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High and Low Resolutions

A low resolution mode is obtained by inserting a small right angle prism near the slit position bending the light path directly of the second mirror. Therefore, the prism works as disperser giving a resolution of R = 2500.

To switch between high and low resolution modes without rotating the two fibers, these are distant 1.1 mm and tilted of 85$^o$ with respect to the grating grooves.

Figure2.4 shows the order format on the CCD for the two modes.

Figure 2.4: The right angle prism switching between the two dispersion modes.
\includegraphics[width=8cm]{CAOS/high.eps} \includegraphics[width=8cm]{CAOS/low.eps}

Figure 2.5: Raytraced spectral format onto the CCD. In the two column, interorder separations between adjacent orders (in pixels) and the slit height (in pixels) are shown. Box represents the CCD area.
\includegraphics[width=14cm]{CAOS/orders.eps}

The insertion of the prism does not introduce big aberrations in the spectrograph. After refocusing the spectrograph through the transfer collimator (with a shift along the mirror optical axis about 6 mm), optical quality decrease a little bit. However, due to the fact that spectrograph is used in a low resolution mode, this indeed does not seem a big issue.

Figure 2.6: Spectrograph spot diagrams on the CCD focal plane. First three configurations (columns) show the LOW RES mode, while the other configurations show the HIGH RES mode. Boxes are 4x4 pixels wide.
\includegraphics[width=14cm]{CAOS/spots.eps}

A high precision (1$\mu$) motorised exchange mechanism can allow to switch among the HIGH RESOLUTION MODES and the LOW RESOLUTION MODE.


next up previous contents index
Next: Throughput and limiting magnitude Up: CAOS - The new Previous: Camera optimization   Contents   Index
Innocenza Busa' 2005-11-14