---
title: Large Fiber Array Spectroscopic Telescope (LFAST)
url: https://www.emergentmind.com/topics/large-fiber-array-spectroscopic-telescope-lfast
type: topic
---

# Large Fiber Array Spectroscopic Telescope (LFAST)

Searching arXiv for LFAST and related spectroscopic facility papers.
The **Large Fiber Array Spectroscopic Telescope (LFAST)** denotes a family of spectroscopic-facility concepts centered on extreme photon collection, fiber feeding, and massively multiplexed spectroscopy. In the recent literature, the name **LFAST** is used explicitly for a **scalable array telescope** in which many small mirrors each feed an individual optical fiber, and the fibers are then reformatted for optical and infrared spectrometers [2509.00181]. Earlier southern-hemisphere facility studies—especially the ESO **Spectroscopic facility** and the US **SpecTel** concept—describe a closely related design philosophy: a dedicated spectroscopic survey telescope in the 10–12 m class, optimized end-to-end for a very large fiber-fed focal plane, simultaneous high- and low-resolution spectroscopy, and optional panoramic integral-field capability [1708.03561; 1907.06797]. Taken together, these works define LFAST as an architectural approach rather than a single finalized instrument.

## 1. Terminology and conceptual scope

The explicit use of the name **LFAST** appears in work on a **scalable array telescope** whose elements are multiple mirrors feeding fibers and, through those fibers, spectrometers [2509.00181]. In that formulation, the telescope is not a monolithic aperture but an aggregate photon-collecting system: many individual mirrors collect light, each mirror feeds one optical fiber, and the outputs from many fibers are combined into a single rectangular fiber that feeds one or more optical and infrared spectrometers.

A closely allied concept appears in the earlier literature under different names. The ESO **Spectroscopic facility** is presented as a novel facility dedicated to **massively-multiplexed spectroscopy**, with a very wide field Cassegrain focus optimized for fibre feeding, an 11.4 m pupil, and a gravity-invariant focus for a giant IFU [1708.03561]. The later **SpecTel** study recommends a southern-hemisphere spectroscopic facility comprising a large diameter telescope, fiber system, and spectrographs **collectively optimized for massively-multiplexed spectroscopy**, with an explicit baseline of an **11.4-meter aperture**, a **five square degree field of view**, and **15,000 robotically-controlled fibers** [1907.06797].

This suggests that **LFAST** functions in the literature both as a specific named project and as a broader design lineage. In both variants, the defining characteristics are the same: end-to-end optimization for fiber spectroscopy, very high multiplex, broad wavelength coverage, and explicit coupling to large imaging and astrometric surveys.

## 2. Architectural forms

Two architectural realizations recur in the cited work.

The first is the **large-aperture survey telescope** form. In the ESO and SpecTel studies, the facility is a 10–12 m class southern-hemisphere telescope dedicated uniquely to spectroscopic surveys, with a corrected **2.5 degree diameter** field of view, a large Cassegrain focal plane, and a survey design in which the telescope, corrector, focal plane, fiber-positioning system, and spectrographs are conceived simultaneously [1708.03561]. The baseline SpecTel formulation is explicit: an **11.4-meter aperture, optical spectroscopic survey telescope with a five square degree field of view** [1907.06797].

The second is the **scalable array telescope** form. In the 2025 LFAST paper, each element of the array consists of multiple mirrors each feeding to an individual fiber; each LFAST unit uses **20 primary mirrors**, each with **0.76 m diameter**; one 20-mirror unit is equivalent in collecting area to a **traditional 3.5 m aperture**; and the full planned system consists of **132 such 20-unit systems**, giving a total collecting area of about **\(1200\,\mathrm{m^2}\)** [2509.00181]. The motivation is explicitly economic as well as scientific: traditional telescope costs scale steeply with diameter, approximately as
$$
\mathrm{cost} \propto D^n,\qquad n \approx 2.0\text{--}2.8,
$$
while support structure costs can scale roughly as \(D^3\) [2509.00181].

A plausible implication is that the two forms address the same throughput problem from different engineering directions. The 10–12 m facility maximizes **etendue** and multiplex within a single large telescope, whereas the array concept attempts to reproduce ELT-scale collecting area through modular fiber-coupled sub-apertures.

