Otter Creek Observatory -- Jefferson Community College -- Louisville Metro Parks

THE OBSERVATORY FACILITY

Otter Creek Observatory is designed to accommodate use by students and interested members of the public. It features a roll-off roof rather than a dome. It is designed to minimize thermal effects that degrade the view through the telescopes. It is powered by the Sun.

The observatory has four principle instruments:

· A pair of long-focus refractors sharing a single mount --the observatory’s primary instrument system.

· An 11" Celestron C-11 SCT--the observatory's "hands-on" instrument.

· A computer-controlled 16" Meade LX200 SCT--the observatory's best instrument for faint objects.

· A Coronado 60mm Hydrogen-Alpha (H-a) Solar Telescope--the observatory's "showcase" instrument for day programs.

In addition, the observatory boasts an abundance of support equipment for these telescopes. This equipment includes film and CCD cameras, a video system, eyepieces, solar filters, auxiliary telescopes, and more. Last but not least is a gnomon for observing solar motions.

The Structure:

Otter Creek Observatory features a roll-away roof rather than the dome that is traditionally associated with astronomical observatories. The roof is operated by a crank winch and cable. The roll-away roof offers a number of advantages over the traditional dome. It allows freer access to the sky--the whole sky is visible from within the observatory, rather than only a narrow slit of sky. This allows for the simultaneous operation of the observatory's three telescopes, and lets those not familiar with the sky see where in the sky those telescopes are pointing. The roll-away roof also allows the observatory building to come into thermal equilibrium with (i.e. reach the same temperature as) the outside air as rapidly as possible.

This last consideration is very important. Otter Creek Observatory is constructed of concrete blocks, which are in turn filled with poured concrete. The roof structure and door are heavy and extremely strong. The facility was built this way in order to be able to withstand both weather and vandals, but such a strong structure has a drawback when it comes time to use to observatory--it takes a long time to "cool off". In physics terms, it has a high thermal inertia. If the building is significantly warmer than the outside air, the building will warm the surrounding air, which will expand and rise upward via a process known as convection. This produces convection currents in the air which distort the view through the telescopes. When you look across a hot parking lot or a charcoal grill and see the view "shimmering in the heat" you are seeing this phenomenon. The effect is much weaker in the observatory than in a parking lot, of course, but when it is magnified through the telescopes it becomes noticeable.

White exterior of reflects radiative heat and keeps the building's temperature at a minimum during the day time. To minimize this problem, the observatory is painted a high-gloss white on the outside. This reflects heat radiation and minimizes how much the building warms up during the day. The interior of the building, on the other hand, is painted a flat black. Anyone who has worn black clothes or been in a black car knows that black objects absorb heat radiation well and warm up more quickly in the sun. Black objects also emit heat radiation well and cool off more quickly at night. Thus the black interior helps the building cool when the roof is rolled away. Furthermore, the open roof allows air to flow around the walls easily and cool the building by convection, too. Thus the building is designed to heat up as little as possible during the day and cool down as rapidly as possible at night.

Another interesting facet of the observatory facility is that it is solar powered. A small solar panel charges a bank of 12V batteries when the sun is up. These batteries then provide 12V DC power directly to the observatory. They also provide 120V AC power through a set of power inverters. The 12V DC is used to run systems such as the drive mechanisms on the Meade 10" telescopes. The 120V AC is used to run systems such as the control computer for the Meade LX200 telescope. All of the observatory's power comes from the Sun--a pollution-free energy source.

The refractors:

The primary instrument at Otter Creek Observatory is a 6" diameter, 108” focal length (F/18) refracting telescope owned by Jefferson Community College that is paired with a 4” diameter, 60” focal length (F/15) refractor that shares the same mount. The paired telescopes are designed to give members of the public side-by-side views of celestial objects at both higher and lower magnifications.

The telescope system was constructed “from scratch”, using resources of both Jefferson and Otter Creek Park. The only commercially produced parts in the telescopes are the lenses and the high-precision Crayford focusing mechanisms (each of which features 10:1 reduction mechanisms for ultra-fine focusing). The mount mechanism was constructed by a team of mechanical engineering students from the University of Kentucky who built it as their senior design project. The mount can raise and lower by approximately 2 feet in order to allow the height of the telescopes to be adjusted for more comfortable viewing. The hexagonal wooden tube for the 6” telescope was constructed with the aid of Jefferson’s wood products program, using wood products program equipment. The 4” telescope and its mounting rings were constructed by observatory staff. Staff also worked on fine-tuning the mount for actual use and who worked on the 6”. Additional work on the project came from numerous volunteers, including Jefferson faculty members, Otter Creek Park maintenance staff, and others.

The refractors were built under budget, so no corners needed to be cut for cost purposes. They are as good as they can be. They are designed to last a lifetime and beyond.

