Instrument Science Report STIS 98-04
Spectroscopic Mode Image Quality of
STIS I. First Order Modes
Kailash Sahu, Steve Hulbert, Howard Lanning, Jennifer Christensen
January 30, 1998
ABSTRACT
This ISR reports the spectroscopic mode image quality of STIS first-order modes as determined from the on-orbit observations as part of the cycle-7 calibration program. Calibration spectra with an internal lamp were obtained for 7 first-order gratings using a narrow
(2-pixel wide) slit, and were used to extract instrumental line profiles at different positions
of the CCD and the MAMA detectors. The line-widths, measured at different positions of
the detectors for different gratings, are presented graphically, and are found to be close to
2 pixels for all modes. The line profiles (shown in normal and log scales) are fairly well
represented by a single Gaussian. The encircled energies are plotted as a function of the
number of pixels, which show that most of the power is contained within +/- 2 pixels of the
line center.
1. Introduction
The spectroscopic mode image quality for different STIS modes was determined on
ground and found to be well within the specification that the FWHM should be 2.5 pixels
convolved with the diffraction width for each mode. It was important to determine
whether there was any change during the launch and to determine the image quality onorbit. This was done through the cycle-7 calibration phase using programs 7077 and 7078
designed for the CCD and the MAMA detectors, respectively. This ISR gives the details of
the results obtained from this program for the first-order modes.
2. Observation
CCD Modes:
Observations were taken using the internal line lamp with the 52”x0.1” slit, which
projects to 2 pixels in the dispersion direction at the detector. Observations with the CCD
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detector were made on 17 March 1997 for the first-order gratings G230LB, G230MB,
G430L, G430M, G750L and G750M. Observations were taken at one central wavelength
setting for each grating. Deep exposures were taken to check for extended wings of the
lines and to ensure a complete field coverage for the lines.
MAMA modes
Observations for the MAMA modes were obtained between July 4 and July 6, 1997.
Calibration lamp spectra were taken at a single central wavelength setting with the
G140M, G140L, G230L and G230M gratings using with the 52”x0.05” slit, which
projects to 2 pixels in the dispersion direction at the detector. Details of the observations
are presented in Table 1.
Table 1: Details of the observations
Grating
Slit width
(arcsec)
Central
wavelength
(A)
Date of
observation
Exp. time
(s)
Setting
(Amp)
Lamp
G230LB
0.1
2375
March 17
10
Line
10.0
G230MB
0.1
2697
March 17
30
Line
10.0
G430L
0.1
4300
March 17
5
Line
10.0
G430M
0.1
4451
March 17
50
Line
10.0
G750L
0.1
7751
March 17
5
Line
10.0
G750M
0.1
7795
March 17
30
Line
10.0
G140L
0.05
1425
July 4
600
Line
10.0
G140M
0.05
1371
July 4
1000
Line
10.0
G230L
0.05
2376
July 6
900
Line
3.8
G230M
0.05
2338
July 5
900
Line
10.0
3. The data reduction and results
Dispersion solutions:
Full details of the dispersion solutions and their accuracies for different modes are
described in a separate ISR by Lanning et al. (1998).
Line profile analysis:
For the analysis of the line profiles, well exposed lines were chosen spanning the
whole region of the detector. Typically 5 lines were selected, which cover the region close
2
to both edges as well as the central part of the detector. For each line, line profiles were
investigated at 3 positions, the top, middle and the bottom part of the detector.
4. RESULTS
Line-widths:
Line profiles were extracted from the wavelength calibrated files, but the results are
presented in terms of pixels which makes it easier to compare the results for all gratings in
a consistent manner. Gaussians were fitted to the extracted line profiles, and the resulting
FWHM values (in pixels) are presented in Figs. 1 to 3. Fig. 1 shows the results for the
CCD L-modes, Fig. 2 shows the results for the CCD M-modes, and Fig. 3 shows the
results for the MAMA L and M-modes.
