Instrumentation Concepts Ground-based Optical Telescopes

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

2. IAG/USP (Keith Taylor)‏ Imaging Fourier Transform Spectrographs (IFTS) FTS = Michelson Interferometer: IFTS = Imaging IFTS over solid angle, . • Beam-splitter produces 2 arms; • Light recombined to form interference fringes on detector; • One arm is adjustable to give path length variations; • Detected intensity is determined by the path difference, x, between the 2 arms.

3. IAG/USP (Keith Taylor)‏ [1 + cos(2x)] I     B() = I(x) = I(x).(1 + cos2x).dx B().(1 + cos2x).d - - and x 1 (, ) = 2 2 IFTS theory (simple version) Given that frequency,  = 1/ (unit units of “c”): Phase difference between two mirrors = 2x So recorded intensity, I, is given by: Now, if we vary x in the range:   x/2  , continuously then: These represent Fourier Transform pairs. Spectrum B() is obtained from the cosine transformation of the Interferogram I(x)

4. IAG/USP (Keith Taylor)‏ ) I ( = [1 + cos(2x)] 1 = R0 = 2  2xmax   IFTS reality (simple version) • At x = 0: the IFTS operates simply as an imager; • White light fringes – all wavelengths behave the same • At all other x-values, a subset of wavelengths constructively/destructively interfere • For a particular , the intensity varies sinusoidally according to the simple relationship: In reality, of course, x goes from 0  xmax which limits the spectral resolving power to: eg: if xmax = 100mm and  = 500nm then: R0  1.105

5. IAG/USP (Keith Taylor)‏ IFTS in practice Since we are talking here about an imaging FTS then what is it’s imaging FoV? Circular symmetry of the IFTS is identical to the FP and hence: 2l.cos = m And also: R >> 2 limited only by the wavelength variation, , across a pixel: However, in anaolgy to the FP  Phase-correction is required in order to accommodate path difference variations over the image surface.

6. IAG/USP (Keith Taylor)‏ Pros & Cons of an IFTS Advantages: • Arbitary wavelength resolution to the R limit set by xmax; • A large 2D field of view; • A very clean sinc function, instrumental profile • cf: the FP’s Airy Function • A finesse N = 2/ which can have values higher than 103 Disadvantages: • Sequential scanning – like the FP. However, the effective integration time of each interferogram image can be monitored through a separate complementary channel, if required; • Very accurate control of scanned phase delay is required • Especially problematic in the optical • At all times, the detector sees the full spectrum and hence each interferogram receives integrated noise from the source and the sky • This compensates for the fact that all wavelengths are observed simultaneously which is why there is no SNR advantage over an FP; • Also sky lines produce even more noise, all the time.

7. IAG/USP (Keith Taylor)‏ Michelson Interfermeter(N = 2 interference ; n >>1)

8. IAG/USP (Keith Taylor)‏ Hybrid and Exotic Systems • FP & IFTS are classical 3D imaging spectrographs • ie: Sequential detection of images to create 3D datat cubes: • FP = Wavelength scanning • IFTS = Phase delay scanning Examples of this are: Integral Field Units (IFUs). These can use either: • Lenslets • Fibres • Lenslets + Fibres • Mirror Slicers There are, however, techniques which use a 2D area detector to sample 2D spatial information with spectral information, symultaneously. These we refer to as: Hybrid Systems

9. 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 Tiger SIFS MPI’s 3D

10. 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)

11. 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?

12. IAG/USP (Keith Taylor)‏ Tiger (in practice) Enlarger Detector Camera Lenslet array Collimator Grism

13. 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

14. 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

15. IAG/USP (Keith Taylor)‏ Tiger Results (SAURON – WHT)

16. IAG/USP (Keith Taylor)‏ Fibres in Astronomy Optical fibre technology offers the astronomical spectrograph designer vast opportunities. Astronomical Spectroscopy is the art of recording spatial and spectral information simultaneously onto a 2D area detector. In other words it requires the re-formatting of information to suit the detector and the astronomical goals. If we could arbitrarily define the geometry of our detectors (even to make them 3D!) then none of the sophisticated optical design would be necessary. This is where fibres come into their own … They are the “perfect” image re-formatters, taking any shape of object and re-forming it into a spectrograph slit.

17. IAG/USP (Keith Taylor)‏ Generally used for astronomy (dia >50m) Protective buffer Special case for Adaptive Optics (dia ~10m) Types of Fibre Fiber operates as an optical wave-guide Operates by total internal reflections

18. IAG/USP (Keith Taylor)‏ Note central obstruction Focal Ratio Degradation(FRD) Input f-ratio = Output f-ratio (A is preserved) But not, unfortunately, in a fibre • Note: • Input f-ratio is not preserved; Fin (slower) > Fout (faster) • Central obstruction is filled in • Ain < Aout ; to compensate, R decreases or d increases

19. IAG/USP (Keith Taylor)‏ sinmax =  n2f n2c n0 Protective buffer nf nc n0 Numerical Aperture (NA) For the fibre to operate as an optical waveguide, total internal reflection (TIR) has to be maintained throughout the passage of light along the fibre. TIR then requires: Note: tanmax = 1/2Fin = NA, the numerical aperture: NA ~ 0.22 (Fin is slower than > 2.3) for normal fibres

20. IAG/USP (Keith Taylor)‏ Using Fibres to link Telescopesto Spectrographs. • Advantages • Spectrograph independent from telescope. Bench Spectrographs, no weight or volume restrictions. • High spectral stability. • Fibres are easy to use and install (once prepared!) • Possibility to perform two-dimension spectroscopy with fibre bundles. • Drawbacks • Transmission losses. • Focal Ratio Degradation. • Circular aperture losses. • Poor sky subtraction. • Fixed “slit aperture”. • Difficult to prepare if not proper tools are available. • Fragile!

21. IAG/USP (Keith Taylor)‏ Fibre slicer(the simplest approach) A simple fibre re-formatter from sky to spectrograph slit Re-formatted onto a long-slit of the spectrograph 2D array of fibres at the telescope focal plane

22. IAG/USP (Keith Taylor)‏ Fibre slicer attributes and examples • Captures light over a full seeing disk (and more) without degrading the intrinsic resolving power of the spectrograph; • Facilitates spatially resolved spectroscopy; • No requirement to centre a point object on a slit; • No requirement to match slit width to the seeing; • Effectively detaches spectral and spatial information; • Facilitates spatially integrated spectroscopy • Integral field spectroscopy (IFS) • Supplies robust spectrophotometry • Objects aligned along the slit • Examples: • F.I.S. (on AAT - 1981) 100 fibres • SILFID (on CFHT - 1988) 400 fibres • HEXAFLEX (on WHT – 1991) 61 fibres • 2D-FIS (on WHT – 1994) 125 fibres

23. IAG/USP (Keith Taylor)‏ 400 individual fibres All wavelengths are aligned Fibre Spectral Image

24. IAG/USP (Keith Taylor)‏ fibre fibre = DFfibre … and now some numbers! Clearly for a fibre diameter, fibre , each individual fibre aperture (fibre) on the sky is given by: where Ffibre is the input focal ratio of the fibre. • Example: • Take fibre =0.5” ; D = 8m and Ffibre = 5 •  fibre ~80m • This integral field unit (IFU) fibre can be retro-fitted to existing long-slit spectrographs, however there are 3 problems: • Focal ratio degradation (FRD) which requires fast f-ratios • Collimator speeds which are matched to normal Cassegrain f-ratios, which require slow f-ratios • Spatial information is lost in the inter-fibre gaps

25. IAG/USP (Keith Taylor)‏ Micro-lens array Lenslet/fibre coupling Coupling fibres with micro-lenses If  = spatial sampling on sky (subtended by micro-lens), then fibre = .DT.Ffibre lens = .DT.FTel

26. IAG/USP (Keith Taylor)‏ The SIFS IFcourtesyC&L de Oliveira, (LNA)‏

27. IAG/USP (Keith Taylor)‏ Fspec Fin Fin fiber therefore Ffibre = 1 + lens Down-side oflenslet/fibre coupling Ffibre The fibre Fin (slower) > Fspec (faster) because of FRD But fibre receives light from micro-lens significantly faster than Fin (where: Ffibre (faster) < Fin (slower) - see red rays) Take: fibre = dia. of fibre lens = dia. of micro-lens Conclusion – don’t make lens too small Use macro-lenses (!)

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

29. IAG/USP (Keith Taylor)‏ Slicer Promo (The End)