where c is the speed of light in vacuum. PHOTBW gives the rms band of the filter in Angstroms (see the Synphot User's Guide for a detailed definition of both parameters).
18.2.2 Fluxes and Magnitude Zeropoints
The NICMOS calibration pipeline provides two photometric parameters for the conversion of countrates into fluxes. These parameters are found in the keywords PHOTFNU and PHOTFLAM in the header of the calibrated image. PHOTFNU is given in units of Jy sec DN-1 and PHOTFLAM in units of ergs cm-2 Å-1 DN-1. Because NICMOS calibrated data are given in countrate, i.e., DN sec-1, the countrate to flux conversion is simply achieved by multiplying the countrate by the PHOTFNU or PHOTFLAM value, depending on which units are desired for the final calibrated image.
In the header of your calibrated images, there are three additional photometric parameters that characterize the filter used for the observation (PHOTPLAM and PHOTBW) and provide the ST magnitude zero point (PHOTZPT). PHOTPLAM gives the value of the pivot wavelength of the filter in Angstroms. This wavelength is source-independent and is the wavelength for which:
The magnitude of an object can be determined in the ST system (e.g., based on a constant flux per unit wavelength) using the photometric zero-point keyword PHOTZPT (= -21.1) simply by:
system (e.g., based on a constant flux per unit frequency) is obtained by applying the following expression:
Details about plans to define an HST JHK system and compute the photometric transformations to ground-based systems are given in "Magnitudes and Photometric Systems Transformations" on page 18-8.
18.2.3 Photometric Corrections
Differential Photometry
The photometric values provided in the headers are obtained from measurements of standard stars in the central regions of the detectors. Both high frequency (pixel-to-pixel) and low frequency (large-scale structures) sensitivity variations will be corrected using on-orbit flats. Preliminary SMOV differential photometry characterization of NICMOS cameras indicate that residual large scale deviations could amount to ~2%, except in the corners that might be higher. A Cycle 7 calibration program has been designed to measure with a fine grid the photometric deviations from the average as a function of wavelength, for each camera. A correction image might be generated as a product of this program, if measurable deviations are found. Pixel Centering
As with many other array detectors, the sensitivity of the NICMOS detectors is lower near the edges of the pixels than in their centers. It is as though there were small regions of reduced sensitivity along the intra-pixel boundaries. In practical terms this effect means that for a source whose flux changes rapidly on a size comparable with or smaller than the pixel size, the measured countrate, and therefore flux, will depend on where the center of the source lies with respect to the center of the pixel. Because this position is not known a priori, this effect will introduce some uncertainty in the flux calibration for a point source. This uncertainty will be largest (no more than a few percent, we expect) for NIC3 at short wavelengths, in which the PSF is undersampled. For high precision photometry and to compute the amount of photometric uncertainty in a particular camera and filter combination due to this effect, subpixel dithering is recommended. PSF Variations
The point spread function (PSF) of the telescope changes with time, and these changes will affect photometry using very small (less than 3-4 pixel radius) apertures. Changes in focus observed on an orbital timescale are 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. As a result of the stress produced by the solid nitrogen on the instrument, NICMOS detectors have been moving, and keep moving, along the focus direction. The motion of the cameras is monitored twice a month and NICMOS focus updates can be periodically implemented, if required. Although preliminary results from SMOV indicate that the breathing effects on small aperture photometry are below our measurement precision (a few percent), the subject is still under investigation. Aperture Correction
It is often difficult to measure the total flux of a point source due to the extended wings of the PSF, difraction spikes, and scattered light. Such measurements are particularly difficult in crowded fields where the extended wings of well resolved sources can overlap with each other. An accurate method of measuring the integrated flux in these situations could consist of several steps:
Empirical PSFs could also be used for the above mentioned method. However, there are no plans to obtain PSF profiles for all camera and filter combinations as part of the Cycle 7 calibration plan. Empirical PSFs for the central regions of the detectors can be obtained from the calibrated images obtained for the Cycle 7 absolute photometry (proposal 7691) and photometric monitoring (proposal 7607) programs.
18.2.5 Absolute Photometry for Emission Line Filters
The narrow band filters in NICMOS are intended primarily for observations of emission or absorption lines in sources. Because the photometric conversion factors PHOTFNU and PHOTFLAM for all NICMOS filters are obtained from continuum observations of emission-line free standard stars, the flux in erg sec-1 cm-2 of an emission line is given by the expression:
Figure 18.1: Estimating Absolute Flux Variation
The examples above compute the countrate in the NIC3 F212N filter for a H2 (2.12 micron) emission line having a gaussian profile of 40 Angstroms and a peak flux of 1.0 x 10-13 erg sec-1 cm-2 A-1. The integrated flux will then be 4.2 x 10-12 erg sec-1 cm-2. In the first example the H2 emission line is at zero redshift and centered on the filter while in the second example the line is redshifted by 80 Angstroms. If the emission line is centered on the filter, the H2 flux will produce 7421.1 DN sec-1 while the countrate will be ~90% of this value (i.e., 6662.3 DN sec-1) for the redshifted emission line. The expression for the Fluxline above can be directly applied to the first case, while a correction factor 1.11=(7421.1/6662.2) is needed in the second case.
18.2.6 Absolute Spectrophotometry with NICMOS Grisms
The accuracy of the absolute spectrophotometry with NICMOS grisms depends on three different limiting factors:
The absolute flux calibration of the spectral energy distribution of the standard stars in the 0.8 to 2.5 µm wavelength range is known to 2-5%. Characterization of the grisms' absolute sensitivity during SMOV indicates that the absolute calibration of grism spectrophotometry for bright sources, i.e. well above background, will have a total 20-30% uncertainty. For Grism C the uncertainties could be even higher because of the large thermal background in this wavelength range (1.4 to 2.5 µm).
stevens@stsci.edu Copyright © 1997, Association of Universities for Research in Astronomy. All rights reserved. Last updated: 11/13/97 17:28:21