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Instrument Science Report WFC3-2000-11

WFC3 IR Channel Operations
I.N. Reid, P.M. Knezek, C. Lisse + the WFC3 team June 4, 2001 ABSTRACT Wide Field Camera 3 is a new imaging camera being prepared for operations with the Hubble Space Telescope following Servicing Mission 4. This ISR describes the science capabilities and operation of the infrared channel of the instrument (0.8 - 1.7 micron). Matched against the NICMOS instrument, WFC3 provides improved sensitivity (an increase by a factor of 2-3) and a significantly wider field of view (a factor of seven).

1. Introduction and Instrument Description
The HST Wide Field Camera #3 (WFC3) instrument is currently scheduled for launch in early 2004. This fourth generation camera will be the last in a series of wide field cameras flown on HST, and will maintain operations until spacecraft de-orbit (nominally 2010). It is intended as a general purpose facility instrument, designed to provide HST with high quality imaging capability. The camera will have both a near-infrared channel (IR, see Table 1), and an ultraviolet/optical channel (UVIS), discussed in a separate Instrument Science Report (ISR WFC3-03-2001). The IR channel, the subject of this ISR, builds on the NICMOS heritage, with a wider field of view and higher throughput, although a spectral range limited to the non-thermal regime, shortward of 1.7 microns. In principle, WFC3 can serve as a backup for imaging programs designed either for ACS/WFC or for wavelengths shortward of 1.7 microns on a revived NICMOS. 1.1 IR Capabilities The superior performance of WFC3 stems from improvements in detector technology. In particular, it will have lower read noise, higher quantum efficiency, and larger formats. This will allow for much deeper and wider-field studies than possible


previously. Table 1.WFC3 IR Channel Instrument Specifications
Detector Field Size Pixel Size Spectral Range Dark Current Readout Noise Operating Temp. WFC3/IR 135"x135" 132 mas 0.8 to 1.7 microns <0.4 e-/pix/sec <15 e-/pix/readout 150K NICMOS/3 51"x51" 200 mas 0.8 to 2.5 microns 0.4-2.0 e-/pix/sec 30-35 e-/pix/readout 75K NICMOS/2 19"x19" 75 mas 0.8 to 2.5 microns 0.4-2.0 e-/sec/pix 30-35 e-/sec/readout 75K

WFC3 is designed to take full advantage of the imaging capabilities provided by HST, providing both UV sensitivity not available with the WFC mode of ACS and a near-IR channel. The observer can therefore choose between observations with the UVIS 4096x4096 (160 arcsecond-square) CCD or the near-infrared 1024-square (135x135 arcseconds) array. The latter has an f/10 focal ratio with 0.132 arcsec pixels. The Rockwell HgCdTe detector provides seven times the areal coverage of NICMOS-3 and an increase of a factor of 2-3 in quantum efficiency. The detector will be read out using a 1Kx1K multiplexing amplifier (MUX) connected to the backside of the array through indium bump bonds. The latter is similar to devices currently being tested by the University of Hawaii for both the Gemini project and for NGST. An innovation in the IR detector is setting the long wavelength cutoff at 1.7 microns, a shorter wavelength than usual for HgCdTe devices. This allows the detector to operate at relatively warm temperatures (150K) with low dark current. As a result, WFC3 can use simple, low-cost solid-state thermoelectric cooling systems (TECs) and the NICMOS thermal radiator, rather than the expendable cryogens required for thermal-IR observations. The relative throughputs of WFC3-IR and NICMOS are shown in Figure 1.


Figure 1: Relative throughput of WFPC2, NICMOS, ACS and WFC3

1.2 IR schematic description WFC3 will reside in a radial bay in HST, the location currently occupied by Wide Field & Planetary Camera #2 (WFPC2). In order to reduce the instrument design costs, the hardware design is based largely on previous HST heritage. Indeed, WFC3 involves direct re-use of several WF/PC parts, such as the enclosure envelope, the filter selection mechanism (SOFA) and parts of the optical bench, besides incorporating many aspects of the ACS/WFC hardware and software design. The optical layout for WFC3 is shown in Figure 2.


