Instrument Science Report NICMOS 2005-003
A Test of Possible NICMOS
Non-linearity
Bahram Mobasher and Adam Riess
August 15, 2005
ABSTRACT
Observations of standard stars with NICMOS grism (G096), when compared with the
same standards taken by STIS (G750L), show a disagreement over the wavelength range
0.8 to 1 micron. Further comparison with ACS F850LP data confirms thisnon-linearity to
be due to NICMOS in the sense that fainter stars in NICMOS appear too faint relative to a
brighter normalisation. We investigate this over a wide magnitude range by compiling
samples of stars (over the range 8 < J < 17) and galaxies (spanning the range 17 < J <
25), with available data with both NICMOS (F110W and F160W) and ground-based JHbands. We find no difference between the NICMOS F160W and ground-based H-band
magnitudes. However, in the case of NICMOS F110W and J-band filters, allowing for differences in the filter response functions, we find a trend at the bright-end (J < 17), confirming a non-linearity but smaller than that was seen in Bohlin effect by a factor of 2-3.
No significant slope is seen at the faint-end. This non-linearity is interpreted as being due
to trapping and the difference in exposure times between the bright and faint objects
(Riess 2005). We conclude that most of the observations will not be strongly affected by
this effect as the source count rates are sufficiently high so that the fraction of charge lost
to traps is negligible. The most affected observations are for faint sources with relatively
short exposures.
Introduction
Recently, a set of standard stars in the range 9.47 < V < 16 were observed with NICMOS
grisms G096 and G141, in an attempt to extend the wavelength coverage of the HST spectrophotometric standards to 1.7 microns (H-band) (Bohlin Lindler & Riess 2005). When
Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.
Instrument Science Report NICMOS 2005-003
observations in the G096 grism were compared with those of the same standards taken by
STIS G750L, a disagreement was found over the wavelength range 0.8 to 1 micron, in
common between the two detectors (Bohlin effect). Bohlin, Lindler & Riess (2005) concluded this apparent non-linearity existed in the NICMOS detector at wavelengths less
than 1.1 microns at the level of ~5.6% per dex, dissipating at longer wavelengths. The
sense of the effect was that fainter stars in NICMOS appear too faint relative to a brighter
normalization. The effect was confirmed by Bohlin, Lindler & Riess (2005) to exist in
NICMOS and not STIS by verfying its size and direction in a comparison between ACS
F850lp photometry and overlapping NICMOS spectrophotometry. The same apparent
non-linearity was also seen in an intra-NICMOS comparison of image pairs of stars taken
with the flat field lamp on and off, implying a change in apparent flux with count rate
(Bergeron, private communication).
If real, extrapolation of this apparent non-linearity from the NICMOS calibration sources
of F110W=12 mag (P330E and G191b2b) to fainter, science-level, fluxes of F110W~2327th mag would imply a measured flux error of ~0.2-0.3 mag, significant for many of the
deep NICMOS science observations performed at these magnitudes. (In practice, NICMOS observations of point sources become background dominated at F110W=23 so any
such non-linearity would be quenched below this flux level by the sky.) Since most of the
NICMOS observations are performed in this flux range, it is essential to re-examine the
question of non-linearity over a large range in flux.
In this ISR we aim to examine the presence of this non-linearity using independent
datasets from NICMOS and ground-based detectors. Furthermore, we extend the flux
range, studied so far, by compiling data for different sources (i.e. stars and galaxies)
observed by both NICMOS and other near-IR detectors over the same wavelength range,
spanning over 15 astronomical magnitudes. All the magnitudes in this study are in the AB
system.
The Sample
To carry out the proposed test here, we need observations of the same sources with both
NICMOS and another infrared detector over a similar wavelength range. To increase the
dynamic range of the observed flux, our sample utilizes bright calibration stars at the
bright end and compact galaxies to cover the fainter fluxes.
High flux regime: We compared the observed flux in the high count regime by comparing
apparent magnitudes previously published by Stephens et al (2000) of stars, observed with
both NICMOS (NIC2) in F110W and F160W and ground-based near-IR detectors in J and
H-bands. The ground-based detectors are operated in the sky-dominated regime due to the
unavoidable high OH emission in the atmosphere, so potential non-linearity in the ground-
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based data would be negligible. Stephens et al. (2000) derived a color correction valid for
stars over a limited color range from spectrophotometry as well as from empirical fitting.
For the F110W, the transformation is: F110W=J+0.26*(F110W-F222M), applicable at
F110W-F222M < 3.1, where J is the ground-based J-band whose throughput is defined by
the window in the atmospheric OH emission at 1.2 microns, and F222M is a NICMOS
medium throughput passband centered at 2.22 microns. For F160W, a similar transformation yields: F160W=H+0.12*(F110W-F222M). The Stephens et al sample includes 23
stars with 8 < J < 17 mag.
