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Hubble Space Telescope
WFC3/UVIS Photometry

Two Chip Photometry

Motivated by the very different quantum efficiency of each of the WFC3/UVIS e2v detectors, WFC3 UVIS photometric calibrations are now determined independently for each of the CCD detectors in the WFC3/UVIS channel. New flat fields for the full frame filters were created, lflats (low frequency flat fields) were recomputed using the dithered observations of Omega Centauri, but treating each chip separately. Flatfields are normalized to the median value of each detector. New inverse sensitivity values  have been calculated for the 42 ‘full array’ filters  using observations for the white dwarf standard stars (G191B2B, GD153 and GD71) obtained between 2009 and 2015. 

UVIS ‘full frame’ filters are: 

Changes from 2012 Photometry:

There are no changes to the pixel area maps (geometric distortion).

Table of Contents

I. Proposal Information & Documentation Provides links to manuals and web pages to help with preparing your Phase 1 & Phase 2 proposals..
II. Data Analysis Information & Documentation Provides links to manuals and web pages for retrieving and analysing WFC3 UVIS data
III. Calibration Files & Calibration Information Links for obtaining the newest calibration files for the UVIS channel
IV. Two-Chip-related Instrument Science Reports (ISRs) List of ISRs directly relevant to the WFC3/UVIS Two Chip photometry. ISRs are in-house refereed technical papers which address specific topics related to instrument and telescope performance or data analysis techniques. This grouping of ISRs has been compiled to provide additional information to observers beyond what is found in the Instrument Handbook.
V. FAQ List of FAQs related to everything from Proposal Preparation to Data Analysis. This guide is based on questions submitted to the Help Desk. The FAQ page will continually evolve over time to include more Q&As as we receive feedback from users.
VI. Other Useful Resources Here you will find links to other useful webpages.

I. Proposal Information and Documentation

WFC3 Instrument Handbook Overview of instrument capabilities. Useful for Phase 1 & 2 planning. PDF version
WFC3 Data Handbook Guide to using the data obtained with WFC3. Useful for post-observations. PDF version
Phase 2 Proposal Instructions Guide for Preparing Phase 2 proposals. WFC3 information is located in Chapter 14.
Astronomer's Proposal Tool (APT) Training Tools A collection of video tutorials for using APT. See the APT homepage for obtaining the latest version. Aladin is the default sky atlas used with APT. See the Aladin homepage for more information on using this software.
Exposure Time Calculator (ETC) User Manual Provides general overview of the ETC for all instruments.
WFC3 UVIS Imaging Exposure Time Calculator Used for estimating the exposure time for UVIS imaging observations
WFC3 Dithering Strategies Web based guide for designing dithering patterns for WFC3.
Activating a Target of Opportunity (ToO) Proposal Describes how an HST observer activates a ToO proposal.

II. Data Analysis Information & Documentation

Retrieving your Data These three links all provide access to proprietary data for PIs as well as publicly available data.
Multimission Archive
Duplication Checker
WFC3 Pipeline Information Provides a brief description of the WFC3 data pipeline, along with notes on updates.
Do I Need to Recalibrate? Web guide on how to check if calibration files for your datasets have changed recently.
Hubble Observation Problem Reports Guidelines for submitting a Hubble Observation Problem Report (HOPR). This is used for: 
1) Problems with implementation, scheduling execution, or data processing of your observations;
2) Requests for failed observations to be repeated
Description of UVIS2.0
UVIS 2.0 User’s Guide
Drizzling two-chip images
Overview of Two-Chip Photometry, and description of WFC3/UVIS reduction pipeline
Step-by-step guide to reducing UVIS imaging data. 
CALWF3 documentation 
DrizzlePac main site
WFC3/UVIS Astrodrizzle Examples:
Optimizing Image Alignment for Multiple Visits
Optimizing Image Sampling for a Single Visit
Aligning Images in Different Filters by Defining an Output Reference Frame
Alignment of Sparse Fields Using Headerlets
SExtractor Catalogs to Align ACS/WFC Images
Space Telescope Analysis Newsletters (STANs) Archive of newsletters sent to observers with updates which may affect observation planning or analysis.
WFC3 Advisories Important advisories regarding the status of the WFC3 instrument.
Publication Guidelines for HST Observations Short summary of what to include in your publications once your data analysis has been completed.

III. Calibration Files and Calibration Information

WFC3 Reference Files This web page contains the current calibration and reference files used in the CALWF3 pipeline.
Zeropoints for UVIS Filters These tables list the inverse sensitivity values (PHOTFLAM1, PHOTFLAM2, VegaMag, STMag, ABmag), pivot wavelengths, and bandwidths for the UVIS filters. 
Encircled Energy and Aperture Corrections Table
These tables list the encircled energy and aperture corrections for the WFC3/UVIS channel.
Encircled Energy Magnitude Correction Plot
This plot shows the change in magnitude from an infinite aperture for a given encircled energy fraction.
HST Keyword Dictionary Reference web page for looking up the meanings of all keywords currently used for HST instruments.

IV. WFC3/UVIS Instrument Science Reports

V. FAQs 

Q: Do I need to use a different pixel area map (PAM)?

A: No. Use the same PAM files. The geometric distortion has not changed. PAM files are available at

Q: I’ve requested and downloaded my UVIS image data from MAST ("the Archive"). What zeropoints should I use?

A:Use the value in the header keyword PHOTFLAM, which is identical to PHTFLAM1. Data processed by the current pipeline uses calwf3 v3.3 (check the value of the header keyword CAL_VER) which automatically normalizes UVIS2 to UVIS1.

VI. Other useful resources

Old WFC3 zeropoints