Photometry with the new 1.5-meter telescope of the Rozhen Observatory

D. Marchev; E. Ovcharov; E. Semkov; I. E. Dimitrova; J. Marti; M. Minev; M. Moyseev; N. Petrov; R. K. Zamanov

REVIEW 3 major objections 4 minor 14 references

Photometry with the new 1.5-meter telescope of the Rozhen Observatory

T0 review · 3 major / 4 minor · reviewed 2026-08-05 · deepseek-v4-flash

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Pith's one-line read The new 1.5-meter Rozhen telescope achieves ±0.02 mag V-band photometry on 60-second exposures for stars from 13th to 17th magnitude.

desk verdict A solid, honest facility report whose central photometric numbers are repeatability statistics that need a more explicit sample definition before they become routine performance claims. read the letter →

arxiv 2508.18752 v1 pith:5GBPLNQA submitted 2025-08-26 astro-ph.IM astro-ph.SR

E. Semkov , N. Petrov , M. Minev , M. Moyseev , D. Marchev , E. Ovcharov , J. Marti , I. E. Dimitrova

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R. K. Zamanov

This is my paper · ORCID

classification astro-ph.IMastro-ph.SR PACS 95.55.Cs95.75.De

keywords AZ1500telescopeRozhenObservatoryphotometricaccuracycommissioningtime-domainastronomyrecurrentnovaTCrBUBVRIphotometryground-basedopticaltelescopes

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

The paper reports that the new 1.5-meter AZ1500 telescope at Rozhen Observatory is in regular operation and that its design and performance are suitable for precise photometry. On 60-second exposures it reaches V-band accuracy of ±0.02 magnitudes for stars between 13th and 17th magnitude, with ±0.05 magnitude accuracy in U and B bands over their useful ranges. Tracking measurements give average drifts of 6.5 arcseconds per hour in right ascension and 4.6 arcseconds per hour in declination. The claim is backed by consecutive-exposure scatter curves, by astrometric solutions with sub-0.02-arcsecond residuals, and by early science runs on the recurrent nova T CrB, the X-ray binary candidate IRAS 18293-0941, and other targets. If correct, this puts a mid-size ground-based facility into the niche of time-domain and follow-up astronomy.

What carries the argument

The load-bearing object is the AZ1500 telescope system itself: a 1.5-meter Ritchey-Chrétien (two-mirror, coma-free hyperbolic design) on an alt-azimuth mount, with quartz optics and two Nasmyth foci. The load-bearing measure is the empirical photometric error curve: for each band, the standard deviation σ of 10 consecutive exposures of ~30 field stars plotted against average magnitude. This σ(m) curve converts the aperture, optics, filters, detector, and pointing stability into a single accuracy specification (e.g., ±0.02 mag in V for 13 < mV < 17 with 60 s exposures). Supporting machinery is a T-Point star-calibration model that gives ~5 arcsec pointing accuracy, and CCD-position tracking l

What would settle it

Take repeated 60-second V-band exposures of a set of non-variable stars in the 13–17 magnitude range on many nights spanning typical Rozhen seeing and airmass, and compute the same 10-exposure σ; if the resulting scatter exceeds ~0.02 mag for mid-range stars, or tracking drift exceeds half the seeing within a few minutes on most nights, the quoted precision would not describe normal operation.

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Extended reading notes

Core claim

On the paper's own terms, the central finding is that the AZ1500 — a 1.5-meter Ritchey-Chrétien telescope with quartz optics on an alt-azimuth mount — is a working precision photometric instrument. The authors demonstrate this by measuring the standard deviation of ten consecutive exposures of roughly thirty field stars in UBVR bands: V-band reaches ±0.02 mag over 13 < V < 17 with 60 s exposures, while shorter exposures cover brighter stars and U and B reach ±0.05 mag over their stated ranges. Tracking was characterized on seven one-hour runs, yielding average shifts of 6.5 arcsec/hour in RA and 4.6 arcsec/hour in Dec. Early science results — UBV flickering of T CrB, multi-epoch photometry o

Load-bearing premise

The accuracy claims assume that the standard deviation of ten consecutive exposures of roughly thirty field stars, taken on a few unspecified nights and targets, fairly represents the telescope's routine performance across seasons, seeing, and airmass.

