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arxiv: 2604.19023 · v1 · submitted 2026-04-21 · 🌌 astro-ph.IM

Seeing at Timau National Observatory Based on ERA5 Dataset

Rhorom Priyatikanto , Gerhana P. Putri , Clara Y. Yatini , Isfahani Rusyda , Evan I. Akbar , Agustinus G. Admiranto , Ferdhiansyah Noor , Siti Maryam
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Elyyani Abd Rachman
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Pith reviewed 2026-05-10 01:53 UTC · model grok-4.3

classification 🌌 astro-ph.IM
keywords astronomical seeingERA5 datasetTimau National Observatorysite characterizationatmospheric seeingoptical astronomyequatorial telescopereanalysis data
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The pith

ERA5 reanalysis data point to a median seeing of 0.79 arcseconds at the Timau observatory site.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper examines twenty years of ERA5 atmospheric data to characterize the astronomical seeing at the future home of Indonesia's 3.8-meter optical telescope. The analysis finds generally favorable conditions with a median value of 0.79 arcseconds, best during March and December, and more variable in the May-September dry season. These results help guide the scheduling and expectations for the observatory's optical imager and near-infrared camera, indicating the site has good potential for equatorial astronomy even if it does not match the clearest sites elsewhere.

Core claim

The ERA5 dataset, despite underestimating seeing at a validation site, yields a median seeing of 0.79 arcseconds at Timau with optimal conditions in March and December and greater variability during the dry season, providing essential information for planning observations with the new telescope.

What carries the argument

Long-term ERA5 reanalysis of atmospheric parameters used to derive seeing estimates at the Timau site.

Load-bearing premise

That the systematic underestimation seen in ERA5 at Eltari Airport does not prevent it from giving accurate absolute seeing values at Timau.

What would settle it

On-site seeing measurements collected at Timau over multiple years showing a median value substantially higher or lower than 0.79 arcseconds would contradict the ERA5-based estimate.

Figures

Figures reproduced from arXiv: 2604.19023 by Abd Rachman, Agustinus G. Admiranto, Clara Y. Yatini, Elyyani, Evan I. Akbar, Ferdhiansyah Noor, Gerhana P. Putri, Isfahani Rusyda, Rhorom Priyatikanto, Siti Maryam.

Figure 1
Figure 1. Figure 1: Topography of West Timor where Timau Observatory is situated. The 0.25◦ × 0.25◦ degree2 grid of ERA5 data is laid over the map. terval between pressure levels is 25 hPa. Hourly data from 1 January 2002 to 31 December 2021 were fed into the anal￾ysis and the seeing estimation method described in the next section. Timau (123.9472◦ E, 9.5971◦ S) is not located at the cen￾tre of the ERA5 rectangular grid. For … view at source ↗
Figure 2
Figure 2. Figure 2: summarizes the number of radiosonde observations each month. Before thorough analysis, we estimated meteorological pa￾rameters (mainly temperature and horizontal wind speed) at different pressure levels, ranging from 10 to 1000 hPa, with 5-hPa intervals. This range covers altitudes up to 37 km above sea level. Linear interpolation was implemented on the ERA5 and radiosonde datasets [PITH_FULL_IMAGE:figure… view at source ↗
Figure 3
Figure 3. Figure 3: Vertical profile of east-west (a), north-south (b), and horizontal (c) wind speed above Eltari from ERA5 (solid) and radiosonde (dashed) data during four different seasons. most of the year. However, it was notably stronger during the June-July-August period. A distinct pattern of more promi￾nent southward wind was observed outside of the December￾February period. This southward flow appeared to contribute… view at source ↗
Figure 4
Figure 4. Figure 4: Comparison between seeing from ERA5 and radiosonde [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Atmospheric profile over Timau as indicated by temperature (a), horizontal wind speed (b), and refractive index structure constant (c). The median and the inter-quartile range (IQR) are from 2002-2021 aggregated at different pressure levels. 200 250 300 Temperature [K] 0 10 20 30 40 50 Height [km] (a) Seasonal Temperature and Wind from ERA5 2002-2021 Nighttime Above Timau 180 200 20 25 30 40 20 0 20 East-W… view at source ↗
Figure 6
Figure 6. Figure 6: Seasonal variation of wind profile over Timau. conclusion. Assuming the underestimation factor from the Eltari radiosonde analysis holds true for Timau, the ERA5- based median seeing of 0.79 arcseconds should be multiplied by 1.3, resulting in a corrected value of 1.03 arcseconds. This adjusted figure aligns more closely with direct measurements and can serve as a valuable reference for future observatory … view at source ↗
Figure 7
Figure 7. Figure 7: Overall distribution (a), seasonal change (b), and daily variation (c) of seeing at Timau. During dry season, the surface wind blows from East to West. The percentage of westerly winds with speeds greater than 10 m/s reaches almost 40% in June, July, and August (Priy￾atikanto et al. 2024). While passing Mount Mutis, the winds may introduce additional turbulence that reduces seeing con￾ditions at Timau. The… view at source ↗
read the original abstract