Low flux regime: to cover the low flux regime, we use NICMOS (NIC3) images of galaxies in the Hubble Ultra Deep Field (HUDF)- (Thompson et al 2005; Robberto et al 2005)
for which ground-based near-IR (JH) ISAAC data, obtained as a part of the GOODS
project, are also available. The HUDF NICMOS images were matched with the ISAAC
and a total of 398 galaxies with both NICMOS (F110W and F160W) and ISAAC (JH)
magnitudes were found.
We verified that the ISAAC zero-points from the GOODS CDFS catalog were correct by
comparing their matching bright stellar magnitudes between ISAAC and the 2MASS surveys. A dozen stars were matched between the ISAAC and 2MASS catalogs (the number
limited by their different flux sensitivity) and the zeropoints were found to agree to better
than 0.05 mag in J and H.
The filter response functions for F160W and H (1.6 microns) bands are very similar and
for the spectral energy distribution (SED) of the galaxies involved, magnitude differences
resulting from differences in the bandpasses are negligible. . However, the F110W filter
extends more to the shorter wavelengths (bluer colors) than the standard ISAAC J-band
and therefore, does require an SED-based correction (similar to a color correction) before
their respective magnitudes can be compared. We discuss the correction to galaxies
F110W magnitudes in the following section.
Correction between F110W and J(1.2) Filters for HUDF GalaxiesConversion of galaxy magnitudes between NICMOS-F110W and ISAAC-J filters is complicated by the fact that galaxies are composite systems with different stellar populations.
Therefore, depending on the redshift and type of a given galaxy, these filters will sample
different parts of its SED. Although the filters overlap for much of the wavelength range,
the difference between the shapes of their response functions would lead to different magnitudes through the two filters. To correct for this difference we employ the same
algorithms used for the photometric redshift measurements from the GOODS program
(Mobasher et al 2004).
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Using the available optical and near-IR data for each of the 398 galaxies, we fit the
observed and template SEDs to estimate the redshift and spectral type of each galaxy. The
reliability of this technique is demonstrated in Figure 1 where we compare the estimated
photometric and spectroscopic redshifts for a sub-sample of HUDF galaxies with available
such data. Individually, the redshifts agree at < 10% level. Of greater relevance to our
application, the redshifts agree statistically to better than 1% for GOODs survey data
(when using a few hundred galaxies with available multi-wavelength photometric data and
spectroscopic redshifts). The template SEDs consist of E, Sb, Sc, Irr and starburst. The filter response functions for F110W and ISAAC J-band are then multiplied by the best fit
SED for each galaxy and the integrated magnitudes through each of the two filters are estimated. The difference between the magnitudes from the two filters was found and used to
correct the respective magnitudes for differences in the shapes of filter response functions.
The NIMOS(F110W)-ISAAC(J) color difference as a function of redshift and for different spectral types used in this study is presented in Figure 2.
Results
Using the sample in the above section , we now examine the presence of non-linearity in
NICMOS filters. Combining the low and high-flux regime data, we can explore a large
dynamic range, from 8 to 23 mag and hence, the non-linearity over 15 magnitudes (many
of the UDF sources are at mag > 23 ; but because they are sky dominated, the flux seen by
the NICMOS detector does not get any fainter than J~23 mag). In the following we consider the two filters in turn:
F160W filter: Figure 3 shows the comparison between NICMOS(F160W) and ISAAC(H)
measurements for stars and galaxies separately, covering a wide magnitude range 8 <
F160W < 17 (for stars) and 18 < F160W < 26 (for galaxies). There is a good agreement
between the magnitudes from the two detectors, both in their zeropoints and as a function
of apparent magnitudes. Importantly, there is no apparent discontinuity in the Y-axis at
the magnitudes the stars and galaxies meet (H=19 mag). The observed agreement between
the two datasets is impressive as no correction is applied to the magnitudes due to differences in filter response functions and that the calibrations between the two sets are done
completely independently. However, this result alone does not conflict with those from
Bohlin et al (2005- Figure 4) who found good linearity present in the wavelength range of
the F160W filter over a smaller dynamic range.
F110W filter: Figure 4 shows the comparison between NICMOS(F110W) and ISAAC(J)
measurements for stars and galaxies separately, covering a wide magnitude range 9 <
F110W < 18 (for stars) and 18 < F110W < 27 (for galaxies). These cover an effective
range of 15 magnitudes from J=8 mag to J=23 mag, beyond which the detected flux is sky-
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dominated. Again, stars and galaxies show a continous sequence at J~18 mag with no evidence of a break, confirming the consistency of the data from the two flux regimes.
