WISPR Design

Optical Design

Lakin Jones — Wed, 16 May 2018 20:24:25 +0000

The instrument’s telescope design is monolithic (with no moving parts) and uses radiationtolerant glass lenses mounted in lens barrels. It is based on the SECCHI/HI design and consists of two telescopes, the inner and outer telescopes with the optical parameters shown in the table below. The optical layout is shown in the figure below (right side). The resolution is optimized for the FOV center, 33.5◦ and 79◦, for the inner and outer telescope, respectively. BK7 was selected for the first lens element because it was shown to be sufficiently resistant to dust impacts. The bandpass for each telescope is selected using a combination of long/short wavelength cutoff filters deposited on internal lens surfaces similar to SECCHI/HI.

Side view of the WISPR instrument showing the exterior (F1–F3) and interior (I1–I7) baffles, and the two telescope assemblies. The dimensions and FOV of the two telescopes and CIE are also shown. For instrument safety, no part ofWISPR can exceed the shadow line even under the maximum possible spacecraft offpoint of 2 deg

WISPR lens assemblies for the Inner (left) and Outer (right) telescopes showing the ray tracing results through the various lens surfaces

As can be seen from the table below, the current optical design is excellent. It provides both fast lenses (low F#) and high spatial resolution (∼2 pixels) for the inner and outer telescopes, respectively. This means that WISPR is potentially capable of capturing images at spatial resolutions of <2 arcmin (2200 km or ∼3 arcsec from 1 AU), which are comparable to eclipse imaging from Earth. This is truly remarkable for a wide-field coronal telescope and the capability will be exploited as mission and solar condition allow. However, the current observing plan is to obtain images with 2°ø2 binning, as is done for SECCHI/HI, to increase the SNR and reduce the telemetry load. Higher image binning (4 °ø 4) will be required at large heliocentric distances to maintain a minimum SNR of 5 at the outer edge of the FOV.

WISPR Optical Design

Telescope	FOV	Spectral Range (nm)	Entrance Pupil (mm)	F#	# of lenses	RMS Spot Size (μm)
Inner	40◦ × 40◦	490–740	7.31	3.83	5-element	19
Outer	58◦ × 58◦	475–725	8.08	4.04	6-element	20

The baffle design rejects the incident solar radiation using a combination of the heat shield leading edge, front baffle assembly, and aperture light traps. Scattered radiation from the spacecraft is eliminated using the interior and peripheral baffle assemblies. The WISPR baffle design is based on the successful SECCHI/HI instrument design (Socker et al. 2000). The combination of the heat shield leading edge and the series of three linear occulters in the front baffle assembly attenuate the stray light that reaches the entrance aperture. Figure 16 shows the inner telescope normalized irradiance of the diffracted light from the heat shield/front baffle assembly combination at the worst off-pointing case of 2◦ during science operations at the minimum perihelion of 9.86 Rs. The worst-case diffracted stray light on the detector is predicted to be 7.5e-13 B/Bsun, which increases to 1.4e-11 B/Bsun when all the other sources of stray light are accounted for (dust damage on the first lens, F-corona, and scattering from the two FIELDS antennas). This is still 55 times lower than the requirement of 7.9e-10 B/Bsun. To deal with the sharp brightness gradient of the corona close to the limb, the last baffle (F3) in the forward baffle assembly imposes some vignetting of the innermost part of the Inner Telescope FOV from 60 % at 13.5◦ to 30 % at 14◦. Also, the wide-field lens creates natural vignetting (increasing as cos4 of the angle from the boresight).

