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As promised,  a picture of the "apparatus" is posted below.

The roof-top prism is standing vertically in the corner of a cut-down cardboard box, with the 1.5m focal length cylindrical prism about 4 inches in front of it. Note that this is not a fully developed and finished device, this is an experimental set-up to see if the idea worked.

Normally, having a dichroic coating put on any optical component is prohibitively expensive. In this case, I found some thin roof-top prisms with dichroic coatings on ebay from a vendor in China. They were probably rejects or surplus, but they were going for a few bucks each. The coatings were not on the front face, however, but on one of the side faces that includes the right angle. So, I stuck two prisms together using a couple of small pieces of double sided sticky tape to make one large prism. You can see the join in the middle of the front face of the prism. The cylindrical lens I already had from some project in the mists of time, so it ended up as a very cheap instrument. It all worked out exactly as planned.

I will now contemplate a more permanent set-up. Lining it up should be fun....

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