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Delving into the Historical Ca II K Archive from the Kodaikanal Observatory: the Potential of the Most Recent Digitised Series

T0 review · 2 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read The paper establishes that the newer 16-bit digitisation of the Kodaikanal Ca II K archive is superior, while the shared quality degradation in both scans is intrinsic to the original plates.

desk verdict A careful and useful comparison of the two Kodaikanal digitizations; the recommendation to prefer the 16-bit series is well supported, but the 'intrinsic artifact' conclusion is somewhat stronger than the evidence. read the letter →

arxiv 1908.05493 v1 pith:HJ6WMNH3 submitted 2019-08-15 astro-ph.SR

Theodosios Chatzistergos , Ilaria Ermolli , Sami K. Solanki , Natalie A. Krivova , Dipankar Banerjee , Bibhuti K. Jha , Subhamoy Chatterjee This is my paper · ORCID
classification astro-ph.SR
keywords CaIIKobservationsKodaikanalarchiveplageareasdigitisationimagequalitysolarcycleinstrumentaleffects
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

This paper asks whether the two recent digitisations of the century-long Kodaikanal Ca II K photographic archive differ because of the scanning or because of the original plates. The authors process both datasets with identical photometric calibration and plage-segmentation software, then compare image-quality metrics and derived plage areas. They find that the slow degradation seen in the older 8-bit scan—worsening spatial resolution, increasing disc eccentricity, growing large-scale inhomogeneities, and changing centre-to-limb variation—also appears in the newer 16-bit scan, so it is intrinsic to the archive. The 16-bit series has more consistent photometry, roughly twice as many images, and higher resolution after 1990, making it the dataset to prefer.

What carries the argument

The load-bearing instrument is a single processing pipeline applied unchanged to both datasets. Images are converted to density, photometrically calibrated by fitting the quiet-Sun centre-to-limb variation to a modern reference, corrected for limb darkening, and segmented into plage using a threshold of 8.5 times the quiet-Sun contrast standard deviation. Because the pipeline is held fixed, any systematic difference between DS1 and DS2 must be attributed either to the digitisation or to the photographic material, and the matching secular drifts in eccentricity, resolution, inhomogeneities, and centre-to-limb variation residuals point to the latter.

What would settle it

Scan a stratified sample of the same Kodaikanal plates (for example, 20 plates per decade from 1904–2007) with a modern calibrated scanner and recompute the same metrics—disc eccentricity, spatial resolution, large-scale inhomogeneities, centre-to-limb variation residuals, and plage areas. If the secular trends shrink or vanish, the shared trends in DS1 and DS2 were partly scanner artefacts; if they reproduce at the same slope, the archive itself is degrading.

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Extended reading notes

Core claim

The central claim is that the two digitisations of the same physical archive carry the same time-dependent quality trends, and therefore those trends belong to the original photographic observations rather than to either scanner. Plage areas derived from DS1 and DS2 with identical techniques agree to an RMS difference of about 0.006 in disc fraction on daily values (0.005 for observations taken on the same day and time), with Pearson coefficients near 0.95–0.98, and both sets track the modern Rome/PSPT coverage in the overlap period, with Kodaikanal slightly lower. On this evidence DS2 supersedes DS1: it provides more than double the images (48,928 versus 22,158), more stable transmittance, and better spatial resolution after 1990, while its remaining imperfections are properties of the plates themselves.

Load-bearing premise

The conclusion depends on the assumption that the two digitisation campaigns, carried out more than a decade apart with different hardware, do not share some systematic artefact that happens to produce the same slow drift in image quality; only then does the matching degradation point to the original plates.

Editorial extensions

If this is right

  • The 16-bit DS2 series should be the default dataset for future Kodaikanal plage-area studies, with DS1 retained mainly for years where it has better coverage, such as 1907, 1909, 1911, 1913, 1955, 1973, and 1993.
  • Long-term activity reconstructions from Kodaikanal data must account for time-dependent image quality, since disc eccentricity, spatial resolution, and large-scale inhomogeneities drift secularly in both digitisations.
  • Errors in the date and time metadata affect both series, so daily-mean analyses or careful metadata cleaning are needed before comparing Kodaikanal data to other archives.
  • The scatter among published Kodaikanal plage-area series stems mainly from different calibration methods and different definitions of the solar radius and normalisation area, not from the digitisation itself.
  • The archive's intrinsic degradation limits the precision of century-long plage-area trends and makes cross-calibration with other Ca II K archives necessary.

