# Why — or why not — build a Stevick-Paul?

A short, blunt list. For detail see [`BUILD_NOTES.md`](BUILD_NOTES.md) and
[`DESIGN.md`](DESIGN.md).

## Reasons TO build one

- **Unobstructed aperture.** No central obstruction and no spider — maximum contrast,
  no diffraction spikes, better mid-spatial-frequency MTF for planetary/low-contrast work.
- **Aberration-free image.** All third-order aberrations (spherical, coma, astigmatism,
  distortion) vanish except a gentle field curvature; the raytraced 8" f/12 is
  diffraction-limited (RMS 0.77 µm vs an 8.05 µm Airy radius).
- **Perfectly achromatic** — it is all-reflecting, like a Newtonian.
- **Only three mirrors to figure** — a paraboloidal primary, a convex sphere, a concave
  sphere, plus a flat. At f/15 the primary may even be left **spherical**.
- **Long focal ratio is easy on eyepieces** and gives high power for planets.
- **Compact for its focal length** — especially the *folded* (FSP) version.
- **It scales up well** (the design warns only against scaling *down*).
- **Rare and rewarding.** It teaches tilted-component optics and precise geometric
  alignment; not many exist, and the design is genuinely elegant.

## Reasons NOT to build one

- **Not a first project.** Four precision surfaces — two of them *powered* spheres — is a
  big step up from a single Newtonian mirror.
- **The convex secondary is the crux.** Convex surfaces cannot be Foucault/Ronchi tested
  directly; you must make the concave tertiary first and use it as a contact/interference
  test plate. This is the step that sinks most attempts.
- **Off-axis alignment is a new skill.** There is no star-test collimation procedure; the
  mirror tilts are locked to the primary tilt and you align to geometry, not to a blurry
  star.
- **The diagonal fit is razor-thin.** The flat must thread between the collimated beam and
  the converging cone with only a fraction of an inch of clearance — get it wrong and it
  shadows the aperture. You *must* derive it from a full-scale drawing.
- **The focal plane is tilted (~9°) and gently curved**, and vignetted ~½ magnitude at the
  field edge. The eyepiece (or sensor) must be tilted to match. Poor choice for wide-field
  imaging.
- **It cannot be made fast.** Focal-plane tilt scales as 1/f-number, so the practical floor
  is about **f/10**; the design range is f/10–f/15 only.
- **More light loss.** Four reflections vs two for a Newtonian (~0.85 vs ~0.92 with
  enhanced coatings).
- **Essentially no support ecosystem** — no kits, no parts, almost no community, and the
  design software is long gone. You are largely on your own.
- **Long, awkward, non-standard mounting**, and no simple tube plan.
- **High cost, time, and risk** for what at 8" is an incremental gain over a good,
  carefully made slow Newtonian.

## Bottom line

Build one if you are an **experienced mirror maker who wants the challenge and the
unobstructed, coma-free image**, ideally as a **folded f/12–f/15 with a spherical
primary**. If this is your first telescope, build a Newtonian first — the SP will still be
there when you're ready.