## 3. Optical design, focal plane geometry, and multiplex

The large-aperture LFAST-style studies place exceptional emphasis on the optical train between sky and fiber. In the ESO concept, the telescope has a **Field of View (FoV) of 2.5 degrees diameter and a 11.4m pupil**, and is described as **the largest etendue telescope** [1708.03561]. The corrected focal plane is about **1.43 m in diameter at F/2.86**, with a plate scale of roughly **160 micron/arcsecond**, described as **an excellent aperture for fibre coupling** [1708.03561]. The three-lens corrector includes an ADC and has good performance in the **360-1300 nm** wavelength range; the working principle is described as one in which **the last two lenses and the focal plane tilt around a circle of 5m radius** to compensate for atmospheric dispersion, with coma corrected by translating the secondary mirror [1708.03561].

SpecTel gives the same geometric core in more detail. The primary mirror consists of **78 ELT primary segments**, the secondary is **4.2 m**, and the telescope uses a **three-lens corrector** that also serves as an **ADC** [1907.06797]. The corrector provides a corrected **2.5 degrees, 1.43m diameter Cassegrain focal plane at F/2.86 and a platescale of 570 mm/deg** [1907.06797]. The scale comparison with existing facilities is explicit: the focal plane has **3.1 times the area of DESI’s prime focus**, and the authors state that it would be possible to duplicate DESI’s fiber positioner design to produce a **15,000 fiber system** [1907.06797].

The array-telescope LFAST uses a different front end but an equally fiber-centric backend. Each mirror feeds **one optical fiber**; each fiber has an **18 \(\mu\)m core diameter**, corresponding to an on-sky footprint of about **1.4 arcsec**; and the outputs from many fibers are then **combined into a single rectangular fiber** [2509.00181]. In this architecture, focal-plane geometry is displaced from a single large corrected field to the problem of reformatting many fiber outputs into a spectrometer-compatible slit.

## 4. Fiber system, positioners, and spectrographs

The fiber system is the central enabling technology in all three papers. In the ESO facility, the large focal plane can host **up to 16.000 fibres**, and using the DESI positioner mechanism, **more than 15000 positioners can be hosted, just using existing technology** [1708.03561]. The same study proposes a two-tier multiplex allocation in which each point of the focal plane can be reached by three fibres, yielding a **2:1** ratio between high-resolution and low-resolution fibres, so that with **15000 fibers** the facility could observe **5000 objects … at high resolution and 10000 at low resolution simultaneously** [1708.03561].

SpecTel retains the **15,000 robotically-controlled fibers** baseline and makes extensibility explicit. It states that **miniaturized electric motors for a higher density of fiber positioners** at a pitch of **5 mm** would allow a **4X increase in fiber number**, enabling instrumentation of **60,000 fibers** [1907.06797]. The paper therefore frames the baseline as a first stage rather than a hard upper limit.

The spectrograph plans are similarly modular. SpecTel describes an alternative design inspired by MOONS in which **600 fibers of 160 micron diameter (1 arcsec projected aperture)** feed the spectrograph through a **300 mm diameter collimating mirror**; the **IR arm covers 965 to 1330 nm with R~4000**; and optical extension to **360 nm** is possible through **two optical channels with R~3000 or three optical channels with R~4000** [1907.06797]. The same paper states that **high resolution spectrographs would naturally be considered as part of the conceptual design process**, and cites use cases requiring **high resolution (R ~ 20,000 - 40,000)** spectra at **S/N~80** for **30-85M stars** to **V~15.5 - 17** [1907.06797].

The ESO study specifies the top-level spectroscopic envelope differently but compatibly: **R=20-40000 for High Res, R=1000-3000 for Low Res**, over **360-1000 nm** [1708.03561]. Its low-resolution optical design uses curved detectors and an **F/1.1 camera**, with **\(\sim 600\) one arcsecond fibres** per spectrograph, and **less than 20 such spectrographs** to cover all low-resolution fibers [1708.03561].

For the array-telescope LFAST, the bottleneck is not focal-plane patrol but efficient reformatting into a slit. The 2025 study focuses on **optimizing coupling between the fiber bundle and the spectrometer slit input**, because losses at this stage directly reduce the usable photon budget [2509.00181]. In that sense, the spectrograph problem migrates from large-scale fiber positioner density to micro-optical coupling precision.

