Substantial differences were observed between the noise-to-signal ratios (NSRs) obtained from PSF-fitting and aperture photometry. Across our sample, the ratio 𝜎PSF/𝜎aper spans more than two orders 10 11 12 13 14 15 16 17

(a)

| 18 N-Cams 24 N-Cams Kepler |  |  |
| --- | --- | --- |
|  | 50 ppm l | imit |
|  |  |  |

σPSF/σaper
10 1
1-hour NSR (ppm)
(b)
Num. Con.
(c)
Con. Ratio
10−2 10−4

[Figure: Figure 4. Noise-magnitude plot for the sample of 152 stars. The noise is the measured 3𝜎-clipped scatter in the detrended and then 1-hour binned outof-transit light curves. Of the two noise-to-signal ratios calculated from the light curves of the two photometry extraction methods, the lowest noise result is shown for each star. The stars observed by 6, 12, 18, and 24 cameras are denoted by triangles, circles, squares, and diamonds, respectively. The scatter in the real Kepler data is calculated in the same way, shown as pentagons with lines connecting the real Kepler and simulated PLATO scatters for the same stars. The modelled (Cabrera et al. 2026) PLATO performances for 6, 12, 18, and 24 cameras are shown as solid lines. The 50 ppm threshold for Earth-analogue detection is indicated by a horizontal dashed line.]

(d)
Aperture (pix)
(e) Aperture PSF
3 4
β1hr
10 11 12 13 14 15 16 17 Host Brightness (V)
life (BOL) condition ‘Quick noise model’ of Cabrera et al. (2026)1. Additionally, the ratio of observed to modelled noise is shown against contamination ratio in the upper panel of Fig. 5. Across our sample, we observe reasonable agreement with the model predictions across the range of host brightnesses. The median ratio of observed-to-predicted noise is 1.061, indicating that the model marginally underpredicts noise levels on average. This noise excess is not generally due to stellar variability. By applying the same detrending approach as in Section 2.6 to the raw, injected stellar signals, we find that the levels of intrinsic stellar variability on one-hour timescales fall in the interval 6.2 ppm ≤𝜎★,1hr ≤32.4 ppm. Subtracting in quadrature the stellar noise from the total one-hour noise for each star reduces the median noise ratio to 1.059. Thus, the stellar component makes a negligible contribution to the noise of all but the brightest stars in our sample. As shown by Cabrera et al. (2026), the noise properties of PLATO are expected to degrade between its BOL and end-of-life (EOL) states. This evolution is modelled in PlatoSim and may explain the remaining noise excess. Two notable outliers are the dimmest stars in V, which exhibit substantially lower noise than predicted by the model. These systems, Kepler-1624 and Kepler-267, also have the reddest V −P colours in the sample, with values of 1.40 and 1.66 mag respectively, compared to the sample median of 0.44 mag. Their large V −P colours imply that they are significantly brighter in the PLATO bandpass than their V magnitudes would suggest, naturally leading to lower photometric noise. Conversely, a notable fraction of stars show significantly larger noise levels than predicted. This effect appears to be correlated with

[Figure: Figure 3. Plots of, as a function of host brightness: (a) The ratio of the PSF-photometry-derived to aperture-photometry-derived 1-hour photometric noise. (b) The number of contaminating stars within 45 arcsec brighter than 5 magnitudes dimmer than the targets. (c) The mean ratio of contaminant to target flux within the photometric apertures. (d) The mean size in pixels of the Marchiori et al. (2019) derived photometric apertures. (e) The noise redness factor 𝛽1hr as defined in Eq. 1 for each photometry method. For each star, the value associated with the photometry method that yields the lower 1-hour noise is shown with greater opacity.]