath is covered with insulation and the telescope pier is also insulated in this crawl space. The observing floor is not a tight fit around the telescope pedestal. The gap has been filled with perforated metal to allow ventilation.

The dome was purchased with a layer of rubber foam insulation on the inside surface. Unfortunately, the walls of the dome support building are not insulated. The siding and the steel support structures do warm up on the East, South, and West sides during the day. This heat requires considerable time to dissipate and introduces image degradation. The architects were reluctant to insulate the inside surfaces of the siding citing condensation concerns. We believe that these surfaces need to be insulated.

Powered ventilation was designed into the observatory. Unfortunately, between the architects and the general contractor, the ducting for the fan has so much efficiency loss that the fan is ineffective. Plans are now underway to add fans located in the crawl space that will draw air through the open dome shutter and exhaust through louvers in the dome siding under the observing floor. The amount of ventilation needed at a good site is about one telescope mass of air per hour. We believe at the typical campus observatory that this number should be at least 4 times greater. The thermal mass of the observatory also needs to be considered. At a high thermal mass observatory, the ventilation is mostly cooling the building since the telescope thermal mass is a small fraction of the building thermal mass.

ELECTRICAL CABLE ROUTING:
The telescope pier was designed with internal conduits leading from underneath the telescope pedestal and exiting the north face of the pier. Telescope and instrument cables rout through the telescope structure, through the polar axle, and into the pedestal. The cables then enter the conduit in the pier and run to the control room. It is very difficult to pull cables through small diameter conduits, so the conduits should be at least 4-inches in diameter and 6-inches is better. Typical building conduits are less than 1-inch in diameter so the architects and general contractors are unfamiliar with providing conduits of the size needed for an observatory. You cannot have too much conduit cross section.

At CSI, the pier has conduits running from the top surface out through the north face of the pier within the crawl space. The cables then run across the insulated concrete crawl space floor and then through a conduit into the control room. The conduit in the control room wall is sealed off with some closed cell foam to prevent warm air from the control room passing into the observatory.

Any conduits entering the telescope pier must have a small gap where they enter the pier or they will transmit vibrations from the building into the pier. This is not a problem at CSI.

PEOPLE and TELESCOPE EYEPIECE ACCESS:
The primary function of the CSI observatory and planetarium is astronomical education. This involves observing at the eyepiece, CCD imaging, and some astronomical measurements. The observatory supports astronomy classes and public outreach. The observatory hosts many star parties open to the community that have been extremely well received by the people in southern Idaho. The observatory also provides full access to the telescope eyepiece for people confined to a wheelchair by using the DFM Engineering ARE-125 Articulated Relay Eyepiece and a wheelchair lift to the observing floor.

The observing floor is accessed by 4 steps from the east side of the observatory or by using the wheelchair lift. Visuals observers form a queue from the East towards the northern quadrant of the observing floor. As they approach the telescope, they are handed the eyepiece end of the ARE-125 where they may observe the object by looking in a convenient direction. They then pass the eyepiece to the next one in line and walk West and South around the telescope and exit down the stairs. This traffic pattern coupled with the ease of observing using the ARE-125 allows a large number of visitors to see many objects on a typical public night.

Small groups may be seated in a semicircle of chairs placed under the telescope. The eyepiece of the ARE-125 is then passed around and each observer may observe the object in a convenient manner.

One area of improvement for the telescope access would be to provide a thermal barrier between the actual observatory and the entrance foyer. Right now this area is open and results in a larger volume of the observatory open to the building thermal environment. We believe that a clear plastic "strip" door would provide sufficient control of the air exchange between the foyer and the observatory and would result in better images.

PERSONNEL AND EQUIPMENT SAFETY:
Personnel safety is provided by closing the gap between the telescope pedestal and the observing floor, controlling the flow of people around the telescope, and by controlling the furniture on the observing floor. The present entrance way steps and hand rails are adequate.

Placing the observing floor at the proper height relative to the telescope and having an equatorial fork mount telescope eases observing and enhances personnel safety. The fork mount telescope has little eyepiece swing resulting in less height change of the eyepiece.

Using the ARE-125 Articulated Relay Eyepiece allows almost all observing to be performed without an observing ladder or stand of any kind. Also, because the ARE-125 allows the observer to look in a comfortable direction independent of the telescope position, the observer does not have to get into uncomfortable positions to look at the object. This reduces the chance of the observer falling and highly increases the enjoyment of observing.

Equipment safety is provided primarily by the telescope limit switch system and is highly enhanced by using the ARE-125 Articulated Relay Eyepiece. Designing the observing floor at the proper height simplifies maintenance of the telescope and the mirror handling equipment. The wheelchair lift provides a convenient and safe way to transport the primary mirror from the observing floor to the building floor.

FUTURE INSTRUMENT PROVISIONS:
Our experience shows that present instruments require a considerable number of cables running from the telescope to the control room and various other areas of the building. We believe that the number of cables in the future will increase, so the telescope and the observatory should be designed to allow access for running cables. At CSI access is relatively easy with the crawl space under the observing floor and the large conduits running through the pier to the telescope pedestal.

SUMMARY:
The College of Southern Idaho observatory works very well. The telescope is free from building induced vibrations, the people flow is very efficient, and the observing is comfortable. Full access to the eyepiece for people in wheelchairs has been achieved, and the seeing is better than expected. The telescope and the observatory have been enthusiastically received by the students and the general population of southern Idaho.

Improvements to the observatory are planned. These include better ventilation, controlling the environment better between the entrance foyer and the observatory, and insulation of the walls on the east, south, and west sides.

 

For additional information, please see the following links:

Observatory Design

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