Advanced Camera for Surveys Instrument Handbook for Cycle 14
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4.1 Imaging OverviewChapter 4:
Imaging
4.2 Which Instrument to Use?
4.2.1 Comparison of ACS and WFPC2
4.2.2 Comparison of ACS and NICMOS
4.2.3 Comparison of ACS and STIS
4.3 Caveats for ACS Imaging
4.3.1 Throughputs and Limiting Magnitudes
4.3.2 Limiting Magnitudes
4.3.3 Signal-To-Noise Ratios
4.3.4 Saturation
4.4 Wide Field Optical CCD Imaging
4.4.1 Filter Set
4.4.2 Long Wavelength Halo Fix
4.5 High-Resolution Optical and UV Imaging
4.5.1 Filter Set
4.5.2 Multiple Electron Events
4.5.3 Red Leaks
4.6 Ultraviolet Imaging with the SBC
4.6.1 Filter Set
4.6.2 Bright-Object Limits
4.6.3 Optical Performance
4.6.4 Red Leaks
4.7 ACS Point Spread Functions
4.7.1 CCD Pixel Response Function
4.7.2 Model PSFs
4.7.3 Encircled Energy
4.7.4 Geometric Distortions
4.7.5 PSFs at Red Wavelengths and the UV
4.7.6 Residual Aberrations
4.8 Two-Gyro Guiding
In this Chapter we focus on the imaging capabilities of ACS. Each imaging mode is described in detail. Plots of throughput and comparisons to the capabilities of WFPC2 and STIS are also provided. Curves of sensitivity and exposure time to achieve a given signal-to-noise as a function of source luminosity or surface brightness are referenced in this chapter, but presented in Chapter 10. We note the existence of bright-object observing limits for SBC channel imaging; these are described in detail in Chapter 7, including tables of the SBC bright-object screening magnitudes as a function of mode and spectral type.
4.1 Imaging Overview
ACS can be used to obtain images through a variety of optical and ultraviolet filters. When the selected ACS camera is the WFC or the HRC, the appropriate filter in one of the two filter wheels is rotated into position and a clear aperture is automatically selected on the other filter wheel. For SBC imaging the single filter wheel is rotated to the required position. A number of apertures are defined for each ACS camera. In general, these refer to different target positions on the detector.
Table 4.1 and Table 4.2 provide a complete summary of the filters available for imaging with each detector. Figures 4.1 through 4.5 show the filter transmission curves. In Figure 4.9 we show the integrated system throughputs.
The CCD filter wheels contain filters with two different sizes. Some filters (F435W, F475W, F502N, F550M, F555W, F606W, F625W, F658N, F660N, F775W, F814W, F850LP and G800L) are full-sized filters that can be used with both WFC and HRC. Others (F220W, F250W, F330W, F344N, F892N, POL0UV, POL60UV, POL120UV, POL0V, POL60V, POL120V, PR200L) are smaller, giving a full unvignetted field of view when used with the HRC, but a vignetted field of view of only 72"×72" when used with the WFC. Use of the small UV filters is not supported with the WFC due to the unpredictable behavior of the silver coatings shortward of 4000Å.
The Ramp Filters are designed to allow narrow or medium band imaging centered at an arbitrary wavelength. Each ramp filter is divided into three segments, of which only the middle segment may be used with the HRC. See Ramp filters for more details on these filters.
Note that although the CLEAR filters are specified in the filter wheel tables, users do not need to specify these filters in their HST proposals; they are added automatically in conjunction with the desired filter in the complementary wheel. In the SBC filter wheel, every third slot (#1, 4, 7, 10) is blocked off, so that in the case of a bright object limit violation, it is only necessary to rotate the filter wheel to an adjacent slot to block the incoming light.
With either the WFC and HRC it is possible to select a filterless observation by specifying CLEAR (this is an "available-but-unsupported" filter) as the filter name, although the image will be of degraded quality. Rough wavelengths and widths when used with the WFC or HRC are listed in Table 4.1 under CLEAR entries. Use of CLEAR will provide slightly degraded PSFs with the HRC and seriously degraded PSFs for the WFC. More details on PSFs with use of CLEAR are provided in
ACS ISR 2003-03. Applications are expected to be rare, but a valid use could be astrometry of extremely faint targets with the HRC when color information is not required.
