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Optics

The optical function is the core of the system and the reason for its existence. All supporting systems are derived from the optical requirements. The optical system may be summarised as follows:

One of the Nasmyth foci of the alt-azimuth WHT is dedicated to the cross-dispersed echelle spectrograph UES and its supporting facilities.

The light path comprises:

1. WHT primary, secondary, Nasmyth flat. These are always in the beam.

2. Image (de)rotator, to control by computer the orientation of the slit projected on to the sky. This unit may be mounted or removed during the day. It must be present for extended sources; it must be absent for polarimetry (planning of this is still in a very early stage); for other applications, the trade-off is between ease of operation and throughput. Acquisition and guiding is not yet implemented without the derotator.
   As of Jan 1994 UES is always used WITH the derotator

3. Nasmyth Acquisition and Guiding unit (A&G), always present, comprising the following computer-controlled functions:
   • Acquisition and guiding facilities (manual and/or automatic, object itself or offset)

   • Calibration light injection

   • Filtering (wavelength band, neutral attenuation, polarisation)

4. UES itself, comprising the following computer-controlled functions:

   • Slit unit:

     • adjustable slit width

     • adjustable slit length (Dekker masks on a slide)

     • adjustable slit orientation (limited range; for aligning a selected spectral order with the CCD columns)

   • Shutter

   • Collimator (choice of UV or standard; focus)

   • Hartman shutter for focusing the collimator

   • Cross-dispersion prisms (to separate echelle spectral orders horizontally)

   • Echelle grating for high spectral dispersion vertically (choice of 2 echelles for optimal trade-off between spectral coverage and sky subtraction; 2 angle adjustments for optimisation of the portion of the spectrum recorded by the detector)

5. Camera (folded Schmidt type)

6. Detector (choice of CCD or IPCS-II, but changes must take place during daytime)

In routine use from the observer's terminal, UES is configured for the desired observation, the telescope is pointed to the desired object, the object is centred and (auto)guiding is started. After calibration exposure(s), science exposure(s) and more calibration exposure(s), the observational sequence is complete and the next object is tackled. The digital images will have been saved for later reduction, but can also be inspected by quick-look facilities.

The format of a UES spectrum (and the portion of it recorded by the detector) are shown in fig. 6.

Polarisation facilities do not exist as yet but are planned, for point sources only. For such observations the derotator will have to be removed from the beam and a polarisation module will have to take its place. A polarisation model of the WHT Nasmyth optical train will have to be developed as part of polarisation commissioning. All of this is technically feasible, but will take time and effort to implement.