TECHNICAL
REPORT
Title: Consequences of Etalon Detuning for TFI Science Doc #: JWST-­‐STScI-­‐002190, SM-­‐12 Date: July 15, 2010 Rev: -­‐ Authors: A. R. Martel, A. Fullerton Release Date: 28 September 2011 Phone: 410-­‐
338-­‐4888 1. Abstract
We present a detailed analysis of the consequences of the position-dependent bandpasses
(PDBs) that are inherent to optical systems that incorporate a Fabry-Pérot etalon, with
particular emphasis on the Tunable Filter Imager (TFI). Simulations are generated for a
variety of input spectral energy distributions: a flat spectrum, Vega, a planetary nebula
(NGC 6543), and a high-redshift QSO. We show that even though the systematic change
in wavelength across the field of view (“detuning”) is small, the corresponding variations
in the integrated counts may be significant (up to ~25%) when the target is imaged near a
wavelength characterized by a sharp spectral gradient. The detected flux can decrease or
increase as a function of position depending on the interplay between: a) the shape of the
source continuum across the bandpass, b) the location of a feature within the bandpass, c)
the proximity of the bandpass to the edge of the blocking filter (which favors one wing of
the Airy function over the other); and d) the change in the intrinsic shape of the bandpass.
The PDB of the TFI has important implications for science observations. In particular, a
judicious choice of dithers or mosaics may be necessary to mitigate these variations.
Accurate photometry of astronomical sources may require substantially different
corrections to be applied at different locations in the field of view.
2. Introduction
Martel & Fullerton (2011) presented the basic operating principles of the TFI etalon
and discussed the concept of wavelength “detuning”. Since the etalon in the TFI is not
used in a telecentric configuration, light corresponding to different radial distances from
the optical axis enters with a range of incident angles, !. This behavior imposes a
positional dependence on the peak wavelength of the bandpass such that
2!
!!
! ! =
cos ! = !! cos ! ≈ !! 1 −
,
!eff
2
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Aeronautics and Space Administration under Contract NAS5-03127
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(1)
JWST-STScI-002190, SM-12
where d is the gap between the plates of the etalon; !eff is the “effective order”1;
!! ≡ 2! !eff is the wavelength at the center of the field of view (i.e., where ! = 0°);
and where the small-angle approximation for the value of cos ! has been used. As a
result, even though an etalon is tuned to transmit a wavelength !! on-axis, the wavelength
of the peak transmission is shifted systematically (“detuned”) toward bluer wavelengths
in a circular pattern centered on the optical axis . For the TFI, 0° ≤ ! ≤ 4.2°, which
reduces !! by only ! ! 2 ≈ 0.3%.
In addition to the location of the transmission maximum, off-axis rays change the
shape of the bandpass transmitted by the etalon; i.e., the Airy function. This dependence
can be seen explicitly by combining Equations (34) and (3) of Martel & Fullerton (2011):
!!
2!eff,!
= 1+
!
!
!
sin2
2
2!eff,!
= 1 +
!
!
2!" cos ! − !!
sin2
!
! !; !eff,!
(2)
!!
(3)
where !eff,λ is the “effective finesse” of the etalon and !! is the phase change upon
reflection at each surface. Both these quantities may be wavelength dependent. The
presence of the cos ! term causes the shape of the bandpass to depend on the distance of
an object from the optical axis.
In the following sections we analyze the consequences of this position-dependent
bandpass (PDB) for the detected counts of typical astronomical sources when they are
observed at different positions in the field-of-view (FOV) of the TFI. In particular, we
present more realistic simulations generated for a variety of spectral energy distributions
(SEDs) observed with the TFI. Our goal is to assess the impact of the continuum and
strong spectral features on the variations of both the wavelength and the observed counts
across the FOV for typical astronomical sources.
