5.3 Photometric Calibrations
Atmospheric absorption bands limit the wavelength ranges over which one can usefully observe in the near-infrared from ground-based telescopes. NICMOS, however, can observe at any wavelength where its detectors are sensitive, and hence its filter bandpasses do not conform to those of instruments used at ground-based observatories. Instead, its filters were designed to meet the anticipated scientific demands. Thus in practice NICMOS does not have filters matched to any of the ground-based photometric bands. The photometric calibration of NICMOS data is discussed in this section. Cases of continuum sources, emission lines, and grism spectra will be presented.
5.3.1 Units for NICMOS Photometry
Given the multitude of units and systems that have been used for infrared photometry (magnitudes, Jy, W m-2 µm-1, erg sec-1 cm-2 µm-1, etc.) and given the lack of a standard for ground-based infrared filters, NICMOS has adopted the approach of calibration to physical units, i.e. Janskys (Jy), or Jy arcsec-2 for surface brightness. Details on how to transform different sets of units can be found in Appendix 2 of the NICMOS Instrument Handbook or obtained using the NICMOS Unit Conversion Tool (available on the STScI NICMOS web pages).
5.3.2 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. The definitions of these keywords are discussed in section 3.3.4 of the HST Introduction. Very briefly, for NICMOS these are the bandpass-averaged flux densities (in F
for PHOTFLAM or F
for PHOTFNU) for a source that would produce a count rate of 1 DN sec-1. 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.
A fundamental challenge in calibrating NICMOS photometry is the inherent uncertainty in absolute flux calibration in the near-infrared, whether from the ground or from space. Unlike the situation at optical wavelengths, there really are no absolutely calibrated spectrophotometric flux standards at near-infrared wavelengths. Ground-based photometric calibration observations are mostly limited to relative magnitude determinations in more or less standard bandpasses (JHK) defined by atmospheric absorption windows. But the absolute flux calibration of primary infrared standards has generally been based on reference to some assumption that a particular type of standard star, e.g., solar-type stars, A0 stars, or white dwarfs, has a spectrum similar to something else that is believed to be well understood, such as the Sun or a well-calibrated stellar atmosphere model. As an example, see Campins, Rieke & Lebofsky (1985), who used the solar analog approach to derive absolute flux calibrations in the near-infrared. These calibrations are uncertain at a level of several percent, setting a fundamental limit on the accuracy of any absolute flux calibration in the near-infrared, including that of NICMOS.
In the case of NICMOS, the photometric calibration is based on observations of solar analogs and hydrogen white dwarfs (WDs). The stars P330E (solar analog) and G191B2B (WD) define the primary calibration, with a few other stars observed as cross-checks. Bohlin, Dickinson & Calzetti (2001) have recently recalibrated the absolute flux density distribution of the HST NICMOS standards. The optical and UV portions of their spectra were measured with high quality HST STIS observations, which were then carefully matched to stellar atmosphere models for extrapolation to near-infrared wavelengths. The infrared extrapolations were then checked by comparison to the most up-to-date ground-based broad band photometry for these stars, normalized to flux density using the Campins et al. (1985) calibrations (adjusted for small bandpass differences).
The STScI NICMOS group and NICMOS IDT have recently reanalyzed all photometric calibration data taken in orbit with NICMOS during Cycles 7 and 7N. Unlike earlier efforts, the new work uses a larger number of measurements, applies uniform and up-to-date NICMOS calibration software and reference files, and employs a more careful treatment of the NICMOS photometric aperture corrections1 than was used for the original calibration effort. Table 5.1 through table 5.3 give the current best estimates of the PHOTFNU and PHOTFLAM values for NICMOS filters. These new values update previous numbers given in older versions of the PHOTTAB calibration reference files, as well as an interim set of calibrations presented in an earlier edition of this Handbook (version 4.0). The new calibrations are believed to be accurate to better than <5%, thus satisfying the absolute calibration requirements for NICMOS. For most filters, the relative calibration accuracy should be better, <3%, with the possible exception of some narrow band filters where uncertainties in absorption line strengths for the solar analog and white dwarf models may result in greater calibration errors.
