Every plate is assign a class designation to indicate the type of exposure that was taken. The following table comes primarily
from the Harvard Annals, Vol 26, page 9. This list is divided into three groups, indicated by the
- Revised classes for Nov 1933, Cambridge. January 1934, Bloemfontein
- Examples of classifications in combination
- Plate classifications for B plates only. Martha Hazen wrote:
MLH note 24 Nov 1993: These classes were used for northern hemisphere plates taken before the Bache moved south. I have not ascertained what codes were used after this.Alison Doan notes in November, 2010: This list
pertains only to B 1 - 3300, according to Martha's prediction, and about 200 early C's and X 1-3800. I just looked at the Annals on ADS and it looks as if this system was used briefly after the move south, too (til 1891). The two classes that persist decades later are Class I (spectra, also often class o) and Class H, chart/long exposure direct.
The list from the above references is incomplete and does not account for all of the classes that appear in the database. These
additional classes are marked with the
derived note and given a description that follows the examples given in the above
reference. Nevertheless, there are still several classes that must be marked
unknown class until additional information is
Curator Alison Doane notes that
chart plate is an ordinary plate that documents the sky, like a patrol plate.