Photometry¶
THATCH performs aperture photometry on calibrated HST drizzled images at known transient positions.
Method¶
- Source position: WCS from the FITS header maps the transient RA/Dec to pixel coordinates
- Aperture photometry: Circular aperture (r=5 pixels) with local background estimated from an annulus (r=10-15 pixels)
- Aperture corrections: Encircled energy fractions from the WFC3 and ACS Instrument Handbooks correct for flux outside the aperture
- AB magnitude calibration: Uses PHOTFLAM, PHOTPLAM, and PHOTZPT header keywords for flux calibration
Aperture Corrections¶
| Instrument | Pixel Scale | r=5px | EE Fraction |
|---|---|---|---|
| WFC3/UVIS | 0.04"/pix | 0.20" | 0.80–0.86 |
| WFC3/IR | 0.13"/pix | 0.65" | 0.94–0.95 |
| ACS/WFC | 0.05"/pix | 0.25" | 0.84–0.86 |
Validation¶
THATCH photometry achieves 0.02 mag RMS agreement with published values, validated against Cowperthwaite+2017 and Lyman+2018 measurements of AT2017gfo:
Left: 1:1 comparison of THATCH vs published AB magnitudes. Right: residuals showing median offset of -0.012 mag and RMS of 0.022 mag.
Multi-Band Light Curves¶
THATCH extracts light curves across all available HST filters for each object:
Multi-band HST light curves for 6 transients extracted by THATCH, spanning Type Ia SNe, core-collapse SNe, a kilonova, and a jetted TDE.