Instrumentation Concepts Ground-based Optical Telescopes Keith Taylor (IAG/USP) Aug-Nov, 2008 Aug-Nov, 2008 Aug-Sep, 2008 IAG/USP (Keith Taylor)‏ IAG-USP (Keith Taylor) 1

IAG/USP (Keith Taylor)‏ Integral Field Units Three principal types of IFUs at UV, optical and near IR wavelengths: Reflective Refractive (microlenses) Optical fibre Also combinations of microlenses and fibres. Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

Why do we want to use an image slicer? To get spatial information on resolved sources. Usually these image slicers are called Integral Field Spectrographs. To preserve light from extended sources and sources whose image profile is broadened by the atmosphere. Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ Image Slicers Slit spectrographs are inherently restricted because light from outside of a narrow slice of the sky does not enter the instrument. This entrance slit can be long and in some circumstances it can even be curved. However in one direction it is narrow. Many images, including in many cases the images of point sources (broadened by seeing) are wider than this. Image slicers reformat the image, allowing more of it to pass through the slit. Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

Lenslet array (example) LIMO (glass) Pitch = 1mm Some manufacturers use plastic lenses. Pitches down to ~50m Used in SPIRAL (AAT) VIMOS (VLT) Eucalyptus (OPD) Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

Integral Field Spectroscopy Extended (diffuse) object with lots of spectra Use “contiguous” 2D array of fibres or ‘mirror slicer’ to obtain a spectrum at each point in an image SIFS Tiger MPI’s 3D Aug-Nov, 2008 IAG/USP (Keith Taylor)‏ 7

IAG/USP (Keith Taylor)‏ Mirror Image Slicers Pioneered by MPI (3D) (Gensel) Compact Efficient Slicer of choice but … Cannot be retrofitted to existing spectrographs Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ Image Slicers Principle of a simple image slicer, arranging several slices of the sky in a line along the entrance slit of the spectrograph. Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

Reflective Image Slicer Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

Reflective Image Slicer Consists of a stack of reflectors of rectangular aspect, tilted at different angles. Relay mirrors reimage the light reflected off these reflectors, and arrange them in a line to form a pseudo slit. The stacked reflectors need not be plane, often they have some power to keep the instrument compact. Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ 3D spectroscopy Integral Field Unit: How to have a projection of a 3D volume to a 2D plan? Spatial reformatting: Slicers l Y X Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

How to “slice” the target? Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ Instrument Status New Optical design Dichroics earlier possible: Smallest size (2mm) Better instrument optimization (sampling) Easier focal plane Shorter instrument (300mm) Implementation phase in a compact volume Shoehorn needed to enter in the shoebox Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

Optical design (IR Path) Relay optics Slicer Unit Prism Collimator Camera Detector Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ Slicer Design (IR) Slicer Unit Pupil & Slit mirror Slicer Unit Pupil & Slit mirror Relay optics Collimator Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

Optical design (IR Path) Relay optics Slicer Unit Prism Collimator Camera Detector Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ Hybrids & Exotica PYTHEAS (Georgelin et al – Marseille) Based on a cross between TIGER (lenslet array IFU) Fabry-Perot Tunable Echelle Imager (Bland & Baldry) Cross-dispersed Echelle Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

Fabry-Perot (reminder) Light enters etalon and is subjected to multiple reflections Transmission spectrum has numerous narrow peaks at wavelengths where path difference results in constructive interference need ‘blocking filters’ to use as narrow band filter Width and depth of peaks depends on reflectivity of etalon surfaces: finesse Aug-Nov, 2008 IAG/USP (Keith Taylor)‏ 19

Fabry Perot (reminder) What you see with your eye Emission-line lab source (Ne, perhaps) – note the yellow fringes Orders: m (m-1) (m-2) (m-3) The central or “Jacquinot” spot Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