## 5. Integral-field capability and alternative observing modes

A notable feature of the large-aperture LFAST-style facilities is the inclusion of a central-field integral-field mode rather than exclusive reliance on pre-selected fiber targets. In the ESO concept, **a gravity invariant focus for the central 10 arc-minutes is available to host a giant integral field unit (IFU)**, and the top-level requirements specify a **Panoramic IFU: 3x3 arcmin FoV, with R∼5000 and coverage in the blue down to 325 nm** [1708.03561]. The low-resolution system must also be usable by the panoramic IFU, and simultaneous low- and high-resolution observations over the whole FoV are specifically required.

SpecTel states that by inserting mirrors ahead of the corrector, **the central 10 arcminutes can be directed to a gravity-invariant Coude focus hosting a panoramic IFU**, with the floor below the telescope rotating to compensate for IFU field rotation [1907.06797]. It also gives the scientific rationale for that mode: alongside traditional multi-fiber surveys based on targets pre-selected from deep photometry, a **panoramic IFU** offers the opportunity for **statistically-complete deep emission line and other serendipitous searches** [1907.06797].

This dual-mode architecture is significant because it broadens the operational meaning of LFAST. The facility is not restricted to catalog-driven spectroscopy; it also admits contiguous-field spectroscopy in the central region, which is especially relevant for emission-line searches, dense environments, and studies where source pre-selection is incomplete.

## 6. Scientific programs and survey role

The science case attached to the large-aperture concepts is organized around four themes: **assembly history of the Milky Way and dark matter**, **galaxy evolution in the cosmic web and intergalactic medium**, **cosmology**, and **the transient universe** [1907.06797]. The strongest recurring driver is Galactic archaeology in the Gaia era. Gaia is providing radial velocities for only **10%** of its sources, leaving **more than a billion stars** with excellent astrometry and photometry but no spectroscopy [1907.06797]. The ESO study similarly identifies **high resolution spectroscopy of GAIA stars** as a primary case and discusses surveys covering **85 million stars at limiting magnitude 17** [1708.03561]. SpecTel states that a **15,000 fiber instrument** on a 10–12 m telescope could observe **650 million stars in only a year** for moderate-resolution kinematic work [1907.06797].

For extragalactic astronomy and cosmology, the motivation is the conjunction of deep imaging and insufficient spectroscopic follow-up capacity. By **2025-2030**, deep imaging surveys will provide **0.2 arcsec image quality over more than 15,000 deg2 from Euclid** and multi-band photometry to **AB~27 from LSST** [1907.06797]. LSST is said to image the southern extragalactic sky in six filters, providing a photometric catalog of **> 4 billion galaxies** [1907.06797]. The proposed facility is intended to supply the missing combination of **large aperture + wide field + high multiplex + better resolution/SNR** relative to current or planned instruments.

Concrete cosmological survey modes are listed in SpecTel: photometric-redshift calibration with **20,000 galaxies to I~25.3**, dense redshift surveys of **20,000 galaxies deg-2** over **14,000 deg2**, and high-redshift clustering of **at least one hundred million galaxies** at **1.5<z<4** [1907.06797]. For cosmic web mapping and Ly\(\alpha\) tomography, the paper argues that representative-volume mapping over **1 < z < 4** is a formidable challenge that **can only be achieved with a dedicated wide-field 10-12m class facility**, and gives the requirement of covering about **200 deg2** from **AB~23 at z~1 to AB~25 at z~4** to span about **1 Gpc3** [1907.06797].

Transient follow-up is tied particularly to LSST. SpecTel notes that LSST will find **more than a million transient events** in five years, and estimates that if roughly ten live events per field are followed while a galaxy survey studies ten fields a night to **AB~23**, then roughly **10% of all visible LSST transients** could be observed via merging fiber allocations **on the fly** [1907.06797]. The ESO study similarly notes that for transients such as SNe, **more than 400 events will be present in the FoV at any time** [1708.03561].

The array-telescope LFAST paper does not elaborate a comparably broad survey program, but its framing is consistent with the same scientific logic: astronomy is often **photon-starved**, and scientific reach improves most directly by collecting more photons [2509.00181]. That formulation aligns the array concept with the survey-driven facilities, even though the cited paper is primarily an instrumentation study.

