2 Spectrograph

  The principles discussed in the introduction define FOCES as a stationary échelle spectrograph mounted at some remote laboratory in the dome building. This isolation of the échelle spectrograph is of threefold nature:

• first of all the spectrograph is $\approx\! 18$ m away from the Cassegrain focus of the telescope. The interruption of the optical light path therefore requires a relay in form of an optical fibre with advantages and disadvantages discussed above;
• the spectrograph is also isolated mechanically. This has two advantages: it decouples the instrument from the movement of the telescope and thus avoids mechanical flexure. It also provides the base for an efficient reduction of mechanical vibrations;
• finally, it puts the spectrograph in a controlled thermal environment. Consequently, thermal drifts of the instrument during an observing night due to temperature changes in the dome are avoided. The spectrograph can be isolated, first by holding the temperature in the laboratory constant, and second by controlling the heat development in the spectrograph itself if necessary. In fact, the latter control proved unnecessary since encoders and motors were selected to produce minimum heat.

This concept has only two severe disadvantages. The optical fibre link will swallow photons that in a direct mounting of the instrument would reach the detector, and our fibre design does not allow a long slit that may be needed for sky background correction of very faint objects. Both disadvantages are taken into account. The light losses due to fibre transmission (see below) are more than compensated by the possibility to use a transmission camera instead of the typical Schmidt camera of a directly mounted instrument. Moreover, FOCES is intentionally designed for a S/N $\ge$ 30 which normally limits the sky background to a small fraction of the total signal ($\le$ 10%).

FOCES is not really a high-resolution spectrograph. Budgetary and technical restrictions forced us to compromise the astronomical requirements with beam size, fibre diameters and available CCD chips; therefore the échelle spectrograph will work at the 2.2 m telescope with a standard resolution of R = 40600 providing a two-pixel resolution element. This limit is set by the $24~\mu$m pixel distance and the area of the 1024² Tektronix CCD chip. The maximum spectrograph resolution product at the 2.2 m telescope is $R \phi = 60300$ with the slit width $\phi$ entered in arcsec; at the 2.2 m telescope the standard slit width corresponds to $130~\mu$m which subtends a 1.5 arcsec angle on the sky. Thus a CCD chip with 2048² pixels of $15~\mu$m distance would yield a 2 pixel resolution of R = 65000 with increased spectral coverage, however, implying significant light losses at the entrance slit due to reduction to 0.8 arcsec slit width. The spectrograph is also designed to be mounted alternatively at either the Calar Alto 2.2 m or the 3.5 m telescope, however, with a reduced throughput or resolution at the 3.5 m.

2.1 Optical design

The optical layout of the FOCES spectrograph follows a white pupil design that has been documented by Baranne (1988). The advantages are discussed there in detail so we need only emphasize that one of its important features for us is the intermediate image of the spectrum which can efficiently be cleaned from scattered light emanating from the échelle grating. Baranne et al. (1996) have built a very similar instrument (ELODIE) for the Observatoire de Haute-Provence. It uses a $\tan \theta = 4$ échelle grating in combination with a 10 cm spectrograph beam and a prism-grism cross-disperser. Due to the strong support obtained from the European Southern Observatory the optical layout of FOCES emerged to become very similar to the one proposed for the UV-Visual Echelle Spectrograph built for the ESO Very Large Telescope (Dekker et al. 1992). The optical characteristics of the FOCES design are given in Table 1, and the layout is shown in Fig. 1. A few principles differ from those specified for the UVES.

  

Table 1: Basic FOCES data

1.

The area of the entrance slit is immediately aside the folding mirror which makes the UVES-type entrance folding mirror unnecessary. The entrance slit and its spectral image are therefore very near together.

2.

The spectrograph beam is only 15 cm, therefore the échelle grating is correspondingly smaller, with an R2 échelle fitting the goals of FOCES.

3.

Instead of a low-order grating we use a double prism for cross-dispersion. This accounts for a less strongly changing interorder distance, and it significantly reduces local straylight in the spectrum.

4.

Finally, FOCES has a transmission camera of an only moderate f/3 design.

2.1.1 Echelle grating

The échelle grating is blazed at $\phi_B = 65^{\circ}$ and has 31.6 lines/mm. Its ruled area corresponds to an overfilling by the 15 cm beam of 34 mm. This 11% linear overfilling leads to only 4.5% vignetting of the beam which optimizes the product of throughput and resolution. As is evident from Fig. 1 the spectrograph works in an extreme near-Littrow configuration of the échelle which is possible with a non-zero angle transverse to the direction of dispersion, $\gamma = 0.7^{\circ}$. The obvious advantages of the quasi-Littrow configuration (cf. Schroeder 1987) include an increased efficiency of the échelle grating.

