REVIEW 3 major objections 83 references
Machine learning on DES photometry classifies quasars at 99 percent precision and estimates redshifts to z approximately 4.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · grok-4.3
2026-06-30 18:41 UTC pith:W3V6TSY4
load-bearing objection They cross-matched DES DR2 to SDSS DR16, ran standard KNN for quasar classification and a hybrid regressor for photo-z, and released numbers on a new 872k-object catalog. the 3 major comments →
Photometric classification of quasars from DES and photo-z estimation with Machine Learning
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Using KNN on PSF magnitudes in the g, r, i, and z bands, the method achieves high-precision quasar classification against stellar contaminants. A hybrid approach combining boosted decision trees and decision tree regressors then estimates photometric redshifts for 872,372 objects, with a cleaned subset of 675,683 suitable for large-scale structure studies in 0 < z < 3, remaining reliable at z ≈ 4.
What carries the argument
K-Nearest Neighbors classifier using four-band PSF magnitudes, combined with a hybrid boosted decision tree and decision tree regressor for photometric redshift estimation.
Load-bearing premise
The cross-matched SDSS spectroscopic sample accurately represents the photometric properties and class distributions of the entire DES point-source population.
What would settle it
Spectroscopic observations of objects classified as quasars by the KNN model that reveal a true precision substantially below 0.99 would falsify the classification performance claim.
If this is right
- The full sample of 872,372 objects supports cosmological applications at high redshift.
- The cleaned sample of 675,683 objects is suitable for large-scale structure studies between redshift 0 and 3.
- High precision classification reduces contamination from stars and galaxies in the quasar sample.
- Recovery of a distinct population at z ≈ 4 extends the reach for quasar studies.
Where Pith is reading between the lines
- Similar machine learning pipelines could scale to future surveys with even larger volumes of photometric data.
- The method might benefit from incorporating additional bands or morphological information to improve recall.
- Cross-validation with other spectroscopic surveys could further test the robustness of the training labels.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper presents photometric classification of quasars in DES DR2 using KNN on PSF magnitudes in g,r,i,z bands, achieving a recall of 0.77 at 0.99 precision via cross-match with SDSS DR16 spectroscopic labels on 168,738 objects. It also describes a hybrid ML approach combining ANNz boosted decision tree and scikit-learn decision tree regressor for photo-z estimation, producing a catalog of 872,372 objects claimed reliable for cosmology at z≈4, and a cleaned sample of 675,683 suitable for large-scale structure studies in 0<z<3, with a stacked outlier classifier to reduce catastrophic errors.
Significance. The development of a large quasar catalog from DES data using machine learning techniques could be significant for cosmological investigations if the performance claims are robustly supported. The hybrid ML method for photo-z and the use of a stacked outlier classifier represent methodological strengths that, if properly validated, enhance the utility of the catalog for studies at high redshift.
major comments (3)
- [Abstract] Abstract: The quoted classification performance (recall of 0.77 at 0.99 precision) is presented without reference to the cross-validation strategy, data split ratios, or how the test set was constructed from the 168,738 objects, which is necessary to evaluate whether the metrics reflect true predictive power rather than in-sample fit.
- [Abstract] Abstract: The representativeness of the SDSS DR16 cross-matched sample for the full DES DR2 point-source population is not demonstrated through any comparison of magnitude distributions, color loci, or selection functions, which is load-bearing for the claim that the classifier and photo-z estimates apply reliably to the 872,372 object catalog.
- [Abstract] Abstract: The hybrid photo-z method is described at a high level without specifics on model combination, hyperparameter tuning, or quantitative validation metrics (e.g., sigma, outlier fraction) for the redshift estimates, undermining the assertion of reliability at z≈4.
Simulated Author's Rebuttal
We thank the referee for their constructive comments, which highlight areas where the abstract could better convey the methodological details and validation steps already present in the main text. We address each point below and have made revisions to strengthen the manuscript.
read point-by-point responses
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Referee: [Abstract] Abstract: The quoted classification performance (recall of 0.77 at 0.99 precision) is presented without reference to the cross-validation strategy, data split ratios, or how the test set was constructed from the 168,738 objects, which is necessary to evaluate whether the metrics reflect true predictive power rather than in-sample fit.
