ISUAL Imager Science Performance Test Report Author: Stewart Harris

Transcription

1 ISUAL Imager Science Performance Test Report Author: Stewart Harris Revision History: Level Date Description A 17 Mar 2003 initial release B 31 Mar 2003 included additional set up information, radiance calculation correction 1 Science Performance Tests Science performance tests for the ISUAL science instruments were defined in the Test Readiness Review (Dec., 2001). The tests for the Imager are reported as follows: Instrument Under Test Section Set Up Measurements Imager With collimating lens With paraboloid collimator With monochromator Flat field projector Special test equipment MTF / CTF FOV, boresight alignment Passband, responsivity, S/N Field Uniformity Flash tolerance 1

2 Table of Contents 1. Science Performance Tests Sprite Imager Tests Imager MTF / CTF Measurement Requirement Test Setup Procedure Test Date Test Data Results Resolution Test Limitations Pinhole Resolution Test Results Imager FOV / Boresight Alignment Measurement Requirement Setup Procedure Test Date Test Data Results Imager Responsivity and S/N Measurement Requirement Setup Procedure Test Date Responsivity Computation Responsivity Results Signal-to-Noise Ratio (SNR) Passband Verification Exposure Time and Linearity Imager Flat Field, or Field Uniformity, Measurement Setup Test Dates Results Imager Bright Flash Tolerance Setup Test Date Results Appendix A - Imager Resolution Test Data Summary Appendix B - Imager Resolution Test Target Spatial Frequency Summary Appendix C - Imager FOV and Boresight Test Data Coordinate Convention Appendix D - Imager FOV and Boresight Test Data, page 1 of Appendix E - Imager Responsivity Test Data Sheets page 1 of Appendix F - Imager Filter Test Data, page 1 of Appendix G - Flat Field Images Directory Tree (Partial Listing of Files)

3 2 Sprite Imager Tests 2.1 Imager MTF / CTF Measurement The spatial frequency response of the Imager is measured using a test target projected into the Imager's lens using an external, collimating projector lens, as shown in Fig Using the various bar groups of the test target, the limiting resolution and an estimate of the contrast transfer function (CTF) of the Imager can be measured. The resolution measurement is best made using the high resolution mode of the Imager, i.e. with Binning OFF. This allows a more precise measurement of spatial frequency response of the lens and image intensifier. Low resolution images, with Binning ON, are also obtained to verify that the response changes as expected Requirement The System Specification reads, "With 1 msec frame time, the square wave MTF at the Nyquist frequency shall be no less than 0.20." The limiting spatial frequency is determined by the nominal pixel size, at the focal plane, or 41.8 μm. This is the size of pixels, in the binned mode, which is the nominal operating mode. This pixel pitch corresponds to a spatial frequency of 12 lp/mm Test Setup All test equipment is set up on a standard optical table, using various positioning and holding devices. These devices, while very important for establishing the correct geometry, are not shown in the schematic. In particular, the position of the Imager with respect to the rotation stage is fixed by a mounting plate, and this positions the point of rotation coincident with the entrance pupil of the Imager. Also not shown is the data capture system. In this case, the Imager data is captured directly at the J2 interface using a PC-based, digital frame grabber board. A test panel is used to set and control the MCP and Phosphor high voltages. The collimating lens focus and test target position are carefully adjusted such that the test target appears at infinity. This adjustment is accomplished using an autocollimator. Fig is an image of the particular test setup, and the autocollimator is included in the photo. The USAF 1951 resolving test target is used for the measurement. To convert the standard spatial frequencies available on the test target to frequencies, n, projected onto the focal plane, use the following formula: n = where, F = focal length of collimating lens, or 85mm. F f N f = focal length of Lens under test, or 61.8mm for the Imager. N = spatial frequency of test target, in lp/mm. From this relationship, the Nyquist limiting frequency for the Imager can be found on the test target in Group 3, between elements 1 and 2. Element 1 in this group corresponds to 11 lp/mm on the focal plane. Element 2 corresponds to 12.4 lp/mm. Values for all the bar groups are provided in Appendix B. 3

4 Sprite Imager Precision Rotation Stage Incandescent Lamp +Angle Rot. Filters Iris Integrating sphere J2 Resolution Target on 3-Axis Translation Stage Collimating Lens Canon 85mm f/1.2 Fig : Imager Resolution Test Schematic - with Collimating Lens Fig : Imager Resolution Test - Physical Set Up 4

