Observing
How to Align an Equatorial Mount
Polar alignment is the difference between a mount that tracks and a mount that merely rotates. Three methods, in increasing order of precision.
An equatorial mount has one axis — the polar or right ascension axis — that must point at the celestial pole for tracking to work. When it does, the sky's rotation becomes a single rotation about a single axis, and one motor at sidereal rate keeps a target centred. When it does not, the target drifts, and the drift is worse at longer focal lengths and longer exposures.
How accurate do you need to be?
| Activity | Required polar alignment error | Practical method |
|---|---|---|
| Visual observing at low power | ≤ 1° (60 arcmin) | Rough alignment by eye, or a compass and latitude scale |
| Visual at high power, GoTo accuracy | ≤ 15 arcmin | Polar scope |
| Unguided imaging, short focal length | ≤ 5 arcmin | Polar scope, refined |
| Unguided imaging, 1000 mm+ | ≤ 2 arcmin | Drift alignment |
| Guided imaging, any focal length | ≤ 5 arcmin is fine | Polar scope plus a good guide calibration |
The encouraging part: if you are guiding, you do not need heroic alignment. The guider corrects the residual. What you do need is consistency, so that the guider is not fighting a large, variable error.
Method one: rough alignment
Level the tripod. Not for optics — for repeatability, and so the azimuth and altitude adjustments move independently.
Set the mount's latitude scale to your site latitude. Check it against a phone, not against memory.
Rotate the mount in azimuth until the polar axis points north (south in the southern hemisphere). A compass gets you within a few degrees; remember magnetic declination.
Look through the polar scope housing, or simply sight along the axis, and confirm Polaris sits near the centre of the field.
That is enough for visual work and for GoTo accuracy after a proper star alignment. It is not enough for unguided imaging.
Method two: the polar scope
A polar scope is a small telescope inside the mount's polar axis with a reticle showing the position of Polaris relative to the north celestial pole. Polaris is about 0.7° from the true pole, so you do not centre Polaris — you place it on the marked offset for the current date and sidereal time.
Rotate the RA axis so the polar scope's reticle is oriented correctly — usually with the small circle at the top or bottom depending on the mount.
Compute the hour angle of Polaris for your date and time, or use an app that draws the reticle for you.
Adjust altitude and azimuth to place Polaris on the computed position within the reticle circle.
Lock both adjustments firmly. Check afterwards that tightening did not shift the axis — it often does.
Verify the polar scope is itself aligned with the RA axis before you trust it. Rotate the RA axis through 360° with the scope fitted; if Polaris traces a circle rather than staying put, the scope needs collimating via its own small grub screws.
A polar scope alone typically gets you to 5–15 arcmin. That is genuinely sufficient for guided imaging and for most GoTo work. Drift alignment is for unguided exposures.
Method three: drift alignment
Drift alignment uses the sky as the reference. The principle: a star near the meridian and near the celestial equator drifts in declination if your polar axis is misaligned in azimuth, and a star near the horizon in the east or west drifts in declination if it is misaligned in altitude. Correct the drift and you have corrected the axis.
Do a rough alignment first. Drift alignment refines; it does not replace.
Put a medium-high power eyepiece in, with the reticle or crosshair if you have one.
Choose a star near the meridian and within about 20° of the celestial equator, and centre it.
Watch for drift over two or three minutes. North drift means one azimuth error, south means the other.
Adjust azimuth in small increments, re-centre, and repeat until no drift is visible over five minutes.
Now choose a star low in the east or west, again near the equator, and repeat using the altitude adjustment.
Alternate the two axes — they interact slightly — until drift is imperceptible in both.
Software-assisted alignment (plate-solving or a drift-alignment routine in your capture program) does the same measurement far faster and more precisely than the eye. If you already run a laptop at the mount, use it — the manual method is still worth knowing, because it is what you do when the laptop will not boot.
After alignment: balance and backlash
Alignment is worthless if the mount is unbalanced. Balance in both axes with the full imaging load in place, and with the telescope roughly where it will sit during the session. Slight east-heavy bias is conventional for imaging, so that the drive gear stays loaded on one side.
Measure backlash too: command a small guide pulse in each direction and watch how long the star takes to respond. Enter that figure into your guiding software rather than guessing it.
Alignment accuracy figures are guidance. Achievable results depend on mount mechanics, payload, seeing and how carefully each step is performed.
This article is educational reference material. Figures are typical approximations, and no observing result is guaranteed — your equipment, sky and conditions will differ.