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)