Table 4.1: ACS WFC/HRC Filters in Filter Wheel #1
Table 4.2: ACS WFC/HRC Filters in Filter Wheel #2
Table 4.3: ACS SBC Filter Complement
Figure 4.1: ACS Broad-band filters
Figure 4.2: ACS SDSS filters
Figure 4.3: ACS UV and Medium-Band filters
Figure 4.4: ACS Narrow-Band filters
Figure 4.5: ACS SBC filters
4.2 Which Instrument to Use?
In this section, we compare briefly the performance of HST instruments with imaging and spectroscopic capability in the UV to near-IR spectral range. Important imaging parameters for all instruments are summarized in Table 4.4, followed by different sections where the ACS characteristics are compared to each other instrument.
Table 4.4: Characteristics of HST Imaging Instruments
4.2.1 Comparison of ACS and WFPC2
Advantages of each instrument may be summarized as follows:
- Wider field of view, 202"×202" vs. 150"×150" or less.
- Higher throughput at wavelengths >3700Å (see Figure 4.6).
- Better resolution: ACS/HRC offers 0.027" pixels vs. 0.046" on WFPC2 (PC).
- Better dynamic range: lower and well sampled read noise, larger sampled full well depth.
- Spectroscopic and coronagraphic observations are possible.
- ACS ramp filters have a higher throughput and FOV than those in WFPC2 (see Figures 4.6-4.9) and offer complete wavelength coverage from 3710Å to 10,710Å.
- Polarization observations on ACS can be made with 3 polarizer angles of 0º, 60º, 120º over the whole HRC FOV.
- For high contrast imaging, the WFPC2 PC has a higher scattered light floor than ACS HRC.
- ACS has more uniform PSFs over the entire field-of-view.
- ACS has better CTE and a lower dark current.
Figure 4.6: Comparison between the system efficiency (or throughput) of ACS WFC and WFPC2 for the filters: Johnson B, Johnson V, Broad V and Broad I. The solid lines are for ACS and the dotted lines for WFPC2. ACS total system throughput is at least a factor of 3-4 better than WFPC2 at these wavelengths.
- Some special filters are available that are not found in ACS. These are the narrow filters (F343N, F375N (OII), F390N, F437N, F469N, F487N, F588N, F631N, F673N, F953N). ACS can do narrow-band imaging with the ramp filters, with a smaller FOV and has higher throughput and lower read noise.
- Wide-field UV observations are possible with the following filters: F122M, F160BW, F170W, F185W, F218W, F255W, F300W, F336W.
4.2.2 Comparison of ACS and NICMOS
ACS and NICMOS have a small overlap in imaging capability for filters at around 9000Å. At longer wavelengths NICMOS must be used; at shorter wavelengths either ACS, WFPC2 or STIS must be used. The following table compares the detection efficiency of ACS and NICMOS in the wavelength region where they both operate. Count rates for a V=20 star of spectral class A1 are given for all filters at common wavelengths; the signal-to-noise (S/N) is also given for a 1 hour exposure of this same star using a 5x5 pixel aperture in each case.
Table 4.5: Near-IR capabilities of ACS compared to NICMOS
Figure 4.7: Comparison between the ACS and WFPC2 ramp filters. The crosses and the open circles are for the ACS narrow and medium band ramps. The open squares are for the 4 WFPC2 ramps. For each of the ACS ramps the peak throughput that was calculated for eleven central wavelength values is plotted. For the WFPC2 ramps, the peak throughput calculated every 100Å within the field of view of any of the 4 chips and a 0º filter rotation angle (as mapped in Figs. 3.4 and 3.5 of the WFPC2 Instrument Handbook, version 3.0), is plotted.
4.2.3 Comparison of ACS and STIS
Both ACS and STIS are capable of imaging over the same wavelength range, between 1200Å and 11,000Å. At much longer wavelengths NICMOS must be used.
Advantages of each instrument may be summarized as follows:
- Wider field-of-view at optical and near-infrared wavelengths, 202"×202" vs. 50"×50" or less.
- Greater selection of filters, including polarizers, are available.
- Higher sensitivity is possible.
- Better CTE and lower dark current.
