Slitless spectroscopy is

spectroscopy Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum. Spectro ...

done without a small slit to allow only light from a small region to be diffracted. It works best in sparsely populated fields, as it spreads each point source out into its spectrum, and crowded fields can be too confused to be useful for some applications. It also faces the problem that for extended sources, nearby emission lines will overlap. This technique is a basic form of

snapshot hyperspectral imaging Snapshot hyperspectral imaging is a method for capturing hyperspectral images during a single integration time of a detector array. No scanning is involved with this method, in contrast to push broom and whisk broom scanning techniques. The lac ...

. Slitless spectroscopy is used for astronomical surveys and in fields, such as

solar physics Solar physics is the branch of astrophysics that specializes in the study of the Sun. It intersects with many disciplines of pure physics and astrophysics. Because the Sun is uniquely situated for close-range observing (other stars cannot be re ...

, where time evolution is important. Both types of application benefit from higher speed operation of a slitless spectrograph: conventional spectrographs require multiple exposures, scanning the slit across the target, to acquire a complete spectral image, while a slitless spectrograph can capture a complete image plane in one exposure. The

Crossley telescope The Crossley telescope is a reflecting telescope located at Lick Observatory in the U.S. state of California. It was used between 1895 and 2010, and was donated to the observatory by Edward Crossley, its namesake. It was the largest glass ref ...

utilized a slitless spectrograph that was originally employed by

Nicholas Mayall Nicholas Ulrich Mayall (May 9, 1906 – January 5, 1993) was an American observational astronomer. After obtaining his doctorate from the University of California, Berkeley, Mayall worked at the Lick Observatory, where he remained from 1934 to 1 ...

. The ''

Henry Draper Catalogue The ''Henry Draper Catalogue'' (HD) is an astronomical star catalogue published between 1918 and 1924, giving spectroscopic classifications for 225,300 stars; it was later expanded by the ''Henry Draper Extension'' (HDE), published between 192 ...

'', published 1924, contains stellar classifications for hundreds of thousands of stars, based on spectra taken with the objective prism method at

Harvard College Observatory The Harvard College Observatory (HCO) is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Harvard University Department of Astronomy. It is located in Cambridge, Massachusetts, United St ...

. The work of classification was led initially by

Williamina Fleming Williamina Paton Stevens Fleming (15 May 1857 – 21 May 1911) was a pioneering Scottish astronomer, who made significant contributions to the field despite facing gender biases. She was a single mother hired by the director of the Harvard Co ...

and later by

Annie Jump Cannon Annie Jump Cannon (; December 11, 1863 – April 13, 1941) was an American astronomer whose cataloging work was instrumental in the development of contemporary stellar classification. With Edward C. Pickering, she is credited with the creation of ...

, with contributions from many other female astronomers including

Florence Cushman Florence Cushman (1860–1940) was an American astronomer specializing in stellar classification at the Harvard College Observatory who worked on the ''Henry Draper Catalogue''. Life Florence was born in Boston, Massachusetts, and received h ...

. Slitless spectrographs encounter an unusual form of

specular reflection Specular reflection, or regular reflection, is the mirror-like reflection (physics), reflection of waves, such as light, from a surface. The law of reflection states that a reflected ray (optics), ray of light emerges from the reflecting surf ...

at the grating, which leads to anisotropic image distortion called

Littrow expansion Littrow expansion and its counterpart Littrow compression are optical effects associated with slitless imaging spectrographs. These effects are named after Austrian physicist Otto von Littrow. In a slitless imaging spectrograph, light is focused w ...

or compression. The distortion occurs because the normal rules of specular reflection do not apply to reflective gratings operated far from the non-dispersive reflection angle.

References

{{Reflist Astronomical spectroscopy

See also

References