NICMOS Instrument Handbook for Cycle 11
 |
 |
 |
 |
|
| NICMOS Instrument Handbook for Cycle 11 | |||
|
Grism Spectroscopy
NICMOS provides grism imaging spectroscopy in the spectral range between 0.8 and 2.5 µm with Camera 3.1 NICMOS is used in this mode of operation without any slit or aperture at the input focus, so all objects in the field of view are dispersed for true multi-object spectroscopy. The grisms reside in the NIC3 filter wheel, therefore the spatial resolution of the spectroscopy is that of this Camera. The filter wheel contains three grisms (G096, G141, G206), of infrared grade fused silica, which cover the entire NICMOS wavelength range with a spectral resolving power of ~200 per pixel.
A grism is a combination of a prism and grating arranged to keep light at a chosen central wavelength undeviated as it passes through the grism. The resolution of a grism is proportional to the tangent of the wedge angle of the prism in much the same way as the resolution of gratings are proportional to the angle between the input and the normal to the grating.
Grisms are normally inserted into a collimated camera beam. The grism then creates a dispersed spectrum centered on the location of the object in the camera field of view. Figure 5.8 shows an example of grism spectra of a point source using G096, G141, and G206. The target is the brightest source in the FOV, although many other sources yield useful spectra as well. The band along the bottom of the images, about ~15-20 rows wide, is due to vignetting by the FDA mask, while the faint dispersed light on the right edge of the G206 grating image is due to the warm edge of the aperture mask.
The two shorter wavelength grisms exploit the low natural background of HST while the longest wavelength grism is subject to the thermal background emission from HST.
Figure 5.8: Grism slitless spectroscopy of a point source, using G096, G141, and G206.
The basic parameters of the NICMOS grisms are given in Table 5.3.
Table 5.3: Grism Characteristics G096 200 0.964 5.219 0.8 - 1.2 45.0 G141 200 1.401 5.5889 1.1 - 1.9 30.769 G206 200 2.058 5.6944 1.4 - 2.5 21.05
Multi-Object Spectroscopy
Grism observations are carried out in a similar manner as other NICMOS imaging. A target is selected and imaged with one of the grisms. We encourage all grism observers to dither their observations in the direction perpendicular to the dispersion. Dithering parallel to the dispersion may result in loss of data off the edge of the detector. However, for the case of emission line point sources, one should dither in both directions. This will improve both the line flux and wavelength measurement of the line. Dither spacing should be a non-integer number of pixels, e.g 2.1 arcsec (10 and a half pixels) and more than four positions should be observed. Dithering the target on the detector will minimize image anomalies such as grot affected pixels, cosmic ray hits, pixel sensitivities, and residual persistence images.
Although multi-object spectroscopic observations can stand alone with no supporting observations, we recommend pairing them with a direct image in NIC3, through an appropriate filter, at the same pointing. This provides the location of each object in the field and aids in the identification of their individual spectra. Because of this natural pairing it is anticipated that most spectroscopy observations will be at least a two image pattern, that is, direct and grism images at the first dither point, move to next dither position, direct and grism images at the second point, etc. The new pattern syntax (see Chapter 11) makes this possible.
The direction of dispersion is perpendicular to the radial direction in Camera 3 where the radial direction is defined by a vector originating at the center of the field of view for Camera 3 and pointing toward the center of the HST OTA axis. In complex fields, such as extended objects and crowded fields, individual spectra of targets may overlap and cause confused images. In such cases, it may be possible to alleviate the superposition of spectra by requesting a specific orientation of the telescope during the Phase II Proposal submission. For complex fields or extended targets, observations of the same field at 3 or more different spacecraft orientations (roll-angles) are advisable, to deconvolve overlapping spectra. It is essential that matching direct images be obtained in this case. It should be recognized that specifying an orientation for a grism observation creates constraints on the number of visibility windows available for scheduling. If different orientations are needed over a short period of time to unscramble the source spectra, telescope scheduling will be difficult.
Grism Calibration
The NICMOS spectroscopic grism mode calibrations were determined from on-orbit observations (Cycle 7 and 7N calibration program IDs: 7695, 7696, 7806, 7903, 7959). Observations were obtained with each grism in and out of Camera 3 campaign mode. Wavelength calibration was carried out by observing planetary nebulae, Vy 2-2 (before January 1998) and HB12 (after this date). The inverse sensitivity curves were derived from observations of the white dwarf G191-B2B and G-dwarf P330E. Grism calibration data reductions were performed at the Space Telescope European Coordinating Facility (ST-ECF). The ST-ECF NICMOS web page is
http://ecf.hq.eso.org/nicmos/nicmos.html.Relationship Between Wavelength and Pixel
Table 5.4 gives the dispersion relationship in the form:
where wavelength is in microns and the 0 pixel is at the central wavelength defined by the position of the object in the direct image. The relationship is plotted in Figure 5.9. The actual location of the positive and negative pixels will be dependent on the grism orientation and the location of the source in the image. The grisms were aligned as accurately as possible along a row or column of the array. We do not expect any distortion or curvature in the spectrum.
