Injection method

Injection method#

The injection method passes a user-supplied truth catalog through the full survey pipeline — the same pipeline used for stream injection. It applies photometric errors and the survey selection function object by object.

Advantages

  • Full per-object photometric errors in all simulated bands.

  • Easy to tune: swap the truth catalog or the survey to test different assumptions.

  • Magnitude errors and per-band noise are propagated exactly as for stream stars.

Limitation — Requires truth catalogs (true positions and magnitudes) for each population. These are not bundled with streamobs.

Single-survey usage#

from streamobs.background import Background

bg = Background(
    survey,
    method='injection',
    source_type='both',           # 'stars', 'galaxies', or 'both'
    bands=('g', 'r'),
    catalog_stars=df_true_stars,
    catalog_galaxies=df_true_galaxies,
)
catalog = bg.generate(phi1_limits=(-20, 20), phi2_limits=(-2, 2))

The output is a DataFrame with survey-namespace-prefixed magnitude and flag columns (e.g. lsst_yr4_r_obs, lsst_yr4_flag_observed), identical in format to a stream injection output.

Multi-survey usage#

Pass a list of two surveys to use different photometric models for each band (e.g. LSST for the color band, Roman for the reference band).

from streamobs.surveys import Survey
from streamobs.background import Background

lsst  = Survey.load('lsst',  release='yr4')
roman = Survey.load('roman', release='dc2')

bg = Background(
    surveys=[lsst, roman],
    method='injection',
    source_type='stars',
    bands=('g', 'F158'),          # lsst covers g, roman covers F158
    catalog_stars=df_true_stars,  # must contain lsst_g_true, lsst_r_true,
                                  # roman_F158_true (+ completeness bands)
)
catalog = bg.generate(phi1_limits=(-20, 20), phi2_limits=(-2, 2))

The output has separate columns for each survey namespace (e.g. lsst_yr4_g_obs, roman_dc2_F158_obs, lsst_yr4_flag_observed, roman_dc2_flag_observed).

Required truth columns#

The truth catalog must contain a <namespace>_<band>_true column for every injected band and for each survey’s completeness_band (if not already one of the injected bands). For example, if LSST’s completeness_band is r, but you only inject g, you still need lsst_r_true so that detection flags can be computed correctly.

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