3. Simulations
In the following section, we explore the consequences of “detuning” by examining its
effect on the following SEDs, all of which are specified in the standard ‘flam’ units of
erg/s/cm2/Å:
a) A constant (“flat”) spectrum in “wavelength” units
b) Vega
c) The emission-line spectrum of a planetary nebula (PN)
d) A high-redshift Ly! emitter
1
See Section 2.1 of Martel & Fullerton (2011) for the definition of “effective order.”
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3.1
An Ideal Etalon with a Flat Spectrum
3.1.1 Peak Wavelength
Our first simulation attempts to reproduce the expected behavior for the ideal case of a
uniform (“flat”) input SED. In Fig. 1, Airy profiles are generated for the optical axis of an
etalon with a constant reflectance and transmittance similar to those of the TFI:
! = 0.85 and ! = 0.15. The reflective finesse is therefore !! ≈ 20. A 180° phase shift
is introduced to mimic the phase changes at the TFI plates. For an etalon gap of 4.0 µm,
the peaks of the Airy profiles in the operational interference orders shortward (! = 3)
and longward (! = 1) of the non-functional regions of the etalon occur at 2.0 µm and
4.0 µm, respectively. The input spectrum is a flat distribution normalized to unity and
expressed in the standard ‘flam’ units of erg/s/cm2/Å. The blocking filters are also fixed
to unity and are assumed to be symmetric about the peaks of the profiles, 1.8 – 2.2 µm
and 3.4 – 4.6 µm.
These simulations indicate that:
1. The Airy profile is always asymmetric in the sense that the red “half” of the
profile contains more counts than its blue counterpart.
2. Detuning: As " increases from 0° to 4°, this asymmetry remains but the peak
shifts to bluer wavelengths. To track the location of the peak, a 2nd -order
polynomial was fit to the upper 20% of the output spectrum (magenta in Fig. 1) as
a function of " . This wavelength detuning is shown in Fig. 2. We find that the
! for both the blue
! and
! red sides of the non-functional region of the TFI
detuning
etalon is an excellent match to Equation (1). In fact, all three lines (blue, red, and
dashed) overlap perfectly in Fig. 2. The maximum detuning is ~0.3%, as
!
expected.
3. PDB: As ! increases and the output spectrum shifts to the blue, the bandpass
becomes slightly narrower; i.e., its resolving power increases. Since the counts are
integrated under the profile over a fixed wavelength range defined by the blocking
filter, a net decrease is expected. This is shown in Fig. 3. The maximum change is
~0.5% for the flat input spectrum of this simulation.
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Figure 1 Simulations for ! = 0° on the blue (top) and red (bottom) side of the TFI etalon’s
non-functional region. The inputs are: Airy profiles (dashed black), flat input spectrum
(green), and normalized blocking filter (cyan). The output spectrum (magenta = green x
dashed x cyan) is shown in the bottom panels as well as the second-order fit to the peak.
Constant input parameters typical for the TFI are assumed: ! = 0.85 !! ≈ 20 ; ! =
0.15; ! = 1; and an etalon gap of 4.0 !m.
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Figure 2 Wavelength detuning for the simulations described in Fig. 1 in absolute (top) and
relative (bottom) wavelength. Blue: shift in the peak of the profile on the short-wavelength
side of the non-functional region of the TFI. Red: same but for the long-wavelength side.
Dashed: detuning described by Equation (1). The three lines overlap exactly in the bottom
panel.
Figure 3 Relative change in integrated counts due to PBD. Blue: output spectrum to the
short wavelength side of the TFI non-functional region; red: same but to the long
wavelength side.
3.1.2 Average Wavelength
For simulations with SEDs of some astronomical sources, the peak of the output
spectrum may not correspond to the peak of the bandpass provided by the etalon. Such
cases arise, e.g., if the peak of the emission line is highly distorted or asymmetric (as for
Ly! in a QSO, see §3.2.4) or if multiple emission lines fall within the bandpass, causing
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the software to confuse the true location of the peak. For these spectra, we will instead
estimate the centroid of the output spectrum (after multiplying with the Airy function and
the blocking filter) with a flux-weighted average. By default, this average is calculated
over the full range of the blue and red regions, 1.5 – 2.5 µm and 3.2 – 5.0 µm,
respectively, and so may include some contribution from any “leaks” outside the
blocking filter’s primary bandpass. An example is shown in Fig. 4 for the simulation of a
flat input spectrum. Because of the asymmetry in the profiles, the centroid is always
skewed to wavelengths slightly longer than the peak of the bandpass (top panel) and so
the ideal situation described by Equation (1) is not expected to be recovered (bottom
panel).
