If TUBETYPE is 'OPEN', ignore this field.
If TUBETYPE is 'CLOSED', TUBELEN should contain the length (in meters) of the telescope tube (or dewcap, if a refractor), measured from the front edge of the objective to the front end of the tube (or dewcap). Thus, for a reflector, TUBELEN is just the length of the telescope tube measured from the front surface of the primary mirror. For a refractor, it is the effective length of the dewcap.