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Hubble Space Telescope
NICMOS STScI Analysis Newsletter 35

| STScI Analysis Newsletter
| 21 February 2006
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-Two new Instrument Science Reports regarding Non-linearity


NICMOS ISR 2006-001: NICMOS Count-rate Dependent Nonlinearity Tests  
using Flatfield Lamps

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  


NICMOS ISR 2006-002: NICMOS Count Rate Dependent Non-Linearity in  
G096 and G141

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  

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