LEARNED IT · 09/26
Compass north is eight degrees off, and the mount has no way to know
A phone compass points at magnetic north. At Goolwa that is about 8 degrees east of true north, and an alt-az goto inherits the error whole. Measured it at 7.4 degrees by centring Venus by hand and reading how far the mount had to move.
The AZ-GTe starts from an assumption and never checks it: tube level, pointing true north. There is no GPS, no magnetometer, no accelerometer, no level sensor anywhere in the mount. Two motor step counters, and dead reckoning from wherever the tube happened to be at switch-on. Nothing in the system can catch a bad start.
So the start has to be right, and a phone compass cannot make it right. Magnetic north is not true north. At Goolwa the difference is roughly 8 degrees east, and that offset goes straight into every goto for the rest of the night.
The measurement
On 12 September, after ninety minutes of empty frames, I centred Venus by hand with the arrow keys and then read the raw axis counts back off the mount:
| correction needed | |
|---|---|
| azimuth | 7.4° |
| altitude | 0.9° |
7.4 degrees against a declination of about 8. The small altitude figure is the tripod not being quite level, which is the second-order error and clearly not the one that matters.
For scale, the SV205 on the Mak sees 0.2 by 0.13 degrees. A 7.4 degree miss is thirty-seven fields of view. There was never a moment where the planet was about to drift into frame. Even a 25 mm eyepiece at about 1 degree is seven fields short.
Why tracking still works when goto doesn't
This is the part that keeps fooling me, and it fooled me again tonight.
Goto needs the alignment to be accurate. Tracking only needs it to be roughly right. Once the mount believes it is on Venus it drives both axes at the rates Venus needs, and those rates come from where Venus really is in the sky, not from where the tube is actually pointing. A few degrees of pointing error changes the rates by a hair.
So a badly aligned mount tracks beautifully — on empty sky. Smooth, confident, and wrong.
The check that proves nothing
I "verified" the pointing by reading the axis counts, converting them to altitude and azimuth, and comparing against the ephemeris. They matched to a few hundredths of a degree, and I took that as proof the tube was aimed correctly.
It proves nothing. The conversion from axis counts to a sky position assumes home was level and true north — the exact thing in question — so it can only ever agree with itself. What the match really showed was that the mount was tracking at Venus's rate, which is true whether the alignment is perfect or seven degrees out.
Any alignment check has to come from outside the mount's own model. Look through the eyepiece. Plate solve. Put your eye on the sky. Do not ask the mount whether the mount is right.
What to do instead
Don't bother getting home perfect. The reliable move is:
- Point roughly north and level, close enough.
- Find one bright thing by hand — Venus, the Moon, Sirius — with an eyepiece or the red dot finder.
- Sync or one-star align on it. Thirty seconds.
- Every goto after that inherits the correction, including the 7.4 degrees.
If setting home by compass anyway, swing about 8 degrees west of where the needle points to land on true north. Declination drifts slowly and is different everywhere, so it is a site number, not a universal one.
Related: [[declination-sets-the-ceiling]] is about a completely different declination — the target's, and how high it can get. Two words, two meanings, both of them cost me a session.