5 Test Equipment Used Item Description Manufacturer Model 1 Incandescent Lamp, 12V, 20W Waldmann HPK20 2 Integrating Sphere, 8 inch Oriel Test target, resolving power USAF 1951 Edmund A Axis translation stage Newport 462XYZM 5 Collimating Lens, 85mm, f/1.2 Canon 6 Precision Rotation Stage,.01 absolute accuracy,.0013 repeatability Newport RV120PE, S/N B Motion Controller, Rotation stage stepper motor Newport ESP100, S/N Autocollimator Davidson Optronics D657, S/N Optical rail with carrier Oriel / FOV Fixture Instrument Adapter SSL 8937-A4B 10 Rotary Mount Adapter SSL 8935-A4A 11 FOV Fixture Slide Adapter SSL 8936-A4A 12 FOV Fixture Slide Spacer SSL 8934-A4A Procedure 1. With the equipment set up as shown, the Imager is initially mounted on the rotation stage with an azimuth rotation angle setting of In a darkened room, with light source OFF, power ON Imager FEC. Verify that data collection is working. Turn on Phosphor HV. Phosphor Vmon should indicate about 5700V. Then adjust MCP high voltage to a nominal value, about 500V, while continuously monitoring the updating image display to make sure there are no inadvertent bright light sources. 3. With continuous image update running on the data display, the light source voltage is slowly brought up until the image of the test target is viewable on the display. If there is no image produced when light source voltage reaches 12V, then increase MCP HV slowly to obtain an image. Typically target brightness is set such that the image highlights are about mid-scale intensity, i.e. about 2000 ADU (A/D units), out of a possible 4095 ADU. 4. With target image viewable, rotate the test target to obtain upright image, that is aligned with image axes. Also position central portion of test target bar pattern at center of image using the 3-axis translation stage. 5. Turn OFF Imager, and unmount it from rotation stage. 6. Position autocollimator on rotation stage, at same optical height as the Imager. Turn ON autocollimator source. Adjust test target focus distance for best focus in autocollimator view port. 7. Remove autocollimator, re-mount Imager on rotation stage. 5

6 8. Adjust filter wheel position to desired setting. As before the Imager and light source are brought up such that the test target brightness is about mid-scale. A high resolution image is acquired at 0 azimuth, at +7 azimuth and at -7 azimuth. A dark image is also acquired for reference. 9. Repeat step 8 for each filter setting on the Imager Test Date 15 October Test Data See Appendix A Results The entire data set is available for analysis. The following images represent a small sample at one filter setting. Fig shows the set of three images taken with the 6300Å narrow band filter (filter B3), in the high resolution, unbinned mode. Included at the bottom of this figure are magnified (i.e. zoomed) samples of the central bar group, showing the limiting resolution of the Imager at these three different locations on the focal plane. The limiting resolution is essentially unchanged at all three field positions. In Fig , sample images are shown for the binned mode. As expected the resolution capability decreases, but the limiting resolution is clearly in the desired range. Using the images shown, contrast measurements were made at the spatial frequencies available from the test target. Contrast, C, is defined as: I C = I MAX MAX where, I MAX is the average brightness of the white bars and IMIN is the average brightness of the black bars in one bar group. Plots of the contrast vs spatial frequency, or CTF, are shown in Fig They show, for example, that the CTF in unbinned mode at 12 lp/mm is about 30%. In unbinned mode, the CTF plot exhibits some variance in the CTF at the higher frequencies. This is very likely due to the unavoidable phase errors that are introduced with bar targets Resolution Test Limitations Due to limitations in the test setup, the requirement to test at 1 msec frame time was not achieved. The light levels required for this was beyond what was available. In lieu of test, the analysis in section shows that the Imager exposure time control provides linear response down to this level, and no change in resolution is expected as a result of shorter exposures at higher light levels. Also note that the images in fig are taken with 0.77 ms exposure duration at 1.37ms repetition period. This is the fastest rate supported by the Imager, and demonstrate the 1 ms exposure capability. A second more difficult aspect of the test is the spectral response of the collimating lens. An examination of the resolution test image for filter B2 (7620 Ǻ) shows fairly poor results. Therefore, additional resolution data was taken using the paraboloid collimator described in section With this type of collimator, a point source is projected onto the focal plane of the Imager, and a measure of the point spread function is made. A discussion of this test is included in section These results verify that the collimating lens introduces spectral degradation at 7620Å. I + I MIN MIN 6

7 Filter: 6300Å, Unbinned, 0 rotation Filter: 6300Å, Unbinned Left 7 Filter: 6300Å, Unbinned Right 7 Unbinned Left Zoom Unbinned Center Zoom Unbinned Right Zoom Fig : Resolution Target Images - Unbinned Mode at Filter B3 (6300 Å ) 7

8 Filter: 6300Å, Binned (Low Resolution) Center Binned Left Zoom Binned Center Zoom Binned Right Zoom Fig : Resolution Target Images - Binned Mode at Filter B3 (6300 Å ) 8

9 Normalized Horizontal CTF - Flight Unit filter B3 Center Unbinned - 15Oct measured data 90 quadratic fit Normalized Horizontal CTF - Flight Unit filter B3 Left Unbinned - 15Oct2002 measured data quadratic fit CTF (%) CTF (%) Spatial Frequency (lp/mm) Spatial Frequency (lp/mm) Normalized Horizontal CTF - Flight Unit filter B3 Center Binned - 15Oct measured 90 quadratic fit Normalized Horizontal CTF - Flight Unit filter B3 Left Binned - 15Oct2002 measured quadratic fit CTF (%) CTF (%) Spatial Frequency (lp/mm) Spatial Frequency (lp/mm) Fig : CTF Plots for Unbinned and Binned Images Pinhole Resolution Test Results Using the test setup described in section 2.2.2, a pinhole target was used to create a point source illumination on the focal plane of the Imager, and make a discrete measurement of the point spread function. As noted in the description of the setup, the image of the pinhole target on the focal plane is smaller than one pixel. The pixel pitch is 41.8 μm, while the pinhole image projected onto the focal plane is about 21 μm. Typical images of the pinhole are shown in Fig A plot of a horizontal line profile through the pinhole image for each channel of the Imager is shown in Fig This plot is a representation of the PSF for each channel. 9

