The Roman Space Telescope Grism and Prism assemblies will allow the wide-field instrument (WFI) to perform slitless, multi-object spectroscopy across the complete field of view. These optical elements play a critical role in the High Latitude Wide Area and High Latitude Time Domain Surveys, which are designed to produce robust spectroscopic redshifts for millions of objects over the mission lifetime. To facilitate the characterization of these assemblies, a dedicated test bed was designed and utilized to perform a wide variety of spectroscopic measurements over the full range of operational wavelengths and field angles. Characterized features include, but are not limited to dispersion magnitude, dispersion clocking, encircled energy, total throughput, and bandpass edges. We present the results of this experimental campaign in which the Grism and Prism assemblies met or exceeded many of their design requirements and discuss measurement limitations.
We have published the optical design and early test results of the Roman Space Telescope grism spectrometer in previous SPIE proceedings. We report the follow-on activity of the spectral and radiometric calibrations, including the calibration methods, experiment designs, and test equipment calibration, such as the light source and detectors used in the test. The grism calibration includes the throughput versus wavelength, which is largely determined by the diffraction efficiency of the two diffractive surfaces. It also includes spectral resolution, point spread function, and relative radiometric measurements. The measured results are presented. The comparisons between the test data and the theoretical simulations are also presented. The tests and results presented are from the engineering test unit in ambient room temperature environment. The thermal/vacuum tests are planned to verify the results when the flight unit is ready.
We have presented the optical design and early test result of WFIRST grism spectrometer in previous SPIE conferences. This paper reports the follow-on activity of the spectral and radiometric calibrations, including the calibration methods, experiment designs, and the light source and detector calibration. The real grism calibration includes the throughput versus wavelength, which is largely determined by the diffraction efficiency of the two diffractive surfaces. It also includes spectral resolution, point spread function, and encircle energy measurements. The measured data are presented. The comparisons between the test data and the simulation from theory, or optical model, are also presented.
The WFIRST wide field instrument (WFI) includes a slitless spectrometer, which plays an important role in the WFIRST mission for the survey of emission-line galaxies. WFI is building engineering design and environmental test (EDU and ETU) units to reduce risk for the flight grism unit. We report here on successful build and test of the EDU grism. The four-element EDU grism consists of two prism elements and two diffractive elements that provide R700 dispersion. The elements were fabricated with alignment fiducials and integral flats to allow opto-mechanical alignment in six-degrees of freedom. Each element in turn, was installed onto a hexapod and positioned to its nominal orientation relative to the grism deck, then bonded into individual cells. Alignment measurements were performed in situ using theodolites to set tip/tilt and a Micro-vu non-contact Multisensor Measurement System was used to set despace, decenter and clocking of each element using the hexapod. After opto-mechanical alignment, the grism wavefront was measured using an Infrared ZYGO interferometer at various field points extending over a 20 by 14- degree (local) field of view. Using modeled alignment sensitivities, we determined the alignment correction required on our Element 2 prism compensator and successfully minimized the field dependent wavefront error and confocality. This paper details the alignment and testing of the EDU grism at ambient and cold operating temperatures.
Interferometers using computer-generated holograms (CGHs) have become the industry standard to accurately measure aspheric optics. The CGH is a diffractive optical element that can create a phase or amplitude distribution and can be manufactured with low uncertainty using modern lithographic techniques. However, these CGHs have conventionally been used with visible light and piezo-shifting interferometers. Testing the performance of transmissive optics in the infrared requires infrared CGHs and an infrared interferometer. Such an instrument is used in this investigation, which introduces its phase shift via wavelength-tuning. A procedure on how to design and manufacture infrared CGHs and how these were successfully used to model and measure the Wide-Field Infrared Survey Telescope grism elements is provided. Additionally, the paper provides a parametric model, simulation results, and calculations of the errors and measurements that come about when interferometers introduce a phase variation via wavelength-tuning interferometry to measure precision aspheres.
A spherometer is often used to precisely measure the radius of curvature of a spherical surface. It can also measure the vertex radius of a more complex surface such as an off-axis parabola (OAP). This paper provides a reliable algorithm to find the vertex radius of an OAP by solving a few equations based on the test geometry. This algorithm can also be easily expanded to any conic surface with high-order aspheric coefficients. The algorithm was verified by measuring an 8-inch diameter OAP and comparing the results with its known prescription. Results show good agreement. An example of measuring the vertex radius of a 4-m diameter OAP is also presented. In addition to this, a calculation was done to show that the coma and astigmatism are independent of the clocking of the spherometer on the optic.
The slitless spectrometer plays an important role in the WFIRST mission for the survey of emission-line galaxies. This
will be an unprecedented very wide field, HST quality 3D survey of emission line galaxies1. The concept of the
compound grism as a slitless spectrometer has been presented previously. The presentation briefly discusses the
challenges and solutions of the optical design, and recent specification updates, as well as a brief comparison between
the prototype and the latest design. However, the emphasis of this paper is the progress of the grism prototype: the
fabrication and test of the complicated diffractive optical elements and powered prism, as well as grism assembly
alignment and testing. Especially how to use different tools and methods, such as IR phase shift and wavelength shift
interferometry, to complete the element and assembly tests. The paper also presents very encouraging results from
recent element tests to assembly tests. Finally we briefly touch the path forward plan to test the spectral characteristic,
such as spectral resolution and response.
