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Near Infrared Camera and Multi-Object Spectrometer Instrument Handbook for Cycle 17 > Appendix A: ImagingReference Material

Appendix A:
Imaging
Reference Material
This appendix provides basic throughput information and plots for the imaging and polarimetric filters. The corresponding information for the grism elements is provided in Chapter 5. It is important to note that the information provided here is approximate. The precise values will change as each CDBS update provides more accurate data. Users should follow the instructions below to generate their own data using in STSDAS while keeping in mind that their calculated values should be close to those provided here.
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:
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.
All throughput curves and band parameters in this section were built with the DQE curve corrected to 77.1 K. The values for most of these parameters come directly from the synphot task in STSDAS. The graph (grtbl) and component (cmptbl) tables used for these calculations were:
grtbl = “k511557nm_tmg.fits
cmptbl = “m7t1443mm_tmc.fits
The above 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 (77.1 K, at the NDWTMP11 Mounting Cup temperature sensor). 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 of the original DQE curves were made based on the ratio of the mean value of lamp-flat images taken at the current operating temperature of 77.1 K to flats taken at the above temperatures. The correction factor as a function of wavelength is then multiplied by the original curve to get the new curve. Note that the DQE can vary by factors of 2–3 across each 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
These outputs can be easily generated from bandpar. For example the following command generates these outputs for the F160W filter on camera 3:
bandpar nicmos,3,f160w
The FWHM is defined as the intersection between the transmission curve and the half-maximum-transmission value. Since the transmission curve shape is strongly affected by the DQE (particularly for the broad filters), the measured FWHM will change with temperature. This is NOT the FWHM value returned by the bandpar task.
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 V6.0, using the following parameters, in each of the filters:
  1.80 mm PAM for NIC1
  0.20 mm PAM for NIC2
default TinyTim V6.0 throughput tables and the G2V source spectrum

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