WISPR Details

Design Philosophy

Lakin Jones — Tue, 15 May 2018 15:46:31 +0000

The WISPR Instrument Module (WIM) and its subassemblies. Two telescopes cover the WISPR FOV: the Inner and Outer telescope. Three baffle systems (Forward, Interior, and Aperture Hood) provide stray light control. The CIE controls the two APS detectors and is described in Sect. 3.3.1. The Door Latch release is the only WISPR mechanism. Most of the subassemblies are briefly described in Sect. 3

The WISPR design draws its heritage from the SECCHI heliospheric imagers aboard the Solar Terrestrial Earth Relations Observatory (STEREO; Kaiser et al. 2008) mission and from the SoloHI imager (Howard et al. 2013) under development for ESA’s Solar Orbiter mission scheduled for launch in 2017 (Müller et al. 2013). In fact, SoloHI provides many of the design elements and subsystems for adaptation into the WISPR design.

The WISPR instrument is being designed to live within a challenging set of science requirements and resource constraints. In order to achieve the necessary science, WISPR needs to take rapid sequences of images with highly variable signal content across an almost 90◦ FOV. To achieve this,WISPR uses a combination of baffle systems to greatly reduce incoming stray light, two optical systems to cover the large scene with uniform sensitivity, a novel low-powered radiation-hardened Active Pixel Sensor (APS) detector for each telescope, and an electronics chain with enough bandwidth to process images from both detectors and throttle the data down to meet spacecraft data transfer limits. The electronics and software are designed to meet the science requirements based on the conditions and environments predicted from 0.25 to 0.046 AU, while still allowing the flexibility to adapt to circumstances and observations beyond those requirements.

To optimize the science return of the mission, WISPR is located on the ram-side of the SPP spacecraft viewing the coronal structures to be encountered by the in-situ instrumentation (Fig. 8). This is also the reason that the radial FOV extends to 90 degrees elongation. This accommodation may expose the instrument to higher dust flux, during perihelion, than an anti-ram location but it is essential for providing the proper observations of the largescale structures that are being measured by the other SPP instruments, including the sources for any energetic particle events. Efforts are under way to understand and minimize the risk to the instrument from the inner corona environment as we discuss in Sect. 2.4. The adoption of a two-telescope design is driven by the need to accommodate the FIELDS antennas (Bale et al. 2014, this issue), which are located in front of WISPR, just behind the heat shield. With a single wide-angle lens system, two of the antennas would intrude into the unobstructed FOV of the lens leading to unacceptable stray light levels. Covering the WISPR FOV with two lens systems allows a more efficient masking of the reflections from these antennas and enables the safe operation of the instrument. This is discussed in more detail in the optical design section (Sect. 3.1).

WISPR Details

WISPR Pub Number 1

System Description

Lakin Jones — Sun, 13 May 2018 17:31:29 +0000

System Description Lakin Jones Sun, 05/13/2018 - 13:31

The WISPR instrument comprises two modules: (1) the WISPR instrument module (WIM), shown in Fig. 9, includes the structure, baffles, door, telescopes, focal plane arrays (FPA) and the camera interface electronics (CIE), and (2) the Instrument Data Processing Unit (IDPU) which consists of the Data Processing Unit (DPU) and the Low Voltage Power Supply (LVPS). The electronics functional block diagram is shown in Fig. 10. TheWISPR Camera Interface Electronics (CIE) is an adaptation of the SoloHI electronics. The data is transferred from the WIM to the IDPU via a serial data interface similar to Camera Link, is compressed and packetized and is then transferred to the onboard Solid State Recorder via SpaceWire.

WISPR Electronics functional block diagram. The IDPU (left) is located inside the spacecraft. The CIE (right) is located on the WISPR telescope.

