hereafter 2MASS 0345 [@kirkpatrick97], BRI 0021$-$0214, hereafter BRI 0021 [@irwin91], three 3-7 Myr objects, $\sigma$ Orionis 51 [@zapatero00], $\sigma$ Orionis 71 [@barrado02], $\sigma$ Orionis J053849.5$-$024934, hereafter $\sigma$ Orionis J053$-$024, and five 1-2 Myr objects, KPNO-Tau 1 [@briceno02], KPNO-Tau 4 [@briceno02], KPNO-Tau 12 [@luhman03], 0457$+$3015, and 2MASS 0535$-$0546 [@stassun06].

The calibrators were all chosen because they have similar spectral types to the ONC PMOs, with the exception of the eclipsing binary 2MASS 0535$-$0546 (type M7) which was felt to be worthy of observation owing to the precisely known surface gravities of the components.

The three $\sigma$ Orionis objects were selected because there was fairly good prior evidence that they were bona fide cluster members, as opposed to contaminating field dwarfs in front of this rather diffuse cluster. This evidence was either from previous spectroscopy of alkali metal lines ($\sigma$ Orionis 51, [@mcgovern04]) or from detection of mid-infrared excess and/or spectroscopic evidence ($\sigma$ Orionis 71, $\sigma$ Orionis J053$-$024, [@caballero07]). Being older, they can be used to compare objects of slightly greater mass and gravity.

Further selection criteria for the 1-2 Myr calibrators in the Taurus-Auriga molecular cloud complex and field dwarf calibrators were that they are bright objects with little or no 