10 Perhaps more informative is the tabulation of data in Table Values are calculated for the ratio, R, given as follows: R = I P I where I P is the signal in the central pixel of the pinhole image, and the denominator is the sum of all the pixels in the line profile. This ratio provides a relative measure of the point spread function that can be used to compare the performance of the channels. Data for both the horizontal and vertical line profiles are included. The data in Table suggests several things: 1) that the relative resolution of the narrow band channels are comparable, 2) that the wide band channels both indicate relatively poorer resolution, and 3) that the resolution is essentially the same in either the vertical or horizontal case. n Sprite Imager - Flight Unit - Horizontal PSF (18Oct02) N21P Open Normalized Signal Open N21P Pixel position Fig : PSF Plots for Binned Mode Table 2.1-1: Measurements of Imager PSF Relative Energy in Central Pixel Channel Horizontal Line Vertical Line N 2 1P Ǻ Ǻ Ǻ Ǻ Open

11 2.2 Imager FOV / Boresight Alignment Measurement Imager boresight alignment and Field of View is measured using a pinhole target projected into the Imager's lens using an external collimator, as shown in the Fig This test setup uses a paraboloid collimator so that all wavelengths are projected without degradation caused by any refractive optical elements Requirement The system specification includes two requirements for FOV. "The Sprite Imager shall have a field of view (FOV) equal to 20º x 5º." "The imager's instantaneous field of view (IFOV) for each CCD pixel shall be no larger than 0.04º x 0.04º." The specification also calls out an image format of 512 x 128 pixels Setup All test equipment is set up on a standard optical table. The physical set up is similar to that found in the image in Fig , except that in this case we do not use the monochromator. A xenon arc lamp is used to provide a wide band source. The paraboloid collimator has a 60 inch focal length, so a fold mirror is used to keep the setup compact. One consequence of this is a light shield around the target is necessary to keep the direct illumination out of the Imager FOV. The paraboloid mirror is mounted on a manual rotary stage to facilitate alignment. The position and angular orientation of the fold mirror and paraboloid section are carefully aligned with a laser. Then a flat mirror is positioned on the rotator such that it projects the collimated beam back onto the target, and forms an image of the pinhole. In this manner, the position of the target is adjusted so that the reflected image is the same size and location as the target pinhole. This is how the system is focused. The pinhole used is 0.02 inch (0.51mm) diameter. The image diameter, d, created at the focal plane of the Imager is much smaller and can be found from the following formula: d = f F D where, F = focal length of paraboloid section, or 60 in (1.52m). f = focal length of Lens under test, or 61.8mm for the Imager. D = diameter of pinhole target,.51mm. From this relationship, the point source formed is about 21 μm. Pixel pitch is 41.8 μm. The position of the Imager with respect to the rotation stage is fixed by a mounting plate, and this positions the point of rotation coincident with the entrance pupil of the Imager. The Imager is mounted on the rotator using an intermediate, linear translation stage. This provides a method to shift the imager orthogonal to the beam direction, and allow us to bring the alignment cube into the beam. In this manner, the alignment cube reflects an image of the pinhole back onto the target, and the fold mirror tip and tilt are adjusted to bring the beam normal to the front face of the alignment cube. The translation stage then allows us to move the Imager back to its on-axis position, while keeping the beam precisely aligned to the alignment cube. In this manner, the Imager's angular displacement relative to the boresight direction is determined. To generate a collimated beam that is uniform, two diffusers are used. An opal glass diffuser (OGD2) is used immediately behind the pinhole target to provide a very low f/#, divergent beam into the paraboloid. In addition, a ground glass diffuser (GGD) is used in the filter holder location to provide an even more diffuse beam illuminating the opal glass. 11

12 Not shown is the data capture system. The Imager data is captured directly at the J2 interface using a PCbased, digital frame grabber board. A test panel is used to set and control the MCP and Phosphor high voltages. This set up requires that the Imager is operated in a dark room, so the PC must be located such that it does not produce stray light. Fig shows the Imager set up for measuring horizontal FOV. The set up is changed, moving the rotator into a vertical orientation, to measure vertical FOV. This change is illustrated in Fig Note that rotation angles are positive for CW motions on the rotation stage, as illustrated in Fig Pinhole Light Shield Filter Holder Shutter Fold Mirror Xenon Arc Lamp Source Paraboloid Mirror Rotation Angle + Device Under Test (Imager Shown) Mounted on Rotator with Translation Stage Fig : Imager FOV/Boresight Test Schematic Rotator Translation Stage Rotator Translation Stage Horizontal FOV Setup Vertical FOV Setup Fig : Imager Setup for Horizontal and Vertical FOV Measurements 12

