May 14th, 1935 was not a normal day in Los Angeles. It was finally opening day at Griffith Observatory. For years locals had watched the large concrete and steel structure slowly take shape on the south slope of Mt. Hollywood. Patrons could now enjoy planetarium shows for the affordable sum of 25¢, view images of the sun collected from the coelostat, and study exhibits on geology, astronomy, chemistry, and mathematics.
To many visitors that first week, the prime attraction must have been the imposing 12” diameter Zeiss refracting telescope in the copper-clad East dome. Eager patrons had to wait until May 17th for stubborn clouds to clear and telescope viewing to open for the first time. The scene 84 years ago may have been similar to a typical night at the observatory today; relaxed men and women form a line outside the telescope dome while restless children mill about. Chatter is light and easy as a cool spring breeze pushes in from the south, the direction of a burgeoning City of Los Angeles. A hush comes over visitors as they enter the telescope dome. A child climbs a few steps of stairs, dutifully closes one eye, and peeks through the telescope. Hesitantly at first, but then eagerly! The planets and stars accessible to anyone who cares to look!
This scene has played itself out countless times over the last 84 years at Griffith Observatory. The building invites individuals to become astronomical observers through many different instruments built into the structure itself, none more obvious than the iconic 12” Zeiss telescope. To date, over 7.5 million people have looked through the telescope, making it most viewed through telescope in the world. This is the story of that very special telescope. An instrument that was not made for any one person but for anyone who wants to get just a little bit closer to the stars.
The Origin Story
In 1908 Griffith J. Griffith and his friend John Anson Ford found themselves enjoying the view through what was then the largest telescope in the world, the 60” diameter reflector telescope on top of Mt. Wilson, just outside of Pasadena. Griffith, already a well known and at times controversial benefactor to the city of Los Angeles, was awestruck by the experience. Ford quoted him, “Man’s sense of values ought to be revised. If all mankind could look through that telescope, it would revolutionize the world”.
By the end of his life in 1919, Griffith had established a bequest of $750,000 to build an observatory on Mt. Hollywood with a “Hall of Science,” a “Theater of the Heavens,” and a telescope at least 12” in diameter. In addition, he required that the building be free to enter, a tradition that Griffith Observatory proudly upholds today. Though Griffith originally hoped the structure could be completed by 1915, it took until 1931 for legal issues with his bequest to be resolved and enough funds raised for construction to begin. The building would open and be named after its benefactor four years later.
Selecting a Grand Public Telescope
Griffith left instructions that the observatory must include “a telescope at least 12 inches in diameter,” but other than that planners had much leeway in selecting a telescope. The selection process was guided by the foundational principle that the telescope must be appropriate for general public use. Public telescopes must offer observers clear and crisp views of recognizable objects such as the moon and planets. Research telescopes, on the other hand, are designed to see faint objects very far away and therefore require a large diameter. A bigger diameter means the telescope can collect more light and dimmer objects can be seen. These professional instruments are large, ungainly, and often not appropriate for general use.
The designers of the building had two types of telescopes to choose from: refracting or reflector. Refracting telescopes utilize a glass lens to redirect (i.e., refract) light and focus it at the rear of the telescope. A reflector telescope achieves much the same result but with a curved mirror. Both designs offer advantages and disadvantages. Reflector telescopes have dominated the realm of research telescopes since the early 1900s because large mirrors can be made and supported more easily than large lenses. Although more expensive, refractor telescopes can provide sharper views of bright objects, a desired characteristic for the public telescope. As Dr. Dinsmore Alter, the first director of Griffith Observatory, describes in a 1955 edition of The Griffith Observer, “Technically a 12-inch reflecting telescope would have fulfilled the conditions [of the bequest], the trustees planned for a 12-inch refractor, which is much the more expensive but which, in general, is better for public use”.
