A new prime focus corrector for the WEAVE project for the William Herschel Telescope is being produced. The corrector consists of six lens elements, the largest being 1.1 m in diameter. It also incorporates an Atmospheric Dispersion Corrector. Testing procedures for the WEAVE prime focus corrector lens elements are described here. Critical issues encountered in practice, including the influence of the lens size, wedge and weight on the testing procedure are discussed. Due to large lens dimensions, a dedicated test tower and lens support system has been developed to measure the optical surface form errors of the concave surfaces and the transmitted wavefront of each lens. For some of the lens elements, sub-aperture measurements have been performed using an off-axis Hindle sphere and the resultant OPD maps have been stitched together. The challenge of testing a wedged lens with a combination of a long radius convex surface and a short radius concave surface has been resolved by using another lens from the system as an auxiliary lens. The practice of testing convex surfaces via internal reflection/transmission through the lens element has been avoided entirely in this case and some discussion justifying the choices of metrology approach taken is given. The fabrication and acceptance testing of the lens elements has been completed within the expected time and budget, and all elements have been shown to meet requirements.
A high precision Co-ordinate Measuring Machine (CMM) is an ideal instrument for aligning mid to large (400 to
600 mm) diameter multiple element lens assemblies. The CMM has many advantages over simpler dial gauge and rotary
table setups. For example, these traditional methods do not necessarily make it easy to separate the out-of-roundness of a
lens or its mounting cell, from a misalignment of the lens and cell. With a CMM, the 'as made' geometry of both the
lenses and their mounting cells can be determined before the mounting and alignment process begins. By considering the
actual shape of the lenses and cells, adjustments can be made during the alignment process to ensure that the complete
assembly meets the designer's tolerances. This paper discusses CMM alignment techniques used and experience gained
while assembling large lens corrector assemblies (for example, the three element Prime Focus Unit for FMOS, the
Subaru Fibre Multi-Object Spectrograph) destined for installation in astronomical telescopes.
Any measurement of an artefact on a CMM will include contributions from the systematic errors of the machine, the random errors in the machine and also the actual geometry of the artifact. If an artifact is rotated (or translated) within the CMM measuring volume, such that the chosen measurement points on the artifact continue to map onto the same machine coordinates, then the component of the measurement due to the artifact geometry will rotate, whilst the component due to the machine systematic errors will remain in the same position.
In the self-calibration technique, a set of measurements of this type is made, and from these the systematic errors of the machine can be identified. The measurement accuracy of the artifact geometry is then only limited by the random component of the machine error, which is generally smaller than the systematic errors of the machine.
This paper reviews self-calibration techniques and assesses their feasibility for improving the uncertainty of form measurement of large optical surfaces on a coordinate measuring machine.
Electronic distance measuring instruments (EDMs) are devices used by surveyors where calibrated tape measures are not adequate or appropriate. Modern EDMs are generally accurate and reliable, are commonly capable of measuring up to 6 km, and may be combined with an electronic theodolite in a total station unit. Precise traceable calibration of EDMs is possible using a linear displacement interferometer, for example, with the respective reflectors in back-to-back configuration. Calibration data may be analysed for scale error and cyclical error. The distances so calibrated are usually constrained by the length of laboratory (and/or straight rails) available, as well as by the maximum working distance of the interferometer, but may be extended further with caution by the introduction of mirrors to fold the EDM beam. This paper describes the apparatus used to calibrate an EDM up to 200 m in a 60 m laboratory, and investigates some of the problems and artefacts that can arise, for example, from unwanted intermediate reflections of the EDM beam.
Line scales such as engineering rules and steel tapes are still used for many routine measurements despite the existence of more sophisticated devices. MSLNZ's Automatic Line Scale Measuring Instrument is based on a heterodyne laser interferometer used in a configuration that compensates for Abbe errors. The position of each scale graduation is detected by monitoring the change in the diffuse reflection of a focused line of diode laser light, as a motorized trolley travels along above the scale. The signal from the diffuse reflection is used to trigger the laser measurement system at the edge of each graduation. The instrument is capable of measuring the position of every graduation on a rigid scale up to four meters in length with an uncertainty of Q(10e-6 m, 7.5e-6L) (95% confidence level). Measurement time (after set up) for a one-meter scale is less than 2 minutes.
This paper describes the design of a facility to calibrate electronic distance measuring instruments (EDMs), as used in surveying (electronic theodolites) and large scale industrial measurement, over the range of 0 to 60 m. The combined uncertainty of the system at 60 m, estimated at the 95 percent confidence level, is expected to be 0.4 mm. The EDM is compared with a heterodyne laser measurement system in a back-to-back configuration. The trolley carrying the optics travels on aluminium rails. In order to improve the straightness of the path followed by the reflectors during measurement, the trolley optics are mounted on a two-axis motorized translation stage which uses a quadrant diode to track an additional guiding laser beam parallel to the required path. Once programmed, the trolley tracking electronics are autonomous and no connection is necessary to external power or control sources for that. However, radio frequency remote control of the motor propelling trolley motor would assist the measurements.
This paper describes improvements to a Hilger gauge block interferometer, including a fiber optic feed for a traceable laser radiation, automatic wavelength selection, and a CCD camera for fringe observation. The gauge measuring process is now semi-automated, computer controlled, more accurate and significantly faster and easier to operate than the original design.