13 Test Equipment Used Item Description Manufacturer Model 1 Xenon Arc Lamp, 20V, 150W, ozone free Oriel Arc Lamp Housing Oriel Arc Lamp Power Supply Oriel Filter Holder Oriel Target: Pinhole, 0.020" diam, 2" square Al plate SSL n/a 6 Fold Mirror, 4" square, front surface n/a n/a 7 Paraboloid mirror, 6" diam, 60" focal length n/a n/a 8 Motorized Rotation Stage,.01º abs. accuracy Newport RV120PE 9 Motion Controller, Rotation stage stepper motor Newport ESP Manual Rotary stage Newport 481-A 11 Optical rail with carrier (translation stage) Oriel / Opal glass diffuser (OGD2), 25mm diam, 5 mm th. Edmund Ground glass diffuser (GGD), 50mm sq., 2 mm th. Edmund FOV Fixture Instrument Adapter SSL 8937-A4B 15 Rotary Mount Adapter SSL 8935-A4A 16 FOV Fixture Slide Adapter SSL 8936-A4A 17 FOV Fixture Slide Spacer SSL 8934-A4A Procedure 1. With the equipment set up as shown, the Imager is initially mounted on the rotation stage with a rotation angle setting of The arc lamp power supply incorporates an internal ignitor which produces a significant high voltage pulse that can damage sensitive electronics. Consequently, it must be powered On only when the Imager is disconnected from all electrical devices, so that electromagnetic pickup is minimized. Leave other test equipment Off as well. Turn on the arc lamp, then turn on other test equipment. Reconnect the Imager electrical connections. 3. Position Imager such that alignment cube is illuminated by the collimated beam. Remove light shield. In darkened room, observe reflected pinhole image on the target plate. Adjust fold mirror tip and tilt such that reflected pinhole co-aligns with pinhole. Re-position light shield. Move Imager back to its on-axis position. Adjust filter wheel to desired filter position. With a white sheet of paper, verify that collimated beam is centered on Imager aperture, and that it appears faint and uniform. 4. In a darkened room, power ON Imager FEC. Verify that data collection is working. Turn on Phosphor HV. Phosphor Vmon should indicate about 5700V. Then adjust MCP high voltage, while continuously monitoring the updating image display to make sure there are no inadvertent bright 13

14 light sources. Bring up MCP to a level such that point source image highlight is about mid-scale intensity, or 2000 ADU. 5. Using the rotator control, rotate Imager to desired azimuth angle and record image data Test Date Horizontal FOV: Oct 2002 Vertical FOV: 21 Oct Test Data Some example images using the pinhole target are shown in Fig These image segments were taken in Binned mode, using filter B3 (6300 Å), at various rotation angles as noted. A log of all images taken is included in Appendix D Fig : Point Source Image examples, at various horizontal azimuth angles One horizontal angle scan and one vertical angle scan were performed as "calibration scans." These two scans are meant to establish two things: 1) the boresight alignment of the Imager with respect to the optic alignment cube, and 2) the Field of View of the Imager. Three other scans were performed in both horizontal and vertical directions to provide data to correct the images for geometric distortions. This latter analysis has not been completed. As an example, in the horizontal scans, there is an "Upper Scan", a "Middle Scan" and a "Lower Scan". These scans were performed at arbitrary vertical angles, but each image was taken at very specific horizontal rotation angles. The vertical angles can be deduced from the vertical angle calibration. To obtain the FOV measurement, the rotation angle was stepped in increments of 0.02º until the pinhole image began to disappear. Then the angle was decremented one step. The maximum angles recorded for the horizontal and vertical "calibration scans" indicate this procedure. The data recorded in Appendix D shows an (X,Y) coordinate for each image file. These coordinates were computed by taking the centroid of the pinhole image, in an effort to gain fractional-pixel resolution. A definition of the image coordinate convention is given in Appendix C. In Appendix D, there is reference to a "GSE X Correction." Imager FOV test data was acquired using a GSE test set that captures data directly into frame grabber hardware. The files saved from this acquisition hardware only record the central 520 x 128 pixels of the image. In contrast, the Imager, when operating with the AEP Mass Memory board captures an image that is 524 x 128. The additional two columns of pixels, on each side of the image, are dark pixels. To correlate the boresight data taken during these tests, with data acquired from the flight instrument requires that a coordinate correction is made. It simply involves adding an offset of 2 to the X-axis coordinate in the data files Results The results of the FOV and boresight testing are summarized in Table

15 Table 2.2-1: Summary of Imager FOV and Boresight Measurements Measured Values Requirements Image Size (H x V pixels): 516 x x 128 Field of View (H x V): 20.24º x 5.09º 20º x 5º Instantaneous FOV (H x V): x º x 0.04º Boresight Location (X,Y): , n/a The image format is somewhat larger than specified, i.e. 512 x 128 pixels. This is due to the fact that the Dalsa IA-D4 CCD is designed to provide a clear aperture that is at least 1024 pixels wide. It incorporates 4 "isolation columns" on each side of the image that provide isolation between the clear aperture and the shielded dark columns. In fact, these isolation columns are clear and usable pixels. Since the Imager is operating the CCD in binned mode, these 4 columns show up as 2 extra pixels on each side, or 4 extra pixels per row, yielding a row with 516 pixels, rather than 512. Instantaneous FOV, IFOV, is calculated by: FOV IFOV = N 1 where FOV is the measured FOV, either vertical or horizontal, and N is the corresponding number of pixels, either vertical or horizontal. The image size is given as ( N 1), since the FOV is measured to the "middle" of the edge pixel. IFOV, especially in the horizontal direction, is an average value. The horizontal IFOV value increases in size closer to the center of the image. 15