Understanding the seeing conditions is crucial for astronomical observations using a ground-based telescope. This study analyzes long-term atmospheric data (2002-2021) from the ERA5 dataset to assess the seeing conditions at the new Timau National Observatory in Indonesia, which will house a 3.8-meter optical telescope. While the ERA5 dataset shows good agreement with radiosonde data for temperature and wind speed, it tends to underestimate seeing at Eltari Airport, Kupang. Despite this discrepancy, the ERA5 data suggest a median seeing of 0.79 arcseconds at Timau, with optimal seeing conditions in March and December and greater variability during the May to September dry season. These findings are crucial for the planning and operation of the observatory, which requires excellent seeing conditions for its three-band optical imager and a near-infrared camera. Although the seeing at Timau is not as good as some other observatories, the conditions at Timau make it an observatory that has good prospects for equatorial regions.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The manuscript analyzes 20 years (2002-2021) of ERA5 reanalysis data to characterize seeing conditions at the proposed Timau National Observatory in Indonesia for a 3.8 m optical telescope. It reports good ERA5-radiosonde agreement on temperature and wind speed at the validation site (Eltari Airport) but notes systematic underestimation of seeing there; nevertheless, it derives a median seeing of 0.79 arcsec at Timau, identifies optimal conditions in March and December, and concludes the site has good prospects for equatorial astronomy despite not matching premier observatories.

Significance. If the seeing derivation from ERA5 profiles is reproducible and the noted validation bias is shown not to affect Timau, the long-term statistics would provide a valuable baseline for an under-characterized equatorial site, aiding telescope planning and operations. The use of publicly available reanalysis data over two decades and direct comparison to independent radiosonde measurements are positive features that support reproducibility.

major comments (2)
  1. [Methods / Results] The procedure for deriving seeing (in arcseconds) from ERA5 variables such as vertical profiles of wind speed and temperature is not described anywhere in the manuscript. Since seeing is not a direct ERA5 output and requires a turbulence parametrization or integral (e.g., involving C_n^2 or Fried parameter), the reported median of 0.79 arcsec cannot be reproduced or its uncertainties assessed.
  2. [Abstract and Validation section] The abstract and validation discussion explicitly state that ERA5 underestimates seeing at Eltari Airport, yet the same uncorrected ERA5-derived values are used for Timau without bias correction, site-specific validation, or sensitivity analysis showing that the underestimation does not apply (or applies equally) at Timau. This leaves the absolute median seeing and the claim of suitability for a 3.8 m telescope dependent on an untested assumption.
minor comments (1)
  1. [Abstract] The final sentence of the abstract is awkwardly phrased ('the conditions at Timau make it an observatory that has good prospects'); a clearer wording would improve readability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their detailed and constructive comments on our manuscript. We have carefully considered each point and revised the manuscript to improve the clarity and robustness of our analysis. Our point-by-point responses are provided below.

read point-by-point responses

  1. Referee: [Methods / Results] The procedure for deriving seeing (in arcseconds) from ERA5 variables such as vertical profiles of wind speed and temperature is not described anywhere in the manuscript. Since seeing is not a direct ERA5 output and requires a turbulence parametrization or integral (e.g., involving C_n^2 or Fried parameter), the reported median of 0.79 arcsec cannot be reproduced or its uncertainties assessed.

    Authors: We agree with the referee that the derivation procedure was not described in the original submission, which prevents reproducibility. We have added a comprehensive description of the method in the revised Methods section, specifying the parametrization used to compute the seeing from ERA5 temperature and wind profiles, including the integration for the Fried parameter. This addition ensures that the median value of 0.79 arcsec and associated uncertainties can be reproduced. revision: yes

  2. Referee: [Abstract and Validation section] The abstract and validation discussion explicitly state that ERA5 underestimates seeing at Eltari Airport, yet the same uncorrected ERA5-derived values are used for Timau without bias correction, site-specific validation, or sensitivity analysis showing that the underestimation does not apply (or applies equally) at Timau. This leaves the absolute median seeing and the claim of suitability for a 3.8 m telescope dependent on an untested assumption.

