Sources of Uncertainty
There are various sources of uncertainties when performing photometry with NICMOS that could affect the final accuracy of the measurements. In the following, several systematic uncertainties and special cases are mentioned.
Absolute Spectrophotometric Standards
The white dwarf G191-B2B and the solar analog P330E are NICMOS primary spectrophotometric standards. To assess the accuracy of the G191-B2B model in the near-infrared, two atmosphere flux distributions for exactly the same physical parameters were computed independently by two different experts in this field. The largest differences in the continuum fluxes of the two independent models are 3.5% in the near-infrared at 2.5 µm (Bohlin (1996; AJ 111, 1743)).
The spectral energy distribution of P330E in the 0.4µm - 0.8µm range is the same as the solar reference spectrum, within the uncertainties of the FOS measurements (Colina & Bohlin (1997; AJ 113, 1138)). Also the near-infrared spectrum of P330E, created by rescaling the reference spectrum of the Sun (see Colina & Bohlin 1997 for details), agrees to within 2% - 3% with ground-based near-infrared photometry.
In summary, the accuracy of the absolute spectral energy distribution of NICMOS primary standards introduces a systematic uncertainty of about 2% - 3% in the absolute calibration of the filters.
Relative Photometry Accross Detectors
The photometric values provided in the headers of the images are obtained from measurements of standard stars positioned in the central regions of the detectors. The results of the relative photometry characterization of NICMOS cameras (see section 5 above) indicate that relative photometry to better than 2% can be achieved for all filters in cameras NIC1 and NIC2, and for long wavelength filters (>= 2.0 µm) in NIC3.
Intra-pixel Sensitivity Variations
No evidence of intra-pixel sensitivity effects has been observed in cameras NIC1 and NIC2. However, as mentioned already (see section 6), the intra-pixel sensitivity affects NIC3 photometry in the 1.0 - 1.8 µm wavelength range and errors as large as 10-20% can be present, if images are taken without a subpixel dithering strategy. Therefore, subpixel dithering is recommended for high precision photometry.
Changes in focus are observed on an orbital timescale due mainly to thermal breathing of the telescope. In addition to this short term PSF variation there is an additional long term NICMOS component, as the cryogen evaporates and the dewar relaxes. This last effect is critical for NIC3 images where the focus of the camera has changed significantly during Cycle 7. In addition, PSF changes as a function of position in the detector. All these effects are believed to affect at the few percent level photometry obtained with small apertures. However, no quantitative measurements are available and therefore TinyTim simulations are recommended to study these effects, if high precision is required.
The photometric conversion factors provided in the header of NICMOS images have been obtained by doing aperture photometry on standards using fixed radius apertures (see section 3 for details). It is often difficult to measure the total flux of a point source using large apertures where the flux contribution from the extended wings of the PSF, diffraction spikes, and scattered light is also included. This is in particular true in crowded fields where the extended wings of well resolved sources could overlap with each other. To take into account aperture correction effects it is advisable to use TinyTim PSFs to measure the encircled energy curve of growth as explained in the NICMOS Data Handbook.
Color Dependence of Flatfields
The strong wavelength dependence of NICMOS flat-fields limits the photometric accuracy of sources with extreme colors observed in broad-band filters. Simulations with a very red source (J-K = 5 equivalent to a 700K black-body) indicates that these photometric errors are small, around 3% or less (NICMOS Instrment Handbook for Cycle 18) Targets redder than J-K ~5 could have photometric errors in excess of 3% for some of the filters.
Velocity shifts and Photometry with Narrow-band Filters
The photometric conversion factors for all NICMOS filters are obtained from observations of continuum, emission line free, standards. The integrated flux in erg*sec-1*cm-2 can be obtained as a function of the full width half maximum of the filter and the PHOTFLAM parameter as explained in the NICMOS Data Handbook. However, if the target has large velocity shifts the emission line does not coincide with the peak transmission of the filter, the line flux will be in error (few to several percent, depending on the filter and velocity shift) and a correction to account for the displacement is required. A method is indicated in the NICMOS Data Handbook.