Figure 2: WFC3 optical layout

Like WF/PC and WFPC2, light is sent to WFC3 through the use of an external plane pick-off mirror (POM) which intercepts the central portion of the incoming f/24 beam from HST, and directs it to a field mirror which allows selection of either the UVIS or the IR channel. After the beam is redirected by a fold mirror, the hyperbolic IR M1 mirror images the pupil to the elliptical IR M2 mirror. The latter is equipped with tip/tilt and focus mechanisms, and these two mirrors constitute a re-imaging relay which is used to place and control the position of the HST pupil onto the cold stop. The latter, located ~0.5 mm in front of the Refractive Corrector Plate (RCP), blocks emission from the HST secondary and spiders and reduces background radiation at the detector. The RCP is an anamorphic asphere that corrects the spherical aberration introduced by the HST primary mirror. The f/10 beam passes through the IR filter wheel, which limits the bandpass of the transmitted light, the IR detector window, and finally onto the IR HgCdTe focal plane array (FPA). The entire optical train is supported by a thermal control subsystem, as well as by control and data handling electronics subsystems.The block diagram for the WFC3 assembly can be seen in Figure 3.


Figure 3: WFC3 block diagram

2. Science with WFC3/IR
WFC3 will provide the opportunity for a wide range of high-impact scientific observations and discoveries. The instrument will be used primarily as a low noise, high sensitivity imaging device, but can also provide low-resolution imaging using optical and IR grisms. The discussion below describes a representative sample of the diverse scientific programs that will be carried out using the WFC3 IR channel, both alone and in conjunction with observations at UVIS wavelengths. 2.1 Science Overview 2.1.1 Galactic

WFC3 is well suited for galactic astronomy. The IR channel will permit detection of low-luminosity obscured sources, such young stellar objects (YSOs) and brown dwarfs in molecular clouds and star forming regions, while the high angular resolution will allow imaging of protoplanetary disks, tracing the formation and evolution of other planetary systems. The 1-2 micron region is a highly effective wavelength regime for searching for low-mass stars and even cooler brown dwarfs, particularly since the suite of filters


offered can identify sources with methane absorption, characteristic of temperatures below 1400K. The wide field offered by WFC3 will provide scope for surveys of both open clusters and Galactic globulars, while the system can also be employed in searching for low-mass companions to known nearby stars. WFC3 will also prove invaluable in probing gas dynamics during stellar birth and death. Equipped with a large set of narrow- and medium-band UV, optical and IR filters, optimised for study of key emission lines in HII regions, stellar winds, supernova remnants, planetary nebulae and interstellar clouds, WFC3 will extend the spectral range, sensitivity and field of view of previous HST instrumentation. Combining high-resolution imaging at optical and near-infrared wavelengths will permit detailed study of the physical structure of photoionised regions, shocks and collimated outflows. The nearinfrared capabilities will be particularly important in studying the dusty circumstellar environments of mass-losing evolved giants and supergiants, examining when and where asymmetries in nebulae and/or stellar winds develop, and how those features might be related to the location within the Galaxy. 2.1.2 Planetary Science

WFC3 can be used for many applications in studying the physical characteristics of bodies within our own Solar System. The instrument will support the continuing campaign of Mars probes, including detailed mapping of Mars to search for evidence of water absorption at 1.4 microns, two-color photometric searches for silicate minerals, and global monitoring of Martian weather (dust storms, polar cap size). The atmospheres of the giant planets can be probed through observations in the methane bands at 1.08 and 1.58 microns, molecular hydrogen emission at 1.24 and 1.5 microns, and the He line at 1.08 microns. WFC3 will play a crucial role complementing Cassini observations of Saturn and Titan, starting in 2004, and will also be used to study the Uranian equinox in 2007. Finally, the wide wavelength coverage offered by WFC3 will be invaluable in imaging asteroids, comets, Centaurs and Kuiper belt objects, searching for water ice and silicates, determing their sizes, colors, albedos and surface structures, and providing clues to their composition and origins. 2.1.3
st

Extragalactic (low z)

Understanding star formation is one of the main challenges facing astronomy in the 21 century. WFC3 will prove a highly effective tool in this enterprise through its ability to supply high resolution, panchromatic imaging of star formation regions in nearby galaxies, where our external viewpoint offers a global perspective. Ultraviolet imaging provides direct information on the recent star formation history, identifying O and B stars, formed within the last 100 Myrs, which are responsible for the bulk of the observed ionisation, photodissociation and kinetic energy input. The more mature population can


be traced through imaging at visual and far-red wavelengths, while the near-infrared can identify young supergiants and embedded sources within dusty star-forming regions (Figure 4). Combined with radio and X-ray data, such observations will afford powerful insight into galaxy evolution.

