New Instrumentation Concepts – Ground-based Optical Telescopes
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Transcript of New Instrumentation Concepts – Ground-based Optical Telescopes
New Instrumentation Concepts – Ground-
based Optical Telescopes
Keith Taylor(IAG/USP)
September, 2010
Synopsis of Lectures
Basic Principles: Fundamentals ; Basic Technologies; Basic
Techniques Introduction to Astronomical
Instrumentation Imaging ; Spectroscopy ; Interferometry
Advanced Instrumentation Techniques 2D techniques ; 3D techniques ; Hybrid
techniques ; Classical Spectroscopy ; Integral Field Spectroscopy ; Robotics
What is the purpose a telescope?
Collect and analyze photons over a region of sky At what wavelength ()?
Over what bandwidth ()? At what Spectral Resolution ()?
Over what Field of View (FoV)? At what Spatial Resolution? Point sources? Single or Multiple? or Diffuse?
At what Temporal Resolution? Polarization of source?
Note: Objects generally very distant and extremely faint Every photon counts Time to do observation/experiment must be << a human life-time # of telescopes << # of astronomers who want to use them # telescopes may be >> # of good ideas on how to use them?
What’s the message here?
Telescopes are a rare and expensive resource; They are not simply there to gather photons that
would otherwise be best left in the most secure storage medium in existence (the sky);
Emphasis should be on effectiveness rather than efficiency;
Access to telescopes is highly competitive Not always a good match between observational
goals to available instrumentation; Conflict between general purpose and targeted
instrumentation; As instrumentation becomes more powerful there
is a move from studies of individual objects to statistically astronomy and cosmology.
The “Art” of Observational Astronomy
The detection of ultra-distant objects The detection of ultra-faint signals
Plane-wave of light from distant objects Has to be intersected and focused into an instrument
of some type which analyses the information in a useful way.
How can this be done most effectively? Make the collecting area as big as possible (?); Use an instrument that is optimized for the collection
of the required information. eg: Imaging ; Spectroscopy ; Interferometry ; Polarimetry Object morphology - Single or Multiple or Diffuse? What about the time domain?
Limits to spatial Resolution of a Telescope
Resolving power: Wave nature of light => The telescope aperture produces fringe rings that set a limit to the resolution of the telescope.
min = 1.22 (/D)
Resolving power = minimum angular distance min between two objects that can be separated.
For optical wavelengths, this gives
min ~0.1 arcsec / D[m]
min
Seeing (ground-based telescope)
Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images.
Bad seeing Good seeing
Spatial resolution can only be recovered with Adaptive Optics
The Best Location for a Telescope?
Far away from civilization – to avoid light pollution
Chilean Andes ; Island Volcanos (Hawaii or La Palma)
Mount Wilson
Mount Palomar
Kitt Peak
(not any more!)
The Best Location for a Telescope
On high mountain-tops – to avoid atmospheric turbulence ( seeing) and other weather effects
Paranal Observatory (ESO), Chile
So what does a “modern” telescope look like?
Traditional primary mirror: sturdy, heavy to avoid distortions.
Secondary mirror
The 4m class (c1975)
The 4-m Mayall Telescope at Kitt Peak National Observatory
(Arizona)
Equatorial mount(1 axis of rotation)
Advances in Telescope Design (c1980 and beyond)
2. Simpler, stronger mountings (“Alt-azimuth mountings”) to be controlled by computers
1. Lighter mirrors with lighter support structures, to be controlled dynamically by computers
Floppy mirror Segmented mirror
Modern computer technology has made possible significant advances in telescope design:
Examples of Modern Telescope
Design of the Large Binocular
Telescope (LBT)
The Keck I telescope mirror
Examples of Modern Telescope Design (2)
8.1-m mirror of the Gemini Telescopes
The Very Large Telescope (VLT)
The Future(Needs AO to exploit D4
science)
E-ELT – 42m(ESO)
TMT – 30m(Caltech, UC, Canada)
GMT – 25m(Multi-institutes US+)
THE ELECTROMAGNETIC
SPECTRUM Wavelength, frequency, energy units
(convenient working units in any band typically yield numerical values in the range 1–10000) Radio: cm, GHz, or MHz Far-IR/Sub-mm: μm or mm IR: μm UVOIR: °A, μm, or nm EUV: eV or °A X-Ray: keV Gamma Ray: MeV
The Electromagnetic Spectrum
Need satellites to observe
Wavelength
Frequency
High flying air planes or satellites
UVOIR Astronomy
Uniqueness: Best developed instrumentation; Best understood astrophysically; Highest density of astrophysical information;
By accident or design? – don’t ask S. Hawkins? Provides prime diagnostics on the most important
physical tracers. UVOIR observations/identifications are
almost always prerequisites to a thorough understanding of cosmic sources in other EM bands.
