ic ▁mirror ▁but ▁able ▁to ▁be ▁manufact ured ▁using ▁mass - production ▁techniques . ▁To ▁compens ate ▁for ▁the ▁aber r ations ▁the ▁mirror ▁design ▁introdu ces , ▁V ixen ▁adds ▁a ▁" field ▁correct or ▁l ens " ▁– ▁actually ▁a ▁three - element ▁correct or ▁in ▁the ▁draw ▁t ube ▁of ▁the ▁foc user ▁which ▁also ▁reduces ▁field ▁curv ature ▁for ▁wide ▁field ▁applications . ▁The ▁design ▁results ▁in ▁an ▁image ▁that ▁is ▁free ▁of ▁com a ▁and ▁ast ig mat ism . ▁ ▁These ▁characteristics ▁have ▁led ▁to ▁the ▁V IS AC ▁being ▁referred ▁to ▁as ▁a ▁" Po or ▁man ' s ▁R itch ey – Chr ét ien ". ▁In ▁fact , ▁given ▁that ▁it ▁has ▁no ▁ast ig mat ism ▁and ▁field ▁curv ature , ▁it ▁performs ▁better ▁than ▁a ▁true ▁RC . ▁There ▁is ▁the ▁down side ▁of ▁the ▁potential ▁for ▁chrom atic ▁aber r ation ▁due ▁to ▁the ▁ref ract ive ▁elements , ▁but ▁it ▁is ▁hardly ▁notice able . ▁ ▁This ▁particular ▁design ▁is ▁also ▁unusual ▁in ▁that ▁it ▁is ▁a ▁C asse gra in ▁design ▁but ▁has ▁a ▁fixed ▁primary ▁and ▁ref ractor - style ▁ra ck - and - pin ion ▁foc user ▁which ▁removes ▁the ▁image ▁shift ▁issues ▁seen ▁with ▁other ▁cat ad io pt ric ▁designs . ▁ ▁These ▁features ▁together ▁make ▁for ▁a ▁teles cope ▁that ▁is ▁very ▁well ▁su ited ▁to ▁ast roph ot ography ▁either ▁at ▁the ▁native ▁f / 9 ▁or ▁using ▁the ▁optional ▁foc al