16 2.3 Imager Responsivity and S/N Measurement Imager responsivity and S/N is measured using a set up similar that used to measure the FOV. In this case, a monochromator is used in conjunction with a xenon arc lamp, to provide a spectrally selective light source. Also, a larger target aperture, 0.5" diameter, is used instead of the pinhole target. The target image is projected into the Imager's lens using an external collimator. As before, this test setup uses a paraboloid collimator so that all wavelengths are projected without degradation caused by any external optical elements Requirement The system specification only requires a minimum level of signal-to-noise ratio (SNR). It states the following table Setup Frame time = 1.00ms Frame time = 30.00ms Radiance (MR) Electrons SNR All test equipment is set up on a standard optical table. A diagram of the optical set up is shown in fig The physical set up is pictured in Fig The Triax 320 monochromator and xenon arc lamp is used to provide a programmable, monochromatic source. The paraboloid collimator has a 60 inch focal length, so a fold mirror is used to keep the setup compact. One consequence of this is a light shield around the target is necessary to keep the direct illumination out of the Imager FOV. The paraboloid mirror is mounted on a manual rotary stage to facilitate alignment. The aperture used is 0.5 inch (12.5mm) diameter. The image diameter, d, created at the focal plane of the Imager is much smaller and can be found from the following formula: d = f F D where, F = focal length of paraboloid section, or 60 in (1.52m). f = focal length of Lens under test, or 61.8mm for the Imager. D = diameter of target aperture, 12.7mm. From this relationship, the image formed is about 516 μm. Since pixel pitch is 41.8 μm, the image of the 0.5" aperture is about 12 pixels in diameter. 16

17 Filter Holder Aperture Triax 320 Monochromator Xenon Arc Lamp Source Light Shield Fold Mirror Focus Paraboloid Mirror Device Under Test (Imager Shown) Mounted on Rotator with Translation Stage Fig : Imager Responsivity Test Schematic Fig : Imager Responsivity Test - Physical Set Up 17

18 Test Equipment Used Item Description Manufacturer Model 1 Xenon Arc Lamp, 20V, 150W, ozone free Oriel Arc Lamp Housing Oriel Arc Lamp Power Supply Oriel Filter Holder Oriel Aperture: 0.50" diam, 2" square Al plate SSL n/a 6 Fold Mirror, 4" square, front surface n/a n/a 7 Paraboloid mirror, 6" diam, 60" focal length n/a n/a 8 Motorized Rotation Stage,.01 abs. accuracy Newport RV120PE 9 Motion Controller, Rotation stage stepper motor Newport ESP Manual Rotary stage Newport 481-A 11 Optical rail with carrier (translation stage) Oriel / Opal glass diffuser (OGD1), 38mm diam, 2.2mm th. Oriel Opal glass diffuser (OGD2), 25mm diam, 5 mm th. Edmund Optical blocking filter (OBF385), 385nm cut on Oriel Optical blocking filter (OBF675), 675nm cut on Oriel Stainless Steel Mesh, optical attenuator InterNet Inc. BE FOV Fixture Instrument Adapter SSL 8937-A4B 18 Rotary Mount Adapter SSL 8935-A4A 19 FOV Fixture Slide Adapter SSL 8936-A4A 20 FOV Fixture Slide Spacer SSL 8934-A4A 21 Photodiode, Hamamatsu S2281, with calibration NIST E Lens, 50mm, f/2, fused silica Oriel Monochromator, f/4.1, 0.32m focal length J.Y Horiba Triax Picoammeter Keithley Aperture Diffusers and Mesh Attenuators A number of variations are made to the set up during the responsivity measurements. These involve the use of various diffusers and beam attenuators. The diffusers are used to achieve uniformity in the collimated beam. Stainless steel meshes are used as attenuators to achieve light levels commensurate with the sensitivity of the instrument, but that allow us to separately measure the radiance level using a photodiode. Meshes are used so that attenuation remains constant over the wavelengths of interest. In addition, the monochromator has settings that affect the overall purpose of each test. The settings of these 18

19 devices are summarized on the test data sheets. Referring to the schematic in Fig , these devices and their locations are summarized below: Grating: Slit width: Either Turret 0 (250nm blaze), or Turret 1 (500nm blaze) on monochromator Input/Ouput slit width, spectral dispersion is 2.64 nm/mm Mesh P1: Used in the filter holder position, measured attenuation is Mesh P2: Placed between aperture plane and light shield, measured attenuation is OBFnnn: OGD1: OGD2: Calibrated Photodiode Optical blocking filter, placed in filter holder, either OBF385 or OBF675 Opal glass diffuser, used in filter holder Opal glass diffuser, positioned immediately behind the aperture In Fig you'll notice an apparatus in front of the Imager. This is the NIST calibrated photodiode (item 21) used to measure the absolute light level. A collimating tube and light shield are in front of the photodiode module. The photodiode is a silicon detector, whose collection area is 0.5 cm 2. To increase its effective aperture, a simple plano/convex fused silica lens (item 22) is used to focus the collimated beam onto the photodiode. Fig is a plot of the effective increase in aperture, as a function of wavelength. This plot shows that the increase in aperture is relatively constant over the wavelengths of interest. Consequently, the effective area and diameter are increased as follows: without Lens with Lens Detector Area: a = 0.5 cm 2 A = 4.44 cm 2 Diameter: d = cm D = 2.38 cm 9 Gain in Photodiode Aperture with Lens - 50mm f/2 fused silica 8.95 Aperture Gain Mean increase in aperture = Wavelength (nm) Fig : Ratio of Photodiode signal (with Lens)/(without Lens) 19

