Spectrometer The instrument used for the astronomers MinGyu Kim

Primary applications of spectroscopy SED Spectral classification Radial velocity determination Spectral line formation -abundance analysis-from equivalent widths -line profile analysis(thermal, collisions, rotation) -zeeman effect(magnetic fields) -stellar age(Li abundance, etc.)

The basic spectrograph Slit : isolates portion of sky that is imaged in a single wavelength Collimater : makes beam parallel Dispersing element : diseperses light as a function of wavelength Camera : forms image of object(star or slit) on detector

Dispersion The important design characteristics of a spectrograph are the dispersion(dθ/dλ), which defines how widely the various wavelengths are spread out, and the resolution, which describes the minimum difference in wavelength that can be determined.

dx=dθ·fcam →dx/dλ=dθ/dλ·fcam Here, dx/dλ is properly defined as linear dispersion, astronomers define linear reciprocal dispersion dλ/dx[A/mm]. 50~200A/mm : low dispersion(spectral classification) 10~50A/mm : medium dispersion(radial velocities) <10A/mm are high dispersion(line profiles)

Resolving power is the ability of the components of an imaging device to measure the angular separation of the points in an object. The imaging system’s resolution can be limited either by aberration or by diffraction causing blurring of the image. The minimum angle that can be resolved is δθ=1.22·λ/d radians(angular resolution). The angular resolution may be converted into a spatial resolution, ᅀ l=1.22fλ/D Resolution If blurred image wider than the slit is coming from the universe as below, the resolution is already determined from that.

Types of Spectroscopes Spectroscope -utilized for visual observation of spectra Spectrograph -photographic recording of spectra Spectrometer -obtaining digital recording

Classifications in detail Low Resolution spectrographMedium Resolution spectrograph High Resolution spectrograph PrismGratingEchelleGrism Classify with resolution Classify with the device dispersing light

Prism-based Using principle of refraction, a prism shaped like a triangular. This happens because a ray of light is refracted as it passes from air to glass, and this refraction is slightly different, depending on the specific wavelength of light.

Disadvantage Incoming light is not linearly dispersed, so that the distances between the various constituent wavelengths are not equal.

Diffraction grating-based A diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions(spaced usually from 200 to 1,000 per mm). The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as the dispersive element.

gratings are classified as either reflection or transmission gratings. -Reflection gratings -Transmission gratings :the transmitted beam is diffracted into multiple orders.

Dispersion increases with order m, and the red light is dispersed through a greater angle than blue. In the higher orders, the end of the blue overlap the red. It may be necessary to insert a colored glass filter to isolate the order of interest. The useful spectral range(over which orders will not overlap) is approximately λred- λblue=λcentral/m

Disadvantage Successive, fainter spectra are referred to as a first-order spectrum, second-order spectrum, and so on. This effect, of course, may give rise to significant light losses. Incoming light is not linearly dispersed, so that the distances between the various constituent wavelengths are not equal.

Echelle Desires both high spectral dispersion and broad wavelength coverage. Light passed throw the slit go into the cross dispersers and disperse the light as above figure.

Grism Here a grating has been bonded to the surface of a prism. Incidence angle and the transmittance angle is approximately same so, it disperse the light just put it along the ray. The deviation of the beam by the prism is compensated by the deviation of the central wavelength in the grating resulting in a spectrum centered on the system optical axis.

prism and grating are non linear dispersion(each are stretch out to one direction), however, if you use grism, there is no deviation so either direction is evenly dispersed.

Classification with resolution

High resolution spectrograph(HRS) It can discern fine features in the spectra from astronomical sources by spreading the spectrum out more than the other spectrographs. Low resolution spectrograph(LRS) To see the faint sources and background of the universe.

Medium resolution spectrograph(MRS) MRS allows for higher resolution spectroscopy than the LRS and better sensitivity than the HRS and, therefore, bridges a gap between these two instruments.

Applications for astronomers

A small basic spectroscope A simple grating-based slit spectroscope. Such an instrument on a moderate telescope is capable of providing reasonable spectra of the brighter stars, sufficient for spectral classification, etc.

Conventional Cassegrain spectroscope A grating-based slit spectroscope attached to the Cassegrain focus of a large telescope. It uses a dichroic mirror to split the incoming light into a red and a blue beam, each of which is then separately directed into a spectroscope optimized for those wavelengths. Dichroic mirror,depending on material made of, reflects some light and transmit other light.

Transmission grating spectroscopes Designed to obtain the spectra of up to 100 objects simultaneously at low dispersions and down to about magnitude 23. The spectroscope uses lenses throughout in order to avoid the obstructions caused by the secondary mirrors in reflecting systems. The entrance aperture is a mask with up to 100 precisely positioned slits to match the positions of objects in the FOV. Because of the width of the entrance field, the optics of this spectroscope need to have their aberrations corrected over a much wider FOV than those for normal spectroscopes and this is the reason uses transmission optics.

COUDE spectroscopes This instrument uses reflection gratings. Three separate imaging elements are incorporated into the one mounting and can be selected by changing the angle of the grating. Much better dispersion and spectral resolution possible compared with Cassegrain spectroscope.

Grism-based spectroscope It is designed for low dispersion spectroscopy uses when a high efficiency instrument is required for the study of the spectra of very faint objects.

Multi-Object spectroscopes The TGS can obtain the spectra of up to 100 objects simultaneously, however, a mask containing slits positioned to match the objects to be studied and this has to be made up for every observation.

So, a more flexible system is to use optical fibers to direct the light from the primary mirror to the instrument.

It can observe up to 30objects simultaneously over a 1deg field and the fiber optics are moved to the calculated positions of those objects by computer controlled positioning arms.

Three fibers are carried by each arm. One intercepts the light from the object to be observed(2.6’’). The 2 nd the light from the nearby sky to provide background subtraction(2.6’’). The 3 rd provides a separate direct image for positioning purposes(36’’ by 36’’).

Echelle grating spectroscope An echelle grating can give high dispersions and spectral resolutions, but in order to achieve these it usually has to be used at high spectral orders and so must be allied to a cross disperser. Spectroscope use an echelle grating as the primary disperser and then a low dispersion spherical grating which acts both as the cross disperser and as the imaging element.

Infrared spectroscopes The main difference between such an IR and those of visible is the need to reduce background noise. This require cooling system. It is encased in a vacuum chamber and cooled down below 80K.

Spacecraft-borne spectroscopes Any spectroscope operating at wavelengths shorter than about 350nm, in the longer IR or microwave regions, has to be lifted above the Earth’s atmosphere. Two detectors : one is for shorter wavelength and the other is for longer. Two alternative imaging concave mirrors Two cross dispersing concave gratings

Fabry-Perot spectroscopes A filter isolates one of the transmitted orders, and the beam is re-imaged. Fabry-perot etalons are most likely to be found in an observatory in use as narrow band filters. In operation, the spacing of the etalon is varied to scan its transmitted wavelength over the free spectral range of the order being used. Etalons : various, with gaps ranging from 15 to 500um, velocity resolutions ranging from 7 to 150km/s, spectral resolutions ranging from 500 to 60,000. Dispersion is caused by the interference between the multiple reflections of light between the two reflecting surfaces. Constructive interference occurs if the transmitted beams are in phase, and this corresponds to a high-transmission peak of the etalon