Tiger (Courtes, Marseille) Technique reimages telescope focal plane onto a micro-lens array Feeds a classical, focal reducer, grism spectrograph Micro-lens array: Dissects image into a 2D array of small regions in the focal surface Forms multiple images of the telescope pupil which are imaged through the grism spectrograph. This gives a spectrum for each small region of the image (or lenslet) Without the grism, each telescope pupil image would be recorded as a grid of points on the detector in the image plane The grism acts to disperse the light from each section of the image independently So, why don’t the spectra all overlap? Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ Tiger (in practice) Enlarger Detector Camera Lenslet array Collimator Grism Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ Avoiding overlap -direction The grism is angled (slightly) so that the spectra can be extended in the -direction Each pupil image is small enough so there’s no overlap orthogonal to the dispersion direction Represents a neat/clever optical trick Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ Tiger constraints The number and length of the Tiger spectra is constrained by a combination of: detector format micro-lens format spectral resolution spectral range Nevertheless a very effective and practical solution can be obtained Tiger (on CFHT) SAURON (on WHT) OSIRIS (on Keck) True 3D spectroscopy – does NOT use time-domain as the 3rd axis (like FP & IFTS) – very limited FoV, as a result Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ PYTHEAS PYTHEAS (Georgelin et al – Marseille) Based on a cross between TIGER (lenslet array IFU) Fabry-Perot Goal True 3D imaging Given by a lenslet array IFU system Wide wavelength range Given by a classical Grating or Grism High Spectral resolution Given by a Fabry-Perot Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

Scientific Motivation Ideal 3D imager should have: High Spatial Resolution Large telescope (with Adaptive Optics) Large Field-of-View (comparable with interesting sources) High Spectral Resolution Easily obtained with FPs Long wavelength coverage Easily obtained classical spectroscopy Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

PYTHEAS (Optical Scheme) Magnified field imaged onto a mirolens array FP dissects spectral information into multiple orders Grism disperses these orders in same way as TIGER FP is scanned over a FSR to give full wavelength coverage Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

PYTHEAS = Combination of … TIGER’s true 3D capability Simultaneous: 2D Spatial + 1D Wavelength FP’s quasi-3D capability through encoding wavelength with time In this way one achieves high spectral and spatial resolution over a wide wavelength range but not simultaneously Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ PYTHEAS – How it works Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

PYTHEAS - Results Enlargement of Na Doublet range. Local Interstellar + Globular components Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

Tunable Echelle Imager (TEI – Baldry & Bland) Consider what a spectrograph does to this image if it is placed at the input aperture of the spectrograph: Assume galaxy is a continuum, then becomes Spectra from each point overlaps – total confusion … This is why we use a slit becomes Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

But what if the galaxy is monochromatic? Then … becomes x So lets move the slit at the spectrograph input … becomes and, in fact … x becomes x Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

Crossing gratings with FPs So, if we want to do imaging and spectroscopy simultaneously: ie: Integral Field Spectroscopy We have to make objects appear monochromatic Crazy … how can we do that? So how about making them multi-monochromatic? This is exactly what a Fabry-Perot does Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

Multi-monochromatic FP images dispersed by grating spectrograph becomes x Scan the FP and then … becomes Aug-Nov, 2008 IAG/USP (Keith Taylor)‏ x+dx

Reminder of X-dispersed Echelle X-dispersed echelle grating spectrometers allow high resolution and lots of spectral coverage Achieve this by having two orthogonal gratings One gives the high resolution (in y-axis) the other spreads the spectrum across the detector(in x-axis) Slit is consequently much shorter Aug-Nov, 2008 IAG/USP (Keith Taylor)‏ 37

IAG/USP (Keith Taylor)‏ X-dispersion Orders are separated by dispersing them at low dispersion (often using a prism). X-dispersion is orthogonal to the primary dispersion axis. With a suitable choice of design parameters, one order will roughly fill the detector in the primary dispersion direction. With suitable choices of design parameters it is possible to cover a wide wavelength range, say from 300-555nm, as shown in the figure, in a single exposure without gaps between orders. Illustrative cross-dispersed spectrum showing a simplified layout on the detector. m = 10-16 The vertical axis gives wavelength (nm) at the lowest end of each complete order. For simplicity the orders are shown evenly spaced in cross-dispersion. Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

So now replace grating with a cross-dispersed echelle Crossed with an FP gives Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ A TEI scan Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ TEI: Option #1 Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ TEI: Option #2 Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ TEI: Option #3 Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

TEI configurations (from Baldry & Bland) Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

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IAG/USP (Keith Taylor)‏ Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

Highly efficient use of detector Aug-Nov, 2008 IAG/USP (Keith Taylor)‏

IAG/USP (Keith Taylor)‏ The neatest trick OH sky-line suppression imaging In this example, 90% of OH energy is suppressed. Huge gain in SNR against sky continuum Aug-Nov, 2008 IAG/USP (Keith Taylor)‏