Although the SPPDI unit itself is rugged, engaging in interferometric measurements can be challenging. As with any optical interferometer, operation of the SPPDI requires a stable, vibration-free environment with low ambient temperature variations. In addition, a successful interferometric setup requires that the SPPDI and the optical system under test be mounted in stable optical mounts with precision angle and linear motion adjusters.
There are optical benches (mounting platforms) and optical component mounts available from a variety of suppliers which offer the required degree of precise motion control. A web search will quickly reveal a variety of suppliers which serve this market. Alternatively, there are surplus equipment suppliers that offer used equipment obtained from large governmental or commercial laser labs, for a fraction of the cost of new equipment.
A 2-inch diameter Telescope Adapter is provided with the SPPDI to facilitate direct attachment of the SPPDI to the 2″ eyepiece holder typically provided with higher-end commercial telescopes. Such an arrangement is shown in the photo on the left.
For larger refractors, such as the 5″ refractor shown in the photo on the right, the prime (far-field) focal plane for the telescope objective is so far distant from the rear of the scope that two extension tubes are required in order to position the SPPDI at prime focus. This does not lend itself to precise positioning of the SPPDI with respect to the optical axis of the objective lens. Use of the diagonal mirror eyepiece holder typically provided with these telescopes would enable prime focus to be reached without the use of extenders, but could compound the errors in the objective lens with errors in the diagonal mirror.
So, to avoid these complications, the SPPDI is mounted separately on a three-axis translation stage setup as shown in the photo on the right. This requires that the telescope first be brought into autocollimation with the “optical flat” (high precision flat mirror) used for double-pass interferometry. This is easily accomplished by means of a low cost laser alignment tool typically sold for the purpose of aligning the primary mirror in Newtonian type telescopes. Then the SPPDI is positioned along the x, y, and z axes by means of the translation stages. For reflector type telescopes with low-profile focusers, the SPPDI may be easily positioned at prime focus. Note that the SPPDI pinhole aperture is located at a depth of about 32.5 mm inside the SPPDI enclosure.
As the telescope focal length increases, isolation from ambient vibration becomes increasingly difficult. It is very helpful to have all equipment co-located on a stable platform, such as on an optical table like the one shown in the photo on the right.
The 10″ optical flat seen in the photo above left is mounted in a fork mount obtained by disassembling a large (11″ aperture) commercial Schmidt-Cassegrain type telescope. The optical tube assembly was replaced with a purpose-built saddle capable of supporting a 12-inch diameter optical flat. The electronic slow motion control associated with the fork mount is used to bring the telescope under test into near autocollimation against the optical flat. Final precision alignment is achieved by means of a set of three 1/4×20 jack screws mounted on the ends of three long lever arms which support and raise and lower the tripod feet.
Measurement Methods and Use-Cases
1.) CoC: Center of Curvature.
This use-case is typically used for measuring concave reflective optics, such as spherical or parabolic mirrors. Mirrors with focal ratios (“speeds”) as fast as f/2 (i.e., f/4 at CoC) may be measured directly by the SPPDI without any ancillary optics interposed between the test article and the SPPDI.
Very large aspheric optics may be tested with the SPPDI, as long as the blur circle diameter (due to geometrical aberrations) does not exceed 1 mm at CoC. There is no size limit for mirrors or optical systems that are diffraction-limited or near diffraction-limited at CoC.
Unavoidable aberrations will occur within the SPPDI internal beam splitter cubes and tilt plates. These aberrations typically increase in proportion to the size of the test beam footprint on the SPPDI internal optics. Fortunately, these internal aberrations are easily measured and compensated for by use of a high precision concave spherical reference mirror, such as those available from Kerry Optical Systems.
Two very “fast” (f/0.5 and f/1.27) 2″ diameter high precision (<1/10th wave) fused silica concave spherical mirrors are available from Kerry Optical Systems for use in measuring and compensating for the SPPDI internal aberrations. A wavefront obtained for the reference mirror, at the same focal ratio as for the test article, may be subtracted from the wavefront of the test article, to yield a high accuracy wavefront map for the test article.
2.) DPAC: Double-pass At Autocollimation.
This use-case is applicable for measuring either transmissive optics or reflective optics when an accurate optical flat of sufficient size is available to reflect the output collimated beam back through the test article.
The SPPDI will not accept test beams that are faster then f/4 due to the physical size limitations of various internal components. So, if the test article has a “speed” faster than f/4, it will be necessary to interpose beam forming optics (aka, a “transmission sphere”) between the SPPDI and the test article. Microscope objectives work well for this purposes, but exhibit various aberrations that need to be measured and compensated for. This can be done with the aid of the Kerry Optical Systems high precision reference mirrors as discussed in the CoC use case.
Beam Forming Optics
The SPPDI is physically limited to measuring beams that are f/4 or “slower.” So, beam forming optics must be used when the incident test beam is “faster” than f/4. This figure shows the components that comprise the Kerry Optical Systems Tube Lens Collimator or Transmission Sphere, when used with a user-supplied microscope objective.
The Transmission Sphere comprises two parts: (1) a collimating “tube lens” mounted in the small (1.25″ OD) end of the supplied COTS eyepiece extender; and (2) a compression mount that fits in the eyepiece end of the COTS eyepiece extender. The “tube lens” receives the diverging beam from the SPPDI and collimates the beam to a diameter that just overfills the rear aperture (typically 8 or 9 mm) of interchangeable microscope objectives that screw into the RMS threaded end of the compression mount.
Each microscope objective produces a converging output beam which has a focal ratio governed by the NA (numerical aperture) of the microscope objective. For example, a 10x objective with an NA of 0.25 will produce an output beam from the microscope objective with a focal ratio of f/2. “Faster” output beams can be achieved with higher NA objectives.
The Transmission Sphere may be used in combination with either our precision reference mirror or our precision Random Ball Test Kit, to produce a correction wavefront, which can then be subtracted from the wavefront obtained for the test article. This process allows removal of the errors associated with the Transmission Sphere or Random Ball, as well as the SPPDI.
IMPORTANT: The Transmission Sphere produces accurate results ONLY for diffraction-limited or near diffraction-limited optical systems. Geometrical aberrations or large optical errors in the test beam introduce retrace errors that cannot be adequately compensated for.
The interferogram and associated wavefront for the test article should be obtained first, with a DFTFringe outline that is suitable for the test article. This same outline must then be used in obtaining the correction wavefront for the reference sphere interferogram. This will typically mean that the outline used in producing the wavefront for the reference sphere will be substantially smaller than the full diameter of the reference sphere interferogram.
Wavefront “Sense”
An interferogram is a topographic map of a wavefront. Just as in reading a topographic map of a mountain, it is often difficult to know whether traversing the map from fringe to fringe leads uphill or downhill. For optical interferometry, sophisticated and expensive optics can be introduced in the beam path that eliminates the confusion. Fortunately, there is a very simple method for determining the wavefront “sense” which does not require expensive add-on optics. The method involves defocusing the wavefront, and observing the algebraic sign of the Zernike Z3 Defocus term, as displayed by DFTFringe. If the interferometer has moved closer to the test article, the algebraic sign of the Z3 term should become more negative, but all other Zernike terms should remain approximately the same. If the Z3 term becomes more positive, then the DFTFringe “Invert” function should be launched to invert the defocused wavefront. If the Z8 terms of the defocused and non-defocused wavefronts no longer have the same algebraic sign, then the non-defocused wavefront should be inverted.
