Physics
Scientific paper
Feb 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006nicm.rept....1d&link_type=abstract
Instrument Science Report NICMOS 2006-001, 14 pages
Physics
Hubble Space Telescope, Hst, Space Telescope Science Institute, Nicmos
Scientific paper
We investigate the recently discovered NICMOS count-rate dependent non-linearity(Bohlin et al. 2005) using the flatfield lamps to artificially increase the count rate. A starcluster field was imaged in a lamp off-on-off sequence in all cameras in a selected set offilters, followed by a series of darks to investigate persistence and to clean the imagesfrom any remaining charge for the next orbit. Subtracting the lamp-off images from thelamp-on images clearly shows residual ADUs at the star positions, indicating that ahigher background (and thus total) count rate increases the number of ADUs registeredfrom an object. We model the non-linearity with a power law (count-rate ? flux?) and fitthis model to the data. Both NIC1 and NIC2 (NIC3 was not tested in this program) shownon-linearity, becoming stronger at shorter wavelengths, but with larger amplitude thanpredicted by the Bohlin et al. NIC3 measurements. The non-linearity in NIC1 and NIC2amounts to 0.06-0.10 mag offset per factor ten change in incident flux for the shortestwavelength (F090M and F110W), about 0.03 mag/dex at F160W, and less at longerwavelengths. Archival data from Cycle 7 are also analyzed, showing that the nonlinearity has not changed in NIC2 F110W, and suggesting that this effect is independentof detector temperature.
Bergeron Eddie
Bohlin Ralph
de Jong Roelof S.
Riess Adam
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