The telescope was purchased from the Carl Zeiss Company (thus its current moniker of the Zeiss telescope) in 1931 for the princely sum of $14,900, about $250k in today’s dollars. It was the first material item purchased for the observatory and the last item to be installed. The Zeiss Company was, and still is, one of the premier suppliers of optical systems in the world making them an obvious choice for the manufacturer of a high-quality refracting telescope. Furthermore, the German company utilized a type of mount known as an offset fork stress compensating system. In this configuration the movement of the eyepiece is small when compared to the telescope's movement across the sky, thereby making it a good choice for a public instrument.
Today the Zeiss Company manufactures high quality optics for devices such as cameras and microscopes, but has not made large telescopes for decades. Therefore, the instrument at Griffith Observatory is both unique and rare. There is only one other Zeiss telescope of similar design in the United States (located in Pittsburg) along with a handful abroad. Unfortunately, much of the design documentation on the Zeiss telescopes was lost after WWII so limited original information on the telescope is available today.
It will come as no surprise that the performance of a refracting telescope is highly contingent on the quality of the primary lens. Fortunately, Zeiss is one of the best. The optical design of the observatory’s telescope can be described as a 12” diameter apochromatic doublet lens. Those are complex terms but the underlying idea is simple. Refractor telescopes with a single lens suffer from a type of distortion called chromatic aberration, meaning that a star will have a slight smearing of colors around it as opposed to appearing as a single point of light. This happens because different colors of light deflect at slightly different angles when going through glass; the same reason why white light separates into a spectrum of colors when going through a prism. The problem can be corrected by cleverly using more than one lens to correct for the difference in diffraction. A telescope with this feature is known as apochromatic. Either two or three separate lenses, known as a doublet or triplet respectively, can be used to correct for chromatic aberration. Since the Zeiss Telescope is an apochromatic doublet, that means it actually uses two separate lenses together to form a sharper image with fewer color aberrations.
Any telescope or camera lens has a parameter called focal ratio, which is simply a ratio between the focusing length of the lens or mirror (i.e., focal length) and the diameter of the optic. Although not as important as the diameter of the lens, focal ratio does have an effect on telescope performance. Higher focal ratios typically lead to telescopes with sharper images and a narrower field of view, whereas lower ratios result in a wider field of view and a better ability to see dim objects. A typical backyard telescope has a focal ratio from 5 to 10, and modern research telescopes can have ratios as low as 2 or 3. The Zeiss telescope, however, has a focal ratio of 16.7, more than double that of most telescopes. That makes the Zeiss telescope very well suited for viewing bright objects such as the moon and planets, ideal objects for a public audience.
To further increase optical performance, the telescope has four equally spaced blackened baffles (donut-shaped disks) attached inside the tube of the telescope. This prevents internal reflections within the telescope by “cutting off the unneeded edges of the converging rays of light which are bent to a focus,” explains Dr. Alter in the same article mentioned previously.
The last piece of the optical puzzle is the eyepiece. There are many different types and styles of eyepiece that won’t be covered in detail here. What is important to know is that the eyepiece can be easily swapped in and out of the telescope, and it determines the magnification of the object being viewed. Achievable magnifications with the Zeiss telescope range from 125x to 500x. However, magnification power is limited by the Earth’s own atmosphere. In much the same way air over a hot car looks wavy, turbulence in the atmosphere can cause objects through the telescope to look fuzzy and distorted. This problem gets worse with higher magnifications. Most nights, the telescope is limited to magnification in the 200x-300x range. It is important to remember though that the most important parameter of a telescope is resolution, not magnification. And more resolution is achieved with a larger diameter telescope.
The alignment of all these optical components is critical to the correct functioning of the instrument as a whole. Unfortunately, large refractors commonly sag under the weight of their own lens. However, the Zeiss telescope has a secret behind its enameled white shell. Dr. Alter once again enlightens us, “The telescope tube in any position is braced by a system of eight lengthwise rods. Individual weights attached to the eye-end of the rods automatically pull on the rods that happen to be uppermost and release on the lower ones. Thus, even the usual slight bending of the tube by the pull of gravity is counteracted”. Or put more simply, weighted rods within the telescope counteract structural distortions caused by gravity. This critical feature means that the primary lens of the telescope and eyepiece remain aligned no matter what direction the telescope is pointed.
