Tracking the Sky: Why Equatorial Mounts Exist
Every long-exposure astrophoto has the same problem: the sky moves. To keep pinpoint stars in a 30-second frame you need to compensate. An equatorial mount is one elegant answer, and it explains why most serious astrophotography rigs look strange.
The problem: two-axis motion
As Earth rotates, stars trace circles across the sky. In your local frame — the alt-az frame where 'up' is up and 'east' is east — that motion is split awkwardly across two axes. A star rising in the east climbs in altitude and moves southward in azimuth at the same time, at rates that depend on where it is.
An alt-az mount can be motorized to track this, but it takes coordinated motion on both axes and the field-of-view rotates over time. Not a problem for visual observing; a major problem for cameras.
The trick: align one axis with Earth's
Tilt the mount so one of its axes points at the celestial pole — parallel to Earth's rotation axis. Now that axis (the right-ascension or polar axis) can be turned at one steady rate, opposite to Earth's rotation, and the entire sky stays put. The perpendicular axis (the declination axis) is used only to point at a new target and then locked.
That is an equatorial mount. From Miami's latitude (25.7° N), the polar axis is set 25.7° above the horizon and pointed north. The counterweight, which balances the telescope on the opposite side of the polar axis, is what gives equatorial mounts their unmistakable silhouette.
Types of equatorial mount
| Class | Payload | Typical price |
|---|---|---|
| Star tracker | ≤ 3 kg (camera + short lens) | $300 – 500 |
| Entry EQ | ≤ 7 kg (small refractor) | $500 – 900 |
| Mid-weight EQ | 10 – 20 kg (imaging rig) | $1,500 – 3,500 |
| Heavy EQ | 25 – 45 kg (large OTA + camera) | $5,000 – 12,000 |
| Strain-wave / direct-drive | harmonic drive, no counterweight | $3,000 – 8,000 |
As a rough rule, a mount is happy at about half its rated payload for imaging. Overloaded mounts oscillate; under-loaded ones don't self-correct backlash well. Buy heavier than you think you need.
Polar alignment: the ritual
The mount only tracks perfectly if its polar axis is truly parallel to Earth's. Aligning it — 'polar alignment' — is a five-minute chore on every session.
- The three ways to polar-align
- 01Polar scope + Polaris: rough-align by putting Polaris in the small scope built into the polar axis, at a specific position on a reticle that indicates true north. Good to about 5 arcminutes.
- 02Plate solving (SharpCap, ASIAIR, etc.): the camera takes a shot, software identifies the star pattern, and gives you exact corrections. Good to under 1 arcminute in a few minutes.
- 03Drift alignment: the classical method. Point at a star near the meridian and celestial equator, expose for a minute, note north-south drift, adjust azimuth. Repeat near the eastern horizon for altitude. Perfect but slow.
Auto-guiding: the second-order correction
No mount is mechanically perfect. Gears have periodic errors of a few arcseconds. For exposures longer than 30–60 seconds at typical focal lengths, you need auto-guiding: a small second telescope with its own camera watches a guide star and sends fine corrections to the mount every second.
The standard software is PHD2 (free). A modest guiding setup — a 50 mm guide scope and a small monochrome camera — costs under $250 and turns a decent mount into an imaging platform capable of 5-minute unguided-looking frames.
Frequently asked
- Do I need an equatorial mount for a phone star tracker?
- Modern star trackers (Star Adventurer, SkyGuider Pro, MSM Nomad) are essentially miniature equatorial mounts — you point their polar axis at Polaris and they turn a single axis to cancel Earth's rotation. That's the entire mechanism.
- Why is a mount so much more expensive than the telescope?
- Because the mount's mechanical performance — arcsecond-level pointing and tracking of significant mass — is what actually determines whether your images are sharp. A great telescope on a wobbly mount is a wobbly telescope.
- Can I use an equatorial mount from the southern hemisphere?
- Yes — polar-align to the south celestial pole. There's no bright star there, but multiple polar-alignment techniques (Sigma Octantis reticles, plate solving) work fine.