the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Jul 2024
HARP2 Pre-Launch Calibration Overview: The Effects of a Wide Field of View
Abstract. The HyperAngular Rainbow Polarimeter (HARP2) is a wide field-of-view (FOV) polarimeter built for the NASA Plankton Aerosol Cloud and Ocean Ecosystem mission launched in early 2024. HARP2 measures the linear Stokes parameters across a 114° × 100° (along-track by cross-track) FOV. In the Fall of 2022, HARP2 underwent calibration at NASA Goddard Space Flight Center (GSFC) Calibration Laboratory (Code 618). HARP2 was characterized for radiometric and polarimetric response across its FOV. We have used telecentric calibration methodology on prior iterations of HARP that involved the normalization of pixels across the FOV such that calibration parameters determined at the center of the charged coupled device (CCD) detector can be used across the entire scene. By using a dual-axis yaw/pitch motorized mount, we devised two scan patterns to evaluate this methodology for HARP2. The results show that pure intensity measurements do indeed vary minimally across the FOV and therefore can utilize the flat-field normalization (telecentric) technique. On the other hand, images of polarized targets change significantly across the FOV, and calibration parameters determined at the center of the detector used in the wide FOV perform significantly worse than calibration parameters determined at or near to the location of the test (up to 5 % mean absolute error in degree of linear polarization, DoLP). We evaluated the use of a paraboloid fit of the polarized calibration parameters, at discrete FOV locations, to determine those parameters at a pixel-level resolution. According to the wide FOV results, this process shows a marked improvement for fully polarized (DoLP = 1) calibration data to less than 1 % uncertainty after using the paraboloid fit. These results are important for the development of any wide FOV polarimeter, especially those like HARP2 which use a front lens which causes significant barrel distortion and a division of amplitude central optical element leveraging multiple reflections. Full characterization of the source of these optical effects remains a part of future work.
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RC1: 'Comment on egusphere-2024-2024', Anonymous Referee #1, 24 Jul 2024
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This manuscript provides an important contribution to the reporting of the essential pre-launch (polarimetric) calibration efforts for the HARPS2 multi-angle polarimeter onboard PACE. It describes a significant variation of the polarimetric response across the field of view. However, the current version of the manuscript appears quite incomplete, for two main reasons:
1) It remains unclear what the status of this paper will be in the context of the complete description of the polarimetric performance and calibration of HARP2. In particular, its relation to the McBride et al. (2023-2024) paper should be specified. But neither paper provides a complete overview of all contributors to the overall polarimetric uncertainty. Will there be one final paper that will provide a complete overview? For this paper, at the very least an overview of relevant instrumental polarization effects and other systematics (as a function of incident angle across the field of field; see below) should be provided, and from this, the pertinent effects should be identified.
2) The final results of polarimetric error are only discussed in a hand-waving fashion, and not interpreted at all. There is an allusion to “barrel distortion” without even the start of a physical description of how this effect would cause the polarization effects. To be accepted as a publication in this journal, at least a qualitative interpretation of the results is required.
Some specific comments/questions:
- The requirement of 0.5% absolute polarimetric accuracy should be split up according to its constituent effects that are described by the 3x3 system matrix that relates the measured Stokes parameters [I,Q,U] to the “true” values. This matrix can be derived from the M and M^{-1) matrices (one of which I understand to be the “polarized calibration matrix”?), as M^{-1)*M - I, with all possible errors and variations upon those incorporated in M. This 3x3 matrix can be normalized by the first element. The diagonal elements Q->Q and U->U (“scaling”) then describe how well linear polarization is measured. Q<->U describes systematic rotations of the coordinate system (which may not be that relevant). I->Q,U (“zero point”) describes instrumentally induced polarization. And Q,U->I describes the influence of polarization on the radiometry. It appears that this manuscript predominantly deals with the first effect.
- The main polarimetric systematic (which can be formulated as I->Q,U) is due to any differential effect between the three beams: transmission, sampling, optical aberration, imperfect non-linearity correction. The latter is alluded to in Sect. 3.2, but the propagation of the residuals to the polarimetry has not been performed.
- At least the instrumentally induced polarization will likely vary significantly with incidence angle across the field of view. Is this effect considered? Have measurements of only the integrating sphere without a calibration polarizer been performed to map these effects?
- Note that also Q,U can get lost to Stokes V due to birefringence in glass or reflections on metallic mirrors. Have such effects been considered?
- Often, the systematics for Q and U can be considered equivalent. But this is not the case here, because of the sub-optimal three-beam splitting at angles [0,45,90] (instead of [0,60,120]). Because of this, the noise propagation to U is sqrt(2) worse. Moreover, Stokes U is highly susceptible to systematic effects in determining Stokes I from the [0,90] channels. Do you see any effects due to this “asymmetry”?
- l 118: I would hope that your integrating sphere is not reflective but scattering with high efficiency…
- l 119: No beam of light is fully “unpolarized” even if only due to photon noise. Down to which level did you confirm polarization consistent with zero?
- l 125: Please specify the wire-grid polarizer. Does it still have an extinction ratio >1000 in the blue band? And what is the effect of the 13 deg tilt? Perhaps in terms of 3D projection of the polarizer axis onto the beam?
- Sect. 3.3 Please refer to https://opg.optica.org/ao/abstract.cfm?uri=ao-39-10-1637 https://opg.optica.org/ao/abstract.cfm?uri=ao-47-14-2541 for the formalism describing noise propagation and optimality of these matrices.