Line widths were measured across the entire field of view of the detectors and were
found to be 2.0 +/- 0.2 pixels. This is consistent with that expected from the use of a 0.1
arcsec slit and is well within the specifications. There is no correlation between the line
width and the position on the detector which implies that there is no significant tilt at the
detector plane.
Line profiles
Figs. 4 to 15 show typical shapes of the observed line profiles. Note that line profiles
were examined at various positions of the detector, and for many lines. No significant differences were found dependent on the position on the detector. A typical line profile for
the G750L mode is shown in linear scale in Fig. 4, where the solid line is the observed
profile, the dashed line is the Gaussian fit and the dotted line is the residual. It is clear from
the figure that the line profiles do not show any sign of extended wings. Figs. 5 to 9 show
the same for the other first order CCD gratings, and Figs. 10 and 11 show equivalent plots
for the first-order MAMA gratings. Figs. 12 to 15 show the line profiles for the different
CCD and MAMA modes in the logarithmic scale which would emphasize the presence of
any wings at lower intensities. As seen in the figures, the line profile is well represented by
a single Gaussian, without any significant contribution for a second component in the first
order modes. Note that a scattering component was seen for the GHRS Echelle modes
which could be represented by an extra Gaussian component (Cardelli et al. 1990, 1993).
The STIS first-order modes do not show such a scattering component.
The figures clearly show that the scattering component for the first-order modes is very
small, and the LSF is close to what is expected when a 2-pixel wide slit is used. It should
be emphasized that all the analyses presented here are valid only for the line-profiles taken
with a two-pixel wide slit. With the use of a wider slit, the line profile is expected to be
broader. It should also be noted that these results are for observations taken with internal
lamp calibrations. The analysis of the line profiles obtained with external target is the sub-
3
ject of a separate investigation. However preliminary analysis of observations of an
external target shows results similar to what is found for the internal lamp and the expectation is that the line profiles for external targets would be close to the line profiles observed
with the internal lamps.
Based on these results, the recommendation for the GOs would be to use a 2-pixel
wide slit to obtain optimal instrumental line profiles. For point sources observed with a
wider slit, the line profile will be affected by the PSF, and will be a convolution of the PSF
with the inherent LSF of the instrumental setup. The principal effect of using a wider slit,
however, is not to broaden the FWHM of the line profile (since the PSF is peaked), but to
add low-intensity wings to the LSF.
Encircled Energies
Fig. 16 to 19, show the encircled energies as a function of the number of pixels from
the line center. Figs. 16 and 17 show the plots for the CCD L and M modes, and Fig 18
and 19 show the same for the MAMA L and M modes respectively. In all cases, most of
the power is contained within +/-2 pixels from the line center. The central 2 pixels typically have 60% of the flux and the central 4 pixel enclose >90% of the total flux in the line
profile. This emphasizes the earlier conclusion that any scattered component in the line
profiles contributes very little to the total flux.
4
Figure 1: FWHM of the line profiles for the CCD L-modes, as measured by fitting Gaussians to the observed line profiles.
5
Figure 2: The fwhm of the line profiles for the CCD M-modes.
6
Figure 3: The fwhm of the line profiles for the MAMA L and M-modes.
7
Figure 4: Representative line-profile for the G750L mode in linear scale. The solid curve
is the observed profile, the dashed line is the Gaussian fit and the dotted curve is the residual.
