This is a used, minty, Pentax SDHF-105. D=105mm; F=700mm. The design consists of an SD doublet and a single field flatener mounted rearward in the OTA. The coatings are Pentax's legendary SMC. Though the design requires longer than a simple doublet to cool down, once at equilibrium it provides sharp, totally flat fields edge to edge with excellent contrast (no matter where Luna is near the edge of the FOV!).
Regards,
Jim
-------------------- "I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me."