RGO 50 is the product of a solar observation programme begun in 1873 at the Royal Observatory, Greenwich and continued, from 1953 to 1979, at Royal Greenwich Observatory, Herstmonceux. It primarily consists of a sequence of glass plate negatives of daily observations. Where no plate was produced at Greenwich or Herstmonceux the gap has been filled by sourcing plates from observatories elsewhere around the world, particularly the Indian observatories at Kodaikanal and Dehra Dūn. Observation times for these plates may be given in Universal (Coordinated) Time (UT) or a location specific time such as Indian Standard Time (IST) or Pacific Standard Time (PST). A sequence of 6798 plates from Royal Observatory, Cape of Good Hope, South Africa (1918-1977) was amalgamated with the sequence at an unknown date. Plates produced at Greenwich from circa 1873-1917 are thought to have been destroyed in the First World War. After 17 January 1977 the plates were produced at roughly weekly intervals.
The class is a continuation of the main part of RGO 51: RO and RGO Solar Plates Contact Prints (1873-1917), which is itself essentially a continuation of RGO 67: Kew Observatory solar photographs (1858-72).
Some additional material in other formats such as 35mm film was obtained from a variety of sources and used to complete or augment the series.
This catalogue has been written to include entries for dates where no plate is known to be extant to allow these gaps to be marked explicitly. Exceptions to this rule are:
- 18-20 September 1920 and 10 April 1947: there are no plates extant and no numbers assigned
- RGO 50/1231-1250, 1464, 3200-3999, 5505, 8921-8930, 9266, 9643, 12929, 14377-14576, 16841-16940, 19391-19399 and 20255: there are gaps in the reference code sequence but there are no plates missing (i.e. date sequence is continuous)
-RGO 11002-11062: the numbers were assigned to dates duplicating plates RGO 10941-11001 and have thus been deleted
Additionally there are entries coded RGO 50/888, 1888, 6434, 6435, 16710 but these do not represent an extant plate.