observationsvolym

observationsvolym is the region of space or parameter space accessible to a given set of observations under a specific survey design and instrument sensitivity. In astronomy and cosmology, the term describes the comoving or physical volume within which objects can be detected by a survey, given flux or magnitude limits, sky coverage, and redshift range. The volume depends on cosmology and the instrument’s sensitivity and is central to deriving number densities and luminosity functions from observed counts.

Formally, if a survey covers a solid angle Ω and spans redshifts z1 to z2 with a selection function S(z,L) giving the detection probability for an object of luminosity L at redshift z, the observed volume is V_obs = ∫Ω ∫_{z1}^{z2} S(z,L) dV/dz dΩ dz. For flux-limited samples, L_min(z) is set by the flux limit; the maximum redshift z_max for a source of luminosity L satisfies L = 4π d_L^2(z_max) F_lim × K(z_max). This framework allows converting counts into space densities and calibrating selection effects.

Examples are common in galaxy surveys such as SDSS and DES, where the survey volume can reach tens to hundreds of cubic gigaparsecs for bright populations, with smaller volumes for fainter or more narrowly defined samples. The concept also applies to other observational sciences where data collection defines the explored region of parameter space, including particle physics experiments with detector acceptance or ecological surveys limited by detection thresholds.

See also: selection function, completeness, luminosity function, comoving volume.