The night sky of the pretelescopic era was a decidedly uncomplicated place. The heavens contained the fixed stars, the luminous band of the Milky Way, the moon and the five wanderers or planets, all known since before recorded history. Occasionally a brilliant comet swept across the sky; there were also shooting stars which appeared and disappeared in the blink of an eye. And very rarely, a brilliant ‘guest star’ would appear where no star had shone before and then slowly fade over the following months, disturbing the otherwise immutable starry vault.

Beyond this were a handful of cloudy spots in the sky, nature unknown, but like the stars they did not move and so could not be atmospheric phenomena. There was the hazy cloud of stars in the tail of Leo, named Coma Berenices, and the compressed grouping of tiny stars known as the Pleiades. There was a hazy patch of light in the Cassiopeia Milky Way which would one day be known as the Double Cluster. And finally there was the luminous patch in Cancer called the Praesepe.

By the time that William Herschel embarked on his career in astronomy, the telescope had been in use for over 160 years. Progress in improvement and refinement of the instrument had been painfully slow, however, with a combination of factors responsible for the circumstance.

The primary problem with the refractor telescopes of the seventeenth and eighteenth century was spherical and chromatic aberration, a result of the fact that these were single lens instruments that could not overcome their inherent faults. The solution that was found was to increase the focal lengths of the instruments. Where the simple Galilean telescopes of the mid-seventeenth century had apertures of under 2 inches and focal lengths of 2 or 3 feet, instrument makers found that increasing the focal lengths by a factor of 5 or 10 times helped reduce the effects of the aberrations, though they were not nearly eliminated. Over the course of the succeeding decades, astronomers began using telescopes with focal lengths of well over 100 feet and in some cases over 200 feet. Not surprisingly, these were clumsy and cumbersome instruments to use; it was difficult enough aiming the telescope at a selected object, let alone following the object across the sky for any length of time. It was only in the mid-eighteenth century when the first achromatic telescopes were developed that the long refractor telescopes passed into history.

On the occasion of William herschel’s election to the Royal Society on 7 December 1781, his friend William Watson presented him with a copy of Charles Messier’s ‘Catalogue of nebulae and star clusters’, the now famous list of 103 objects which appeared in the Connaissance des Temps for 1784.

At the time, Herschel was preparing to embark on his third survey of the heavens. The first, carried out in the years before 1779, was a survey of all stars of magnitude 4 or brighter, examined with a 7-foot reflector of 4.5-inch aperture. The second survey had begun in August 1779, and was an examination of all stars brighter than magnitude 8 with a 7-foot reflector of 6.2-inch aperture. The primary result of that survey was the compilation of a catalogue of double and multiple stars, numbering 269 in all, which was due to be published in the Royal Society’s Philosophical Transactions early in 1782. An unexpected byproduct of the survey had been Herschel’s discovery of the planet Uranus, the event which transformed Herschel into a professional astronomer of world renown.

Visual observations

The visual observations for the following guide were carried out by the author during two principal time periods: 1992–2001 and 2006–2010. In 1992, a computer printout of the Herschel catalogue was obtained from David and Brenda Branchett of the Astronomical League. The list had been generated by Father Lucian Kemble to facilitate an ongoing project to observe the entire Herschel catalogue with modern amateur telescopes.

All objects in this guide were located by the starhopping method, whereby the observer uses star charts to proceed from a known location to a new target. The present author’s procedure was to record observations on a preprinted form. In addition to identifying the target, the date and time observed, viewing conditions, location, telescope and magnification used, a written description of the object was included and further supported by a field sketch with surrounding stars placed as accurately as possible. Later, each observation was compared with a Digitized Sky Survey (DSS) image of the object in question. If the star patterns and general appearance of the object matched the DSS image, the object was recorded as seen. In the overwhelming majority of cases, the first attempt was successful. However, there were occasional faint objects or objects located in crowded fields that were not successfully recorded the first time. If the DSS image indicated a negative observation, the procedure was repeated until success was attained.

It is the evening of 5 September 1784. As darkness descends on the village of Datchet, the bustling activity of the day slowly draws to a close. A few gentlemen, mostly in pairs, walk the streets at a leisurely pace, taking their evening constitutionals, quietly discussing the day’s events. Their way is lit by a few widely spaced lamps, but these will be extinguished shortly; no need to waste precious fuel lighting streets long after everyone has gone to bed. The doors of the village pub open and close with diminishing regularity as the last of the patrons leave and make their way home. By 9:30 p.m. the village is largely quiet, save for the occasional bark of a dog.

It is a beautiful, star-lit night with a slight breeze and the air is crisp, suggesting that the cool days of autumn are not far off. After several days of cloud the moonless skies are clear and the brilliant summer Milky Way arches overhead, seen in all its glory. A short way from the village centre stands a solid two-storey house with a low stone wall extending away down the road. The house is dark save for the light of a single lamp near an open window on the second floor. In the garden the silhouette of a curious structure can be seen rising into the night sky, a peculiar construction of slanted spars and cross-beams, integrated with a pair of ladders on both sides. It appears to be more than 20 feet high and in the centre is a considerable tube, like a cannon but far larger, pointed almost vertically into the sky. There is a small platform off to the side of the tube near the top and one can see that there is a man standing there, leaning over the tube in the darkness. He is silent for a while but then calls out quietly to the ground below and all of a sudden the structure moves ever so slightly, turning on the casters supporting it. There is another man, this one standing at the base of the structure, who next turns a hand crank slowly just above his head. He stops immediately upon a second command and the night is again silent for several minutes.

Preface

This book makes extensive use of images from the Digitized Sky Survey (DSS) and the author wishes to thank Lynn Kozloski, Space Telescope Science Institute, Scott Kardel, California Institute of Technology, and Sue Tritton, Royal Observatory Edinburgh, for their permission to use images originally obtained by the Oschin Schmidt Telescope of Palomar Observatory and the United Kingdom Schmidt Telescope of the Anglo-Australian Observatory in the catalogue portion of this book.

We extend our appreciation and thanks to Andreas Maurer and the Antique Telescope Society for permission to use material from their publication A Compendium of All Known William Herschel Telescopes in the chapter entitled ‘The telescope maker’. Mr Maurer also graciously provided the photograph of the 10-foot telescope built by William Herschel and presented to Göttingen University, as well as the photograph of the replica of the 25-foot Spanish telescope.