27 28 29 30
PV: 518 nm
RMS: 77 nm
100 × 100 mm
249 nm -278 nm
PV: 527 nm
RMS: 77 nm
100 × 100 mm
Figure 1 shows an example freeform optic that is used in an
unobstructed reflective telescope design. The majority of the
surface sag is described by a best-fit sphere, with concave radius ~200 mm over a 100-mm-diameter clear aperture. The remaining form has ~100 µm of mostly asymmetric deviation
from this sphere.
The surface was first measured with a custom CGH and a
4 in. f/1.5 transmission sphere. Alignment of the CGH to the
interferometer was simple because of the inclusion of alignment
features on the hologram. Part alignment, however, was quite
tedious. In a traditional sphere test, the alignment errors are
X tilt, Y tilt, and power, and the adjustments are basically or-
thogonal (that is, three alignment knobs will independent-
ly adjust each of the tilts and power). The freeform surface
alignment, though, includes six degrees of freedom that are
not orthogonal, making the fringe nulling operation quite a
bit more challenging.
The surface was also measured without null optics on QED’s
ASI(Q) running prototype freeform software. The basic principle of the test is the same as a non-null stitching test of a rotationally symmetric asphere. Rather than trying to match the
reference wavefront to the test surface with null optics, the surface is instead tested against a spherical wavefront. Stitching is
required because the vast majority of aspheres have too much
aspheric departure from a sphere to be acquired with a single
measurement. In this example, a 6 in. f/3.5 transmission sphere
is employed, and a lattice of 47 subapertures acquired to adequately cover the part (see Fig. 2a). The part alignment is soft-ware-assisted, and the subapertures are acquired and stitched
together automatically (without user intervention).
The two different interferometric techniques agree quite well.
Figure 2b shows that the figure error maps qualitatively agree,
and scalar metrics such as peak-to-valley (PV) and root-mean-square (rms) also compare favorably.
Close examination of the data, however, reveals some subtle and
important differences between the measurements. For the next
analyses, we removed a 36-term Zernike polynomial fit from
each map to remove the low-order form and highlight mid-spatial frequency features on the surface. One difference between
the two maps is the number of resolution elements: subaper-
ture stitching enables more magnification than the full-aper-
ture CGH test. In this example, the stitch test has 2000 pix-
els across the diameter of the surface, while the CGH test has
600. The impact of this is apparent if we zoom in on a 10 mm
region on the part: the features in the stitched map are much
sharper (see Fig. 3).
FIGURE 2. Lattice design and simulated fringes for an off-axis subaperture (a), with CGH (left) and ASI(Q) (right) measurements (b).