Figure 4: Panchromatic galactic structure - NGC 2442 from B to K

WFC3 will also better calibrate the extragalactic distance scale through red and nearinfrared observations of Cepheid variables and post-AGB stars in the Virgo cluster, determination of the location of the tip of the red giant branch in later-type galaxies, and measurement of surface brightness fluctuations in other nearby galaxies and brightest cluster members. Other programs will study the relationship between age and metallicity in nearby galaxies, both from system to system and as a function of position within each galaxy, while multi-wavelength imaging of the ultraluminous star clusters seen in merging galaxies can test whether those objects are actually proto-globular clusters. Finally, the high spatial resolution and large FOV of WFC3 is also well suited for studies of interstellar dust in external nearby galaxies using distant galaxies as the background. 2.1.4 Galaxy Clusters and Cosmology (high z)

WFC3 is ideal for the detection and study of several important classes of high redshift galaxies. High-resolution, near-infrared imaging is an essential pre-requisite for analysis of the evolution of massive ellipticals and the formation of spiral disks at z>1. Deep, wide-field IR imaging with WFC3 can also be used to search for galaxies beyond z=6, where the Lyman-alpha forest and Lyman limit absorption suppress completely the flux at optical wavelengths. The dark background offered by space-based observations gives WFC3 a critical advantage over ground-based IR telescopes in this type of survey. These observations can provide a direct test of CDM theories of the formation of galaxies and large-scale structure in the early universe. WFC3 will also likely be the instrument of choice for studying galaxy evolution between z=1 and z=3, the epoch when star formation is thought to have peaked. Infrared observations are essential for mapping the older population at those redshifts, where


optical observations probe the rest-frame UV, and are therefore biased towards active star-forming regions. Surveys of galaxy clusters over a range of redshifts can be used to address such issues as the evolution of stellar populations in different environments. Finally, WFC3 will probe into the farthest reaches of the Universe through ultradeep imaging of "random" pieces of the sky, a follow-on to the Hubble Deep Field.

3. Operations
To complete the observations listed in Section 2, the instrument will be required to perform a number of commandable operations. The overwhelming majority of HST observations have been obtained using a limited set of observing modes. In order to maximise simplicity and minimise costs, while preserving scientific return, WFC3 has been designed to provide only the most-utilised observing modes. The overall configurations are described below in detail: 3.1 The IR detector A full discussion of the near-infrared detector properties is given in a separate ISR (ISR WFC3-03); here, we summarise the main properties. The 1024x1024 Rockwell HgCdTe IR Focal Plane Array (FPA) is mounted on a HAWAII-IR multiplexer which is composed of four independent quadrants, each having one output amplifier. Pixels on each quadrant are addressed through two digital shift registers, horizontal and vertical respectively. The readout rate is 100 kHz, with a total frame readout time of ~10 seconds. Each register requires two clocks, one (LSYNC/FSYNC) initialising the shift register to the first column(row) of the quadrant, the other (PIXEL/LINE) incrementing (in unit steps) the addressed column (row). The outermost 5 pixels are not exposed to the sky and serve as reference pixels. Thus, the effective size of the largest science image is 1014x1014 pixels. The array is read out at the start of each exposure. This `zero read' provides the baseline value for each pixel, the equivalent of a bias exposure in optical CCDs. The array can be read out while an integration is in process without destroying the data contents (non-destructive readout, NDR). Multiple NDRs are useful both in reducing readnoise and in increasing the dynamic range of an individual exposure. Up to 15 NDRs will be possible for WFC3 during a single observation. 3.2 Operating Modes