Diffraction profile for the combination of heat shield and forward baffle system. A1 is the entrance aperture of the inner telescope

The aperture light trap, including baffles AE1 and AE2, closes out the aft side of the entrance aperture and defines the aft Unobstructed Field Of View (UFOV) angle. The aperture light trap captures diffracted light from the F1 and F2 baffles, but does not directly intercept any diffracted light from the heat shield leading edge. The aperture light trap baffles are oriented toward the forward baffles such that no single reflection from the light trap directly enters the A1 aperture. The peripheral baffles limit stray light from surrounding spacecraft surfaces entering the interior baffle cavity. Following STEREO/HI, the interior baffles are CFRP panels coated with Aeroglaze Z307 to attenuate reflected stray light in the instrument. In addition, the interior baffles are oriented to prevent any single reflection of scattered light from spacecraft surfaces outside the aft UFOV from reaching the A1 aperture. The instrument is designed to remain below the direct solar radiation that comes over the heat shield leading edge from the sun disk edge throughout the entire PSP orbit for the worst-case off pointing. The shadow line in Fig. 15 defines this 8.07◦ solar exclusion zone based on the solar disk radius of 6.07◦ at the minimum perihelion of 9.5 Rs, the maximum failure mode off pointing of 2.0◦.

The baffle design directly drives the instrument volume. The design uses realistic baffle tolerances (e.g. 80 μm Z/220 μm X for F2/F3 baffles to F1 baffle; heat shield leading edge to F1 baffle tolerance given in Table 3 WISPR Instrument Characteristics) based on SECCHI/ HI and SoloHI experience. In addition, the instrument design includes a forward UFOV angle from the F1 baffle of 9.12◦ to avoid the heat shield leading edge for the worst-case tolerances. Overall, the current optical design meets the stray light requirements, even in the worst-case configurations of the FIELDS antennas and dust impacts.

Instrument Stray Light Control

Left: Model predictions of the stray light levels at Beginning-(BOL) and End-Of-Life (EOL) for the WISPR telescopes. The EOL predictions assume damage to 0.6 % of the lens area and use lab BSDF measurements from the dust-impacted glass. The higher levels in the inner telescope are a result of the much brighter scene at those elongations

Left: The improvement in stray light levels resulting from the single (top panel) to two-telescope (bottom panels) design change and from the optimization of the peripheral baffles

The control of stray light due to spacecraft accommodations has been the major focus of the WISPR team during the preliminary design phase of the project. The WISPR imager concept was a single wide-field lens, requiring an UFOV of 180◦. However, the FIELDS instrument needed to place its antennas on the sunward side of the spacecraft to sample the solar wind undistorted by the spacecraft charging effects. As a result, two of the antennas impinged either directly into theWISPR FOV or extended into the UFOV allowing diffracted sunlight to enter the aperture at unacceptable levels. In addition, the tips of the antennas will get so hot (∼1800 ◦C) that they will radiate in the visible region of the spectrum creating another (and novel) source of stray light. The only solution for allowing the instrument to operate was to baffle directly these two sources of stray light. In order to achieve this without sacrificing most of its FOV, theWISPR field-of-view was split into two separate imaging assemblies as discussed above.

This change allowed the design of peripheral baffles that capture the diffracted and radiated light from the antennas and reduce the stray light to acceptable levels as shown in Fig. 17. This is a preliminary result, however. The stray light modeling is performed via Monte-Carlo techniques with the FRED Optical Engineering software using a CAD model of the instrument and FIELDS antennas. This approach allows not only the modeling of the antenna diffracted and radiated light but also the testing of various coatings for the baffle surface and even the modeling of the effects of dust impacts during the mission as we see in Sect. 2.4. These new stray light modeling methods, driven by the need to accommodate occulting-like imagers in crowded spacecraft environments, far exceed the corresponding modeling efforts in past coronagraphs and imagers where tight controls of structure intrusions in the UFOVs were possible. They demonstrate that visible light imagers can be accommodated and operate safely even when structures intrude into their direct UFOVs.

WISPR Pub Number 1

WISPR Design

Mechanical Design

Lakin Jones — Wed, 16 May 2018 15:46:51 +0000

The WIM consists of a primary structure made from composite face sheets with a honeycomb core, which encloses two Focal Plane Assembly (FPAs) boxes, holding the detectors and detector readout boards (DRBs), two boxes for the baffles (interior and forward) and a plate for the peripheral baffle. In addition, the CIE is contained in a box attached to the rear of the primary structure and the radiators are mounted to the right side. The door mounts to the top and opens to the left.

