Stevick-Paul Telescope

Design notes, a buildability assessment, and an exact raytrace of the Stevick-Paul telescope (SPT) — an unobstructed, all-reflecting, tilted-component design that is free of third-order aberrations. This repository focuses on an 8” f/12 instrument.

The arrangement

Incoming collimated light → paraboloidal primary → convex spherical secondary → concave spherical tertiary → focus, extracted by a flat diagonal. The two spheres are of mating curve (radius = 0.4 × the primary radius of curvature). All mirrors are used off-axis so the aperture is unobstructed.

Exact raytrace layout of the 8" f/12 Stevick-Paul

Exact raytrace result

The geometry was reconstructed from the archived design manual plus Bob Novack’s labelled Folded-Stevick-Paul diagram, then refined by exact raytracing. The one unknown — the principal-ray angle out of the tertiary — solved to 9.000°, which is the focal-plane tilt. The resulting system is:

  • Radii: +192” paraboloid, +76.8” convex sphere, +76.8” concave sphere
  • Spacings: e1 = 57.6”, e2 = 76.8”, e3 = 38.4”; offsets a1 = 6”, a2 = 12”
  • Principal-ray angles: 2·T1 = 5.945°, 2·T3 = 4.459°, 9.000°
  • Verified diffraction limited: geometric spot RMS 0.77 µm on-axis, ~1.06 µm at 1° field, versus the 8.05 µm Airy radius (λ = 550 nm, f/12)
  • Diagonal placement checked by raytrace: the flat threads tangent between the collimated beam and the converging cone with no aperture obstruction (a naive placement blocked ~10% of the collimated beam — see TRACE.md §5)

Spot diagrams (tiled with the Airy disk):

Spot diagrams at 0.00, 0.25 and 0.50 degrees field

Airy disk diameter versus wavelength and f-number:

Airy disk diameter plots

Repository contents

File Description
why-why-not.md Short list: reasons to build — or not build — a Stevick-Paul
FSP.md How to build the folded variant (FSP), with CAD drawings
DESIGN.md Collected facts, equations, history, and tradeoffs
BUILD_NOTES.md Buildability assessment, f/12 vs f/15, risk register, why f/10+ only
TRACE.md Exact prescription, method, results, and caveats
sp_layout.png / .svg / .dxf CAD-style meridional layout
sp_spotdiagrams.png Spot diagrams at 0.00°, 0.25°, 0.50° field
sp_airy.png Airy disk diameter vs wavelength and vs f-number
sp_model.py Reproducible conic raytracer and build_sp()
make_outputs.py Regenerates all figures

Reproducing the figures

python3 -m pip install numpy scipy matplotlib
python3 make_outputs.py

Sources

The original Amateur Telescope Makers Journal #3 article (1993) and Stevick’s own raytracer are not available; this reconstruction is based on the archived material at:

This is an independent reconstruction, not the original prescription; it reproduces Stevick’s published values and his claimed aberration-free image.