Integral Field Spectroscopy
In normal spectroscopy a slit or a single aperture is placed into the focal plane which is re-imaged through the spectrograph onto the detector. While this measure is necessary to arrange detector space in dispersion direction to allow the spectra spread, the disadvantage is that only a point or (in case of a slit) a line through the object is sampled at the same time. To cover an object two-dimensionally requires several consecutive exposures with altered slit positions. This method is not only time consuming, but also prone to errors in pointing and in stability as all sorts of changes can happen while these lots of exposures are done.
Integral-field spectroscopy aims at addressing these problems by allowing spectroscopy of a two-dimensional area of the image plane in a simultaneous way. As the spectrum of each spatial element (also referred to as a "spaxel" to distinguish from the detector pixels) is taken at the same time, the result is stable with respect to temporal changes. Furthermore effects of pointing errors and atmospheric dispersion can be addressed later during data reduction as no light is lost if the field covered is large enough. Each spaxel can be referred to as being a slit. However, as the spectra of several adjacent spaxels can be added together, there are no slit losses caused e.g. by bad seeing conditions.
There are different techniques to realize an integral field unit ("IFU") technically. Aim is to sample the light of the spaxels and to re-arrange it in a way that the spectra of different spaxels not not overlap on the detector. The image above shows those techniques:
- Pure lenslet coupling: The image of the telescope is projected onto a microlens array. In each microlens ("lenslet") focus an image of the pupil appears. The foci of the lenslets are in the focal plane of the spectrograph collimator. To assure that spectra of the same line do not overlap, the dispersion direction needs to be slightly angled with respect to the rows and columns of the array. The advantage of this design is its simplicity and its high throughput. On the down side the detector space is not optimally used, and at large arrays the band width has to be restricted to shorten the spectra to avoid overlapping.
- Fibre lenslet coupling: This technique uses one optical fibre per lenslet focus to collect the light sampled by the particular spaxel. As fibres provide the flexibility of chosing the output location, the light outputs of all spaxels can be put into a line, the "pseudoslit". The advantages are the full use of the detector, the longer allowed spectral bandpass (just limited by detector size and spectrograph characteristics) and the possibility to match the fibre output focal ratio using microlenses at the fibres. Using this, fibre-lenslet IFUs can be curtailed to virtually any existing spectrograph as a powerful add-on. On the down side, optical fibres introduce a higher loss due to coupling issues and the non-preservation of the Etendue by the fibre inherent effect of focal ratio degradation.
- Image slicers: The image slicer is an optically different approach on how to realize an IFU, in most cases based on mirror optics. The image of the object hits a mirror array that dissects slices of the image to send them to different channels. Eachslice is re-imaged onto a slit via a pupil mirror. Each slice is imaged along a row forming a large pseudo slit, with the dissection in the second dimension happening by the detector pixels similar to a classical long-slit spectrograph. The systems made by CfAI use the so-called advanced image slicer where the surfaces are powered to achieve a higher degree of correction and efficiency. Image slicers have a very high efficiency, they can be built with large spaxel sizes as no manual interaction is necessary when machining the components monolithically from metal, as done at the CfAI. The current disadvantage is the higher scatter for visual wavelengths when comparing with lenslet IFUs. Hence slicer IFUs are restricted to the infrared regime at the moment, but research is going on to get smoother slicer surfaces.
IFU projects at the CfAI
The CfAI has constructed many ground-breaking Integral Field Spectroscopy systems, using novel designs and construction methods, such as monolithic precision machining of optical surfaces for image-slicing IFUs. Some of these are listed on the spectroscopy projects page.
concentrates on the design and construction of fibre-lenslet and image slicer based IFUs.
Fibre-lenslet based IFUs
- SMIRFS-IFU: Prototype IFU and technology demonstrator for UKIRT, 72 elements, optimized for the near infrared
- TEIFU: 1000-element IFU for the 4.2m William-Herschel-Telescope
- GMOS-IFU: 2 fields of 1000 and 500 elements to enable simultaneous background subtraction. Two built for Gemini North and Gemini South
- IMACS-IFU: 2 fields of 1000 elements each to enable background subtraction and beam switching. Located at the Magellan-1 6.5m telescope IMACS spectrograph.
- GNIRS-IFU: 21 slices, IFU for the GMOS spectrograph of GEMINI North.
- KMOS-IFU: 24 deployable IFUs of 15 slices each, currently in construction
- JWST NIRSpec IFU: 30 slice IFU for the James Webb Space Telescope's NIRSpec spectrograph, currently in construction