goalpaca

Go libraries and tools for ASCOM Alpaca astronomy devices: clients, device servers, simulators, and conformance checks.

Requires Go 1.23 or later. Uses only the standard library and supports Linux, macOS, and Windows.

Try the simulators

From a checkout, serve all ten device types on HTTP port 11111:

go run ./cmd/alpacasim

Connect your Alpaca application to 127.0.0.1:11111, or use its discovery feature. Each device type has device number 0. Open http://127.0.0.1:11111/setup for the device list and setup pages.

Discover servers from another terminal:

go run ./cmd/alpacadiscover -timeout 2s

Useful simulator options:

Flag	Purpose
-port 11112	Change the HTTP port
-discovery off	Disable UDP discovery (enabled by default)
-ipv6	Also answer IPv6 multicast discovery
-quiet	Disable per-request logging

Discovery uses UDP port 32227. Clients on another machine need access to both the HTTP port and discovery port. Direct addresses work without discovery.

Use the client library

Add goalpaca to your Go module:

go get github.com/mikefsq/goalpaca

Read a camera's sensor width:

package main

import (
    "fmt"
    "log"

    "github.com/mikefsq/goalpaca/client"
)

func main() {
    cam := client.NewCamera("127.0.0.1:11111", 0)
    if err := cam.SetConnected(true); err != nil {
        log.Fatal(err)
    }
    defer cam.SetConnected(false)

    width, err := cam.CameraXSize()
    if err != nil {
        log.Print(err)
        return
    }
    fmt.Println(width)
}

The client supports all ten Alpaca device types and both JSON and ImageBytes camera transport. See client examples and API documentation.

Write a driver

See DRIVERS.md for device interfaces, hardware lifecycle, configuration, standalone binaries, and testing. The simulators provide working implementations without hardware.

Packages and tools

Package	Purpose
alpaca	Protocol types, errors, and ImageBytes encoding
client	Typed clients and discovery
server	HTTP serving, discovery, setup forms, and protocol validation
registry	Driver registration and construction from configuration
devicemain	Command-line entry point for standalone drivers
sim	Simulators for all ten device types
conformance	Device checks based on ConformU

cmd/discover_proxy answers discovery for servers using goalpaca's registration extension; see discovery relay setup. cmd/fault_proxy injects faults for testing client recovery; see its usage guide.

Standalone drivers using devicemain accept -discover to scan attached hardware and print JSON before configuration loading or device construction. The response contains driver, supported, identity keys in preference order, and devices with label and configuration values. Unsupported scanners return supported: false; a supported scan with no matches returns an empty array. Scan errors appear in JSON and cause a nonzero exit. Scanners receive a ten-second context deadline and must leave devices already in use undisturbed. Custom launchers can call devicemain.Discover for the same output contract. This hardware scan is separate from -discovery and client network discovery.

Tests

go test ./...
go test -race ./...

The suite covers protocol handling, clients, simulators, and ConformU-derived checks. Hardware drivers need their own tests for capabilities, limits, and failure handling.

References

• Go API documentation
• Vendored Alpaca OpenAPI specifications

License

MIT, © 2026 @mikefsq. Vendored ASCOM specifications retain their upstream MIT notices.