The mechanical design of the Inner Telescope FPA showing the main components of the FPA. The F4 baffle provides additional stray light rejection for the Outer Telescope

Focal Plane Assembly (FPA) The FPA provides physical mounting, optical positioning, electrical connections, and thermal cooling for the APS detector (Fig. 18). TheWISPR FPA design is a slight modification to the SoloHI FPA to account for the smaller detector, the APS control electronics, a warmer operating temperature, and a shorter distance to the radiator plate. The APS detector is cooled passively by conducting heat through a cold finger to a radiator with a view to deep space. A 10 ◦C temperature drop between the radiator and the detector is expected based on the SECCHI/COR2 performance. No difficulties are expected on obtaining temperature <−55 ◦C since the SECCHI/COR2 CCD is operating at <−70 ◦C.

Baffles

The mechanical design incorporates three baffle systems (forward, interior, and peripheral), all of them made of Al 6061. The forward baffles are attached to the truss structure with a series of clips and include shims for individual baffle alignment. The clips and screws are located on the outside edge of the baffles well outside the FOV. The interior baffles are assembled as a unit, which is then is mounted in the interior of the primary structure via pivot mounts on the sides and a mounting flexure in the front with shimming capability. The function of the peripheral baffle is to prevent stray light from the FIELDS antennas entering into the instrument. It is basically an Al plate with cutouts around the two telescope apertures. Those cutouts define the FOV of the instrument.

Door

The one-shot WISPR door is a slight modification of the SoloHI door (Fig. 19). It is composed of several CFRP layers. The door blank is made using an invar mold for coefficient of thermal expansion (CTE) matching. It is mounted on the primary structures via two hinges. The Ejection Release Mechanism (ERM) is the sole WISPR mechanism (Fig. 19, right). It is a shape memory release device with a redundant firing circuit.

Instrument Mounts

WISPR is mounted on the +X, +Y (ram-side) panel of the PSP spacecraft with four bipod mounts (Fig. 20). The PSP spacecraft is a hexagonal design and hence there is no panel facing directly towards the ram direction. WISPR is rotated by −20◦ about the Z relative to the panel to optimize the coverage of structures to be encountered by the spacecraft. The somewhat unconventional adoption of two legs per mount (hence bipod) provides the necessary stability. The composite (Ti-Al) tube mounts keep the instrument primary structural frequency >80 Hz, address CTE mismatch between panel and instrument, and maintain the instrument to spacecraft alignment.

Left: The SoloHI door. Right: Mechanical components of the door shown on the SoloHI instrument. The WISPR and SoloHI doors will be identical except for size

View of WISPR from the rear of the spacecraft showing its orientation relative to the +X +Y panel

WISPR Pub Number 1

WISPR Design

Electrical Design

Lakin Jones — Fri, 11 May 2018 17:13:22 +0000

The WISPR electrical design builds upon the SoloHI development program and consists of two major components: the Camera Electronics (CE), provided by NRL, and the Instrument Data Processing Unit (IDPU), provided by the Johns Hopkins University/Applied Physics Laboratory (JHU/APL). Each component comprises several subsystems, which we describe briefly below.

Camera Electronics

The WISPR Camera Electronics control and read out the APS detectors for both telescopes and send raw camera images to the IDPU for processing. They consist of the Camera Interface Card (CIC), which communicates between the IDPU and the two telescopes, and the image acquisition circuitry for the two telescopes. The latter comprises the APS detector, the Detector Interface Board (DIB) and the Detector Readout Board (DRB) enclosed within the FPA for each telescope (Fig. 10).

