Jack Topper
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Hadley telescope

My build of the 3D-printed Hadley reflector, from the community’s open design.

General arrangement of the Hadley 114/900 Newtonian telescopeSide elevation. Parallel starlight travels to a 114 mm primary mirror 1,060 mm along the frame, reflects as a converging cone 770 mm back to a 45 degree diagonal mirror, and turns up 130 mm to the focal plane inside the eyepiece. Three rods tie four printed rings together; the mirror sits in a cell with three spring-loaded collimation screws.FOCAL PLANEØ114770130f = 770 + 130 = 900 · f/7.9STARLIGHT
My Hadley build: purple printed rings on metal rods, resting on a plywood mount
My build, 2023.

I built this telescope from Jonathan Kissner’s open Hadley design (Maff on Printables), with modifications of my own. Printed rings and three metal rods hold a 114 mm mirror with a 900 mm focal length. A small diagonal mirror sends the light out the side to the eyepiece.

Why a $20 spherical mirror is enough

Slowest focal ratio at which a spherical mirror is good enoughThe curve N equals the cube root of D over 512 lambda rises from about f/4.5 at 25 mm to f/10 at 300 mm. Hadley 1721 at 149 mm f/10.7 and the 3D-printed Hadley at 114 mm f/7.9 sit above it; Newton at 33 mm f/4.8 sits on it; the same 114 mm mirror at f/4 sits well below.04080120160200240280320f/2f/4f/6f/8f/10f/12f/14MIRROR DIAMETER D (mm)SPHERE IS GOOD ENOUGHerror at best focus ≤ λ/4NEEDS A PARABOLOIDN = (D / 512λ)^⅓NEWTON 1668HADLEY 1721HADLEY 3DSAME MIRROR AT f/4

A spherical mirror is easy to make, but it focuses light from its edge slightly short of light from its centre. A paraboloid fixes that and costs far more. The sphere’s error at best focus is D / (2048 N³). Keeping it under a quarter wavelength needs a focal ratio of at least ∛(D / 512λ), which is f/7.4 for a 114 mm mirror. Hadley runs at f/7.9, so the cheap sphere performs like a parabola.

Where the design comes from

Newton built the first reflecting telescope in 1668. John Hadley built the first one that worked at a useful size in 1721, and in a side-by-side test it matched the magnification and detail of the Royal Society’s 123-foot Huygens telescope. It’s the same layout as this one, three centuries earlier.