Telescope
Focal Plane
Slit
Spectrograph
The generic telescope/spectrograph
collimator
Dispersing element
camera
detector
Where is the re-imaged pupil?
(= Image of Telescope formed by Collimator)
Collimator Camera
Det
Pupil
Astro. Spectrograph (Schematic)
D
A (D)
() 1()
a (d)
F1 F1F2
T’scope
CollCam
Det
x ()
lx
ly y
Fundamental Parameters A = Telescope collecting area
D = diameter = solid angle subtended at telescope aperture
= angle a = beam area of collimator
d = diameter 1 = acceptance solid angle at spectrograph
= angle F1 = focal-ratio of telescope F2 = focal-ratio of spectrograph camera dp = pixel-size pf detector
of linear dimensions: lx-by-ly
The wavelength resolving power (R) of an astronomical spectrograph is given by:
R = /d Etendue (information flux) through any
optical system (eg: telescope to spectrograph) is conserved and given by:
A = a1 (or: D. = d.) (1)
Source with surface brightness, (ergs.s-
1.cm-2.sterad-1) then flux gathered by spectrograph is:
..A. (ergs.s-1) (2)
where: A = Etendue = “Information Throughput” Luminosité = A (= L) LR-product = AR (a general “figure of
merit”)
Pre-area detectors:Says nothing aboutpixelation of data
CCD Array Camera
Semiconductor fabrication limits the size of a CCD detector
To get a large area need to mosaic detectors together
Subaru Mosaic CCD Camera
NB: A implies single circular apertures, but …
Area detectors (eg: CCDs) allows 1-D (y) of spatial information 1-D (x) of spectral () information
Now a given pixel-size (dp) is given by:dp = d.F2.d = D.F2.d (3)
While spatial and spectral multiplexes are given by:
Mx = lx/(2dp) ; My = ly/(2dp) (4)
We can therefore re-define a figure of merit as:
AR Mx My (5) where: A = Entendue A = Luminosité A.R = the LR-product
So: (LR) Mx My = AR Mx My (6)
From equn.(1)
Nyquist sampling
Now includes areadetector advantage
From equations 1 to 6our figure of merit becomes
…(LR) Mx My = ..(/4)2.D..(d/d).d. Mx My
(7)
= ..(/4)2.D..(dp/d).(1/F2). Mx
My (8)
= .(/4)2.R.(lx.ly/4).(1/F22)
(9)
This figure of merit implies that there is no advantage to Large telescopes (D) or Large Spectrographs (d)
But it is dependant on: Camera f-ratio (F2
2) which should be minimized (ie: as fast as possible), and
Detector format (lx.ly) which should be maximized
AngularDispersion
PixelDispersion
Bigger telescopes givesmaller pixels on the sky
Cram more lightinto a given pixels
So … need larger telescope to deliver finer
spatial resolution Practical constraints:
1. Input aperture: x 1” (seeing limit)2. Pixel-size: dp ~20m (fabrication constraint)
3. Camera f-ratio: F2 2 (refractive) and > 1 (Catadioptic)
Constraints 1&2 (with Equn (3)) implies F2.D ~ 8m … and even 8m requires f/1 (Schmidt)
cameras and what do you do for ELTs
Conclusion: Large telescopes do not improve information
gathering capacity but do give improvements in “Information Density”
in units of ergs.s-1.cm-
2.arcsec-2
Need to offer improvements in spatial resolution through the use of Adaptive Optics (AO)
dp = D.F2.d
The Large Telescope Game
Once D > 4m then either (or both) F2 < 2 (not easy) < 1” (requires AO)
For D ~8m and above, AO is essential Unless objects are spatially resolved (like faint
galaxies) For spectroscopy (gratings or FPs)
d/d is intrinsic (ie: fixed for a given configuration)
This means that D d (1/F2) … double bind: The larger the telescope …
The larger the spectrograph, and … The faster the camera Spectrograph cost Dn where n is “large”!
