Slitless

Slitless refers to a mode of spectroscopy in which light from every object in a field is dispersed without using a slit to isolate a single source. A dispersive element, such as a grism or prism, is placed in the optical path so that spectra from all objects project onto the detector simultaneously. Without a slit, the spectral resolution is determined mainly by the angular size of each source along the dispersion direction, making extended objects yield lower resolution while compact sources can show higher resolution.

Because there is no slit to confine light, slitless spectroscopy provides spectra for many objects in a single exposure, enabling efficient field-wide surveys of emission lines and redshifts. It is particularly useful for identifying emission-line galaxies, measuring redshifts of faint galaxies, and detecting high-redshift Lyman-alpha emitters. The technique is especially powerful on space telescopes, where the absence of atmospheric effects and sky lines enhances sensitivity and stability. Notable implementations include the Hubble Space Telescope with ACS grisms (G800L) and the WFC3 grisms (G102, G141), used in programs such as PEARS and 3D-HST.

Challenges include spectral overlap between neighboring objects, variable spectral resolution tied to object size, and complex background subtraction. Data reduction commonly relies on auxiliary direct imaging to locate sources and specialized software to model and extract spectra while mitigating contamination, such as the aXe package.

Slitless spectroscopy is complementary to traditional slit spectroscopy. Its strength lies in survey efficiency and unbiased sampling, while its limitations concern resolution, contamination, and redshift precision in crowded fields.