The Spectrograph Optics

Since a monolithic spectrograph poses several technological challenges, our baseline concept subdivides the focal surface into 4 equal quadrants, each having a diameter of 0.7 degrees on the sky, and each feeding a separate spectrograph. This modular spectrograph approach offers a lower cost and lower risk to building large spectrographs, thus providing a possible design pathway for even larger instruments on future telescopes such as MaxAT. Even with this modular approach, the optical components of the design are large--the collimator optics and grating are roughly 0.4-m in diameter. Figure 17 shows the collimator design for two of the four spectrograph channels. The design of one of the f/2.4 cameras is shown in Figure 18.

The anti-reflection coatings on the lenses will probably be SolGel/MgF₂. With these coatings it is possible to keep reflection losses below 1.5% per surface over the entire spectral region (0.36-1.7microns).

  

Figure 17: The ray trace diagram for the SWIFT spectrograph, showing the collimator design for two arms of the spectrograph. In this preliminary design, where the focal surface is subdivided into four 0.7^o circular adjacent fields, the apex of the field is unused. We are currently investigating alternate designs which maximize the field usage.

  

Figure: The ray trace diagram for the SWIFT spectrograph camera. In the present design, the camera must be angled away from the grating for optimum efficiency. Having four separate camera modules offers a way to balance the spectrograph load azimuthally.