High-redshift Gamma-ray Bursts Research Articles

Expected Gamma-Ray Burst Detection Rates and Redshift Distributions for the BlackCAT CubeSat Mission

We report the results of an extensive set of simulations exploring the sensitivity of the BlackCAT CubeSat to long-duration gamma-ray bursts (GRBs). BlackCAT is a NASA APRA-funded CubeSat mission for the detection and real-time subarcminute localization of high-redshift (z ≳ 3.5) GRBs. Thanks to their luminous and long-lived afterglow emissions, GRBs are uniquely valuable probes of high-redshift star-forming galaxies and the intergalactic medium. In addition, each detected GRB with a known redshift serves to localize a region of high-redshift star formation in three dimensions, enabling deep follow-on searches for host galaxies and associated local and large-scale structures. We explore two distinct models for the GRB redshift distribution and luminosity function, both consistent with Swift observations. We find that, for either model, BlackCAT is expected to detect a mean of 42 bursts per year on orbit, with 6.7% to 10% of these at z > 3.5. BlackCAT bursts will be localized to an r 90 ≲ 55″ precision and reported to the community within seconds. Due to the mission orbit and pointing scheme, bursts will be located in the night sky and well placed for deep multiwavelength follow-up observations. BlackCAT is on schedule to achieve launch readiness in 2025.

Expected Gamma-Ray Burst Detection Rates and Redshift Distributions for the BlackCAT CubeSat Mission

We report the results of an extensive set of simulations exploring the sensitivity of the BlackCAT CubeSat to long-duration gamma-ray bursts (GRBs). BlackCAT is a NASA APRA-funded CubeSat mission for the detection and real-time subarcminute localization of high-redshift (z ≳ 3.5) GRBs. Thanks to their luminous and long-lived afterglow emissions, GRBs are uniquely valuable probes of high-redshift star-forming galaxies and the intergalactic medium. In addition, each detected GRB with a known redshift serves to localize a region of high-redshift star formation in three dimensions, enabling deep follow-on searches for host galaxies and associated local and large-scale structures. We explore two distinct models for the GRB redshift distribution and luminosity function, both consistent with Swift observations. We find that, for either model, BlackCAT is expected to detect a mean of 42 bursts per year on orbit, with 6.7% to 10% of these at z > 3.5. BlackCAT bursts will be localized to an r 90 ≲ 55″ precision and reported to the community within seconds. Due to the mission orbit and pointing scheme, bursts will be located in the night sky and well placed for deep multiwavelength follow-up observations. BlackCAT is on schedule to achieve launch readiness in 2025.

High-z gamma-ray burst detection by SVOM/ECLAIRs: Impact of instrumental biases on the bursts’ measured properties

Context. Gamma-ray bursts (GRBs) can be detected at cosmological distances, and therefore can be used to study the contents and phases of the early Universe. The 4−150 keV wide-field trigger camera ECLAIRs on board the Space-based multi-band Variable Object Monitor (SVOM) mission, dedicated to studying the high-energy transient sky in synergy with multi-messenger follow-up instruments, has been adapted to detect high-z GRBs. Aims. Investigating the detection capabilities of ECLAIRs for high-redshift GRBs and estimating the impacts of instrumental biases in reconstructing some of the source measured properties, focusing on GRB duration biases as a function of redshift. Methods. We simulated realistic detection scenarios for a sample of 162 already observed GRBs with known redshift values as they would have been seen by ECLAIRs. We simulated them at redshift values equal to and higher than their measured value. Then we assessed whether they would be detected with a trigger algorithm resembling that on board ECLAIRs, and derived quantities, such as T90, for those that would have been detected. Results. We find that ECLAIRs would be capable of detecting GRBs up to very high redshift values (e.g. 20 GRBs in our sample are detectable within more than 0.4 of the ECLAIRs field of view for zsim > 12). The ECLAIRs low-energy threshold of 4 keV, contributes to this great detection capability, as it may enhance it at high redshift (z > 10) by over 10% compared with a 15 keV low-energy threshold. We also show that the detection of GRBs at high-z values may imprint tip-of-the-iceberg biases on the GRB duration measurements, which can affect the reconstruction of other source properties.

