9 Polar Alignment
An equatorial mount only tracks the sky properly when its polar axis points at the celestial pole, the point the whole sky appears to rotate around (close to the star Polaris in the northern hemisphere). Polaris dedicates the POLAR tab to getting it there, and ships two workflows for the job. Both rely on plate solving: taking a short photo and matching its stars against a catalog to work out exactly where the telescope is pointing. TPPA (Three-Point Polar Alignment) is the default and what most setups should use: it sweeps the mount through 30 degrees of right ascension (RA), plate-solves three points along the way, and fits the mount’s polar axis from those solutions. Rudimentary alignment is the second option, a single-target iterative helper designed for setups where TPPA cannot work, typically because the polar region is hidden or the mount cannot slew freely.
Because both workflows depend on plate solving, a solver must be configured before you start. ASTAP (Kleijn, n.d.), the plate-solving program bundled with Polaris, works without any extra setup; PlateSolve3 and the online astrometry.net service need credentials or paths. Solver configuration lives with the rest of the equipment setup in Chapter 5.
9.1 TPPA: three-point polar alignment
TPPA is the plate-solve-driven method. Polaris captures a frame, slews the mount in RA, captures and solves again, and repeats until it has three solved points spanning about 30 degrees of RA. From the arc those three solutions trace, it computes where the mount’s polar axis actually points and how far it sits from the true pole. The result is a live error arrow that tells you which way to turn the altitude and azimuth bolts, the two adjustment bolts that tip the mount’s axis up-down and left-right.
After the initial measurement, refinement mode takes over: a continuous capture-and-solve loop with an on-screen error overlay that updates as you adjust the bolts, so you can watch the error shrink and converge to near zero without re-running the whole procedure.
Because TPPA fits the axis from three independent solutions, it separates polar misalignment from other pointing errors and typically converges in fewer iterations than the rudimentary method. It does, however, require a clear enough sky and a mount that can slew (move under motor power) through the 30-degree RA sweep, while still tracking, without hitting an obstruction.
9.2 Rudimentary alignment: single-target iterative
Rudimentary alignment exists for the situations TPPA cannot handle:
- Balconies and urban observatories where the polar region is blocked.
- Mounts that cannot slew freely in RA without obstruction.
- Setups you have already aligned roughly with a compass and a tilt app on your phone, where you just want a one-target sanity check.
- Manual mounts with no GoTo (no motors that can point the mount at coordinates on command): you point at the target by hand, and Polaris only captures and solves.
The workflow mirrors what experienced operators of the ASIAIR (ZWO’s popular all-in-one imaging controller) already do by hand.
First, coarse-align physically. Use a magnetic-declination calculator (NOAA’s online tool, for example) plus a tilt app on your smartphone, and align the mount as close to true polar as you can eyeball it. This part happens outside Polaris; five minutes with the phone gets you within a degree or two.
Second, pick a known, visible target: anything bright above the horizon, such as Sirius, Vega, or M42. Naked-eye visibility helps, not because the solver needs it (plate solving works fine on faint targets) but because you want to know roughly where the camera is pointing.
Then click Start with Slew to target selected. Polaris commands the mount to GoTo the chosen coordinates, waits for it to settle, captures one frame, and runs a plate solve. It then reports:
- the pointing the mount actually achieved (RA and Dec, where Dec is declination; together they are the sky’s equivalent of longitude and latitude);
- the azimuth error and altitude error, in arcseconds or arcminutes (an arcminute is 1/60 of a degree; an arcsecond is 1/60 of that);
- the total error magnitude, colour-coded green below 1 arcminute, amber from 1 to 5 arcminutes, and red above 5 arcminutes;
- a green dot (where the target should be) and a red dot (where the mount ended up) on the embedded sky map.
Now walk over to the mount and nudge the azimuth or altitude knob a little, in the direction the error indicates. The sign convention:
- Positive azimuth error means the mount is pointing east of where it should be; turn the azimuth knob westward.
- Positive altitude error means the mount is pointing above the target; turn the altitude knob down.
Click Re-capture + solve. Polaris captures another frame at the same mount position, without slewing this time, and re-computes the error. The convergence sparkline below the result block shows the trend across iterations: the bars get shorter and greener as you converge. Repeat until you are satisfied. Unlike TPPA there is no auto-stop; you decide when good enough is good enough.
For manual mounts without GoTo, run the same loop with Use current position instead of Slew to target. You aim the mount by hand and Polaris captures and solves wherever it is pointing.
Why the math works despite being an approximation
A single plate-solved frame cannot distinguish polar misalignment from mount pointing-model error, the small mechanical imperfections every mount has (cone error, where the telescope tube is not quite parallel to the mount’s axis; axes that are not exactly perpendicular; and so on). Rudimentary alignment attributes the entire pointing offset to polar misalignment, which is mathematically wrong, but it works iteratively. After one or two manual knob nudges, the polar component dominates the change between iterations, while the mount-model component is constant and vanishes from the visible error arrow once you are a couple of iterations in. This is the same approximation used by SharpCap (a popular Windows capture program) in its Plate-Solve Polar Alignment and by KStars (the open-source planetarium and imaging suite) in its single-target mode, and the procedure that ASIAIR and N.I.N.A. (another free imaging suite) operators already run by hand.
9.3 Accuracy expectations
The colour coding gives you the coarse scale: green below 1 arcminute, amber between 1 and 5 arcminutes, red beyond. In practice, most operators settle at 30 to 60 arcseconds of total error for visual use or wide-field imaging; serious deep-sky imaging targets less than 30 arcseconds. TPPA’s refinement mode makes the sub-arcminute range straightforward to reach because the overlay updates continuously while you turn the bolts.
9.4 Pre-flight requirements
Before the rudimentary workflow starts, a pre-flight strip at the top of the sub-pane shows a check or cross for each prerequisite:
- Camera connected: required, since the workflow captures frames.
- Mount connected: required for Slew to target mode, optional for Use current position mode (manual mounts).
- Site location: required, because the math needs your latitude and longitude to convert RA/Dec to local altitude and azimuth (how high in the sky, and toward which compass direction). Set it under Settings → Site location.
9.5 Choosing between the two methods
| Situation | Use |
|---|---|
| Full view of the polar region, mount can slew 30 degrees in RA freely | TPPA (more rigorous, fewer iterations) |
| Balcony, blocked polar view, or limited slew range | Rudimentary |
| Manual mount (no GoTo) | Rudimentary with Use current position |
| First-night setup after assembly | Rudimentary for a fast initial check; follow up with TPPA if you have time |
| Permanent observatory, periodic check | TPPA (cleaner result) |
9.6 Troubleshooting and known limitations
The plate solve must succeed. If the frame has too few stars (overcast sky, fogged optics, very short exposure), the solve fails and Polaris asks you to bump the exposure or gain.
Single-target math drifts near the zenith. Near the zenith (the point straight overhead), the azimuth term gets amplified by cos(altitude). Polaris compensates for this in the reported number, but the underlying physics still mean an alignment run near the zenith is less informative than one closer to the celestial equator. Prefer targets at moderate altitude.
There is no auto-convergence threshold. Some users want the loop to stop when the total error drops below, say, 30 arcseconds. Today this is left manual; the sparkline and colour coding are enough signal for most operators. A configurable per-rig threshold is a possible future addition.
For a polar-alignment step in the context of a complete session, see the first-night walkthrough in Chapter 4; general plate-solve failures are covered in Appendix C.