# Earthshine blog

## "Earthshine blog"

A blog about a system to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

## Drift alignment

Mechanical designPosted by Peter Thejll May 02, 2012 03:01PM
Here are some data relevant for performing a better alignment of the mount. By taking pairs of images with 10 minutes between in the East, South and West, while tracking at sidereal rate, it is possible to see which way the centre-of-field drifts and from that deduce where the mount polar axis is pointing and how it should be corrected to point more closely at North. Center-of-field is determined from astrometric plate solutions using astrometry.net.

The center-of-field coordinates have been measured on JD2456049. Field 1 comes before field 2 in each pair. RA and DEC are:

E1: 18:20:11.665, +03:17:26.152
E2: 18:20:08.522, +03:17:49.439 which is 23 arc seconds more Northernly

S1: 13:54:10.407, -19:06:52.553
S2: 13:54:09.741, -19:06:43.784 which is 9 arc seconds more Northernly

W1: 09:13:41.508, -10:49:04.877
W2: 09:13:39.970, -10:49:08.317 which is 4 arc seconds more Southernly

Analysis:

When tracking due East centre-of-field drifted North
When tracking due West centre-of-field drifted South
When tracking near the meridian centre-of-field drifted North

Now, what does this imply? I am thinking it means that our Polar axis points East of true North, and is not elevated enough.

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later:

In order to calculate the angles that the mount needs to be turned by we need a method to analyze such positional data as we have above. I am testing various least-squares approaches in order to determine the 3D rotation matrix. After a lot of work it turns out that accurate fits are difficult to get when the number of positions is small. I generated artificial examples with known rotations and 60 pairs of points and ended up with solutions that were only within +/-1 degree of the known answer.

For the three pairs of points found above, the solutions were very small numbers, easily 0 within the +/- 1, or more, error limit expected.

At least this is consistent with a situation where the polar axis is quite well aligned already, and that a better alignment will require delicate work with the adjustment screws indeed!