Optical and Near-infrared Observations of the Distant but Bright “New Year’s Burst” GRB 220101A

High-redshift gamma-ray bursts (GRBs) are useful to probe the early Universe, but only a few candidates have been detected so far. Here, we report the optical and near-infrared observations of the afterglow of a relatively high-redshift event GRB 220101A, which was triggered on New Year’s Day of 2022, and therefore referred to as the “New Year’s burst.” With the optical spectra obtained by XL2.16/BFOSC and NOT/ALFOSC, we determine the redshift of the burst to be z = 4.615. We find that the optical afterglow of GRB 220101A is one of the most luminous ever detected. Based on our optical and near-infrared data, and combined with the X-ray observations, we perform a multiband fit with the Python package afterglowpy. The jet opening angle is constrained to ∼3.°4, which is consistent with the jet-break time at ∼0.7 day. We also determine the circumburst density of n 0 = 0.15 cm−3 and kinetic energy E K,iso = 3.5 × 1054 erg. In the prompt phase of the burst, we find a “mirror” feature in the lightcurve from 80 s to 120 s. The physical origin of such a mirror feature is unclear.

Photometric redshift estimation for gamma-ray bursts from the early Universe

ABSTRACT Future detection of high-redshift gamma-ray bursts (GRBs) will be an important tool for studying the early Universe. Fast and accurate redshift estimation for detected GRBs is key for encouraging rapid follow-up observations by ground- and space-based telescopes. Low-redshift dusty interlopers pose the biggest challenge for GRB redshift estimation using broad photometric bands, as their high extinction can mimic a high-redshift GRB. To assess false alarms of high-redshift GRB photometric measurements, we simulate and fit a variety of GRBs using phozzy, a simulation code developed to estimate GRB photometric redshifts, and test the ability to distinguish between high- and low-redshift GRBs when using simultaneously observed photometric bands. We run the code with the wavelength bands and instrument parameters for the Photo-z Infrared Telescope (PIRT), an instrument designed for the Gamow mission concept. We explore various distributions of host galaxy extinction as a function of redshift, and their effect on the completeness and purity of a high-redshift GRB search with the PIRT. We find that for assumptions based on current observations, the completeness and purity range from ∼82 to 88 per cent and from ∼84 to $\gt 99~{{\ \rm per\ cent}}$, respectively. For the priors optimized to reduce false positives, only $\sim 0.6~{{\ \rm per\ cent}}$ of low-redshift GRBs will be mistaken as a high-redshift one, corresponding to ∼1 false alarm per 500 detected GRBs.

Measurement methods for gamma-ray bursts redshifts

In the era of multi-messenger astronomy, gamma-ray bursts (GRBs) with known redshifts, especially high-redshift GRBs, are a powerful tool for studying the structure and evolution of the early Universe. We review the background, the history, and the application of measurement methods of GRB redshifts in astronomy. Based on different observation targets, two measurement methods are mainly introduced. One is on GRB afterglow, the other is on GRB host galaxy. There are various processing methods belonging to measurement methods based on afterglow, including spectral measurement method of afterglow and afterglow spectral energy distribution fitting method with improved methods. There are also numerous measurement methods based on host galaxy, such as spectral measurement method of host galaxy, template matching method of host galaxy, some automatic spectroscopic redshift measurement methods, and machine learning methods. We subsequently introduce the principles, effects, and performance of these methods. We enumerate several detection and measurement instruments, which have been used in observation. The characteristics, advantages, disadvantages, and applicability of the GRB redshift measurement methods are summarized and analyzed. Furthermore, we provide a data set of 611 GRBs with measured redshift. The data set has been collected since 1997. Analysis and statistics are presented based on this data set. We summarize the characteristics of GRBs such as location, time, and accuracy. Finally, we introduce Space-based multi-band astronomical Variable Objects Monitor (SVOM) mission dedicated to searching high redshift GRBs. We also introduce the application prospect of various redshift measurement methods in SVOM mission.

GRB 221009A: The BOAT

GRB 221009A has been referred to as the brightest of all time (BOAT). We investigate the veracity of this statement by comparing it with a half century of prompt gamma-ray burst observations. This burst is the brightest ever detected by the measures of peak flux and fluence. Unexpectedly, GRB 221009A has the highest isotropic-equivalent total energy ever identified, while the peak luminosity is at the ∼99th percentile of the known distribution. We explore how such a burst can be powered and discuss potential implications for ultralong and high-redshift gamma-ray bursts. By geometric extrapolation of the total fluence and peak flux distributions, GRB 221009A appears to be a once-in-10,000-year event. Thus, it is almost certainly not the BOAT over all of cosmic history; it may be the brightest gamma-ray burst since human civilization began.