3.2.1 IMAGE (MULTIACCUM) The WFC3 IR channel will have one primary observing mode, which shall be the standard science and external calibration data taking mode. This mode shall be the equivalent of the NICMOS IMAGE mode. A series of 2-16 NDRs are made with specified exposure times at a given pointing. The MUX reads the detector signal into the instrument A/D, which converts the signal to digital form and transmits the information to the CPU, where the data are written to the data buffer. An observation is complete when the instrument clock measures the full preset integration time and the last datum is read. The pixel scale of 0.132 arcsec leads to undersampling of the effective instrumental PSF, but much of the angular resolution can be retrieved using sub-pixel dithering coupled with software image reconstruction using the DRIZZLE algorithm (Fruchter and Hook, 1997), applied to the data post observation. The IR channel will also have internal calibration targets, with a predefined mode to provide control of the lamp sub-system. In MULTIACCUM mode, the observer will need to define the following parameters for successful observing: · Aperture: The specification of the aperture defines where the target is placed on the imaging plane. Spectral elements: The preliminary list of Spectral Elements for the IR channel of WFC3 are listed in Table 2. Gain: This optional parameter specifies the analog-to-digital gain of the HgCdTe array readout electronics in units of e-/DN. The selectable gain values are 2, 4, 8 and 16 eleectrons/DN, giving maximum counts of 131070, 262140, 524280 and 1048560 respectively. Most observations will use gains 2 and 4. Dither-Line-Spacing: This parameter specifies the spacing between points for a single-line dither scan along a 45-degree diagonal, in arcseconds. It is only applicable when Dither-Type=Line has been specified. Dither-Line-Steps: This parameter specifies the number of steps (integer values only) to be observed in a single-line scan. It only applies when Dither-Type=Line has been selected. Dither-Type: Dithering the pointing of the WFC3 IR FPA refers to repeating the same image at slightly different pointings. This can be used both to improve the resolution of an image and to remove detector-based artifacts such as hot pixels. Large dither offsets are undesirable because the geometric distortion in the WFC3 optics will cause the size of the offsets (in pixel units) to vary across the FOV.

·

·

·

·

·


Dithering can be implemented in several ways. · Number of Iterations: The value entered is the number of times this Visit and Exposure Specifications line should be iterated. There are many observational situations when two or more exposures should be take of the same target field. Time Per Exposure: This value is the duration of the exposure. If Number of Iterations is used, the stated Time Per Exposure will be repeated n times, where n is the Number of Iterations. Sample sequence: The dynamic range of the WFC3 IR array can be increased by compiling up to 16 NDRs in the course of a single exposure. The range of exposure times is selected by the observer from a limited number of pre-specified sequences, chosen to optimise the number of NDRs for the historically mostfrequently used NICMOS sample sequences. Subarray: The WFC3 IR channel will support the use of subarrays. A square region can be defined with size d x d pixels, where d = 1024/n and n=2,4,8 & 16. All subarrays are centred on the detector, with an equal number of pixels in each quadrant. On each edge of the detector, the 5-pixel wide reference pixel region, matching the subarray positions, will also be read out; thus, a subarray with a science image of size dxd pixels will produce an output image of size (d+10)x(d+10) pixels. The readout times, equivalent to the minimum integration times per frame, are as follows:

·

·

·

Field size (pixels) 1024x1024 512x512 256x256 128x128 64x64

Field size (arcseconds) 135 x 135 68 x 68 34 x 34 17 x 17 9x9

Readout time (s/frame) 2.889 0.867 0.272 0.097 0.0405

3.2.2. ANNEAL (Restricted) There will be an observation mode for annealing the IR channel which will be patterned after the ACS/WFC anneal mode. This mode will perform a pre-determined warmup sequence, hold at a warm temperature set-point, and then return the detector to its nominal operating temperature. It is intended that both the UVIS detector and the IR


detector will be annealed simultaneously.

3.2.3 Target Acquisition (not supported) There will be no target acquisition mode for WFC3. This does not preclude standard HST target acquisition capabilities such as uplinking a pointing correction in real time (interactive acquisition) or uplinking a pointing correction to be stored onboard for later use (reusable target offsets). 3.3 Channel Selection Selection of the UVIS or IR channels results from the use of a switching mechanism, the Channel Select Mechanism (CSM), which provides transmission to the UVIS channel and reflection to the IR channel. This mechanism will take less than 60 seconds to switch from supporting one channel to the other. WFC3 will not be capable of observing with both channels simultaneously. 3.4 Optical Alignment The WFC3 will have corrector mechanisms for both the UVIS and IR channels in order to correct for the OTA and provide high-quality images. These corrector mechanisms will be aligned on-orbit by focussing, tipping, and tilting, and will remain aligned for the duration of the mission. A procedure will be developed for doing the primary alignment during the SMOV, with repeated adjustments as necessary during the mission. This procedure will be similar to those developed for STIS, NICMOS, and ACS, all of which contain similar corrector mechanisms. 3.5 Bright Objects Although the WFC3 IR FPA cannot be harmed by exposure to any bright sources (other than direct solar radiation), over illumination can cause detector artefacts such as residual images, which may degrade science data. An array pixel has a measured "well depth", corresponding to the total number of photons absorbed/electrons created that the pixel can accept before saturating. When the total charge exceeds this limit, long-lasting effects (minutes to hours) can occur, affecting pixel linearity and dark current as nonideal "trap" states (due to impurities and defects in the detector crystal lattice) are filled by photoelectrons. The A/D converter also has a maximum signal limit, but that limit is typically higher than the pixel well depth. For scientific observations, saturation is a primary concern when observing bright targets. Pixels may become non-linear before reaching the nominal saturation level. Whenever a pixel exceeds this linearity threshold, as determined from pre-flight testing, a