    Authors: The referee raises a valid point about the potential systematic bias. While we cannot perform site-specific validation without additional data at Timau, we have revised the manuscript to include a sensitivity analysis in the Results and Discussion sections. This analysis quantifies the impact of the observed underestimation bias on the Timau seeing statistics. We have also updated the abstract and conclusions to reflect that the reported values are ERA5-derived and may be underestimated, thereby addressing the untested assumption with quantitative discussion of the limitations. revision: partial

Circularity Check

0 steps flagged

No circularity: direct statistical summary of reanalysis data

full rationale

The manuscript computes median seeing and seasonal statistics directly from ERA5 vertical profiles at the Timau site. It reports a comparison to independent radiosonde data at Eltari but does not fit parameters, invoke self-citations for uniqueness, or present any derivation that reduces the output to the input by construction. The 0.79 arcsec median is a summary statistic, not a model prediction. No load-bearing steps match the enumerated circularity patterns.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

The central claim rests on the accuracy of the ERA5 reanalysis product and an unspecified empirical relation that maps its temperature, wind, and humidity fields to astronomical seeing; no new physical axioms or entities are introduced.

pith-pipeline@v0.9.0 · 5517 in / 1011 out tokens · 35648 ms · 2026-05-10T01:53:20.870790+00:00 · methodology

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Works this paper leans on

54 extracted references · 6 canonical work pages

  1. [1]

    2020, Astronomy & Astrophysics, 641, A1

    Aghanim, N., Akrami, Y ., Arroja, F., et al. 2020, Astronomy & Astrophysics, 641, A1

    2020

  2. [2]

    I., Jatmiko, A

    Akbar, E. I., Jatmiko, A. T. P., Putra, M., et al. 2019, Journal of Physics: Conference Series, 1245, 012024

    2019

  3. [3]

    K., Erdo˘gan, M., et al

    Aksaker, N., Yerli, S. K., Erdo˘gan, M., et al. 2020, Monthly Notices of the Royal Astronomical Society, 493, 1204

    2020

  4. [4]

    2004, Publications of the Astronomical Society of the Pacific, 116, 266

    Bakos, G., Noyes, R., Kov´acs, G., et al. 2004, Publications of the Astronomical Society of the Pacific, 116, 266

    2004

  5. [5]

    2022, Annual Report 2022, Tech

    Barcons, X. 2022, Annual Report 2022, Tech. rep., European Southern Observatory

    2022

  6. [6]

    1982, Bulletin of the American Meteorological Society, 63, 277

    Bengtsson, L., Kanamitsu, M., Kållberg, P., & Uppala, S. 1982, Bulletin of the American Meteorological Society, 63, 277

    1982

  7. [7]

    2023, Remote Sensing, 15, 2225

    Bi, C., Qing, C., Qian, X., et al. 2023, Remote Sensing, 15, 2225

    2023

  8. [8]

    J., Koch, D., Basri, G., et al

    Borucki, W. J., Koch, D., Basri, G., et al. 2010, Science, 327, 977

    2010

  9. [9]

    J., & Lindley, D

    Bowen, A. J., & Lindley, D. 1977, Boundary-Layer Meteorology, 12, 259, doi: https://doi.org/10.1007/BF00121466

    work page doi:10.1007/bf00121466 1977

  10. [10]

    2018, Multiple Messengers and Challenges in Astroparticle Physics, 489

    Branchesi, M. 2018, Multiple Messengers and Challenges in Astroparticle Physics, 489

    2018

  11. [11]

    E., Vernin, J., Coqueugniot, Y ., & Caccia, J

    Coulman, C. E., Vernin, J., Coqueugniot, Y ., & Caccia, J. L. 1988, Applied Optics, 27, 155

    1988

  12. [12]

    P., Uppala, S

    Dee, D. P., Uppala, S. M., Simmons, A. J., et al. 2011, Quarterly Journal of the Royal Meteorological Society, 137, 553

    2011

  13. [13]

    Fairall, C. W. 1984, Journal of Atmospheric Sciences, 41, 3472 , doi: 10.1175/1520-0469(1984)041⟨3472:WSEOEA⟩2.0.CO;2

    work page doi:10.1175/1520-0469(1984)041 1984

  14. [14]

