Transit Telescopes, 05 February 2003
Good evening. It's nice to see so many here tonight to take part in this Workshop on transit telescopes. Why have I chosen this topic? Well it's quite simple: there is a transit telescope at Orwell Park Observatory, situated in a room adjoining the equatorial dome. No Victorian observatory was built without a transit telescope and, as a result, in that era they were common instruments; however, nowadays they are generally rare museum pieces.
A transit (or culmination) occurs when a celestial body, during its daily path across the sky, crosses the observer's meridian. All celestial bodies cross the meridian twice a day: upper transit is the crossing closer to the observer's zenith; lower transit is the crossing further from the zenith. Two consecutive transits of a body are, of course, twelve sidereal hours apart. It is possible to observe both upper and lower transit only for circumpolar bodies; other bodies will be beneath the horizon for the lower transit.
Transits have a long and important place in the history of astronomy:
- One of the first transit instruments ever set up could well be the one that still exists today on Salisbury Plain. I am, of course, referring to Stonehenge! The reason that I call Stonehenge a transit instrument is that it could have been used to observe celestial alignments.
- The first person to observe a telescopic transit was Ole Rømer (1644-1710). Working in Copenhagen he pointed his telescope out of a south-facing window in 1675. To aid estimation of the moment of transit, he added a set of cross hairs in the focal plane.
- An early experiment at Greenwich, needed to validate theories of navigation, proved that the Earth rotated in a uniform manner. This was done by observing the transit of Sirius through a fixed telescope. Sirius was chosen as it is bright enough to observe in daylight. A fixed telescope could be used as transits of a body always occur at the same altitude.
Of course, early observations of transits were of low accuracy. But as observing techniques and instrumentation improved, accuracy also improved. Whereas early transit telescopes used a single vertical wire, later transit observers were able to obtain improved accuracy by using instruments with several vertical wires. Always an odd number was used, either three or five or seven: that is one central wire and pairs of wires stepping away from it on either side. The transit was timed over each wire and the average calculated. For a body like the sun or moon that shows a disc, a further gain in accuracy could be obtained by timing the transits of both the preceding and following limbs. A by-product of this approach was to enable estimation of the angular size of the body by recording the difference in times over each wire and taking the average.
In fact, there are several ways in which to use the wires. They can be left unilluminated, in which case a star will wink out as it passes behind, but of course the observer has no warning that the transit is about to occur and may be unprepared for the disappearance. The wires can be illuminated from the front (i.e. bright wires on a dark field), but this can make the timing of transits of faint stars difficult. Alternatively the wires can be lit from behind (i.e. dark wires on a bright field) but again this can make faint stars hard to see.
Early transit instruments in fact comprised two separate devices:
- a relatively large telescope, fixed in the meridian, was used to record the instant of upper transit,
- a smaller telescope, known as a wall quadrant (mural quadrans in Latin), fitted to a graduated arc, was used to record the altitude of the transit.
It was not until 1850 with the Airy Transit Circle (ATC) at the Royal Greenwich Observatory that the two instruments were combined.
The theory of transits is based on the mathematical relationship that Local Hour Angle (LHA) equals Local Sidereal Time (LST) minus Right Ascension (RA). On the meridian, LHA is of course zero, so RA=LST. LST can be derived from Universal Time (UT) so by timing a transit it is possible to estimate the RA of the body observed. Knowledge of RA is of course a prerequisite to creating a map of the sky. As local time is derived from the instant of solar transit it is possible to detemine longitude by comparing an observed transit time of a star with the predicted time of the event at a reference meridian (typically Greenwich).
Historically, a star whose position was known precisely was called a clock star. Observations of transits of clock stars were used to correct clocks. Following the Industrial Revolution, the electric telegraph network spread across the world and was used to distribute time signals from national observatories to whomever was prepared to pay to receive the service. In the mid to late nineteenth century rich industrialists often established country estates where they could live free from the pollution that their factories produced. The estates where often in remote parts of the country not accessible to the electric telegraph, so how could the owners know the time? Good quality clocks were available, but neeed to be set, and to be corrected to ensure that they kept to time. It was to this end that the transit telescope enjoyed a resurgence in popularity as a small, cheap, easy-to-use device that a member of domestic staff could be trained to operate. All the hard mathematics was eliminated by the annual publication of tables with pre-worked solutions leaving the need for only a few simple sums to provide a reliable, accurate, astronomical estimate of time. The book A Treatise on the Transit Instrument as Applied to the Determination of Time by Josiah Latimer Clark, published in 1882, exemplifies this approach.
No measuring tool is free from error and the transit instrument is no exception. As Tycho Brahe knew, it is better to quantify the errors and eliminate them mathematically in the data reduction than to constantly tinker with the telescope in an effort to remove the errors completely. So what are the errors that can affect a transit telescope? There are three main sources: collimation, level and azimuth error.
- Collimation error occurs when the cross-wires are not on the optical axis of the instrument. The error is constant for all altitudes. It can be quantified by reversing the instrument, i.e. picking it up, turning it over and replacing it on the mounting thus changing the sign of the error. It is not possible to do this with the ATC due to its size and weight.
- Level error occurs when one mounting is higher than the other. This error is zero on the horizon and maximum at the zenith. It can be either positive or negative. There two ways to estimate this error. The first is to use a striding level, a sensitive spirit level mounted in a frame so that it can stand on the pivots of the instrument. Errors in the use of the striding level itself are removed by reversing it end-for-end and averaging the readings. The second method is to take a nadir observation, that is to set the telescope pointing vertically downwards over a carefully levelled mirror or mercury-bath and to compare the position of the cross wires with that of their their reflection. In a perfectly adjusted instrument the reflection will coincide with the wires themselves; an instrument out of the vertical will produce a reflection separate from the wires. This task is made easier by the use of a Bohnenberger eyepiece. This device is an eyepiece with a right angle prism fixed inside the barrel. A light is made to shine on the prism which reflects it down the telescope tube to illuminate the wires and cause their shadow to be seen in the eyepiece image. Unfortunately the pillars that support the ATC do not go down to a common foundation; further, the two pillars are made from different materials and so expand and contract at different rates as the temperature changes through the year. As a result, the instrument is particularly susceptible to level error.
- Azimuth error occurs when the transit is not truly in the meridian plane. This error is maximum at the horizon and zero at the zenith. It can be of either sign i.e. either east or west of the local meridian. This error can be estimated from a long run of observations by comparing the observed times of transit against altitude of the body observed. Once the transit instrument is fixed in the meridian plane, a reference mark can be set up on the earth's surface, distant from the observatory, and used to provide an occasional check of the alignment of the central wire. Of course, the check needs to be done in daylight, or the meridian marker illuminated at night. The ATC uses a reference marker in the form of an obelisk on the northern horizon several kilometres away in Chingford. Appropriately this marker stands on a hill known as Pole Hill.
Am I talking about an obsolete subject? No. At the La Palma observation site there is a small dome with a transit telescope in it. The name of this telescope is the Carlsberg Automatic Transit Telescope.
Thank you for your attention.
Orwell Park transit refractor.
Frontispiece of Latimer Clark's book.