Transcript AST Senior Review Major Recommendations - IAG-Usp

Instrumentation Concepts
Ground-based Optical
Telescopes
Keith Taylor
(IAG/USP)
Aug-Nov, 2008
Aug-Nov, 2008
Aug-Sep,
2008
IAG/USP (Keith
IAG-USP
(Keith Taylor)
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.
Aug-Nov, 2008
IAG/USP (Keith Taylor)
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:
x ) = 1 [1 + cos(2x)]
(,
I
2
2
Now, if we vary x in the range:   x/2  , continuously then:

I(x) = B().(1 + cos2x).d
-

B() = I(x).(1 + cos2x).dx
-

and

These represent
Fourier Transform pairs.
Spectrum B() is obtained from the cosine transformation of the Interferogram I(x)
Aug-Nov, 2008
IAG/USP (Keith Taylor)
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:
1
I ( ) = [1 + cos(2x)]
2
In reality, of course, x goes from 0  xmax which limits the spectral resolving
power to:

2xmax
R0 =  =

eg: if xmax = 100mm and  = 500nm then: R0  1.105
Aug-Nov, 2008
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.
Aug-Nov, 2008
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.
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Michelson Interfermeter
(N = 2 interference ; n >>1)
Aug-Nov, 2008
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
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
Examples of this are: Integral Field Units (IFUs). These can use either:
 Lenslets
 Fibres
 Lenslets + Fibres
 Mirror Slicers
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
Tiger
SIFS
MPI’s 3D
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)
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)
Tiger (in practice)
Enlarger
Lenslet array
Aug-Nov, 2008
Detector
Collimator
IAG/USP (Keith Taylor)
Grism
Camera
Avoiding overlap
• 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)
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
SAURON
OSIRIS
(on CFHT)
(on WHT)
(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)
Tiger Results (SAURON – WHT)
Aug-Nov, 2008
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 reforming it into a spectrograph slit.
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Types of Fibre
Fiber operates as an optical wave-guide
Generally used
for astronomy
(dia >50m)
Operates by
total internal
reflections
Protective
buffer
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Special case for
Adaptive Optics
(dia ~10m)
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
Aug-Nov, 2008
IAG/USP (Keith Taylor)
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:
sinmax =
 n2f  n2c
n0
Note: tanmax = 1/2Fin = NA, the numerical aperture:
NA ~ 0.22 (Fin is slower than > 2.3) for normal fibres
Protective
buffer
nf
nc
n0
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Using Fibres to link Telescopes
to 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!
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Fibre slicer
(the simplest approach)
A simple fibre re-formatter
from sky to spectrograph slit
Re-formatted onto a longslit of the spectrograph
2D array of fibres at the
telescope focal plane
Aug-Nov, 2008
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.
• SILFID
• HEXAFLEX
• 2D-FIS
Aug-Nov, 2008
(on AAT - 1981)
(on CFHT - 1988)
(on WHT – 1991)
(on WHT – 1994)
IAG/USP (Keith Taylor)
100 fibres
400 fibres
61 fibres
125 fibres
Fibre Spectral Image
400 individual
fibres
All wavelengths are aligned
Aug-Nov, 2008
IAG/USP (Keith Taylor)
… and now some numbers!
Clearly for a fibre diameter, fibre , each individual fibre aperture (fibre) on
the sky is given by:
fibre =
fibre
DFfibre
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:
1. Focal ratio degradation (FRD) which requires fast f-ratios
2. Collimator speeds which are matched to normal Cassegrain f-ratios,
which require slow f-ratios
3. Spatial information is lost in the inter-fibre gaps
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Coupling fibres with micro-lenses
Lenslet/fibre coupling
Micro-lens array
If  = spatial sampling on sky (subtended by micro-lens), then
fibre = .DT.Ffibre
lens = .DT.FTel
Aug-Nov, 2008
IAG/USP (Keith Taylor)
The SIFS IF
courtesy
C&L de Oliveira,
(LNA)
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Down-side of
lenslet/fibre coupling
Ffibre
Fin
The fibre
Fspec
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:
therefore Ffibre =
fibre = dia. of fibre
lens = dia. of micro-lens
Aug-Nov, 2008
Fin

1 + fiber
lens
Conclusion – don’t make lens too small
Use macro-lenses (!)
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)
Slicer Promo (The End)
Aug-Nov, 2008
IAG/USP (Keith Taylor)