Chapter 4

Chapter 4, The Point Design

Section 4.7.7: High Resolution Optical Spectrograph

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There will likely be a desire to have an R ~ 50,000 optical spectrograph to carry out spectroscopic observations to support studies of the following:

Although this type of instrument was not conceptually explored in the current study, it does have specific requirements that require technological development. It is therefore important to mention some of these.

This instrument could be utilized without significant image correction, but would benefit from the enhanced seeing delivered by a boundary layer adaptive optics (AO) system. However, the preference would be for this instrument to be designed to exploit the conditions not optimal for the most demanding AO applications.

If it is implemented for use with nominally good seeing of 0.5 arcseconds, a conventional R2 echelle instrument will require a beam diameter of at least 1000 mm! An R4 echelle would reduce the beam diameter by a factor of two to 500 mm! In this smaller beam case, the instrument would still require a 3 x 3 mosaic of current off-the-shelf echelles from Thermo RGL (formerly Richardson Grating Lab), which have a dimension of 200 mm along the spatial axis, or width of the grating. RGL currently knows how to make excellent one-dimensional mosaics, but it has stated through private communication that a new technique would need to be developed in order to mosaic echelles into two dimensions. Please note that the length of the echelle remains unchanged at 2 m regardless of its diffraction angle. Enhanced seeing would further reduce the required beam diameter in direct linear proportion to the seeing improvement provided by the AO system.

An alternative approach is to develop an R10 echelle similar to those previously discussed for the high resolution, mid-IR spectrographs. Such a grating would allow R = 50,000 on a 0.5 arcsecond slit with a 200 mm beam. The length of such an echelle would still be 2 m, but the mosaic would only be required along the dispersion axis, or length of the grating. Unfortunately, R10 echelles are not currently fabricated for use in the optical. Thermo RGL, through private communication, has stated that it is willing to explore the possibility of making such a grating.

Future GSMT instrumentation studies will probably explore this particular spectrograph in more detail through externally funded studies.

March 2002