Please note that the NIC1 and NIC2 polarizers have not yet been recalibrated. The values given in table 5.1 through table 5.3 are taken from the older PHOTTAB calibration reference file i7l12297n_pht.fits, and are marked by asterisks. Note also that the phtometric calibration for the extremely broad band filters F140W, F150W and F175W should be used with caution. These bandpasses are so broad that the conversion from source flux to count rate will be very strongly dependent on the color or spectrum of the source.
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The photometric calibration information presented here is appropriate for NICMOS data taken during Cycles 7 and 7N only. In Cycle 11 and thereafter, NICMOS will most likely be operating at a significantly warmer temperature, regulated by the NICMOS Cooling System. Because the detector quantum efficiency is a function of temperature, the photometric zeropoint calibrations will almost certainly be different than those appropriate to Cycle 7 data. New photometric calibrations will be derived as quickly as possible in Cycle 11. Please check the STScI NICMOS web pages for updates on photometric calibration for Cycle 11 data.
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In the header of calibrated NICMOS 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:
where c is the speed of light in vacuum. PHOTBW gives the rms band width of the filter in Å (see the Synphot User's Guide for a detailed definition of both parameters).
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:
where CR is the count rate in units of DN sec-1. On the other hand, the magnitude in Oke's AB system (e.g., based on a constant flux per unit frequency) is obtained by applying the following expression:
As noted above, the NICMOS filter bandpasses do not match those of ground-based filters, and therefore it is not straightforward to derive "standard" infrared magnitudes normalized, e.g., to the spectral energy distribution of Vega. However, it may sometimes be useful to approximate a Vega-based photometric system by using an estimate of the flux density of Vega through the NICMOS bandpasses as a photometric zeropoint. Because there are no direct spectrophotometric observations of Vega or other A0V standards through the NICMOS filter system, our best option is to use a spectrophotometric model for Vega and synthetically integrate it through the bandpasses. Table 5.1 through table 5.3 give bandpass averaged flux densities (in Jy) for the NICMOS filters using a model reference spectrum of Vega (Colina, Bohlin & Castelli 1996, ISR CAL/SCS-008). An approximate Vega-normalized magnitude may be computed as
m = ZP(Vega) - 2.5 log (PHOTFNU × CR × 
F(Vega)
-1)
where ZP(Vega) is the magnitude of Vega. Under the "CIT" infrared photometry scale, Vega is defined to have ZP(Vega) = 0.0, whereas in the "Arizona" system (e.g., Campins et al. 1985, AJ, 90, 896), Vega has ZP(Vega) = 0.02.
Table 5.1: NIC1 Photometric Zeropoints
Table 5.2: NIC2 Photometric Zeropoints
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*These values have not been recalibrated, and are taken from PHOTTAB i7l12297n_pht.fits |
Table 5.3: NIC3 Photometric Zeropoints
5.3.3 Photometric Corrections
Photometric Stability with Time and Temperature
The photometric stability of NICMOS was monitored throughout the instrument's lifetime in Cycles 7 and 7N using repeated observations of the primary standard stars through a limited set of filters in each camera. There is some evidence for a small drift in the throughout with time, amounting to roughly 2% over the lifetime of the instrument. This is to be expected, as the array quantum efficiency is temperature dependent, and the instrument temperature gradually warmed throughout Cycle 7 and 7N. At present, this photometric "drift" is no larger than other known uncertainties in the absolute photometric calibration of NICMOS, and we do not discuss it further here. In the future, STScI may provide information for correcting the photometric zeropoints as a function of the observation date. This will be announced via the NICMOS WWW pages and the Space Telescope Analysis Newsletter (STAN). It is important to note, however, that NICMOS photometric zeropoints will almost certainly be significantly different in Cycle 11 and beyond when the instrument is operating at a warmer temperature with the NCS.
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 are corrected using on-orbit flats. On-orbit differential photometry characterization of NICMOS cameras 1 and 2 indicate that residual, large scale deviations amount to < 2% (rms). Much larger variations are seen for Camera 3, but are due to a different effect, intrapixel sensitivity variations, which we will discuss next.
