1997 HST Calibration Workshop
Space Telescope Science Institute, 1997
S. Casertano, et al., eds.
Scattered Light in the STIS Echelle Modes
W. Landsman
Hughes STX, NASA/GSFC, Greenbelt, MD 20771
C. Bowers
NASA/GSFC, Greenbelt, Maryland 20771
1. Introduction
The STIS echelle spectra obtained during the Early Release Observations (Heap et al. 1997,
Walborn et al. 1997, Jenkins et al. 1997) have non-zero residuals in the cores of saturated
interstellar lines, indicating the need for a scattered light correction. A rough measure
of the magnitude of the needed correction is given in Figures 1 and 2, which show the
ratio of the interorder to the in-order ux in dierent echelle modes in both pre-launch
calibration images of a continuum lamp source (Figure 1), and in post-launch images of
stellar continuum sources (Figure 2). The interorder and in-order uxes are computed by
averaging the central 200 pixels in the dispersion direction. The amount of scattered light
in the interorder region rises toward shorter wavelengths for two reasons: (1) the order
separation decreases toward shorter wavelengths, and (2) the amount of echelle scattering
is expected to have a inverse dependence on wavelength (e.g. Cardelli, Ebbets & Savage
1990). At the shortest wavelengths the fraction of light scattered into the interorder region
can be 10% for the NUV-MAMA and 15% for the FUV-MAMA.
2. Sources of Scattered Light
The strong narrow emission lines of deep platinum line lamp WAVECAL images can be used
to isolate the scattering sources in the echelle modes. Figure 3 shows a very deep NUVMAMA line lamp image taken during the pre-launch calibration, while Figure 4 shows
the deepest line lamp image thus far obtained during ight. Both images were obtained
with the 0:100 by 0:0900 slit. These images do not show extended wings perpendicular to
the orders, indicating that the contribution of cross-disperser grating scattered light is
negligible. Instead, the two main sources of scattered light are the following:
1. A detector halo surrounds the strong emission lines. This halo is never larger than
about 16 pixels in radius. This value should be compared with the interorder spacing,
which is about 15 pixels at the shortest wavelengths of both MAMA detectors, and
about 40-50 pixels at the longest wavelengths. The size of the detector halo is larger
in the NUV-MAMA and has a radius increasing with increasing wavelength.
2. Scattering from the echelle gratings causes a continuum which connects the appearance
of an emission line in dierent orders. Note that this source of scattered light is very
\non-local".
Two minor sources of scattered light can also be seen in Figures 3 and 4. First,
reections in the optical path cause a secondary image of the emission lines, as seen at
about \ve o'clock" o the edge of the haloes of the strong lines. Second, the emission lines
near the bottom edge of the NUV-MAMA detector show vertical spikes, whose origin is
presently uncertain.
1
2
Figure 1. Ratio of interorder to in-order ux for ground calibration data. Each
plot includes data from at least two images (marked with dierent symbols).
Ratio: Interorder / In−order
E140M
E140H
0.20
0.08
0.15
0.06
0.10
0.04
0.05
0.02
0.00
0.00
1300 1400 1500
Wavelength (Å)
1350 1400 1450 1500
Wavelength (Å)
E230M
0.08
E230H
0.10
0.08
0.06
0.06
0.04
0.04
0.02
0.02
0.00
0.00
1800 2200 2600 3000
Wavelength (Å)
Figure 2.
1800 2000 2200 2400 2600
Wavelength (Å)
Ratio of interorder to in-order ux for ight data.
BD +28 4211: E140M
0.15
0.10
Ratio: Interorder /In−order
BD+332642:E140H
0.10
0.08
0.06
0.04
0.05
0.00
1300 1400 1500 1600 1700
Wavelength (Å)
0.02
0.00
1250
HD 107213: E230M
0.020
0.06
0.015
0.04
0.010
0.02
0.005
2400 2600 2800 3000
Wavelength (Å)
1400
BD +75 325: E230H
0.08
0.00
1300
1350
Wavelength (Å)
0.000
2400 2450 2500 2550 2600 2650
Wavelength (Å)
3
Figure 3. A deep pre-launch calibration E230H line lamp spectrum with a central wavelength of 2762 A.
4
Figure 4. A post-launch E230H line lamp spectrum (O42704KGM) with a central wavelength of 2513 A.
5
Figure 5. Laboratory measurement of the scattering prole of the E140M grating
at three dierent wavelengths. The dotted lines show the predicted scattering
proles at 1230 A and 1780 A computed by scaling the 1509 A scattering prole
by ,3 2.
:
−2
Wavelength 1: 1236 Å
Log Normalized Counts
Wavelength 2: 1509 Å
Wavelength 3: 1782 Å
−3
−4
−5
−0.6
−0.4
−0.2
0.0
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Offset from Order Center [order]
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3. Laboratory Measurements of Echelle Scatter
The scattering properties of the STIS echelle gratings were studied prior to their integration
into the instrument. Figure 5 shows the scattering function of the E140M grating at 1230 A,
1509 A, and 1780 A. The amount of scattered light is found to be a strong function of
wavelength, with a ,3 2 dependence. On the other hand, the amount of scattering in
the E140H grating shows little dependence on wavelength (Figure 6), at least for the two
wavelengths measured. The scattering prole of the E140H grating is also atter than that
of the E140M grating, and shows more pronounced order ghosts.
:
4. Status
Unlike IUE or GHRS, the scattering in the STIS high-resolution modes appears to be
dominated by scattering from the echelle grating, rather than from the cross-disperser. The
non-local nature of echelle grating scatter makes a correction algorithm more dicult to
implement. The most complete correction algorithm will likely be an iterative scheme, in
which an initial spectrum is rst extracted without any correction for scattered light, and
then convolved with the echelle scattering function and detector response function. The
amount of scattered light is then computed and used to correct the initial spectrum. A
simpler local correction algorithm (e.g. Bianchi & Bohlin 1984) should still be useful where
there are no strong variations in the continuum, or where the order overlap due to the
detector halo is signicant.
6
Figure 6. Laboratory measurement of the scattering prole of the E140H grating
at 1509 A (solid line) and 1782 A (dotted line).
Log Normalized Counts
−2
−3
−4
−5
−0.6
−0.4
−0.2
0.0
0.2
Offset from Order Center [order]
References
Bianchi, L. & Bohlin, R.C. 1984, A&A, 134, 31
Cardelli, J.A., Ebbetts, D.C., & Savage, B.D. 1990, ApJ, 365, 789
Heap, S. et al. 1997, submitted to ApJL
Jenkins, E. et al. 1997, submitted to ApJL
Walborn, N. et al. 1997, submitted to ApJL
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