Yes Ian. The objective lens is BK7 but a low dispersion glass would be better. It is plano convex and need only be ground and polished, figuring is unescessary. It is F:40 for reduction of lateral chomatic aberration The bino achromat is just that, no special features 7X50 binos are OK but higher powers are obtained with focal length of 330mm Coliour is not so bad as calculation would predict, the images are brilliant lit and the resolution is that for the objective not the bino lens as so is the resolution. there is some edge color due to lateral CA, so the weak lens must be just that.I have seem worse secondary residual color in a commercial grade 5inch refractor! Make one and try it, you will be surprised.. easy! I fold my Hypos down to one third of half the focal length of the weak og. My current Hypo is only 32 inches long in spite of a focal length of 6000mm.