Table 1
Ca II K digitised image archives considered in our study.
| Observatory | Years | Number of images | Spectral width | Disc size | Data type | Pixel scale | Reference |
| [nm] | [mm] | [bit] | [′′/pixel] | ||||
|
|
|||||||
| Arcetri | 1931−1974 | 5141 | 0.03 | 65 | 16 | 2.4 | 1 |
| Coimbra | 1994−1996 | 79 | 0.016 | 87 | 16 | 1.8 | 2 |
| Kodaikanala | 1907−1999 | 22164 | 0.05 | 60 | 8 | 1.3 | 3 |
| Meudon | 1980−2002 | 5727 | 0.015 | 86 | 16 | 1.5 | 4 |
| Mitaka | 1917−1974 | 8585 | 0.05 | variable | 16, 8b | 0.9, 2.9b | 5 |
| McMath-Hulbert | 1942−1979 | 5593 | 0.01 | 17.3 | 8 | 3 | 6 |
| Mt. Wilson | 1915−1985 | 36147 | 0.02 | 50 | 16 | 2.7 | 7 |
| Rome/PSPT | 2000−2014 | 2000 | 0.25 | ~27 | 16 | 2c | 8 |
Notes. Columns are: name of the observatory, period of observations, number of images, spectral width of the spectrograph/filter, the size of the solar disc on the plate, digitisation depth, pixel scale of the digitised file, and the bibliography entry.
These data have more recently been re-digitised with 16-bit data type extending the dataset from 1904 to 2007 (Priyal et al. 2013; Chatterjee et al. 2016).
The two values correspond to the period before and after 02/03/1960, when the images were stored on photographic plates and on 24 × 35 mm sheet film, respectively.
Reference.(1) Ermolli et al. (2009a);(2) Garcia et al. (2011);(3) Makarov et al. (2004);(4) Mein & Ribes (1990);(5) Hanaoka (2013);(6) Fredga (1971);(7) Bertello et al. (2010);(8) Ermolli et al. (2007).
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