FUNCTION GT_DETECTOR,QLDS,nocheck=nocheck
  
  ON_ERROR,2
  IF !debug GT 0 THEN ON_ERROR,0
  
  IF NOT KEYWORD_SET(nocheck) THEN begin
     QLMGR,QLDS,valid
     IF NOT valid THEN  $
        MESSAGE,"Parameter must be a valid Quick Look Data Structure"
  END
  RETURN,QLDS.HEADER.detector
  
END