Finding high-redshift gamma-ray bursts in tandem near-infrared and optical surveys

The race for the most distant object in the Universe has been played by long-duration gamma-ray bursts (GRBs), star-forming galaxies and quasars. GRBs took a temporary lead with the discovery of GRB 090423 at a redshift z=8.2, but now the record-holder is the galaxy GN-z11 at z=11.0. Despite this record, galaxies and quasars are very faint (GN-z11 has a magnitude H=26), hampering the study of the physical properties of the primordial Universe. On the other hand, GRB afterglows are brighter by a factor of >100, with the drawback of lasting only for 1-2 days. Here we describe a novel approach to the discovery of high-redshift (z>6) GRBs, exploiting their near-infrared (nIR) emission properties. Soon after the bright, high-energy prompt phase, a GRB is accompanied by an afterglow. The afterglows of high-redshift GRBs are naturally absorbed, like any other source, at optical wavelengths by Hydrogen along the line of sight in the intergalactic medium (Lyman-alpha absorption). We propose to take advantage of the deep monitoring of the sky by the Vera Rubin Observatory, to simultaneously observe exactly the same fields with a new, dedicated nIR facility. By comparing the two streams of transients, one can pinpoint transients detected in the nIR band and not in the optical band. These fast transients detected only in the nIR and with an AB colour index r-H>3.5 are high-redshift GRBs, with a low contamination rate. Thanks to the depth reached by the Rubin observations, interlopers can be identified, allowing us to discover ~11 GRBs at z>6 per year and ~3 GRBs per year at z>10. This turns out to be one of the most effective probes of the high-redshift Universe.

The Improved Amati Correlations from Gaussian Copula

In this paper, we present two improved Amati correlations of gamma-ray burst (GRB) data via a powerful statistical tool called copula. After calibrating with the low-redshift GRB data, the improved Amati correlations based on a fiducial Λ cold dark matter (ΛCDM) model with Ωm0 = 0.3 and H 0 = 70 km s−1 Mpc−1, and extrapolating the results to the high-redshift GRB data, we obtain the Hubble diagram of GRB data points. Applying these GRB data to constrain the ΛCDM model, we find that the improved Amati correlation from copula can give a result well consistent with Ωm0 = 0.3, while the standard Amati and extended Amati correlations do not. This results suggest that when the improved Amati correlation from copula is used in the low-redshift calibration method, the GRB data can be regarded as a viable cosmological explorer. However, the Bayesian information criterion indicates that the standard Amati correlation remains to be favored mildly since it has the least model parameters. Furthermore, once the simultaneous fitting method rather than the low-redshift calibration one is used, there is no apparent evidence that the improved Amati correlation is better than the standard one. Thus, more work needs to be done in the future in order to compare different Amati correlations.

Breakthrough Multi-Messenger Astrophysics with the THESEUS Space Mission

The mission concept THESEUS (Transient High Energy Sky and Early Universe Surveyor) aims at exploiting Gamma-Ray Bursts (GRB) to explore the early Universe, as well as becoming a cornerstone of multi-messenger and time-domain astrophysics. To achieve these goals, a key feature is the capability to survey the soft X-ray transient sky and to detect the faint and soft GRB population so far poorly explored. Among the expected transients there will be high-redshift GRBs, nearby low-luminosity, X-ray Flashes and short GRBs. Our understanding of the physics governing the GRB prompt emission will benefit from the 0.3 keV–10 MeV simultaneous observations for an unprecedented large number of hundreds of events per year. In particular the mission will provide the identification, accurate sky localisation and characterization of electromagnetic counterparts to sources of gravitational wave and neutrino sources, which will be routinely detected during the 2030s by the upgraded second generation and third generation Gravitational Wave (GW) interferometers and next generation neutrino detectors.

