Focal Reducer R130S/R150S Model # 3633 Shortens the focal length of the R130S from 720mm to 626mm (f/4.8) Shortens the focal length of the R150S from 750mm to 652mm (f/4.3) Focal reducer, The 3633, reduces a telescope's focal length to about 87%. Focal reducers shorten the effective focal length of telescopes to provide wider visual fields and faster f-ratios for astrophotography.