The C-11:

The C-11 is Otter Creek Observatory's "hands-on" instrument. It is a 11" diameter, 2800 mm focal length (F/10) Schmidt-Cassegrainian telescope (SCT) owned by Otter Creek Park. The C-11 is mounted in the left rear corner of the observatory on a modified Cave German equatorial mount and a specially fabricated pier. The mount was donated to Otter Creek Observatory by the University of Louisville. The C-11 was Otter Creek Observatory's first telescope, and served as the observatory's primary instrument until September of 2000.

The C-11 is the observatory's largest telescope, having over 20% greater light-gathering surface area than the 10" Meade. The C-11's mount is durable and simple to operate. It has a basic electric drive on its polar axis, no motor on the declination axis, and no fine adjustment controls on either axis. This means the C-11 is poorly suited for photography or high-power work, but excellently suited for use by interested members of the public who might wish to try their hand at a telescope, rather than simply looking at what the observatory staff is showing through the 10". Thus the C-11 is the observatory's "hands-on" instrument. Otter Creek Observatory is pleased to be able to offer a telescope of this size and quality for hands-on use.

The LX200:

The LX200 is Otter Creek Observatory's best instrument for faint objects (like galaxies or nebulae) and is literally a robotic telescope. It is a 16" diameter Schmidt-Cassegrainian telescope (SCT) owned by the Challenger Learning Center. The LX200 is mounted in the left rear corner of the observatory on a heavy-duty alt-azimuth fork mount. The LX200 is controlled by a support computer provided by Jefferson, and is outfitted with some accessories provided by Jefferson as well.

The LX200 is a much larger and more capable telescope than the other SCT’s in the observatory, having over twice the light-gathering surface area of the next largest telecope. The LX200 also features a fully computer-controlled mount. The mount can be controlled by voice commands through a program called DigitalSky Voice, by Astro-Physics. This allows the LX200 to move on its own from object to object at voice command. The LX200 is also equipped with a SBIG ST-6 CCD camera as well as a low-light video camera and TV monitor for displaying images from the LX200 for many people to view at once (this also allows those who have difficulty seeing through the eyepiece of a telescope to enjoy the view through a telescope). It also has a Crayford focuser for precise focusing.

The primary functions of the LX200 are to serve as a photographic (still or video) instrument, to allow the observatory to serve persons and groups who have considerable background in astronomy or who are wishing to do research at the observatory, and as a demonstration platform to illustrate technologies used in larger observatories. Because of the complexity of the LX200, and the plethora of wires and cables around it, and the power consumption of the LX200 and its support equipment, the LX200 is not always used for viewing by the general public.

The Coronado H-a:

The 60mm Coronado H-a is Otter Creek Observatory's other "showpiece" instrument. It is a 60mm (2.4") diameter, 400 mm focal length (F/6.7) refracting telescope (SCT) owned by Jefferson Community College.

The Coronado is a small, highly specialized instrument with unique capabilities. It has a special filter that allows only a very specific color of light, produced by hydrogen gas, through the telescope. This allows it to reveal amazing features on the Sun. It can only be used as a Solar Telescope – it can not be used for anything else.

The images below show the Sun as seen through a normal solar filter (such as what is often used with the Meade telescopes) and through an H-a telescope.

Image taken through a nomal “white-light” filter:

H-a image taken at the same time as the picture above:

Images from Big Bear Solar Observatory

The Coronado is mounted on a portable equatorial mount.

The primary function of the Coronado is to serve as a visual and photographic (still or video) solar instrument. Because of it’s portability, it also travels as part of the Mobile Telescope program.

Another H-a view:

www.coronadofilters.com

The Gnomon:

The gnomon is located on the south wall of the observatory and is used for solar observations. On sunny days the gnomon casts a shadow of the wall of the observatory which indicates the position of the sun in the sky. As the day progresses, the Sun moves from east to west, and the shadow cast by the gnomon moves from west to east. As the seasons progress, the Sun moves northward in the sky (during the February through June) or southward in the sky (during July through January).

The gnomon is a very useful device for discussing the Sun’s effect on basic aspects of everyday life. The relationship between the position of the sun and temperature in summer and winter, or the connection between the Sun’s motion and our concepts of “A.M.” and “P.M.” can be shown with the gnomon.

The picture at left shows the shadow cast by the gnomon at approximately 12:41 P.M. on March 13, 2004. The dark line shows the track of the shadow that day. The track was started at 10:38 A.M., and there are additional marks at 11:07 A.M., 11:50 A.M., and 12:08 P.M. The times are accurately determined using a time-signal clock.

The vertical dark line indicates “solar noon” – the point halfway between sunrise and sunset when daylight is half over. Note that even at 41 minutes past “noon” on the clock it was still not yet solar noon.