Editorial extensions

If this is right

Time-series monitoring of accreting white dwarfs, recurrent novae, and symbiotic stars can be done at the ±0.02 mag level in V, with 60-second cadence reaching 17th magnitude.
Optical follow-up of gamma-ray and X-ray sources can rely on the telescope's astrometric residuals of 0.01–0.02 arcseconds and stable photometric zero points to identify and characterize counterparts.
Reverberation-mapping campaigns on AGN and blazars, which require densely sampled, precise light curves over long baselines, become practical with this instrument.
M31 nova candidate confirmation via narrow-band H-alpha imaging can be attempted routinely with the telescope's filter wheel and fast exposure capability.
The second Nasmyth focus, if fitted with a low-resolution spectrograph, would extend the same precision to spectroscopy of transients.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

The reported accuracy rests on only a handful of nights and on stars chosen without stated selection criteria; a fair test would re-measure σ(m) on many nights over varied seeing and airmass, and on targets of known constancy, before treating ±0.02 mag as routine.
Because run-to-run tracking scatter spans 0.9 to 14.6 arcsec/hour, the useful exposure time is likely seeing- and tracking-dependent; an autoguider or field derotator upgrade could push the limiting magnitude beyond V=17 for the same total integration.
The method of ten consecutive exposures captures short-timescale noise such as scintillation and tracking wobble but not long-term systematics, so multi-night differential photometry may be even better than the single-run σ suggests — a testable prediction for repeated standard-star observations.
The same σ(m) procedure could serve as a quick commissioning metric for other mid-size telescopes, allowing direct comparison of photometric precision across facilities.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Nothing to see here but a solid, honest instrument report. The new pieces are the first published tracking and photometric error measurements for the Rozhen 1.5 m, plus a few demonstration light curves. That is exactly what a facility paper should be, and the central numbers are directly shown in tables and figures. I believe the telescope does what they say: 60-s V-band exposures give ±0.02 mag scatter for 13–17 mag stars, with U/B/R also characterized. The T CrB flickering light curves and the IRAS 18293-0941 astrometry are nice illustrations, though they are explicitly preliminary.

The soft spots are real but not disqualifying. The photometric error curves in Fig. 2 are computed as the standard deviation of ten consecutive exposures of about thirty field stars. That is a repeatability statistic, not an end-to-end accuracy estimate. Consecutive exposures share airmass, seeing, and tracking conditions, so zero-point, color-term, aperture, and flat-field errors are largely invisible to it. More importantly, the paper gives no selection criteria for the stars or the nights. The tracking data in Sect. 3 show factor >15 run-to-run variation (0.9 to 14.6 arcsec/hr in RA), so a few good nights could dominate the quoted limits. That is the main thing I would want addressed before trusting the quoted magnitude limits as routine performance. The tracking statistics are also sparse: seven one-hour runs, and the 'exposures up to 10 min will provide good results' conclusion does not follow from a typical drift of 1–2 arcsec when the worst case is 14.6 arcsec/hr. The T CrB color–temperature numbers are given without error bars, but the authors flag the analysis as preliminary and promise a forthcoming paper.

The citation pattern looks appropriate. The paper leans on the relevant method references and its own prior work; no issue there. There is no circular reasoning because they are reporting measurements, not fitting a model and calling it a prediction.

Overall: this is a useful facility characterization for the Bulgarian community and for anyone who wants to use the AZ1500 for time-domain follow-up. It deserves a serious referee, mainly to force the authors to specify their star/night sample and to present the repeatability/accuracy distinction honestly. I would send it to peer review with a request for revision rather than desk-reject it.

Referee Report

3 major / 4 minor

Summary. The paper reports the first-light performance characterization of the new 1.5 m AZ1500 telescope at Rozhen Observatory. It presents pointing accuracy, tracking shifts from seven one-hour runs, and photometric error curves derived from the standard deviation of ten consecutive exposures of about thirty stars in U, B, V, and R bands. The central quantitative claims are ±0.02 mag V-band photometry for 13 < mV < 17 with 60 s exposures and average tracking shifts of 6.5 arcsec/hr in RA and 4.6 arcsec/hr in Dec. The paper also gives brief science highlights, including T CrB flickering observations and a derived color-temperature variation. It concludes that the telescope's design and performance are suitable for precise observations of various astrophysical objects.