    Fried, D. 1966, J. Opt. Soc. Am., 56, 1372

    1966

  15. [15]

    2003, Radio Science, 38

    Fukao, S., Hashiguchi, H., Yamamoto, M., et al. 2003, Radio Science, 38

    2003

  16. [16]

    2006, in WIT Transactions on Ecology and the Environment (2006), V ol

    Gawad, A., Zoklot, A.-S., & Abdel Gawad, A. 2006, in WIT Transactions on Ecology and the Environment (2006), V ol. 93, doi: 10.2495/SC060411

    work page doi:10.2495/sc060411 2006

  17. [17]

    1997, ECMWF Re-Analysis Profect Report Series, 72

    Gibson, J. 1997, ECMWF Re-Analysis Profect Report Series, 72

    1997

  18. [18]

    2011, Annual Review of Nuclear and Particle Science, 61, 251

    Goobar, A., & Leibundgut, B. 2011, Annual Review of Nuclear and Particle Science, 61, 251

    2011

  19. [19]

    X., Wright, J

    Han, E., Wang, S. X., Wright, J. T., et al. 2014, Publications of the Astronomical Society of the Pacific, 126, 827

    2014

  20. [20]

    2021, Monthly Notices of the Royal Astronomical Society, 501, 4692

    Han, Y ., Yang, Q., Liu, N., et al. 2021, Monthly Notices of the Royal Astronomical Society, 501, 4692

    2021

  21. [21]

    A., Yang, R., Sarazin, M., & Hickson, P

    Hellemeier, J. A., Yang, R., Sarazin, M., & Hickson, P. 2019, Monthly Notices of the Royal Astronomical Society, 482, 4941

    2019

  22. [22]

    2019, ECMWF Newsletter, 17

    Hersbach, H., Bell, W., Berrisford, P., et al. 2019, ECMWF Newsletter, 17

    2019

  23. [23]

    2020, Quarterly Journal of the Royal Meteorological Society, 146, 1999

    Hersbach, H., Bell, B., Berrisford, P., et al. 2020, Quarterly Journal of the Royal Meteorological Society, 146, 1999

    2020

  24. [24]

    2012, Monthly Notices of the Royal Astronomical Society, 427, 1903

    Hidayat, T., Mahasena, P., Dermawan, B., et al. 2012, Monthly Notices of the Royal Astronomical Society, 427, 1903

    2012

  25. [25]

    Hutt, D. L. 1999, Optical Engineering, 38, 1288, doi: 10.1117/1.602188

    work page doi:10.1117/1.602188 1999

  26. [26]

    2019, The Astrophysical Journal, 881, 19

    Jones, D., Scolnic, D., Foley, R., et al. 2019, The Astrophysical Journal, 881, 19

    2019

  27. [27]

    Khosroshahi, H. G. 2018, Nature Astronomy, 2, 429

    2018

  28. [28]

    G., Lipunov, V

    Kornilov, V . G., Lipunov, V . M., Gorbovskoy, E. S., et al. 2012, Experimental Astronomy, 33, 173

    2012

  29. [29]

    2020, Publications of the Astronomical Society of Japan, 72, 48

    Kurita, M., Kino, M., Iwamuro, F., et al. 2020, Publications of the Astronomical Society of Japan, 72, 48

    2020

  30. [30]

    2022, Research in Astronomy and Astrophysics, 22, 045002, doi: 10.1088/1674-4527/ac4e00

    Li, T.-R., & Jiang, X.-J. 2022, Research in Astronomy and Astrophysics, 22, 045002, doi: 10.1088/1674-4527/ac4e00

    work page doi:10.1088/1674-4527/ac4e00 2022

  31. [31]

    M., Kornilov, V

    Lipunov, V . M., Kornilov, V . G., Zhirkov, K., et al. 2022, Universe, 8, 271

    2022

  32. [32]

    2013, Boundary-layer meteorology, 147, 179 11

    Mahrt, L., Thomas, C., Richardson, S., et al. 2013, Boundary-layer meteorology, 147, 179 11

    2013

  33. [33]

    2018, Nature Astronomy, 2, 930

    Putra, M. 2018, Nature Astronomy, 2, 930

    2018

  34. [34]

    2022, in Ground-based and Airborne Telescopes IX, V ol

    Pirnay, O., Albart, P., Bastin, C., et al. 2022, in Ground-based and Airborne Telescopes IX, V ol. 12182, SPIE, 1327–1338

