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Posts Made By: Bill Blunk

October 29, 2010 09:25 AM Forum: Telescope Making

RACI Finder Prism

Posted By Bill Blunk

Jim Coughlin said:

I'd like to make my own RACI finder, but I'm not sure what kind of prism to use. I find references to Amici prisms, Amici roof prisms, porro prisms, Abbe porro prisms, etc. Any advice would be appreciated.

Likely Jon's suggestion to just buy an Amici prism diagonal is your most logical and cost effective solution. However, if you still want to build one or perhaps need a larger field for your finder (2" eyepieces?), the following may help. Small Amici roof prisms are available from Surplus Shed for $7.50. Because of the delicate roof (easy to chip!) they are harder to mount than a standard RA prism that would yield a reversed R-L field. If the RACI finder you want is larger, finding an adequate prism is more difficult. WWII elbow telescopes can sometimes be found and their Amici roof prisms used. Usually part of the military mount that holds the prism can be utilized. I made a nice 48 mm clear aperture Amici diagonal like that. The porro prism you mentioned will not yield a RACI finder as it gives straight through viewing with an offset (like porro prism binoculars). One option that WILL also work is a single Porro-Abbe prism. These have the advantage of not using a diffraction-causing roof, so bright stars will not have a horizontal diffraction spike on both sides as can happen with an Amici roof prism when used astronomically. Unfortunately, to get a correct image, you need to not only look 90 degrees vertically from the incoming light path, but also must stand at right angles to the side. This type is not commercially available at all, but I made a 2" one for a spotting scope back when 2" Porro Abbe prisms were available at low cost. Viewing with it took some getting used to. The other commonly available correct image diagonal prism is the Schmidt prism. It is also a roof prism, but by additional reflections has a viewing angle at 45 degrees to the original light path. You specified RACI, so these would not meet your RA = Right Angle requirement. I mention this because some commercially available 45 degree correct image Schmidt prism diagonals are misidentified as "Amici prisms". Hope this helps.