Properties of High-redshift Gamma-Ray Bursts

The immense power of gamma-ray bursts (GRBs) makes them ideal probes of the early universe. By using absorption lines in the afterglows of high-redshift GRBs, astronomers can study the evolution of metals in the early universe. With an understanding of the nature of GRB progenitors, the rate and properties of GRBs observed at high redshift can probe the star formation history and the initial mass function of stars at high redshift. This paper presents a detailed study of the dependence on metallicity and mass of the properties of long-duration GRBs under the black hole accretion disk paradigm to predict the evolution of these properties with redshift. These models are calibrated on the current GRB observations and then used to make predictions for new observations and new missions (e.g., the proposed Gamow mission) studying high-redshift GRBs.

Neural networks and standard cosmography with newly calibrated high redshift GRB observations

Gamma-ray bursts (GRBs) detected at high redshift can be used to trace the cosmic expansion history. However, the calibration of their luminosity distances is not an easy task in comparison to Type Ia Supernovae (SNeIa). To calibrate these data, correlations between their luminosity and other observed properties of GRBs need to be identified, and we must consider the validity of our assumptions about these correlations over their entire observed redshift range. In this work, we propose a new method to calibrate GRBs as cosmological distance indicators using SNeIa observations with a machine learning architecture. As well we include a new data GRB calibrated sample using extended cosmography in a redshift range above z > 3.6. An overview of this machine learning technique was developed in [1] to study the evolution of dark energy models at high redshift. The aim of the method developed in this work is to combine two networks: a Recurrent Neural Network (RNN) and a Bayesian Neural Network (BNN). Using this computational approach, denoted RNN+BNN, we extend the network's efficacy by adding the computation of covariance matrices to the Bayesian process. Once this is done, the SNeIa distance-redshift relation can be tested on the full GRB sample and therefore used to implement a cosmographic reconstruction of the distance-redshift relation in different regimes. Thus, our newly-trained neural network is used to constrain the parameters describing the kinematical state of the Universe via a cosmographic approach at high redshifts (up to z ≈ 10), wherein we require a very minimal set of assumptions on the deep learning arquitecture itself that do not rely on dynamical equations for any specific theory of gravity.

Probing the initial mass function of the first stars with transients

ABSTRACT The emergence of the first, so-called Population III (Pop III), stars shaped early cosmic history in ways that crucially depends on their initial mass function (IMF). However, because of the absence of direct observational constraints, the detailed IMF remains elusive. Nevertheless, numerical simulations agree in broad terms that the first stars were typically massive and should often end their lives in violent, explosive deaths. These fates include extremely luminous pair-instability supernovae (PISNe) and bright gamma-ray bursts (GRBs), the latter arising from the collapse of rapidly rotating progenitor stars into black holes. These high-redshift transients are expected to be within the detection limits of upcoming space telescope missions, allowing to place effective constraints on the shape of the primordial IMF that is not easily accessible with other probes. This paper presents a framework to probe the Pop III IMF, utilizing the cosmological source densities of high-redshift PISNe and GRBs. Considering these transients separately could provide useful constraints on the Pop III IMF, but tighter bounds are obtainable by combining PISN and GRB counts. This combined diagnostic is more robust as it is independent of the underlying Pop III star formation rate density, an unknown prior. Future surveys promise to capture most high-redshift GRBs across the entire sky, but high-redshift PISN searches with future telescopes, e.g. Roman Space Telescope, will likely be substantially incomplete. Nevertheless, we demonstrate that even such lower bounds on the PISN count will be able to provide key constraints on the primordial IMF, in particular, if it is top-heavy or not.

SkyHopper mission science case I: Identification of high redshift Gamma-Ray Bursts through space-based near-infrared afterglow observations