The échelle is kept in a gimbal mount with all three axes fully adjustable as shown in Fig. 2a. It is oriented upside down to avoid dust contamination of the surface. The adjustment is accurate to within $\pm 10$ arcsec on each of the axes. Two of the axes are controlled manually with screws, while the transverse horizontal axis can be read out and moved with a DC motor using a gray code absolute encoder. This allows a careful centering of the blaze function on the detector. Intentionally, it also allows decentering the orders such as to cover near-infrared spectral regions at the order ends that are outside normal coverage (i.e. at wavelengths > 750 nm where the full spectral coverage ends).

  

Figure 1: Optical layout of the FOCES spectrograph with entrance slit, collimators, échelle grating, folding mirror, prism/grism cross-disperser, and camera

  

Figure 2: Spectrograph components: a) échelle mounting from above (top) and from aside (bottom), b) mounting of cross-disperser tandem prism used for single-fibre mode, c) mounting of blue and red grism used in combination with the prisms for higher cross-dispersion in dual-fibre mode

2.1.2 Collimators and folding mirror

The collimator consists of a pair of off-axis paraboloids cut from a single parabolic f/2 Zerodur$^{\rm\scriptscriptstyle TM}$ blank with a focal length of 1524 mm. The diameter of the collimators is 254 mm, and they are mounted in aluminium housings that allow directional fine tuning with precision screws. For optimal performance the mirror surfaces are silver-coated with a special cover avoiding corrosion. The overall reflectivity is more than 99% at maximum (near 6000 Å), but decreases to 90% near 4000 Å. The double paraboloid arrangement is very efficient in removing most of the aberrations due to the tilted incidence of the beam (see Dekker et al. 1992).

Between the two collimators the spectrograph beam is folded on a small plane Zerodur$^{\textstyle\rm\scriptscriptstyle TM}$ mirror of 100 $\times$ 10 mm size, in the immediate vicinity of which an intermediate image of the échelle spectrum is observed. This offers the unique advantage to free the spectrum from most of the scattered light produced at the échelle and other surfaces and edges simply by inserting a diaphragm which passes only the light falling through the intermediate image. The resulting improvement gives FOCES nearly the quality of a monochromator as is shown in Sect. 4.

2.1.3 Cross disperser

After recollimation on the second collimator the beam enters a cross-dispersing tandem prism (cf. Fig. 2b). The prisms are made of LF5 with a basis length of 160 mm and a width of 112.6 mm. LF5 has been chosen because its transmission in the near ultraviolet is very high as compared with other flint glasses, and because its angular dispersion is high enough. The prism angle is 33$^{\circ}$, and the prisms are used near minimum deviation. The strong cross-dispersion required implies the tandem prism arrangement to avoid problems with total reflexion that would be present on a single 55$^{\circ}$ prism. The symmetric position is read out and adjusted with an accuracy of 2 arcmin.

  

Figure 3: Full view of recorded single-fibre spectrum of a cool star displaying 70 échelle orders ranging from 3900 to 7000 Å, with a single bad column on the CCD chip of the test camera. At bottom right the inset shows a magnified section of the échelle image near the atmospheric ${\rm O}_2$ band at 6800 Å. The spectrograph slit is set to full two-pixel resolution (R = 40600), corresponding to 1.5'' at the focal plane. The seeing was $\approx 2.5''$ as is evident from the cross-order width

The prisms are anti-reflection coated with an efficiency better than 95% over the full visible wavelength region. Compared with a grating as a cross-disperser the tandem prism thus has a gain in efficiency of approximately 20 to 30%. It also removes the global cross-order scattered light level on the detector. Moreover, the prism is also first choice because it allows a much more economic use of the CCD real estate; whereas a grating produces an order separation $\Delta y \propto \lambda^2$ the prism order separation varies only $\propto \lambda^{-1}$ clustering in the red instead of the blue. The second aspect is particularly important since the blue signal is always considerably fainter than the red and the interorder definition therefore much more significant.

While the prism cross-disperser provides a sufficient separation of the échelle orders for observations with a single fibre, FOCES can also be used in a dual-fibre mode that requires nearly twice the cross-dispersion. This is achieved with an additional grism that can be moved into the beam immediately in front of the prisms as shown in Fig. 2c. The resulting échelle image pattern is shown in Fig. 3 for a short single-fibre exposure of Procyon.

2.2 Mechanical design and stability

The spectrograph is mounted on an optical bench made of ferromagnetic stainless steel which weighs $\approx$ 300 kg. While its mechanical properties are optimized for frequencies below 50 Hz to compensate for small perturbations such as encountered in dome buildings, there are resonances near 140 Hz and higher, which are damped pneumatically using a system of shock absorbers to mount the bench on. The overall mechanical stability is therefore very high and makes the spectrograph particularly useful for radial velocity work.

Thermal stability is enforced by the design itself. FOCES is set up under controlled thermal conditions such as found in a Coudé laboratory of a telescope dome. The room temperature is not assumed to vary by more than 2 K. The spectrograph with the optical bench is housed in a cover additionally isolated with polystyrene. No active cooling of the spectrograph is needed since the internal heat from motors is negligibly small; this is guaranteed by switching off the active electronic components immediately after they have been used; this is possible due to use of absolute encoders.