flag in the datafile will be set to identify this condition. The WFC3 IR channel does not have a shutter, so to protect the detector from overillumination by Earth light, the IR Filter Select Mechanism (FSM) will be rotated to place the BLANK position in the beam. During observations which involved earth occultation, this procedure will be initiated by the ground system before the Earth limb (whether dark or illuminated) comes into view. Calibration lamp exposures can be taken during Earth occultation with the CSM in the UVIS position, preventing extrenal light from entering the IR channel. After calibration exposures are complete, the FSM will move to the BLANK position before moving the CSM. 3.6 Spectral Elements There will be 18 selectable IR filters on a single filter wheel, including a BLANK blocking filter, for taking dark frames and for IR channel protection (see Table 2). The WFC3 IR focus is designed for placement of a single spectral element in the optical path. Table 2.WFC3 IR Channel Filter List
Broadband F125W (J) F140W(J+H) F160W(H) F075W(wide R) Mediumband F098M (SDSS z) F127M(continuum) F139M (water/methane) F153M (water.ammonia) F167N ([Fe II] cont.) F164N ([FeII]) Narrowband F126N ([Fe II]) F128N (Paschen beta) F130N (Paschen beta cont.) Special Filters G102 ("blue" grism) G141 (IR grism) BLANK

3.7 Calibration Like other HST instruments, WFC3 requires calibration data, which must be maintained and updated periodically to compensate for imperfections in the optics and detectors. Periodic calibration campaigns will be scheduled and conducted by STScI, using calibration proposals written by instrument scientists or Guest Observers. All of the calibration exposures will be taken using the standard observing modes described in section 3.1. For both UVIS and IR channels, at least three types of calibration data will be required: 1. Photometric calibration ­ determining the quantum efficiency of WFC3 as a function of wavelength. Absolute calibration can be determined from observations of standard stars with well-determined flux distributions; flatfield images will be obtained to track variations in pixel-to-pixel sensitivity, matching against reference images taken during the ground calibration campaign. Detector


non-linearity, saturation and dynamic range measurements will also be made during these tests. 2. Spatial calibration ­ measurement will be made of the point spread function (PSF) of unresolved sources as a function of wavelength, coupled with periodic verification of the plate scale and geometric distortion through observations of standard astrometric starfields. 3. Dark current calibrations ­ measurements designed to monitor the base electronic signal from the detector (no light incident). Full-frame images will be taken to maintain pixel-by-pixel calibrations. Some of these calibration measurements require observations of external sources, while others can be done using internal lamps which can be used to illuminate either the UVIS or IR detector (Figure 2). The calibration subsystem is designed to permit monitoring of detector performance, besides providing a means of assessing correct operation of key instrument systems (filters, detector gain, linearity and cosmetics). For highest efficiency, internal calibrations should be executable in parallel with science and/or calibration observations for other SIs, wherever possible in occultation, so as to give minimal interference with science observations. There are a number of steps necessary in order to produce calibrated images. The initial, basic steps are listed below and described briefly. For a more complete explanation of the steps, please see the HST Data Handbook. 1.) Application of the Static Mask: This step flags static (i.e. known) bad pixels. The data themselves are not changed in any way. 2.) A/D Correction: This step uses the analog-to-digital converter to take the observed charge in each pixel and convert it to a digital number. 3.) Zero Level Removal: The zero-read exposure provides a measure of the level of the electronic pedestal, 4.) Dark Image Subtraction: This step removes any thermally-induced dark current, as well as a glow from the field flattening lens. This is done by default for all exposures over 10 seconds. 5.) Flatfield Multiplication: This steps corrects for the fact that the number of electrons generated in any given pixel by a star of a given magnitude depends on the quantum efficiency of the pixel as well as any large scale vignetting of the field-of-view caused by the telescope and the camera optics. This is done by multiplying the data image by an inverse flat field.


