Instrument Description

As is apparent from the optical diagram in Fig. 1 , IMAPS is a very simple objective-grating spectrograph with only two reflecting surfaces and no transmission elements ahead of the detector. The size of the IMAPS entrance aperture is small (250 cm²), but if it were fed by a moderately large telescope its effective area of about 4 cm² would not be much better since the reflectivities of normal incidence optics are so low in the far uv. Both gratings are coated with aluminum with a thin overcoat of lithium fluoride to prevent the formation of a uv-absorbing oxide layer. This combination gives a good reflectivity longward of 1000Å, but has an inferior performance below 1000Å.

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Figure 2: The electron-bombarded, intensified CCD image sensor in the IMAPS instrument, as described in §3.4. Light entering from the left forms an image on the opaque KBr photocathode. Photoelectrons are accelerated by the $\vec{E}$ field, and they do a single gyration about the axis of the $\vec{B}$ field so that they come to a focus on the CCD. Each electron impact creates a cascade of several thousand secondary electrons within the thinned CCD's active silicon layer (on the back side, away from the gates). [From ( Jenkins et al. 1988) ©1988 SPIE]

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3. Instrument Description