next up previous
Next: Mechanical Collimator Up: High Resolution Spectroscopy in Previous: Brief History of IMAPS

3. 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.

 
Figure: 1 Principal components of the IMAPS instrument. [From (Jenkins et al. 1988) ©1988 SPIE]  

\begin{figure}
\epsscale{.6}
\plotone{Fig.optics.eps}\end{figure}


 
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]  

\begin{figure}
\epsscale{.8}
\plotone{Fig.detector_dwg.eps}\end{figure}



 
next up previous
Next: Mechanical Collimator Up: High Resolution Spectroscopy in Previous: Brief History of IMAPS

12/15/1998