Figure 4 Same as Fig. 2 except that the centroid of the output spectrum is calculated with a
flux-weighted average.
3.2 The TFI Etalon
In this section, we present simulations for the more realistic case of the TFI. The
throughputs of the actual blocking filters (F158M, F175M, F200M, F229M, F246M,
F349M, F425M, and F470M) are used. Except for the flat spectrum and Vega, the
centroid of the output spectra is calculated as the flux-weighted average. To simplify the
analysis and the interpretation, the effective finesse is assumed equal to the reflective
finesse, i.e., defects such as surface irregularities on the etalon plates are ignored. The
net effect of these effects is to degrade the resolving power of the filter; i.e., to increase
the width of the bandpass (Martel & Fullerton 2011).
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3.2.1 A Flat Spectrum
In Figures 5 – 7, we show the results of a simulation with a flat distribution as the input
SED. The etalon gap is finely tuned to 4.0073 µm so the peak of the output profile in the
third order (short-wavelength side of the non-functional region) is located at 2.0 µm at
! = 0° for a more direct comparison with the results of §3.1.1. The corresponding peak
in the first order (long-wavelength side) is at ~3.86 µm. As in §3.1.1, the detuning in
wavelength is tracked with a 2nd-order fit to the peak of the output profile (magenta in
Fig. 5).
From Fig. 6, we find that the detuning in wavelength is slightly less severe than for the
case of an ideal etalon and blocking filter. This is likely due to the structure of the
blocking filter, which favors the red wing of the profile over the blue wing. In fact, for
the TFI, it is impossible to achieve the ideal behavior described by Equation (1). On the
other hand, the change in count-rate is more important for the TFI than for the ideal case,
up to ~2% (Fig. 7) versus ~0.5% (Fig. 3).
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Figure 5 Simulations for ! = 0° on the short-wavelength side of the TFI etalon’s non-
functional region (top) and on the long-wavelength side (bottom). The inputs are: Airy
profiles (dashed black), flat input spectrum (green), and non-normalized blocking filter
(cyan). The output spectrum (magenta = green x dashed x cyan) is shown in the bottom
panels as well as the 2nd-order fit to the peak.
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Figure 6 Flux-weighted wavelength shift for the simulations shown in Fig. 5 in absolute (top)
and relative (bottom) wavelengths. Blue: shift of the profile on the short wavelength side of
the non-functional region of the TFI. Red: same but for the long-wavelength side. Dashed:
detuning described by Equation (1).
Figure 7 Relative change in integrated counts due to PDB. Blue: output spectrum on the
short-wavelength side of the TFI non-functional region; red: same but for the longwavelength side.
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3.2.2 The Spectrum of Vega
In these simulations, the input SED is the spectrum of Vega from the Kurucz 1993
atlas, vega_k93.fits, which can be found in the directory of HST standard spectra of the
Calibration Data System (CDBS) at STScI. It covers a wavelength range of 0.009 – 160
µm with non-uniform sampling. In the near-IR, the spectrum is dominated by a strong
continuum attenuated by several moderately strong absorption lines. To evaluate the
potential impact of the absorption lines on the PDB, simulations were generated for an
etalon tuning that places the absorption line due to Brackett ! (at ~1.95 µm) in the blue
wing of the Airy function (Figs 8 – 10) on the short wavelength side of the non-functional
region of the etalon. Similar simulations are shown in Figs 11 – 13 for a tuning that puts
the same absorption line on the long wavelength side of the Airy function.