Intrapixel Sensitivity Variations and Camera 3
As with many other infrared arrays, NICMOS detector sensitivity varies across the area of each physical pixel. It is higher in the center, and lower near the edges. In practical terms, this means that for a source whose flux changes rapidly on a scale comparable with or smaller than that of a pixel, the measured countrate, and therefore the derived flux, will depend on where the center of the source lies with respect to the center of the pixel.
For NICMOS cameras 1 and 2, the PSF is sufficiently well sampled that intrapixel sensitivity variations do not introduce a large (or at least, not a dominant) effect on point source photometry. Camera 3 images, however, are highly undersampled, and the exact position of a point source relative to the pixel grid can introduce large variations in the measured signal. Measurements made using calibration test data, as well as science data with large numbers of dither positions, show that the flux variation can be as large as 30%. The tighter the PSF is, the larger these variations will be. For that reason, "in focus" images taken during the two Cycle 7N refocus campaigns are more subject to the impact of intrapixel sensitivity variations, although the effect is still appreciable for non-campaign data as well. Similarly, stellar images through the shorter wavelength filters (where the telescope diffraction limit is smaller) show a greater variation due to this effect than do longer wavelength data. In addition, the intrapixel sensitivity variations can introduce complications for measuring image centroids, "pulling" the centroid away from pixel corners and edges toward the center of the pixel.
The effects of intrapixel sensitivity variations and possible approaches to correcting NIC3 point source photometry are discussed in two useful references: NICMOS ISR-99-005 (Storrs et al.), available from the STScI WWW pages, and a paper by Tod Lauer (1999, PASP 107, p.1434). Storrs et al. derive a photometric correction for Camera 3 F110W and F160W stellar images based on a "sharpness ratio," defined as the ratio of the flux in the peak pixel to that in the integrated PSF. This method is simple to apply and avoids uncertainties due to centroiding errors. Lauer derives a two dimensional pixel response map which can be used to correct photometry if the position of a point source can be measured accurately.
Intrapixel sensitivity variations should have substantially less effect on spatially extended sources. In general, given well dithered data sampling many different sub-pixel positions, the variations due to intrapixel sensitivity should average out, but given the rather large amplitude of the effect for point sources (especially for short wavelength, in-focus NIC3 observations), many dither positions (>10) are probably needed before the error on the mean count rate is smaller than other sources of photometric uncertainty in NICMOS.
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Camera 3 intrapixel sensitivity variations can have a major impact on photometry of point sources. For data taken with fewer than 10 dither positions, this uncertainty may dominate photometric errors unless steps are taken to correct it. We strongly recommend that Camera 3 users concerned with photometry of point sources read the references cited here.
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Temporal and Spatial PSF and Focus Variations
The point spread function (PSF) of HST changes with time, and these changes will affect photometry using small apertures. Changes in focus observed on an orbital timescale are due mainly to thermal breathing of the telescope. In addition, there were long-term variations in NICMOS focus as the cryogen evaporated and the dewar relaxed. The solid nitrogen introduced stress on the instrument, and NICMOS detectors moved along the focus direction as this stress diminished. The change was especially rapid during the SMOV period. The focus was largely stable thereafter, however, throughout most of the instrument lifetime.
The dewar distortion had the greatest effect on the focus of Camera 3, pushing it beyond the range of the Pupil Alignment Mechanism (PAM). For this reason, two special Camera 3 refocus campaigns, each three weeks long, were undertaken, one from 12 January through 1 February 1998, and the second from 4 June through 28 June 1998. During these periods, the HST secondary mirror was moved in order to bring Camera 3 into the range where its correct focus could be reached by the PAM.
The focus history of the telescope at the time of your observation can be determined using the NICMOS History Tool (see section 5.2).
