Reflector Telescope

RFO's 40 inch reflecting telescope.

The West Wing houses a 40″ reflecting telescope. This telescope’s focal ratio is f/3.6, with a focal length of 3,600 mm. The entire telescope weighs about one third of a ton. It is supported below the building by three concrete piers.

Mounted on the side of the giant 40″ scope is an 8″ reflecting spotting scope which also functions for observing. There is also a Telrad finder. The telescope is an alt-azimuth mounting with a computer drive integrated with planetarium software so that the operator can choose a target and command the telescope to slew to that point in the sky. The software of the drive keeps the altitude and azimuth of the telescope constantly adjusted so that it tracks an object in the sky.

The telescope mirror was purchased from Jeff Baldwin. The Project 40 team finished rough grinding, fine grinding, polishing and figuring the mirror on a hand-built polishing machine. Optical testing used a large zone mask for initial figuring and two years of interferometric measurements and figuring to reach the final curve. The telescope mirror was coated with NASA-grade aluminum and aluminum oxide as a donation by Viavi (formerly JDSU and OCLI) in Santa Rosa. The mount was hand-crafted of tubular steel welded together to form a rocker box. The altitude and azimuth drive design uses a SiTech controller and interface to a Windows 7 computer running TheSky X for finding and slewing to objects. Each axis has a computer-controlled server motor plus gear train with the required torque to move the massive mounting.

A diagram of the 40 inch telescope showing how it works

How it works: The light from an object enters at one end of the telescope and travels the length of the tube where it is reflected off a 40″ diameter parabolic mirror. The light then travels back in the opposite direction where it hits a tilted secondary mirror near the end where the light first entered. The secondary is set at an angle of 15º. The light is reflected from the secondary mirror to a diagonal tertiary mirror just outside the telescope tube, and is again reflected to the final direction through a coma-correcting lens stack and an eyepiece for use by the observer. 

Refractor Telescope

RFO's 8 inch refractor telescope.

The white dome houses a two-meter-long refractor telescope. Attached to the side of the telescope are a spotting scope (generously donated by Stellarvue) and a Telrad finder. A tracking motor moves the telescope to counteract the Earth’s rotation, so that an object in the eyepiece will stay there over a period of time as the Earth moves. This telescope made its way to RFO from Dominican University in San Rafael.

How it works: Light enters at one end of the telescope through an 8″ piece of refracting glass which slightly bends the light. The light rays converge together at the opposite end of the telescope where a small 45º mirror directs the light out to an eyepiece.

Diagram showing how the refractor telescope works.

Robotic (CCD) Telescope

RFO's 20 inch robotic (CCD) telescope.

The East Wing houses a beautiful Ritchey-Chrétien 20” telescope, donated to RFO by the University of San Francisco in 2016. The RC design is the most popular choice for professional observatories: even the Hubble Space telescope is of the same design. This telescope is not set up for visual observing: camera equipment is attached to the back of the scope, and RFO docents use the system to do astrophotography and research. The entire system is controlled by computer software.

This sensitive camera system (called a “CCD” for charge-coupled device, referring to the camera’s chip) is capable of imaging objects hundreds of millions of light-years away. It can also image asteroids and comets to help ascertain their positions and orbits, and can provide data on a star’s light, giving astronomers insight into the star’s physical properties. Contributions to professional research projects are made with data generated from image analysis. Take a look at our Astrophotography and Research projects.


Telescope: RCOS 20RC, 20-inch Ritchey-Chrétien, carbon truss
 AstroPhysics GTO3600
Camera: Research grade 16.2 megapixel Atik 16200
Spectroscope: DSS7 mounted on an SBIG STl-8XME
Autoguider: Lodestar Autoguider mounted on a 400mm refractor
Filters (9): Optec LRGB, Ha, and photometric BVRI
Focal length: 4115 mm (f/8.23)

Diagram showing how to robotic (CCD) telescope works.

How it works: In the diagram at right, light enters from the left and reflects off the primary mirror at the rear of the scope, bouncing back to the secondary mirror. The light is reflected back through an opening in the primary mirror, coming to focus inside the camera.

RFO thanks the University of San Francisco for their generous donation of this equipment.

Solar Telescopes
RFO has two methods for observing the Sun—safe direct observation through heavily filtered telescopes, and radio antennae which record fluctuations in activity.

RFO's solar scopes

The telescope at right rear is a powerful 80mm Lunt refractor with a red hydrogen-alpha filter. It images the solar chromosphere (atmosphere) and shows prominences, filaments and granules.

The nearer scope is an 80mm Orion refractor with a neutral density filter. It images the solar photosphere (surface) and shows sunspots in high detail. Our “Sun Spotter” apparatus is in the center.

How they work: The light path diagram for our 8″ Refractor applies to both these refractors.

Funds for the Lunt telescope were provided by the California State Parks Foundation and private donations. The telescope was dedicated in memory of beloved observatory docent and solar astronomer Merlin Combs.


RFO's radiotelescope

The Radio Telescope consists of antennae attached to the Observatory’s walls which feed radio and audio signals to the indoor classroom computers.

We can hear the Sun’s activity and see radio graphs projected onto an indoor screen for public observing. Radio astronomer Dean Knight examines the fluctuations appearing on today’s graph.

About Telescopes

There are three basic types of telescope design (refracting, reflecting, and catadioptric), and the Robert Ferguson Observatory operates all three types.

Size refers to the primary mirror or refracting glass used in the telescope–not the length of the telescope or the magnification.  The larger the primary mirror or refracting glass, the more light is gathered. The “object”—a galaxy, for example—appears brighter to the observer.

Magnification is dependent on the focal length of the telescope and the specific eyepiece used.

Filters of various types can be used to help emphasize certain features of the object being observed. Oddly enough, a green filter can bring out surface features of the red planet Mars.

Other devices attached to a telescope may include one or more “spotting scopes”—smaller telescopes that provide a wider field of view to assist the operator in finding objects.  Telrads are devices which generate a red laser ring “target.”  When the operator moves the telescope so that the desired object is centered in the target, it will also be visible in the telescope’s eyepiece.

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