5 Verifying the BSM pointing requirement s necessitat ed the use of three separate o ptical references: a threeaxis displacement -measuring interferometer from the Zygo Corporation, a Leviton Inter -target Differential  Electronic Autocollimator (IDEA) , and a Leica T3000 Theodolite. There was no single metrology solution  that could provide high resolution, high accuracy, and absolute position  verification over the BSM mirror’s  full range of motion. Rather, it was necessary to utilize information from multiple reference instruments  in  order to verify the requirement.  

The Zygo interferometer setup consisted of a 3- axis High Stability Plane Mirror Interferometer , driven by a  Zygo laser and measured by a Zygo ZMI4100 measurement board (Figure 4). This setup provided both the  highest time and spatial resolution, achieving a 10- kHz cadence and having an advertised resoluti on of just  over 20 nanoradians. Unfor tunately , its field -of-view (FOV) of ±3 milliradians , compared to the   5.6 milliradian range to be measured,  result ed in the need to realign the interferometer  to capture the full  FOV. The pitch axis was prone to significant non- linearity , in part caused by a polarization effect of  the BSM  mirror's dielectric coating, which was tuned to the 532- nm ATLAS laser, but not the  633-nm Zygo laser   wavelength. This non- linearity meant that only the yaw axis data of the Zygo was trusted, and led to  inconveniently taking Zygo measurements of both BSM axes using only the Zygo yaw axis.  The Zygo has  no reference, so is not an absolute, but rather is an  incremental sensor. The characteristics of the Zygo  made it  impractical for TVac testing , but it was extensively used for the bulk of testing . 

The IDEA autocollimator ha s a FOV of roughly ±10 milliradians , is very compact and lightweight, has  a very  high accuracy of 2.5 m icroradians, and is very stable. IDEA makes use of a reference flat  to measure  against, which minimizes  measurement drift, makin g it an absolute measurement instrument. N o optical  adjustments are required to measure the differ ence between the reference flat and the mirror of interest,  which in this case was the BSM Mirror. An aperture select mechanism is required to alternate IDEA’s view  between the reference flat and the BSM mirror.  The IDEA characteristics  made it ideal for m easuring  thermal stability during Thermal Vacuum ( TVac) (Figure 5), though it was also used for non- TVac testing.  IDEA is a low bandwidth device,  able to achieve a maximum cadence on the order of 5 Hz .    The theodolite offered similar performance as IDEA , though large and bulky , but unlike IDEA, it required  manual operation for each measurement. Theodolites have significantly more heritage as an optical  reference than IDEA , which is a fairly new development not yet  commercially available . Measurements  taken with a theodolite were used to determine the BSM sensor scal e factors and axis orientation.    Optical measurements during TVac were taken using IDEA , whose wider FOV could tolerate gross motion  of the BSM mount due to temperature changes in the vacuum cha mber.  IDEA measurements were used  to make temperature- dependent adjustments to the scale factor , determined before TVac . Performance  over the full range was quantified using interferometer data which had been post -processed to remove nonlinearitie s. Processing a combination of data from these three instruments verified BSM pointing  performance over its full required range of motion and temperature.    The photo of Figure 6  shows optical references used b y the measurement instruments. The BSM c ube was  used by the theod olite as a position reference. The reference flat was used by IDEA as its position  reference.    As position knowledge and pointing requirements for simil ar systems in the future become more  demanding, verif ying these requirements will require not only the synthesis of measurements from multiple  references, but also deep  understanding of the limitations of each reference chosen.