  We present results from a Hubble Space Telescope (HST) program characterizing
the atmospheres of the outer two planets, in the HR8799 system. The images were
taken over 15 orbits in three near-infrared medium-band filters - F098M, F127M
and F139M - using the Wide Field Camera 3. One of the three filters is
sensitive to water absorption band inaccessible from ground-based observations,
providing a unique probe of the thermal emission from the atmospheres of these
young giant planets. The observations were taken at 30 different spacecraft
rolls to enable angular differential imaging, and the full data set was
analyzed with the Karhunen-Loeve Image Projection (KLIP) routine, an advanced
image processing algorithm adapted to work with HST data. To achieve the
required high contrast at sub arcsecond resolution, we utilized the pointing
accuracy of HST in combination with an improved pipeline designed to combine
the dithered, angular differential imaging data with an algorithm designed to
both improve the image resolution and accurately measure the photometry. The
results include F127M ($J$) detections of the outer planets, HR8799 b and c and
the first detection of HR8799 b in the water-band (F139M) filter. The F127M
photometry for HR8799 c agrees well with fitted atmospheric models resolving a
long standing difficulty to model the near-IR flux for the planet consistently