Mechanical Design and Structure
Visitors to Griffith Observatory often describe the telescope as looking like it should be mounted on a battleship and this is not an unreasonable description. The gunmetal gray steel support structure is an imposing sight for first time visitors and still commands respect from those of us who are fortunate enough to work with the telescope regularly. The telescope mount is responsible for supporting and aligning the telescope. No small task given that the moving parts of the instrument weigh an impressive 4.5 tons!
The telescope employs a type of mount known as an equatorial configuration. One axis of rotation is oriented parallel to the rotation of the earth, or put another way, is pointed at the North Star. This axis is called right ascension. A second axis, the declination axis, is orientated in a perpendicular direction. Together, right ascension and declination form a coordinate system for objects in the sky similar to latitude and longitude on Earth.
Next time you’re out late, pay attention to how the stars move in the sky over time. The motion of the stars is due to Earth’s spin on its axis. Since the Earth’s rotation axis points at the North Star, objects in the sky also appear to move around the North Star. Therefore, tracking objects in the sky becomes easy with an equatorial mount. The telescope only has to move at a constant speed around the right ascension axis to keep an object in the telescope for an entire night of observing. This is a critical feature for a telescope that serves the public. Without it, the Zeiss would need to be manually re-located every 20–30 seconds!
Visitors to the Zeiss dome are able to see the single motor that drives the telescope on the left side of the telescope pedestal. The drive motor, still original from 1935, is unassumingly small and produces only 1/97th of a horsepower. However, it can easily rotate the 9,000lb bulk of the telescope! Only a small motor is needed to turn the telescope because it is precisely balanced. Balancing is the job of the four spherical shaped counterweights hanging on the side of the telescope. The counterweights are oriented such that no matter what orientation the telescope is in, the balance point stays directly on top of the axis of rotation. It’s like having a well-balanced see-saw that only moves if you push it slightly in one direction or another. In addition to the visible counterweights, there is a third even larger counterweight hidden below floor level. It attaches to the main north-pointing axis of the mount and keeps the entire telescope from toppling over.
The telescope mount is in turn placed on top of the telescope pedestal, a concrete pillar positioned in the middle of the dome. Fortunately, visitors need not be concerned about the structural integrity of this large structure and telescope on the roof of the building. In 1933, while Griffith Observatory was still under design, a large earthquake struck Long Beach causing loss of life and tens of millions of dollars in damage. This event did not go unnoticed in Los Angeles and encouraged the designers to make Griffith Observatory more robust to earthquakes. The walls of the building were thickened and more steel re-bar was added into the substructure. This resulted in a very sturdy structure on which to place the Zeiss telescope while also making the building very resilient to future earthquake activity.
The Zeiss gets a Twin
In the 1930s and 40s a unique vehicle roamed the streets of Los Angeles. The car, owned by Shelly Stoody, was typical for the time except for a 9.5” diameter, 12’ long Zeiss telescope mounted to the roof. It is unknown today how Shelly procured the 1920s era telescope but regardless, he would drive his showpiece to various amateur astronomy events and presumably share views of the sky. In 1955 Stoody sold the telescope to Griffith Observatory.
The 9.5” telescope was installed parallel to the 12” Zeiss in September 1955 and has remained there ever since. The two telescopes complement each other quite nicely. Using the smaller telescope in tandem with the 12” can provide two types of views of the same object. For example, a low magnification view through the 9.5” shows the whole moon, and a high magnification view through the 12” offers closeups of particular lunar features. Despite its smaller size, the 9.5” telescope has excellent optics and can occasionally outperform its larger cousin, especially when a larger field of view is desired. The smaller telescope was also useful during a scientific research program at Griffith Observatory during the time of installation. It acted as a guide telescope for the larger 12” telescope, aiding astronomers in the detection of flare stars.
