Aligning an equatorial mount (three options)

Aligning an equatorial mount (three options)

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An equatorial mount should be set up so that the hour axis is parallel to the Earth's axis,
i.e., the upper end of the hour axis must point exactly north, and the angle between the axis
and the horizontal (the polar latitude) must be equal to the geographical latitude of the observation site. The polar latitude can be roughly set in advance using the scale on the mount or, better yet, with a digital spirit level. Precise polar alignment, also called polar alignment, is a prerequisite for
- astrophotography with long focal lengths and long exposure times
- the use of setting circles when searching for celestial objects.

Polar alignment with the polar finder, the Kochab method
Quick and convenient.

The polar finder is a small finderscope, usually built into the hollow hour axis, sometimes
also mounted on a special bracket parallel to the hour axis. For accurate alignment, the polar finder must be illuminated so that its markings can be seen. Please note that the declination axis lies across the hour axis; it has a hole at the height of the polar finder scope. The declination must be adjusted so that the hole clears the view through the polar finder scope.

For a rough alignment, the polar finder scope is aimed at Polaris. For a precise alignment,
it must be taken into account that Polaris is approximately 40' from the pole. The distance is marked on the polar finder scope,
but in which direction? Here, you can orient yourself using the star Kochab (beta UMi). The
celestial north pole is almost exactly on the straight line connecting Polaris and Kochab, i.e., Polaris and Kochab are opposite each other at the pole. However, the image in the polar finder scope is rotated 180°, so Polaris must appear to be on the same side as where we see Kochab with the naked eye.

Procedure: 1. Look at the Little Dipper with the naked eye and imagine a clock face, centered at Polaris. At which numeral is Kochab? 2. Look through the polar finder scope and adjust the mount so that Polaris is at the same numeral where you just saw Kochab.

North alignment using the Scheiner method, "Einscheinern"
Well-known and proven, but time-consuming. (after Julius Scheiner, 1897)

You need a crosshair eyepiece and align a star precisely with the crosshairs. Then you let the mount track for a while. As long as the mount is not precisely aligned, the star will drift away from the crosshairs. From the direction of the drift, you can determine in which direction you need to adjust the mount's setup.
(More details in Wikipedia: Scheiner method)
North alignment with Excel, the Lüthen-Kahlhöfer method
Digital, direct, convenient.

A camera with a standard lens is attached to the telescope and pointed at the celestial pole. Then,
two images are taken, differentiated by rotating the camera around the hour axis.
For three stars, the rectangular X-Y coordinates are measured on both images. We also need
the right ascension and declination of the three stars, the longitude and latitude of the observatory, and
the time of the image. Analysis using an Excel spreadsheet yields the necessary corrections
for the north direction and the polar altitude in degrees and the corresponding number of turns of the adjustment screws.

The Excel spreadsheet and detailed instructions are available for free download at:
www.sternwarte-nms.de, Menu: Info/Privacy Policy / Downloads / North Alignment with Excel

Jürgen Kahlhöfer (copyright free)