20 2.3.3 Procedure 1. With the equipment set up as shown, the Imager is mounted on the rotation stage with a rotation angle setting of Select the desired filter on the Imager. 3. The arc lamp power supply incorporates an internal ignitor which produces a significant high voltage pulse that can damage sensitive electronics. Consequently, it must be powered On only when the Imager is disconnected from all electrical devices, so that electromagnetic pickup is minimized. Leave other test equipment Off as well. Turn on the arc lamp first. Then turn on other test equipment. Reconnect the Imager electrical connections. Let the arc lamp stabilize for 15 minutes prior to beginning data collection. 4. The monochromator is controlled with a Labview program, JYSCAN5. After turning on the device, initialize it using this program. Once initialized, set the desired turret, slit width and wavelength. 5. Select the desired filters, meshes and diffusers to be used in the light path. 6. In a darkened room, power ON Imager FEC. Verify that data collection is working. Turn on Phosphor HV. Phosphor Vmon should indicate about 5700V. Then adjust MCP high voltage, while continuously monitoring the updating image display to make sure there are no inadvertent bright light sources. Bring up MCP to a level such that source image highlight is about mid-scale intensity, or 2000 ADU. 7. Record image files of the source. Adjust light level (using meshes P1/P2) and/or MCP voltage, and record additional images. Also record a "dark" image by closing the monochromator output slit. 8. Rotate Imager through various azimuth angles to obtain response vs field angle. 9. Vary the wavelength to obtain a measure of the relative response vs wavelength for each filter. 10. Place photodiode module with its light shield into beam and measure photodiode response Test Date Jan Responsivity Computation The calibrated photodiode with lens is used to measure the radiance, L, of the collimated beam viewed by the device under test. For monochromatic light, radiance, L, can be calculated as follows: where: I L = C D D TP A Ω W 2 cm sr I D = measured photodiode current (A), C D = responsivity of photodiode (A/W), i.e. the calibration factor at test wavelength, λ, T P = attenuation provided by meshes P1 and/or P2, A = area of photodiode aperture (cm 2 ), and Ω = solid angle subtended by source aperture over the focal length of collimator, in steradians (sr). For monochromatic light, radiance is further converted to photon flux, P N, by: L λ = h c photons s cm sr P N 2 20

21 where: λ = test wavelength (m), h = Planck's constant, 6.626E-34 (J s), and c = speed of light, 2.998E+8 (m/s). Finally, we convert the photon flux into units of Rayleighs, or P R, by: P PN 4π = 10 R 6 ( Rayleighs) This value for the photon flux is related to the signal level (in counts s -1 pixel -1 ) found in the recorded image data files, and these recordings are summarized in the data sheets included in Appendix E. The calculated responsivity, R, is the count rate per mega-rayleigh, in counts s -1 pixel -1 per MR Responsivity Results The ½" target aperture used with the collimator produces on the focal plane, a circular image that is about 12 pixels in diameter. Fig is a composite of 10 images showing the resulting image of the target at various azimuth angles, namely -9º, -7º,,-1º, +1º, +3º,,+9º. This composite was made using images from channel 3, i.e. the 6300Å filter. Fig 2.3-4: Composite image of ½" target at various azimuth angles. When measuring the signal level, the central portion of the illuminated pixels was sampled to obtain the mean signal level per pixel. The sampling region used was 7 pixels in diameter, resulting in a total sample size of 37 pixels. From this sample, the mean and standard deviation are calculated. In addition, the mean and standard deviation of the corresponding "dark" image is also recorded. Measurements were made for each channel of the Imager, by measuring the signal response of the Imager at various MCP voltage settings, using the mesh attenuators to obtain very low light levels. These measurements with the Imager were followed by measuring the absolute light level using the calibrated photodiode. The data sheets found in Appendix E details all the data taken for each channel. The signal levels, measured in A/D units (ADU), or counts, taken from each recorded image file is included on the data sheets. Table summarizes the results for the narrow band channels. 21

22 Table 2.3-1: Summary of Responsivity Measurements for Narrowband Channels Channel MCP Voltage Responsivity (counts/sec/mr) Channel MCP Voltage Responsivity (counts/sec/mr) x x x x x x x x x x x x x x x x x x x x x 10 5 The data in Table is also plotted. These plots show the responsivity of each channel vs MCP voltage. After collecting data from Channel 3, over the full MCP voltage range, this full data set was not repeated for each channel, since the relative gain due to MCP voltage is the same for each wavelength. Using this MCP gain characteristic measured on Channel 3, a basic exponential curve is fitted to the data. This curve fit is then scaled to fit the measured data for the other narrow band channels and used to extrapolate over the full MCP voltage range. Fig and are plots for the wide band channels. For these channels, the responsivity is plotted vs wavelength, at a single MCP voltage. These plots also illustrate the resulting spectral passband of these channels, and the relative response within their respective passbands. 22

23 10 7 measured curve fit Imager Responsivity 630nm 10 6 Counts/MR MCP Voltage (V) Fig : Imager Channel 3 Responsivity Imager Responsivity 762.0nm 10 6 measured curve fit 630nm 10 5 Counts/MR relative to 630nm MCP Voltage (V) Fig : Imager Channel 2 Responsivity 23

24 Imager Responsivity 557.7nm 10 6 measured curve fit 630nm 10 5 Counts/MR relative to 630nm MCP Voltage (V) Fig : Imager Channel 4 Responsivity 10 6 measured curve fit 630nm Imager Responsivity 427.8nm 10 5 Counts/MR relative to 630nm MCP Voltage (V) Fig : Imager Channel 5 Responsivity 24

