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| Variations of PS1 Calibrators. |
Variations between PS1 calibrators in ZTF
In previous work we have seen that there are significant colour dependent residuals that vary between fields and even between quadrants in the same field. Additionally, PSF residuals (characterized by chi and sharpness) vary with colour and between quadrants.
CMDs of field stars and PS1 calibrator stars in quadrants of ZTF field 296. Field stars are blue and calibrators red.
In the plot above we show the stars used for calibration for a halo field. Here we see that stars as faint as r=21 are used and that bright/nearby disk main sequence stars are avoided based on the single age, single distance selection of M13 stars, see the ZTF ZSDS Figure 7. Some stars are also removed based on variatiablilty. However, the ZSDS notes that stars fainter than average r=20 are not used for calibration.
CMDs of field stars and PS1 calibrator stars in quadrants of ZTF field 487. Field stars are blue and calibrators red. Left top: CCD 16, q2. Right top: CCD 07, q3. Left Bottom: CCD 1, q1. Right bottom: CCD 4, q4.
In the plot above we show the variations in the photometric calibrators used in different readout channels within the Galactic disk field 487. Here we see a wide variety in the colour, magnitude range and types of stars used for calibration. In the dense, low reddening fields bright main sequence disk stars, and giant branch stars, are selected. In fields with higher reddening only fainter disk main sequences stars are selected. Stars g-r > 1.8 are not selected due to a g-i < 3.0 cutoff.
The result of these selections is that a not only is different population of calibrators used in different fields, but different stellar types depending on Reddening.
CMD for all PS1 calibrator stars in ZTF field 487.
In the plot above I show the combined distribution of all PS1 photometric calibrators for ZTF field 487. The overall distribution is not representative of any population and is essentially a mixture of stars with different properties.
Discussion
The figures above demonstrate that the population of PS1 stars currently used by ZTF for photometric calibration varies significantly between fields and even within individual fields.Ideally it would be best to use a single population, such that systematics errors due to variations in age and metalicity could be corrected. However, this is not possible for an all-sky survey. One possibility is to use WD spectrophotometric calibrators such as used to anchor the HST system. These stars consist of DA-type WDs that have spectra that are well definied by the combination of their effective temperatures and log(g).
Significant work has recently been undertaken on building a primary calibration catalog from these sources. However, the sample of well characterised WDs within the magnitude range approriate for ZTF (13 < mag < 18) currently stands at ~12. It is thus impossible to calibrate each ZTF field to such stars. Nevertheless photometry for ZTF fields could be tied to the fields containing these stars.