- MAMAs can be used in Time-Tag Mode.
- FUV-MAMA gives higher S/N than SBC due to the lower dark current.
- An OII filter centered at 3727Å is available that allows deep, high-resolution OII imaging.
- Narrow band filters at 2800Å and 1900Å allow imaging in MgII and CIII, respectively.
- Selectable aperture (slit) size for the MAMAs means that bright object concerns are lessened.
True to its name, ACS significantly enhances the imaging capabilities of HST. Due to the combination of sensitivity and field of view ACS has become the instrument of choice for UV/optical imaging on HST.
Figure 4.8: Comparison between the system efficiency of ACS SBC and STIS FUV-MAMA. For the ACS SBC the total system throughput for the f122m, f125lp and f165lp filters is plotted in the solid lines. For the STIS FUV-MAMA the system throughput for the Clear (25mama) and Lyman-(f25lya) filters are given with the dashed lines.
4.3 Caveats for ACS Imaging
There are a few characteristics of ACS that should be taken into account when imaging with ACS:
- The HRC and WFC filters are housed in two filter wheels shared by both cameras. As a consequence, when a filter is chosen for the primary camera the filter used in the parallel camera is not independently selectable (see Table 8.4).
- The ACS cameras are designed to be used with a single filter, and for this reason unfiltered imaging or imaging through two filters leads to significantly degraded imaging quality (particularly in the WFC) and is not normally used except for polarization observations, or bright target acquisitions with the HRC. The polarizer filters were designed with a zero optical thickness so that they can and should be used with another filter.
- The geometric distortion of the WFC is significant and causes the projected pixel area to vary by ± 9% over the field of view. This distortion affects both the photometric accuracy and the astrometric precision and must be accounted for when the required accuracy is better than 10%.
- The ratio of in-band vs. out-of-band transmission for the ACS CCD UV filters is similar to that of WFPC2, once the two detector QE curves are taken into account (the red leak on ACS F330W is very small). This implies that for imaging in the UV of intrinsically red objects the effect of filter red leaks needs to be calibrated.
- The cosmic ray fluxes for HRC and WFC are comparable, respectively, to those of the STIS CCD and WFPC2. As with these instruments typical imaging observations will need to be split or dithered for cosmic ray rejection.
- Hot pixels are a significant issue for WFC due to a lower than expected rate of removal through anneals. Section 7.2.6 provides further details and a recommendation that separate exposures with small dithers be considered as a means of helping to remove residual hot pixels.
- The large format of the WFC requires significantly more shifts to read out data than with STIS or WFPC2, therefore the impact of decreasing Charge Transfer Efficiency will be encountered earlier. Section 7.2.7 details current expectations, which for Cycle 14 are expected to remain modest.
- The default GAIN setting for WFC primary observations is
GAIN=1. This allows for good sampling of the readout noise but it does not allow one to reach the full well counts of WFC. The readout noise for the WFC is still better than critically sampled at GAIN=2, which provides sampling of the full well depth as well (by contrast all WFPC2 results were obtained with a GAIN falling at least a factor of 3 short of critically sampling the readout noise). For HRC primary observations, the default gain isGAIN=2. For the HRC GAIN=4 is needed to sample the detector full well depth, but this does result in modest undersampling of the readout noise. For HRCACQdata, the default setting isGAIN=4. Users may select the GAIN they wish to use for their ACS observations by using the GAIN optional parameter in their Phase II proposal. However, not all GAIN settings are supported (see Section 2.5).- At wavelengths longward of ~8000Å, internal scattering in the HRC CCD produces an extended PSF halo. This should affect only a minority of observations since at these wavelengths the WFC camera should normally be preferred. The WFC CCDs include a front-side metallization that eliminates the large angle, long wavelength halo problem for
~<9000Å. (For observations of red targets with the F850LP refer to Section 6.3.2).
- The ACS filter complement is not as rich as that in WFPC2. In particular, the Strömgren filter set and several narrow band filters available in WFPC2 (F375N, F390N, F437N, F469N, F487N, F588N, F631N, F656N, F673N, F953N) are not available on ACS. In general, these filters were not heavily used by the GO community. For most applications they can be replaced with the ACS medium and narrow ramps but it is conceivable that for some specialized applications the WFPC2 will still be preferred.
