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This answer to When was the first true Gregorian telescope built? explains:

Newtonian uses one concave and one flat mirror (or just one concave).

Gregorian uses two concave mirrors, and

Cassegrain uses one concave and one convex mirror.

All these mirrors should be parabolic in an ideal situation. First Gregorian was made by Hooke in 1673.

This was 5 years before John Hadley was born. Hadley made a parabolic mirror in 1721, for a Newtonian telescope btw.

There was certainly known that for Newtonian telescope optics, at some point when the f/no becomes small enough spherical aberration will limit the resolution rather than diffraction, and that a more parabolic shape can avoid this particular problem.

When grinding optical surfaces, near-spheres are the easiest, the tend to come naturally. There are ways to make them aspherical known to present-day amateur telescope makers.

But I don't know anything about how Hadley's first parabolic mirror was made parabolic, nor how it was tested to verify its asphericity, nor what it was used for; a Newtonian telescope or a more complex system with two or more curved surfaces.

Question: How did Hadley do it and why exactly? How did Hadley make the first parabolic telescope mirror, verify its asphericity, and for what kind of telescope was this work done?

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  • $\begingroup$ Can you explain your last question? I thought it was pretty clear these were all astronomical 'scopes; only the Moon and planets (and their moons) are resolvable objects, so those would be what can be viewed more clearly (on-axis) with a parabolic mirror. $\endgroup$ May 19 at 12:32
  • $\begingroup$ @CarlWitthoft I see what happened, I'd clarified the question in the body of the question but the title didn't get fixed. It now reads the same as in the body of the telescope. $\endgroup$
    – uhoh
    May 19 at 12:58
  • $\begingroup$ @CarlWitthoft What about double stars as resolvable objects? $\endgroup$
    – D. Halsey
    Jun 23 at 23:06
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    $\begingroup$ @D.Halsey I'm assuming you mean resolve as two separate point sources; I used to have a great table of the brighter star-pairs and their angular separation, which can be compared to the angular resolution possible without adaptive optics. Sadly lost that somewhere in the past. $\endgroup$ Jun 24 at 12:51

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