HST Primer: Scientific Instruments ACS, WFC3, and FGS
An overview of the Advanced Camera for Surveys (ACS), the Wide-Field Camera 3 (WFC3), and the Fine Guidance Sensors (FGS).
The Advanced Camera for Surveys (ACS)
The Advanced Camera for Surveys (ACS) was installed during Servicing Mission 3B in March 2002. Failures in the ACS CCD Electronics Box and Low Voltage Power Supply in January 2007 made the WFC and HRC channels inoperable (the SBC continued to function). The problem was partially fixed during Servicing Mission 4 in May 2009, restoring use of the WFC but not the HRC.
In its current state, the camera provides ultraviolet and wide field optical imaging through two channels that have adjacent but not contiguous fields of view. The locations of these channels in the HST focal plane are presented in HST Primer: System Overview.
The Wide Field Channel (ACS/WFC) provides high throughput and wide field imaging. The camera has a 202" ×202" field of view from 350 nm to 1100 nm. Total system throughput is 42% at 600 nm (see HST Primer: Scientific Instrument Comparisons). The WFC detector is a pair of butted (2K by 4K), thinned and backside-illuminated, SITe CCDs with a red-optimized coating, a long wavelength halo fix, and 15 × 15 μm pixels. The plate scale is 0.050 arcsec/pixel. The WFC PSF is critically sampled at 1160 nm and undersampled by a factor of three at 370 nm. Well-dithered observations with the WFC should result in a reconstructed PSF FWHM of 0.1" to 0.14".
The Solar Blind Channel (ACS/SBC) is optimized for high resolution, solar-blind far-UV imaging. The camera has a 34" × 31" field of view from 115 nm to 170 nm and a peak efficiency of 6% (see HST Primer: Scientific Instrument Comparisons). The detector is a solar-blind CsI Multi-Anode Microchannel detector Array (MAMA) with 25 × 25 μm pixels. The plate scale is 0.032 arcsec/pixel.
In addition to these prime capabilities, ACS also provides imaging polarimetry, spectropolarimetry, and slitless spectroscopy, as described in HST Primer: Additional Observing Modes. Further information can be found in the ACS Instrument Handbook.
ACS proposers can benefit from reading Advice for Planning ACS Observations, which is ACS Instrument Science Report 2019-07 (https://hst-docs.stsci.edu/acsoam). There can be observational details that are specified in Phase II but must be requested in Phase I because they impose restrictions on telescope scheduling or use limited resources.
Wide Field Camera 3 (WFC3)
The Wide Field Camera 3 (WFC3) was installed in May 2009 during Servicing Mission 4, replacing the highly accomplished WFPC2. This camera provides ultraviolet, optical, and near-infrared imaging through two independent channels. These channels cannot be operated simultaneously, although they can be operated sequentially within the same orbit. Their location in the HST field of view can be seen in HST Primer: System Overview.
The Wide Field Ultraviolet-Visible Channel (WFC3/UVIS) is a high throughput, panchromatic camera with a field of view of 162" × 162", sensitive to wavelengths from 200 nm to 1000 nm. The total system throughput with this camera is 28% at 600 nm (see HST Primer: Scientific Instrument Comparisons). The detector is a pair of butted, 2K by 4K, thinned and backside-illuminated CCDs with an ultraviolet-optimized anti-reflective coating and 15 × 15 μm pixels. The plate scale is 0.04 arcsec/pixel. In addition to wavelength optimization, the primary differences between the WFC3 and ACS CCDs include a lower read noise (3 e- for WFC3, 4 e- for ACS) and a smaller inter-chip gap (465 μm rather than 750 μm). The UVIS channel provides 62 broad-, medium-, and narrow-band filters, and one grism.
The Wide Field High-Throughput Infrared Channel (WFC3/IR) has a 136" × 123" field of view over the wavelength range of 800 nm to 1700 nm. The total system throughput with this camera is 50% at 1600 nm. The detector is a 1K by 1K HgCdTe Teledyne array with 18 × 18 μm pixels. The plate scale is 0.13 arcsec/pixel. The detector has 12 e- RMS read noise in a 16-sample non-destructive readout sequence, or 21 e- RMS read noise in the difference of two samples (a correlated double sample). The IR channel provides 15 broad-, medium-, and narrow-band filters and two grisms.
WFC3 has slitless imaging spectroscopic modes in both channels and five UVIS quad filters as described in HST Primer: Additional Observing Modes.