The Office of Instruction and Assessment at the University of Arizona currently offers a Certificate in College Teaching Program. The objective of this program is to develop the competencies necessary to teach effectively in higher education today, with an emphasis on learner-centered teaching. This type of teaching methodology has repeatedly shown to have superior effects compared to traditional teacher-centered approaches. The success of this approach has been proven in both short term and long term teaching scenarios. Students must actively participate in class, which allows for the development of depth of understanding, acquisition of critical thinking, and problem-solving skills. As optical science graduate students completing the teaching program certificate, we taught a recitation class for OPTI 370: Photonics and Lasers for two consecutive years. The recitation was an optional 1-hour long session to supplement the course lectures. This recitation received positive feedback and learner-centered teaching was shown to be a successful method for engaging students in science, specifically in optical sciences following an inquiry driven format. This paper is intended as a guide for interactive, multifaceted teaching, due to the fact that there are a variety of learning styles found in every classroom. The techniques outlined can be implemented in many formats: a full course, recitation session, office hours and tutoring. This guide is practical and includes only the most effective and efficient strategies learned while also addressing the challenges faced, such as formulating engaging questions, using wait time and encouraging shy students.
A software configurable optical test system (SCOTS) based on deflectometry was developed at the University of Arizona for rapidly, robustly, and accurately measuring precision aspheric and freeform surfaces. SCOTS uses a camera with an external stop to realize a Hartmann test in reverse. With the external camera stop as the reference, a coordinate measuring machine can be used to calibrate the SCOTS test geometry to a high accuracy. Systematic errors from the camera are carefully investigated and controlled. Camera pupil imaging aberration is removed with the external aperture stop. Imaging aberration and other inherent errors are suppressed with an N -rotation test. The performance of the SCOTS test is demonstrated with the measurement results from a 5-m-diameter Large Synoptic Survey Telescope tertiary mirror and an 8.4-m diameter Giant Magellan Telescope primary mirror. The results show that SCOTS can be used as a large-dynamic-range, high-precision, and non-null test method for precision aspheric and freeform surfaces. The SCOTS test can achieve measurement accuracy comparable to traditional interferometric tests.
When defining surface irregularities more often than not it is useful to represent these as slope errors rather than surface
errors. However, surface error specifications are more commonly used even though they do not accurately represent
performance but simply physical characteristics. An example of slope measurement technique and specification is a
method developed by the University of Arizona called SCOTS (Software Configurable Optical Test System) which has
showed to be accurate, while very fast and easy to setup. It directly measures the transverse ray aberration/slope of an
optical system using transmission deflectometry.
SCOTS (software configurable optical test system) is a useful tool that can provide lens manufacturers with the ability to
evaluate the net performance of a lens system without the use of complex metrology systems and setups. This technique
is based on measuring the transverse ray aberrations of rays to obtain wavefront information using transmission
deflectometry, the refractive equivalent of reflection deflectometry.
Some work using deflectometry on refractive surfaces has been briefly reported in the past, where the power of a single
lens has been the measurement objective. Results showing the use of deflectometry on reflective optical surfaces, such
as the primary mirror of the Giant Magellan Telescope (GMT) show that this method has a large dynamic range in
which measurement accuracy is comparable with those of interferometric methods; generating interest on our part, to
investigate deflectometry for refractive systems in more detail.
In this paper, we focus on reporting initial tests using SCOTS by measuring simple refractive elements, such as 1"
diameter biconvex BK7 lenses. Results indicate a good agreement when comparing them with equivalent
MATLAB/ZEMAX wavefront measuring models, which include the measured lens parameters, where the estimated and
measured wavefront RMS values and spherical aberration Zernike coefficient agree on average to within 10nm. We also
investigate the effect of the chromatic aberration on the refractive optical system by collecting data using three different
wavelengths: 620nm, 550nm and 450nm. The alignment of the test setup was done rapidly and we used an LCD screen
with a pixel pitch of 0.1905mm. The camera used for the measurements was a simple digital CCD camera.
SCOTS (software configurable optical test system) is a useful tool that can provide lens manufacturers with the ability to
evaluate the net performance of a lens system without the use of complex metrology systems and setups. This technique
is based on measuring the transverse ray aberrations of rays to obtain wavefront information using transmission
deflectometry, the refractive equivalent of reflection deflectometry.
Some work using deflectometry on refractive surfaces has been briefly reported in the past, where the power of a single
lens has been the measurement objective. Results showing the use of deflectometry on reflective optical surfaces, such
as the primary mirror of the Giant Magellan Telescope (GMT) show that this method has a large dynamic range in
which measurement accuracy is comparable with those of interferometric methods; generating interest on our part, to
investigate deflectometry for refractive systems in more detail.
In this paper, we focus on reporting initial tests using SCOTS by measuring simple refractive elements, such as 1"
diameter biconvex BK7 lenses. Results indicate a good agreement when comparing them with equivalent
MATLAB/ZEMAX wavefront measuring models, which include the measured lens parameters, where the estimated and
measured wavefront RMS values and spherical aberration Zernike coefficient agree on average to within 10nm. We also
investigate the effect of the chromatic aberration on the refractive optical system by collecting data using three different
wavelengths: 620nm, 550nm and 450nm. The alignment of the test setup was done rapidly and we used an LCD screen
with a pixel pitch of 0.1905mm. The camera used for the measurements was a simple digital CCD camera.
NASA's James Webb Space Telescope (JWST) will be a premier space science program for astrophysics following
launch scheduled for 2014. JWST will observe the early universe, with emphasis on the time period during which the
first stars and galaxies began to form. JWST has a 6.5 m diameter (25 square meters of collecting area), deployable,
active primary mirror operating at cryogenic temperatures.
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