1. The definitions are:
(int opd) = (delayed arm length) - (fixed arm length)
(int opd) = 2[LD + SD] +C ... for tel B = delayed.
Thus at LD = SD = 0 (home) we get
C = x(del,home) - x(fix,home)
where x is the optical length of the respective arm.
Each instument will have its own value of C depending on the lengths of the optical paths leading to its beam-combination point.
2. For the IR table the C value is approximately tabulated in
my 19 Oct 95 memo; at 38 m baseline it is
C(IR) = -1814.1 cm.
3. For the Fluor instrument it is a custom to write the C value
as the sum of 2 parts, a magic constant plus a Fluor offset (fixed in value),
C(F) = MC + FO
which at 38 m has values
C(F) = -1704.3 + 39.65 cm
= -1664.6 cm.
4. The difference is
C(F) - C(IR) = 149.5 cm
which is due to about 128 cm greater length
in the Fluor delayed arm length, plus about 21.5 cm (perhaps 20.3) greater
in the delayed arm glass fiber in the combination box.
5. In autocollimation the paths are slightly different, and we
found for 38 m baseline in Nov 1999 that
MC(auto) = -1709.5 cm,
i.e., about 5.2 cm more negative than the value
for a star, giving in general
MC(auto) = MC -5.2 cm.
6. At the 21 m baseline we expect (see 19 Oct 95 memo)
C(IR) = 202.7 cm
which leads to
MC = C(F) - FO
= C(IR) + 149.5 -39.65
= C(IR) + 109.9
= 312.6 cm.
7. In Jan 2000 the stub delay dihedral was moved about 1.5 cm
toward the corner, so the fixed arm length is 2*1.5 = 3.0 cm shorter, so
in Jan 2000 we have
C(2000) - C(1999) = -x(fix,home,2000) + x(fix,home,1999) = 3.0 cm.
8. From the above we compile a table of expected results.