The Isaac Newton Telescope
hen the Royal Observatory was founded, in 1675, the village of Greenwich was located in open countryside, several miles from London. As the capital grew, urbanization steadily approached and Greenwich declined as a favorable site for the nighttime study of planets and stars. By the end of the nineteen forties, factory smoke and mercury vapor street lighting forced a decision to relocate the Observatory and after an extensive search, Herstmonceux was selected as its new home.
- Now located at the Observatorio del Roque de los Muchachos, on the Canary Island of La Palma, the historic Isaac Newton Telescope was the focus of the Author's visit- seen here, looking north, at sunset.
Why this site was selected remains a mystery to most who stop and consider it. Since the mid-1800's, most observatories have been located on remote mountains where the air turbulence is minimal and the number of clear nights is exceptional. Herstmonceux is located near sea-level and to make matters worse, the Observatory was situated near a marsh- predictably, the dampness occasionally interfered with observing sessions.
- The Isaac Newton 2.54 meter telescope, f/3.29 at Prime focus, gives a 40 arcminute unvignetted field of view.
- Photo credit: Isaac Newton Telescope Group
Even before the Royal Observatory was moved from Greenwich, British astronomers had pleaded for a telescope sizeable enough to conduct research that would be competitive with instruments like the 200-inch on Mt. Palomar. So, during the early sixties, funds were finally obtained for the construction of a 100-inch telescope that would be located in Herstmonceux. The University of Michigan donated a 98-inch blank and this was used to create the primary mirror of what Queen Elizabeth II would eventually christen as the Isaac Newton Telescope
, during its dedication ceremony in 1967. The instrument came to be known as the INT.
Through the early years of the nineteen-sixties, astronomers were mostly earth bound to the telescopes accessible by travel over land. However, as the price of air travel declined, it became more commonplace for astronomers to journey abroad and use other instruments as guests. For the British, this was an eye-opening experience because it gave them opportunities to conduct research with telescopes located in more favorable conditions than those available back in the UK. So, almost as soon as the Isaac Newton Telescope began regular operation, most agreed that the instrument was located in the wrong place! For example, only one-third of the nights in Herstmonceux were favorable for astronomical research.
- The Author stands beneath the Isaac Newton Telescope to provide a sense of scale.
- Photo credit: Julio Carballo Bello
Therefore, in the early seventies, a search commenced for a better site to locate the telescope in the northern hemisphere where it could be placed into more frequent use. Several locations were tested, including Hawaii, but the Island of La Palma soon gained favor and plans were made to establish a major observatory there with Spain, Denmark and Sweden. In 1979, the INT was removed from its dome in Herstmonceux so it could be refurbished and fitted with a new, larger, and optically superior 100 inch mirror made from low-expansion material. The mechanical components of the INT were shipped to La Palma in 1981 and, within a year, the new mirror followed. First light was captured in early 1984, and, by the end of May that same year, the first guest astronomer had used the scope.
The decision to move the telescope was not without detractors and some believed that it would have been cheaper to have built a new instrument rather than move the INT to the Canary Islands. Their arguments cited that the moving costs were excessive because of the re-engineering work required for the telescope to operate at a lower latitude, the figuring of its improved 100 inch Zerodur mirror and the need to build a new temperature controlled building. The telescope also encountered other, unexpected problems. For example, local workmen from La Palma discovered a small gap separating the foundation of the telescope from the rest of the building. Interpreting this as a construction oversight, they irreversibly sealed the gap with concrete. Unfortunately, they did not know that the space was intentional to prevent vibrations within the building from transferring to the telescope- a common design feature even found in amateur observatories.
- The telescope's original 98-inch mirror was replaced with a 100-inch primary when the instrument was relocated from Herstmonceux to La Palma during the early nineteen-eighties.
Once the news of the workman's ill-conceived solution spread, many astronomers predicted the telescope's performance would be compromised- rumors and reports about this persisted for years. Dr. Martínez-Delgado, my host and team leader, has collected data with the INT throughout his career and has never described any experiences about vibrations affecting the results of his research efforts in our conversations. Similarly, although all three of us were tromping about the control room just outside and within twenty feet of the telescope's location, none of the vibrations we must have induced registered on the data our team obtained- this is particularly noteworthy when considering that we took several half-hour exposures! So, earlier concerns and reports about the INT's reduced value must have been exaggerated, at best.