As ! increases, the peak of the output profile (magenta in Figures 8 and 11) shifts to
bluer wavelengths by ≲ 0.25% (Figures 9 and 12). The location of the absorption line in
the Airy profile therefore does not affect the wavelength detuning. However, the
corresponding change in the integrated counts is unusual. On the blue side of the nonfunctional region of the etalon, the integrated counts increase away from the optical axis
by up to ~2%, while on the red side, they decrease by up to ~1% (Figs 10 and 13).
How can we explain this behavior? For the small range of shifts of the profiles to the
blue, all the output spectra should suffer a similar loss from the absorption lines. Hence,
we hypothesize that to first order, the change in integrated counts is dominated by the
interplay between the shape of the input spectrum and the edge of the blocking filter
(neglecting the fine structure of the blocking filter and the narrowing of the Airy profile
with !).
• On the blue side of the non-functional region, as ! increases the Airy profile
slides up the steep, blue SED of Vega and gains counts. But simultaneously, the
profile loses counts in its far blue wing due to suppression by the blocking filter.
The net result is a gain in counts as ! increases. The closer the Airy profile is to
the blue edge of the blocking filter, the greater the loss in the blue wing, and the
larger the overall increase (~0.2% vs ~2% in Figs 10 and 13).
• On the red side of the non-functional region, the Airy profile is much closer to the
blue edge of its blocking filter. In this case, the loss in the far blue wings
dominates over the gain from the Vega spectrum, resulting in a net loss in counts.
Again, the closer the profile is to the edge of the blocking filter, the greater the
loss in the wings (~0.7% vs ~1% in Figs 10 and 13).
However, other simulations with Vega show that a range of behavior can be obtained
by moving the Airy profile across the blocking filter, especially near the edges. A
rigorous analysis should include the competing contribution of the stellar continuum
across the bandpass, the absorption lines, the fine structure in the throughput of the
blocking filter, the proximity of the bandpass profile to the edge of the blocking filter, as
a well as the PDB itself.
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Figure 8 Simulations for ! = 0° on the short wavelength side of the TFI etalon’s non-
functional region (top) and on the long wavelength side (bottom). The inputs are: Airy
profiles (dashed black), Vega spectrum (green), and non-normalized blocking filter (cyan).
The output spectrum (magenta = green x dashed x cyan) is shown in the bottom panels as
well as the 2nd-order fit to the peak. The etalon gap is tuned to 3.9 µm so that the Br!
absorption line falls in the blue wing of the Airy profile (uppermost panel).
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Figure 9 Flux-weighted wavelength shift for the simulations shown in Fig. 8 in absolute (top)
and relative (bottom) wavelengths. Blue: shift of the profile on the short wavelength side of
the non-functional region of the TFI. Red: same but for the long-wavelength side. Dashed:
detuning described by Equation (1).
Figure 10 Relative change in integrated counts due to PDB. Blue: output spectrum for the
short-wavelength side of the TFI non-functional region; red: same but for the longwavelength side.
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Figure 11 Simulations for ! = 0° on the blue side of the TFI etalon’s non-functional region
(top) and on the red side (bottom). The inputs are: Airy profiles (dashed black), Vega
spectrum (green), and non-normalized blocking filter (cyan). The output spectrum
(magenta = green x dashed x cyan) is shown in the bottom panels as well as the 2nd-order fit
to the peak. The etalon gap is tuned to 3.85 µm so that the Br! line falls in the red wing of
the Airy profile (uppermost panel).
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Figure 12 Flux-weighted wavelength shift for the simulations shown in Fig. 11 in absolute
(top) and relative (bottom) wavelengths. Blue: shift of the profile on the short wavelength
side of the non-functional region of the TFI. Red: same but for the long-wavelength side.
Dashed: detuning described by Equation (1).
Figure 13 Relative change in integrated counts due to PDB. Blue: output spectrum for the
short-wavelength side of the TFI non-functional region; red: same but for the longwavelength side.