4.3.1 Throughputs and Limiting Magnitudes
In Figure 4.9 below, we show the throughput of the two unfiltered ACS CCD cameras: WFC and HRC. Superposed on this plot, we show the unfiltered WFPC2 (WF4) and the clear STIS throughputs. In Figure 4.8 the ACS SBC system throughput is compared to that of the STIS FUV-MAMA.
Figure 4.9: ACS CCD System Throughputs + OTA Versus those of STIS and WFPC2
4.3.2 Limiting Magnitudes
In Table 4.6, we give the V magnitude, in the Johnson-Cousins system, reached for an A0V star during a one-hour integration (CR-SPLIT=2) which produces a signal-to-noise ratio of 10 integrated over the number of pixels needed to encircle ~80% of the PSF flux. More precisely, for the WFC a boxsize of 5x5 pixels (0.2 arcsec) was used , for the HRC a 9x9 pixel boxsize (0.2 arcsec), and for the SBC a 15x15 pixel boxsize (0.5 arcsec). The last column gives the limiting magnitude assuming an optimally weighted PSF fit. The observations are assumed to take place in
LOW-SKYconditions for the Zodiacal light and SHADOW of the Earthshine. Note that the assumed sky backgrounds are therefore much better than average conditions; these are best case limits.
Table 4.6: ACS limiting V magnitudes for A stars
4.3.3 Signal-To-Noise Ratios
In Chapter 10, we present, for each imaging mode, plots of exposure time versus magnitude to achieve a desired signal-to-noise ratio. These plots, which are referenced in the individual imaging-mode sections below, are useful for getting an idea of the exposure time you need to accomplish your scientific objectives. More accurate estimates will require the use of the ACS Exposure Time Calculator.
4.3.4 Saturation
Both CCD and SBC imaging observations are subject to saturation at high total accumulated counts per pixel: the CCDs, due either to the depth of the full well or to the 16 bit data format, and the SBC, due to the 16-bit format of the buffer memory (see Section 7.2.1 and Section 7.4.1). In Chapter 10, saturation levels as functions of source magnitude and exposure time are presented in the S/N plots for each imaging mode.
4.4 Wide Field Optical CCD Imaging
The Wide Field Channel of ACS was designed primarily for high throughput observations in the visible. The use of protected silver mirror coatings, the small number of reflections and the use of a red sensitive CCD have provided the high throughput required for this camera at the expense of a 3700 Å blue cutoff. The WFC detectors are two butted 2k by 4k thinned, backside-illuminated, SITe CCDs with a red optimized coating and long-
The optical design of the camera introduces a two-component geometric distortion. The detectors themselves are at an angle with respect to the optical axis. This produces an 8% stretching of one pixel diagonal compared to the other. As a result WFC pixels project on the sky as rhombuses rather than squares. These effects are purely geometrical and are routinely corrected in the ACS data reduction pipeline. The second component of geometric distortion is more complex. This distortion causes up to ±9% variation in effective pixel area and needs to be taken into account when doing accurate photometry or astrometry as the effective area of the detector pixels varies nonlinearly with field position.
4.4.1 Filter Set
WFPC2 and Johnson-Cousins filters
All of the most commonly used WFPC2 filters are included in the ACS filter set. In addition to a medium and a broad V band filter (F550M and F606W), there is a complete Johnson-Cousins BVI set (F435W, F555W, F814W).
Sloan Digital Sky Survey filters
The Sloan Digital Sky Survey (SDSS) griz filter set (F475W, F625W, F775W, F850LP) are designed to provide high throughput for the wavelengths of interest and excellent rejection of out-of-band wavelengths. They were designed to provide wide, non-overlapping filter bands that cover the entire range of CCD sensitivity from the blue to near-IR wavelengths.
Narrow Band filters
The H
] (F502
), and [NII] (F660N) narrow band filters are full-size, and can be used with both WFC and HRC.
Ramp filters
ACS includes a complete set of ramp filters which provide full coverage of the WFC wavelength range at 2% and 9% bandwidth. Each ramp filter consists of 3 segments. The inner and outer filter segments can be used with the WFC only, while the central segments can be used by both WFC and HRC. Unlike the WFPC2 where the desired wavelength is achieved by offsetting the telescope, the wavelength of ACS ramps is selected by rotating the filter while the target is positioned in one of the pre-defined apertures. The monochromatic field of view of the ramp filters is approximately 40" by 80". Details of how to use the ramp filters are given in Section 8.5.2.
