Photometric Calibration & Spectral Validation of the Solar Ultraviolet Imaging Telescope onboard Aditya-L1

Janmejoy Sarkar; Soumya Roy; A N Ramaprakash; Rushikesh Deogaonkar; Sreejith Padinhatteeri; Durgesh Tripathi; Avyarthana Ghosh; Raja Bayanna Ankala; Gazi Ameen Ahmed

arXiv · 2503.02560

Photometric Calibration & Spectral Validation of the Solar Ultraviolet Imaging Telescope onboard Aditya-L1

Janmejoy Sarkar·Soumya Roy·A N Ramaprakash·Rushikesh Deogaonkar·Sreejith Padinhatteeri·Durgesh Tripathi·Avyarthana Ghosh·Raja Bayanna Ankala·Gazi Ameen Ahmed

View original source ↗Read source document ↗Export metadata ↓

Abstract

The Solar Ultraviolet Imaging Telescope (SUIT) is one of the seven payloads on board Aditya-L1 mission of the Indian Space Research Organization (ISRO). SUIT provides full and partial disk images of the Sun in the 200-400 nm wavelength range. This would help us probe the solar atmosphere at different heights and understand the mass and energy transfer process between its layers. For the first time, SUIT will also help us measure spatially resolved solar spectral irradiance at this wavelength band, which is significant for studying the sun-climate relationships. To perform these studies, it is necessary to photometrically calibrate the payload and validate the spectral coverage of the various bandpasses. We perform the photometric calibration and spectral validation of 8 bandpasses using light of known intensity and spectral coverage. For photometric calibration, the telescope throughput is modeled using sun-as-a-star spectrum from SOLSTICE and SOLSPEC. The modeled throughput is compared with in-lab measurements taken with light of known intensity. The ratio of measured photoelectrons gathered with the modeled prediction agree within 20%. For spectral validation, readings are taken across the transmission spectrum of each filter, keeping adjacent readings independent of each other. The relative intensity measured at each wavelength is seen to trace the modeled telescope bandpass for that filter. These tests could not be performed for filters with bandpasses operating below 250 nm (NB01, BB01 and BB02), primarily due to heavy atmospheric attenuation in these wavelengths leading to decreased SNR of the data. The experimentally measured results agree closely with the modeled values, validating SUIT's optical performance and presenting the reliability of the developed throughput model.

Explore related subjects

astro-ph.IM astro-ph.SR

Keep this discovery

Explore connections, maps & timelines

BibTeX RIS

Janmejoy Sarkar, Soumya Roy, A N Ramaprakash, Rushikesh Deogaonkar, Sreejith Padinhatteeri, Durgesh Tripathi, Avyarthana Ghosh, Raja Bayanna Ankala, Gazi Ameen Ahmed. 2025-03-04. Photometric Calibration & Spectral Validation of the Solar Ultraviolet Imaging Telescope onboard Aditya-L1. https://arxiv.org/abs/2503.02560

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2026

Multi-Scale Contrastive Attention for Light-Curve Representation Learning

Current and next-generation time-domain surveys demand automated techniques capable of analyzing millions of light curves, observed in multiple filters, without relying on exhaustive human annotation or scarce spectroscopic follow-up. We present Astra-CLR, an attention-based, self-supervised contrastive learning framework which enables the representation of raw light curves into a highly discriminative latent space. Pre-trained on $\sim$2.1 million unlabeled Zwicky Transient Facility light curves, the framework utilizes partial light curves as input sequences to generate asymmetric, multi-scale temporal views (explicitly contrasting shorter sequences against longer ones) forcing the network to learn a robust "local-to-global" mapping strategy. Furthermore, we introduce a novel multi-view late fusion architecture that extends the model to efficiently handle longer light curves with larger numbers of observations while accommodating the different cadences associated with each filter. The discriminatory power of the resulting representations was evaluated by using them as input to a Multinomial Logistic Regression classifier, trained to identify 12 broad classes of variability. Final accuracy achieved $\sim 0.70$. When applying a label-efficient, partial top-layer fine-tuning strategy, the topological structure of the latent space is significantly refined, boosting results to $\sim$0.77. Astra-CLR is the first publicly available multi-filter time-series Transformer trained exclusively on real ZTF light curves. Results presented here demonstrate that it provides an ideal foundation for the development of end-to-end pipelines, taking into account color evolution and respecting the inhomogeneous nature of astronomical light curve sampling.