Reading between the lines

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

  • A third digitisation of the same plates with modern equipment may not add much scientific value beyond DS2 for most purposes, because the quality bottleneck appears to be the photographic material itself; a testable extension would be scanning a stratified sample of plates to check whether any remaining artefacts disappear.
  • The common time-dependent trends could in principle be modelled as a quality-correction function and applied to plage areas, but that is an extension beyond what this paper attempts.
  • If plate degradation is intrinsic, then other photographic solar archives of similar age may hide comparable latent trends, so cross-archive composites may need to include plate-age or observational-epoch covariates.
  • The suggestion that Kodaikanal's effective bandwidth may be broader than the nominal 0.5 Å, or that the line was observed off-centre, is a testable inference that could be checked by examining the shape of the centre-to-limb variation over time.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

2 major / 5 minor

Summary. This paper compares two digitizations of the Kodaikanal Ca II K spectroheliogram archive: DS1 (22,158 mostly 8-bit JPG images, 1907--1999) and DS2 (48,928 16-bit FITS images, 1904--2007). The authors apply the same photometric calibration, limb-darkening compensation, and plage-segmentation pipeline to both series, and also to Rome/PSPT data as a modern reference. They quantify disc eccentricity, spatial resolution, large-scale inhomogeneities, CLV-fit chi-squared, and density/contrast statistics, and derive plage areas. The main findings are: (i) both series show similar time-dependent degradation (increasing eccentricity, worsening resolution, growing inhomogeneities, changing CLV), which the authors attribute to intrinsic properties of the original photographic material; (ii) DS2 has better and more consistent image quality and roughly twice as many images; (iii) plage areas from DS1 and DS2 are highly correlated (Pearson 0.95 daily) but differ by RMS ~0.006 of disc area, with cycle-dependent offsets; and (iv) differences among published plage-area series are largely traceable to different processing choices and solar-disc definitions. The authors conclude that DS2 should be preferred.

Significance. If the conclusions hold, this is a valuable reference for users of the Kodaikanal archive and helps explain the discrepancies among previously published plage-area series. The consistent application of one pipeline to both digitizations is a real strength, as it removes processing differences as an explanation for DS1--DS2 differences, and the image-quality metrics are quantified with uncertainties. The explicit statement that the plage-area errors are tentative, not formal, is appropriately cautious. The external Rome/PSPT comparison provides a useful modern anchor. The practical recommendation that DS2 be preferred is well supported by its larger sample, better post-1990 resolution, and more stable density statistics. The main weakness is that the causal attribution of the common time-dependent trends to the original plates, rather than to the shared processing pipeline, is asserted more strongly than the evidence supports.

