Separate fixed backgrounds raise one control color contrast in an October 2025 BD+05 4868 Ab MIRI visit

We test image extraction and control windows in one public JWST MIRI visit of the known disintegrating planet BD+05 4868 Ab. The visit covers 9 to 11 October 2025. We extract all 48,850 native integrations with three source widths and four background choices. The saved data have 2,205 minute bins. A fixed quality mask keeps 63 channels at 8–10 micrometers and 85 at 10–12 micrometers. All twelve extractions give positive local red-minus-blue log-flux contrasts in two selected cores. The ranges are 685–2,152 ppm and 1,136–1,386 ppm. The nine extractions with backgrounds that vary with time give core point values above all eighteen overlapping control points within each extraction. Separate fixed levels in each exposure instead give control values as large as 1,457 ppm. A test prompted by these results holds the exposure 1 background level fixed for the whole visit. At control center phase 0.29, the three widths change from +809, +1,195, and +1,457 ppm to −94, −25, and −96 ppm. Only the fixed background level changes. One core is equal, and the other changes by 16–29 ppm. Other large control variation remains. This establishes a processing effect for the specified statistic and windows. It does not show that one fixed level is a correct sky model. Conditional noise intervals do not include unknown calibration errors or earlier window selection. The cause of the core feature remains unresolved. We supply full-visit minute spectra, original pixel samples, source hashes, code, figures, and commands for reproduction.

JWST · MIRI · BD+05 4868 Ab · image extraction · background subtraction · control windows · disintegrating planet · public data · reproducibility