Integral field spectroscopy

IFU targets

This is a brief description of the principles of integral field spectroscopy, mainly using fibre-lenslet techniques. It is meant to serve as a link to the GEMINI Multiobject Spectrographs and Durham Astronomical Instrumentation Group: Spectroscopy Programme pages which contain a list of current instrumentation projects . It is not meant to be comprehensive and apologies are offered for not giving credit or references to the the many other workers in this field.

Click here for James Turner's IFU data reduction software page

For a summary, please see the paper given at the 1998 SPIE large telescope conference in Kona Sweden in March 1998 (gzipped postscript 0.5Mb). An older paper given at the 1996 SPIE large telescope conference in Sweden in May/June 1996 (Postscript 0.3Mb) is also available.

A general paper on sampling and background subtraction (0.4Mb, gzip postscript). in fibre-lenslet IFUs has been accepted by PASP (Allington-Smith and Content).

Beyond longslit spectroscopy

Traditional spectroscopy is based on dispersing the image of a slit (single or multiple) so that a spectrum is produced for whatever fraction of the light from the target of interest falls within the aperture defined by the slit. If the slit is extended in length beyond the confines of the target, then it is also possible to record the spectrum of adjacent sky to subtract from that of the object - particularly important if the object is fainter than the sky, which is very frequently the case. While this is satisfactory for many applications, it makes poor use of the incident light when the object is extended, either intrinsically or due to poor seeing. In these cases, what is really required is the ability to record a spectrum from each part of an extended object.

This cannot be done with a longslit except in one dimension defined by the length of the slit. However the longslit can be stepped in position across the target by moving the telescope and recording separate exposures for each position. But this is time-consuming since the effective exposure time is multiplied by roughly the ratio of the object size to the width of the slit.

Other techniques are available such as Fabry-Perot scanning. This allows a large object to be surveyed in a single exposure but only at a single wavelength (which depends on position within the field) so that the required data volume with axes labelled by x-position, y-position and wavelength must be built up via a series of exposures. As with stepped longslit spectroscopy, this is an inefficient use of telescope time.

Techniques which record spectra from each part of an object simultaneously are termed Integral Field Spectroscopy (Figure 1). The terms two-dimensional spectroscopy or three-dimensional imaging are also used, although, strictly, they include non-simultaneous techniques as well.

Basic techniques

There are three main techniques.

IFS techniques

As will be seen below, it is possible to combine some of these techniques to get the best combination of features.

In some cases, these techniques require the telescope focus to be magnified so that either the physical size of the devices can be made more manageable and/or to adjust the focal ratio of the input telescope beam. This requires the use of fore-optics , generally a pair of achromatic lenses.

Integral field spectroscopy in Durham

Integral field techniques which are being studied by the Durham Astronomical Instrumentation Group (AIG) include. Durham is currently embarked on three projects to provide integral field units (IFUs). The most ambitious is that for the GEMINI Multiobject Spectrographs (GMOS).

Basic parameters

The above discussion concerns only the technical implementation. At this stage we need to consider the general science requirements . Naturally enough, these vary from project to project but, taking some sort of sum over these, it appears that the basic requirements are:

More specifically, GMOS's requirements are for 0.2 arcsec sampling, a field of view with an area of at least 50 arcsec-squared and separate object and background fields separated by about 2 arcmin.

Technical issues

Jeremy Allington-Smith
11 Jan 1996 - last revision 30 June 1998