Now Argus had a hundred eyes in his head and never went to sleep with more than two at a time, so that he kept watch of Io constantly.
Thomas Bulfinch, 1855.
This cookbook is an introduction to and overview of fibre spectroscopy and, in particular, the techniques and software available for reducing observations made with fibre-fed spectrographs. It is intended as a starting point for astronomers with such observations to reduce. No prior knowledge of fibre spectroscopy is assumed.
Fibre-fed spectrographs, or fibre spectrographs for short, have become common in recent years and several are available as common-user instruments for the UK astronomical community. A traditional astronomical spectrograph can observe only a single astronomical object at a given time. The essential feature of fibre spectrographs is that they can observe many objects simultaneously; typically over a hundred objects for a modern instrument. Furthermore, the spectra obtained with fibre spectrographs are similar, in terms of wavelength range, resolution, sensitivity and accuracy to those obtained with traditional single-object spectrographs. Thus, fibre spectrographs act as a ‘telescope multiplier’; a given set of objects can be observed with a fibre spectrograph in much less time than with the equivalent single-object spectrograph. For example, the 2dF fibre spectrograph on the 3.9m Anglo-Australian Telescope can observe 400 spectra simultaneously. In a single observation it can acquire spectra which would require 400 consecutive observations using a traditional single object spectrograph.
Of course there are limitations on using fibre spectrographs to observe multiple objects. In particular, multiple objects can only be observed if they are simultaneously in the field of view of the telescope. Also, in order to obtain the maximum advantage from the instrument the various objects being observed should be of similar brightness, otherwise the duration of the observation must be set by the faintest object and some of the advantage of simultaneous observation is lost. These constraints favour telescope designs with wide fields of view. Nonetheless, the increase by one or two orders of magnitude in the number of spectra which can be acquired in a given amount of observing time has lead to a veritable revolution in astronomical spectroscopy. Numerous large scale surveys and smaller individual projects which would hitherto have been infeasible are now regularly carried out. Multiple-object spectroscopy is now common, and, indeed, on some telescopes the norm rather than the exception.
The basic principle of fibre spectroscopy is simple. A set of optical fibres are positioned in the focal surface of the telescope so that each is illuminated by one of the objects being observed. The other ends of the fibres are positioned along the entrance slit of a spectrograph. The basics of fibre spectrographs are discussed further in Section 5. Fibre spectroscopy is not the only technique for simultaneously observing the spectra of many objects. Other techniques include multi-slit spectroscopy and spectroscopy with objective prisms. These techniques are beyond the scope of this cookbook. However, Section 9 gives a brief summary of their advantages and disadvantages relative to fibre spectroscopy in order to help you to judge whether fibre spectroscopy is appropriate for your purposes. Conversely, optical fibres have uses in astronomical spectroscopy other than allowing multiple targets to be observed simultaneously. For example, they can be used to bring the light from a single target to a large, stable, floor-mounted spectrograph for high-precision radial velocity determinations or interferometry. Again, these techniques are beyond the scope of this cookbook. Fibre spectroscopy has been carried out at wavelengths ranging from the ultra-violet (Å) to the infrared ( micron).
The first fibre-fed astronomical spectrograph was Medusa, built by Hill et al. at the Steward observatory and first used in 1979. Since then many instruments have been built; Parry gives a list and the early development of the subject (up to 1988) has been reviewed by Hill. Several fibre spectrographs are available to the UK astronomical community as common-user instruments. The principal ones currently available are the Anglo-Australian Telescope (AAT) 2dF, WYFFOS/AUTOFIB2 on the William Herschel Telescope (WHT) and FLAIR on the UK Schmidt Telescope (UKST). These instruments are briefly described in Section 7. Additional instruments are being built or are planned. For example, the 6dF should replace FLAIR on the UKST around the year 2001. Numerous fibre spectrographs are also available or being developed at foreign observatories. The most ambitious fibre spectroscopy instrument under development is the Chinese LAMOST project which will have a dedicated 4m Schmidt telescope and be able to observe up to 4000 objects simultaneously.
A variation on the conventional fibre spectrograph is the Integral Field Unit (IFU). In a traditional fibre spectrograph the fibres are individually positioned so that they are illuminated by the objects being observed. In an IFU the fibres are simply packed in a regular grid positioned in the focal surface of the telescope and thus produce a set of spectra at a grid of points on the sky. Often the fibres will be arranged in a closely-packed grid (that is, a hexagonal or honeycomb pattern) rather than the more conventional rectangular grid1. IFUs are not yet in common use, though they are likely to become important in the future. For example, the GMOS spectrographs being built for the Gemini telescopes include an optional IFU as one of their modes of operation. IFUs are not considered further here, though many of their features are similar to traditional fibre spectrographs.
This cookbook is an overview and is not specific to any particular instrument. However, it does contain both numerous references for further information and some worked examples. The structure of the cookbook is:
If you are familiar with the principles of fibre spectroscopy then you can omit Part I and proceed straight to the worked examples. On Starlink systems example datasets are distributed with the cookbook so that you can try the examples for yourself.
Fibre-fed spectrographs are a relatively recent innovation and are rarely described in textbooks on astronomical instrumentation. However, they are mentioned briefly in Astronomical Observations by Walker, pp167-169 and pp115-116.
There have been a number of conferences in whole or part about fibre spectroscopy and proceedings are usually available. These conferences include the following:
Note that, as mentioned above, wide-field spectroscopy includes a number of techniques, of which fibre spectroscopy is but one (although perhaps currently the most important). Most of the proceedings carry progress and status reports for the major instruments as they are built and subsequently operated; it is possible to see them developing over a number of years.
In general the more recent proceedings are the most useful. In particular, Wide-Field Spectroscopy and Fiber Optics in Astronomy III include excellent reviews by Parry[40, 41], which are strongly recommended.
The construction and properties of optical fibres are beyond the scope of this cookbook; for details see the reviews by Barden, Heacox and Connes and Nelson. Of course, the major uses of optical fibres are outside astronomy. For an accessible introduction to these wider uses see Hecht’s Understanding Fiber Optics and for the history of the subject see his City of Light. If you do read any non-astronomical literature about optical fibres you should be aware that various different types are available, only some of which are usually used in astronomical instrumentation.
A brief note about lexicography is probably in order. The word ‘fibre’ (or ‘fiber’) is spelt differently on opposite sides of the Atlantic. Both spellings are common in the literature. In this cookbook I have used the British spelling throughout except that I have tried to follow the preferences of authors and editors for the titles of manuals, conference proceedings etc.
Technical terms are shown in a bold font like this the first time that they are used. Also:
items appearing in graphical windows, such as those used by 2dFDR, are shown in a sans serif font like this.
1The principal advantage of a hexagonal grid over a rectangular one is that for a given size of fibre it achieves a denser sampling of the region of sky observed. Also, each fibre is equidistant from all its nearest neighbours, which is a desideratum of some aspects of information theory. However, the representation and analysis of data sampled on a hexagonal grid is more complicated than the rectangular case.