1.2m (48'') Telescope Optical System

Created 08/27/97 by NC
Updated 08/19/18 by EF

In 2013, we replaced the old primary, see this link for a description of the specification of the new mirror, which was a drop-in replacement for the old one but of much higher quality.

Here is the 1.2m (48'') Optical System, both as designed, and as built, for the old mirror. Units are inches.

Design Built
D1 48
R1 -180.743 -180.78
R2 -60.2484 -60.342
d 67.611 67.566
K1 -1.040231
K2 -3.083152
m 4.091
BFD 25.50
f 369.7011
F 7.7021
D2 13.0
D2(IR) 11.6
Rcorr -7.56
BFD with Corr 26.377
Thick. of Corr .275
BFD (from corr) .347
F(w/corr) 7.98
f(w/corr) 383.14
Note: The corrector listed is a simple field flattener, not the doublet currently in use (since Aug 97).

Note: Using the BFD without the corrector, the distance from the mounting flange to the focal plane is
MFFP=25.50-7.17-8.01=10.32in = 262.1mm
25.50 is BFD w/o corrector
7.17=8.15 (thickness of M1) - 1.6 (central sag of M1)
8.01=17.63 (height of cell) - 9.62 (top of cell to bottom of M1)

The primary central hole is 13'' in diameter, with the inner 0.5" used for an alignment surface. Likewise the outer 0.5" of the mirror is masked. Therefore the usable surface is from R=7 to R=23.5. The honeycomb cells are 7.5'' in diameter; their ribs are 3/8'' thick. The front plate is 1.1''; the back face is 0.8'', with 36 holes each centered on the 7.5'' honeycomb cells. Of these holes, 30 are 3.5'' in diameter and the other 6 are 2.275'' in diameter. The thickness of the mirror at the outer radius is 8.15''. The surface tolerance was 0.1 wave RMS at 632nm throughout the surface.

New Mirror: SOML (PDF, 2013 finish document)

Old Mirror: 6:1 DRAWING (PDF, 1988 as cast)

The secondaries have alignment surfaces that are 3'' in diameter, including the central hole (1'' in diameter).

The neutral point of this telescope can be found via
NP= L/tan a
where a = -L/s2/[1+1/(m-b)/(m-1)]
and a is the angle of tilt, L is the linear amount of translation off axis of the secondary, m is the magnification (4.019), b is the ratio of back focal distance to the primary focal length (0.2821), and s2 is the distance from the secondary vertex to the prime focus (579.6mm). Chose a small L (1mm) or a to derive this point. For the 48", the neutral point is 534mm behind the secondary vertex, or 21.04 inches.

Baffles for 3'' field:

Primary: 8.94'' diameter, rising to 32.24'' above the primary vertex. That is for an infinitely thin tube. The height of tube should be compensated for the tube thickness by decreasing it by

Delta h = 7.428 * thickness

Secondary: 15.3'' in diameter, extending 4.59'' in front of secondary vertex. Tube length should be decreased by

Delta h = 2.709 * thickness

to account for the finite thickness of material.

  • Drawing of secondary mirror and its mount

  • Drawing of mirror separations