Focus Shift in z-band

In summer 2001 new PT filters for u, g, r, and i band were installed while the existing z-band filter was retained. The new filters (u, g, r, and i) were specified to be 5.0 mm thick while the old z band filter is 5.4 mm thick. The thickness difference causes the focus shift shown in the two images below.

i band, in focus

z band, not focused

These images were obtained November 11, 2001 (MJD 52225) about two minutes apart. Telescope focus was adjusted less than 30 minutes before. The i band image is reasonably focused but the z band is two or three units out (where our tolerance is +/-1 unit; the elongation is a sign of astigmatism in the secondary mirror).

November 15, 2001