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From here:

Herschel 40 foot telescope model Herschel 40 foot telescope model

click for larger

Herschel's 40 foot telescope had a diameter of 48 inches (122 cm) and was f/10. It was not a good mirror (cast in metal) and apparently not used that much.

But my question is about where the eyepiece was planned to be held during normal use (which may not have happened). The Wikipedia article says only:

The telescope was located on the grounds of Observatory House, Herschel's house in Slough, between 1789 and 1840. The first observation with the telescope was on 19 February 1787, when Herschel pointed the then-incomplete telescope towards the Orion nebula, which he observed by crawling into the telescope and using a hand-held eyepiece: "The apparatus for the 40-foot telescope was by this time so far completed that I could put the mirror into the tube and direct it to a celestial object; but having no eye-glass fixed, not being acquainted with the focal length which was to be tried, I went into the tube, and laying down near the mouth of it I held the eye-glass in my hand, and soon found the place of the focus. The object I viewed was the nebula in the belt of Orion, and I found the figure of the mirror, though far from perfect, better than I had expected. It showed four small stars in the nebula and many more. The nebula was extremely bright."

The above quote describes some tests of the mirror when the telescope was not yet complete, I'm asking about the plan for the completed configuration.

Question: Where did William Herschel plan to put their head when looking through the famous 40-foot telescope? Was the plan to peer over the edge of the tube or try to get less off-axis; closer to the center and basically partially block the aperture with the upper body? Or was there going to be a diagonal mirror like a Newtonian telescope?

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  • $\begingroup$ The origin of this question is in the writing of this answer to DIY telescope to observe nebulae $\endgroup$
    – uhoh
    May 17 '21 at 22:56
  • $\begingroup$ I believe Richard Holmes' "The Age of Wonder" has a long chapter on Herschel's work. It's a good read. $\endgroup$ May 18 '21 at 12:02
  • $\begingroup$ The observer will look as is shown in your right hand side picture: over the edge of the tube. The eyepiece of a Newtonian does not have to be at the center of the tube. $\endgroup$ May 18 '21 at 14:01
  • $\begingroup$ @AlexandreEremenko In the case of spherical or parabolic mirrors (i.e. axisymmetric), there will be some distortion moving even half-way between the center and edge. I'll see if I can calculate it tomorrow, but I think it would be quite large. Since the mirror was cast metal rather than polished glass, I wonder if the shape was corrected for off-axis distortion somehow? Or for best resolution did Herschel or other observers actually sometimes move close to the center? $\endgroup$
    – uhoh
    May 18 '21 at 14:22
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    $\begingroup$ @uhoh Who is "Sir Herschel"? The 40 foot telescope was built by William Herschel and it was his son John who was knighted. $\endgroup$ May 18 '21 at 17:00

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