Photometry

Photometry#

Here are brief descriptions of WFC3 Notebooks for photometry:

WFC3/UVIS Filter Transformations with stsynphot#

We present how to calculate photometric transformation coefficients between WFC3/UVIS wide-band filters and any other non-HST filter system for a given object spectrum. This tool uses the latest WFC3 synthetic throughput tables and replaces functionality provided in the WFC3 Photometric Conversion Tool, which is no longer supported. For more detail on photometric transformations to other systems, see WFC3 ISR 2014-16.

Flux Unit Conversions with synphot and stsynphot#

We present a framework for users to convert between multiple magnitude and flux unit systems, and plot the results on the selected spectrum. This tool is based on the NICMOS unit conversion form and replaces the HST Unit conversion tool, which was developed in 2014. The updated tool incorporates the latest WFC3/UVIS (WFC3 ISR 2021-04) and WFC3/IR (WFC3 ISR 2020-10) photometric calibration as well as recent changes in the Vega spectrum of up to ~1.5% (Bohlin et al. 2020). See Section 9.5.2 of the WFC3 Data Handbook for more information.

Synthetic Photometry Examples for WFC3#

This notebook replaces pysynphot examples from the 2018 version of the Data Handbook and demonstrates how to use stsynphot for a few use cases:

  • Compute the inverse sensitivity, zeropoint, and encircled energy correction for any WFC3 ‘obsmode’

  • Renormalize a spectrum to 1 count/sec in a given bandpass and output the predicted magnitude or flux for a different bandpass

  • Determine the color transformation between two bandpasses for a given spectrum

  • Compute color terms for UV filters for a blue versus a red standard star observed on UVIS2

WFC3/UVIS Time-dependent Photometry#

For UVIS images retrieved after October 15, 2020, new time-dependent photometry keyword values (PHOTFLAM, PHTFLAM1, PHTFLAM2 and PHTRATIO) are populated in the image header and must be applied separately for each observation epoch. This is a change from prior calibration, where a single set of keyword values were provided for each filter, independent of date. For more detail on the new calibration, see WFC3 ISR 2021-04.

In this tutorial, we show how to use the time-dependent calibration to compute aperture photometry on UVIS calibrated, CTE-corrected images (flc.fits, hereafter FLC) obtained at three epochs, spanning a total range of ~8 years and showing a loss in sensitivity of ~2%.

Alternately, the FLC science arrays may be ‘equalized’ to account for sensitivity changes prior to computing photometry, where a reference set of keywords may be then used for all images. This photometric ‘equalization’ must be performed before combining any set of FLC images with AstroDrizzle which span multiple epochs in time.

Calculating WFC3 Zeropoints with stsynphot#

We present how to calculate photometric zeropoints using stsynphot for any WFC3 detector, filter, date, or aperture. This tutorial is especially useful for calculating Vegamag zeropoints, which require an input spectrum. The notebook is also useful for computing time-dependent WFC3/UVIS zeropoints for any observation date, as the values listed in WFC3 ISR 2021-04 are defined for the reference epoch. As of mid-2021, the WFC3/IR zeropoints are time-independent.

WFC3/UVIS Pixel Area Map Corrections for Subarrays#

We present how to apply pixel area map (PAM) corrections on FLT or FLC observations. The notebook also provides a comprehensive function that takes science data and outputs the PAM corrected data.

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