StepSolve

A headless plate-solving appliance for Raspberry Pi. It continuously captures sky images, solves them to determine exact telescope pointing (RA/Dec), and publishes those coordinates over LX200 TCP for SkySafari — optionally syncing them to an OnStepX mount controller.

The key principle: the product is the headless service, not the UI. It starts at boot, runs unattended, and survives power cuts. The web dashboard is a phone-friendly "how's it going?" window.

System context

                    ┌───────────┐
                    │ SkySafari │
                    └─────┬─────┘
                Go To     │     ▲
                (LX200)   ▼     │ Position readout
                    ┌───────────┐         ┌──────────────┐
                    │  OnStep   │◄────────│  StepSolve   │
                    │  (mount)  │  :CM#   │  (this app)  │
                    └─────┬─────┘         └──────▲───────┘
                          │ Move                 │ Capture & solve
                          ▼                      │
                    ┌───────────┐         ┌──────┴───────┐
                    │ Telescope │────────►│  Pi Camera   │
                    └───────────┘         └──────────────┘

SkySafari connects independently to OnStep (for Go To) and StepSolve (read-only position validation). StepSolve corrects OnStep's alignment after each solve via :CM#.

Operating modes

Mode	Capture	Solve	LX200
Solve	Continuous	Yes	Publishes solved RA/Dec
Demo	No	Yes (bundled images)	Publishes real solve results
Idle	No	No	Returns last known position

Demo mode runs the real solver against bundled sky images — useful for testing the full pipeline on the Pi without a live sky.

Development (macOS)

# Run tests (no hardware or star catalogs needed)
dotnet test tests/

# Run the app (starts idle by default in Development environment)
dotnet run --project src/

# Web dashboard
open http://localhost:5001

# Test LX200 protocol
echo -n ':GR#' | nc localhost 5002

The app detects macOS and skips rpicam-still. In Solve mode on Mac, it cycles through bundled demo images instead of capturing.

Deployment (Raspberry Pi)

See deploy/PI_SETUP.md for the full setup guide.

First install — flash Pi OS, then from your Mac:

PI_HOST=pi@stepsolve.local bash scripts/deploy.sh --install

Subsequent deploys (build + rsync + restart in ~30 s):

bash scripts/deploy.sh

Cut a release (builds, packages, and publishes to GitHub Releases — users can then update via the dashboard):

bash scripts/release.sh v1.0.0

The service is a single self-contained binary managed by systemd. No runtime dependencies on the Pi beyond rpicam-still (bundled with Raspberry Pi OS) and optionally solve-field for the Astrometry backend.

Solver backends

Three backends are supported, selected via Solver:Backend in configuration:

Backend	Binary	Notes
astrometry	solve-field	Default. Most robust; requires index files.
tetra3	Python subprocess	Fast; good for Pi. Pure Python, no native deps.
cedar	Python subprocess	Tetra3 fork with optimised database; back-end wired, not in UI yet.

Backend switches take effect on the next solve cycle — no restart needed.

Configuration

Settings live in appsettings.json (defaults), appsettings.Development.json (Mac overrides), and appsettings.runtime.json (persisted UI changes). The web dashboard's Settings panel writes to the runtime file; those values survive restarts and override the defaults.

Key settings:

Setting	Default	Description
StepSolve:Mode	solve	Operating mode
Solver:Backend	astrometry	Active solver
Solver:FovEstimateDeg	0	Camera diagonal FOV — set this for your hardware
Camera:ShutterUs	1000000	Shutter speed in microseconds
OnStep:Enabled	false	Enable mount sync after each solve

Ports

Port	Protocol	Purpose
5001	HTTP / WebSocket	Web dashboard and REST API
5002	TCP (LX200)	SkySafari connection

Both are advertised via mDNS (stepsolve.local) when Avahi is configured.

Documentation

• docs/PRD.md — full specification: API reference, solver details, LX200 protocol, deployment, architecture
• docs/REWRITE_PLAN.md — migration rationale from SkySolve (Python predecessor)