Polarizer filters
The WFC/HRC filter wheels contain polarizers with pass directions spaced by 60º, optimized for both the UV (POL0UV, POL60UV and POL120UV) and the visible (POL0V, POL60V and POL120V). All the polarizer filters are sized for the HRC field of view, so will induce vignetting when used with the WFC, where the FOV will be about 72" by 72". More information on the use of the polarizers is given in Chapter 5.
Grism and Prism
The CCD channels also have a grism (G800L) for use with both WFC and HRC from 5500Å to 11,000Å, and a prism (PR200L) for use with the HRC from 1600Å to 3500Å. Again, these are described more fully in Chapter 5.
4.4.2 Long Wavelength Halo Fix
The PSF of the STIS and HRC CCDs are characterized by a significant halo at long wavelengths which is due to photons crossing the CCD and being reflected back in random directions by the front side of the CCD. The problem becomes noticeable beyond 8000Å because only long wavelength photons can transverse the CCD without being absorbed. The so-called halo fix for the WFC consists of a metallization of the front side of the CCD which essentially reflects photons back to the original pixel.
Inflight calibrations observing stars with a broad color range, in particular very red stars, have shown that a significant halo does set in above 9000Å. A full discussion of this may be found in Gilliland & Riess,
2002 HST Calibration Workshop, p61. In the F850LP filter, in particular, extremely red stars show a progressive loss of flux in small to moderate sized apertures as a function of color. This halo effect is only partially treated by the Exposure Time Calculator. Observers can use synphot (see Section 6.3.2) to most accurately calculate the photometry of red sources in the SDSS z-filter.4.5 High-Resolution Optical and UV Imaging
The High Resolution Channel of ACS is the prime ACS camera for near-UV imaging. HRC provides high throughput in the blue and a better sampling of the PSF than either the WFC or other CCD cameras on HST. The HRC pixel size critically samples the PSF at 6300Å and is undersampled by a factor 3.0 at the blue end of its sensitivity range (2000Å). In this capability, the HRC functionally replaces the Faint Object Camera as the instrument able to critically sample the PSF in the V band. For this reason, although we expect that most of the usage of HRC will be for UV and blue imaging, HRC can also be convenient for imaging in the red when the PSF sampling is important. As an example, better PSF sampling is probably important for accurate stellar photometry in crowded fields and we expect that the photometric accuracy achievable by the HRC will be higher than that achievable with the WFC. Well-dithered observations with the HRC should lead to a reconstructed PSF FWHM of 0.03 arcsec at ~4000Å, increasing towards longer wavelengths. HRC also includes a coronagraph that will be discussed in Chapter 5. The HRC CCD presents a long wavelength halo problem similar to the STIS CCD since the front-side metallization correcting the halo problem for the WFC CCDs was implemented only after the HRC CCD had been procured. Although most of the HRC imaging is likely to occur in the UV, users should be cautioned to take into account the effects of the long wavelength halo when using the HRC in combination with near-IR filters.
4.5.1 Filter Set
The HRC-specific filters are mostly UV and blue. The set includes UV and visible polarizers (discussed in Chapter 5), a prism (PR200L, discussed in Chapter 5), three medium-broad UV filters (F330W, F250W, and F220W) and two narrow band filters (F344N and F892N). Use of the UV filters with the WFC is not supported because of the uncertainty of the WFC silver coating transmission below 4000Å.
All broad, medium and narrow band WFC filters can be used with the HRC whenever a better PSF sampling is required. In general, where their sensitivity overlaps the throughput of WFC is higher than that of HRC. Only some of the WFC ramp filters can be used with the HRC since only the middle ramp segment overlaps with the HRC FOV. In particular, HRC can use the FR459M and FR914M broad ramps, and the FR505N [OIII], FR388N [OII] and FR656N (H
4.5.2 Multiple Electron Events
Like the STIS CCD but unlike WFPC2, the HRC CCD is directly sensitive to UV photons and for this reason is much more effective in detecting them. However, whenever a detector has non-negligible sensitivity over more than a factor two in wavelength, it becomes energetically possible for a UV photon to generate more than one electron, and so be counted more than once. This effect has indeed been seen in STIS and also during the ground testing of the HRC detector. The effect is only important shortward of 3200Å, and reaches a magnitude of approximately 1.7e-/photon at 2000Å. Multiple counting of photons has to be taken into account when estimating the detector QE and the noise level of a UV observation, since multiple photons cause a distortion in the Poisson distribution of electrons.