The orientation and position of the spectra relative to the direct object has been measured in-orbit and was found to be similar to the Thermal Vacuum measurements except for a small 0.5o rotation. The dispersion parameters have remained fairly constant during the in-orbit observations. They are significantly different from the pre-flight measurements, and the current best estimates of the dispersion relations are those determined from on-orbit observations.
Figure 5.9: Wavelength Versus Pixel Number for each Grism. Note that the actual location of the central wavelength on the detector depends on the position of the source.
Table 5.4: Grism wavelength to pixels relationship G096 -0.00536 0.9487 G141 -0.007992 1.401 G206 -0.01152 2.045
Sensitivity
Background radiation will be a greater concern for grisms than for imaging observations. Every pixel on the array will receive background radiation over the spectral bandpass of the particular grism, while the source spectrum will be dispersed over many pixels. Therefore, the ratio of the source to background flux will be much lower for the grisms than for the regular imaging mode filters. The background rate per pixel (sky + telescope) expected with NCS operations is presented in Table 5.5 below for the three grisms. Observing a source with flux at all wavelengths equal to the peak response for each grism will result in a peak count rate equal to the background. The increase in the background flux for the G206 grism is dramatic. Grisms G096 and G141 should therefore be used whenever possible. Despite its broad wavelength coverage, the G206 grism should be used for the longest wavelengths only. Dithered observations, especially when the field is uncrowded, can often be used to remove the background quite well. Thus breaking observations into several spectra, taken on different parts of the detector, is strongly recommended.
Table 5.5: Grism Background Radiation (sky + telescope)
Figure 5.10 gives the sensitivity of each grism as a function of wavelength, as measured for the standard star P330E in June 1998 (left panels) and renormalized to the DQE expected in Cycle 11 onward, after installation of the NCS (right panels). The signal was measured in an aperture of 10 pixels (2 arcsec) in the spatial direction. Table 5.6, 5.7, and 5.8 present the basic information for the three NICMOS grisms. In the same tables, the best direct imaging filter to associate with each grism is given with the list of its characteristics.
Figure 5.10: Grism Inverse Sensitivity Curves, G096 (top), G141 (middle), and G206 (bottom), both measured (June 1998 observations, left) and predicted with the expected Cycle 11 DQEs (right)
Note that for the G206 grism, the large thermal background means that exposures can never be longer than about five minutes, even for faint sources, because the detector will be saturated by the background. See Chapter 4 for more details on the thermal background seen by NICMOS. The dithering/chopping strategies described in Chapter 11 for background removal should be used with this grism.
.
Table 5.6: Grism A: G096 0.9673 0.9911 1.0010 0.4029 0.8-1.2 69.8 Direct Imaging Filter F110W 1.0998 1.1035 1.2035 0.5915 0.8-1.4 94.9
Table 5.7: Grism B: G141 Thermal background is important. 1.414 1.5100 1.4020 0.7914 1.1-1.9 74.7 Direct Imaging Filter F150W 1.5035 1.5069 1.6355 0.8020 1.1-1.9 97.7
Intrapixel Sensitivity
The same intrapixel sensitivity problem which affects NIC3 images (see Chapter 4) will affect the grism spectra since the dispersion direction is not exactly aligned with the detector rows: as the heart of the spectrum crosses from one row to the next, the flux will dip by 20 percent or so. This effect is not obvious in emission line spectra but can be very clear in continuous spectra. The frequency of the dip and the placement of the sensitivity minima within the spectrum will depend on exactly where the spectrum falls on the detector, and the angle between the dispersion direction and the detector X axis. Note that the former changes with the dithering position, and the latter is temporally variable. As noted earlier, the grisms and the detector appear to have rotated with respect to each other by a half a degree between the two NIC3 observing campaigns. A correction procedure has been added to NICMOSlook.
Grism Decision Chart
The decision chart given in Figure 5.11 helps guide the proposer through the construction of a grism observation.
Figure 5.11: Grism Decision Chart
1 NICMOS Instrument Science Report, NICMOS ISR-97-027
|
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |
| |
|
|
|
,