- l 305: This “expected angular response” should really be explained! At least a qualitative comparison with the data should be part of this paper. Moreover, are you certain that there are no other effects that can explain the calibration measurements?
- Fig 8-9: Because DoLP cannot be >1, the statistics in these plots are weird. Please elaborate on their relevance for measurements of scenes with DoLP<1.
Citation: https://doi.org/10.5194/egusphere-2024-2024-RC1 -
RC2: 'Comment on egusphere-2024-2024', Anonymous Referee #2, 31 Aug 2024
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The manuscript entitled "HARP2 Pre-Launch Calibration Overview: The Effects of a Wide Field of View" by Noah Sienkiewicz and co-workers concerns the pre-launch calibration of HARP2, the HyperAngular Rainbow Polarimeter now flying on the recently launched NASA Plankton Aerosol Cloud and Ocean Ecosystem (PACE) mission. In synergy with SPEXone, the other multi-angle polarimeter on board, HARP2 will yield unprecedented and world-wide aerosol retrievals, invaluable for human health and global climate studies. Characteristic for HARP2 is its wide field-of-view (FoV). Previously, for prior HARP instances, the so-called 'telecentric' calibration approach was followed, where the calibration parameters determined at the center of the FoV was used across the entire FoV, via a flat-field normalization technique of the pixels. The current contribution now reveals that this approach was investigated for HARP2, and that it was found to be valid for pure intensity measurements, but not for the polarized calibration parameters. Instead, a paraboloid fit of the calibration parameters needed to be implemented, to improve the polarized calibration data across the FoV to within the required uncertainty.
As this contribution concerns improvements of the calibration of an instrument currently flying, this is highly relevant and of great interest to the spectropolarimeter society. The paper is also well written and the scientific results are presented clearly and concisely. Therefore, I recommend the manuscript to be accepted for publication in AMT, subject to the following very minor revisions:
Specific comments
- My feeling is that the title of the contribution does not match its content very well. Instead of providing an overview of the calibration, I find that the paper focusses on the difference with the previous approach. Therefore, I freely suggest: "HARP2 Pre-Launch Calibration: Dealing with polarization effects of a Wide Field of View" or something along these lines.
- Abstract, p. 1: While probably obvious, it is not clearly stated that the presented, improved approach using the parabolic fit of the polarized calibration parameters will effectively be used to process the incoming flight data. The authors should elaborate on how this will work in practice (implementation in L0-L1 processor).
- P. 5, line 135: It is said that an additional "27th sector" was taken at the end of each scan at the center position, with both shutters sequentially actuated. For all clarity: so these are two extra measurements (diffuser and dark), but for the same position of the turn-tilt stages, correct? This could be clearly stated, to avoid confusion.
- P. 6, line 184 (Figure 2): It is stated that a wide FoV effect is visible in Figure 2 due to a non-uniformity of the sphere illumination. Indeed, I can see the brighter area's at the right for each sector in the left panel of Figure 2, but this is not indicated. Please clearly indicate this in the Figure and Figure caption.
- P. 7, top: The binned super-pixel used to generate the response curve was taken to be as close to the center of the sphere aperture as possible for each scan position, to avoid the non-uniformity of the sphere illumination. A better approach would have been to improve the uniformity of the illumination by implementing a diffuser in the sphere, or by modifying the optical fiber positioning (or aiming). Has this been considered? If so this could be briefly mentioned.
- P. 7, line 199: Cross-band contamination: the cross-band contamination is weak and the contamination is ruled out by selection, in the SRF determination. This can be done here because monochromatic light is used (GLAMR) and the bands are (intendedly) illuminated one by one. In white light illumination (radiometric calibration for instance), all bands are illuminated at once, so the cross-band contamination will play a role. Is this contamination negelected? Please comment.
- P. 7, line 218: "...brute force search as the 26-Sector scan had much more stable pointing than the 9-Sector scan used for the SRF." It is not clear to me what is meant here. Why is the pointing for the 26-Sector scan more stable? Besides, pointing should not be critical since the instrument is looking into a sphere. Please clarify.
- P. 12, top: It is stated that the existence of an elevated background illumination signal in HARP2,
discovered after the launch of PACE, is briefly addressed. Where is this done? Only here in the conclusion? If included, this deserves more attention (a paragraph on it wouldn't be out of place).
- P. 16, Figure 1: It would be instructive to indicate both the Grande and Venti spheres in this figures, with their respective output ports, along with the FoV of the instrument. This to give an idea of the proportions. Please also indicate the size of the polarizer used.Technical corrections:
- Abstract, p. 1: please introduce the PACE acronym in the first sentence: "...built for the NASA Plankton Aerosol Cloud and Ocean Ecosystem (PACE) mission launched..." (insert "(PACE)").
- P. 2, line 35: Correct "MAPS" to "MAPs"
- P. 2, line 59: Correct typo: "...were asked to meet a and a 0.5% absolute accuracy..."
- P. 10, line 309: Correct typo: "2-dimnsional"
- P. 11, line 335: Reference is made to Figure 9. I think this should be Figure 8.
- P. 11, line 347: Correct possible typo: "overserved" vs. "observed".
- P. 19, caption Figure 4, last line: Correct typo: "show" vs. "shows"Citation: https://doi.org/10.5194/egusphere-2024-2024-RC2
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