Today, the INT features a 2.54m diameter Primary mirror, with an overall focal length of 7.5m, that provides a corrected f/3.29 Prime and f/15 Cassegrain focus. An unvignetted 40 arcminute field of view (about the size of the first or last quarter moon) is obtained at the Prime focus whereas the Cassegrain focus reveals a 20 arcminute piece of the sky. This telescope is considered a wide field instrument by those who take images at its Prime focus because it captures a much larger portion of the sky than most telescopes in service of a similar size. Both Prime and Cassegrain foci are equipped with instrument rotators and autoguiders. The autoguiders continuously analyze a separate image of an often nondescript star just outside the Primary camera's view then signal small corrections that improve the telescope's tracking accuracy from <1 arcsecond in 3 minutes (<2 arcseconds in 10 minutes) to better than 0.3 arcseconds. The optical tube assembly and its mount weigh a prodigeous 94 tons yet the pointing accuracy of the INT has a precision of only 5 arcseconds- that equates to hitting a target on the moon, located 238,000 miles from Earth on average, with a error of only four miles. The weight of the instrument has been distributed so it remains in balance regardless of its position. Therefore, only a small set of motors are required for it to slew at .5 degrees per second in RA and 1.3 degrees per second in declination when moving from one target in the sky to another.
The instrument's precision polar alignment and tracking accuracy make it possible to obtain excellent imaging results without engaging the autoguider if the exposure is limited to a few minutes! For example, during the second night of our observing session, thickening clouds prevented the selection of a suitably bright guide star that would position our target at a favorable location on one of the imaging chips. So, instead of abandoning our project, we found ourselves relying on the accuracy of the mount's alignment and tracking capabilities by taking a series of one and two minute unguided exposures. The results were not disappointing.
The telescope is equipped with several detectors including the Intermediate Dispersion Spectrograph
which sits at the Cassegrain focus of the instrument. The other major detector is the Wide Field Camera, located at the Prime focus- this was the detector used during our three day observing run.
- The INT's Wide Field Camera, positioned at Prime focus, can be used to create a mosaic image covering a one-half square degree of sky.
The Wide Field Camera
consists of four thinned 2k X 4k CCD chips with 13.5micron pixel diameters that correspond to 0.33 arcseconds per pixel. The edge to edge limit of the mosaic, if you ignore the approximate 1 arcminute inter-chip spacing, is 34.2 arcminutes. Interestingly, binning and windowing are not supported. A separate, smaller 400 X 288 pixel CCD chip enables automatic guiding of the instrument- the autoguide window is 60 X 60. Typical autoguiding exposures are between one and five seconds duration.
Most major observatories have a person on duty, known as the operator, who is responsible for the night-time control of
the telescope, the facilities and to assist astronomers in collecting astronomical data. For example, when a visiting astronomer arrives for their nighttime observing session, they are permitted to watch but not touch any of the controls that govern the instrument, its detectors, the dome or any other aspect of the observatory's functioning. Thus, the researcher is provided a chauffeur to transport him or her and their project throughout the heavens. Several years ago, the administrators of the Isaac Newton Telescope decided to forego this luxury in a cost cutting decision so that, now, astronomers are required to get behind the wheel and drive themselves. Some consider this a hassle but I was absolutely ecstatic when I learned about this situation!
- The telescope is normally parked in a horizontal position so that the Wide Field Camera's cooling port is accessible from the interior dome catwalk.
At the beginning of our first night observing run, we were greeted by the telescope's Astronomer on Duty who reviewed a list of instructions about pointing and slewing the telescope, opening and closing the dome, operating the camera and dealing with weather related or other unexpected conditions. For example, the Wide Field Camera has to be cooled with liquid nitrogen every evening and again before the visiting astronomer departs the next morning. Maintaining the camera's chips at tremendously low temperatures reduces noise produced by even the finest imaging sensors to vanishingly low levels and since there is no permanent staff at the INT, the visiting astronomer is expected to keep the camera chilled. Cooling the camera requires the donning of protective clothing so that the super cold liquid does not accidentally splash onto the astronomer who has been conscripted into service. Once sufficiently protected from the possibly of freeze burns, a two foot long hollow probe is inserted and held in the camera's cooling port while refrigerant slowly fills its internal reservoir. Producing a cloud of blue-white condensation, this process continues for about five minutes or until overflowing liquid nitrogen is glimpsed.
