Astronomical spectrographs. ASTR320 Wednesday February 20, 2019

Transcription

1 Astronomical spectrographs ASTR320 Wednesday February 20, 2019

2 Spectrographs A spectrograph is an instrument used to form a spectrum of an object Much higher spectral resolutions than possible with multiband imaging A typical spectrograph has the following parts: Entrance aperture, slit-shaped or fiber-fed (round) Optical system to collimate diverging light; make rays parallel so that all rays approach dispersing device at same angle Dispersing device (e.g., prism or diffraction grating) Camera to focus the image of the dispersed light onto a detector (photographic plate, CCD)

3 Spectrographs

4 Dispersing element Can be a prism, etalon, grating, or grism We ll talk about reflective diffraction gratings as an example

5 Diffraction grating Based on the concept of diffraction with multiple slits: By increasing the number of equal slits (in spacing and width) into the beam, we narrow the width of the individual peaks As the number of slits is increased to large numbers, we can make the peaks arbitrarily narrow Thus, by increasing the number of slits we can arbitrarily set the resolution, or the narrowness of the lines

6 Diffraction grating When coupled with the fact that the locations of the sets of peaks shift as a function of wavelength, you can see that the ability to discriminate or resolve different wavelengths directly depends on the number of slits, and the interference order, m. Portion of the image structure for a single bichromatic point source viewed through several slits. From Kitchin's Astrophysical Techniques. Thus, one can increase the wavelength resolution by: increasing the number of slits the beam "sees" (i.e., decreasing the groove or slit spacing, d, which is the same as increasing the number of grooves or slits per inch). increasing the interference order, m, that you choose to observe.

7 Reflection grating Basically a large number of long, very very thin parallel mirrors Made by scoring a piece of glass with many fine, parallel, closely spaced grooves or slits (~ several x 10 3 or more per inch) that act to make multislit diffraction patterns, one diffraction pattern for each wavelength

8 Reflection grating The grating as a whole acts as a mirror, but is convoluted with the diffracting effect of all of the parallel reflecting lines For a given grating line density, diffraction order, and angle α of collimated input, each wavelength has a unique diffraction angle β: m = d(sinα + sinβ) This separates the wavelengths to produce the spectrum recorded by the spectrograph

9 The grating equation Same as diffraction/interference equation, except now consider two pathlengths, the incoming and outgoing rays, which together must have a combined pathlength difference that is an integer multiple of the wavelength in order to create a maximum (and a half-integer multiple for a minimum): m = d(sinα + sinβ) Where d = space between adjacent grooves or slits α = angle of incidence of collimated beam to grating. β = angle of emergence of a ray of certain wavelength from the grating. λ = wavelength m = order of interference (most commonly a small number) = number assigned to each fringe in the diffraction pattern

10 Long slit spectrograph The most basic type of spectrograph Slit is used to block light from everything except for the object of interest (not a diffractive slit!)

11 Long slit spectrograph

12 GMOS/Gemini mask design software Multi-object spectrographs Clever instrument design can enable observation of hundreds of objects in a single exposure SDSS plug plate

13 GMACS for GMT: multi-object spectroscopy

14 GMACS optical layout

15 GMACS design concept

16 Cross-dispersed echelle spectrographs

17 G-CLEF High Resolution Spectrograph High Resolution Spectrograph and Precision Doppler Spectrometer Smithsonian Astrophysical Observatory R = 50, ,000 Spectrograph Core Science: PRV, Exoplanet Spectra, Abundances, IGM/ICM

18 Maunakea Spectroscopic Explorer: Massively multiplexed spectroscopy

19 MSE low-resolution spectrographs Low-resolution spectrographs High-resolution spectrographs

20 MSE high-resolution spectrographs Low-resolution spectrographs High-resolution spectrographs

21 MSE fiber positioners Fiber positioners MSE focal plane