GNIRS in test
The SpectrographUsing a prism (or a grating), light can be split up into different wavelengths (colors!) to produce a spectrum.
Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed object
Grating Spectrograph
Simple grating spectrograph
Spectrum extracted along a slit so ‘imaging’ in one dimension
Off source light along slit used to measure and subtract sky background
What you get
Optical long slit spectrum of a galaxy
Minimal data reduction so far
Can see galaxy, bad pixels and sky lines Need off source
signal to measure and remove Target
Sky lines
Target
Considerations for Spectroscopy
Basic parameters - resolution and central wavelength for spectrum
Slit width (if selectable) affects resolution
Wavelength range Set by combination of detector geometry
and resolution Some spectrographs provide large range
others (eg. PACS on Herschel) provide only a few 1000kms-1 range, so centering on your line critical
Detectors for Opical/near-IR
(current) Photon Counters:
Image tube + TV camera + real-time discrimination (not solid state)
eg: IPCS - c70s to c80s CCDs now dominate - Hi QE but …
Integrate signal on detector – no time resolution Finite read-noise Finite read-time
EMCCDs – new generation of Photon Counters CCD-like QEs V. high frame-rates
DQE - the key to gooddetectors
Detector quantum efficiency - the fraction of incident photons detected - is the key measure for the effectiveness of a detector;
Traditional photographic plates, while large in size, have DQE of only about 10%
CCDs and similar semiconductor devices can have DQE as high as 90% (though wavelength dependent) Like having a telescope with 9 times the
collecting area
CCDs
CCDs combine photon detection with integration and multiplexing
Incident photons excite charge carriers which are stored and integrated in a capacitor
CCDs are also uniquely effective in transferring charge from 2D to 1D charge ‘clocked’ from pixel to pixel and
read out at fixed point ideal for multiplexing
CCD Array Camera
Semiconductor fabrication limits the size of a CCD detector
To get a large area need to mosaic detectors together
Subaru Mosaic CCD Camera
Near-IR Detectors
CCDs use Silicon as their substrate Valance to conduction bandgap in silicon is 1.1eV so
restricted to detecting photons with wavelength < 1 micron
Need different materials for infrared InSb for 1 to 5 micron, HgCdTe for 1 to 2.5 micron Detector elements bonded to Si CCD system to provide
multiplexing readout
IR Arrays vs. Optical
IR arrays are smaller, more expensive Readout has to be faster because of
higher backgrounds Use of different materials can push to
longer wavelengths More difficult to work with, less helpful
characteristics, more expensive At longest wavelengths have to stress the
detector to produce lower energy band gaps
Telescope TrackingEquatorialMounts
German
Fork
English Alt-AzimuthMounts
Telescope FociiPrime Cassegrain NasmythCoude
Equatorial vs. Alt-Azimuth
Alt-Az: • Simpler, more efficient, mechanical structure• Required for 8m class telescopes•Tracking requires 2-axes control• Only possible with computer control• Field rotation while tracking
Equatorial:• 1-axes control (Declination axis stationary)• Simple RA rotation• No image rotation while tracking• Mechanically asymmetric wrt gravity• Only suitable for “small” telescopes (4m or less)
Telescope Guiding
TelescopeField of View Off-set guide
probe patrol regions
InstrumentField of View
Tracking is generallynot good enough over
long exposures
Fourier Transform Spectrometer
• As translation mirror scans an interference pattern is produced that is the FT of the source spectrum• Scan distance defines the resolution of the spectrum• Advantage - get spectrum of whole field• Disadvantage - get broad band noise
Refracting/Reflecting Telescopes
Refracting Telescope:
Lens focuses light onto the focal plane
Reflecting Telescope:
Concave Mirror focuses light onto the focal
plane
Almost all modern telescopes are reflecting telescopes.