The INT is located at the crest of an immense caldera
that crowns La Palma and therefore is subject to (seemingly) ever-present winds that gust throughout the day and night. The noise from these sometimes gale-force breezes fills the dome room with a cacophony of sounds that ranges from cymbal crashes to the sighs of a bow randomly drawn across the strings of a base violin. These sounds are a constant accompaniment that occasionally produces a startle during the camera cooling job. Interestingly, the observatory rules allow for observations to continue until the winds exceed 50 miles per hour!
- Visiting astronomers are required to chill the Wide Field Camera with liquid nitrogen before and after their nighttime observing session. The Author is seen donning special protective clothing prior to handling the super-cold liquid.
- Photo credit: Julio Carballo Bello
- Liquid nitrogen feeds up through the long, thin black tube, seen right of center. A long probe at the end is inserted into the camera cooling port until the coolant begins to overflow.
- Photo credit: Julio Carballo Bello
Both the telescope and dome operation were originally controlled by a dedicated computer, called the Telescope Control System
. Located on the third floor of the instrument's building, the control station is immediately adjacent to the dome room that houses the telescope. A large picture window provides a view of the going's-on under the dome but it remains closed during the observing session. This original control system is only partially used today because management of the telescope and autoguider has been handed off to PC's and servers. Opening and closing the dome, parking and manual slewing of the telescope is still performed by controls on an engineering rack that sports analog dials and manual knobs. Awakening the observatory is a delicate dance of over ten sequential steps choreographed by a set of thoroughly thumbed printed instructions. When performing this function, it was impossible to avoid comparison with my own modest setup, now, almost half a world to the west.
For example, the telescope is nominally parked in a horizontal position so the Wide Field Camera's cooling port can be accessed from the catwalk ringing the dome's interior. To make the scope functional, first, it must be slewed to a more or less vertical position. Next, the shutter is pulled open then the telescope is synced so the dome follows it's movements. After about five minutes of this exercise, the observatory is ready for the exposure of the observing session's sky flats. This is not too dissimilar from the routine I follow when readying my remote New Mexico observatory for nighttime operation- but the amount of exhilaration I felt standing in the INT control room was significantly greater!
- Dr. Martínez-Delgado seated in the control room of the Isaac Newton Telescope. The telescope no longer has a full time operator thus visiting astronomers must learn to control all aspects of its operation.
I was unable to resist the temptation of standing beneath the same stars the telescope was watching but, with my first venture outside on the observatory's rooftop, I found this to be an initially bewildering experience. The sky above the observatory had a vacancy of light more profound than anywhere I have previously visited other than the inside of a deep underground cave. I literally could not see my hands or feet- walking was an exercise to be performed with caution lest I trip over or bump into some unsuspected object and fall flat on my face or tumble over the edge of the third-floor observational deck.
Conversely, as my eyes adjusted to the darkness, the sky overhead began to fill with slowly brightening embers- the light of distant suns that seemed to number in the tens of thousands! There was no romantic twinkle to this glitter- each point of light seemed immovable and resolutely nailed in place.
- Looking southeast from the Author's room at the Residence, dawn breaks following the end of the final observing session at the Observatorio del Roque de los Muchachos, on the Canary Island of La Palma. The two objects that resemble satellite antennae are actually the Cherenkov MAGIC I and MAGIC II gamma-ray telescopes.
On my first venture into the apparent abyss, my eyes caught the curious, white glow of a soft triangular cloud stretching along the western horizon and extending upward to a peak midway to the zenith- I assumed this was the remains of twilight reflecting off a patch of fog. However, Dr. Martínez-Delgado soon corrected this perception and explained I was witnessing the Gegenschein or Zodiacal light- the rarely seen scattering of the Sun's rays on a ring of interplanetary dust aligned with the planets' orbital track. It became noticeably bright as my pupils enlarged to gulp more of the sparse light.
On a subsequent rooftop visit, I witnessed the majesty of the Milky Way arching nearly overhead from Cygnus to Scorpio, rising in the south east. The dark lanes that part this region were silhouetted against a blizzard of stars positioned much farther in the distance. As if viewing a long exposure photograph, I could discern the general shape and traced many edges of this occulting material. This view of the Milky Way's closest dust lanes from the building's observational deck made them remarkably real.
Overall, the three day observing run enabled the team to compile enough data that only a DVD could contain it. Following a short sleep, after the last imaging session, a taxi arrived and transported us down the mountain to the La Palma's airport where a thirty minute flight awaited, bound for Tenerife and my evening appointment.