( ( If ( ( the telescope ) ( is ( collimated ( but ( ( the image ) ( is oval ) ) ) ) ) ) ) ( , ( that ( ( may ( mean ( ( the mirror ) ( has ( ( astigmatism , ) ( which ( results ( when ( ( ( light rays ) ( from ( ( the ( same zone ) ) ( of ( a mirror ) ) ) ) ) ( ( do not ) ( come ( to ( ( a focus ) ( in ( the ( same plane ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN If) (S (NP (DT the) (NN telescope)) (VP (VBZ is) (VP (VBN collimated) (SBAR (CC but) (S (NP (DT the) (NN image)) (VP (VBZ is) (ADJP (JJ oval))))))))) (, ,) (NP (DT that)) (VP (MD may) (VP (VB mean) (SBAR (S (NP (DT the) (NN mirror)) (VP (VBZ has) (NP (NP (NN astigmatism)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBZ results) (SBAR (WHADVP (WRB when)) (S (NP (NP (JJ light) (NNS rays)) (PP (IN from) (NP (NP (DT the) (JJ same) (NN zone)) (PP (IN of) (NP (DT a) (NN mirror)))))) (VP (VBP do) (RB not) (VP (VB come) (PP (TO to) (NP (NP (DT a) (NN focus)) (PP (IN in) (NP (DT the) (JJ same) (NN plane)))))))))))))))))) (. .)))
If the telescope is collimated but the image is oval, that may mean the mirror has astigmatism , which results when light rays from the same zone of a mirror do not come to a focus in the same plane.
(ROOT (S (NP (NNP Plane) (NNS mirrors)) (VP (VBP work) (SBAR (IN because) (S (NP (DT the) (JJ light) (NNS rays)) (VP (VBP create) (NP (NP (DT a) (JJ virtual) (NN image)) (PP (IN behind) (NP (DT the) (NN mirror)))))))) (. .)))