This is a string, either ' OPEN' or ' CLOSED'. (Only the first letter is actually used.) Use ' OPEN' if the tube is an open framework, and ' CLOSED' if it is an opaque cylinder. If (as in some telescopes) the part near the declination axis is closed, but the front end of the tube is open, use ' OPEN'. This information is needed to determine when the Moon can shine directly on the objective.
If the telescope is a refractor used with out a dewcap, use ' OPEN'. A refractor used with a dewcap should be marked ' CLOSED'; the dewcap information will be used in the TUBEDIAM and TUBELEN columns (see below).