The volume available on-board small satellites limit the optical aperture to a few centimetres, which limits the GroundSampling Distance (GSD) in the visible to approximately 3 m at 500 km. We present the latest development of a laboratory demonstrator for a deployable telescope that will triple the achievable ground resolution and quadruple the photometric capability from a CubeSat imager. In this paper, we present the overall opto-mechanical design of a Cassegrain telescope with a segmented primary mirror with a 30 cm baseline. The segments are folded for launch and unfold in space. To enable diffraction-limited imaging, piston, tip, and tilt (PTT) on each of the mirror segments should be below 12 nm RMS. The key challenge is to ensure phasing, and this precision level will require an active phasing stage. We present laboratory results of deployment and active phasing of the primary mirror segments. The initial deployment is performed using shaped memory alloy that deploy mirror segments. We demonstrate a repeatability below ±4.5 μm, enabling the four PSFs (one for each mirror segment) to be imaged on the detector simultaneously. An alignment step using compact and calibrated capacitive sensors allows for a control of the mirror positions in PTT below the wavelength. Finally, we investigate the sensitivity of misalignments of a deployable secondary mirror and show that it is well within reach of the technology developed in this study.
Metasurfaces, which are the 2D version of metamaterials, have revolutionised compact optics. Using subwavelength periodic nanostructured dielectrics, the refractive index and absorption properties of metasurfaces can manipulate light to a degree surpassing conventional bulk materials. Using metasurfaces, the phase, polarization, spin (for circularly polarised light), amplitude and wavelength of light can all be arbitrarily tailored to imitate a lens, which we refer to as a metalens (ML). MLs allow a larger choice of materials for optical components and have five major advantages over traditional refractive lenses – superior resolution, miniaturisation, lighter weight, multifunctionality and cost. In recent years, numerous metasurfaces with useful functionalities have been proposed, and although novel in their approach they still have very few real-world applications. One such application is within infrared laser systems, which have real-world use such as laser designators.
In this work, we demonstrate polarisation-insensitive metalenses working at λ = 1064 nm, with a d = 1 mm aperture size and four different F-numbers (f# = 0.5 - 5). The lenses are made using amorphous silicon (a-Si) pillars on top of a fused silica substrate, in order to function with a high efficiency (>60%) and little loss – where previous metal-based (plasmonic) metalens devices suffered from low efficiency (<10%) and high loss. The a-Si pillars range from 70-360 nm diameter atop a fused silica substrate, which are fabricated using electron beam lithography (EBL) and reactive ion etching processes.
The characterised lenses are shown to have almost diffraction-limited focal spot sizes, agreeing with the theoretical values of λ.f/d, and focussing efficiencies of 60%. Furthermore, we have designed large area lenses with aperture d = 10 mm, where the number of pillars per lens exceeds 550 million. By using an efficient Python script, we are able to make these 100 mm2 samples with just 14 hours of EBL writing time. The 10 mm lenses have focal lengths of 5 mm and 20 mm (F-numbers of 0.5 and 2 respectively). Such large area lenses are of considerable interest to many commercial applications where superior resolution and a light weight are beneficial, including laser designators/targeting, airborne and aerospace applications, as well as handheld devices.
Metasurfaces have revolutionized the definition of compact optics. Using subwavelength periodic structures of nanostructured dielectrics, the refractive index and absorption properties of metasurfaces – which are 2D metamaterials – can manipulate light to a degree not possible with conventional bulk glasses and crystals. The phase, polarization, spin (for circularly polarized light), amplitude and wavelength of light can all be manipulated and crafted to user-specified values to mimic the action of a lens, which we refer to as a metalens (ML). MLs have four major advantages over traditional refractive lenses – superior resolution, lighter weight, miniaturization and cost. Many metasurfaces with useful functionalities have been proposed in recent years, yet although novel in their approach have few real-world applications. One such market is the use within infrared laser systems, such as laser designators. In this work, we demonstrate metasurface lenses working at a wavelength of λ = 1064 nm, with aperture d = 1 mm and four different Fnumbers (focal length f = 0.5, 1, 2 and 5 mm). The lenses are composed of 700nm high a-Si pillars – ranging from 70- 360 nm diameter – which are fabricated using electron beam lithography (EBL) and reactive ion etching processes, on top of a fused silica substrate. Such lenses are shown to have diffraction-limited performance, with focal spot-size agreeing with theoretical values of λ‧f/d. Furthermore, we have designed large area lenses with aperture d = 10 mm, where the number of pillars per lens exceeds 550 million. By using an efficient Python script, we are able to produce these 100 mm2 samples with just 14 hours of EBL writing time.
This paper investigates the potential role of small satellites, specifically those often referred to as CubeSats, in the future of infrared astronomy. Whilst CubeSats are seen as excellent (and inexpensive) ways to demonstrate and improve the readiness of critical (space) technologies of the future they also potentially have a role in solving key astrophysical problems. The pros and cons of such small platforms are considered and evaluated with emphasis on the technological limitations and how these might be improved. Three case studies are presented for applications in the IR region. One of the main challenges of operating in the IR is that the detector invariably needs to be cooled. This is a significant undertaking requiring additional platform volume and power and is one of the major areas of discussion in this paper. Whilst the small aperture on a CubeSat inevitably has limitations both in terms of sensitivity and angular resolution when compared to large ground-based and space-borne telescopes, the prospect of having distributed arrays of tens (perhaps hundreds) of IR-optimised CubeSats in the future offers enormous potential. Finally, we summarise the key technology developments needed to realise the case study missions in the form of a roadmap.