Mirror-alignment on a shoestring

I don’t have the right tools for this, so how do I then proceed?

The spectrograph looks like this:

geometry-overview

The light enters from a fiber port near the top left corner and is reflected of off a mirror (not installed at the time the photo was taken). The beam from the fiber is horizontal and must be turned 90° in the xy-plane, and 6,6° upwards in the xz-plane (x being left-right, y being top-bottom and z orthogonal to the baseplate).

The distance from the mirror to the center of the 1st parabolic mirror, which is where the two pencil-lines in the photo cross each other is 111,5 mm in the xy-plane and 112,3 mm in space. It will be impossible to align the first mirror with a ruler – which is all I have.

The precision of my ruler is around 1/4 of a mm, which is around 2% on a distance of 11 cm. However, if I “extend” the spectrograph to 2 m, this number decreases to less than 0,1%.

This is shown in this next photo:

DSC03317

The laser beam comes in from the top right and is reflected by the mirror on a 3D-printed kinematic mount. At a distance of 11,5 cm from the mirror the laser beam must be 13 mm higher on the z-axis, but at 2 m from the mirror this translates to 23,2 cm.

Obviously it’s much easier to achieve an elevation of 23,1 cm with acceptable tolerance compared to 1,3 cm, when all you have is a ruler..

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