COL 11(Suppl.), S22202(2013) CHINESE OPTICS LETTERS September 30, 2013
Null testing toroidal surface and biconic surface
with cylinder compensator
Zhuang Liu (
444
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)
1,2
and Yan Gong (
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)
1∗
1
State Key Laboratory of Applied Optics, Changchun Institute of Optics
§
Fine Mechanics
and Physics, Chinese Academy of Sciences, Changchun 130033, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
∗
Corresponding author: gongy@sklao.ac.cn
Received July 11, 2013; accepted August 14, 2013; posted online September 25, 2013
Toroidal surface and biconic surface are employed increasingly, however their profile cannot be null tested
easily for they are non-rotationally symmetrical. Null testing method with cylinder compen sator is pro-
posed to solve this problem. The theory of this method is revealed. The errors of this method are present.
Three typical testing optical systems with cylinder compensator are demonstrated at last. The design
results and total error indicate that this method is feasible.
OCIS codes: 220.1000, 220.1250, 220.3620, 220.4840.
doi: 10.3788/COL201311.S22202.
Toroidal surface and biconic surface are increasingly used
in modern advanced optical instruments. The toroidal
surface which has different radii in sagittal and tangen-
tial plane can reduce effectively astigmatism and coma.
Thus it is employed widely in off-axis or multi-pass opti-
cal system. Biconic surface which has two more degrees
of freedom than toroidal surface can improve optical sys-
tem’s performance greatly. They will be widely used in
future.
The toroidal surface and biconic surface cannot be
tested directly by interferometer without any compen-
sators because they are non-rotationally symmetrical.
There are two common methods to test these types of
surfaces. One method is use o f profilometry. The surface
is measured one point or one line at a time by profilom-
etry, profile of the surface can be generated by mapping
out several hundred points or lines. This process is not
exp ensive, but it do es not offer the detail of an inter-
ferometric map. Precision of this method is several tens
of nanometers. Accuracy will degrades when table’s in-
stability increases due to long distance (in excess of a
couple inches). Ano ther option available is null testing
with computer generated holograms (CGH), figures can
be measur e d interferometrically. This metho d is the com-
monest method, but hologram required the customized
program for different surface s and lithography etching
on chrome made it time-consuming, laboring, and expen-
sive. One hologram canno t be used to different profiles
[1]
.
Cylinder surface is also non-rotatio nally symmetrical.
It is curving in one direction and flat in orthogonal di-
rections. As a compensator, cylinder lens can turn the
spherical wavefront from one point (the cat-eye) into non-
rotationally symmetrical wavefront which fits the tested
toroidal surface or biconic surfac e and take the place of
CGH in null testing. Compared with hologram, cylinder
lens can be got easily, and one cylinder le ns can be used
for different profiles.
Methods of null testing toroidal surface and biconic
surface with cylinder compensator are discussed in de-
tail. The compe nsation theor y of toroidal surface and bi-
conic surface is shown, and tolerance ana ly sis is covered
as well. Three null testing optical systems with cylinder
compensator for three typical surfaces are designed. De-
sign r esults demonstrate this method’s feasibility.
The curve of toroidal surface in the tangential plane is
defined by
Z =
y
2
R
1 +
q
1 −
y
2
R
2
. (1)
This curve is then rotated about an axis parallel to the
Y axis a nd intersecting the Z ax is with a distance ρ from
the vertex. Toroidal surface has 2 radii: radius R in sagit-
tal plane and rotate radius ρ in tangential plane. In most
cases, the toroidal s urface is concave. We se t the cylin-
der lens’ generatr ix perpendicular to the tangential plane
to correct the wavefront from the cat-eye, and discuss its
theory in sagittal plane and tangential plane respectively.
In sagittal plane, the cylinder can be considered as a
plane plate lens. The plane plate lens whose thickness
is not neglected will lead to the longitudinal displace-
ment and wavefront aberration. As a compensato r, the
primary aberratio n is 3rd-order spherical wavefront aber-
ration.
As shown in Fig. 1, the longitudinal displacement L
produced by passage through a plate of thickness T and
refractive index n is easily found by Snell’s law for small
angle of incidence to be
Fig. 1. Layout of compensation optical system in sagittal
plane.
1671-7694/2013/S22202(5) S22202-1
c
2013 Chinese Optics Letters