Amplitude Calibration System
                         Prototype Development Plan
                     From: Science IPT Calibration Group
              Butler, Mangum, Martin-Pintado, Wootten, van Dishoeck 

Prototype 1: Semi-Transparent Vane System
             
             The tests of the first prototype of a semi-transparent
             vane (STV) calibration system have been made at the IRAM
             30m telescope, with encouraging results.

             Further tests with this system are required to confirm
	     that the STV concept can work at the 3% level.  The
	     test that needs to be made is to check that the losses in
	     the vane are due to absorption to within 3%.  This test
	     will require a measurement of the transmission of the
	     vane on an astronomical source with very good weather
	     conditions.  Tests are needed at frequencies for which both
             ALMA and the 30m operate, with 1.3mm and 3mm most critical.
             These tests could be completed on the existing device in
             a few nights of good weather.

Prototype 2: Wire Grid Amplitude Calibration System

             Design and construction of a test prototype for a more
             advanced device containing a polarization grid has been
             stalled.  This second prototype will allow us to study the
             anticipated advantages of using a grid, which are
             anticipated to allow calibration accuracy of 1%.  In
             fact, in the previous observing tests made with Prototype
             1 at the IRAM 30m telescope, some preliminary
             measurements with the grids of the 30m receivers have
             been made.  These tests were quite encouraging, and we
             are confident that tests could be made which would allow
             for a precision of about 1%.  Tests are needed at 
             frequencies for which both ALMA and the 30m operate, with 
             1.3mm and 3mm most critical.

Prototype 3: Multi-Load Amplitude Calibration System
             
             This more advanced design would incorporate the concepts
             detailed in ALMA Memo 461.  Tests will include comparison
             testing of three types of couplers: semi-transparent
             vane; wire grid; dielectric film.  Testing will be
             performed on the IRAM 30m telescope. Tests are needed at
             frequencies for which both ALMA and the 30m operate, with
             1.3mm and 3mm most critical.  The design of the 30m
             telescope may only allow tests at 1.3mm.  An ALMA design
             will follow, with tests on the ALMA prototype front ends
             at the OSF.