1.25" Telescope eyepiece with K-Mirror for image derotation

What is a K-Mirror?

A K-mirror is a system of 3 plane mirrors mounted on a common motor axis which runs parallel to the chief ray of the system. If looking at the system parallel to the mirror surfaces, where only the edges of the mirrors remain visible, the middle mirror and the front and back mirror look like the backbone and legs of a capital-K; this illustrates the origin of the name.

The principal use of the element is to rotate a beam that hits the first mirror on some optical axis, hits the middle and exit mirror, and leaves the system on the same principal axis. A frequent implementation occurs in the derotation stages of optical telescopes where a beam angle implied by the optical axis of the telescope is undone to keep its orientation aligned with some downstream optics. Because there is an odd number of mirrors, the overall effect also includes a flip of the image.

Demonstration:
https://www.youtube.com/watch?v=G9--9Awmr-A

How to replicate this project:

1. Print in 0.2mm layers
2. Use 25x25x1.2mm First surface mirrors (there are cheap decorative normal 25x25mm mirrors tiles with a white back paint, you can use a hot air soldering iron to careful remove the back paint, and you have a first surface mirror for very cheap)
3. Use the side slots to slide the mirrors into place
4. Glue everything in place
5. if you have problems with the top mirror leaking outside light, consider wrapping the whole print in Tin foil to block external light.

Why you need a K-Mirror when a dove prism does the same?

• Dove prisms are expensive
• K-mirror works with a wider spectral range (thermal imaging)

All in all this is a 5$ solution to rotating a beam, dont expect wonders (u need better mirrors for that then scratched off decorative tiles).