An engineering milestone, not a sky result
PLACID has completed internal alignment and preliminary acceptance testing, but the result is still an engineering milestone rather than a report of astronomical performance. The instrument was installed on the DAG Nasmyth platform in 2025, and tests with a calibration light source were completed in early 2026. On-sky commissioning was still waiting for TROIA XAO operation, with first science targeted for late 2026 or early 2027.
The preprint combines on-site preliminary acceptance measurements with simulations and uses a target list and exposure-time calculator to estimate potential observations. It reports no on-sky science result, so those estimates remain prospective.
The trade-off in the masks
PLACID is built to work in H band at 1.65 micrometres and Ks band at 2.15 micrometres. Its 8-bit spatial light modulator, or SLM, has 1,920 by 1,152 pixels. The instrument has a field of view measuring 16 by 9.6 arcseconds, reported optical throughput above 20 percent, and an H-band diffraction scale of 85.084 milliarcseconds.
During alignment, while TROIA work had not yet produced an output beam, the setup used a monochromatic 633-nanometre fibered HeNe-laser beam as a backup calibration source. The first PSF, or pattern made by a point source, measured with a monochromatic 1,550-nanometre source qualitatively matched the theoretical pattern very well. The paper called it a success, although it contained more speckles than the simulation.
Acceptance testing compared five focal-plane masks: FQPM, Roddier, and vortex masks with charges 2, 4 and 6. The team derived raw azimuthally averaged normalized-residual-intensity curves from background, coronagraphic and non-coronagraphic PSFs. These curves provide a radial view of residual signal around the center, but they did not include coronagraphic throughput.
The reported on-axis performance order was Roddier, VC2, FQPM, VC4 and VC6. VC6 had the best normalized intensity in that comparison, but its inner working angle, a measure of how close to the center a mask can probe, was large. FQPM and VC2 were similar. The reported highest contrast did not exceed 10 to the power of minus five under variable background conditions.
The simulations put numbers on that trade-off. In H band, the modeled inner working angle was 53.048 milliarcseconds for Roddier and 303.178 milliarcseconds for VC6, with FQPM at 89.355 milliarcseconds and VC2 at 121.915 milliarcseconds. These are modeled values, and on-sky commissioning was still pending.
Models point to targets, with a warning
To map a possible discovery space, the target list required a declination of at least minus 24 degrees, visual magnitude no greater than 13, and angular separation no more than 16 arcseconds. The exposure-time calculator assumed a signal-to-noise ratio of 5, instrument transmission of 20 percent and a pessimistic Strehl ratio of 75 percent, an image-quality assumption used in the calculation.
Under those assumptions, the model placed kappa And b, HR 8799 b and c, AB Aur b and 1RXS 1609 b among apparently achievable detections. HR 8799 d and e and WISPIT 2 b were limiting cases, while AF Lep b and 51 Eri Ab were likely to remain undetected in the model. The authors hoped post-processing would improve contrast by around a factor of 10, but presented that as a hoped-for gain rather than a demonstrated result.
The authors explicitly warn against treating the exposure times as firm observing plans. The calculator omitted models for the point-spread function and saturation, ADI self-subtraction and field rotation, atmospheric variability, detector non-linearity, flat-field errors, imperfect background subtraction and temporal speckle evolution. It also treated residual speckles as photon noise that could be averaged down indefinitely, leaving the resulting values potentially optimistic or otherwise unreliable.
Binary mode awaits the same test
The update describes a binary-observing interface. Users enter the separation and position angle of the two components; during field rotation, the secondary mask moves while the primary stays fixed. The paper says both stellar PSFs require reference differential imaging, or RDI, or a combined ADI-RDI approach.
For now, the report describes PLACID as prepared for its next engineering phase. The authors report successful internal alignment, normalized-intensity levels outperforming factory acceptance, ongoing TROIA closed-loop tests, and software and observation tools described as ready. Their schedule targeted on-sky work for August or September 2026, with first science aimed at late 2026 or early 2027. Whether the internal-source results and model forecasts survive real observing conditions is still unanswered.
Paper data and sources
Original title: The PLACID active coronagraphic imager instrument: commissioning status
Authors: Ruben Tandon, Lucas Marquis, Liurong Lin et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-28
DOI: 10.1117/12.3102879
Original paper · Full text