NICMOS IntraPixel Sensitivity Chun Xu and Bahram Mobasher

1. NICMOS IntraPixel Sensitivity Chun Xu and Bahram Mobasher Space Telescope Science Institute Abstract We present here the new measurements of the NICMOS (Camera 3) intrapixel sensitivity after the installation of the cryo-cooler in Cycle 11. We find a 27% decrease in the intrapixel sensitivity from Cycle 7 to Cycle 11 for both J (F110W) and K (F160W) bands. The decrease in the intrapixel sensitivity could not be attributed to the change of the PSF as the latter is less than 5%. We also discuss a possible cause for the decrease of the intrapixel sensitivity. In the end we present the formula to correct this intrapixel sensitivity. Introduction Accurate photometry depends, in part, on the width of the Point Spread Function (PSF) compared to the pixel size. If the PSF is much smaller than the pixel size and if there is any sensitivity variations within a pixel, then the detected flux is dependent on the positions of the PSF within the pixel. This could lead to significant uncertainties in photometric accuracy of point-like sources. In case of the NICMOS, this effect is negligible for NIC1 and NIC2, as the PSF core is much broader than their pixel size and hence, to a large extent, the PSF covers the entire pixel. However, NIC3 with a pixel size of 0.2 arcsec, could undersample the PSF at all wavelengths, leading to different total fluxes for a given star at various dither positions. The purpose of this study is to characterize the nature of this effect and to correct it. We analyzed data from both Cycle 7 (pre-cryo-cooler) and Cycle 11 to study the change of this effect due to the installation of cryo-cooler. The Analysis We define normalized total flux as Fn=Ftot /<Ftot>, where Ftot is the total flux of a star in one dithered exposure and <Ftot> is the average of the total fluxes of that star over all dithered exposures. We use median instead of average in practice. We define sharpness ratio as S=Fpk/Ftot , Where Fpk is the peak flux of one pixel near the center of a stellar image in one dithered exposure and Ftot is the total flux of that star in the same exposure, as defined previously. As discussed earlier on, S indicates the subpixel position of a stellar image while Fn reflects the relative total flux in one dithered exposure. Ideally, if there is no intrapixel sensitivity variations, we don’t expect Fn to change with S, however, if there is intrapixel sensitivity variations and if the size of the PSF is comparable or smaller than that of a pixel, then we expect a trend in the sense that higher sharpness ratios correspond to higher normalized fluxes. The steeper the slope, the higher the intrapixel variations. Figure 1 shows an example how the normalized flux changes with the sharpness ratio for different dithered exposures of a single star. The Strategy of the Correction In the PSF undersampling cases, if lights hit the center of a pixel, the resulted PSF should be sharper than when lights hit near the edge or corner of a pixel, because in the latter case, the lights will spread out among a few pixels thus lead to a broader PSF. One the other hand, if there exists intrapixel sensitivity variations, the measured total flux will be different in these two cases, even for the same star. Thus we expect a correlation between the FWHM of the PSF and the measured total flux. The aim of this study is to find such correlation and use this correlation to correct the observed flux based on the measured FWHM of a PSF. In practice, we use a parameter called sharpness ratio rather than FWHM in the correction procedure since the former is easier to assess. The sharpness ration is defined as the peak flux of a pixel near the center of a PSF vs the total flux within PSF. Either the sharpness ratio or the FWHM is practically an indicator of the intrapixel position that a PSF falls on a pixel. Figure 2. The fits to both Cycle 2 and Cycle 11 data. As discussed previously, the parameter a (slope) is a good indicator of the intrapixel sensitivity. Table 1 and Figure 2 clearly show that this value decreased from Cycle 7 to Cycle 11 for both F110W and F160W. The decrease of a implies either a decrease of intrapixel variation or an increase of the PSF. After a careful comparison of the PSF in both cycles, we rule out the latter case and conclude that the decrease of the intrapixel sensitivity variations from Cycle 7 to Cycle 11 is intrinsic (cf: Xu & Mobasher 2003). The decrease of the intrapixel sensitivity is in line with the expectation that after the installation of the cryo-cooler, the increase of the operation temperature leads to a higher electron mobility so smoothes the differences of the detector quantum efficiency (DQE) within a pixel. This effect is mathematically similar to convolving with a function of larger FWHM. The Correction of the Intrapixel Sensitivity Based on the parameters in Table 1, we can easily correct to the measured total stellar flux in individual exposures to averaged flux, the latter is considered the true flux of a star. The formula is simple: Fc = F x (aSm+b)/(aS+b) where Fc is the corrected flux and F is the measured flux, S is measured sharpness ratio in that exposure, a, b, Sm are parameters in Table 1. This correction is also applicable to any observation at their individual dithered exposures. The users only need to measure the total flux and sharpness ratio and plug them into this formula. The Data and Reduction This study was initiated by Storrs et al. (1999) using Hubble Deep Field South (HDf-S) data, including data from proposal 8058,8073,8074,8087 and 8076 in Cycle 7. The data were taken using NIC3 with filters F110W and F160W at different dither positions. After the installation of cryo-cooler in Service Mission 3, this study was continued in Cycle 11 (proposal 9638) through the observations of star cluster NGC 1850, using NIC3 with the same 2 filters. The observations are dithered at 25 steps so the stars in the field of view fall onto different subpixel positions. The data are calibrated using calnica 4.1.1 through the pipeline. The total flux of a star and the peak pixel flux are measured using Sextractor (Bertin & Arnouts 1996). The aperture radius is set to 2.5 pixels, roughly twice of the assumed PSF FWHM of 1.4 pixels. For a given star, if the measured FWHM is larger than 2.8 pixels in certain dithered exposures, these exposures are discarded to assure the data quality. Figure 1: Normalized flux vs. sharpness ratio for a selected star. A linear fit to the data is over plotted. Results and discussion We apply our method to both Cycle 7 and Cycle 11 data and find that the intrapixel sensitivity decreased from Cycle 7 to Cycle 11, after the installation of the cryo-cooler, as a result of the increase in the operation temperature. The results are presented in Table 1 and Figure 2. Table 1. Linear fit to the data. References: Bertin, E., and Arnouts, S., 1996, A&AS, 117, 393 Storrs, R., Hook, S., Stiavelli, M. et al., 1999, ISR NICMOS-99-005 Xu, C, Mobasher, B., 2003, ISR NICMOS-03-009