"A new 48-inch-diameter corrector plate was made by Grubb-Parsons in England and installed in the telescope. This new plate produces better images over a wider range of wavelengths than the original." See: http://www.nps.gov/history/history/online_books/butowsky5/astro4f.htm
For details of this telescope see: http://www.astro.caltech.edu/palomarnew/sot.html