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Throughput

At UV and visible wavelengths, losses in the derotation optics arise as a result of the four air-glass surfaces (2 per cent loss at each surface) and the reflection at the flat aluminium mirror (approx 15 per cent loss). The throughput of the system is therefore expected to be about 75 per cent, with the losses dominated by the reflection at the flat mirror.

The throughput of the system cuts off sharply in the near infrared, beyond about 2 microns, due to absorption by the fused silica material (Heraeus Homosil) used in constructing the prisms. This material also absorbs at wavelengths between 1.3 and 1.5 microns. It transmits well in the UV down to below the atmospheric cutoff at 3000Å.

It is worth noting that the losses in the WHT derotation optics are significantly greater than those in the optical derotator used with UCLES (in principle an echelle of similar design to UES) at the AAT. The smaller field and longer f/ratio at the AAT makes it possible to construct a monolithic derotator, in which the reflection at the flat mirror is replaced by a third total internal reflection. The only losses in the AAT derotator are therfore those due to the two air-glass surfaces on the two external faces. Unfortunately, it is not possible to adopt a similar approach on the WHT, due to the larger field and faster f/ratio. The advantage however with the derotator for UES, is that due to the larger field, offset autoguiding is possible.