The 1873-1917 prints in this class were made from glass plate negatives which are no longer extant. The plates were the product of a solar observation programme begun in 1873 at the Royal Observatory, Greenwich but are thought to have been destroyed in the First World War. The sequence of images is a continuation of RGO 67: Kew Observatory solar photographs (1858-72) and is itself continued by RGO 50: Royal Observatory Greenwich and Royal Greenwich Observatory, Herstmonceux Solar Plates (1918-79).
The 1955 prints are of images made at different times of day from the plates in RGO 50 and thus do not replicate that series. The 1956-72 prints were made from plates extant in RGO 50. Gaps in the sequence of RGO 50 were filled by sourcing plates from other observatories and thus the prints in RGO 51 also include some images created in locations other than Greenwich and Herstmonceux.
From the early 20th century the prints show the hand drawn circles, arrows and reference codes made on the glass plate negatives to mark sun spots and foculae.