The IDPU controls the two cameras, the door deployment and the operational heaters, receives the analog data, digitizes it to 14 bits, removes cosmic rays, and adds individual images together to increase SNR. The IDPU is described in detail in Sects. 3.4–3.4.2. The WISPR instrument concept is in effect a miniaturization of the SECCHI/HI concept with adaptations from the SoloHI design. The WISPR telescope volume (54.3 (L) °ø 21.7 (W) °ø 26 (H) cm) is about 2.5 times smaller than the SECCHI/HI volume (72 (L) °ø 42 (W) °ø 24 (H) cm). It is the smallest Heliospheric Imager to date with capabilities that meet or even exceed the performance of the SECCHI/HI. It is a two-telescope system, similar to SECCHI/HI, with an inner telescope extending from 13.5◦ to 53◦ and an outer telescope extending from 50◦ to 108◦ (Fig. 9). The instrument uses the spacecraft heat shield as the first occulter and hence the alignment between the heat shield and the first occulter baffle, F1 is a critical element for the successful control of the stray light (see Sect. 3.1). The inner FOV cutoff is set at an elongation of 13.5◦ from Sun center, corresponding to a heliocentric distance of 2.3 Rs at 9.86 Rs perihelion. The cutoff is dictated by two requirements: (1) to remain within the heat shield umbra (8◦, including a 2◦ maximum spacecraft offpoint), and (2) to accommodate the instrument on the spacecraft bus at a reasonable height and with reasonable mass. The overall instrument characteristics are shown in the table below.

WISPR Instrument Characteristics

Telescope Type	Wide-angle lenses, aperture stop placed in front of lens: Inner: f = 28 mm, aperture = 42 mm2, 490–740 nm (bandpass) Outer: f = 19.8 mm, aperture = 51 mm2, 475–725 nm (bandpass)
Plate Scale	1.2–1.7 arcmin/pixel (inner-outer)
FOV	95◦ radial × 58◦ transverse, inner field limit 13.5◦ from Sun center
Image Quality	Predicted RMS spot including allowable tolerances at 20◦ from boresight: Inner: 19.5 microns (2.34 arcmin) Outer: 19.9 microns (3.38 arcmin)
Detector	APS, 10 micron pitch, 2048 ×1920 pixels
Baffle Design/Stray Light Rejection	Front heat shield edge, forward baffle and diffraction light trap designed to reject incoming solar radiation, interior baffles and aperture enclosures designed to reject scattered solar radiation from spacecraft structures, and thermal radiation from antennas. Average predicted stray light: <2×10−9 B/Bs @ 9.86 Rs and <2×10−12 B/Bsun @ 0.25 AU, well below the K +F corona
Pointing	Instrument axes aligned to spacecraft to <0.5 deg, F1 and heat shield leading edge placement error <13 mm. Baffles achieve adequate rejection with 2◦ excursion from sun center at perihelion
Calibration	<20 % absolute radiometric, platescale <4 %, pointing: accuracy 5 arcmin (3σ), jitter 0.8 arcmin (1σ), windowed stability 1.6 arcmin (1σ)
Mass	WISPR Instrument Module (WIM) 9.8 kg; Instrument DPU (spacecraft provided) 1.1 kg
Average Power	7 W (including 4 W operational heater power)
Envelope	WIM Module: 58 cm × 30 cm × 46 cm (door closed)
Avg TLM Rate	Allocated data rate 26.6 kbps (during 10-day operational periods); 23 Gbits per orbit

A set of forward occulters (Forward Baffle Assembly) is located on a ledge to reduce the diffraction from the heat shield. An internal baffle assembly reduces this stray light component further as well as stray light diffracted from the FIELDS radio antennas and other spacecraft structures. Another set of baffles is located at the apertures of the two telescopes to prevent any further reflections from reaching the detectors. Because of the orbit profile, the WISPR stray light rejection requirements vary as a function of elongation angle and heliocentric distance by about an order of magnitude. The most stringent requirement is 1.8°ø10−12 B/Bsun at the outer edge of the FOV (90◦ elongation) at the largest distance from the Sun (0.25 AU). The sophisticated baffle design allows WISPR to meet this requirement and allows for high signal-to-noise ratio (SNR) imaging ranging from SNR = 20 at the inner FOV at closest perihelion to SNR = 5 at the largest distance and FOV angles. The detectors are 2048 °ø 1920 format APS CMOS devices developed for the SoloHI program (Howard et al. 2013). APS devices are much less susceptible to radiation damage than the more common CCD devices and are therefore the best option for this mission. They also come with significant savings in terms of power and mass. These devices are described in more detail in Korendyke et al. (2013). The devices are cooled to −60 ◦C via a passive radiator. A one-shot door protects the baffles and optics from contamination during ground operations, launch, and early flight operations.

