Astronomical Instrumentation Often, astronomers use additional optics between the telescope optics...
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Transcript of Astronomical Instrumentation Often, astronomers use additional optics between the telescope optics...
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
12345678
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