Plate Solving

The "match this image of stars to a sky catalogue and compute exact pointing + rotation + pixel scale" tool. Plate solving is the secret sauce that makes modern astro automation work: after a goto, the session manager solves the current image, computes the offset, and corrects the mount until the target is dead-centre. Same trick underlies polar alignment and astrometry-based satellite tracking.

Plate solving is mostly invisible — it lives inside Mount & Session Control and is called by N.I.N.A / Ekos / SGP / Voyager / ASIAIR. But knowing which solver is doing the work matters for speed and reliability, especially on a Pi (see Pi-based Astro Control).

Free / OSS solvers

• ★ ★ Astrometry.net — Apache, the OSS foundation; published 2010 (Lang, Hogg, Bovy, et al.). Builds quad-based hashes from star catalogues (Tycho-2, 2MASS, UCAC4, Gaia DR3) and matches arbitrary star fields blind (no initial guess needed). Web service at nova.astrometry.net plus local binaries. The fallback that always works given enough time.
  • https://astrometry.net
• ★ ★ ASTAP (Astrometric STAcking Program) — freeware, Win/Linux/Mac. The 2026 default solver inside N.I.N.A and Ekos. Hint-based (you supply approximate RA/Dec/scale); blazingly fast (sub-second on a modern PC); ships matched star DBs (G05, V17, H18); also does FITS viewing and stacking. Closed-source binary, freeware, no nag. Han Kleijn maintains it actively.
  • https://www.hnsky.org/astap.htm
• ANSVR (Andy Galasso's local astrometry.net wrapper for Windows) — bundles astrometry.net + Cygwin so it runs locally on Windows. Slower than ASTAP but truly blind. Free.
• All Sky Plate Solver — free Win wrapper around astrometry.net for SGP/APT users.
• PlateSolve2 — free Win solver bundled with PlaneWave's PWI; legacy but still works.

Hybrid / cascade

• ★ Most session managers use a cascade: try ASTAP first (fast, hint-based), fall back to ANSVR or astrometry.net if it fails (blind, slow). N.I.N.A and Ekos both support this out of the box.

Paid

• PinPoint (DC-3 Dreams) — paid (~$100). Closed-source Windows astrometry; very fast; widely used in commercial photometry. Mostly redundant for amateurs given ASTAP exists for free.
• TheSkyX Image Link — bundled with TheSkyX Pro; integrates with Bisque mount automation.
• MaxIm DL Image Link — bundled.

Web / cloud

• nova.astrometry.net — free upload-and-solve service; great for "I just want to know what this is"; rate-limited; needs internet. Good for legacy DSLR / film scans where local index files are awkward.
• Astrobin Plate-solve-on-upload — see Citizen Science & AstroBin; auto-plate-solves any image you upload and overlays the catalog grid.

Index files / catalogue downloads

• Astrometry.net index files — by FoV (4200 / 4203 etc.); ~50 GB to install all bands; ~1 GB if you only do one focal length. Pi setups should install a single matched band to save SD space.
• ASTAP star databases — G05 (~120 MB) for wide-field, V17 / H18 for narrower fields; ~1 GB total covers everything.
• Gaia DR3 chunked subsets — modern solvers increasingly draw from Gaia DR3 directly; ASTAP 2024+ has Gaia-based DBs.

Speed reality

• ASTAP with hint — 0.2–1.0 s on a modern desktop, 1–3 s on a Pi 5.
• ASTAP blind — 5–30 s.
• Local astrometry.net — 5–60 s.
• Cloud astrometry.net — 10 s to several minutes (queue depending).

On a Pi at the scope

• ASTAP runs fine on Pi 4 / 5 (ARM build official); install the G05 DB and you're set.
• Local astrometry.net also runs but slowly; install only the index files matching your FoV.
• Ekos auto-detects ASTAP when present; default is built-in StellarSolver which is itself an astrometry.net fork — competitive with ASTAP, no extra binary required.

What's changing in 2024–2026

• ASTAP has effectively become the default thanks to N.I.N.A and Ekos integration; nova.astrometry.net is the fallback.
• StellarSolver (Robert Lancaster, ~2021) is a built-in astrometry.net library used directly by KStars/Ekos — no external binary, runs everywhere KStars runs.
• Gaia DR3 / DR4 indexed solvers — denser star catalogue; better solving in dust lanes / globular clusters.
• Smart scopes do plate solving in firmware — invisibly; you just point at "M31" in the app.

License / pricing summary

• Free: astrometry.net (Apache), ASTAP (freeware), ANSVR (freeware), All Sky Plate Solver, PlateSolve2, StellarSolver (GPL inside KStars).
• Paid: PinPoint (~$100), bundled inside TheSkyX / MaxIm DL.

Pick this if…

• Default 2026 pick: ASTAP (used inside N.I.N.A, Ekos can use it).
• Linux/Mac, single binary, no install drama: StellarSolver (already inside KStars).
• Truly blind, no clue what the scope is pointed at: astrometry.net (local or web).
• Backup for ASTAP: ANSVR or astrometry.net.
• Paid only if your manager forces it: PinPoint inside Bisque tools.
• Pi at the mount: ASTAP + G05 catalog, or just rely on Ekos / StellarSolver.