    2022

  35. [35]

    L., Skillen, I., Cameron, A

    Pollacco, D. L., Skillen, I., Cameron, A. C., et al. 2006, Publications of the Astronomical Society of the Pacific, 118, 1407

    2006

  36. [36]

    G., Djamaluddin, T., Rachman, A., & Wijaya, D

    Priyatikanto, R., Admiranto, A. G., Djamaluddin, T., Rachman, A., & Wijaya, D. D. 2024, PASA, xx, xx

    2024

  37. [37]

    S., Hidayat, T., et al

    Priyatikanto, R., Mumpuni, E. S., Hidayat, T., et al. 2023, Monthly Notices of the Royal Astronomical Society, 518, 4073

    2023

  38. [38]

    R., Winn, J

    Ricker, G. R., Winn, J. N., Vanderspek, R., et al. 2015, Journal of Astronomical Telescopes, Instruments, and Systems, 1, 014003

    2015

  39. [39]

    1979, Journal of Optics, 10, 299

    Roddier, F. 1979, Journal of Optics, 10, 299

    1979

  40. [40]

    H., & DeBenedictis, D

    Ruggiero, F. H., & DeBenedictis, D. A. 2002, in DoD High Performance Modernization Program Users Group Conference, 10–14

    2002

  41. [41]

    2019, Current Science, 117, 365

    Sagar, R., Kumar, B., & Omar, A. 2019, Current Science, 117, 365

    2019

  42. [42]

    D., Zakiar, M

    Sakti, A. D., Zakiar, M. R., Santoso, C., et al. 2023, Plos one, 18, e0293190

    2023

  43. [43]

    2022, Journal of Physics: Conference Series, 2214, 012013

    Saputra, M., Danarianto, M., Murti, M., et al. 2022, Journal of Physics: Conference Series, 2214, 012013

    2022

  44. [44]

    1990, Astronomy and Astrophysics, 227, 294

    Sarazin, M., & Roddier, F. 1990, Astronomy and Astrophysics, 227, 294

    1990

  45. [45]

    2002, in European Southern Observatory Conference and Workshop Proceedings, V ol

    Sarazin, M., & Tokovinin, A. 2002, in European Southern Observatory Conference and Workshop Proceedings, V ol. 58, European Southern Observatory Conference and Workshop Proceedings, ed. E. Vernet, R. Ragazzoni, S. Esposito, & N. Hubin, 321

    2002

  46. [46]

    B., Flatau, P

    Szkolka, W., Baranowski, D. B., Flatau, P. J., et al. 2025, Climate Dynamics, 63, 88

    2025

  47. [47]

    I., Silverman, R

    Tatarski, V . I., Silverman, R. A., & Chako, N. 1961, Physics Today, 14, 46

    1961

  48. [48]

    2002, Publications of the Astronomical Society of the Pacific, 114, 1156

    Tokovinin, A. 2002, Publications of the Astronomical Society of the Pacific, 114, 1156

    2002

  49. [49]

    W., Lykke, K

    Tonry, J., Stubbs, C. W., Lykke, K. R., et al. 2012, The Astrophysical Journal, 750, 99

    2012

  50. [50]

    S., Kallberg, P., Simmons, A., et al

    Uppala, S. S., Kallberg, P., Simmons, A., et al. 2005, Quarterly Journal of the Royal Meteorological Society, 131, 2961

    2005

  51. [51]

    1986, Proceedings of SPIE - The International Society for Optical Engineering, 628, doi: 10.1117/12.963521

    Vernin, J. 1986, Proceedings of SPIE - The International Society for Optical Engineering, 628, doi: 10.1117/12.963521

    work page doi:10.1117/12.963521 1986

  52. [52]

    2006, Atmospheric Science: An Introductory Survey: Second Edition (Singapore: Elsevier), 1–488

    Wallace, J., & Hobbs, P. 2006, Atmospheric Science: An Introductory Survey: Second Edition (Singapore: Elsevier), 1–488

    2006

  53. [53]

    2022, Remote Sensing, 14, 3085

    Xu, M., Shao, S., Weng, N., & Liu, Q. 2022, Remote Sensing, 14, 3085

    2022

  54. [54]

    2023, Monthly Notices of the Royal Astronomical Society, 522, 1419

    Zhu, L., Zhang, H., Sun, G., et al. 2023, Monthly Notices of the Royal Astronomical Society, 522, 1419

    2023