3.8 Dithering Strategies Dithering strategies will likely be similar to those adopted by ACS and will be defined in a later document. 3.9 Calibration Program Instrument calibration will be provided using a large number of observations of standard stars (Hubble Guide Star Catalogue, SAO Bright Stars) made throughout the mission. The NICMOS absolute spectrophotometric standards G191-B2b, a hot white dwarf, and P330E, a solar analogue (Coline & Bohlin, 1997), are expected to serve the same purpose for WFC3. A number of other standards, spanning a range of spectral types from O to M, L, will be used to determine colour-dependent terms in the photometric calibration. The expected relative expected to be within 1% constant pixel size across Extremely bright sources pixel-to-pixel accuracy. photometric accuracy across the field-of-view (FOV) is AFTER correction has been made in software for the nonthe detector field-of-view caused by the WFC3 optics. (J < 11) will suffer blooming effects that will degrade this

For point sources, the expected absolute photometric accuracy of the final processed data will be < 5%; to reach this level of accuracy, however, careful processing of the data in order to remove systematic errors due to the undersampled PSF and gain variations due to source centroid positions in a pixel will be necessary.

4 Control and data handling
A simplified model of STScI operations is that observers specify visits to a specific target with a particular set of exposures. The ground system commands the vehicle to point at that target and arranges for exposurs to be scheduled at times when orbital constraints allow unobstructed viewing of the target field. Exposures specify an SI and configuration. For WFC3, the configuration includes the selected channel (UVIS or IR), spectral element, subset of FOV to be used and exposure duration. In the case of the IR channel, the command software will appear identical to that used for NICMOS Multiaccum observations. A simplified view of the dataflow in the HST system is shown in Figure 5. The STScI processing, planning and scheduling system produces command uploads, which are uplinked to the spacecraft via NASA/GSFC. Each command is tagged with a specific time of execution. The NSSC-1 computer, part of the SI Control and Data Handling subsystem (SI C&DH) interprets those stored commands and relays them to the SIs. The


SSM Computer interfaces with NSSC-1 and is responsible for engineering cuntions, such as pointing control, slew execution, loss-of-lock monitoring and telemetry.

Figure 5: HST data flow

Science and engineering data produced by the Sis are output on separate channels. The science data are processed by the Science Data Formatter (SDF), which normally forwards the data to the Solid State Recorder (SSR) for temporary storage. When a TDRS contact occurs, data from the SSR are downlinked and processed using ground systems. It is also possible to plan real-time observations, where the data are forwarded directly from the SDF to the ground. SI engineering data are processed through the NSSC-1 and eventually saent to the ground via a separate low-rate data channel. Data volume issues with respect to WFC3 observations are discussed in a separate ISR.

5 Summary
WFC3 will provide HST with high-resolution imaging and low-resolution field spectroscopy capabilities in the ultraviolet, visual, red and near-infrared wavelength regimes. This ISR provides an overview of the basic characteristics of the near-infrared channel, spanning the wavelength region from 0.8 to 1.7 microns.

References
Calzetti, D and Noll, K. 1998, Review of NICMOS Performance, ISR NICMOS-98014 Cheng, E., and MacKenty, J. 1997. WFC3 Instrument Concept.


Fruchter, A.S. and Hook, R.N. 1997, SPIE 3164, 120 Henry, R. 1999, WFC3 OP-01 document, Revision 2.1 MacKenty, J, 1999. Hubble Space Telescope Wide Field Camera 3 Contract End Item Specification (Part I) Massey, P. et al. 1997. Direct Imaging Manual for Kitt Peak NICMOS Instrument Handbook, May 1999 Robberto, M. et al 2000, First results from the new HgCdTe MBTE detectors for WFC3-IR channel, WFC3 ISR 2000-04 Soderblom, D. 1998. WFPC2 Phase II Proposal Instructions, Cycle 8 Stiavelli, M. et al 1998, Dithering strategies for ACS, ISR ACS-98-02 Stiavelli, M., Hanley, C., and Roberto, M., 1999. WFC3 Near-IR Channel: PSF and Plate Scale Study, WFC3 ISR 1999-01

Instrument Science Report WFC3-2000-01