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3.2.3 A Planetary Nebula Spectrum
Simulations were generated for the input spectrum of a planetary nebula. A synthetic
spectrum of NGC 6543 (The Cat’s Eye Nebula) was created by modeling the H and He
emission lines as Gaussian profiles with integrated fluxes and a weak, flat continuum
tabulated in Hora et al. (1999) and Bernard-Salas et al. (2003). Other lines, such as Fe,
are not included. For our purposes, an exact spectrum is not necessary: we simply want to
characterize the consequences of the PDBs for an astronomical target with strong, narrow
emission lines and a weak continuum.
The results of the simulations are shown in Figures 14 –16. The etalon was tuned to a
gap of 4.25 µm so that the Brα line at ~4.05 µm was situated in the red wing of the Airy
profile on the long-wavelength side of the non-functional region of the etalon. Since
many lines lie within the bandpass on the blue side, we measured the position-dependent
change in wavelength by using the flux-weighted average (§3.1.2).
We find that even though the detuning in wavelength across the FOV is small (Fig.
15), the corresponding variation in the counts can be very significant (up to a ~25% loss
on the red side; Fig. 16) when the bandpass encompasses a strong emission line. This
change is attributable to the steepness of the Airy function: as ! increases, the peak of the
Airy function detunes to bluer wavelengths and in effect “slides” along the fixed
emission line. As a result, large variations in the integrated counts under the narrow line
are recorded. These changes are more pronounced near the peak of the bandpass, where
the gradient d! d! is steepest. On the short-wavelength side of the etalon’s nonfunctional region, the effect is less pronounced since the loss in counts from the strong
line at 2.17 µm in the red wing is partially compensated by the gain in counts from the
line at 2.06 µm in the blue wing.
This interpretation is confirmed when the etalon gap is tuned so that Brα falls in the
blue wing of the Airy function on the long-wavelength side of the etalon’s non-functional
region (Figs. 17 – 19). In this case, the change in counts shows a strong increase away
from the optical axis, up to ~30% (Fig. 19), for the same small detuning in wavelength.
Although this is opposite to the behavior seen when the emission line was placed in the
red wing, it is consistent with expectations.
In orbit, the user will likely place the target at the center of the field (! = 0°) and tune
the etalon so the peak of the bandpass matches the wavelength of the emission line for
maximum throughput. For Brα, the etalon gap would correspond to 4.276 µm. The
simulation indicates that the change in the observed counts is significant, ~8% at
" = 4.0°, since the Airy profile is shifted to the blue. Hence, if the source were observed
again at another location in the field, the integration time would need to be increased
accordingly to compensate for the loss in flux.
!
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Figure 14 Simulations for ! = 0° on the blue side of the TFI etalon’s non-functional region
(top) and on the red side (bottom). The inputs are: Airy profiles (dashed black), planetary
nebula spectrum (green), and non-normalized blocking filter (cyan). The net bandpass (blue
= dashed x cyan) and the output spectrum (magenta = green x dashed x cyan) are shown in
the bottom panels as well as the location of the flux-weighted average (vertical dotted line).
The etalon gap is tuned to 4.25 µm so that Brα at ~4.05 µm falls in the red wing of the Airy
profile.
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Figure 15 Flux-weighted wavelength shift for the simulations shown in Fig. 14 in absolute
(top) and relative (bottom) wavelengths. Blue: shift of the profile on the short wavelength
side of the non-functional region of the TFI. Red: same but for the long-wavelength side.
Dashed: detuning described by Equation (1).
Figure 16 Relative change in integrated counts due to PDB. Blue: output spectrum for the
short-wavelength side of the TFI non-functional region; red: same but for the longwavelength side.
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Figure 17 Same as Fig. 14 but for a tuning of the etalon gap that places Brα in the blue wing
of the Airy profile.
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Figure 18 Same as Fig. 15.
Figure 19 Same as Fig. 16. The change in integrated counts due to PDB shows a large
increase as a function of !.
