Astronomical Instrumentation

Often, astronomers use additional optics between the telescope optics and their detectors. This is called the instrumentation. It can range from a verysimple colour filter to a highly-sophisticated multi-beam spectrograph.Observatories try to provide instruments with a range of spectral resolutioncovering a wide range in wavelength.

Spectral resolution, defined as can be as low as 4 for imaging in the

I-band filter, where =800 nm and =200 nm, and as high as 1000000 fora ultra high-resolution spectrograph, with velocity resolution of 300 m/s.

Instruments available for the Subaru Telescope (Japan)

Imagers

These are the simplest instrument. Even though imaging instruments can workwithout any optics, they usually do contain several optical components:

• Filters – select a certain wavelength range

• Focal Reducers – change the scale at the detector. For a large telescope the field is usually small (size inversely proportional to telescope diameter), and in this way the scale at the detector can be enlarged.

As well as components like baffles, coronographs, etc., to avoid contaminationfrom ambient light.

Spectrographs

Refraction: Refraction: Snell’s Law: n1 sin(1) = n2 sin(2)

2

1 n1

n2

n1 = refractive index in region 1

n2 = refractive index in region 2

n = c / v = vacuum /medium

Diffraction grating:Diffraction grating:

Telescope

Focal Plane

SlitSpectrograph

SpectrographSpectrograph

collimator

Dispersing element

camera

detector

Grating Equation: mλ = d(sinß + sin α) m = order number λ=wavelength d = groove distance α=incident angle ß=diffracted angle dß /dλ = m /d cosßAngular dispersion can be increased either by increasing the number of grooves/mm (smaller d) or by working at high orders m (Echelle spectrographs).

The higher the orders, the smaller will be the FREE SPECTRAL RANGE: two wavelengths in fact will overlap as soon as mλ = (m+1) λ’

Dλ = λ’ - λ = λ/m

The higher the order, the shorter is the FREE SPECTRAL RANGE. Given angular dispersion and wavelength, the larger is the number of lines/mm, the larger is the FREE SPECTRAL RANGE.

Types of grating spectrographsTypes of grating spectrographs

• Grating & prism spectrographs with collimator/camera optics - long spectrum (linear format)

• Echelle spectrometers - cross-dispersion, square format

• Objective prism or grating - slitless spectroscopy

• grism (grating/prism) - insert into optical path of a camera

• Integral-field spectrographs

• Multi-object spectrographs

Cross Dispersion

12345678

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Echelle grating(used in high order #)

Cross-disperser (used in low order)slit

Detector focal plane

Keck HIRES:HH 444

[SII]

Telluric O2

H [NII]

[SII]

The Solar Spectrum (from Kitt Peak’s McMath-Pierce Solar Telescope): 2960 – 13000 angstroms

Integral Field Spectroscopy – Obtaining Spectra in 2D

Pupil array

Spectra

Integral Field SpectroscopyIntegral Field Spectroscopy

(e.g. SAURON)

Reflect: X = n /4 n = 1, 3, 5, ….

X

Transmit: X = n /4 n =0, 2, 4, ….

Interference filtersInterference filtersTunable Fabry-Perot filtersTunable Fabry-Perot filters

Detectors

Detectors in Astronomy

The most common arrays currently in use are:

- CCDs (Charged Coupled Devices)

- Photomultipliers

- Photographic Plates (although very seldom used)

- Infrared Array Detectors

Detector properties to be considered:

CCD Photo- IR Array Phot. Multiplier plate------------------------------------------------------------------------------------------Quantum Efficiency 80% 10-20% 80% 0.1%

Size 6cm 5cm 2cm 50cm

Resolution 9m no 10m 10m

Readout Noise 1-2 e- no 10 e- -

Dark Current negl. Low low low

Linearity linear linear needs non-lin. correction

Dynamic Range high (105) low high low

CCDsCCDs

• Properties - Cosmic Rays: 5 to > 103 e- produced by each charged particle usually effects 1 or few pixels. non-gaussian charge distribution (different from stellar image or PSF) - Well depth: 5 x 104 to 106 e- - Pixel size: 6 m to 30 m - Array size: 512 x 512 to 4096 x 4096

Charge TransferCharge Transfer05

10

05

10

5100

V

Charge Coupled Devices (CCDs)Charge Coupled Devices (CCDs)

Output amplifier