Northern hemisphere ETX alignment

These fields of view, through two different finders, are centered on the North celestial pole. They show the position that Polaris should occupy in your finder's field of view at your longitude and your local time (below) if you want to point your telescope accurately due North.

Field of view 6 degrees

This straight-through finder is supplied with the ETX 90. Polaris appears to rotate around the pole anti-clockwise.

Field of view 8 degrees

This right-angle finder is supplied with the ETX 125 and, because of the mirror, the view is left-right reversed. Polaris appears to rotate around the pole clockwise.

Effect of error

If your telescope is perfectly level the only alignment error comes from taking the direction of Polaris as due North. The maximum error occurs when your local sidereal time (LST) is either 08:30 or 20:30 and Polaris is well to the left or right of the pole. At times around 02:30 and 14:30 LST, when Polaris is apparently directly above or below the pole, the error is small.

When the error is worst, it increases from zero at your zenith, reaching one degree at the altitude of Polaris, and a maximum value of arctan(pi / (180 * cos(latitude)) at your horizon. The effect is particularly bad if you are at a high latitude.

This table shows how many degrees away from the correct position your telescope will be, given your latitude (left hand column) and the altitude of an object and assuming the worst case one degree mis-alignment.
Degrees in error
Altitude

0 10 20 30 40 50 60 70 80 90

Latitude 0 1.0 1.0 0.9 0.9 0.8 0.6 0.5 0.3 0.2 0.0

10 1.0 1.0 1.0 0.9 0.8 0.7 0.5 0.3 0.2 0.0

20 1.1 1.0 1.0 0.9 0.8 0.7 0.5 0.4 0.2 0.0

30 1.2 1.1 1.1 1.0 0.9 0.7 0.6 0.4 0.2 0.0

40 1.3 1.3 1.2 1.1 1.0 0.8 0.7 0.4 0.2 0.0

50 1.6 1.5 1.5 1.3 1.2 1.0 0.8 0.5 0.3 0.0

60 2.0 2.0 1.9 1.7 1.5 1.3 1.0 0.7 0.3 0.0

70 2.9 2.9 2.7 2.5 2.2 1.9 1.5 1.0 0.5 0.0

80 5.7 5.7 5.4 5.0 4.4 3.7 2.9 2.0 1.0 0.0

Degrees in error
		Altitude
		0	10	20	30	40	50	60	70	80	90
Latitude	0	1.0	1.0	0.9	0.9	0.8	0.6	0.5	0.3	0.2	0.0
10	1.0	1.0	1.0	0.9	0.8	0.7	0.5	0.3	0.2	0.0
20	1.1	1.0	1.0	0.9	0.8	0.7	0.5	0.4	0.2	0.0
30	1.2	1.1	1.1	1.0	0.9	0.7	0.6	0.4	0.2	0.0
40	1.3	1.3	1.2	1.1	1.0	0.8	0.7	0.4	0.2	0.0
50	1.6	1.5	1.5	1.3	1.2	1.0	0.8	0.5	0.3	0.0
60	2.0	2.0	1.9	1.7	1.5	1.3	1.0	0.7	0.3	0.0
70	2.9	2.9	2.7	2.5	2.2	1.9	1.5	1.0	0.5	0.0
80	5.7	5.7	5.4	5.0	4.4	3.7	2.9	2.0	1.0	0.0

This page updated 11th October, 1999. email rpb@pobox.com. Home page.