Polar alignment

Your location

Latitude Longitude (east +)

Longitude matters: 1° off moves Polaris by 2 minutes on the dial.

Date and time

Live

Choose a date and time

Your polar scope

My polar scope shows an inverted image

Number at the top of the reticle

Celestial pole (centre)
Where the star goes
Its motion over the next hour

Where to put the star

on the reticle dial, on the circle.

You are in the southern hemisphere: the visible pole is the south celestial pole, and the guide star is Sigma Octantis (see below).

Near the equator the pole is very low over the horizon: you need a clear horizon, and atmospheric refraction slightly upsets the alignment.

Guide

An equatorial mount follows the sky by turning a single axis, provided that this axis points at the celestial pole. Polaris is not exactly at the pole: it is about 0.65° away and circles it once a day. This page works out where it is right now in your polar scope reticle.

First: an upright reticle

The dial only means something if the reticle is upright, with its 0 (or 12) at the top, vertical. On German equatorial mounts this is the case when the right ascension (RA) axis is in its home position: counterweight bar straight down, tube above the mount.

To check it once and for all, in daylight: set the counterweight bar vertical (a spirit level held against it helps), look through the polar scope at the vertical edge of a distant building or a pole, and see whether the reticle’s 0–6 line is parallel to it. If not, loosen the ring or screws that hold the reticle, turn it until the 0 is at the top, and tighten.

Then, every time you align, put the RA axis back in that position before looking: the position given here applies directly, with no calculation.

The dial allows for the polar scope’s image. Almost all polar scopes invert it: like a small astronomical telescope, they turn the image upside down (a 180° rotation). To check, look at a distant object in daylight: if the top of a tree appears at the bottom, the image is inverted. If not, untick “My polar scope shows an inverted image”.

The method step by step

Level the tripod (bubble centred). Otherwise every azimuth adjustment slightly changes the altitude, and the other way round.
Turn the mount so its polar axis points roughly at the visible pole: north in the northern hemisphere, south in the southern hemisphere. A compass is enough (magnetic north is a few degrees off true north), or Polaris with the naked eye.
Set your latitude on the mount’s altitude scale.
Remove the polar scope caps. On many mounts you also need to turn the declination axis to line up the hole through it. Switch the reticle light on, as dim as possible.
Put the RA axis in its home position, counterweights down: the reticle is upright, 0 at the top.
Without touching the RA and declination motions or the tripod, turn only the azimuth (left-right) and altitude (up-down) bolts to bring the star onto the circle, at the position shown here. Make the last adjustment by tightening the bolts against each other so it stays put.
Leave the tripod alone. Then point at your targets with the RA and declination motions: tracking now only needs the RA axis to turn.

How precise does it need to be?

For visual observing a rough alignment is enough. Within 1 or 2° (latitude on the scale, mount facing north, Polaris roughly centred in the polar scope), an object stays in a low-power eyepiece for many minutes; you just re-centre it now and then. You do not even need the polar scope.

For photography the error shows. A polar alignment error E makes stars drift by at most E × 2π × t / 1,436 min during an exposure of length t: with E = 5′, about 7″ in 5 minutes. A polar scope used well gives a few arcminutes: enough for a camera lens, often borderline for long exposures at long focal lengths. Astrophotographers then aim for better than one arcminute.

The atmosphere lifts anything low in the sky a little (about 1′ at 45° altitude): the methods below take it into account, the polar scope does not. It does not matter visually.

For imaging: drift alignment and plate solving

Drift alignment (Bigourdan’s method): follow a star near the meridian and the celestial equator with an illuminated reticle eyepiece or a camera, and correct the azimuth from its drift in declination; then repeat with a star low in the east or west to correct the altitude. It is slow but precise, and you do not need to see the pole.

Plate solving: capture software recognises the stars on two or three images taken while turning the RA axis, works out where the axis points and tells you which way to turn the bolts. Open-source programs do this, for example N.I.N.A., Ekos (KStars) or the drift tool in PHD2.

Southern hemisphere: Sigma Octantis

In the south no bright star marks the pole. The nearest, Sigma Octantis, about 1° from the pole, is only magnitude 5.4: visible to the naked eye only under a really dark sky, and barely in a small polar scope. This page gives its dial position the same way; it turns clockwise.

Many reticles therefore draw the “Octans trapezium”: Sigma, Chi, Tau and Upsilon Octantis, four stars of magnitude 5 to 6 around the pole. Turn the RA axis to orient the drawing like the trapezium in the sky, then adjust azimuth and altitude until each star falls into its small circle.

Star positions: Hipparcos catalogue (ESA, new reduction by F. van Leeuwen, 2007), with proper motion. Precession, nutation and sidereal time: the open-source Astronomy Engine library (Don Cross). Place search: OpenStreetMap’s Nominatim.