LNIB Astrovid EASYTESTER Ronchi Eyepiece - $20 shipped
The EASYTESTER 1 consists of a classical Ronchi grating mounted in a standard 1-1/4" eyepiece tube. When the device is focused on a bright star, a pattern of parallel lines appears that can be used to determine the optical quality of the system and identify many of the common faults that plague commercial telescopes these days. If the lines are straight and parallel to the edge,one can conclude that the mirror or objective lens is well figured and that the secondary (and) corrector plate are properly figured. If the lines appear to be anything other than straight and parallel, a quick comparison to the defect chart, will identify the type of error involved.
The EASYTESTER consists of a 133 line grating mounted in a 1 1/4" diameter tube made of instrument quality brass and a plastic eye cap. The grating can be moved up or down and locked into position with a setscrew to accommodate various backfocal distances.