NICMOS Grisms

NICMOS provides slit-less grism imaging spectroscopy in the spectral range between 0.8 and 2.5 microns with Camera 3 (see NICMOS ISR-97-027). All objects in the Camera 3 field-of-view are dispersed for true multi-object spectroscopy. There are three grisms (G096, G141, G206), each of infrared grade fused silica, which cover the entire NICMOS wavelength range with a spectral resolving power of ~200 per pixel. The grisms reside in the Camera 3 filter wheel, and the spatial resolution of the spectroscopy is the same as for Camera 3 imaging (~0.203"/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. For more information on the NICMOS grisms, visit the Space Telescope European Coordinating Facility's NICMOS web page.