Near Infrared Imaging of the Hubble Deep Field with The Keck Telescope
  Two deep K-band ($2.2 \mu m$) images, with point-source detection limits of $K=25.2$ mag (one sigma), taken with the Keck Telescope in subfields of the Hubble Deep Field, are presented and analyzed. A sample of objects to K=24 mag is constructed and $V_{606}-I_{814}$ and $I_{814}-K$ colors are measured. By stacking visually selected objects, mean $I_{814}-K$ colors can be measured to very faint levels; the mean $I_{814}-K$ color is constant with apparent magnitude down to $V_{606}=28$ mag. 
David W. Hogg, G. Neugebauer, Lee Armus, K. Matthews, Michael A.   Pahre, B. T. Soifer, A. J. Weinberger (Caltech)