Astronomical Instrumentation Often, astronomers use additional optics between the telescope optics and their detectors. This is called the instrumentation. It can range from a very simple colour filter to a highly-sophisticated multi-beam spectrograph. Observatories try to provide instruments with a range of spectral resolution covering 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 for a 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 work without 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 contamination from ambient light.

Spectrographs

Refraction: Snell’s Law: n 1 sin(  1 ) = n 2 sin(  2 ) 22 11 n1n1 n 2 n 1 = refractive index in region 1 n 2 = refractive index in region 2 n = c / v = vacuum /  medium

Diffraction grating:

Telescope Focal Plane Slit Spectrograph Spectrograph 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 cosß Angular 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 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 Echelle grating (used in high order #) Cross-disperser (used in low order) slit Detector focal plane

Keck HIRES: HH 444 [SII] Telluric O 2 HH [NII] [SII]

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

Integral Field Spectroscopy – Obtaining Spectra in 2D

Pupil array Spectra Integral Field Spectroscopy (e.g. SAURON)

Reflect: X = n /4 n = 1, 3, 5, …. X Transmit: X = n /4 n =0, 2, 4, …. Interference filters Tunable 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 9  m no 10  m 10  m Readout Noise 1-2 e - no 10 e - - Dark Current negl. Low low low Linearity linear linear needs non-lin. correction Dynamic Range high (10 5 ) low high low

CCDs CCDs Properties - Cosmic Rays: 5 to > 10 3 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 10 4 to 10 6 e - - Pixel size: 6  m to 30  m - Array size: 512 x 512 to 4096 x 4096

Charge Transfer V

Charge Coupled Devices (CCDs) Charge Coupled Devices (CCDs) Output amplifier