Camera Interface Card

The CIC provides the electrical interface to the IDPU, routing of command/telemetry within the instrument, coordination of the inner/outer telescope readouts, signal chain and 14-bit A/D conversion of video from the two telescopes, and local analog telemetry acquisition. An RTAX1000SL FPGA provides the logic for the CIC. The CIC supports a Camera Link Interface (CLI) to GSE for early testing, and an interface to the IDPU, which provides: (1) a Command/Telemetry serial interface with 3.3 V LVDS async UART 19.2k BAUD, and (2) a serial pixel interface (SPI) with LVDS interfaces for serial header and video, 40MHz clock, and DVAL/LVAL/FVAL signals sent to the IDPU. The SPI supports a 2 Mpixels/sec readout with a ≤256 bytes header.

Active Pixel Sensor

The WISPR imaging detector is based on the Active Pixel Sensor (APS) developed by Sarnoff Corporation for the SoloHI investigation (Fig. 21). Table 5 summarizes the WISPR APS imaging specification. The detector is radiationhardened (operational after >1 Mrad exposure), has excellent performance in read noise, dark current, and full well capacity, and simplifies the drive electronics compared to CCDs. The APS detector includes the readout preamplifiers, the Double Correlated Sample and Hold circuitry, multiplexers and switches to access and read individual pixels. The capability to access individual pixels nearly eliminates the charge transfer efficiency (CTE) degradation from radiation damage and the image smearing in shutterless operation, present in the SECCHI/HI images. The device can operate under two gain modes: a high gain mode with full well >120,000 e− and ∼40 e− read noise, and a low gain mode with full well >20,000 e− and ∼7 e− read noise.

Left: WISPR APS Detector Design. The top and bottom halves (960 × 2048) can be read independently.
Right: The APS/DIB flight package. The two yellow flex cables connect the sensor to the DRB

WISPR APS Detector Performance Capability

Parameter	Capability
Format	2048×1920
Pixel (size, type)	10 μm, 5T PPD
Operating Temperature Range	<−55 ◦C
Technology	Jazz 0.18 μm
Power	<500 mW at 3.3 V
QE	>34.3 % average over 470–755 nm
Radiation Tolerance	Tested to 100 Krad
Read Noise (EOL, 95 % of pixels)	7–13 e−/pix
Dark Current (EOL, 95 % of pixels)	1.57–1.9 e−/s/pix
Linear Full Well (95 % of pixels)	20,000–21,300 e−/pix
Readout Rate	2 Mpix/s
Digitization	14-bit ADC
Cosmetics	95 % of pixels meet EOL requirements
Readout Modes	Progressive scan, global reset
Redundancy	Independent operation of each 960 ×2048 half

The WISPR APS detector utilizes the detector designs developed by the Solar Orbiter SoloHI program. The pixel design had been advanced in a series of ‘sandbox’ test runs under a Sarnoff development program (Korendyke et al. 2013). The performance of the detectors before and after radiation has been evaluated and documented during the SoloHI development program. The result of these tests raised the maturity level to TRL 6. To minimize dark current and potential radiation damage, the detectors will operate at moderately low temperatures (<−55 ◦C) using a cold finger passive radiator. The flight device fabrication has been completed and the selection and burn-in of flight candidates is underway. The WISPR program requires at least 4 flight devices (2 flight models and 2 flight spares).

Detector Interface and Readout Boards (DIB/DRB)

TheWIPSR Instrument Data Processing Unit comprises two cards (DPU and LVPS) in a Magnesium alloy enclosure

The DRB generates the readout sequencing and collects the raw video from the DIB, sets the (adjustable) bias signals for the APS, monitors the detector temperature and controls the operation of the calibration LEDs. An RTAX1000SL FPGA provides the logic for the DRB.

