The Officina Stellare Extreme 1.5x Extender for RiFAST400 or Larger: (Not the RiFAST300)
Designed by master optician Massimo Riccardi of Officina Stellare, this 4 lens design extender will bring your powerful RiFAST400 telescope from f/3.8 up to f/5.7, increasing resolution and keeping the same tiny-star's shape.
This extender is fully corrected for a wide field of 42 mm. The KAF 1680X chip can continue be used with only a small amount of vignetting and quality decrease on the extreme borders. It's fully diffraction limited from 450 nm to 730 nm.