High Signal-to-Noise, Differential NICMOS Spectrophotometry

We report analysis for NICMOS CAL/9642, High S/N Capability Characterization.The purpose of this three orbit test was to establish NICMOS time domain stability in a domain not previously tested for NICMOS. Observations of the bright (H = 6.13) G0 V star HD 209458 were obtained using 1.8 second MULTIACCUMs with G141 on NIC3. Exposure-to-exposure stability summed over the full spectrum provides S/N of 2000, i.e. we demonstrated 0.5 mmag photometric precision treating the observations simply as broad-band, time-series photometry. Orbit-to-orbit means in a 0.2 micron band taken in ratio to neighboring continuum, without applying any corrections for correlated variations in contemporaneous auxiliary parameters, differed by only 3x10-4. A much better result was obtained with application of such corrections, but the result cannot be considered secure with comparison of only two orbits data. There is an excellent chance that in applications with sufficient flux to generate the required Poisson limit, that well planned NICMOS observations can provide differential spectrophotometric stability better than 1 part in 10,000 if not even significantly better.