Authors: We agree that the abstract would benefit from a brief mention of the validation procedure. The full manuscript (Section 3.2) describes the use of 5-fold cross-validation with an 80/20 train/test split on the 168,738 cross-matched objects to ensure the metrics reflect out-of-sample performance. We will revise the abstract to include a short reference to this cross-validation strategy. revision: yes
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Referee: [Abstract] Abstract: The representativeness of the SDSS DR16 cross-matched sample for the full DES DR2 point-source population is not demonstrated through any comparison of magnitude distributions, color loci, or selection functions, which is load-bearing for the claim that the classifier and photo-z estimates apply reliably to the 872,372 object catalog.
Authors: This is a fair observation. While Section 2 details the cross-matching procedure and selection criteria, the manuscript does not include explicit side-by-side comparisons of magnitude or color distributions between the spectroscopic subsample and the full DES DR2 point-source population. We will add such a comparison (via a new figure or table) in the revised version to directly address representativeness. revision: yes
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Referee: [Abstract] Abstract: The hybrid photo-z method is described at a high level without specifics on model combination, hyperparameter tuning, or quantitative validation metrics (e.g., sigma, outlier fraction) for the redshift estimates, undermining the assertion of reliability at z≈4.
Authors: We acknowledge that the abstract is high-level. Sections 4 and 5 of the manuscript provide the specifics on the hybrid combination of ANNz boosted decision tree and scikit-learn decision tree regressor, the hyperparameter tuning process, and quantitative metrics including sigma, bias, and outlier fractions, with particular validation at z≈4. We will expand the abstract to incorporate key validation metrics and a note on the hybrid approach. revision: yes
Circularity Check
No significant circularity; standard supervised ML on external labels
full rationale
The paper cross-matches DES DR2 photometry with independent SDSS DR16 spectroscopic labels to obtain a training set of 168738 objects, trains KNN for classification and a hybrid regressor for photo-z, reports metrics (recall 0.77 at 0.99 precision) on that labeled sample, and applies the fitted models to generate the larger DES catalog. This is ordinary supervised learning whose performance metrics are measured against held-out external labels rather than by construction; no equation or claim reduces the output to the input by definition, no self-citation chain is load-bearing, and no ansatz or uniqueness result is smuggled in. The derivation chain is self-contained against the external SDSS benchmark.
Axiom & Free-Parameter Ledger
free parameters (2)
- K in KNN
- hyperparameters of boosted decision tree and regressor
axioms (1)
- domain assumption SDSS DR16 spectroscopic classifications are accurate and unbiased for the matched DES objects
read the original abstract
This paper presents a comprehensive study of quasar photometric classification and redshift estimation using machine learning techniques. We cross-matched photometric data from the Dark Energy Survey Data Release 2 (DES DR2) with spectroscopic classifications from the Sloan Digital Sky Survey Data Release 16 (SDSS DR16), yielding an initial sample of 168,738 point-like objects. Using a K-Nearest Neighbors (KNN) algorithm with PSF magnitudes in the $g$, $r$, $i$, and $z$ bands, we achieved high-precision quasar/galaxy classification against stellar contaminants, reaching a recall of 0.77 at 0.99 precision. Photometric redshifts were subsequently estimated using a hybrid machine learning approach combining a Boosted Decision Tree from ANNz and a Decision Tree Regressor from scikit-learn. The resulting catalog spans redshifts from $z \approx 0.5$ to $z > 3$, with a distinct population recovered at $z \approx 4$. A stacked outlier classifier was developed to mitigate catastrophic redshift errors. The full photometric redshift sample contains 872,372 objects and remains reliable for cosmological applications at $z \approx 4$. The cleaned catalog contains 675,683 objects and is suitable for large-scale structure studies in the range $0 < z < 3$. This robustly characterized quasar catalog provides a valuable resource for future cosmological investigations.
Figures
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