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Stefan this reminds me of an antique brass refractor I saw once for measuring double stars where sone distance down the OTA (in front of the focal plane) there was an optically flat placed in the beam, but the plate was sawn in half so that one half could be tilted by a micrometer to displace half the beam laterally, crating a double image. By varying the tilt the effect could be adjusted so double stars (appearing as 4 stars) could be overlapped visually with just a plain crosshair eyepiece - instead of a bi-filar micrometer.
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