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To all those interested in the history of optics and/or astronomy.  If you happen to be visiting Bath [UK] then do make time for a visit to the Herschel House in New Kings Street [about 5 minutes walk from the Roman baths].  At this spot, William Herschel, a noted musician at the time, made his own 7inch newtonian telescope, and with it discovered Uranus in 1781.

This is quiet georgian terrace that runs roughly east west [as shown in map below].  The house itself is on the south side of the street.  You are able to view the receptions rooms, ground floor, basement and the small town garden.  The rooms are furnished in typical georgian fashion, and have various displays.  In the garden, attached to the house, is his workshop - with some of the flagstones shattered when the furnace broke dumping molten speculum onto the damp floor!

Fortunately, the house is on a hillside, with the slope going down from the north, so the view to the south is good - though I imagine that smoke from the houses must have been quite a problem.

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Besides being an outstanding observer, Herschel was a prolific telescope maker, making many scopes for wealthy customers, and you can see a number of interesting items such as a sub aperture pitch lap - apparently he was the first to use this in the polishing process.  The workshop contains replicas of the furnace, a workbench and an observing tool for measuring double star separations.

Newton was the first to make a working reflector, and a few others followed him, but Herschel perfected the art in this house - originally working in the reception rooms, before being banished to the shed.

Something I had not realised was that many of his telescopes were designed with the mirror tilted wrt the tube, to avoid the need for a secondary mirror.  This can be seen most clearly in the illustrations of his 40inch telescope, where his head is shown peeking over the rim of the scope, looking down toward the mirror.  I presume he had a fixed mount for the eyepiece.

This nicely demonstrates the engineering problems that he faced and overcome.  Speculum has, when freshly polished, a reflectivity of about 68%.  This drops to 60% or less when tarnished - a rapid process for speculum.  A second mirror means that the system only transmits 35-45% of the light to the observer.  Clearly Herschel was ready to accept the reduced resolution at the center of the field, due to the tilt, to gain extra light.  Interestingly, his ‘Uranus’ scope is in the classical Newtonian configuration.

You can easily model the tilted or obscured Herschelian scopes in WinLens3D, and compare them with the Newtonian. 

the telescope had a 7 foot focal length, making it an f/14 system.  At these low apertures the difference between a sphere and a parabola is very small [.1microns].  When grinding a mirror, a sphere surface is the natural result, so the polishing process performs the aspherisation.  Apparently Herschel monitored this by undertaking a star test with a movable sub aperture.  When the parabolic form is reached, this could be moved anywhere over the mirror without affecting the image quality. 

One final comment - it’s nice to see that on the day of writing, stunning results are just coming in from the Herschel Space Observatory.  Given that Herschel was a keen nebulae observer, it is very appropriate that these observations should be of gas clouds birthing stars!

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