FROIA

An ultralight large aperture space telescope concept

It is exciting to imagine a space telescope with the optical acuity to resolve a dime on the surface of the Earth, or from low Earth orbit to see structures like the lunar rover. As we describe here, a hybrid telescope/interferometer can have the coronagraphic sensitivity to detect a 1m-scale satellite intruder a million times fainter and only 20m away from a geostationary satellite. The astronomical capabilities of such a telescope will revolutionize the search for exolife and the direct imaging of exoplanets. We suggest here a way forward to achieving these, while deploying the world’s largest optical telescope — in space.

A narrow field-of-view Fizeau telescope with novel curvature-polished thin mirrors can create a larger, lighter, and potentially more straightforward optical system than the two decades-old design of the James Webb Space Telescope (JWST). The concept here, which we have scaled from our 2 m diameter smallsat-deployable space optical telescope, comfortably fits in a SpaceX Starship payload bay. With its thin optics and small mechanical mass, it could be nearly an order of magnitude lighter and use much less volume than JWST. Parametric cost scaling models suggest it could be built for about $100M. We call this the Fizeau Reconstruction for Optically Increased Acuity or FROIA. The envisioned telescope has a Modulation Transfer Function (MTF) that samples >90% of the MTF optical baselines of a 15 m filled aperture at >1% amplitude. Thus, it will nominally achieve the full image resolution of the corresponding filled-aperture telescope for moderate signal-to-noise scenes. Its total pupil area of 40 m2 would also be significantly larger than the 25 m2 JWST light collecting area. However, each 1.6 m FROIA sub-aperture diameter would be comparable to the 1.3 m JWST hexagon mirror segment diameters.

The 0.1” angular resolution observations obtained with the JWST demonstrated that a deployable space telescope with 18 sub-apertures (in this case illuminating a conventional monolithic secondary mirror) could achieve a point spread function (PSF) corresponding to the MTF and diffraction limit of its 6.5 m nearly-filled pupil. It is notable that the 80% Strehl and high spatial frequency image fidelity JWST obtains in the near-IR required several complex iterative pointing and phasing algorithms that ultimately achieved its optics’ roughly 50 nm position tolerance. The JWST actuated, and optically or mechanically sensed, 131 degrees of freedom of its primary (M1) and five for the secondary mirror (M2) in order to achieve this.

Figure 1: FROIA, stowed, within the Starship payload envelope (left), and the deployed telescope (right)

FROIA’s 20 thin primary mirrors (M1) are identical, have a diameter of 1.6 m, and their centers are unequally spaced on a 13.6 m diameter circle in the deployed configuration. Each of these off-axis parabola segments illuminates its own small concave off-axis elliptical secondary mirror segment (M2) that relays the prime focus of the parent parabola to the Gregorian focus. The optical system behaves like a Fizeau interferometer with phased, optically independent, off-axis telescopes forming overlapping Gregorian-focus images. Figure 1 shows a sketch of the telescope volume and deployed M1 mirror segments. Here four M1 support arms with three joints and five mirror segments each, allow the M1 optics to fold out into a ring attached to the core structure of the spacecraft bus. The 20 small elliptical M2 segments are mounted near one end of the rigid structure, while the Gregorian focal plane is near the opposite side of this core-telescope volume. Figure 2 shows an optical diagram that samples the geometrical ray paths of one FROIA sub-aperture. Table 1 describes some basic optical characteristics of the telescope.

FROIA Optical Characteristics

Value

Unit

M1 parabola focal length

5.0

m

M1 outer diameter

15.2

m

15.2m pupil fill factor

20%

Filled aperture MTF effective fill factor

91%

M1 sub-aperture diameter (x20)

1.6

m

M2 conic (ellipsoidal) radius

0.4

m

M1-M2 vertex distance

5.0

m

M2 sub-aperture diameter (x20)

6.0

cm

M2-image distance

4.6

m

Diffraction-limited field-of-view

15.0

arcsec

Effective focal length

112.0

m

Table 1: FROIA Optical Characteristics

Figure 2: FROIA 1.6m M1 Sub-aperture and 6cm M2 segment

While only about 20% of the area of the corresponding 15 m diameter telescope pupil is covered by the FROIA M1 optics, the optimal circular arrangement of these smaller phased mirrors creates an MTF that covers >90% of the MTF baseline domain of the filled aperture, as shown in Figure 3. Consequently, a straightforward FROIA image deconvolution/reconstruction algorithm can achieve the full 15 m 0.006” angular resolution at a wavelength of 500 nm: more than twice that of JWST.

Figure 3: A FROIA pupil function from 20 1.6 m circular apertures on a 13 m diameter ring with small random angular displacements from a regular lattice (left) yields an MTF that is greater than 1% over more than 90% of the 15 m filled-aperture optical baselines (right).

To save mass, the FROIA optomechanics are relatively sparse compared to JWST and are therefore less mechanically stiff. This is mitigated by the Fizeau optics that includes an M2 subaperture for each M1 segment that dynamically corrects subaperture position errors. These 20 small (6 cm) M2 mirrors provide high bandwidth active optical system control to high precision. For example, the positional error of M1 segments due to the dynamic flexure of the external support arms can be compensated — up to at least 10 µm decenter errors. Wavefront and M1-M2 phasing information will be derived from the image scene using recently proven Machine Learning algorithms. This scenario contrasts dramatically with the 50 nm tolerance for JWST segments that must be maintained over the primary mirror optical surface.

The JWST launch mass was about 6200 kg. However, its optical telescope assembly (without post-focus instruments) had a mass of about 1400 kg, including mirror optics mass of about 700 kg to which the M1 assembly contributed 360 kg, or nearly half. FROIA uses non-abrasively (curvature) polished thin mirrors. For example, the total mirror mass of its 2 or 3 mm-thick mirrors will be less than half the 6.5 m JWST mirrors. With a less complex instrument complement and bus than JWST and no complex sunshade needed (if FROIA operates in the visible and near IR), the entire 15 m diameter telescope should have a mass of less than 1000 kg that easily fits within a Starship. In fact,  a more complex 25m deployable structure that uses the Fizeau concept could probably be launched by a Starship for a few $100M.

The Laboratory for Innovation in Optomechanics and the Institute of Astrophysics in the Canary Islands Spain is prepared to undertake a rapid evaluation of the leading FROIA risk areas. Such a go-nogo design study could be done for significantly less than $1M. We note that if a launch vehicle for a 1m-scale small-satellite was available then a 2.5m diameter $5M Fizeau telescope could be built and launched as both a scientifically important space telescope, and a compelling demonstration of the technology concepts.

Our Collaborators

VisSidus Technologies, Inc.

Perfecting the process of making large-scale optical mirrors

Company

Optics
AI
Telescope

Contact

jeff.reykuhn@yahoo.com

MorphOptic
2540 Kekaulike Ave.
Kula, HI 96790