Significance. If the performance figures hold, the AZ1500 telescope becomes a competitive mid-size facility for time-domain photometry of 13-17 mag objects, filling a niche between smaller robotic telescopes and 4-8 m class facilities. The paper is a useful facility report: the data are presented in tables and figures, the photometric error definition is explicit, and the tracking data are shown for individual runs. However, the central claims rest on internal repeatability rather than end-to-end accuracy, and the statistical treatment is informal. The science highlights are illustrative rather than definitive. The paper's value is in documenting the new facility, but the quantitative claims need firmer statistical grounding to support the stated performance.

major comments (3)

[Section 4, Eq. (1) and Fig. 2] The quoted photometric accuracy is the standard deviation of ten consecutive exposures of ~30 stars. This is a measure of internal repeatability within a single field and night, not end-to-end photometric accuracy. Zero-point, color-term, aperture, flat-field, and long-term atmospheric variations are not included. No selection criteria are given for the stars or the nights, and the tracking data in Section 3 show run-to-run scatter from 0.9 to 14.6 arcsec/hr in RA, so representativeness is not demonstrated. The claimed magnitude limits (e.g., ±0.02 mag for 13 < mV < 17 with 60 s) are therefore not supported as routine performance. Please specify the fields used, the star and night selection criteria, and quantify systematic error contributions.
[Section 3] The tracking analysis is statistically informal. The average shifts s1=6.5 and s2=4.6 arcsec/hr are computed from only seven one-hour runs with highly variable values (0.9 to 14.6 arcsec/hr in RA). Removing the largest value is arbitrary and changes the mean substantially; the quoted 6.5 ± 4.1 arcsec/hr is not a standard deviation of a well-defined sample. The statement that 'exposures up to 10 min will provide good results' is not robust: at the largest measured drift, a 10-min exposure accumulates ~2.4 arcsec of drift, comparable to a typical photometric aperture radius. Please report the full distribution, median, and worst-case drift, and relate the tracking requirement to the aperture used for photometry.
[Section 5.1] The T CrB color-temperature results have no error propagation. The temperatures T = 13675 K and T = 8085 K are quoted without uncertainties, yet they are derived from dereddened U-B colors using a recipe with no stated error bars. The conclusion that the flickering source becomes redder and cooler when brighter rests on only two epochs and is not quantitatively supported. Please provide error bars on the colors and temperatures, or soften the claim to a tentative observation. The 'average dereddened U-B0 colour' phrase also needs a definition of the averaging procedure.

minor comments (4)

[Section 4] In the R-band sentence, '12 ≤ mB ≤ 17' should presumably be '12 ≤ mR ≤ 17'. Also, the figure legend for Fig. 2 lists V-band with two exposure times (1 s and 60 s) but the color convention (green vs black) is not described clearly in the caption.
[Section 3] The symbols s1 and s2 are defined as shifts along RA and Dec, but no sign convention or direction is given. The calculation 'a shift with half of the seeing (0.5 arcsec) takes typically 4 minutes' is not derived from the quoted averages; please show the arithmetic or state the assumption.
[Section 2] The coordinates '41041′48.4′′ N, 24044′18.4′′ E' appear to be missing the degree symbol; they should read 41°41′48.4′′ N, 24°44′18.4′′ E. The article header and running text contain several typographical spacing issues (e.g., 'T ome 78', 'T able 1') that should be corrected.
[Section 5.1] The T CrB light curves and Table 1 are useful, but the min, max, mean, stdev, amplitude, and merr columns are not all defined; in particular, 'ampl' and 'merr' are only loosely described. Define each column explicitly.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the paper's central claims are direct measurements of tracking drift and photometric scatter, with no fitted input relabeled as prediction and no load-bearing self-citation chain.

full rationale

The paper is an instrument-performance report, not a model-derived prediction. The central quantitative claims are (i) average tracking shifts of 6.5 arcsec/hr in RA and 4.6 arcsec/hr in Dec, obtained by measuring star positions on CCD frames over one-hour runs (Sect. 3), and (ii) photometric scatter of ±0.02 mag for 13 < mV < 17 in 60-s V-band exposures, obtained by computing the standard deviation of 10 consecutive measurements of ~30 field stars (Eq. 1, Fig. 2). Both are empirical statistics computed from data; neither quantity is defined in terms of the conclusion it is supposed to establish. No parameter is fitted to a subset of data and then presented as a prediction of a closely related quantity. The inference that exposures up to 10 min give good results is a direct propagation from measured drifts and an assumed 1-arcsec seeing, not a circular step. The color-temperature conversions in Sect. 5.1 use published recipes from Nelson et al. (2011) and Bruch (1992), which are external references, not self-citations. The self-citations present ([1], [6], [10]-[13]) are contextual references to prior observations, first light, or science program contexts and are not load-bearing for the performance conclusions. A legitimate concern is that the Sect. 4 scatter measures internal repeatability over consecutive exposures rather than end-to-end accuracy, and the star/night selection is undocumented; that is a correctness/sampling-caveat issue, not circularity. The paper makes no claim that a derived result is independent of its inputs while actually being identical to them. Therefore the appropriate circularity score is 0.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

