  In this article we describe a recent effort to cross-calibrate data from an
infrared detector at the McMath-Pierce Solar Telescope and the Facility
InfraRed Spectropolarimeter (FIRS) at the Dunn Solar Telescope. A synoptic
observation program at the McMath-Pierce has measured umbral magnetic field
strengths since 1998, and this data set has recently been compared with umbral
magnetic field observations from SOHO MDI and SDO HMI. To further improve on
the data from McMath-Pierce, we compared the data with measurements taken at
the Dunn Solar Telescope with far greater spectral resolution than has been
possible with space instrumentation. To minimise potential disruption to the
study, concurrent umbral measurements were made so that the relationship
between the two datasets can be most accurately characterised. We find that
there is a strong agreement between the umbral magnetic field strengths
recorded by each instrument, and we reduced the FIRS data in two different ways
to successfully test this correlation further.
