The EnMAP telescope is an off-axis telescope made of three aspherical mirrors and a folding mirror mounted on bipods. Following a highly precise mechanical placement process , final alignment is performed by position correction of a single compensator element. The mirror position change by shimming is demonstrated to be reproducible within 1 μm.
The measurement of aspheric surfaces in a Fizeau interferometer implies a sometimes dramatic increase in dynamic
range, in terms of acceptable slope and departure, which can run the risk of introducing substantial measurement errors.
Common approaches to relaxing the dynamic range requirement include reducing the area of the surface measured in a
single measurement and stitching together the partial results, or using compensation techniques with the help of additional
components like null-lenses or computer generated holograms. This paper reviews these methods, with special
attention to the questions of degrees of freedom for misalignment. These considerations lead to a proposed method that
uses the inherent symmetry of the problem to scan along the optical axis, gathering measurements at zones of normal
incidence. These measurements are independent from each other; their ensemble represents directly the surface-deviation
in normal direction to the surface and the result is in the object coordinates of the design surface. Using an absolutely
calibrated spherical reference surface, the result is absolute. It is shown that this is very different from the technique of
stitching of zones, even when Intrinsic Coma is preserved through partially overlapping measurement regions.
A new method for measuring the surface of aspheric optics using a combination of two interferometric technologies is
presented. The metrology method provides a 3D measurement with high data density in a short measurement time
without the need for special tooling. The measurement technique is inherently insensitive to ray trace error and can be
used on the shop floor. It covers a large range of aspheric departures and delivers very small measurement uncertainties.
There are many sources of possible errors in phase shifting algorithms. One of those errors that may greatly influence the results is the detuning of the sampling points. Here we will describe a procedure to make the frequency interval in which the detuning error is smaller than a certain tolerances, as large as possible.
Using highly coherent laser sources in interferometry often leads to speckles in the interferograms. These speckles constitute a noise on the fringe phase and, hence, lead to a reduction of wavefront measurement precision. They arise from the light scattered by random imperfections of the optical surfaces. A new technique was developed at Carl Zeiss to reduce the effects of speckles in the laser interferometer DIRECT 100 by a virtual reduction of the spatial coherence regarding the speckle contrast. In the technique presented here the direction of the illuminating light beam in the interferometer is modulated while averaging wavefronts (not intensities) with the real-time wavefront averaging capability of DIRECT 100, resulting in a virtually larger extent of the light source. The fringe contrast is independent of this beam modulation, whereas the speckle contrast in the accumulated wavefront is determined by the virtual extent of the light source. Thus, speckle effects not only from the imaging part of the optical train but also from the illuminating part are reduced.
Research at Carl Zeiss has led to some innovative solutions in the field of optical test methods and interferometry. One example is the method of `direct measuring interferometry' (DMI), which was developed to overcome the problems of vibration and air turbulence when testing big astronomical primaries and is now the heart of the Carl Zeiss laser-interferometer DIRECT 100. Since DMI offers real-time capabilities for the wavefront evaluation, a built-in frame-memory can act as an `electronic hologram' and opens very elegant ways for in-situ correction of small residual errors, for easy aspherical testing, a very simple way of two- wavelength-interferometry, or a new discipline of time-resolved interferometry.
With the invention of a new phase measuring technique, 'Direct Measuring Interferometry' (DMI), almost all practical difficulties of quantitative interferometry in production environment are solved to a large extent.
A new Zeiss interferometer, the Direct 100, is described. This interferometer contains important improvements that are especially applicable to measurements in the presence of vibrations, measurements requiring the highest spatial resolution, integration of testing into manufacturing, use of interferometry for optimum assembly of complex systems, high-precision measurements in air with sub-nm accuracies, absolute calibration of residual errors, testing of aspheres with partial compensation and electronic hologram, measurement of fast processes, and temporally resolved measurements. The measurement and evaluation principle of the Direct 100 are described along with its optics, mechanics, and electronics.
We will report on a new interferometer developed at Carl Zeiss, which has real-time measuring
capability with instant visualization of results, is nearly insensitive to vibrations, has a variable fringe spacing
from one lambda to lambda/1O (lambda represents the wavelength of the light used in the interferometric
test), and can give lambda/100 accuracy through a simple calibration procedure. It can be handled with the
same ease and in just the same way as conventional interferometers.
Surface deviations of spherical mirrors from a best fitting, mathematically ideal sphere were measured to an absolute precision of 0.25 nm rms. Because of the long radius of curvature, a Hindle-type arrangement was used as interferometric setup, resulting in a test arm length of about 1.4 m. A special calibration procedure was implemented to eliminate systematic, setup-dependent errors. A very fast data acquisition technique was combined with real-time wavefront averaging to eliminate the effects of random errors, such as wavefront variations due to the turbulent atmosphere in the beam path. For the evaluation of one mirror surface, all in all 400,000 individual wavefront measurements at 400 x 400 points were combined, requiring an overall measurement time of only one to two days.
From October 1986 to April 1988 Carl Zeiss has fabricated the 3.5m primary mirror of the "New
Technology Telescope" (NY!') for the European Southern Observatory (ESO). This was the first time at
Carl Zeiss that at no stage of manufacturing a skilled optician was necessary for doing manual corrections
on the mirror. In the contrary, all the shaping and fine correction was done by computer controlled
machine work. Computerized interferometry was the tool to deliver the necessary data for closing the
As a rule of thumb, in a sound fabrication process the logarithm of the rms-values of the remaining
figure-errors drops linearly with time. For the LOT (Large Optical Telescope, Iraq) 3.6m-primary mirror
with an f-number of 3.5, which was fabricated at Carl Zeiss in a traditional way and finished in 1984, the
rms-value was brought down by a factor of two in about 81 days. For the ESO-N1T the time for cutting
the rms-value to half was 35 days3.
Next month the fabrication of another 3.5m mirror, the primary for the Telescopio Nationale Galileo
(TNG), will start. Although the NTE' was a very big success, we have used the time since 1988 to further
improve our measuring techniques. First we will report on the metrology applied during fabrication of the
NTT, then we will describe the tremendous improvements in resolution and speed now ready for the TNG.