Abstract Long-duration gamma-ray burst (GRB) afterglow observations offer cutting-edge opportunities to characterise the star formation history of the Universe back to the epoch of reionisation, and to measure the chemical composition of interstellar and intergalactic gas through absorption spectroscopy. The main barrier to progress is the low efficiency in rapidly and confidently identifying which bursts are high redshift ( $z > 5$ ) candidates before they fade, as this requires low-latency follow-up observations at near-infrared wavelengths (or longer) to determine a reliable photometric redshift estimate. Since no current or planned gamma-ray observatories carry near-infrared telescopes on-board, complementary facilities are needed. So far this task has been performed by instruments on the ground, but sky visibility and weather constraints limit the number of GRB targets that can be observed and the speed at which follow-up is possible. In this work we develop a Monte Carlo simulation framework to investigate an alternative approach based on the use of a rapid-response near-infrared nano-satellite, capable of simultaneous imaging in four bands from $0.8$ to $1.7\,\unicode{x03BC}$ m (a mission concept called SkyHopper). Using as reference a sample of 88 afterglows observed with the GROND instrument on the MPG/ESO telescope, we find that such a nano-satellite is capable of detecting in the H-band (1.6 $\unicode{x03BC}$ m) $72.5\% \pm 3.1\%$ of GRBs concurrently observable with the Swift satellite via its UVOT instrument (and $44.1\% \pm 12.3\%$ of high redshift ( $z>5$ ) GRBs) within 60 min of the GRB prompt emission. This corresponds to detecting ${\sim}55$ GRB afterglows per year, of which 1–3 have $z > 5$ . These rates represent a substantial contribution to the field of high-z GRB science, as only 23 $z > 5$ GRBs have been collectively discovered by the entire astronomical community over the last ${\sim}24$ yr. Future discoveries are critically needed to take advantage of next generation follow-up spectroscopic facilities such as 30m-class ground telescopes and the James Webb Space Telescope. Furthermore, a systematic space-based follow-up of afterglows in the near-infrared will offer new insight on the population of dusty (‘dark’) GRBs which are primarily found at cosmic noon ( $z\sim 1-3$ ). Additionally, we find that launching a mini-constellation of 3 near-infrared nano-satellites would increase the detection fraction of afterglows to ${\sim}83\%$ and substantially reduce the latency in the photometric redshift determination.

The THESEUS space mission: science goals, requirements and mission concept

THESEUS, one of the two space mission concepts being studied by ESA as candidates for next M5 mission within its Comsic Vision programme, aims at fully exploiting Gamma-Ray Bursts (GRB) to solve key questions about the early Universe, as well as becoming a cornerstone of multi-messenger and time-domain astrophysics. By investigating the first billion years of the Universe through high-redshift GRBs, THESEUS will shed light on the main open issues in modern cosmology, such as the population of primordial low mass and luminosity galaxies, sources and evolution of cosmic re-ionization, SFR and metallicity evolution up to the “cosmic dawn” and across Pop-III stars. At the same time, the mission will provide a substantial advancement of multi-messenger and time-domain astrophysics by enabling the identification, accurate localisation and study of electromagnetic counterparts to sources of gravitational waves and neutrinos, which will be routinely detected in the late ‘20s and early ‘30s by the second and third generation Gravitational Wave (GW) interferometers and future neutrino detectors, as well as of all kinds of GRBs and most classes of other X/gamma-ray transient sources. Under all these respects, THESEUS will provide great synergies with future large observing facilities in the multi-messenger domain. A Guest Observer programme, comprising Target of Opportunity (ToO) observations, will expand the science return of the mission, to include, e.g., solar system minor bodies, exoplanets, and AGN.

A Roadmap to Gamma-Ray Bursts: New Developments and Applications to Cosmology

Gamma-ray bursts are the most powerful explosions in the universe and are mainly placed at very large redshifts, up to z≃9. In this short review, we first discuss gamma-ray burst classification and morphological properties. We then report the likely relations between gamma-ray bursts and other astronomical objects, such as black holes, supernovae, neutron stars, etc., discussing in detail gamma-ray burst progenitors. We classify long and short gamma-ray bursts, working out their timescales, and introduce the standard fireball model. Afterwards, we focus on direct applications of gamma-ray bursts to cosmology and underline under which conditions such sources would act as perfect standard candles if correlations between photometric and spectroscopic properties were not jeopardized by the circularity problem. In this respect, we underline how the shortage of low-z gamma-ray bursts prevents anchor gamma-ray bursts with primary distance indicators. Moreover, we analyze in detail the most adopted gamma-ray burst correlations, highlighting their main differences. We therefore show calibration techniques, comparing such treatments with non-calibration scenarios. For completeness, we discuss the physical properties of the correlation scatters and systematics occurring during experimental computations. Finally, we develop the most recent statistical methods, star formation rate, and high-redshift gamma-ray burst excess and show the most recent constraints obtained from experimental analyses.

