Practical Astronomy Recap: Equatorial Mounts

At our recent Auckland Astronomical Society practical astronomy session, I wanted to tackle a question I hear all the time from newcomers and experienced observers alike: Why do I actually need an equatorial mount, and how on Earth do I set one up down here in the Southern Hemisphere?

Whether you missed our meeting or just need a refresher before setting up your gear in the backyard tonight, here is my breakdown of how tracking works, why field rotation ruins astrophotos, and how to dial in your alignment.

Do Telescopes Even Need to Move?

Before we jump into the mechanics of telescope mounts, I always like to start with a reality check: you don’t actually need to point a telescope.

It sounds counterintuitive, but some of the most impressive observatories on the planet stay completely fixed:

• Liquid Metal Mirrors: India’s 4-meter International Liquid Mirror Telescope spins a pool of mercury at 7.5 RPM to create a parabolic liquid mirror. Because mercury spills if you tilt it, the telescope points strictly at the zenith (straight up) and lets the Earth's rotation sweep targets across its field of view.

• Giant Radio Dishes: China’s 500-meter FAST dish is locked into a natural sinkhole, using suspended cable receivers to sample incoming signals.

• Transit Circles: The historic Airy Transit Circle at Greenwich only moves up and down along the prime meridian. By timing when stars crossed that single vertical line, historical astronomers set the standard for global timekeeping (Greenwich Mean Time).

However, if you want to look at a specific deep-sky object for more than a few seconds, your telescope has to follow the sky.

The Alt-Az Problem: Field Rotation

Most entry-level setups—including popular Dobsonian telescopes—use an Alt-Azimuth (Alt-Az) mount. They move up/down (altitude) and left/right (azimuth). While intuitive to point, they suffer from a major physical limitation for photography: field rotation.

Because the Earth spins on an inclined axis relative to your local horizon, an Alt-Az mount tracking a target causes the field of view to slowly twist over time. While perfectly fine for visual observing or short planetary snapshots, any deep-sky exposure longer than 15 to 30 seconds will turn sharp stars into curved trails.

The Equatorial Solution: Building a "South Pole" Telescope

An Equatorial (EQ) Mount solves field rotation with a simple geometric trick: we tilt the mount's rotation axis so that it aligns parallel with the Earth's axis of rotation.

When you align the mount's Polar Axis to point directly at the Celestial Pole:

1. You effectively turn your mount into a local "South Pole telescope."

2. You eliminate field rotation entirely.

3. Tracking requires motorizing only one axis (Right Ascension) to turn at a steady rate matching the Earth's rotation.

How to Find True South in New Zealand

In the Northern Hemisphere, polar alignment is easy—you point your polar scope at Polaris, a bright star sitting right next to the North Celestial Pole. In the Southern Hemisphere, we aren't so lucky. Our pole star, Sigma Octantis, is far too faint to see with the naked eye.

Here are my favorite practical methods to locate True South from your backyard:

1. The Visual Cross-Check: Find the long axis of the Southern Cross and extend an imaginary line toward the bright star Achernar. The midpoint between them marks the South Celestial Pole. You can also extend that same line through the Southern Cross and intersect it with a perpendicular bisector drawn between the Pointers (Alpha and Beta Centauri).

2. Beware Your Compass: A standard magnetic compass in New Zealand points anywhere from 19.5° to 25° away from True South depending on where you are in the country. If you don't account for magnetic declination, your alignment will be way off.

3. The Solar Noon Trick: Hang a plumb line on a sunny day. Look up the exact time of solar noon (when the sun crosses your local meridian) on a site like TimeAndDate.com. At that precise minute, the shadow cast by your plumb line points dead True South. Mark that line on your driveway or lawn, and you'll have a permanent reference mark for your tripod legs every night!

Dialing in Precision Tracking

Once you're pointed South and tilted to your local latitude (roughly 37° here in Auckland), how do you ensure sub-arcsecond tracking for multi-minute astrophotography?

• Drift Alignment: By watching a star’s drift near the Eastern horizon (to adjust your altitude) and high overhead near the meridian (to adjust your azimuth), you can dial in precise polar alignment manually.

• Camera Helpers: Modern electronic polar aligners (like the iPolar) or plate-solving software make finding the invisible pole trivial compared to the old visual polar scope days.

• Autoguiding: Even with a great polar alignment, mechanical imperfections or thermal flexure cause drift during long exposures. Using software like PhD2 ("Push Here Dummy 2"), a secondary guide camera locks onto a target star and feeds instant micro-corrections to your mount in real time.

Final Thoughts

Modern smart scopes like the Seestar or Dwarf are brilliant pieces of kit and make getting into the hobby easy out of the box in Alt-Az mode. But understanding how an equatorial mount works remains the single best tool in an astrophotographer's kit. Once you align your mount to the Earth's axis, you let physics do the heavy lifting for crisp, pinpoint stars every time.

Clear skies, and see you at the next meeting!