The central claim requires only standard photometric practice. The fragile items are the representativeness of the small star and night sample, the manufacturer's diffraction-limited assumption, and the use of literature color-temperature recipes. No new theoretical entities or fitted model parameters are introduced.

assumptions (4)

domain assumption The UBVRI passbands are defined by the stated mean wavelengths and effective widths, and standard photometric transformations apply.
Used throughout Sects. 4 and 5 to interpret magnitudes and colors; no transformation equations or extinction coefficients are given in the paper.
domain assumption The measured shifts of field stars over one-hour runs are dominated by telescope tracking errors rather than source variability or detector artifacts.
Section 3, tracking accuracy; the targets include a recurrent nova and a YSO candidate, but the measured positions are assumed to reflect mount performance.
ad hoc to paper The standard deviation of ten consecutive exposures of about thirty stars is an unbiased estimator of photometric accuracy, and the selected stars and nights are representative.
Section 4 and Fig. 2; selection criteria for stars and nights are not documented, and the tracking data in Sect. 3 show high run-to-run scatter, so representativeness is an unverified premise.
domain assumption The optical system is diffraction-limited as stated by the manufacturer, so image quality is not the limiting factor in the photometric measurements.
Section 2 states 'quartz optics that guarantees diffraction-limited performance'; no PSF or Strehl measurement is reported in this paper.

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Cite this review

Pith. "Pith review of Photometry with the new 1.5-meter telescope of the Rozhen Observatory." pith.science (2026). https://pith.science/paper/5GBPLNQA

@misc{pith2026250818752,
  author       = {Pith},
  title        = {Pith review of: Photometry with the new 1.5-meter telescope of the Rozhen Observatory},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/5GBPLNQA}},
  note         = {Machine review of arXiv:2508.18752}
}

read the original abstract

The new 1.5~m telescope (AZ1500) is operating at the National Astronomical Observatory Rozhen, Bulgaria. This paper gives an overview of the telescope and presents a snapshot of the current performance. Science observations are under way, and we give brief highlights from a number of programs that have been enabled.

Figures

Figures reproduced from arXiv: 2508.18752 by the authors.

**Figure 1.** The new dome and AZ1500 telescope. 3 Tracking accuracy To give an estimation of the telescope guiding we measured the position of a few stars on the CCD during 1 hour continuous observations. The shifts in pixels were transformed into arc-seconds. The results for a few nights are: 28 Mar. 2024 UT21:19-22:19, T CrB s1 = 14.6 ′′ , s2 = 9.8 ′′ , 28 Mar. 2024 UT22:19-23:20, T CrB s1 = 1.7 ′′ , s2 = 1.1 ′′ , 15 Apr. 2024… view at source ↗

**Figure 2.** Standard deviation (σ) versus the average magnitude in U, B, V, and R bands. σ represents the errors of the photometry. The colours are: U – magenta, B – blue, V – green (exposure 1 sec), black (exposure 60 sec), R – red. The exposure time (exp-time) is marked on each panel. For more details see Sect.4. 4 The accuracy of the photometry The AZ1500 telescope is equipped with a filter wheel with optical filters in the … view at source ↗

**Figure 3.** Flickering of T CrB in U, B, V bands observed with the 1.5 m telescope. The amplitude of the variability is of about 0.3 mag in U-band and 0.1 mag in B−band. UBV photometry of the recurrent nova T Coronae Borealis (NOVA CrB 1866, NOVA CrB 1946) is already published in [6], where the observations [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

14 extracted references · 14 canonical work pages

[1]

2024, Contributions of the Astronomical Observatory Skalnate Pleso, 54, 15

Minev, M., Petrov, N., & Semkov, E. 2024, Contributions of the Astronomical Observatory Skalnate Pleso, 54, 15

work page 2024
[2]

T., 2002, Society of Photo-Optical Instrumentation Engineers (SPIE) Con- ference Series, 4848, 125

Wallace, P. T., 2002, Society of Photo-Optical Instrumentation Engineers (SPIE) Con- ference Series, 4848, 125

work page 2002
[3]

