Newly calibrated white-light data starts from 1921 till 2011. The Ca-K database starts from 1907-2007 whereas the H-alpha digitized images are available since 1912-1979.
We have chosen to limit the size of our download package to 1000 images, taking into account available buffer space for storing the package on the server as well as a reasonable number of images that can be displayed to the user within a browser window.
The difference is in their observing mode. The prominence data was taken by blocking the disc in-order to observe the prominences which are the faint limb features, otherwise not visible due to the bright disc emission. More details are available in the data (hyperlink) page.
The tolerance value is used when there is no image available for a given cadence (days/weeks/months). A search is then conducted in the database (across the tolerance range centered around the cadence) to fetch the nearest available image.
Generally the typical frequency is one image per day. There are occasions though when multiple images (separated by 30 to 60 minutes) have also been recorded.
This has been assigned by the individual operators after inspecting the images visually at the time of digitization. The 'quality' has been scaled between 1 to 9 where 1 being the best quality image and 9 being the worst. As mentioned above, this factor is somewhat 'subjective' and thus users are free to set their own criteria to judge the quality of an image.