Focal length
Focal length
Disadvantages of Refracting Telescopes
• Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect).
Can be corrected, but not eliminated by second lens out of different material.
• Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be
supported at the edges
The Powers of a Telescope (1):
1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to diameter squared:
A = (D/2)2
D
The Powers of a Telescope (3)
3. Magnifying Power = ability of the telescope to make the image appear bigger.
The magnification depends on the ratio of focal lengths of the primary mirror/lens (Fo) and the eyepiece (Fe):
M = Fo/Fe
A larger magnification does not improve the resolving power of the telescope!
Imaging: what to consider
Field of view vs. angular resolution Do you want to cover a large field or do
detailed study of one object? What about seeing and pixel size
restrictions? Photometric accuracy
Do you just want morphology or do you want to measure flux as well?
Passbands and colours Do you want one or more passband?
Spectroscopy: what to consider
What kind of spectral feature are you after? Emission, absorption
Are you looking for line centre and equivalent width only or do you need more detailed line shape and/or photometry? One or many targets?
Simple Astronomical Imager
Simplest astronomical instrument
Takes images of the sky
Can add filters for limited spectral resolution
Can add polarimeter Can read out
repeatedly to get time resolved information
Filter Systems
Various different filter systems in optical/IR UBVRIJHKLM – Johnson/Cousins optical
to mid-IR ugriz - Sloan optical bands Extras and modifications
y - UKIDSS IR band K’ and K* - modifications to K
HST, Spitzer etc. defined by wavelength rather than name
InterferometryRecall: Resolving power of a telescope depends on diameter D:
min = 1.22 /D.
This holds true even if not the entire surface is filled out.
• Combine the signals from several smaller telescopes to simulate one big mirror
Interferometry
STELES echelle spectrograph(for SOAR)
Primary disperser (echelle grating)
Secondary (orthogonal) disperser (VPHG)
Redchannel
Bluechannel
High Resolution and lots ofSpectrum
• X-dispersed echelle grating spectrometers allow high resolution and lots of spectral coverage• Achieve this by having two gratings
• One gives the high resolution the other spreads the spectrum across the detector• But the slit is consequently much shorter
Multiobject Spectroscopy
To get spectra for lots of objects at once can use two approachesMultislit - have several slits in the image plane and get spectra for all of them
Use fibres or some other way of moving light from different parts of the image and reformatting them along the slit
Fibre Fed Systems
AAT 2dF (now AAT 2dF (now replaced by replaced by AAOmega)AAOmega) Pickoff fibres Pickoff fibres
positioned by robotpositioned by robot Include sky fibres Include sky fibres
for each objectfor each object
Multislit spectroscopy
Example of multislit spectrometer
Easier to achieve at telescope (can use holes in a mask) but preparation and reduction more complex
Need to ensure spectra don’t overlap
Integral Field Spectroscopy
One object, lots of spectra Use fibres or ‘image slicer’ to obtain a
spectrum at each point in an image
Other kinds of spectrometers
Narrow band filters Image a field in a single narrow band Use enough narrow bands and you
have very low res. spectroscopy Fabry-Perot
Effectively acts as a narrow tunable filter
Can thus image a field in emission lines of choice (eg. TTF)
Fabry-Perot 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
Typical Instrument Domains
CCD ImagingCCD = Charge-coupled device
• More sensitive than photographic plates• Data can be read directly into computer memory, allowing easy electronic manipulations
Negative image to enhance contrasts
False-color image to visualize brightness contours
Adaptive OpticsComputer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence
Proof
Stars Plasma (to 105K)
UVOIR Astronomy
Definition: UVOIR = the "UV, Optical, Near-Infrared"
region of EM spectrum Shortest wavelength: 912 Å (or 91.2 nm) --- Lyman
edge of H I; interstellar medium is opaque for hundreds of Å below here
Longest wavelength: ~3µm (or 3000 nm) --- serious H2O absorption in Earth's atmosphere above here
Ground-based UVOIR: 0.3µm (or 300nm) < < 2.5µm (or 2,500nm)