criteria for population, isolation on the sky, and compactness. He identified 100 HCG’s on the POSS R plates which satisfy 3 criteria:

-   $N \geq 4$ where $N$ is the number of members within 3 magnitudes of the brightest galaxy, $m_{1}$.

-   $\theta_N \geq 3 \theta_G$ where $\theta_{G}$ is the angular radius of the smallest circle which contains the geometric centers of all the suggested group members. The radius $\theta_N$ is the angular radius of the largest concentric circle which contains no further galaxies with $m < m_1 + 3$.

-   $\mu_G < 26$ where $\mu_G$ is the total magnitude of the galaxies in the R band averaged over the circle of radius $\theta_G$, in magnitudes arcsecond$^{-2}$.

Hickson examined the entire POSS including regions at low Galactic latitude. Later, Hickson et al.  (1992) measured radial velocities of galaxies in HCG’s, and defined a set of 92 HCG’s with $N \geq 3$ where $N$ now refers to the number of members within 1000 km s$^{-1}$ of the median group velocity. The median redshift of these HCG’s is 0.030. Prandoni, Iovino & MacGillivray (1994) applied Hickson’s original criteria to automated plate scans and identified 59 candidate compact systems.

These catalogs of systems 