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 photometryVersion 1.0Licence MIT1 runUsed on 1 mystery

Questions it answers

Did the star fade over the twentieth century?

Code Python

Download module (.py)colorterm_test.pyView source

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)

2 datasetsDASCH 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

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.