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3.2.4 A High-Redshift Lyα Emitter
For this application, the etalon is tuned to observe the Lyα emission line of a highredshift QSO. Such a program is under consideration by the TFI Science Team as part of
their guaranteed observing time. We use the QSO spectrum of Telfer et al. (2002) as
input. It is a combination of an HST radio-quiet composite below 1275 Å and the Sloan
Digital Sky Survey median composite above 2000 Å and an average of the two in
between. The H I opacity and Lyα forest are modeled according to the prescription of
Madau (1995). At the high redshift of our simulations, the continuum blueward of Lyα is
nearly completely attenuated; i.e., the Lyα forest imposes a “step” function on the SED.
The simulations are shown in Figures 20–22. We assume the observer is searching for
a QSO at a redshift of z = 13 so that the Lyα line will be located at the wavelength of
highest resolution of the TFI, ~1.71 µm (Martel & Fullerton 2011). An etalon tuning of
3.35 µm places the peak of the “step” function just slightly blueward of the peak of the
Airy function (Fig. 20). Once again, we find that even though the detuning in wavelength
!
is small (≲ 0.04%
at Lyα; Fig. 21), the corresponding change in counts is severe, up to
~25% at Lyα (Fig. 22). As for the simulations of the planetary nebula, we attribute this
behavior to the sharpness of the Airy bandpass, particularly near its peak.
Additional simulations offer more insights. If the etalon is tuned such that Lyα falls
further in the blue wing of the Airy function (gap of 3.4 µm), then most of the weak
continuum blueward of Lyα is outside the bandpass and the position-dependent changes
are small, ≲ 1%. In fact, the counts increase with increasing ! because the Airy profile
shifts to shorter wavelengths and includes increasing amounts of flux from the red wing
of the broad Lyα profile. On the other hand, if the tuning is such that the Lyα peak is in
the far red wing (gap of 3.3 µm), then a signfiicant decrease in counts is observed (up to
20%). However, unlike the previous simulations, the flux-weighted wavelength
increases with increasing !. This is expected since the Airy function shifts further into
the highly attenuated continuum of the Lyα forest as ! increases, which necessarily gives
more weight to the red wing of the Airy function.
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Figure 20 Simulations for ! = 0° on the blue side of the TFI etalon’s non-functional region
(top) and on the red side (bottom). The inputs are: Airy profiles (dashed black), QSO
spectrum at z = 13 (green), and non-normalized blocking filter (cyan). The net bandpass
(blue = dashed x cyan) and the output spectrum (magenta = green x dashed x cyan) are
shown in the bottom panels as well as the location of the flux-weighted average (vertical
dotted line). The etalon gap is tuned to 3.35 µm so that Lyα falls in the peak of the Airy
!
profile.
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Figure 21 Flux-weighted wavelength shift for the simulations shown in Fig. 20 in absolute
(top) and relative (bottom) wavelengths. Blue: shift of the profile on the short wavelength
side of the non-functional region of the TFI. Red: same but for the long-wavelength side.
Dashed: detuning described by Equation (1).
Figure 22 Relative change in integrated counts due to PDB. Blue: output spectrum for the
short-wavelength side of the TFI non-functional region; red: same but for the longwavelength side
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4. Implications of a Position-Dependent Bandpass
These simulations indicate that the integrated counts of an astronomical source can
vary significantly over the FOV of the TFI owing to the interplay between the PDB of the
etalon and the SED of the source. These variations may have serious repercussions for
both the implementation strategy and the results of an observing program. Here, we
identify some issues that will require further consideration:
1. Dithers and mosaics: Except for coronagraphic observations, TFI observations
will be dithered. For small dithers (a few pixels), no photometric correction is
necessary within the pattern itself, although shifts along the concentric circle
might be preferable to completely mitigate the effects of PDB. Of course, an
overall correction to the flux may be necessary if the dithered source is located
significantly away from the optical axis. For mosaics where several full fields of
the TFI are observed with large offsets (say, ¼ of the FOV) but with some
overlap, care must be taken to place the target(s) of interest at the same off-axis
position in each field. Otherwise, the target will exhibit different integrated counts
in each field, greatly complicating the analysis and the corrections.