The “drift and shift” (DASH) method of observations was introduced for WFC3/IR GO programs in Cycle 24. It enables users to make large, shallow mosaics within one orbit by guiding on gyros alone after the first exposure, thus eliminating the costly overhead of multiple guide star acquisitions. For DASH mode observations, a minimum of 6 minutes of time under FGS control after the end of the GSAcq and prior to the FGS-Pause activity is required. See Section 7.10.6 in the WFC3 Instrument Handbook.
The “drift and shift” (DASH) method of observations was introduced for WFC3/IR GO programs in Cycle 24. It enables users to make large shallow mosaics within one orbit by guiding on gyros alone after the first exposure, thus eliminating the costly overhead of multiple guide star acquisitions. For DASH mode observations, a minimum of 6 minutes of time under FGS control after the end of the GSAcq and prior to the FGS-Pause activity is required. See Section 7.10.6 in the WFC3 Instrument Handbook. Alignment of the individual IMA reads can be challenging when there are very few point sources. For a summary of DASH reduction, see section 9.5.2 in the Data Handbook.
The capability of commanding the UVIS shutter to use exclusively one of the two sides of the shutter blade in short exposures, to avoid vibration and PSF smearing, was implemented early in cycle 22. This option (BLADE=A in APT) will be made available to the observer when it is critical to the scientific success of a program. See Section 6.11.4 in the WFC3 Instrument Handbook.
Adding a flash at the end of a UVIS exposure (post-flash) greatly increases the detection of faint sources in low background observations where CTE losses would otherwise remove much or all of the flux from those sources. Most UVIS observers should consider using post-flash for all UV, narrow-band, and relatively short medium- and broad-band exposures where the detection of faint sources is required. Please refer to the ETC calculations for updated recommended background levels, in particular regarding the setting of the FLASH parameter in WFC3/UVIS observations. Further information on CTE and post-flash is available in the WFC3 Instrument Handbook and at the WFC3 UVIS CTE webpage.
The spatial scanning observing technique was introduced for WFC3 in Cycle 19. This mode can be used to turn stars into well-defined streaks on the detector or to spread a stellar spectrum perpendicular to its dispersion. It is useful for:
- Observations requiring high temporal sampling and/or time resolution.
- More efficient observation and higher S/N observations of exoplanets.
- High precision relative astrometric observations.
- Imaging and spectroscopy of brighter sources than previously possible.
For exoplanet observations, please specify the filter to be used for the accompanying direct image in the Phase 1. For grism pre-imaging in general, pre-imaging should use the specific band recommended for a particular grism, if possible. For infrared grism observations, it is advised to use one direct image at the beginning and one at the end of each orbit to account for time variable background in IR, ideally done in between each dither. Depending on scientific need, observers may want to consider multiple bandpass direct images.
See the WFC3 Instrument Handbook for further discussion of spatial scanning.
Fine Guidance Sensor (FGS)
There are three Fine Guidance Sensors (FGSs) onboard HST. Two FGSs are used to point the telescope at a target and hold that target in the primary scientific instrument's field of view. This task can be performed with a 2 to 5 milliarcsecond pointing stability. The third FGS (FGS1R in HST Primer: System Overview) is used as a sub-milliarcsecond astrometer and a high angular resolution interferometer. This instrument has two operating modes:
- In POS (Position) mode, the FGS measures the relative positions of objects in its 69 square arcminute field of view to a precision of ~1 milliarcsecond for targets with magnitudes 3.0 < V < 16.8. Position mode observing is used to determine relative parallax, proper motion, and reflex motion of single stars and binary systems. Multi-epoch programs have resulted in parallax measurements accurate to 0.2 milliarcseconds or less.
- In the TRANS (Transfer) mode, the 5" × 5" instantaneous field of view of the FGS is scanned across an object to obtain an interferogram with high spatial resolution. This is conceptually equivalent to an imaging device that samples an object’s PSF with 1 milliarcsecond pixels. The scientific goal of the TRANS mode is to study binary star systems (measure the separation, position angle, and relative brightness of the components), as well as to determine the angular size of extended objects such as the disks of resolvable giant stars or asteroids down to ~8 milliarcseconds.
By using a “combined mode” observing strategy, employing both POS mode (for parallax, proper motion, and reflex motion) and TRANS mode (for determination of visual and relative brightnesses of components in a binary), it is possible to derive the total and fractional masses of binary systems, indicating the mass-luminosity relationship for the components. Additional information can be found in the FGS Instrument Handbook.
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McCullough, P., and MacKenty, J. 2012, "Considerations for using Spacial Scans with WFC3"