major comments (2)
  1. [Section 3.2 / Section 5] The conclusion that the time-dependent trends common to DS1 and DS2 (disc eccentricity, spatial resolution, large-scale inhomogeneities, CLV fit chi-squared) are intrinsic to the original observations is not fully supported, because both series are processed with the same analysis pipeline. The same limb-detection routine feeds the eccentricity and radius estimates, the same 98%-power-spectral-density estimator yields the resolution, and the same iterative background/CLV fitting produces the inhomogeneity and chi-squared metrics. A systematic bias in any of these routines that scales with plate scale, solar radius, dynamic range, or noise level could be impressed on both series and could mimic intrinsic secular degradation. The Rome/PSPT control covers only 1996--2007, so it cannot validate the 1904--1995 portion of the trends. Please either process both series with a second, independent analysis chain and check that the common trends persist, or add sensitivity tests of the metrics to pipeline choices (e.g., limb-detection threshold, PSD window/cutoff, background polynomial order) and soften the causal wording in Sections 3.2 and 5 from "should be ascribed to" to "are consistent with" the original photographic observations.
  2. [Section 4.1 / Figure 13] The DS2--Rome/PSPT overlap comparison, which is the main external validation of the DS2 plage areas, shows an RMS difference of 0.01 in disc fraction and a pronounced seasonal pattern (Rome/PSPT higher in winter, DS2 higher in summer), with a maximum absolute difference of 0.08. The paper reports this pattern but does not analyze its origin. Since a seasonal systematic in DS2 would affect the reliability of the recommended series, please investigate whether this modulation reflects a calibration effect (e.g., CLV or flat-field residuals) and explicitly discuss its impact on the claim of a "good match" between DS2 and Rome/PSPT.
minor comments (5)
  1. [Section 2.1] The sentence "We find only 13,835 images of each set referring to the same solar observations" is ambiguous; please rephrase to clarify that these are paired scans of the same plates, one image from each digitization.
  2. [Section 3.2 / Figure 9] The phrase "normalised to the maximum value from the respective digitisation" is ambiguous; state whether the normalisation is to the maximum over all images of each series or to a per-image maximum.
  3. [Section 3.2] The sentence "The maximum (RMS) error in the plage areas for the average disc eccentricity found for Kodaikanal data is 0.013 (0.0005)" is hard to parse; please spell out the maximum error and the RMS error separately.
  4. [Section 2.2] The sentence "The multiplicative factor for identifying plage was chosen to be 8.5" should explicitly state that this factor multiplies the standard deviation of the quiet-Sun intensity values, as implied later in the text.
  5. [Section 4.2] There is a typo in "Priyalet al. (2017)"; the word "al." should be separated from "Priyal".

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the DS1/DS2 comparison is internally controlled and externally anchored to Rome/PSPT; the shared-pipeline ambiguity is an external-validity risk, not a circular reduction.

full rationale

The paper's central comparison is not circular. DS1 and DS2 are independent digitizations of the same Kodaikanal plates, made with different hardware, bit depth, epoch, and operators (Makarov et al. 2004 vs Priyal et al. 2014). Both series are then processed with one pipeline and one fixed segmentation threshold (8.5 sigma), a threshold chosen from Rome/PSPT versus SRPM comparison rather than fitted to the Kodaikanal data, so the DS1/DS2 agreement is a controlled comparison rather than a fitted prediction. The photometric calibration is anchored to an external modern reference, Rome/PSPT, and the overlap period 1996-2007 provides an independent benchmark for the plage-area comparison. The conclusion that common time-dependent trends in eccentricity, resolution, inhomogeneities, and CLV chi-square are intrinsic to the original Kodaikanal observations is an inductive inference from two independent realizations of the same archive, not a definitional identity. A systematic bias shared by the single processing pipeline is a legitimate alternative explanation for some of the common trends, and the text itself concedes in Section 3.2 that the quiet-Sun sigma test is 'not a conclusive test' because instrumental or observational issues could also contribute. That is an external-validity or correctness risk, not circularity, since no output quantity is mathematically identical to an input by construction. The self-citations to Chatzistergos et al. (2018a, 2019a) supply the processing method and previously published error estimates; they do not import an unverified uniqueness theorem, forbid alternative interpretations, or substitute for the comparison performed in this paper. Therefore no circular step is present.

Assumptions & free parameters 2 free parameters · 3 assumptions · 0 invented entities

The central DS1-DS2 comparison depends on the shared processing pipeline and the inference that common trends are intrinsic. No invented entities. The only fitted parameter is the segmentation threshold, which the paper argues does not affect the relative comparison.