## 7. Enabling micro-optics and developmental status

The 2025 LFAST study shifts attention from facility-level architecture to the micro-optical details required to make a fiber-array telescope practical [2509.00181]. Its core problem is the efficient coupling of many fibers into spectrometer slits with minimal loss. The authors use a **two-dimensional microlens array**, with each microlens aligned to an individual fiber, and turn to **two-photon polymerization (2PP)** because standard fabrication approaches are insufficiently flexible for the required scale and complexity.

The fabrication platform is a **Nanoscribe Photonic Professional / GT2-class system** at Penn State, using a **780 nm** laser with pulse duration **80–100 fs** and repetition rate **80 MHz**, a **25×, NA 0.8 objective**, and a **25 mm × 25 mm × 0.7 mm fused silica** substrate [2509.00181]. The transparent resin **IPX-Clear** has transmission from **350 nm to 1550 nm** greater than **95%**, with minimum transmission still greater than **88%** for a **1 mm thick sample** within the LFAST wavelength regime of **390\,\mathrm{nm} \text{ to } 1700\,\mathrm{nm}** [2509.00181]. The abstract highlights that 2PP can produce microlenses with wavefront aberrations as small as **\(\lambda/20\)**.

The initial prototype is a **\(2 \times 10\)** microlens array with **pitch = 250 \(\mu\)m**, **radius of curvature = 400 \(\mu\)m**, **diameter = 245 \(\mu\)m**, **sag = 19.2 \(\mu\)m**, and total designed height **25 \(\mu\)m** after adding a base [2509.00181]. The fabrication workflow includes acetone and IPA cleaning, **O\(_2\) plasma etch for 5 min**, IPX-Clear thermalization for **10 min**, post-print bakes at **65°C**, **95°C**, and **65°C** for **2 min each**, development in **mr-dev 600** for **20 min**, an IPA bath for **2 min**, and UV cure for **10 min** [2509.00181].

The principal fabrication issue is **dose optimization**. The paper gives the simplified relation
$$
D = \frac{LP^n}{v},
$$
where \(LP\) is laser power, \(v\) is write speed, and \(n\) is typically **2–4**, while noting that the formula omits voxel-size dependence, overlap between slices, and time-dependent polymerization effects [2509.00181]. For the 25× objective, the approximate voxel size is
$$
l = 3.3\,\mu\mathrm{m},\qquad d = 0.6\,\mu\mathrm{m}.
$$

The reported metrology shows incremental improvement. Adapted 2GL shell-like parameters produced a lens with a **peaked profile** deviating from a spherical best fit by nearly **\(2.5\,\mu\mathrm{m}\)**, although surface roughness in the center was as low as **30 nm** [2509.00181]. Varying **PiezoSettlingTime** between **5 s** and **30 s** reduced deviation below **\(0.9\,\mu\mathrm{m}\)** in the best case, but individual lens print time exceeded **2 hours**, which is impractical for a final system requiring **2640 microlenses** [2509.00181]. A later **shell-and-scaffold** approach, using a representative scaffold with **1 \(\mu\)m** slice, **0.5 \(\mu\)m** hatch, and **17.5 mW** laser power, plus a shell at **0.2 \(\mu\)m** slice/hatch, yielded for the most recent **\(2 \times 10\)** prototype an **average deviation from spherical = \(0.87 \pm 0.12\,\mu\mathrm{m}\)** [2509.00181]. The authors plan **iterative pre-compensation** to reduce spherical deviation to **< 0.5\,\mu\mathrm{m}**, with an ideal target of **< 0.25\,\mu\mathrm{m}** [2509.00181].

This developmental status is important for interpreting LFAST as a whole. The 2017 and 2019 facility studies are explicitly **conceptual design** exercises, including staged implementation from a **15,000 fiber system** and moderate-resolution spectrographs to later **high-resolution spectrographs and a large IFU at the Coude focus**, and potentially a final stage with **60,000 fibers** [1907.06797]. The 2025 paper shows that, in the array-telescope variant, critical backend technologies such as microlens-aligned fiber reformatting are still being optimized experimentally [2509.00181]. A plausible implication is that LFAST remains best understood as an evolving spectroscopic systems concept whose key components—collecting architecture, focal-plane strategy, spectrograph modularity, and coupling optics—are still under active design refinement.

Source: https://www.emergentmind.com/topics/large-fiber-array-spectroscopic-telescope-lfast