OnStep & DIY Mount Firmware

The "I have a manual mount and a Mega 2560 and a soldering iron" tier — open-source firmware that drives stepper motors over a microcontroller and emulates LX200 / SkyWatcher-protocol goto. Replaces the proprietary hand-controller stack on cheap mounts (Vixen GP, Celestron CG-5, EQ3, EQ5), retrofits direct-drive on heavy fork mounts, and runs custom builds. Pairs well with MCU Platforms and Arduino / PlatformIO.

For the software talking to the mount over LX200/serial see Mount & Session Control; for Pi-based control hardware that often hosts OnStep see Pi-based Astro Control; for higher-level smart-scopes that hide all this see Smart Telescopes.

OnStep — the OSS reference

• ★ ★ OnStep (Howard Dutton) — GPL, runs on Teensy 3.x / 4.x, ESP32, STM32 Blue Pill / Maple, Arduino Mega 2560. Implements full goto + tracking + autoguide + meridian flip + PEC + ASCOM/INDI/Alpaca interfaces. Drives almost any stepper-motor mount with appropriate driver shields (TMC2130 / TMC5160 silent steppers ideal). The DIY mount-control standard since ~2015.
  • https://onstep.groups.io (community wiki + active forum)
  • https://github.com/hjd1964/OnStep
• ★ OnStepX — newer rewrite by Howard Dutton; Teensy / ESP32 / STM32 focused; cleaner code structure; adds support for absolute encoders, focuser/rotator add-ons, dome control, weather. Most new builds in 2024–26 use OnStepX.
• ★ SmartHandController (SHC) — companion ESP32-based hand-controller with screen + buttons that talks to OnStep over Wi-Fi / Bluetooth / serial. Replaces the SynScan-style hand control. Free.
• OnStep Wi-Fi addon — ESP8266/ESP32 module exposing OnStep over LAN/Wi-Fi as a TCP LX200 server. Talks to KStars, SkySafari, Stellarium directly.

OnStep variants / deployments

• Mini OnStep — minimal Mega 2560 + DRV8825 build; the "I have $40 and an EQ5" recipe.
• MaxPCB / MaxESP / MaxSTM — Howard Dutton's reference PCBs; most clean DIY builds use one.
• PrimaLuceLab Eagle / "Esatto" wrappers — paid commercial repackagings of OnStep-style control. Closed.

AstroEQ

• AstroEQ (Tom Carpenter) — free Arduino-based goto retrofit; predates OnStep; specifically targets SkyWatcher EQ3-2 / EQ5 / HEQ5 motorisation. Less universal than OnStep but simpler for that specific use-case.
  • https://www.astroeq.co.uk

TeenAstro

• TeenAstro (Charles Lemaire) — Teensy-based; OnStep-adjacent; supports more advanced encoder feedback and direct-drive motor control. Smaller community than OnStep.
  • https://groups.io/g/TeenAstro

RDP / commercial controllers

• Pegasus PowerBox / NYX-101 mount controller — commercial; closed; integrates power + USB hub + dew control + mount control.
• PrimaLuceLab Eagle — paid (~€1500) Pi-with-software-tax box; runs StellarMate-like stack but commercial.

Direct-drive / encoder retrofit

• Mountwizzard4 — free Win/Mac/Linux GUI that talks to OnStepX / 10Micron / SkyWatcher; especially useful with absolute encoders for "build your own 10Micron".
• PointXP / pointing model files — many DIY direct-drive builds add an external pointing model; OnStepX has limited model support, MountWizzard4 adds more.

Stepper drivers to know

• TMC2130 / TMC5160 (Trinamic) — silent operation, microstepping up to 256, the modern default.
• DRV8825 — older; cheap; loud; works.
• A4988 — older; for very light mounts only.

Power

OnStep boards typically run at 12 V; large mounts pull 2–4 A peak. Use real beefy supplies, not phone chargers; a 12 V LiFePO4 with low-voltage cutoff for field use.

Pi as host

A Raspberry Pi running Astroberry at the mount, hosting INDI, talking to OnStep over USB-serial: this is the standard "OSS observatory in a backpack" build. See Pi Classic Projects.

What's changing in 2024–2026

• OnStepX maturity — feature parity with commercial controllers for most amateur use-cases.
• ESP32-S3 / RP2040 ports — newer MCUs joining the supported list.
• Direct-drive motor support — TMC5160 + absolute encoder builds putting ~30 kg payload mounts within DIY reach.
• 3D-printed direct-drive mounts — community projects (E2NX2, Stargate, Rainbow Astro replicas).
• Alpaca exposure — OnStep's Wi-Fi server now speaks Alpaca alongside LX200; cross-platform clients work.

License / pricing

• OnStep / OnStepX / AstroEQ / TeenAstro / SHC / Mountwizzard4 — all FOSS / GPL.
• Hardware: Teensy 4.1 ~$30, ESP32 ~$5, TMC5160 driver ~$15, MaxPCB v3 PCB ~$60. A complete DIY OnStepX controller is buildable for under $200.

Pick this if…

• OSS goto retrofit on any stepper mount: OnStepX on Teensy 4.1 + TMC5160 drivers.
• EQ3 / EQ5 / EQ6 specifically: OnStepX or AstroEQ; both work.
• Hand controller, not just Wi-Fi: SmartHandController.
• DIY direct-drive mount: OnStepX + absolute encoders + Mountwizzard4.
• Already paying for commercial: 10Micron / Paramount — different category, not DIY.
• Smart scope owner: none of this applies; the firmware is closed.