free parameters (2)
  • plage segmentation threshold = 8.5 (multiplicative factor of QS standard deviation)
    Chosen in prior work so Rome/PSPT plage areas match SRPM segmentation (Section 2.2). The paper states the precise value is unimportant for this comparison because all series are processed consistently.
  • disc normalization radius = 0.98 R (96% of disc area)
    Choice of area used to normalize plage fractions; differs from 0.97 R used by other studies. The sensitivity to this choice is estimated as 0.01 +- 0.04 relative (Section 4.2).
assumptions (3)
  • domain assumption The two digitizations DS1 and DS2 are scans of the same original plates and were produced with sufficiently independent setups, so common time-dependent patterns imply origin in the plates rather than shared digitisation artefacts.
    Underlies the conclusion that identified issues are intrinsic to the archive (Section 5).
  • domain assumption The quiet-Sun centre-to-limb variation of the historical Kodaikanal observations can be mapped onto the Rome/PSPT reference CLV by a per-image calibration curve that is linearly extrapolated to the non-QS regions.
    Used for photometric calibration in Section 2.2. If the relationship is nonlinear outside the quiet Sun, plage contrast and areas would be biased; the paper acknowledges this approximation.
  • domain assumption The same segmentation threshold (8.5 sigma) identifies comparable physical plage regions on all datasets despite differences in bandwidth, resolution, and stray light.
    Central to comparing plage areas across DS1, DS2, and Rome/PSPT (Section 2.2). The paper tests consistency but notes that bandwidth differences modify effective contrast.

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Pith. "Pith review of Delving into the Historical Ca II K Archive from the Kodaikanal Observatory: the Potential of the Most Recent Digitised Series." pith.science (2026). https://pith.science/paper/HJ6WMNH3

@misc{pith2026190805493,
  author       = {Pith},
  title        = {Pith review of: Delving into the Historical Ca II K Archive from the Kodaikanal Observatory: the Potential of the Most Recent Digitised Series},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/HJ6WMNH3}},
  note         = {Machine review of arXiv:1908.05493}
}
read the original abstract

Full-disc Ca II K photographic observations of the Sun carry direct information about the evolution of solar-plage regions for more than a century and are therefore a unique dataset for solar-activity studies. For a long time Ca II K observations were barely explored, but recent digitisations of multiple archives have allowed their extensive analysis. However, various studies have reported diverse results partly due to the insufficient quality of the digitised data. Furthermore, inhomogeneities have been identified within the individual archives, which, at least partly, could be due to the digitisation. As a result, some of the archives, e.g. that from the Kodaikanal observatory, were re-digitised. The results obtained by different authors who analysed the data from the new digitisation of the Kodaikanal archive differ from each other as well as from those derived from the old digitisation. Since the data were processed and analysed using different techniques, it is not clear, however, whether the differences are due to the digitisation or the processing of the data. To understand the reasons for such discrepancies, we analyse here the data from the two most recent digitisations of this archive. We use the same techniques to consistently process the images from both archives and to derive the plage areas from them. Some issues have been identified in both digitisations, implying that they are intrinsic characteristics of the data. Moreover, errors in timing of the observations plague both digitisations. Overall, the most recent 16-bit digitisation offers an improvement over the earlier 8-bit one. It also includes considerably more data and should be preferred.

Figures

Figures reproduced from arXiv: 1908.05493 by the authors.