Line Profile for G750L (slit width = 2 pixels)
Counts
30000
20000
10000
320
325
330
335
340
345
350
355
60
65
Pixel
Figure 5: Representative line-profile for the G750M mode
Line Profile for G750M (slit width = 2 pixels)
35000
30000
Counts
25000
20000
15000
10000
5000
30
35
40
45
50
Pixel
8
55
Figure 6: Representative line-profile for the G430L mode
Line Profile for G430L (slit width = 2 pixels)
5000
Counts
4000
3000
2000
1000
800
820
840
860
Pixel
Figure 7: Representative line-profile for the G430M mode
Line Profile for G430M (slit width = 2 pixels)
15000
12500
Counts
10000
7500
5000
2500
120
125
130
135
Pixel
9
140
145
150
Figure 8: Representative line-profile for the G230LB mode
Line Profile for G230LB (slit width = 2 pixels)
200
175
150
Counts
125
100
75
50
25
770
775
780
785
790
Pixel
Figure 9: Representative line-profile for the G230MB mode
Line Profile for G230MB (slit width = 2 pixels)
700
600
Counts
500
400
300
200
100
0
250
260
270
Pixel
10
280
290
Figure 10: Representative line-profile for the G230L mode
Line Profile for G230L (slit width = 2 pixels)
200
175
150
Counts
125
100
75
50
25
330
332
334
336
338
340
Pixel
Figure 11: Representative line-profile for the G230M mode
Line Profile for G230M (slit width = 2 pixels)
500
Counts
400
300
200
100
0
200
210
220
230
240
Pixel
11
250
260
270
Figure 12: Representative line profiles of the CCD L-modes shown in log scale.
Line Profile for G750L (Log10 scale. slit width = 2pix)
4
Log10 Counts
3.5
3
2.5
2
430
440
450
460
470
480
Pixel
Figure 13: Representative line profiles of the CCD M-modes shown in log scale.
Line Profile for G750M (Log10 Scale. Slit width = 2pix)
8
Log10 Counts
7
6
5
4
780
785
790
Pixel
12
795
Figure 14: Representative line profiles of the MAMA L-modes shown in log scale.
Line Profile for MAMA G230L (Log10 scale. slit width=2pix)
2.25
Log10 Counts
2
1.75
1.5
1.25
1
.75
333
334
335
336
337
338
339
Pixel
Figure 15: Representative line profiles of the MAMA M-modes shown in log scale.
Line Profile for MAMA G140M (Log10 scale. slit width= 2pix)
1.75
1.5
Log10 Counts
1.25
1
.75
.5
.25
0
862
864
866
868
Pixel
13
870
Figure 16: The encircled energy as a function of no. of pixels for the CCD L-mode. Each
bin in the x-axis corresponds to 2 pixels.
Encircled Energy for G750L(slit width = 2pix)
100
Percent Counts
80
60
40
20
0
0
.5
1
1.5
2
2.5
3
BIN ( 2 pix/bin)
Figure 17: The encircled energy as a function of no. of pixels for the CCD M-mode. Each
bin in the x-axis corresponds to 2 pixels.
Encircled Energy for G750M (slit width=2pix)
100
Percent Counts
75
50
25
0
0
1
2
BIN (2pix/bin)
14
3
Figure 18: The encircled energy as a function of no. of pixels for the MAMA L-mode.
Each bin in the x-axis corresponds to 2 pixels.
Encircled Energy for MAMA G230L (slit width = 2pix)
100
90
Percent Counts
80
70
60
50
1
2
3
4
PIXEL
Figure 19: The encircled energy as a function of no. of pixels for the MAMA M-mode.
Each bin in the x-axis corresponds to 2 pixels.
Encircled Energy for MAMA G140M (slit width = 2pix)
100
Percent Counts
80
60
40
1
2
3
4
PIXEL
15
5
References:
1. Cardelli, J.A., Ebbets, D.C., Savage, B.D., 1990, ApJ, 365, 789
2. Cardelli, J.A., Ebbets, D.C., Savage, B.D., 1993, ApJ, 413, 401
3. Sahu Kailash C., Hulbert, S., Lanning, H., Christensen, J, 1998, Bull. Am. Astron.
Soc. “Spectroscopic Mode Image Quality of STIS”, p1240
4. Lanning, H.H., Christensen, J., Beck, T.L., Lindler, D.J., Plait, P., Gull, T.R., Bradley, L., 1998, Bull. Am. Astron. Soc. “STIS Dispersion Solution Calibration and
Accuracy”, p1240
5. Lanning, H.H., Hulbert, S.J., Bradley, L., Gonnella, A., 1998, “STIS Dispersion
Solution Calibration and Accuracy”, STIS ISR (in prep).
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