The 40foot telescope

After the publication of his two great papers on the construction of the heavens, Herschel busied himself on a more mundane level with the construction of his next great telescope, the so-called 40-foot telescope of 49 inches diameter. Already in the early 1780s he had been making smaller telescopes for sale to raise money for the large telescope he dreamed of. But he chafed at the loss of time to work on his own projects. As Caroline noted, ''[H]e was then on the wrong side of forty-five, and felt how great an injustice he would be doing to himself and to the cause of Astronomy by giving up his time to making telescopes for other observers.''43 Sir Joseph Banks, President of the Royal Society, came to his rescue with the promise of a £2,000 grant, which was later followed by a second.

The second half of the decade brought moments of great satisfaction for both Caroline and William, in the midst of all their work on the great telescope. In 1786, while William and Alexander were in Germany on business, Caroline enjoyed some uninterrupted time at her own small telescope and discovered her first comet. This success was repeated in 1788 with a second, and in 1790 with two more. Altogether she discovered eight comets in her spare time, a number not far out of line with that of professional comet-hunters such as Messier, who in the course of a lifetime discovered (or independently co-discovered) 20. By the time she discovered her eighth comet in 1797, Caroline was so familiar with the night sky that she began her nightly searches with the naked eye, relying on her memory of the constellation patterns to bring anything unusual to her notice.44

In January 1787, Herschel tried a simple experiment with his 20-foot, 19-inch aperture telescope that was spectacularly successful. He removed the small secondary mirror in this Newtonian design instrument and arranged his eyepiece so he could view the light directly off the primary mirror (see figure 4.4, the ''Herschelian''). By eliminating one of the reflections in the optical path, he decreased the loss of light that inevitably occurs, also. To make the new arrangement work, he tipped one side of the primary mirror, so that the light could be brought to a focus off to the side of the primary axis of the telescope. With this slightly improved light-gathering ability, the 20-foot telescope showed two previously unknown moons of ''his'' planet Uranus, moons now called Titania and Oberon.45 The experiment encouraged Herschel to design his 40-foot telescope, still in the making, as a ''front view.'' Coincidentally, one of the first observations he made with the 40-foot—in August 1789, before the telescope was completely finished—revealed two new moons of Saturn, Mimas and Enceladus.

Although the mirrors for the 40-foot telescope tarnished quickly, and the instrument as a whole required such time-consuming maintenance that Herschel often preferred to use the 20-foot, the completion of this behemoth must also count as one of Herschel's great rewards during the late 1780s (figure 4.9). Although by this time he had a small army of smiths, carpenters and polishers working for him, he never lost interest in the finest details of its construction. ''There is not one screw-bolt about the whole apparatus but what was fixed under the immediate eye of my brother,'' Caroline noted.46

For William, the second half of the 1780s also brought a happy change in his personal circumstances. In 1788, he married a wealthy widow, Mary Pitt. Financial worries eased, and

Figure 4.9 Herschel's 40-foot focal length reflecting telescope, of aperture 49 inches. (Credit: Royal Astronomical Society.)

William began to allow himself more time for pleasurable activities such as hosting concerts at his house and taking his wife, and, later, his son John on vacations.

The change occasioned by William's marriage was not such a happy one for Caroline, now aged 38, for after 16 years, she was being displaced as a caregiver and companion to William. She mentions her brother's marriage in her memoir only in connection with how busy she was in the months preceding it: ''[I]t may easily be supposed that I must have been fully employed (besides minding the heavens) to prepare everything as well as I could against the time I was to give up the place of a housekeeper, which was the 8th of May, 1788.''47

The pages of her memoir and journal pertaining to the period 1788 to 1789 do not survive, a fact that her family members attribute to her initial bitterness toward her sister-in-law over her new situation, and later ''calmer judgement which counselled the destruction of all record of what was likely to be painful to survivors.''48 But Caroline had adapted to her evolving opportunities as housekeeper, singer, and assistant astronomer, and she adapted once again to her new situation. She later explained to the wife of her nephew John how she came to be granted a salary from the crown to continue her astronomical work: ''I refused my dear brother's proposal (at the time he resolved to enter the married state) of making me independent, and desired him to ask the king for a small salary to enable me to continue his assistant.''49 This salary was granted, making Caroline the first woman to be paid to do astronomical work, although payments proved to be irregular. Caroline moved to her own apartments in town, and commuted on foot to William's house to continue her astronomical duties.

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