2025, Astronomy & Astrophysics, 694, A85

Planquart, L., Jorissen, A., & Van Winckel, H. 2025, Astronomy & Astrophysics, 694, A85

work page 2025
[4]

and Ducci, L

Bozzo, E., Ferrigno, C., Oskinova, L. and Ducci, L. 2022, MNRAS, 510, 4645

work page 2022
[5]

A., de Souza, R

Kuhn, M. A., de Souza, R. S., Krone-Martins, A., Castro-Ginard, A., Ishida, E. E. O., et al. 2021, Astrophysical Journal Supplement Series, 254, 33

work page 2021
[6]

Y., Semkov, E., Minev, et al., 2023, A&A, 680, L18

Zamanov, R., Boeva, S., Latev, G. Y., Semkov, E., Minev, et al., 2023, A&A, 680, L18

work page 2023
[7]

Nelson, T., Mukai, K., Orio, M., Luna, G. J. M., & Sokoloski, J. L. 2011, Astrophysical Journal, 737, 7

work page 2011
[8]

1992, Astronomy & Astrophysics, 266, 237

Bruch, A. 1992, Astronomy & Astrophysics, 266, 237

work page 1992

Show all 14 references

[9]

S., Carrera, F., Guillout, P., et al

Motch, C., Warwick, R., Cropper, M. S., Carrera, F., Guillout, P., et al. 2010, Astronomy & Astrophysics, 523, A92

2010
[10]

P., Nedialkov, P

Ovcharov, E. P., Nedialkov, P. L., Valcheva, A. T., Ivanov, V. D., Tikhonov, N. A., et al. 2008, MNRAS, 386, 819

2008
[11]

C., Kushwaha, P., Strigachev, A., et al

Bachev, R., Tripathi, T., Gupta, A. C., Kushwaha, P., Strigachev, A., et al. 2023, MNRAS, 522, 3018

2023
[12]

2023, Research in Astronomy and Astrophysics, 23, 085006

Ibryamov, S., Zidarova, G., Semkov, E., Peneva, S. 2023, Research in Astronomy and Astrophysics, 23, 085006

2023
[13]

2024, MNRAS, 529, 673

Jankovic, T., Bonnerot, C., & Gomboc, A. 2024, MNRAS, 529, 673

2024
[14]

Bonev, T., Bulgarian Astronomical Journal, 2011, 17, 3. 1Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, Tsarigradsko Shose 72, BG-1784, Sofia, Bulgaria e-mails: nip.sob@gmail.com, mminev@astro.bas.bg, mmoyseev@nao-rozhen.org 2Depar...

2011

Pith tools

Reviewed August 5, 2026 · model on record in the stance chip above.

[1] [1]

2024, Contributions of the Astronomical Observatory Skalnate Pleso, 54, 15

Minev, M., Petrov, N., & Semkov, E. 2024, Contributions of the Astronomical Observatory Skalnate Pleso, 54, 15

work page 2024

[2] [2]

T., 2002, Society of Photo-Optical Instrumentation Engineers (SPIE) Con- ference Series, 4848, 125

Wallace, P. T., 2002, Society of Photo-Optical Instrumentation Engineers (SPIE) Con- ference Series, 4848, 125

work page 2002

[3] [3]

2025, Astronomy & Astrophysics, 694, A85

Planquart, L., Jorissen, A., & Van Winckel, H. 2025, Astronomy & Astrophysics, 694, A85

work page 2025

[4] [4]

and Ducci, L

Bozzo, E., Ferrigno, C., Oskinova, L. and Ducci, L. 2022, MNRAS, 510, 4645

work page 2022

[5] [5]

A., de Souza, R

Kuhn, M. A., de Souza, R. S., Krone-Martins, A., Castro-Ginard, A., Ishida, E. E. O., et al. 2021, Astrophysical Journal Supplement Series, 254, 33

work page 2021

[6] [6]

Y., Semkov, E., Minev, et al., 2023, A&A, 680, L18

Zamanov, R., Boeva, S., Latev, G. Y., Semkov, E., Minev, et al., 2023, A&A, 680, L18

work page 2023

[7] [7]

Nelson, T., Mukai, K., Orio, M., Luna, G. J. M., & Sokoloski, J. L. 2011, Astrophysical Journal, 737, 7

work page 2011

[8] [8]

1992, Astronomy & Astrophysics, 266, 237

Bruch, A. 1992, Astronomy & Astrophysics, 266, 237

work page 1992