2. Photometric corrections: For a given tuning of the etalon, any given target field
will potentially contain hundreds of sources, some imaged in their continuum and
others in an emission line. Generally speaking, the interaction between the PDB
of the instrument and the SED of the source will affect each source differently. A
significant problem for the observer is that the SED of each source is unknown a
priori so any correction to be applied will involve some guesswork. One possible
strategy is to observe the field at several wavelengths in order to construct the
overall shape of the SED, although fine detail in the spectrum, such as narrow
emission lines, will be difficult to capture. Some difficulties may also be
encountered with coronagraphy in an emission line, such as a QSO, since the
occulters are located near the edge of the FOV where the effects of are the most
pronounced. Special photometric calibration may need to be implemented.
3. On-orbit commissioning and calibration: During the on-orbit commissioning
and calibration campaigns for the TFI, the effects of the PDB can be characterized
through observations of a source with a strong but narrow emission line, such as a
planetary nebula or Be star. Changes in the position of the peak of the Airy
function can be measured by scanning the etalon at multiple gaps (3 or 5, say) to
locate the wavelength of the peak flux of the star at several off-axis radii.
Changes in the shape of the bandpass can be tracked by using:
1) A star with a precisely known spectrum that is dominated by a continuum
that is not very steep. The absorption lines should be weak and no emission
lines should be present. A JWST spectrophotometric standard star would be
a good candidate. By observing the star at several off-axis radii at a single
tuning of the etalon, the effects of the changing shape of the bandpass can be
mapped by measuring the total flux of the star at each of these field
positions. The radial profile can then be compared with an optical model.
Large deviations may signify a problem with the instrument’s optics.
Check with the JWST SOCCER Database at: http://soccer.stsci.edu/DmsProdAgile/PLMServlet
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JWST-STScI-002190, SM-12
2) A planetary nebula spectrum might be preferable to a stellar spectrum for
characterizing the position-dependent shape of the Airy function. If the
planetary nebula is observed at several radii with a single tuning of the
etalon that puts the narrow emission line in a wing of the bandpass, then
extreme variations of up to ~10 – 25% will be measured in the integrated
counts. However, measuring the flux of the extended, clumpy, or
filamentary ionized regions of a nebula may prove more challenging and
uncertain than the comparatively simple aperture photometry of a single Be
star, for example.
5. Conclusions
We have simulated the variations in wavelength and integrated counts over the FOV of
the TFI for a variety of SEDs. We find that the effects of the etalon’s PDB can be
significant and sometimes counter-intuitive. The largest effects occur when the incident
SED has steep gradients, due to the presence of strong absorption features in a continuum
(e.g., the Lyα forest in high-redshift Lyα emitters) or strong emission-lines (e.g., as seen
against weak continua in planetary nebulae). Since the Airy function of the TFI’s etalon
is also quite sharply peaked, the exact position of the spectral features associated with
these gradients within the bandpass has a strong effect on the detected counts. The
interplay between fine structure of the blocking filters and the shape of the Airy function
can also be important. These results suggest that a judicious observing strategy and
intensive modeling may be required to perform accurate photometry of sources in the
FOV of the TFI.
References
Bernard-Salas, J., Pottasch, S.R., Wesselius, P.R., & Feibelman, W.A. 2003, A&A, 406,
165
Hora, J.L., Latter, W.B., & Deutsch, L.K. 1999, ApJS, 124, 195
Madau, P. 1995, ApJ, 441, 18
Martel, A.R. & Fullerton, A. 2011, An Introduction to the TFI Etalon, JWST-STScI002059, SM-12
Telfer, R.C., Zheng, W., Kriss, G.A., & Davidsen, A.F. 2002, ApJ, 565, 773
Check with the JWST SOCCER Database at: http://soccer.stsci.edu/DmsProdAgile/PLMServlet
To verify that this is the current version.
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