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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
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