Astronomical Spectroscopy

MEGA SHORT REVIEW OF

Astronomical Spectroscopy
P. Massey & M. M. Hanson

A basic astronomical spectrograph contains: a slit on to which focused the light from

the source; a collimator, which would take the diverging light beam and turn it into para-

llel light; a disperser; and a camera that would then focus the spectrum onto the detector.

Spectroscopy is a fundamental tool in astronomy, since allows to determine the chemical

compositions, physical properties and radial velocities of astronomical sources.

There are different kinds of blocking filters in order to achieve the wanted wavelength

observation and avoid order overlapping.

Conventional Long-Slit spectrographs are the most versatile, allowing to take spectra of

bright or faint sources and obtain spatially resolved spectra of extended sources. Some steps

involved in the reduction of Long-slit spectrographs data are as followed: Over scan (20-40

columns on the right side of the chip) must be fitted and subtracted. Trim the blank areas. If

the CCD camera had bias structure, it must be removed. Then, depending either one want to

flux calibrate or normalise the spectrum, one should choose the best flat normalisation. After

this, one must remove the non-uniformity in the spatial direction (slit illumination function)

and identify the sky and the object. After this, one needs to identify and fit the lines to the

pixel number in order to perform the wavelength calibration. Finally, one must normalise

the spectrum (in order to perform abundance analysis or measuring radial velocities) or flux

calibrate (in order to model the spectral energy distribution or determine reddening among

others) the spectrum.

Echelle spectrographs avoids to waste detector area by cross dispersing the various or-

ders, spreading them across the detector. This brings some difficulties of flat-fielding.

Multi-objects spectrometers uses an slit mask containing dozen or even hundreds of slits

(Multi-slit spectrograph) or multi-object fibre positioner (Fiber-fed Bench-Mounted spectro-

graphs) to allow multiple objects observation. Multi-object reduction is basically the same as

mentioned in long-slit spectrographs, but there are some differences. For fibers, sky subtrac-

tion must be done by having some fibers assigned to blank sky. For multi-slits, there will be

a very challenging reduction, since the wavelength coverage of each slitlet will depend upon

its placement in the field.

Fabry-Perots and Integral Field spectroscopy obtain spatially resolved spectroscopy of an

extended source.

When observing with a NIR spectrometer, there must be some changes in the reduction,

since the detectors are different, but mainly it presents significant background radiation and

strong absorption from the Earths atmosphere. In order to remove this features, telluric stan-

dards observations must be made with similar declination to the target object, but to transit

30-60 minutes before or after in order to get similar air masses.