In addition to the two Zeiss telescopes, there are actually three more telescopes on the same mount. The smallest one is the finder scope. This small 1.5” refracting telescope has a cross hairs stamped on the lens and is used only for aligning the larger telescopes. There are also two 8.25” reflector telescopes mounted next to the 9.5” telescope. These were donated by the manufacturer Celestron and installed during the 2002–2006 renovation of the building. Both telescopes have a camera where the eyepiece normally sits, thereby enabling a live feed to be broadcast elsewhere in the building.
What to See
Thanks to diligent care by the dedicated staff at Griffith Observatory, visitors to the observatory consistently enjoy striking views of the cosmos. The target object for a particular night varies throughout the year and is entirely at the discretion of the telescope operator. Most popular are large, striking objects such as the moon and brighter planets. The Zeiss telescope easily resolves craters, mountains and other features on the moon. The rings of Saturn are clearly visible, as are cloud belts and the four largest moons of Jupiter. These objects are bright enough to cut through the extreme light pollution in Los Angeles, but views are often affected by what astronomers refer to as poor “seeing” conditions. Poor seeing results from light passing through turbulent air, creating a blurry or wavy distortion.
Deep sky objects, a term for just about any object outside of our own solar system, also are featured sometimes. Most popular is the Orion Nebula, a striking star-forming region visible in the winter sky. Brighter star clusters are also occasionally seen through the telescope. However, light pollution makes viewing even relatively bright deep sky objects particularly difficult. It is useful to remember that Griffith Observatory was not meant as a research institution, and even in 1931 designers realized that light pollution from the rapidly growing city would have an effect on viewing. However, the public nature of the building meant that it was much more important to site the observatory in an accessible location as opposed to a desolate mountain peak.
The Zeiss telescope has been used for viewing special celestial events regularly during its lifetime. The first such instance occurred just a couple years after the building opened when the little known Finsler’s Comet was viewed by over 11,000 people in 1937. Unprecedented crowds were brought to the observatory in 1985/86 to view the return of Halley’s comet. And in July 1994, crowds jammed the streets leading to Griffith Observatory to see the impact and aftermath of Comet Shoemaker-Levy 9 on Jupiter. Since 1951, generations of television cameras have been attached to Zeiss telescope for special occasions, such as eclipses, where images have been broadcast to monitors within the building, or more recently, live streamed on the internet. NASA has even utilized the live stream from the Zeiss telescope to broadcast images of special astronomical events to millions of people around the world.
Operating the telescope
Most visitors are intimidated by the controls of the Zeiss refracting telescope and the systems of knobs and wheels can look complicated. However, operating the telescope is not actually so complex!
After a telescope operator decides what to view on a particular night, she must determine the coordinates of the object in the sky. This is done using standard reference guides combined with a simple calculation involving sidereal time (time referenced from the stars as opposed to the sun), or can also be found with any number of electronic applications. The operator knows the current position of the telescope from two large dials on the telescope mount. Large movements of the telescope are done by unlocking both rotation axes using knobs on the telescope, manually slewing the instrument into the correct position, and locking it back into place.
Not only does the telescope move, but the roof does also. Shutters on the outside of the dome can be opened and closed to create a slit for the telescope to view through and the roof rotates 360 degrees to accommodate viewing in any direction. Although the telescope moves automatically to match the movement of the sky, the roof does not and must be repositioned throughout the night.
With the telescope pointed in approximately the correct part of the sky and the roof oriented accordingly, the telescope operator can use the finder scope to hone in on an object. Fine adjustments are done using the controls on the rear of the telescope including a mechanical knob to adjust declination, and electronic buttons to control right ascension.
The last thing to do is insert an eyepiece of appropriate power and focus the telescope. The focuser slowly moves the eyepiece in and out of the main tube until it is positioned in just the right spot to catch and magnify the image from the primary lens without causing distortions. At that point, the operator can open the dome to the public and visitors can look through the telescope.
Science Research comes to the Observatory
The primary purpose of the telescopes at Griffith Observatory has always been to inspire and educate the public. The telescope’s relatively small size and poor location just outside of downtown Los Angeles make it less than ideal for academic research. However, that has not kept some enterprising astronomers from trying! In 1940, two young staff members, George Herbig and George Bunton, designed a planetary camera for the telescope. The camera and telescope combination worked so well that Griffith Observatory was eventually made the source for many illustrations used in astronomy texts and newspapers.
