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Heimdallr, Baldr, and Solarstein: designing the next generation of VLTI instruments in the Asgard suite

Adam K. Taras, J. Gordon Robertson, Fatme Allouche, Benjamin Courtney-Barrer, Josh Carter, Fred Crous, Nick Cvetojevic, Michael Ireland, Stephane Lagarde, Frantz Martinache, Grace McGinness, Mamadou N’Diaye, Sylvie Robbe-Dubois, and Peter Tuthill

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Abstract

High angular resolution imaging is an increasingly important capability in contemporary astrophysics. Of particular relevance to emerging fields such as the characterization of exoplanetary systems, imaging at the required spatial scales and contrast levels results in forbidding challenges in the correction of atmospheric phase errors, which in turn drives demanding requirements for precise wavefront sensing. Asgard is the next-generation instrument suite at the European Southern Observatory’s Very Large Telescope Interferometer (VLTI), targeting advances in sensitivity, spectral resolution, and nulling interferometry. In this paper, we describe the requirements and designs of three core modules: Heimdallr, a beam combiner for fringe tracking, low order wavefront correction, and visibility science; Baldr, a Zernike wavefront sensor to correct high order atmospheric aberrations; and Solarstein, an alignment and calibration unit. In addition, we draw generalizable insights for designing such system and discuss integration plans.

© 2024 Optica Publishing Group

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Supplementary Material (1)

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Supplement 1       Supplementary document

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No data were generated or analyzed in the presented research.

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Figures (10)
Fig. 1.
Fig. 1. Block diagram of the Asgard instrument suite. Modules covered in this paper are shaded green. Heimdallr contains common optics that correct the beam aberrations, sensing ${\rm K}$ band while splitting and distributing the other wavebands. The ${\rm Y}$, ${\rm J}$, and ${\rm H}$ bands go to Baldr on the upper level, which can operate in either ${\rm J}$ or ${\rm H}$, with the Bifrost spectrograph using ${\rm H}$ or ${\rm J}$, and ${\rm Y}$. Baldr is on the upper level, but it shares a common detector with Heimdallr and hence the beams return. Calibration and alignment beams are be generated by Solarstein, with injection mirrors used to switch between sky and the internal source.

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Fig. 2.
Fig. 2. Summary of the Asgard suite and the key functions of each module. Black silhouette for scale. Baldr inset (green) shows a preliminary mask being used to sense the wavefront (right) from a beam with a known aberration (left). Heimdallr inset illustrates the non-redundant pupil pattern and the corresponding image on the detector for a single wavelength.

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Fig. 3.
Fig. 3. Top view of Heimdallr optical layout, with repeated paths for each of the four beams partially transparent for clarity. The common optics [OAP1, (DM), OAP2] compress, delay, correct, and change the height of the beam. After dichroics pick off NOTT and Baldr beams, the motorized focus and knife-edge mirrors align the beams in a non-redundant mask pattern (blue box) that, when imaged on the detector, enables low order wavefront control and visibility science. Fold mirror labels are omitted for clarity. Details of the Narcissus box are shown in Fig. 4.

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Fig. 4.
Fig. 4. (left) Side view of the final elements before the detector, including the filters within the C-RED One. The assembly immediately in front of the C-RED One is referred to as the “Narcissus box.” The N1 optic has holes drilled to allow starlight to pass through. Note that the rays shown reaching N2 are notional–only thermal radiation to and from the cold stop area should exist in this space. (right) Side view of the mechanical design of the Narcissus box, mounted on an ${XYZ}$ translation stage. The model includes custom mounts for tip/tilt adjustment of N2 and K1M2.

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Fig. 5.
Fig. 5. Narcissus mirror design. (left) Map showing part of N1, with K1 holes in black, K2 in green. The red circle shows the area of N1 seen through the cold stop by one particular pixel on the sensor. (right) The color map shows the relative intensity of thermal radiation reaching the sensor (i.e. solid angle of overlap with the holes in N1). K1 is at the top, K2 at the bottom (opposite to the left-hand panel, since the beams cross at the cold stop). Patterns are not centered to allow room for Baldr measurements on the same detector.

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Fig. 6.
Fig. 6. Phase mask design tradeoff for sensitivity to Zernike modes. (left) Sensitivity as a function of phase mask dot diameter for some monochromatic Zernike modes with phase mask depths corresponding to a $\pi /2$ phase shift. The black dashed lines highlight the sensitivities for the traditional $1.06\;\lambda /D$ versus a $2\;\lambda /D$ diameter phase mask. (right) Sensitivity as a function of phase mask phase shift for some monochromatic Zernike modes with phase mask diameter equal to $1.06\;\lambda /D$. The black dashed line indicates ${90^ \circ}$ where the peak sensitivity is found for all modes.

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Fig. 7.
Fig. 7. Top view of the Baldr optical layout used to sense the wavefront of each beam. The beams are incident on the dichroic from the lower level (orange inset) and an ADC is mounted on the underside of the breadboard (omitted for clarity). The dichroic transmits Bifrost beams to fold mirrors (not shown), and reflects either ${\rm H}$ or ${\rm J}$ band light to the OAP, depending on motor configuration. The OAP brings beams to focus on a phase mask that can be positioned in the plane perpendicular to the beam (blue inset). A flip mount enables changing between low and high SNR configurations. A series of knife-edge mirrors positions the beams in a horizontal pattern and returns them to the Heimdallr detector on the lower level.

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Fig. 8.
Fig. 8. Footprint diagram of the four Baldr pupil images on the sensor plane.

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Fig. 9.
Fig. 9. Spot diagrams showing pupil image quality at the sensor plane. The box size is 25 µm on a side; the Airy disc (for pupil imaging) is not shown because it is much larger than the box. The numerical values in the legend are wavelengths in µm.

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Fig. 10.
Fig. 10. Solarstein optical layout, with black dashed line indicating the edge of the upper level breadboard. Using the stage at the top left, a source is selected. The light is injected with an off-axis paraboloid (OAP) through a pinhole, creating an unresolved source. The diverging beam has an aperture stop with a secondary obstruction. Another OAP then collimates the beam at 18 mm diameter. Now the beam is split into four identical, cophased beams using a series of beamsplitters, where each final beam is produced by two reflections and two transmissions. Each beam is then reflected down to the lower level with right-angle mirrors.

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