Although there may be a slight non-linearity at the bright magnitudes (10 < J < 16), it is
less than that seen by Bohlin et al by a factor of 2 to 3 as determined from a line fit to these
data. However, more leverage on the non-linearity is gained by fitting across the full
dynamic range. By dividing the data into magnitude bins, we do not find any significant
slope at the faint part of the diagram. Indeed the average of the bins from 23 mag <
F110W < 27 mag (in which the detected flux rate is constant due to the sky dominance),
the apparent NICMOS F110W mags are consistent with those inferred from the ground to
within a 2% and highly inconsistent with the 25% deficit expected from an extrapolation
of the Bohlin effect. However, a source of uncertainty in directly comparing the present
results with those in Bohlin et al is that the latter is based on the count rates, with the count
rate per pixel being higher in stars than in galaxies with the same magnitudes. Therefore,
the results in Figures 3 and 4, using total magnitudes over a wide magnitude range , provide a stronger confirmation of the NICMOS photometry than that based on the count
rates, as was performed by Bohlin et al. The "face-value" interpretation of this analysis is
that observers using NICMOS to determine magnitudes in similar observations as the
UDF, can derive accurate results by using the NICMOS zeropoints without any correction
to them.
As discussed in a forthcoming analysis by Riess (2005), the underlying cause of the difference between the apparent non-linearity seen by Bohlin et al and the lack of one seen
here appears to be due to the effect of trapping and the difference in the exposure time of
the observations in the UDF and those taken by Bohlin et al. The key difference from the
grism observations appears to be the length of the exposure time in the UDF observations
which are > 1000 sec (the grism observations are typically ~60 sec).
In a separate study A. Riess (2005) has shown that intermediate time-scale trapping and
the associated detrapping seen by Bergeron and Dickinson (2003) with 155 sec time-constant explains both the apparent non-linearity in the grism data and the lack of any seen in
the UDF data. Short exposures of less than ~155 sec of moderate or faint sources will
accumulate charge while suffering an apparent net loss due to trapping. This net loss will
gradually abate as the exposure time exceeds the time constant for detrapping and charge
is reemitted at the same rate it is lost. Continued exposure time will reduce the fractional
loss to the source as additional charge is accumulated at the proper rate. A prediction of
this explanation is the apperance of a smooth rise in the apparent count rate in time intervals of less than 155 sec in the non-destructive reads.
In the Riess study, examination of intermediate reads in both the NICMOS grism data
used by Bohlin et al. and in other long-exposure photometric observations (from GO
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9353) shows an apparent deficit in count rate at shorter exposure times, asymptotically
approaching an enhanced count rate.
Conclusions:
Most observations obtained for science will not be strongly affected by this effect either
because the integration times are significantly longer than the 155 second e-fold time or
because the source count rates are sufficiently high that the fraction of charge lost to traps
is negligible. Scientific observations made with relatively short integrations of faintsources will be most vulnerable.
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Figure 1: Comparison between the spectroscopic and photometric redshifts for UDF galaxies with available optical and near-IR data. The BViz (ACS)+JH(NICMOS)+K(ISAAC)
are used to estimate photometric redshifts and spectral types.
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Figure 2: Changes in F110W(NICMOS)-J(ISAAC) with redshift for different spectral
types of galaxies. Ellipticals (black); Sa/Sb (red); Sc(green); Irr(blue); starbursts
(magenta/yellow)
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Figure 3: Comparison in total magnitudes between the NICMOS (F160W) and groundbased H-band (ISAAC) photometry for stars and galaxies. Stars are taken from Stephens
et al (2000) and galaxies are from UDF with available NICMOS and ground-based data. A
total of 398 galaxies are used. Solid line presents agreement between magnitudes. The
magnitudes for both point sources (stars) and extended objects (galaxies) correspond to
total. The green points correspond to average difference in magnitude bins. Errors are
derived from poisson statistics.
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Figure 4: The same as Figure 3 for two different magnitude range (i.e. for stars and galaxies separately).
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References
Bergeron, E., and Dickinson, M. 2003, Instrument Science Report, NICMOS 2003-010,
(Baltimore: STScI)
Bohlin, R. C., and Gilliland, R. L. 2004 AJ, 128, 3053
Bohlin, R. C., Lindler, D. J. and Riess, A. 2005 Instrument Science Report 2005-002 (Baltimore: STScI)
Mobasher, B. et al 2004 ApJL 600, L167
Riess, A. 2005, Instrument Science Report 2005, Baltimore:STScI (in prep)
Robberto, M. et al 2005 AJ (in prep)
Stephens et al 2000 ApJ
Thompson , R. et al 2005 (in prep)
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