Celestron Erfles revisited

per the discussion of 12-9-02, I have an eyepiece marked Celestron 20mm Erfle 1 1/4 Fully Coated. Circle V on barrel. Advertised 65* AFOV. This eyepiece appears to be in the same housing as the common 25mm ortho, and in fact the lens set also appears to be the same 25mm ortho with the addition of a plano-convex field lens. I found it had severe vigneting when used in f/8 telescopes (reflector and refractor). Both these scopes support a fully illuminated FOV with a 32mm type I UO Konig (much larger field stop). Knowing the same thing occurs when the field stop of common orthos is removed, I suspected that this basically ortho (ala Goerz) implementation could not support such a wide AFOV. I stopped mine down to ~17mm as is used in the common 25mm ortho and this resulted in a ~55* AFOV. True field now is the same as a 25mm ortho but with increased magnification and larger AFOV per the 20mm fl. The field remains sharp to the edge with a little edge distortion present (no distortion in the 25mm configuration with the extra field lens removed). It also Barlows well in this mode with no vigneting. Ron

Interesting, and a bit strange to me, since I didn't notice any such effect in any of the four eyepieces I checked. Whenever the sky clears up (3 days or 3 months???) I will re-check the eyepieces, and look specificly for vigneting effects.