GRISM SPECTROSCOPY

(from "The Scientist,s Guide to NICMOS" by R. I. Thompson")

NICMOS utilizes grisms to achieve multiple object spectroscopy in the spectral range between 0.8 and 2.5 microns. The grisms reside in the filter wheel for camera 3, therefore the spatial resolution of the spectroscopy is similar to the spatial resolution of camera 3. The filter wheel contains three grisms to cover the wavelength range with a spectral resolution of 200. The two shorter wavelength grisms exploit the low natural background of HST while the longer wavelength grism is subject to the thermal emission from the HST and NICMOS optics.

General Grism Description

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. Grisms are normally used to create spectra in a camera by rotating the grism into the normal camera beam. The grism then creates a dispersed spectrum centered on the location of the object in the camera field of view. NICMOS utilizes this mode of operation without any slit or aperture at the input focus so that all objects in the field of view display their spectra for true multi-object spectroscopy. 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. The NICMOS grisms have an interference filter coated on their entrance faces to limit the bandpass of the spectrum. This is necessary to prevent overlap of orders and reduce thermal emission from the telescope. Since the NICMOS grisms do not have an input slit or aperture, there is not a reduction of the background flux found in slit dispersing systems. This is not a significant problem in the shorter wavelengths, but the long wavelength grism has a high background flux.

The NICMOS grisms have a resolution of 200 per pixel. The resolution is a compromise between wedge angles, spectral and spatial coverage, sensitivity to alignment, and limiting flux. The thickness and clearances in the filter wheel along with the requirement of confocality with the images through the filters limits the thickness of the grisms and therefore their wedge angles.

NICMOS Grism Parameters

For a constant resolution the central wavelengths are space in equal logarithmic intervals which sets the central wavelengths. The bandpasses are set by the filters to reduce background and to limit the spatial extent of the spectra. All of the grisms have CaF as their optical material and a 2 mm thickness at the center. The tangent of 5.875 degrees is approximately 0.1, therefore there is about 1.6 mm of wedge in the grism going from 1.2 to 2.4 mm at the extreme edges. The clearances on the filter wheel are tight therefore higher spectral resolutions which require larger wedge angle would be difficult to accommodate.

Expected Sensitivity

Grisms A and B are in spectral ranges with very little background, however, grism C is in the range of strong thermal emission from the HST and NICMOS optics. Since the grisms are slitless each pixel receives the background radiation from the full bandwidth of the grism and filter combination. The increase in the background flux for grism C is dramatic. Use grisms A and B when possible. Grism C is for the longer wavelengths only.

GRISM Background Fluxes

Multi-Object Spectroscopy

Multi-object spectroscopy observations are identical to imaging observations and can be carried out in the same manner as any of the imaging operations discussed above. In multi-object spectroscopy one of the grisms in the filter wheel for camera 3 will be selected in the same manner as selecting any filter. The observations then proceed via one of the readout and operation modes discussed above. Multi-object spectroscopy observations may appear as part of a sequenced observation.

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 OTA axis. Some observations may profit from a particular orientation of the dispersion direction on the sky. It will be the observers responsibility to state a preferred direction of dispersion in the proposal submission. This will generally require a particular roll direction of the space craft to achieve this. A change of spacecraft attitude in either pointing or roll will require either a new single image or a new sequence of observations at the new attitude.

Although multi-object spectroscopy observations can stand alone with no supporting observations, it is anticipated that most observations will be paired with an image in camera 3 at the same pointing with an appropriate filter. This provides the location of each object in the field and aids in the identification of each spectrum. Because of this natural pairing it is anticipated that most spectroscopy observations will be in at least a two image sequenced observation.

Observing time for a spectroscopic observation is identical to that of an imaging observation in the same mode.

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