Created
September 18, 2025 23:58
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Plotting all BOSS exposures taken in a night
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| import os | |
| import numpy as np | |
| from astropy.io import fits | |
| import matplotlib.pyplot as plt | |
| np.random.seed(16) | |
| expand_path = lambda path: os.path.expandvars(os.path.expanduser(path)) | |
| spAll_path = expand_path( | |
| f"$SAS_BASE_DIR/ipl-4/spectro/boss/redux/v6_2_1/summary/daily/spAll-v6_2_1.fits.gz" | |
| ) | |
| def get_specFull_path(spec_file): | |
| _, field_id, mjd, *_ = spec_file.split("-") | |
| field_id, mjd = map(int, (field_id, mjd)) | |
| return expand_path( | |
| "$SAS_BASE_DIR/ipl-4/spectro/boss/redux/v6_2_1/spectra/daily/full/" | |
| f"{str(field_id // 1000):0>3}XXX/{str(field_id).zfill(6)}/" | |
| f"{mjd}/{spec_file}" | |
| ) | |
| # This part takes a long time because the spAll is large and compressed | |
| with fits.open(spAll_path) as spAll: | |
| spec_files = spAll[1].data["SPEC_FILE"] | |
| # Let's select a random visit spectrum | |
| spec_file, = np.random.choice(spec_files, size=1) | |
| specFull_path = get_specFull_path(spec_file) | |
| with fits.open(specFull_path) as image: | |
| # specFull data model is here: https://data.sdss.org/dsi/file/select-specFull/main/section-bhm | |
| # extensions 5 onwards (0-indexed) contain individual exposures | |
| # Let's plot all the individual exposures for that night | |
| cmap = plt.get_cmap("tab10") | |
| fig, ax = plt.subplots(figsize=(8, 8), dpi=300) | |
| offset = 0 | |
| for i, hdu in enumerate(image[5:]): | |
| λ = 10**hdu.data["LOGLAM"] | |
| flux = hdu.data["FLUX"] + offset | |
| ivar = hdu.data["IVAR"] | |
| c = cmap(i) | |
| ax.plot(λ, flux, c=c, ds="steps-mid", label=f"{hdu.header['MJD']} {hdu.header['EXPOSURE']}") | |
| ax.fill_between(λ, flux - 1/np.sqrt(ivar), flux + 1/np.sqrt(ivar), color=c, step="pre", alpha=0.2) | |
| offset += np.nanmedian(hdu.data["FLUX"]) | |
| ax.set_ylim(0, ax.get_ylim()[1]) | |
| ax.set_ylabel(f"Flux + offset (10$^{{-17}}$ erg/s/cm$^2$/Å)") | |
| ax.set_xlabel("Wavelength (Å)") | |
| ax.legend() | |
| figure_path = f"{spec_file[:-5]}.png" | |
| fig.savefig(figure_path, bbox_inches="tight") | |
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Example output: