1. 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

2. 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)‏

3. 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)‏

4. 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)‏

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

6. 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)‏

7. 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

8. 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)‏

9. 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)‏

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

11. 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)‏

12. 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)‏

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

14. 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)‏

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

16. 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)‏

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

18. 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)‏

19. 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

20. 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)‏

21. 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)‏

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

23. 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)‏

24. 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)‏

25. 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)‏

26. 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)‏

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

28. 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)‏

29. 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)‏

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

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

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

33. 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)‏

34. 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)‏

35. 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)‏

36. 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

37. 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

38. 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 nm, 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)‏

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

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

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

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

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

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

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

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

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

48. 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)‏