CaHK Photometry: Metallicity & Gravity

• CaHK photometry is a narrow-band imaging technique centered on the Ca II H and K resonance lines that isolates metallicity signals by compensating for temperature-driven continuum variations.
• It underpins surveys like Pristine and DECam/MAGIC, enabling metallicity mapping of metal-poor halo stars and the calibration of solar Ca II K spectroheliograms.
• Modern implementations include Gaia-linked synthetic systems and dedicated filters that yield precise, multi-parameter diagnostics across diverse stellar populations.

CaHK photometry is narrow-band photometry centered on the Ca II H and K resonance lines near $395\,\mathrm{nm}$, usually combined with broad-band colors so that temperature-dependent continuum variations can be separated from line-strength changes and translated into metallicity-sensitive, and in some regimes gravity-sensitive, observables. In contemporary practice it underlies the Pristine survey on CFHT/MegaCam, Gaia-linked synthetic CaHK systems, DECam/MAGIC metallicity mapping in the southern halo, integrated-light selection of extremely metal-poor globular clusters, and, in a distinct solar application, the photometric calibration of full-disc Ca II K spectroheliograms (Longeard et al., 2019, Martin et al., 2023, Chiti et al., 26 May 2026, Chatzistergos et al., 2017).

1. Physical basis of the CaHK diagnostic

The CaHK method exploits the fact that the Ca II H and K lines remain among the strongest metal-sensitive optical features even at very low metallicity. In the Pristine framework, the relevant filter is centered on the strong Ca II H and K absorption lines, whose strength is highly sensitive to stellar metal abundance in old halo-type stars. For old, metal-poor populations, weaker absorption and reduced line blanketing in this region change the narrow-band flux relative to broad-band bands, while colors such as $g-i$ or Gaia $G_{\rm BP}-G_{\rm RP}$ act as temperature proxies (Longeard et al., 2019, Martin et al., 2023).

In the stellar regime, the practical consequence is that a more metal-poor star appears brighter in CaHK at fixed broad-band color. The resulting metallicity indicator is therefore not CaHK alone but a color-color combination such as $(CaHK-g)-1.5(g-i)$, $[(CaHK-G)_0-2.5(G_{\rm BP}-G_{\rm RP})_0]$, or $\mathrm{CaHK}-g-0.9(g-i)$, each designed to suppress the dominant temperature trend and leave a metallicity-sensitive residual (Longeard et al., 2019, Martin et al., 2023, Chiti et al., 26 May 2026).

The same passband can probe parameters other than metallicity when the stellar type changes. For A-type stars, the flux in the CaHK region is strongly sensitive to surface gravity, and the combination $(u_0-CaHK_0)$ versus $(g_0-r_0)$ separates blue horizontal branch stars from blue stragglers more effectively than the traditional $(u_0-g_0)$ plane (Starkenburg et al., 2019).

In integrated light, the physical interpretation is similar but the observable changes from stellar colors to cluster colors. For M31 globular clusters, $u$ and especially $g-i$0 are treated as local continuum references around the narrow CaHK bandpass, so $g-i$1 and $g-i$2 act as line-strength proxies in integrated light (Heumen et al., 18 Aug 2025).

2. Filters, instruments, and photometric systems

The modern stellar CaHK literature is dominated by a small set of closely related instrumental implementations. The Pristine survey uses a narrow-band Ca H&K filter on CFHT/MegaCam, while the DECam/MAGIC survey uses the N395 filter on DECam. The latter is described with $g-i$3, $g-i$4, and an approximately top-hat transmission profile designed to capture the Ca II H and K lines at $g-i$5 and $g-i$6 (Longeard et al., 2019, Do et al., 15 Apr 2026, Chiti et al., 26 May 2026).

A second system is synthetic rather than directly observed. Gaia DR3 BP/RP spectro-photometry was used to construct $g-i$7 for 219.2 million sources by integrating the reconstructed BP/RP spectra under the MegaCam CaHK filter transmission curve. This produced a Gaia-linked, all-sky, bright-star metallicity-sensitive system and enabled an absolute recalibration of the deeper Pristine photometry (Martin et al., 2023).

A distinct branch of Ca II K photometry arises in solar archives rather than stellar surveys. Historical full-disc Ca II K spectroheliograms are photographic observations of solar chromospheric emission and require image-by-image photometric calibration and centre-to-limb variation compensation before any plage or network photometry is meaningful (Chatzistergos et al., 2017).

System	Platform	Representative role
Pristine CaHK	CFHT/MegaCam	Resolved-star metallicity work in the Milky Way halo
$g-i$8	Gaia DR3 BP/RP + GaiaXPy	All-sky bright-star metallicity-sensitive photometry
N395 / MAGIC	DECam on the 4 m Blanco Telescope	Southern-hemisphere metallicity mapping of halo stars and streams
Ca II K spectroheliograms	Historical solar photographic archives	Solar chromospheric photometry after calibration and CLV correction

Observed depth and coverage differ strongly across systems. In Pristine dwarf-galaxy work, CaHK is typically shallower than the accompanying $g-i$9 and $G_{\rm BP}-G_{\rm RP}$0 imaging and is used reliably only to about $G_{\rm BP}-G_{\rm RP}$1 in the Sgr II analysis and $G_{\rm BP}-G_{\rm RP}$2 in the Draco II analysis (Longeard et al., 2019, Longeard et al., 2018). By contrast, $G_{\rm BP}-G_{\rm RP}$3 is bright-star limited by Gaia XP availability and blue-end signal-to-noise, with $G_{\rm BP}-G_{\rm RP}$4 typically reached around (G_{\rm BP}\sim15\