I got this Aries Chromacor for my 6” f/8 Skywatcher refractor (same as Celestron C-150R, made by Synta). My scope tested about 1/10 wave overcorrected for spherical aberration. This Chromacor corrected it too far and I replaced it with a type N. It is almost unused, mint condition. If your achromat refractor is around 1/6 wave overcorrected, this may be the one for you. Information on how to decide which Chromacor is best (to null out the S.A.) and how to install it is available from the website of astrobuffet and elsewhere. Get apochromatic performance at a fraction of the cost.
New ones are $599+. This is only $450 shipped. ::SALE PENDING::