Figure 1
Figure 1. Examples of the images from the two digitisations of the photographic Ca II K observations of the Kodaikanal observatory taken on 02 January 1936. Left: 8-bit digitisation from data set 1, or DS1 in short; Right: 16-bit digitisation from DS2. The images are shown to their full range of values and were not compensated for the ephemeris. The DS1 image is shown to scale with the DS2 image (i.e. equal number of pixels p… view at source ↗
Figure 2
Figure 2. Number of images per year (top panel) and annual coverage (bottom panel ) of the Kodaikanal data in the two digitised series. DS1 is shown in dotted red, DS2 in solid blue, while the annual coverage by the two archives together is shown in dashed black. Among the photographic archives of Ca II K observations, the one from the Kodaikanal observatory has probably the largest collection of images, covering more than a … view at source ↗
Figure 3
Figure 3. Selected processing steps applied to images from the two digitisations of Kodaikanal observations taken on 02 January 1936 and shown in [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figures from the paper (14 more)
Figure 4
Figure 4. Figure 4: Magnified sections from the raw DS1 and DS2 negative images (first two columns) and calibrated and limb-darkening compensated images (last two columns) of the Kodaikanal observation taken on 02 January 1936 (see Figures 1 and 3) displaying a quiet Sun region (top 2 row…
Figure 5
Figure 5. Figure 5: Comparison of observations from DS2 (upper row) and Rome/PSPT (lower row), both taken on 27 May 2000. Shown are the raw images (left column), processed and limb– darkening compensated images (middle column), and the segmentation masks (right column). The raw images are…
Figure 6
Figure 6. Figure 6: Raw density images from DS1 (left), DS2 (middle), and Rome/PSPT (right) taken on 03 April 1999 illustrating saturated plage regions in the Kodaikanal data. accuracy, and an average pixel-scale of 1.3 00pixel−1 . The second dataset (DS2, hereafter) was derived by Priyal…
Figure 7
Figure 7. Figure 7: Enlargement of observations displayed in [PITH_FULL_IMAGE:figures/full_fig_p008_7.png]
Figure 8
Figure 8. Figure 8: Solar disc eccentricity (top panel ) and spatial resolution (bottom panel ) computed for DS1 (red), DS2 (blue), and Rome/PSPT (black) data as a function of time. Shown are annual mean values (solid lines) along with the asymmetric 1σ interval (shaded surfaces). The hor…
Figure 9
Figure 9. Figure 9: The maximum and the minimum values of transmittance (top panel) and the standard deviation of density values, σD, (bottom panel ) within the solar disc for the DS1 (red) and DS2 (blue) data as a function of time. Shown are annual mean values (solid lines) along with th…
Figure 10
Figure 10. Figure 10: Identified large scale inhomogeneities (top panel ) and reduced χ 2 of the fit to the curve obtained by relating the measured density QS CLV from the Kodaikanal data to a reference QS CLV from Rome/PSPT data (bottom panel ) for DS1 (red), DS2 (blue), and Rome/PSPT (bl…
Figure 11
Figure 11. Figure 11: The standard deviation of the contrast values, [σC ], over the whole solar disc (top panel) and in the quiet Sun only, [σ QS C ], (bottom panel ) for the calibrated and limb-darkening compensated DS1 (red), DS2 (blue), and Rome/PSPT (black) data as a function of time.…
Figure 12
Figure 12. Figure 12: Fractional disc coverage by plage (Panels a and b) as a function of time, derived with the same processing and segmentation parameters from DS1 (red), DS2 (blue), and Rome/PSPT (black) images. Panel a shows daily values, while panel b displays annual mean values (soli…
Figure 13
Figure 13. Figure 13: Top: Plage areas in disc fraction for 26 images taken on the same days found in DS1 (dotted red), DS2 (solid blue), and the Rome/PSPT (dashed black) series plotted against the number of the image. The error bars denote the RMS error in the derived plage areas due to t…
Figure 14
Figure 14. Figure 14: Scatter plots between the plage area values derived from images of DS2 (x-axis) and those from images (y-axis) of DS1 (Panel a) and Rome/PSPT (Panel b). Blue asterisks (orange dots) show the annual (daily) values. The solid black lines have a slope of unity and repres…
Figure 15
Figure 15. Figure 15: Plage areas presented in the literature (y-axis) versus the ones derived here from DS2 (x-axis): (a) Chatterjee, Banerjee, and Ravindra (2016) from DS2; (b) Ermolli et al. (2009a) from DS1; (c) Kuriyan, Muralidharan, and Sampath (1983) from the actual photographs; (d)…
Figure 16
Figure 16. Figure 16: Raw Kodaikanal observation taken on 20 January 1938. Circles enclose the areas considered by Chatterjee, Banerjee, and Ravindra (2016, dashed green), Priyal et al. (2017, dashed green), Priyal et al. (2014, dotted yellow), and in this work (solid blue). SOLA: comparis…
Figure 17
Figure 17. Figure 17: Calibrated images of Kodaikanal observation taken on 20 January 1938 (shown in [PITH_FULL_IMAGE:figures/full_fig_p028_17.png]

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Pith tools

Reviewed August 14, 2026 · model on record in the stance chip above.