1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-29a71 Center Point of Contact: GSFCa71 Submitted by: Wil HarkinsSubject: Star Image Metrology Part II: Irradiance Measurements Practice: The fundamental behavior of a spaceborne (or space-related) optical imaging system is
2、 contained in its response to an impulse, i.e., a point source. The point source image is referred to by a variety of names: star image, point spread function, far field pattern, impulse response, Fraunhofer diffraction pattern. Here we will use “star image.“ This guideline provides information on i
3、nstruments and procedures used in measuring star image irradiance distributions.Programs that Certify Usage: N/ACenter to Contact for Information: GSFCImplementation Method: This Lesson Learned is based on Reliability Guideline Number GT-TE-2406 from NASA Technical Memorandum 4322A, NASA Reliability
4、 Preferred Practices for Design and Test.Benefit:Star image measurements provide the primary means of appraising end-to-end health of an optical imaging system. These tests represent good engineering practice, and are reliable indicators of system performance. Star image tests are thus the final arb
5、iter of quality control. A system passing the star tests discussed here ensures that the optical instrument is performing in a reliable way.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Measurements of irradiance distributions provide important qua
6、ntitative information concerning the fidelity of the star image to theoretical expectations. Such measurements either validate performance, or provide important clues as to the nature of a problem. For example, the problem might be a decentered component or an inappropriate conic constant.Implementa
7、tion Method:1. Description of Star Image Test ApparatusThe basic equipment needed to perform laboratory star image visual measurements is indicated in Figure 1. The optical system on the left half of the drawing is a Star Simulator. This is a Collimator with a back-illuminated pinhole located in the
8、 focal plane. The Collimator can be either reflective or refractive, and its pupil should be larger than the imaging system being tested. The pinhole represents the star, and its diameter should be smaller than the Collimator Airy Disc, (i.e., unresolved). Depending on the application, the pinhole b
9、ack-illumination can be either from a coherent or incoherent source. An example of the former is a laser; the latter, a point arc lamp.Between the source and the pinhole there is usually some coupling optics to maximize the power through the pinhole, and to fill the aperture of the Collimator with l
10、ight.refer to D descriptionD Figure 1: Basic Layout of a Star Image Test Setup Caution: The pinhole acts like a “pinhole camera.“ If a coiled filament lamp is used, an image of the coil will be formed on the collimator pupil and the output irradiance will be quite nonuniform.A support structure is n
11、eeded to hold the optical system under test. This mounting fixture should provide modest azimuth and elevation tilt control to allow alignment of the “test system“ to the Collimator optical axis. The test support fixture should also allow the “test system“ to be accurately rotated through its operat
12、ional field of view. For refractive “test systems,“ the rotation axis should ideally pass through the systems rear nodal point. A mechanical support fixture that accomplishes Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-this is called a T-Bar Noda
13、l Slide. (Reference 1.) However, for catadioptric and reflective systems, rotation through the rear nodal point is impractical because this point is usually far removed from the physical embodiment of the “test system.“ In this Guideline, the rotation axis will be at, or near, the entrance aperture
14、of the “test system.“For irradiance measurements the “test system“ star image can be probed directly by some methods. Other methods need a magnified star image. In the latter case, this can be accomplished using a high quality microscope objective which should be well-corrected for both axial color
15、and spherical aberration. Its numerical aperture must be sufficient to collect all the light emerging from the star image.In the Reliability Preferred Practices Guideline No. GT-TE-2405, Star Image Measurements, Part I: Visual Measurements, we relied on making dimensional measurements by eye on key
16、features of the star image both laterally and axially. In this Guideline, irradiance is the key measurement parameter. The dimensional features discussed in Part I are not lost but contained within this broader measurement context.2. Ideal Star ImagePlease refer to Reliability Preferred Practices Gu
17、ideline No. GT-TE-2405 (Lesson Learned 0718).3.1 Axial Intensity MethodA new technique (Reference 2) has been developed in recent years that makes use of axial intensity scans on star images to extract quantitative information about aberration content. Laboratory embodiments can have a number of for
18、ms. One embodiment is illustrated in Figure 2 which makes use of a commercially available “scanning micrometer eyepiece.“ The star image is magnified with a microscope objective. The magnified image is centered on the entrance face of a fiber optic. The exit face of the fiber is coupled to a detecto
19、r such as a photomultiplier tube (PMT). The output signal of the detector is connected to the y-axis of an xy-recorder. The x-axis is driven by a signal from a linear transducer attached to the Z-axis motion of the microscope. The experiment generates an intensity plot as a function of axial microsc
20、ope position.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD Figure 2: Experimental Configuration for Measuring Spherical Aberration via Axial Intensity Scans We will use spherical aberration as an example to illustrate the ut
21、ility of the axial scan profile. It is well known that the axial intensity of a lens free of spherical aberration has a symmetric axial intensity profile about paraxial (Reference 3) focus as is shown in Figure 3. However, it is not well known that the axial intensity pattern remains symmetric in th