4.5.3 Red Leaks
When designing a UV filter, a high suppression of off-band transmission, particularly in the red, has to be traded with overall in-band transmission. The very high blue quantum efficiency of the HRC compared to WFPC2 makes it possible to obtain an overall red leak suppression comparable to that of the WFPC2 while using much higher transmission filters.The ratio of in-band versus total flux is given in Table 4.7 for a few UV and blue HRC filters, where the cutoff point between in-band and out-of-band flux is defined as the filter's 1% transmission point. The same ratio is also listed for the equivalent filters in WFPC2. Clearly, red leaks are not a problem for F330W, F435W, and F475W. Red leaks are more important for F250W and F220W. In particular, accurate UV photometry of objects with the spectrum of an M star will require correction for the redleak in F250W and will be essentially impossible in F220W. For the latter filter a redleak correction will also be necessary for K and G types.
Table 4.7: In-band Flux as a Percentage of the Total Flux
4.6 Ultraviolet Imaging with the SBC
The Solar Blind Channel is the ACS camera optimized for far-UV imaging. The SBC uses the same optical train as the HRC and is comparable in performance to the FUV MAMA of STIS.
4.6.1 Filter Set
Like the STIS FUV MAMA, the SBC includes a Lyman
4.6.2 Bright-Object Limits
The bright object limits are discussed in detail in Section 7.5.
4.6.3 Optical Performance
The optical performance of the SBC is comparable to that of the STIS FUV-MAMA. The use of the repeller wire increases the quantum efficiency of the detector by ~30% or so, but adds a halo to the PSF.
4.6.4 Red Leaks
The visible light rejection of the SBC is excellent, but users should be aware that stars of solar type or later will have a significant fraction of the detected flux coming from outside the nominal wavelength range of the detector. Details are given below, in Table 4.8.
Table 4.8: Visible-Light Rejection of the SBC F115LP Imaging Mode
which have
which have
4.7 ACS Point Spread Functions
The ACS point spread function has been studied in ground test measurements, using models generated by the TinyTIM software of J. Krist and R. Hook and measured in on-orbit data. As with other HST instruments, the ACS point spread function is affected by both optical aberrations and geometric distortions. Also, point sources imaged with WFC and HRC experience blurring due to charge diffusion into adjacent pixels because of CCD subpixel variations, which reduces the limiting magnitudes that can be reached by WFC/HRC. The SBC PSF and the long-wavelength HRC PSF are additionally affected by a halo produced by the detectors themselves.
4.7.1 CCD Pixel Response Function
The sharpness of the CCD PSF is somewhat degraded by charge diffusion into adjacent pixels. The effect is usually described in terms of the pixel response function (PRF), which gives the distribution of flux from within the pixel into adjacent pixels. Charge diffusion results in ~0.5 mag loss in the WFC limiting magnitude at short wavelengths (the worst case). At longer wavelengths and at all wavelengths for the HRC the reduction in the limiting magnitude is ~0.2 mag or less. Due to variations in the CCD thickness, charge diffusion is not constant over the field of view. At different wavelengths, the CCD pixel response functions can be represented by the following kernels (for the center of the field):
More details on ACS CCD charge diffusion are given in
ACS ISR 03-06.4.7.2 Model PSFs
Table 4.9 and Table 4.10 give ACS model PSFs in the central 5×5 pixel region in two wavelength bands (filters). Numbers listed are the fraction of the total energy received in each pixel. The models have been generated using TinyTIM, taking into account the HST optical aberrations and obscurations as well as the CCD pixel response function. Field dependent geometrical distortions are included. The real PSF will also differ from the model because of the jitter in the HST pointing, HST focus variation (focus breathing), and other instrumental effects, some of which are briefly discussed below.
