Long-slit spectroscopy

In astronomy, long-slit spectroscopy involves observing a celestial object using a spectrograph in which the entrance aperture is an elongated, narrow slit.

When the spectrographic slit extends over the diameter of a nebula, the lines of the velocity profile meet at the edges.

The tapered edges of the velocity profile stem from the fact that the material at the edge of the nebula is moving perpendicular to the line of sight and so its line of sight velocity will be zero relative to the rest of the nebula.

Long-slit spectroscopy can ameliorate problems with contrast when observing structures near a very luminous source.

The structure in question can be observed through a slit, thus occulting the luminous source and allowing a greater signal-to-noise ratio.

Observation through a long slit allows simultaneously taking spectrographs of all parts of the objects which fall onto the slit. When observing spectral lines, different Doppler shifts can be observed along a given spectral line, leading to velocity profiles of the object along the slit.
Typical velocity profiles of several nebulae observed using long-slit spectroscopy.