In addition, the NICMOS PSF varies somewhat as a function of position on the array, and this can affect photometry made either with small apertures (by changing the effective aperture correction as a function of position), or using PSF fitting programs such as daophot unless there are enough stars in the field for the program to derive a positionally dependent PSF. The focus variations, however, are relatively small (see NICMOS ISR 98-005, "NICMOS Focus Field Variations and Focus Centering," Suchkov & Galas 1998). We have used synthetic PSFs from TinyTim (see Aperture Correction) to determine the positional dependence of the aperture correction as a function of position on NIC2, where the focus variations are largest. For F110W (the effect should be largest at short wavelengths), the maximum to minimum variation in the aperture correction for photometry measured within a 2 pixel radius is < 2%. The variations are expected to be smaller for the other cameras and for images at longer wavelengths, although this has not yet been empirically verified.
Aperture Correction
It is often difficult to measure the total flux of a point source due to the extended wings of the PSF, diffraction spikes, and scattered light. Such measurements are particularly difficult in crowded fields where the extended wings of sources can overlap with each other. An accurate method of measuring the integrated flux in these situations could consist of several steps:
- Measure in the image the total counts within a small radius.
- Simulate the TinyTim2 PSF for the particular camera-filter combination and position in the detector. TinyTim allows the user to simulate PSFs with various focus settings, and users concerned with small focus variations should be careful to set these parameters to the values appropriate for their observations. (This will have the largest effect for NIC3 data taken outside the refocus campaigns.)
- Use the simulated PSF to measure the fraction of total flux within the photometry aperture.
To obtain the total flux of the source, the countrate then only needs to be multiplied by the PHOTFNU or PHOTFLAM value and by the inverse of the measured fraction obtained in step three above.
Empirical PSFs could also be used for the above mentioned method. There was no special Cycle 7 calibration program to obtain PSF measurements for all camera and filter combinations. A significant amount of PSF data does exist in the archive, however: 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. The STScI NICMOS group will make a library of empirical PSFs available at some time in the future.
Out-of-Band Leaks
Many very red targets (e.g., protostars) were observed with NICMOS at short wavelengths (~1 µm). For these sources the flux at ~2.2-2.5 µm could be orders of magnitude larger than at ~1.0 µm and therefore exceptionally good out-of-band blocking would be required. Pre-launch tests indicated that for very red sources (temperature ~700 K and lower), some NICMOS filters might have significant red leaks. The suspect filters were F090M, F095N, F097N, F108N, F110M, F110W, F113N, F187N and F190N. A limited set of in-flight tests (in calibration programs 7691 and 7904) were made during Cycle 7, observing very red stars to calibrate the possible effects of out-of-band leaks. These data have never been carefully analyzed, but preliminary indications are that actual red leaks are insignificant or non-existent. We nevertheless caution here that this analysis is only preliminary. Users interested in photometry of sources with extreme colors should check the STScI NICMOS WWW pages for updates, or examine the in-flight test photometry of red stars themselves.
Non-Zero Zeroth Read Correction for Bright Sources
The problem of the non-zero zeroth read for bright sources was discussed in section 3.3 (ZSIGCORR) and section 4.3.2. If significant signal from an object is present in the zeroth read, then this was not properly taken into account in the nonlinearity corrections made for data processed by the OPUS pipeline before 11 November 1997. It is advisable to reprocess the data with the most recent version of the calibration software which includes an additional step (ZSIGCORR) to account for this zero read signal correction. When reprocessing such data, calnica version 3.3 or greater will automatically apply this step to all MULTIACCUM mode observations if both ZOFFCORR and NLINCORR are also being performed. All data retrieved from the Archive via OTFR after 26 September 2001 are automatically processed with this step.
Magnitudes and Photometric Systems Transformations
As was previously mentioned, NICMOS data are calibrated in units of Jy or Jy arcsec-2 for flux densities and surface brightnesses, respectively. The filter bandpasses do not exactly match those of ground-based JHK filters, and therefore if you wish to derive standard JHK magnitudes from NICMOS data, then it will be necessary to apply a color term correction. The recommended NICMOS JHK-analog system is obtained using the F110W, F160W and F222M filters, although the F110W filter especially is quite different from the standard J bandpass (specifically, it is much broader and bluer). Synthetic color terms can be computed using synphot, given real or model spectra for an astronomical source and the NICMOS and ground-based filter bandpasses (see, for example, Holtzman et al. 1995, PASP 107, 1065, where a similar procedure was used to derive color terms for WFPC2 photometry).
