NICMOS Instrument Handbook for Cycle 11

Appendix 1:
Imaging
Reference Material

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This chapter provides basic information and sensitivity plots for the imaging and polarimetric filters. The corresponding information for the grism elements is provided in Chapter 5. The spectral characteristics of the NICMOS flight filters were measured at cryogenic temperature and normal incidence at Ball Aerospace. All filters had their spectral transmission measured from 0.5 to 2.7 microns with a step of 0.001 microns. For each filter, we provide the following information:

1. A plot of the total system throughput, which convolves the filter transmission curve with the OTA, the NICMOS foreoptics and the predicted detector's response under NCS operations. A listing of the central, mean, and peak wavelengths, of the wavelength range, filter width, transmission peak, and the fraction of the PSF contained in one pixel (assuming the source is centered on the pixel) is also given.
2. Sensitivity curves for each filter. The curves are calculated for both an extended and a point source, assuming in both cases a flat spectrum [in F()]. The curves give the flux as a function of time needed to reach S/N=3 and S/N=10, respectively. For the extended sources, the flux is given in Jy/arcsec² and the S/N is calculated per pixel; for the point sources, the flux is given in Jy, and the S/N is calculated in a 0.5" radius aperture (NIC1 and NIC2) or 1" radius aperture (NIC3). Because of the considerable uncertainty in both the dark current behavior and the actual operating temperature of the NICMOS detectors under NCS, we plot sensitivity curves for two limiting cases. The solid lines correspond to a "good" scenario: no dark current bump (0.5 e^-/s) and an operating temperature of 75 K, while the dotted lines represent a "bad" scenario, i.e the dark current is elevated and NICMOS has to operate at a temperature of 78 K. In this case, the dark current reaches levels of 2 e-/s/pixel. The effect of the higher DQE at 78 K actually causes a better performance of the "bad" scenario for short integration times, which is why the two curves intersect. However, the differences between the two scenarios are negligible for all observations except the longest exposures at short wavelengths. For all calculations, the adopted read-out noise is ~30e^-. The sky background, as listed in Chapter 9, is included in all computations. The saturation limit as a function of time is also shown for both scenarios, however, the curves are virtually indistinguishable. For the extended sources, we have computed the flux which saturates the pixel (the aperture), while for point sources we have computed the flux needed to saturate the brightest pixel (assuming the source is centered on the pixel). The range of times given goes from 0.2 to ~3,000 seconds, although we advise observers not to use exposures longer than about 1/3-1/2 orbit (t < 1500-1600 seconds, see Chapter 3).

   Proposers should always use the default settings of the ETC to calculate exposure times (see Chapter 9). The ETC default settings (elevated dark current, operating temperature of 78 K) fall somewhere between the two limiting cases shown in the sensitivity plots.

   
    
   

All throughput curves and band parameters in this section were built with the DQE curve corrected to 78 K. The values for most of these parameters come from the synphot task in STSDAS. The graph (grtbl = k511557nm_tmg.fits) and component (cmptbl = 12m1009hm_tmc.fits) tables used for these calculations are available from CDBS at STScI.

The filter transmission curves and OTA and NICMOS optics reflectivities are the ones listed in these files. The only change made relative to these tables is that the nic?_dqe* table for each camera was modified to reflect changes from the reference temperature of each curve to the nominal NCS operating temperature (78 K). Based on the NICMOS standard star photometry, each DQE curve + filter transmission curve is correct for the temperature at which the standard star observations were made (NIC1: 61.3 K, NIC2: 61.3 K, NIC3: 62.0 K)

The correction from these temps to 78 K was made based on fourth-order DQE vs. temperature fits to the F110W, F160W and F222M (NIC3 only) data from the end-of-cryogen warmup in early 1999 and monitor data over the entire NICMOS lifetime, with the fit in wavelength a linear function (DQE correction factor = a₀ + a₁ *lambda(microns)). The respective coefficients from 61.3 to 78.0 K for NIC1 and NIC2 and from 62.0 to 78.0 K for NIC3 given in Table 1.1.

Table A1.1: DQE correction factor coefficients

Camera	ao	a1
NIC1	1.9849598	0.28142546
NIC2	1.8082418	0.25807383
NIC3	1.8644739	0.29893187

 

The correction factor is then multiplied by the original curve at each wavelength to get the new curve. Note that this is a mean DQE value across the detector (mean of pixels (*,35:*), as the DQE can vary by factors of 2-3 across the array.

The band parameters correspond to the output of the STSDAS task bandpar where:

Central wavelength = PIVWV

Mean wavelength = AVGWV

Peak wavelength = WPEAK

Maximum throughput = TPEAK

The "Wavelength range" values are just measured by-eye from the throughput curves to serve as a rough estimate.

The "central pixel fraction" is the fraction of the total light from a point source that is contained in the central pixel of the point spread function (PSF). This assumes that the source is centered on a pixel, and that the pixel response function is unity across the pixel. All PSFs were made from TinyTim V5.0c, using the following parameters, in each of the filters:

• 2.36 mm PAM for NIC1
• 0.69 mm PAM for NIC2
• -9.50 mm PAM for NIC3
• source centered at pixel 128 128 (detector center)
• aberrations for 31/12/1998
• 10.0" diameter, no subsampling, no jitter
• default TnyTim V5.0c throughput tables and the G2V source spectrum