# Presentation: 705606

## Science/Instrument Overview
Ed Cheng

- Ed Cheng

- WFC3 Systems Requirements Review

## Historical Context

- As a result of the NASA/HQ decision to extend HST operations to 2010, the HST might have been left without a fully functional imaging camera in the last part of the mission.
- The only planned camera at the time is the Advanced Camera for Surveys (ACS) that will be deployed in mid-2000.
- Imaging is a critical scientific function of HST, and has high “visibility.”
- WFC3 will help ensure an imaging capability through 2010.
- Using new technology charge-coupled device (CCD) detectors, the WFC3 will have wide field sensitivity to the near-UV (to 200 nm).
- WFC3 will be deployed during SM4 in mid-2003.
- This rationale drove the WFC3 “baseline” UV/visible (UVIS) design.
- Subsequent community input drove the requirements for a second, infrared (IR) channel, extending wavelength coverage to 1700 nm.

## WFC3 Would Be The Last Camera on HST

## WFC3 and Panchromatic Imaging

## The WFC3 Concept

- Radial instrument.
- Uses the center of the HST focal plane.
- Wide-field and minimal distortion.
- The external radiator optimally supports cold operation of detectors.
- Reuse significant pieces of WF/PC (1) enclosure hardware.
- Supports two imaging channels.
- The ultraviolet/visible (UVIS) channel supports imaging from 200 nm (near-UV) through 1000 nm (deep red) with a 160 x 160 arcsec FOV.
- 2  x 2K x 4K CCD technology with high efficiency coatings.
- The infrared channel (IR) supports imaging from 850 nm (red) through 1700 nm (near-IR) with a 120 x 120 arcsec FOV.
- 1K x 1K Mercury-Cadmium-Telluride (HgCdTe) technology.

## HST Spacecraft Layout

## Scientific Instrument Locations in the HST

## WFC3 Instrument Layout (preliminary)

## Comparison of HST Imaging FOV

## History of the HST Focal Plane

## WFC3 Scientific Implementation

- WFC3 is a “facility-class” instrument.
- There are no guaranteed time observations (GTO) for the scientific staff. (Uses money for development that would otherwise support the staff during GTO activities.)
- Scientific leadership is provided by the Instrument Scientist.
- Analogous to the Principal Investigator (PI).
- All scientific aspects of the instrument are the responsibility of the “Science IPT”.
- Analogous to the Instrument Definition Team (IDT).
- Detector characterization, calibration, Servicing Mission Orbital Verification (SMOV), Early Release Observations (ERO), etc.
- GSFC, Space Telescope Science Institute (STScI), and JPL members.
- A Scientific Oversight Committee (SOC) represents the scientific community and advises the Project and STScI on the scientific priorities of the instrument.

## WFC3 Low-Cost Development Philosophy

**Hardware reuse.**
- WFC3 will reuse as much of the WF/PC (1) hardware as possible.
- WF/PC (1) was deployed with the HST in 1990, and was replaced by the WFPC2 during SM1 in 1993.
- WFPC2 provided spherical aberration correction and improved detectors.
**Design reuse.**
- Where no flight-spare or returned hardware exists, reuse existing designs for WF/PC, STIS, NICMOS, or ACS as much as possible (build-to-print).
**Experience reuse.**
- WFC3 development team members are already expert on HST instruments.
**Build-to-operate.**
- A significant life-cycle cost of instruments is operations and science support of instrument modes.
- WFC3 minimizes these modes, and reuses software and procedures from previous instruments with similar function.

## UVIS Channel Capabilities

- Extends HST wide-field imaging to 200 nm.
- Enables high-sensitivity surveys in this important wavelength range.
- ACS wide-field channel (WFC) cuts off at 350 nm.
- Reinforces HST imaging capability.
- Lower temperature CCDs improve radiation resistance for longer life.
- Minichannel CCD design for low-light sensitivity in a radiation environment.
- Spectral filter suite will continue HST’s productivity in emission line imaging.

## UVIS Channel Characteristics

- 160 x 160 arcsec field-of-view (FOV).
- Simple optical system, yielding high throughput (3 reflections, compared to 5 for WFPC2).
- Mirror coatings optimized for near-UV response.
- 48 selectable filters.
- 2 x (2K x 4K) CCD detector.
- Read noise < 4 electrons/readout (goal of 3 at 50 KHz).
- Dark current < 20 electrons/pixel/hour at -83C.
- UV-enhanced, backside-thinned.

## UVIS Channel Optical Design (preliminary)

## IR Channel Capabilities

- Vast improvement in HST near-IR discovery efficiency.
- ACS discovery efficiency is a factor of 10 over WFPC2.
- WFC3 IR channel discovery efficiency is a factor of 15 to 20 over NICMOS (depending on achieved QE).
- Excellent sensitivity compared with NICMOS.
- Improved HgCdTe QE over previous generation devices.
- Potential for IR channel sensitivity overlap with the CCD.
- Previous generation cut on at 900 nm (extending to 2.5 um or 2500 nm).
- Thinned CdZnTe technology can extend cut on wavelength into the visible.
- Improved camera efficiency is possible from 700 nm to 900 nm because HgCdTe is more sensitive than CCDs in this wavelength range.

## Sensitivity Comparison

## IR Channel Characteristics

- 120 x 120 arcsec FOV, centered on UVIS channel FOV.
- Mirror coatings optimized for the IR.
- 15 selectable filters (goal of 20).
- 1024 x 1024 Mercury-Cadmium-Telluride (HgCdTe) detector.
- 1.8 m long-wavelength cutoff HgCdTe (1.7 m useable).
- Read noise < 15 electrons/readout (goal of 10) at 100 KHz.
- Dark current < 0.4 electrons/second (goal of 0.1).
- Thermoelectric cooling (no cryogens).

## IR Channel Optical Design (preliminary)

## WFC3 Instrument Throughput (preliminary)

## Web Site

- Public information.
- Password-protected portion for WFC3 team use only.
- Used to coordinate WFC3 Project activities.
- Extensive document archive with search capability.

**http://wfc3.gsfc.nasa.gov**

## Baseline CEIS for this SRR

- The baseline CEIS that all subsystems are reporting to includes:
- A UVIS channel that maintains the maximum heritage with the ACS instrument subsystems.
- An IR channel that maintains the maximum heritage with the NICMOS subsystems (with obvious modifications for a 1K x 1K format).
- This is the configuration that we have advertised to the scientific community as well as NASA HQs.
- Our on-orbit experience with previous instruments suggests a number of upgrades that are highly desirable but not included in the baseline CEIS.
- These upgrades will either improve scientific usefulness near EOL, allow the instrument to be more sensitive for low-background scenes, or improve science operations efficiency.

## The Upgrades

## Closure Plan for the Upgrades

- We will conduct cost-capped studies to determine which of these upgrades will fit within:
- Instrument life-cycle cost.
- HST observatory resources.
- These results will be weighed against the quantifiable scientific return from each upgrade.
- All upgrades will be decided upon by December 1, 1999, at which time the implemented upgrades will be executed via a CCR.
- Instrument design will proceed in such a way so that we do not preclude any of the upgrades, if at all possible.