A User-oriented Comparison of the Techniques for 3D Spectros

A comparison of the most popular techniques for 3D spectroscopy is presented in a way which should hopefully be useful for astronomers intending to use these techniques. Integral field spectroscopy, slitless spectroscopy, tunable imaging filters, imaging F

a r X i v :a s t r o -p h /0510524v 1 18 O c t 2005

A comparison of the most popular techniques for 3D spectroscopy is presented in a way which should hopefully be useful for astronomers intending to use these techniques. Integral field spectroscopy, slitless spectroscopy, tunable imaging filters, imaging F

2Sperello di Serego Alighieri sperello@arcetri.astro.it

partially overcome by an obvious development of IFS:the?eld of view,par-ticularly for IFS using?bre–lenslet arrays,can be separated in several disjoint regions,for example to cover several galaxies in a cluster.Examples of this development are the multiple IFS of GIRAFFE[9],and the even more?exible programmable IFS concept[5].Because of its?exibility,IFS is suited for a large number of applications,from kinematical studies of the Galactic centre to stellar population and kinematical studies of distant galaxies[10].

Also slitless spectroscopy is capable of simultaneously recording a (x,y,λ)data cube:originating from the objective prism technique,used on Schmidt telescopes for more than?fty years,it is easily implemented in mod-ern imaging(focal reducer)spectrographs by removing the slit.Therefore it records spectra of all objects over the whole?eld of view,which can be quite large,like the14’x14’?eld of VIMOS[12].The disadvantages are the high sky background,since on every detector pixel the sky is integrated over the whole wavelength range,and the overlap of spectra in the dispersion direction.Still this technique is particularly useful for surveys and searches of special objects, when the sky background is very low,like in space or in small atmospheric windows.For example it has been successfully used with ACS on the HST for GRAPES,a spectroscopic survey of the Hubble Ultra Deep Field down to an AB magnitude limit of z=27.2,leading to the discovery of a large number of emission line objects over a huge redshift range,like AGN and Lymanαgalaxies[15].In this case the e?ects of the spectra overlap has been substan-tially reduced by taking spectra at various position angles.An example of the use of slitless spectroscopy from the ground is the search for Lymanαemitters at z=6.5in the atmospheric window centred at915nm.In this case the spectral range can be limited to the20nm width of the window by using a narrow-band?lter.Therefore both the sky emission and the spectra overlap are greatly reduced and very faint emission line objects can be found down to a line?ux of2×10?17erg cm?2s?1[11].

Energy–resolving detectors are imaging arrays where each pixel has some energy resolution.Therefore these are true3D devices capable of simul-taneously recording the(x,y,λ)data cube,and no spectrograph is necessary. Being mostly photon-counting detectors,they also have a very good temporal resolution.Their main disadvantages are the very limited spectral resolution and?eld of view.Two di?erent technological approaches are being explored in the optical range:the Superconducting Tunnel Junctions(STJ[14])and the superconducting transition-edge sensors[6].Currently STJ detectors us-ing tantalum metal?lms have a good quantum e?ciency in the optical range (about70%),but have a resolutionλ

A comparison of the most popular techniques for 3D spectroscopy is presented in a way which should hopefully be useful for astronomers intending to use these techniques. Integral field spectroscopy, slitless spectroscopy, tunable imaging filters, imaging F

A User–oriented Comparison of the Techniques for3D Spectroscopy3 however produce spectra of many objects in a large?eld,and very suitably ful-?ls the needs of many applications,making it the most popular3D technique. Practically all telescopes have MOS instruments,using either a?bre posi-tioner coupled to a spectrograph,movable slitlets,or a multi-aperture plate. The latter implementation has advantages in terms of better sky subtraction and throughput than?bres,and a larger number of objects and better posi-tioning?exibility than slitlets.A good example is VIMOS on the VLT[12], which is capable of simultaneously recording spectra of1000objects over a 14’x14’?eld of view.The disadvantages are that it requires prior imaging(and mask preparation),that objects have to be preselected(not good for object searches),and that it is not capable of a complete2D coverage of extended objects.

3Scanning Techniques

Tunable imaging?lters cannot simultaneously record the data cube,but require scanning in wavelength.The most used in astronomy is the Fabry–Perot?lter,which uses interference between two glass plates[4].They have very good imaging capability,a large?eld of view and good spectral resolution. They su?er from the so–called phase problem:the central wavelength is not constant over the?eld of view.Therefore reconstructing the(x,y,λ)data cube is not straightforward.Fabry–Perot?lters have been used for a large number of applications mostly on nearby galaxies and nebulae.

Imaging Fourier Transform Spectroscopy(IFTS)is a special tech-nique using the interference of two optical beams.Although it requires several exposures by scanning a movable mirror,and the reconstruction of the(x,y,λ) data cube is not straightforward,but requires heavy computation,neverthe-less the scanning does not imply any loss of photons,which are all recorded over the full?eld of view and wavelength range[3].A disadvantage compared to the simultaneous3D techniques,like the IFS,is that the readout noise a?ects the?nal data cube not just once,but a number of times equivalent to the number of spectral elements.Also each spectral element su?ers the sky noise of the whole bandpass.Therefore IFTS is …… 此处隐藏：5954字，全部文档内容请下载后查看。喜欢就下载吧 ……