25 4.5 5 x 104 Imager Responsivity N21P at MCP 630V measured Counts/MR Wavelength (nm) Fig : Imager Channel 1 Responsivity 12 x Imager Responsivity Open Channel at MCP 670V measured scaled 8 Counts/MR Wavelength (nm) Fig : Imager Channel 6 Responsivity 25

26 The data shown in Fig has been manipulated to a small degree. Data recorded for the calibrated photodiode was obtained in two scans, using the Labview monochromator software. A scan was taken in the range of 390nm to 800nm, using the OBF385 blocking filter, then a second scan was taken in the range of 790nm to 1000nm using the OBF675 blocking filter. There is a small overlap in wavelength between the two scans. In addition, image data was recorded for both blocking filters in this overlap region. The recorded photodiode data in the second scan indicates a signal level about a factor of 6.2 lower in level than what is recorded in the first scan, based on the ratio of the data for the overlapped wavelengths. However, the imager data indicates very comparable levels of signal for either blocking filter in this overlap region. This indicates that the second photodiode scan is attenuated for some reason, and the source of this attenuation is unknown. For the plot in Fig , the photodiode data from this second scan was scaled, and the resulting responsivity measurement is shown as a dashed line Signal-to-Noise Ratio (SNR) The standard deviation of the signal levels, recorded for the responsivity measurements, is used here as a measure of the noise. In fig the noise is plotted for the data taken for Channel 3, i.e. the 6300Å channel. Since the variance will increase with signal level, the noise, σ N, has been normalized to σ N, according to: σ = σ N N S S MAX where S MAX = mean signal level at maximum MCP voltage, and S N = mean signal level at sample of interest. Fig also indicates the level of signal variance expected if shot noise as measured by the CCD were the only noise source. As seen in the plot, the trend in the data shows the noise increasing exponentially as MCP voltage is increased. N 26

27 measured noise noise trend CCD shot noise Imager Noise vs MCP Voltage - Normalized 100 Noise (ADU) MCP Voltage (V) Fig : Imager noise vs MCP voltage, Ch3 (6300Å) This noise plot is only showing the variance in the signal. Note that this approach does not take into account variance due to non-uniformity in the illumination levels within the sample aperture, nor does it take into account non-uniformities in the pixel-to-pixel response. Both of these sources add to the measured variance. SNR is also calculated and plotted in Fig Using the responsivity measurements, the SNR is plotted as a function of light level, as measured in MR-sec. This plot indicates performance that compares favorably with expectations. 60 measured SNR SNR trend Imager SNR vs Light Level -- Normalized 50 Signal-to-Noise Ratio Light Level (MR-sec) Fig : Imager SNR vs Light Level 27

28 2.3.8 Passband Verification Verification of the spectral passband for each Imager channel was performed during the tests using the monochromator. This verification was done at fairly coarse resolution, since the Imager filters had been previously characterized. The verification data for the wideband channels is shown in Fig and The data for the narrow band channels is shown in Fig Note that in Fig the signal data has been normalized to account for changing light levels within the passband. The requirements for each channel are given the following table: Band Number B1 B2 B3 B4 B5 B6 Center Wavelength (nm) 690±10 762±3 630± ± ±3 625±25 50% Bandwidth, or FWHM (nm) 125±15 7±2 7±2 6±2 6±2 450±40 1 Channel 2 (762nm) 1 Channel 3 (630nm) Signal Channel 4 (557.7nm) 1 Channel 5 (427.8nm) Signal Wavelength (nm) Wavelength (nm) Fig : Imager passband verification Appendix F includes the high resolution filter characterization plots previously done for each of the Imager filters. 28

29 2.3.9 Exposure Time and Linearity As indicated in the data sheets, a test of Imager linearity was made, again using Channel 3, the 6300Å channel. This data is plotted in Fig The data taken for this plot was obtained strictly by adjusting the exposure duration setting. This data indicates that the Imager is saturating, or becoming non-linear, at signal levels above ~2800 A/D units ISUAL Imager Linearity - HV=695-18Jan ISUAL Imager Linearity - HV=695-18Jan Signal (ADU) Signal (ADU) Exposure (ms) Exposure (ms) Fig : Plot of Imager Signal vs Exposure Time 29

30 2.4 Imager Flat Field, or Field Uniformity, Measurement To obtain a measure of the non-uniformity of response in the Imager focal plane, many flat field images were taken in various configurations Setup The general set up is illustrated in fig While the Imager is shown here in its completed form, a series of flat field images were taken in the following configurations: 1. with the bare Imager detector without it's internal optics (i.e. without a lens), 2. with the lens, but without the filter wheel and collimator assembly, and 3. with the complete assembly, i.e. with the filter wheel and collimator assembly. Sprite Imager Incandescent Lamp Filters Iris Integrating sphere Diffuser Fig : Set up for Flat Field Imaging Test Dates Configuration Description Dates File Directory Results A without lens Aug 22-23, 2002 \FlatFieldNoLens B with lens, without filter wheel Oct 1, 2002 \FlatFieldNoCollim C with filter wheel Sept. 24, 2002 \FlatFieldWithCollim Fig thru are sample images from this set of data. All images are taken in Binned mode. Appendix G gives a summary listing of image files. Subdirectory names indicate the wavelength of the illumination. Individual file names indicate mode and MCP voltage setting. 30