WISPR Details

WISPR Pub Number 1

Assembly, Integration and Test

Lakin Jones — Sat, 12 May 2018 17:45:58 +0000

Assembly, Integration and Test

The instrument is calibrated at the component and subassembly level as well as “end-to-end” at the instrument unit level. Optical tests will ensure that the baffle surfaces and optical components meet requirements for efficiency, imaging and scattered light. The APS detector is calibrated for quantum efficiency, dynamic range, resolution, and noise. The instrument performance is tested/characterized in the dedicated NRL coronagraph test facilities that contain an 11 m beamline optical test chamber and Class 100 cleanroom. Additional baffling is added to the chamber to allow end-to-end stray light testing of stray light to ∼10−15 B/Bsun, similar to the successful SECCHI/HI end-to-end stray light test. This test was the first test to successfully achieve this level of sensitivity. The chamber is equipped with collimating optics, a precision instrument pointing table and necessary light sources. The laboratory is equipped with optical benches, theodolites, alignment telescopes, optical flats, and light sources. Component transmission and reflectivity are characterized using a Cary spectrophotometer and spectroradiometer. End-to-end calibrations performed under vacuum include: vignetting, radiometric calibration (responsivity), image quality, wavelength range, stray light and flat field. End-to-end calibration activities use the instrument electronics in the flight configuration. All calibrations are directly traceable to NIST using secondary standards. The laboratory calibration and image quality measurements are validated on-orbit using a set of standard stars similar to the procedures we use on the SOHO/LASCO and STEREO/SECCHI instruments. The final calibration using the standard stars will be accurate to ∼3 %, exceeding the 20 % absolute calibration requirement (Thernisien et al. 2006).

WISPR Pub Number 1

WISPR Details

Environmental Challenges

Lakin Jones — Fri, 11 May 2018 17:51:55 +0000

Environmental Challenges

We have little, if any, information for the environment PSP is going to operate in. It is reasonable to expect that the spacecraft will encounter high particle intensities, including elevated numbers of neutrons. The mission total ionizing dose (TID) of radiation is estimated to be 24 krad behind 100 mils (2.54 mm) of Al shielding.

The other concern is interplanetary dust. This is a novel concern for a heliophysics mission because PSP is the first spacecraft to receive dust impacts at a high orbital velocity, about 170 km/s at perihelion at a location where significant amounts of interplanetary dust are thought to be present. Unfortunately, we know little about the dust environment close to the Sun (see discussion in Sect. 1.4). The Helios measurements from 0.7 to 0.3 AU are the only available measurements (Leinert et al. 1981).

WISPR Pub Number 1

WISPR Details

Radiation Effects

Lakin Jones — Thu, 10 May 2018 17:53:43 +0000

Radiation Effects

We used PSP radiation guidelines for a seven-year mission for EEE parts selection. Our designs address single event effect (SEE) induced failure (latchup, burnout, gate rupture, secondary break-down), non-destructive SEE (e.g., non-destructive latchup, minilatchup, and single event functional interrupts) and single eventinduced soft errors (including single event upsets (SEU) or transients in linear devices) and SEE-induced soft errors. All EEE parts meet the TID requirement with a minimum radiation design margin of 2°ø the mission TID (60 krad behind 100 mils of Al shielding). We use no EEE parts having a linear energy transfer (LET) threshold of <25 MeVcm2/mg (SEU) or 100 MeVcm2/mg. The selected APS detector technology (see Sect. 3.3.1) mitigates potential problems of Non-Ionizing Energy Loss (NIEL) and radiation-induced Charge Transfer Efficiency (CTE) losses. Unlike CCDs (LASCO, SECCHI/HI), the photoelectrons are read-out from each APS pixel without shifting through the rest of the detector. Like CCDs, the radiation-induced damage increases the dark current, dark current non-uniformity noise in addition to particle-induced ionization transients (“cosmic rays” are scrubbed on-board as done on SECCHI/HI), temporal variations in pixel dark current and other effects.

