The following is a  guide to understanding the plate IDs for the Mt. Wilson Plate Archive. When the plates were inventoried, it was decided that only information printed on the envelope would be stored in the database. As such, there are many fields that lack information. As an example, searching the PAST database for the observer "Hubble" yields only 55 "hits"! But if one knows that the plate ID suffix "H" stands for Hubble, you can search the plate IDs ending with "H" and get a more respectable 2976 hits. It is hoped the following information will help with such queries. Credit goes to Francois Schweizer for compiling all of this.

Plate IDs 01:  Generalities

In the good old days, each telescope, focus, and/or instrument had a unique prefix and the plate identification (ID). These ID prefixes were adhered to by most observers with the introduction of regular plate logs at the telescopes, post World War II (see Sandage reference below). Unfortunately, in the early Mt. Wilson days, they were often not adhered to, and observers routinely had their own way of labeling envelopes. Only the running plate number was pretty universally used.


  • Sandage, A., The Hubble Atlas of Galaxies (1961), p. 27
  • Arp, H., Atlas of Peculiar Galaxies (1966) (plate listing)

Plate IDs 02: Direct Plates (including Image-Tube Plates)

Plate IDs are of the form X-NNNN-Y, where the prefix X refers to the telescope, the suffix Y refers to the observer, and NNNN is a running number.

Observatory Telescope and Prefix Observers and Suffixes
Mt. Wilson
60-inch S-    (Sixty-inch)
(Observer letters could also be prefixes!)
100-inch H-    (Hooker telescope or Hundred-inch)



-A Adams (guessed)
-B Baade
-Delta Duncan (capital greek Delta)
-H Hubble
-M van Maanen (guess)
-Mi Minkowski
-MH (Milton) Humason
-P Pease
-Ri Ritchey
-S Sandage
200-inch PH-    (Palomar Hale telescope)
48-inch Schmidt PS-    
(Palomar Schmidt camera)
Note: observer name not part of plate ID
60-inch PR-     (Palomar Ritchey-Chretien tel.)
Same as Mt. Wilson, plus the following:
-A Arp
-Bm Baum
-M Minkowski (as used by Arp)
-Z Zwicky (used by Arp)
Las Campanas
40-inch CF-  (Campanas Fourty-inch)
100-inch CD-  (Campanas DuPont)
  CD-IT- (Campanas DuPont Image-Tube)
Observers were listed in log books instead of by suffix. These plates tend to have more complete entries in the database.



S-1732-H Plate #1732, 60-inch Mt. Wilson, obtained by Hubble
S-330-P Plate #330, 60-inch Mt. Wilson, obtained by Pease
H-1607-H Plate #1607, 100-inch Mt. Wilson, obtained by Hubble
H-2524-M Plate #2524, 100-inch Mt. Wilson, obtained by van Maanen
A-93 Plate #93, taken by Adams (?), probably 60-inch Mt. Wilson
PH-20-Bm Plate #20, Palomar 200-inch, obtained by Baum
PS 21122 Plate #21122, Palomar-Schmidt. No observer suffix, but observer field (Arp) is listed in the database.
PR 266 Plate #266, Palomar 60-inch. No observer suffix, but observer (Schweizer) is listed in the database.