I would like to continue this review in a similarly light-hearted vein while giving the reader as comprehensive review as possible of X-ray cluster surveys.
X-ray astronomy began on the 18th of June 1962 with the detection of the X-ray background and Sco-X1 by a sounding rocket experiment (Giacconi et al. 1962). For shorthand in this review, this will be denoted as 0 Anno Giacconi (AG), and subsequent events will be quoted in these units.
During the first three years of sounding rocket experiments from 0 AG several cluster detections were in dispute [e.g., Coma claimed by Boldt et al. (1966) and discounted by Friedman & Byram (1967)], so the first unambiguous cluster detection came in 4 AG when Byram, Chubb, & Freidman (1966) detected M87 / Virgo.
The numerous sounding rocket campaigns that occurred between 1962 and 1975 resulted in several more cluster detections (e.g., Perseus; Fritz et al. 1971) and the discovery that the X-ray emission in Coma was extended (Meekins et al. 1971). Unfortunately the collecting area and exposure time of these experiments ruled out the detection of all but the brightest few clusters.
On 12th December 1970 the first X-ray satellite, UHURU, was launched. The two-year lifetime of the mission allowed the whole sky to be scanned many times, producing the first true X-ray survey. The fourth and final UHURU catalog (4U; Forman et al. 1978) contains 52 clusters, several of which that were not known to be clusters at the time.
2.2. The End of the First Age of X-ray Astronomy?
UHURU was the first of a number of increasingly more complicated experiments that allowed further surveys and dedicated pointed observations. Most notable of these was Ariel-V (Cooke et al. 1978), which made the first iron line detection in a cluster (Mitchell et al. 1976).
The final mission in this series, HEAO-1, made the deepest X-ray survey (Piccinotti et al. 1982), which was the mainstay of X-ray astronomy for the two decades that followed. So at the end of this exciting period of X-ray astronomy, how well does "Edge's Law" stand up? At 17 AG a total of 95 clusters are known, which is well above the 50 required by this point.