On the use of mosaic direct-vision prism for radial velocity measurements

Mosaic direct-vision objective prisms satisfy the three conditions necessary for a whole-sky survey of spectroscopic binaries and short-period variables. We point out that the use of rectangular component prisms instead of square ones may cut by half the light loss caused by frame obstruction. The portion of the aperture not covered by the prism can be used to produce a direct image which can serve as a reference point in the spectral line measurement, thus dispensing with any rotating mechanism.