Reconciling low and high redshift GRB luminosity correlations

The correlation between the peak spectra energy (${E}_{p}$) and the equivalent isotropic energy (${E}_{\mathrm{iso}}$) of long gamma-ray bursts (GRBs), the so-called Amati relation, is often used to constrain the high-redshift Hubble diagram. Assuming Lambda cold dark matter cosmology, G.-J. Wang et al. [Astrophys. J. 836, 103 (2017)] found a $\ensuremath{\gtrsim}3\ensuremath{\sigma}$ tension in the data-calibrated Amati coefficients between low and high redshift GRB samples. To reduce the impact of fiducial cosmology, we use the parametrization based on cosmic age (PAge), an almost model-independent framework to trace the cosmological expansion history. We find that the low and high redshift tension in Amati coefficients stays almost the same for the broad class of models covered by PAge, indicating that the cosmological assumption is not the dominant driver of the redshift evolution of GRB luminosity correlation. Next, we analyze the selection effect due to flux limits in observations. We find Amati relation evolves much more significantly across energy scales of ${E}_{\mathrm{iso}}$. We debias the GRB data by selectively discarding samples to match low-$z$ and high-$z$ ${E}_{\mathrm{iso}}$ distributions. After debiasing, the Amati coefficients agree well between low-$z$ and high-$z$ data groups, whereas the evidence of ${E}_{\mathrm{iso}}$ dependence of Amati relation remains to be strong. Thus, the redshift evolution of GRB luminosity correlation can be fully interpreted as a selection bias and does not imply cosmological evolution of GRBs.

Expanding or Static Universe: Emergence of a New Paradigm

The no-evolution, concordance expanding universe cosmology and no-evolution, static universe tired light model are compared against observational data on eight cosmology tests. The no-evolution tired light model is found to make a superior fit on all tests. Any attempts to introduce evolutionary corrections to improve the concordance cosmology fit on one test often worsen its fit on other tests. Light curve data of high redshift gamma ray bursts and quasars fail to support claims for cosmological time dilation due to expansion. Also, the SCP supernova light curve test results are considered to be flawed by selection effect biases. The big bang theory also has difficulty accounting for redshift quantization, for the multi-megaparsec periodicity seen in the distribution of galaxy superclusters, and for the discovery of galaxies at redshifts as high as z ~11.9. In overview, it is concluded that a static universe cosmology must be sought to explain the origin of the universe. One possible choice is a cosmology that predicts nonconservative tired-light redshifting in intergalactic space, the continuous creation of neutrons in space, the rate of matter creation scaling with both celestial body mass and temperature, galaxies growing progressively in size, and changing their morphology in the manner suggested by Jeans and Hubble.

A possible bright ultraviolet flash from a galaxy at redshift z ≈ 11

In the optical sky, minutes-duration transients from cosmological distances are rare. Known objects that give rise to such transients include gamma-ray bursts (GRBs), the most luminous explosions in the universe that have been detected at redshift as high as z ~ 9.4. These high-redshift GRBs and their associated emission can be used to probe the star formation and reionization history in the era of cosmic dawn. Here we report a near-infrared transient with an observed duration shorter than 245 s coincident with the luminous star-forming galaxy GN-z11 at z ~ 11. The telluric absorption shown in the near-infrared spectrum indicates its origin from above the atmosphere. We can rule out the possibility of known man-made objects or moving objects in the Solar system based on the observational information and our current understanding of the properties of these objects. Since some long-duration GRBs are associated with a bright ultraviolet (UV) or optical flash, we investigate the possibility that the detected signal arose from a rest-frame UV flash associated with a long GRB from GN-z11. Despite the very low probability of being a GRB, we find that the spectrum, brightness, and duration of the transient are consistent with such an interpretation. Our result may suggest that long GRBs can be produced as early as 420 million years after the Big Bang.

Optical design of the visible telescope for the SVOM mission.

This paper describes the optical design of the visible telescope (VT), which is the primary payload for the Chinese-French Space-based multi-band astronomical Variable Objects Monitor (SVOM) mission, for the detection and observation of high-redshift gamma-ray bursts. The VT aims at reaching a limiting magnitude of +22.5Mv with the exposure time of 300 s in the 630 km Sun-synchronous orbit with an inclination of 30°. The VT, also known as the fine guidance sensor for the SVOM, aims to measure the relative performance error (RPE) of the platform during the tracking and provide the RPE to the platform to correct its stability. The optical design is presented in this paper. The mirror manufacture and test results are presented. The optical system performance, tolerance budget, thermal analysis, and stray light design of VT are fully analyzed. Finally, the diffraction encircled energy and point source transmittance are tested in the lab for the finished telescope.