ALPY in the red

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Robin Leadbeater

Note also that that the ALPY optics are specially optimised for the violet end and the sharpness deteriorates noticeably in the red/IR in any case due to chromatic aberrations. (The early  ALPY’s had an IR blocking filter to stop people operating in this region until it was removed after I pointed out the effect the terrible ripples in the passband of the filter had on the instrument response)