22、e presence of spherical aberration.(Reference 3). The pattern is not the same shape as that for the zero aberration case, and the center of symmetry is no longer in the paraxial focal plane. However, the axial separation, d, between the plane of symmetry and the paraxial focal plane is directly rela
23、table to the exit pupil spherical aberration coefficient W040(in micrometers) present through the equation:refer to D descriptionD(3) refer to D descriptionD Figure 3: Axial Inensity of Diffraction Limited System Figure 4 (a) shows an experimental axial intensity plot for a system with significant s
24、pherical Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-aberration. The F-number of the system is F/10.3. The operating wavelength is 0.6328 micrometers. The separation between the plane of symmetry and the paraxial focal plane is 3.07 mm. (Note, pa
25、raxial focus is established by a separate axial intensity scan with a small aperture in the entrance pupil). Using Eq.4, we find that the amount of spherical aberration is 5.7 waves! Figure 4 (b) shows a theoretical plot with the same amount of spherical aberration. The two plots are virtually ident
26、ical. A recent paper (Reference 4) also shows how axial intensity scans of star images with mixed Seidel aberrations can be used to identify and quantify those aberrations.refer to D descriptionD Figure 4: Axial Intensity where W040= 5.7 Waves (a) Experimental; (b) Theoretical. Courtesy Dr. Qian Gon
27、g 3.2 Lateral Irradiance Methods3.2.1 CCD ArraysThe magnified star image formed by the microscope can be focussed on the detector chip of a CCD camera. A video frame-grabber tied into a PC can be used to acquire a frame of data for analysis. Hardcopy can be acquired via a video printer. Software pac
28、kages are commercially available that will allow analysis of the data frame.Among the useful features are: a) power profiles across the star image; b) power centroid location; c) calculation of encircled energy; d) 2-D contour and 3-D power maps. The profiles can be compared to theoretical predictio
29、ns like the one shown later in Figure 8(b). Not only is the size of the Airy Disc obtained, but also the relative power between the central peak and the rings. (Note: Small aberrations rob power from the core and shunt most of it into the first bright ring). In addition, an encircled energy calculat
30、ion should show 84% of the energy within the core (for a circular unobscured entrance pupil).Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Potential problems with some CCD arrays are: dynamic range, linearity, and pixel uniformity. For example, the
31、 ratio of the peak power in the Airy Disc to the peak power in the first bright ring is about 68. Some CCDs do not have enough dynamic range. To see the first few rings means the central core is saturated. An unsaturated core means that the second bright ring, and sometimes the first, are buried in
32、noise. Nonlinearity and pixel nonuniformity imply that profile shapes would be wrong.3.2.2 Fiber Optic ProbeThe fiber probe scanner used in Sec.3.1 to obtain axial profiles can also be used to obtain lateral profiles. The fiber optic probe assembly is mounted on a motion controlled boom. This boom c
33、an be translated horizontally either manually or under computer control. In this way the probe can be scanned through the magnified star image. A transducer provides a voltage signal proportional to the linear position of the boom.The entire scanning micrometer eyepiece can be rotated and the axis o
34、f rotation is the optical axis of the microscope. This allows the observer, who views both the star image and the probe location through an eyepiece attached to the device, to select the orientation of the scan relative to the star image. The boom signal can be used to drive the x-axis of an xy-reco
35、rder while the signal from the PMT/radiometer can drive the y-axis. PMTs have significantly more dynamic range than CCDs, and excellent linearity. This will allow more accurate profiles through the star image. (A commercially available eyepiece of this type can be obtained from EG (b) Operational Pr
36、inciple Technical Rationale:All optical imaging systems used on spaceborne or space-related instruments should undergo an end-to-end systems check to validate performance. Star image testing is the primary means of conducting this validation process. It provides data pertinent to the pass/fail crite
37、ria associated with the optical imaging system.References:1. 1. J. Geary, Introduction to Optical Testing, Tutorial Text TT-15, SPIE Press,(1993).2. J. Geary and P. Peterson, “Spherical Aberration: a Possible New Measurement Approach,“ Opt.Eng. 25, 2, (1986).3. V. Mahajan, Aberration Theory Made Sim
38、ple, Tutorial Text TT-6, SPIE Press, (1991).4. Q. Gong and S. Hsu, “Aberration measurement using axial intensity,“ Opt.Eng. 33, 4, pp 1176-1186, (1994).5. P. Peterson and J. Geary, “Intermediate spherical aberration,“ Opt.Eng. 25, 11, pp 1232-1240, (1986).6. J. Gaskill, Linear Systems, Fourier Trans
39、forms and Optics, John Wiley, (1978).7. J.Goodman, Introduction to Fourier Optics McGraw-Hill (1968).8. J. Geary, M. Yoo, P. Davila, A. Wirth, A. Jankevics, M. Ruda, R. Zielinski, and L. Petrilli, “Comparison of Wavefront Sensors,“ SPIE Proc. Vol. 1776, pp 58-72, (1992).9. D. Vanderberg, W. Humbei,
40、and A. Wertheimer,“Quantitative Evaluation of Optical Surfaces by Means of an Improved Foucault Test Approach,“ Opt.Eng. 32, 8, pp 1951-1954, (1993).10. J. Wyant and K. Creath, Applied Optics and Optical Engineering, Vol. 11, Chap. 1, ed. R. Shannon and J. Wyant, Academic Press (1992).11. Reliabilit
41、y Preferred Practices, Guideline No. GT-TE-2405, Star Image Metrology Part I: Visual Measurements12. Reliability Preferred Practices, Guideline No. GT-TE-2404, Optical Testing Guideline: Fizeau Interferometry.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from I
42、HS-,-,-Impact of Non-Practice: If end-to-end systems measurements are not made on spaceborne or space-related optical imaging systems, then the consequences could be the ultimate failure of the mission in-whole or in-part. The Hubble Telescope should be a constant reminder of this.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-03-29a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
