Synopsis: Different plates used for DASCH photometry used emulsions with different color sensitivities — equivalent to placing different filters in front of a CCD camera. The DASCH data do not make it easy to tell which measurements come from which emulsions, potentially leading to apparent "variability" that is actually a color effect.

Details

Different photographic emulsions are sensitive to different wavelengths of light. For the same reasons that you might place place different filters in front of a CCD camera, photographic astronomers used different emulsions for different observations.

The vast majority of HCO plates use one of three emulsions, which we can refer to by their rough bands of color sensitivity: ”blue“, “red”, and ”yellow”. Most HCO plates were blue-sensitive.

This inhomogeneity in the plates is especially important to keep in mind when looking at DASCH images. If you have a source that suddenly seems to have brightened massively, what you may really be seeing is a switch of emulsions from blue to red — if your source is red too.

For instance, here is a partial lightcurve of TYC 9504-35-1, one of the southernmost DASCH stars in the entire sky:

Around 1972, the lightcurve seems to split into several groups at different brightnesses. This can’t be physical, right?

Indeed it is not. These measurements come from three sets of “Damons South” plates, which used three different emulsions: the “blue“, “red”, and ”yellow” mentioned above. (These are plate series dsb, dsr, and dsy.) This star is relatively red (Gaia DR3 T_eff of around 4700 K), and apparently the APASS catalog has a missing or low-quality color measurement for this star. The different groups of measurements correspond to plates using the different emulsions, with different errors as the calibration scheme attempts to adjust all of the magnitudes to the APASS system (approximately Johnson B) using the information available.

The DASCH photometric calibration process solves for “color terms” that diagnose the kind of emulsion used on each plate. Because the catalogued color term for a source of interest may be inaccurate, it is valuable to understand which measurements came from which emulsions. This information, however, is not currently clearly surfaced in the DASCH data products. You must do a little legwork to infer which plates used which emulsion.

As stated above, most HCO plates are blue. The following items suggest a non-blue plate:

• Membership in the dny, dnr, dsy, or dsr series (which stand for Damons North Yellow, Damons South Red, etc.)
• A plate class of “L1”, “L2”, “L2S”, “L3”, or “L4”.

It is always possible that plates have been mis-labeled, so plates that “should” be blue might not be, and vice versa. The DASCH team intends to surface the calibration color terms that should hopefully make it possible to infer each plate’s emulsion in a reliable, quantitative way.

If you are constructing a long-term light curve for a target, it is safest to only consider plates using blue emulsions. If you see a systematic difference between “blue” magnitudes and “red” magnitudes, that probably indicates that the automated color correction is not correct for your source.

Back up to the list of known issues.