4.1 Introduction
This Chapter describes the theoretical design, and subsequent modeling, o f some new kinds o f spectrograph collimator arrangem ents Collim ator geom etry has significant implications on the feasibility and space envelope o f any proposed instrument. The design o f a collim ator system is therefore a fundam ental starting point for the development o f any new spectrograph. A num ber o f different collim ator arrangem ents w ere investigated for the HROS spectrograph, and the ray tracing results o f these studies are presented here.
As discussed in C hapter 1 the sole purpose o f a collim ator is to align or collim ate light from the spectrograph slit onto the echelle grating. It is im portant that in perform ing this task any aberrations introduced not degrade the final projected spectral line width at the detector. Off-axis or 'offset aperture' parabolic m irrors have often been successfully em ployed within spectrographs as collimators. Section three o f this C hapter reports on the feasibility o f different parabolic m irror arrangem ents, for integral use on the HROS instrument. First in the next section the possibility o f employing dioptric lens collimators is considered for use within high resolution spectrographs like HROS.
4.2 Lens Collimator Design for Optical/UV Spectrographs
Em ploying a dioptric collimator can potentially reduce the space taken up by a N asm yth spectrograph. Lens collimators may also be advantageous for C assegrain instrum ents, again potentially reducing the space requirements for such an instrum ent. W e now consider the feasibility o f lens collimators specifically for a proposed N asm yth version o f HROS.
4.2.1 The Baranne Design for a Nasmyth HROS
A modified 'Baranne' white light pupil option for a N asm yth instrum ent w as suggested for HROS by OSL [72]. In Figure 4.1 we see an example o f this type o f system. Such a system has the advantage o f forming an interm ediate pupil, allowing the spectrum to be split up into separate channels for subsequent secondary collim ation and imaging. For such a system to w ork a primary collim ator w ould be required and is used in double pass. F or HROS the w avelength range o f such a hypothetical lens w ould be from 3000 to 10000 Angstroms. H ow ever it is well know n that few materials exist that have high throughput in the near ultra violet dow n to 3000 A ngstrom s, the design goal for HROS. W e selected a number o f these m aterials for possible application to the design o f such a lens. At the outset, it w as recognized that the design o f a lens with simultaneous chromatic correction over such a w ide w avelength region was unlikely to be successful. H ow ever the results o f such a study w ould serve to show over w hat kind o f wavelength region such a system m ight be feasible. The materials used for this study w ere Fused Silica, Sodium Chloride, and Fluorite. Additionally w e employed the new Schott materials U ltran 20 and 30 that also have high transmission in this region o f the electrom agnetic spectrum . A
theoretical study o f the feasibility o f dioptric collim ators employing these near ultra violet transm itting materials is detailed below.
Main collimator
Slit
Echelle
Segmented Collector mirror
Secondary collimators
Cameras
Figure 4.1 : The M odified Baranne Layout Showing D ioptric Collim ators
The heart o f the Baranne design is the double pass lens in front o f the echelle, w hich is required to w ork throughout the entire w avelength coverage o f the instrum ent. In order to investigate the feasibility o f such a lens w e theoretically evaluated a num ber o f achromatic lens designs. It is evident from Figure 4.1 that the prim ary collim ator will be required to w ork at a small o ff axis angle. This fact w as not accounted for in the following feasibility study as the overriding design problem w as considered to be correction o f chromatic aberrations, (in addition to Spherical A berration) Additionally the results o f this study are relevant to the secondary collim ators that are required to w ork over only a subset o f the overall w avelength range.
4.2.2 Dispersion and Achromatic Lenses
In the late seventeenth century Isaac N ew ton first passed w hite light through a prism, and discovered light to be com posed o f different wavelengths that are dispersed upon refraction [44]. Snell's law explains this simply as being due to the fact that blue light has a higher refractive index and hence undergoes a greater degree o f bending than red during refraction. The refractive index (N) o f any material is defined as the ratio o f the velocity o f light in a vacuum with respect to the velocity within the medium. Early singlet lens designs suffered from blue colour blur as a result o f dispersion and the corresponding intrinsic change o f focus w ith wavelength. In the eighteenth century a number o f different lens designers found it possible to minimize this effect, and unite blue and red wavelengths to a com m on focus by use o f a positive crow n and w eaker negative flint glass combination. In lenses o f this type the greater dispersion provided by the flint glass com pensates for the dispersion introduced at the crow n element. The primary design task considered here is the reduction o f residual longitudinal chromatic aberrations present in achrom atic doublet lenses. A chrom atic doublet lenses are corrected for spherical aberration, and also chrom atic aberration at tw o w avelengths 6563 Angstroms (term ed c) and 4861 A ngstrom s (term ed f). T hat is the lenses are designed to have the same focal length at these tw o wavelengths. This is achieved using the well-known technique referred to as Conrady's D -d m ethod o f achrom atism [9].
The dispersive pow er o f any material can be expressed as a number, often called the A bbe number (V) representing the level o f dispersion w ith respect to a central (d) w avelength at 5893 Angstroms.