▁K lev t so v – C asse gra in ▁This ▁design ▁is ▁based ▁on ▁the ▁M aks ut ov – C asse gra in . ▁It ▁provides ▁correction ▁of ▁aber r ations ▁via ▁a ▁two - element ▁men is cus - sh aped ▁correct or ▁l ens ▁in ▁front ▁of ▁the ▁secondary ▁mirror . ▁This ▁design ▁was ▁originally ▁en vis aged ▁by ▁G . ▁I . ▁Pop ov ▁with ▁a ▁practical ▁implementation ▁by ▁Yu . ▁A . ▁K lev t so v . ▁It ▁is ▁produced ▁with ▁a pert ures ▁of ▁ 8 , ▁ 1 0 . 2 5 ▁and ▁ 1 3   in ches . ▁The ▁ 8 - inch ▁model ▁emp lo ys ▁a ▁ref ractor ▁style ▁ra ck ▁and ▁pin ion ▁foc user , ▁while ▁in ▁the ▁larger ▁designs ▁the ▁primary ▁mirror ▁is ▁moved ▁as ▁in ▁most ▁other ▁small ▁to ▁medium - s ized ▁C asse gra in ▁designs . ▁ ▁V ixen ▁Six th - Order ▁As pher ic ▁C asse gra in ▁( VIS AC ) ▁V ixen ▁produces ▁an ▁ 8 - inch ▁a pert ure - mod ified ▁C asse gra in ▁design ▁( VC 2 0 0 L ) ▁they ▁refer ▁to ▁as ▁a ▁V IS AC ▁( V ixen ▁Six th - Order ▁As pher ic ▁C asse gra in ) ▁that ▁is ▁based ▁on ▁a ▁C asse gra in ▁design ▁with ▁a ▁primary ▁mirror ▁that ▁is ▁" six th ▁order ▁as pher ic " ▁– ▁somewhat ▁like ▁a ▁hyper bol