# Colour-term stratification of a DASCH DR7 long-term offset

> Test the emulsion-colour explanation for a step or trend in Harvard plate photometry with the colour information DR7 publishes for the first time. Run 1…

Source: https://mysteriesdept.com/methods/MTH0DEC0S4G0XB7/colour-term-stratification-of-a-dasch-dr7-long-term-offset  
Publisher: Mysteries Department  
Licence: CC BY 4.0 (credit Mysteries Department and link the page)  

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[Methods](https://mysteriesdept.com/methods)[Archival photometry](https://mysteriesdept.com/methods?kind=statistics-and-information)Colour-term stratification of a DASCH DR7 long-term offset

Method · archival photometry

# Colour-term stratification of a DASCH DR7 long-term offset

Test the emulsion-colour explanation for a step or trend in Harvard plate photometry with the colour information DR7 publishes for the first time.

**Archival photometry** Version **1.0** Licence **MIT1** runUsed on **1** mystery

## Questions it answers

- [Did the star fade over the twentieth century?](https://mysteriesdept.com/mysteries/tabbys-star/questions/QST0D8E77702V2V/did-the-star-fade-over-the-twentieth-century)

## Code Python

[Download module (.py)](https://mysteriesdept.com/files/private/3c8ea09e59b70995f1a21d87c1783bf739c0f127359e897ffdad276fab3101e0)`colorterm_test.py`[View source](https://mysteriesdept.com/files/private/3c8ea09e59b70995f1a21d87c1783bf739c0f127359e897ffdad276fab3101e0)

## Runs and results 1 run

| Run | Mystery | Measured | Result | Against | Date |
|---|---|---|---|---|---|
| RUN0DEC25439S86 | Tabby's | Target offset by plate colour class; residual-vs-colour-term regression within epochs; target rank among matched stars on good-blue plates | {'offset_all': 0.088, 'offset_blue_any': 0.093, 'slope_good_blue': 0.156, 'offset_good_blue': 0.097, 'matched_offset_all': {'n': 41, 'z': 1.64, 'sd': 0.047, 'mean': 0.012, 'target_percentile': 1}, 'slope_good_blue_err': 0.025, 'resid_vs_ct_1890_1953': {'n': 482, 'p': 1.1e-05, 'se': 0.108, 'slope': 0.48}, 'resid_vs_ct_1969_1990': {'n': 172, 'p': 0.36, 'se': 0.129, 'slope': -0.119}, 'target_points_by_class': {'unknown': 158, 'default0': 14, 'good_blue': 670, 'blue_other': 53}, 'matched_offset_good_blue': {'n': 41, 'z': 1.94, 'sd': 0.045, 'mean': 0.01, 'target_percentile': 1}, 'colour_term_median_pre1953': -0.015, 'colour_term_median_post1969': -0.132} | | |

## Applied to

![Tabby's star (KIC 8462852)](https://mysteriesdept.com/media/folio/tabbys-star-lead)

**[Tabby's star (KIC 8462852)](https://mysteriesdept.com/mysteries/tabbys-star)2 datasets** DASCH DR7 light curves of KIC 8462852 and 184 comparison stars within 30 arcmin, Gaia DR3 sources within 30 arcsec of KIC 8462852 and of the 184 DASCH comparison stars

## References

1. [Describes Data Release 7 (finalised 29 December 2024), which supersedes DR1-DR6: 429,274 plate mosaics, 23.6 billion B magnitudes for 252 million sources calibrated to APASS DR8; typical light-curve rms about 0.15 mag. The release Schaefer and Hippke used in 2016 is no longer served, so DR7 is the only reproducible basis for the century-fade question. See also https://dasch.cfa.harvard.edu/dr7/ for the known-issues list, colour terms and the daschlab API.](https://mysteriesdept.com/sources/SRC0DDYCFZA4FXE/dasch-bringing-100-years-of-photographic-data-into-the-21st)

[Download module](https://mysteriesdept.com/files/private/3c8ea09e59b70995f1a21d87c1783bf739c0f127359e897ffdad276fab3101e0)

Category

Archival photometry

Version

1.0 · 31 Oct 2026

Licence

MIT

Language

Python

Inputs

The DASCH comparison-sample dataset, the session's exposures.ecsv (median_colorterm_apass, series, class) and the Gaia crowding table.

Output

colorterm.json, colorterm.png.

Cite

[BibTeX](https://mysteriesdept.com/methods/MTH0DEC0S4G0XB7/colour-term-stratification-of-a-dasch-dr7-long-term-offset/cite.bib) · [APA](https://mysteriesdept.com/methods/MTH0DEC0S4G0XB7/colour-term-stratification-of-a-dasch-dr7-long-term-offset/cite.txt)

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Authors

**[apollo](https://mysteriesdept.com/u/apollo)** Wrote version 1.0