UVIS Encircled Energy

 

Chip-dependent, filter-based encircled energy (EE) curves were derived from aperture photometry of three white dwarf standards G191B2B, GD153 and GD71 (plus the G-type star P330E) and spliced to the 2009 in-flight models to extend the correction from r=1.4” arcsec to r=6.0” (ISR 2016-03).

These EE solutions were used to derive the latest UVIS photometric calibration (ISR 2021-04), with a few modifications. For two filters, F275W and F814W, new EE curves were computed from a drizzle-combined images of GRW+70d5824 observed regularly from 2009 – 2020, corrected for time-dependent sensitivity, resulting in closer agreement between the UVIS1 and UVIS2 solutions. Following these results, EE values for the other UV filters were also modified by ~1%, and EE values for wavelengths larger than 7500 A were changed by ~0.5% to be in closer agreement with the F814W EE.

EE curves for five additional filters (F336W, F200LP, F350LP, F775W, F850LP) have been recomputed with time-dependent corrections (ISR 2022-02), and these will be included in the UVIS calibration at a future date, after obtaining additional observations in a few specific filters.

ASCII (CSV) tables for UVIS1 and UVIS2.

• Row 1: header information, Row 2-: EE fractions by wavelength

• Column 1: Wavelength (Angstroms), Column 2-end: Aperture radius in arcseconds

• UVIS 1

  

  Download CSV

• UVIS 2

  

  Download CSV

For photometry with aperture radii <10 pixels and targets placed in the upper-left corner of Amp A (e.g. UVIS1-C512A-SUB, UVIS1-C1K1A-SUB), we recommend using the UVIS1 EE fractions, since the PSF in this quadrant is slightly out of focus compared to the rest of the UVIS focal plane. For all other cases we recommend using the UVIS2 EE fractions.

Comparison of EE curves (2020, 2016, 2009, 2006)

 

For drizzled images, or flat-fielded images multiplied by the pixel area map (i.e. FLT*PAM), the mean signal in a circular aperture of radius r is:

 

\(Flux = \frac{F_r \cdot PHOTFLAM}{EE(r)}\)

Where Fr is the signal within aperture r in electrons per second, EE(r) is the encircled energy fraction at radius r, PHOTFLAM is the inverse sensitivity at the infinite aperture, whose default value is PHTFLAM1.

The equivalent calculation using magnitudes is:

\(m=m_i + 2.5\times \log{\left (EE(r)\right)} + ZP\)

where m_i is the instrumental magnitude, m_i = -2.5*log(F_r), ZP is the PHOTFLAM equivalent in magnitude units from ISR 2021-04 Table 2, and EE(r) is as above.

 

For example, aperture photometry using a *drz.fits image, for radius r = 3 pixels of a star on the UVIS1 CCD with the F606W filter yields F_r = 950 e^-/s.

The inverse sensitivity of F606W is PHTFLAM1 = 1.1529E-19 erg·s^-1·cm^-2·Å^-1 /(e^-/s). The encircled energy at r = 3 pixels is

EE(r=3) = 0.742 (UVIS1)

In physical units: 

Flux = 950 * 1.1529E-19  /  0.742 =  1.4761E-16 erg·s-1·cm-2·Å-1

In VEGAMAG:

m = -2.5*log(950) + 26.004 + 2.5*log(0.742) = 18.236 mag