Interestingly, Griffith did suggest in his will that the building could include a research component, “The observatory portion of the building will be so constructed that later if it be deemed advisable an additional structure may be erected above [the building] for scientific astronomical studies” . The closest this vision came to fruition was with the studies of the first Astronomical Observer of Griffith Observatory, Paul Roques. He developed a research program to study flare stars, faint red stars that undergo sudden outbursts in brightness for only a few minutes. Roque’s research program ran between 1952 and 1969 and netted the first record of the complete cycle of a flare star outburst, tracing the stages of the event on a chart recorder. Roques also monitored and searched for flare stars photographically with an automatic system between 1960 and 1971. Flare stars were identified by imaging the motion of the star and looking for brief changes in brightness in the photographic plate. One such instance of a flare stars appears in the image below taken by the Zeiss telescope.
Maintenance and Renovations
As with any device that is kept operational for over 80 years, the Zeiss telescope requires regular upkeep and maintenance. The drivetrain of gears and bearings are lubricated regularly to ensure easy movement. Electronic cables and fuses occasionally need to be replaced. Even the telescope lens is occasionally removed and cleaned. However, this is not done frequently since the lens is irreplaceable if it were to be damaged during the operation.
Mechanical components on the telescope sometime break as well. Since the Zeiss company no longer makes large telescopes replacement parts are not readily available. Many times, new solutions must be found and custom parts fabricated from scratch.
Although Griffith Observatory has received extensive renovation over the years, the telescope itself has remained largely unchanged from its original configuration in 1935. It retains the same optics and most of the same drive components featured on opening day. The most notable improvements were made to the electronic drive system of the telescope in the 1980s. It was converted from a direct current to alternating current power supply, thereby increasing the reliability of telescope tracking. The telescope and mount were also repainted in the early 1980s. Lastly, an equivalency station was added during the major building renovations in 2004–2006. Live views from the telescope and audio from the dome are now broadcast into the building below, allowing visitors who can’t access the dome due to mobility issues or other reasons to experience some of the sights and sounds from the telescope.
The scenes on the roof of the Griffith Observatory are similar today as they were 84 years ago. Families come in droves. Tourists visit from afar. Energetic children race about. Griffith Observatory now belongs to the millions that call Los Angeles home. The building was founded on the fundamental concept of humanizing astronomy and the sciences. It is not just about astronomy, but about astronomy and people. At the core of this mission remains the 12” Zeiss telescope, which continues to create space explorers out of casual observers and instill a connection with the universe to which we all belong.
 M. Eberts, Griffith Park: A Centennial History. Spokane, WA: The Arthur H. Clarke Company, 1996.
 “The Zeiss Telescopes — History and Research,” Griffith Observatory. [Online]. Available: http://griffithobservatory.org/exhibits/zeiss_telescope_history.html.
 D. Alter, “The Twin Refracting Telescopes,” The Griffith Observer, vol. 19, no. 12, pp. 142–143, Dec-1955.
 H. King, The History of the Telescope. New York, NY: Dover Publications, 1955.
 H. Simon, “Heavy Telescope Design,” Product Eng., Jul. 1931.
 P. Rouques, “The Observatory that Griffith Built and its Program of Research,” Griffith Observer, Dec-1962.
 P. Rouques, “The Discovery of a New Variable Star,” The Griffith Observer, Nov-1962.
 “Personal Interview with Anthony Cook,” 29-Mar-2019.
 E. C. Krupp, “Going Public,” Inspiration Astron. Phenom. VI, 2010.
Evan Hilgemann is a mechanical engineer at NASA’s Jet Propulsion Laboratory and also curates Explore & Observe, an email newsletter on modern-day exploration of earth and space.
This work was done as a private venture and not in the author’s capacity as an employee of the Jet Propulsion Laboratory, California Institute of Technology. Any views and opinions expressed do not state or reflect those of NASA, JPL, or the California Institute of Technology