Crater damage caused by dust impacts in the three glass types used in our testing. BK7 (left) is a commonly used glass type in space telescopes. BK7 with a diamond coating (DLC, middle) exhibits an additional ring around the crater possibly caused by coating separation from the glass. Sapphire (right) exhibited the least damage but it is an experimental glass type of unproven optical performance.

WISPR Pub Number 1

WISPR Details

Effects of High Speed Dust Impacts

Lakin Jones — Wed, 09 May 2018 17:56:07 +0000

Effects of High Speed Dust Impacts

Given the potentially high dust velocities, the kinetic energy distribution and fluence of the dust particles must inform the instrument design. Since the mass and size distribution is unknown close to the Sun, the design relies on the JHUAPL/UTEP models developed specifically for PSP (Mehoke et al. 2012). The model predicts about 100 impacts from 10- micron particles and 1000 impacts from 0.1-micron particles at the heat shield during the seven years of the mission. It also predicts that most particles will have diameters below 10 microns. Dust impacts can cause increased stray light levels for WISPR in two ways: (1) by damaging the edges of the forward baffles and, (2) by pitting or cratering the surface of the first lens. Additionally, there is an exceedingly small probability (<10−5 for >1 mm particle) of a catastrophic hit by a large particle.

To understand the effects of dust on instrument performance, the WISPR team has undertaken a glass testing and modeling program during the design phase with the help of the German Co-Is (V. Bothmer, PI). The Dust Accelerator at the Max-Planck-Institut für Kernphysik (MPIK) in Heidelberg was used in October 2012 to test three different candidate glass materials for the WISPR optics: BK7, BK7 with a diamond-like coating (DLC), and sapphire. The tests were performed with a variety of iron particle distributions (0.5–3 microns) and speeds (0.5–8 km/s) against three different impact angles (0◦, 45◦, 70◦).

The examination of the impacted glasses showed that sapphire was the most impactresistant material with very small (2 micron diameter; Fig. 11, right) and relatively symmetric craters. The impacts resulted in an unexpected behavior for the diamond-coated BK7. They caused a halo around the impact crater (Fig. 11, middle) that was likely the result of the local detachment of the coating due to the heat produced by the impact. The regular BK7 has relatively small craters (∼5 micron diameter; Fig. 11, left). Overall, the spall diameters are very consistent with the APL/UTEP model and provide confidence in the overall PSP project dust analysis and risk mitigation procedures.

To access the extent of the damaged area we developed an automated software program to measure the size and numbers of craters in the images. The results (see graphic below) show that sapphire is the most robust glass type. However, this type of glass has never been used for space applications before and therefore requires significant development. On the other hand, the standard BK7 suffered only modest damage and it is a well-known material for space options.

Estimation of damage due to dust impacts for the three glass types. The statistics on the top of the figure are derived from the automated image processing software developed specifically for the dust testing. The normalized statistics (per 105 particle hits) are given in the bottom. The sapphire coating is clearly the most robust but it also has the least heritage and development

Since the dust testing was in agreement with the APL/UTEP dust model, we use the model to estimate the percentage of damaged area expected for the objective lens of the WISPR outer telescope, which has the most exposure to dust. The model predicts that 0.6 % of the lens area will be pitted by the end of the mission. This value, representing the worstcase scenario, is then adopted for both the inner and outer telescope objective lenses. To evaluate the effect on the imaging performance we first measure the change in the Bidirectional Scattering Distribution Function (BSDF) (or Harvey-Shack function) in the damaged glass relative to the pristine BK7 BSDF. The laboratory-measured BSDFs revealed that we made conservative assumptions in our stray light estimates during the design phase. Therefore, the stray light calculations for pristine and damaged WISPR lenses were rerun using the measured BSDFs. The resulting beginning and end of life optics performances are shown in Fig. 13.

To summarize, the dust testing has been very valuable for the WISPR design process. It validated the APL/UTEP model (for velocities ∼2–3 km/s), allowed to safely reject exotic materials and coatings as an alternative to regular BK7, led to the development of a realistic BSDF model for evaluating the stray light effects of dust impacts on the imaging performance, and provided an estimate on the approximate damaged area of the WISPR optics. Based on these results, the regular BK7 was adopted as the baseline for the WISPR optics.

WISPR Pub Number 1

WISPR Details