astro-ph.IM↗

arXiv2026

Combining astrometry with pulsar timing: the first joint analysis of very low frequency gravitational waves

The pHz to sub-nHz GW regime remains largely unexplored but is crucial for mapping the early inspiral stage of SMBHBs and probing early-Universe physics. Astrometry and pulsar timing offer orthogonal and deeply complementary secular observables to investigate this frequency band. We aim to present the first joint data analysis combining real astrometric proper motions with binary pulsar timing, to search for and constrain continuous gravitational waves (CWs) sourced by SMBHBs in the ultra-low-frequency regime ($10^{-12} \le f_{GW} \le10^{-9} Hz$). We employ a Bayesian model selection and upper limit estimation framework to combine apparent proper motion displacements of $\sim 1.5 \times 10^6$ quasars from the Gaia CRF3 catalog with the line-of-sight orbital period derivatives ($\dot{P}_b$) of 11 high-precision binary pulsars. To prevent spurious detections, we heavily model instrumental and astrophysical systematics: we propagate the Galactic potential uncertainty for pulsars via Monte Carlo simulations and perform a Vector Spherical Harmonics (VSH) decomposition up to the octupole order (l=3) for quasars. We find no statistically significant evidence for a CWs signal in the joint analysis ($\ln B_{joint} = -0.42 \pm 0.03$). In the absence of detection, we set the tightest constraints to date on CW strain in the pHz band, yielding a 95% upper limit of $h_0\le6.4x10^{-11}$ at a reference frequency of $f_{ref} = 4 \times 10^{-10}$ Hz. The combined dataset achieves full sky coverage and improves single-dataset upper limits by 20%-30%. Combining orthogonal observables successfully breaks spatial degeneracies intrinsic to isolated searches. Furthermore, forecasts from inj.-rec. indicate that with the extended temporal baseline and reduced uncertainties of the upcoming Gaia DR4, this joint framework is poised to break the $h_0 < 10^{-11}$ upper limit barrier for sub-nHz CWs.

astro-ph.IM↗

arXiv2026

Bayesian classification of astronomical spectra with class uncertainties

Context: We developed a probabilistic machine learning method with the aim of performing the O(10)-way classification of low- and high-resolution spectra of stellar and extragalactic targets for the upcoming 4MOST survey. In fulfilment of the survey requirements, this method should be able to express uncertainty in the input data as well as uncertainty introduced in its prediction. Aims: Four different methods are explored: (1) convolutional neural networks (CNNs), (2) the Dirichlet distribution, (3) Monte Carlo dropout (MCD), (4) Bayesian neural Networks (BNNs) + variational inference (VI). Training and validation was performed using labelled spectra from the SDSS database and a custom 4MOST mock dataset. All the methods were compared in terms of the same metrics: accuracy, area under the curve (AUC), expected calibration error (ECE), Shannon entropy, negative log-likelihood (NLL), Brier score, training time, and inference time. Methods: A CNN with simple architecture and about 20,000 parameters was trained to achieve classification accuracies of 91.5% on SDSS data and 92.8% on 4MOST mock data. The direct Dirichlet prediction and VI models tested provide uncertainties on class membership probabilities, but they confuse classes more often. The MCD on a CNN is found to be the most suitable; it boosts the point-estimate accuracies to 92.6% and 93.9%, while still providing fast training and sufficiently fast inference. Compared to a standard CNN, the method additionally provides well-calibrated uncertainties at marginal extra cost.

astro-ph.IM↗