+----------------------------------------------------------+ | NICMOS | STScI Analysis Newsletter | 21 February 2006 +----------------------------------------------------------+ Visit the NICMOS web site http://www.stsci.edu/hst/nicmos "New in the Last 45 Days" for all new information about NICMOS. CONTENTS: -Two new Instrument Science Reports regarding Non-linearity +----------------------------------------------------------+ NICMOS ISR 2006-001: NICMOS Count-rate Dependent Nonlinearity Tests using Flatfield Lamps http://www.stsci.edu/hst/nicmos/documents/isrs/isr_2006_001.pdf Roelof S. de Jong, Eddie Bergeron, Adam Riess, Ralph Bohlin 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 star cluster field was imaged in a lamp off-on-off sequence in all cameras in a selected set of filters, followed by a series of darks to investigate persistence and to clean the images from any remaining charge for the next orbit. Subtracting the lamp-off images from the lamp-on images clearly shows residual ADUs at the star positions, indicating that a higher background (and thus total) count rate increases the number of ADUs registered from an object. We model the non-linearity with a power law and fit this model to the data. Both NIC1 and NIC2 (NIC3 was not tested in this program) show non- linearity, becoming stronger at shorter wavelengths, but with larger amplitude than predicted by the Bohlin et al. NIC3 measurements. The non-linearity in NIC1 and NIC2 amounts to 0.06-0.10 mag offset per factor ten change in incident flux for the shortest wavelength (F090M and F110W), about 0.03 mag/dex at F160W, and less at longer wavelengths. Archival data from Cycle 7 are also analyzed, showing that the nonlinearity has not changed in NIC2 F110W, and suggesting that this effect is independent of detector temperature. +----------------------------------------------------------+ NICMOS ISR 2006-002: NICMOS Count Rate Dependent Non-Linearity in G096 and G141 http://www.stsci.edu/hst/nicmos/documents/isrs/isr_2006_002.pdf Ralph C. Bohlin, Adam Riess, & Roelof de Jong Since the discovery of the NICMOS count rate dependent non- linearity documented in NICMOS ISR 2005-002, additional tests have been conducted to further understand and quantify the effect. Long integrations of up to 25 minutes exhibited the same level of non- linearity as the original 1-3 minute integrations. Observations of the star P041C on high background from an internal flat field lamp provides another measure of the wavelength dependence of the nonlinearity to compliment the measures based on pure hydrogen WD models. +----------------------------------------------------------+ | Help: help@stsci.edu +----------------------------------------------------------+ | To subscribe or unsubscribe to the STAN, send a message | to majordomo@stsci.edu with the subject line blank and | the following in the body: [un]subscribe nicmos_news +----------------------------------------------------------+ | The Space Telescope Science Institute is operated by the | Association of Universities for Research in Astronomy, | Inc., under NASA contract NAS5-26555. +----------------------------------------------------------+
