Previous: Focal modifier lenses
Up: No Title
Next: Performance of the modifier lenses
Previous Page: Focal modifier lenses
Next Page: Performance of the modifier lenses
When designing a spectrograph it is necessary to decide how to match the size of the collimated beam, which is elliptical in shape, to the size of the grating, which is rectangular in shape. Two possible solutions are:
Solutions intermediate between these two extremes are also possible. There are two options available with the UES. With the focal modifier lenses out of the beam, the input f/ratio of f/11.1 results in a collimated beam of diameter 215 mm. The effect of inserting the focal modifier lenses into the beam is to modify the f/ratio of the telescope beam to f/13.9, resulting in the diameter of the collimated beam being reduced to 172 mm. Both of these options result in some overfilling of the grating, as summarised in Table 24.
It can be seen that the light lost due to overillumnation is greater with the lens out of the beam; the geometrical efficency of the instrument without the lens is 80 per cent of that with the lens.
It should be noted that the effect of the UES focal modifier lenses is precisely opposite to the effect of the UCLES focal modifier lenses. The UES focal modifier lenses decrease the degree of overillumination, whereas the UCLES focal modifier lenses increase it.
Inserting the focal modifier lenses can either increase or decrease the overall throughput of the spectrograph, for the following reasons.
The choice of whether or not to use the lenses depends on which of these effects dominates, as discussed in the next section.