Knut Lundmark worked throughout his life with a rich correspondence with the outside world. Large collections of letters can be found at the Lundmark Collection at Lund University Library's Handwriting Department in 236 capsules (Collection LUNDMARK, Knut, revised in turn, not yet finalized) and in the archive at Norrbottens museum, Luleå.
Karin Rönnbäck, archivist at Norrbottens museum, tells about the Luleå collection, which contains several rare documents:
- Here in Luleå we have quite a large collection, donated by Älvsby Hembygdsförening. The collection is originally from Turku University, later presented to Älvsbyn Upper Secondary School, before moving on to the Homestead Society and finally the Norrbotten Museum.
- The most important part is scientific literature, both his own works and others; books, journals, theses, etc., often bound in fine leather bands, one for Professor Lundmark Library.
- In addition, there are about thirty albums with newspaper clippings, ranging from 1907 until about 1945, where Knut Lundmark himself grabbed all articles and notices that caught his interest (of course astronomy).
- Finally, there is a fairly extensive, mostly private correspondence (his own and wife Birgittas), some photographs, photo albums and related matters, farms, etc.
- Sorry, the material is unlisted, but to some extent arranged and searchable for the interested visitor.
- In total, the collection covers 30 shelves in the archives of Norrbottens museum, and a single volume in the Folk Movement's archives in Norrbotten, including the private correspondence and similar documents.
A third smaller collection was included in Martin Johnson's possessions, a collection that was testamentarily transferred to Ulf R Johansson's ownership in autumn 2011 and later on donated to Lund University Library, the Lundmark Collection. A handful of rarities in this third collection are reproduced here. Some of the letters relate to Lundmark's call to the astronomers worldwide that they should participate in the celebration of the Helsingborg-born Copenhagen professor at the observatory Elis Strömgren (1870-1947), a publication which also came out as planned during the Second World War.