WISPR Pub Number 1

WISPR Design

IDPU

Lakin Jones — Thu, 10 May 2018 17:27:09 +0000

The IDPU is mounted internal to the Parker Solar Probe (PSP), on the inside of the bulkhead to which the WIM is mounted. It is a two-slice assembly consisting of the Data Processing Unit (DPU) slice and the Low Voltage Power Supply (LVPS) slice enclosed in a Magnesium Alloy package (Fig. 22). The LVPS provides secondary power to the WIM and the DPU. It receives 28 V switched power from the spacecraft and provides power control for the operational heaters. The DPU provides the primary interface to the spacecraft, breaking complex scheduled command sequences into primitive operational commands for the two cameras. The DPU commands the WIM, processes, compresses and, stores the WISPR images, distributes and collects housekeeping information and communicates with the spacecraft. The DPU also controls the operational heaters. The spacecraft controls survival heater power to the WISPR instrument directly and provides the door opening service. The WISPR IDPU derives its heritage from similar units on RBSP, CRISM and, MESSENGER.

IDPU Electrical

The figure below (Spacecraft-IDPU-WIM Harness and connectors) shows the connections from the spacecraft to the WIM, which consist of 9 cables. There is a power cable from the spacecraft to LVPS, 2 SpaceWire cables from the spacecraft to the DPU, a power cable from LVPS to WIM, Camera Interface cable from WIM to DPU, operational heater cable from LVPS to WIM, housekeeping cable from WIM to IDPU, survival heater cable from spacecraft to WIM, and the spacecraft-monitored thermistors from WIM to spacecraft.

Spacecraft-IDPU-WIM Harness and connectors. The functionality of each cable is also shown

The image below (IDPU Electrical Block Diagram) shows the electrical block diagram of the WISPR IDPU. The LVPS is implemented on a single 6.5__ °ø 4__ board and contains an inrush transient limiter, EMI Filter, a DV-to-DC converter (5VDC to IDPU, 3.3VDC digital and °æ6.6 VDC analog supplies to WIM), heater switch control, housekeeping ADC System, and convertor synchronization and ADC system control (provided by the digital board).

The DPU is implemented on a 6.5__ °ø 4__ board and contains point-of-load convertors, memory modules for the processor and data processing and an Actel RTAX2000 FPGA. The DPU FPGA contains the SKIP processor, which is a programmable FORTH processor, the Image processor, and all the attached interfaces as shown in Fig. 24. The image processing is performed by hardware in the FPGA. The SKIP processor handles housekeeping, and manages the image processor based on schedules commanded by the ground. The FPGA contains:

Clock and Reset distribution to generate Master Reset from redundant power-on reset chips, External Test Reset, and Internal Watchdog.
30 MHz SpaceWire and image processing clock.
7.5 MHz SCIP processor clock.
Multiple memory interfaces.
Image processor w/digital scope accessible test port.
SCIP processor.
Core I/O, which includes: Camera/Test UARTs, LVPS controls for heater switches and housekeeping ADCs, Voltage supply clocks, RMAP and SpaceWire node, and a40 MHz LVDS camera interface (FIFO to image processor).

IDPU Electrical Block Diagram.

All image processing takes place in the image processor (see block diagram below). The processor contains modules for common operations such as bias subtraction, pixel binning, compression, and packetization as well as modules specific to WISPR operations such as frame summing and a cosmic ray scrub operating on two images at a time. It has access to a 3 Gb SDRAM image storage, a 160 Mb SRAM image buffer, and 33.75 Gb of flash bulk storage, sufficient to store the full WISPR data volume for two orbits. The data are transferred to the spacecraft via SpaceWire at an average rate of 250 kbps.

IDPU Mechanical

The overall IDPU dimension is 21.2 cm (L) °ø 11.6 cm (H) °ø 5 cm (D). It weighs 1070 g and consumes 7.3 W (current best estimate). It is designed to operate between −25 ◦C and 65 ◦C and survive from −30 ◦C to 70 ◦C. The chassis, covers and shielding plate are made of 20 mm thick Mg ZK60A and is put together using Ti alloy (6AL4V). The preliminary structural analysis shows that the primary box and board modes are 192 Hz and 150 Hz, which exceed the 80 Hz minimum frequency requirement and demonstrate sufficient frequency separation.

Image processor Block Diagram showing the planned functions (cosmic ray scrubbing, image summing, binning, compression, packetization)

WISPR Pub Number 1

WISPR Design