We present preliminary trigonometric parallaxes and proper motions for 22 L dwarfs and 18 T dwarfs measured using the ASTROCAM infrared imager on the U.S .
Naval Observatory ( USNO ) 1.55–m Strand Astrometric reflector .
The results presented here are based on observations obtained between September 2000 and November 2002 ; about half of the objects have an observational time baseline of \Delta t = 1.3 yr and half \Delta t = 2.0 yr .
Despite these short time baselines , the astrometric quality is sufficient to produce significant new results , especially for the nearer T dwarfs .
Seven objects are in common with the USNO optical CCD parallax program for quality control and seven in common with the ESO 3.5–m NTT parallax program .
We compare astrometric quality with both of these programs .
Relative to absolute parallax corrections are made by employing 2MASS and/or SDSS photometry for reference frame stars .
We combine USNO infrared and optical parallaxes with the best available CIT system photometry to determine M _ { J } , M _ { H } , and M _ { K } values for 37 L dwarfs between spectral types L0 to L8 and 19 T dwarfs between spectral types T0.5 and T8 and present selected absolute magnitude versus spectral type and color diagrams , based on these results .
Luminosities and temperatures are estimated for these objects .
Of special interest are the distances of several objects which are at or near the L–T dwarf boundary so that this important transition can be better understood .
The previously reported early-mid T dwarf luminosity excess is clearly confirmed and found to be present at J , H , and K. The large number of objects that populate this luminosity excess region indicates that it can not be due entirely to selection effects .
The T dwarf sequence is extended to M _ { J } \approx 16.9 by 2MASS J041519 - 0935 which , at d = 5.74 pc , is found to be the least luminous [ log ( L/L _ { \odot } ) = -5.58 ] and coldest ( T _ { eff } \approx 760 K ) brown dwarf known .
Combining results from this paper with earlier USNO CCD results we find that , in contrast to the L dwarfs , there are no examples of low velocity ( V _ { tan } < 20 km s ^ { -1 } ) T dwarfs .
This is consistent with the T dwarfs in this study being generally older than the L dwarfs .
We briefly discuss future directions for the USNO infrared astrometry program .