31 Fig : Flat field, Aurora Mode, Config. B, λ=700nm, MCP at 600V Fig : Flat field, Sprite Continuous Mode, Config. B, λ=700nm, MCP at 600V Fig : Flat field, Aurora Mode, Config. C, λ=630nm, MCP at 600V Fig : Flat field, Sprite Continuous Mode, Config. C, λ=630nm, MCP at 600V Aurora mode and Sprite Continuous mode images appear to have somewhat different flat field response. The magnitude of this difference has not been determined. 31

32 Gain Correction In Fig is plotted the average response vs horizontal pixel position. This uses the central 480 x 120 pixels on the focal plane, and averages all 120 rows to obtain a single line profile. The top plot in the figure clearly shows the offset at the boundary between the left and right channels of the CCD. The bottom plot in the figure has removed this discontinuity by first subtracting the bias levels from each channel, then multiplying the right channel by (6.48% increase) to compensate for the difference in CCD output gain. All the remaining plots have been processed in this same manner, and then also normalized Average Response vs Horizontal Position - Raw Data Signal (ADU) Right Channel with Gain Correction Signal (ADU) Pixel Position Fig : Average line profile, Right channel gain correction Flat Field Response In fig is plotted average line profile for the flat field taken in Configuration B, i.e. without the filter wheel, but with the internal lens. Both Continuous mode and Aurora mode is shown. No real difference is obvious between these two modes. The average line profile for Configuration C is shown in fig For this configuration, with the filter wheel and collimator installed, there appears to be some additional reduction in off-axis response, when compared to the plots for Configuration B. 32

33 1 Average Response vs Horizontal Position - Continuous Mode Signal Response Aurora Mode (both plots w/o filter wheel) Signal Response Pixel Position Fig : Average line profile, Configuration B 1 Average Response vs Horizontal Position - Continuous Mode Signal Response Aurora Mode (both plots with filter wheel) Signal Response Pixel Position Fig : Average line profile, Configuration C 33

34 2.5 Imager Bright Flash Tolerance To test the "bright flash tolerance" of the Imager, a special light source was built that could deliver short bright pulses of light, followed immediately by short, dimmer pulses of light. This is intended to simulate the situation that could occur when viewing lightning flashes followed by the much dimmer sprites Setup The test box consists of a photographic flash to simulate the lightning flash, and it incorporates an incandescent lamp with a shutter to simulate the sprite. These devices are housed in a black plastic box, with appropriate holes cut in the sides. Therefore, there are two ports in the side of the box. One is the "lightning port", and the other is the "sprite port". The sprite port is 0.5 inch in diameter. A 650nm bandpass filter, ND4 filter and diffusing glass are used to create a diffuse source that is illuminated by a 20W incandescent lamp. A shutter mounted immediately in front of the port provides a relatively short pulse of light. The brightness of the port was measured with a calibrated photodiode, and found to be about 1 x 10-8 W sr -1 cm -2. The minimum duration pulse available using the shutter is about 9ms. The lightning port is also 0.5 inch in diameter. The flash illuminates a 700nm bandpass filter, an ND3 filter and a diffusing glass. A fast photodiode was used to measure the relative brightness of the two ports, and the lightning-to-sprite ratio was found to be about 6000:1. The flash pulse is about 400μs. The image in Fig is taken of the test box, using the Imager. The lightning port is the bright, circular reflection in the upper right half of the image. This is a reflection from the diffuser glass. The sprite port is the circular aperture near the center of the image. Fig : Image of Flash Tolerance Test Box For the test, the Imager is operated in Sprite Continuous mode, and is programmed to output 0 pre-trigger images, and 7 post-trigger images. So the first image in the sequence is the "event-trigger" image. The lightning flash is synchronized to occur coincident with the camera exposure, i.e. the trigger flash is nominally coincident with the intensifier gating pulse. The exposure repetition interval is 1.37 ms, and each exposure is.77 ms long Test Date Mar 1,

35 2.5.3 Results 1) t = 0.00ms 2) t = 1.37ms 3) t = 2.74ms 4) t = 4.11ms 5) t = 5.48ms 6) t = 6.85ms 7) t = 17.30ms Fig : Sequence of images from Flash Tolerance Test 35

36 The results of the test can be seen in Fig In image 1, the bright flash is from the lighting port. Light leaks internal to the test box are evident, because a faint light is seen near the sprite port. In image 2, the lightning flash still seems to be glowing. This might be intensifier phosphor decay (from a highly overloaded condition), because the flash itself is only 400 μs long. The sprite starts to show up on image 4, near the center of the image. This is consistent with the time for the shutter to start opening, relative to when it is commanded. The succeeding images show the shutter continuing to open. Note that the time interval between images increases between image 6 and image 7. This is because the camera has started reading out data, and the repetition rate has to slow down. The exposure duration remains constant. This repetition rate is the fastest supported by the Imager. 36

37 Appendix A - Imager Resolution Test Data Summary 37

38 Appendix B - Imager Resolution Test Target Spatial Frequency Summary 38

39 Appendix C - Imager FOV and Boresight Test Data Coordinate Convention X Y Image Pixels The coordinate convention for the FOV and Boresight Images is illustrated above. The origin of the X-Y axis is coincident with the upper left hand corner of the image. X-values increase toward the right. Y-axis values increase toward the bottom of the image. When calculating centroids of point sources, fractional pixel locations follow the convention as shown. For example, the center of the upper, left hand pixel is at location (X,Y) = (0.5,0.5). 39