Also, as part of the Cycle 7 absolute photometry program, we observed a few blue stars (white dwarfs), intermediate color stars (solar analogs), and very red stars covering a large range in color (table 5.4). The calibrated data are available from the archive for users who wish to derive their own color terms to transform their HST fluxes into any ground-based system. The ground-based photometry given in the table below is based on preliminary and incomplete measurements, and will be updated when the final NICMOS photometric calibrations are completed.
Table 5.4: List of Stars for Photometric Transformations
| Name |
H |
J-H |
H-K |
Status |
Program IDs |
| G191-B2B |
12.6 |
-0.10 |
-0.14 |
Primary standard (white dwarf). |
7691, 7816 |
| P330E |
11.6 |
0.28 |
0.07 |
Primary standard (solar analog). |
7691, 7816 |
| OPH-S1 |
7.3 |
1.53 |
0.94 |
Primary standard (red standard). |
7049, 7691 |
| GD153 |
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Secondary standard (white dwarf). |
7904, 7816 |
| P177D |
12.0 |
0.28 |
0.06 |
Secondary standard (solar analog). |
7904, 7816 |
| CSKD-12 |
9.5 |
2.08 |
0.89 |
Secondary standard (red standard). |
7904, 7816 |
| BRI0021 |
11.1 |
0.75 |
0.52 |
Secondary standard (red standard). |
7904, 7816 |
5.3.4 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:
where FWHM is the full width half maximum of the equivalent gaussian filter to the narrow-band filter used (see Appendix 1 of the NICMOS Instrument Handbook), and we have assumed that the continuum has been already subtracted from the total flux in the filter and that the line is centered in the filter. If the emission line is not at the central wavelength of the filter, the line flux will need correction for the filter transmission curve. To estimate the variation in the absolute flux due to the positioning and width of the emission line in the filter bandpass, the synphot task calcphot can be used as shown below. See the Synphot User's Guide for additional help.
Figure 5.1: Estimating Absolute Flux Variation
The examples in figure 5.1 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.
5.3.5 Absolute Spectrophotometry with NICMOS Grisms
The accuracy of the absolute spectrophotometry with NICMOS grisms depends on three limiting factors:
- Accuracy of the spectral energy distribution of the standard stars used to obtain the inverse sensitivity curve.
- Quality of the flat-fielding and background subtraction of the calibration observations.
- Quality of the flat-fielding and background subtraction of the science observation itself.
The major source of uncertainty in the absolute spectrophotometry of a given source comes from the variability and structure of the background. Every pixel on the NIC3 array will receive background radiation over the entire spectral bandpass of the particular grism, while the source spectrum will be dispersed. The accuracy of the background subtraction is limited by our knowledge of the spectral response of each pixel, which is somewhat different from pixel to pixel. Extracted spectra will have to be corrected for the spectral response of each pixel. The accuracy of this correction is again limited by our knowledge of the detector response.
Remember that the spectra are not aligned with the X axis of the detector, and so move from one row to the next. This makes grism spectra especially sensitive to the intrapixel sensitivity variation. The NICMOSlook software (see section 5.8) version 2.7.4 and higher has a capability built in to correct for this variation. It depends critically on the placement of the spectrum, however, and the software handbook describes an iterative procedure to determine the best correction for this problem.
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).
Grism data reduction and calibration are discussed in more detail in section 5.8.
1 The PHOTFNU and PHOTFLAM keywords represent photometry corrected to infinite aperture, i.e., the scaling between stellar flux density and the total count rate measured from a star within a (hypothetical) infinite aperture. The actual standard star measurements are made within a fixed, finite radius aperture and corrected to infinite aperture using mode point spread functions from TinyTim.
2 TinyTim software can be retrieved from the Web at: http://www.stsci.edu/software/tinytim/tinytim.html