Table 4.9: Model ACS CCD PSFs
The SBC PSF is shown Figure 4.11
.4.7.3 Encircled Energy
In general, the ACS channels encircled energy distribution has been found to be within the original instrument specifications. Figure 4.10 and Figure 4.11 show the ACS encircled energy curves derived from on-orbit images.
Figure 4.10: Encircled energy for the CCD channels
Figure 4.11: Encircled energy for the SBC
4.7.4 Geometric Distortions
Geometric distortions produce a significant impact on the shape of the PSF in all three of the ACS channels, as can readily be seen in Figure 4.12 and Figure 4.13, which display WFC and HRC PSF images. The log stretch enhances the spider diffraction patterns, which the distortion renders non-perpendicular, and the outer Airy rings, which appear elliptical. The distortion owes primarily to the tilt of the focal surface to the chief ray at the large OTA field angles of the ACS apertures. The linear, field-independent, approximation for the WFC produces a difference in plate scale of about 8% between the two diagonals of the field and, in the HRC and SBC, about a 16.5% difference in scale between orthogonal directions rotated about 20 degrees from the aperture edges. Field-dependent distortions, measured as actual vs. predicted distances from field center, amount to about 2% peak in the WFC and about 1% in the HRC and SBC.
The distortions render the pixels, as projected on the sky, trapezoidal in shape and their area varies over the field by about 19% and 3.5% in the WFC and HRC/SBC, respectively. These variations have significant ramifications concerning appropriate techniques for flat-fielding and photometric calibration, especially when complicated by resampling in order to combine dithered image sets. A related issue is the manner in which the halation effects of the HRC and SBC detectors are removed and the treatment of spectra from the prisms and grism, which are not subject to the same distortion effects.
More details concerning geometric distortions in ACS can be found in Distortion in the ACS. A brief introduction to CALACS and PyDrizzle which apply corrections for geometric distortion is given in
Chapter 12.4.7.5 PSFs at Red Wavelengths and the UV
As previously noted, the CCDs used in the HRC and WFC suffer from a halo that is caused by very red photons passing through the device and being scattered back into the detector by the mounting substrate. This creates a large halo in HRC images beyond 7000 Å and WFC images past 10,000 Å. At 8000 Å in the HRC, the halo contains about 10% of the light. At 10,000 Å, it contains about 30% and increases the surface brightness of the PSF wings by over an order of magnitude; overwhelming the PSF diffraction rings and spikes.
Long wavelength photons that pass through the CCD can also be scattered by the electrode structure on the back side of the device. This creates two spikes that extend roughly parallel to the x-axis. These spikes are seen at wavelengths longer than 9500 Å in both the HRC and WFC (see Figure 4.14 and Figure 4.15).
In the UV the core of the PSF becomes rather asymmetrical due to midfrequency optical surface errors. In the SBC, a halo is created by charge migration at the microchannel plate surface. This effect, seen previously in STIS MAMA images, broadens the PSF core and redistributes a small portion of flux into a broad halo that can be approximated by a Gaussian with FWHM ~ 20 pixels. The peak flux for a point source centered on a pixel is reduced by 30%-40% depending on wavelength.
The encircled energy curves presented in this handbook and incorporated into the ETC include all of the scattering effects discussed here.
4.7.6 Residual Aberrations
ACS provides excellent optical performance. Residual aberration levels at the center of the field are 1/30 wave (HRC) and 1/20 wave (WFC) RMS at 5500 A (excluding defocus). Coma and astigmatism are minimized at the field center of each camera. The ACS PSF varies far less over the field of view than those of WFPC2 and STIS. WFPC2 especially suffers from a variable obscuration pattern that significantly alters the PSF structure depending on field position. Lacking the additional obscurations present in WFPC2, ACS PSF variations are instead due to changes in aberrations and charge diffusion.
At the extreme corners of the WFC field, increased astigmatism will slightly elongate the PSF core. The axis of elongation will rotate by 90 degrees if the system passes through focus due to breathing. This may affect ellipticity measurements of small galaxies with bright cores at the field edges. Focus variations in the WFC, which alter the amount of light in the peak, are largely due to detector surface height irregularities and amount to the equivalent of 5 microns of breathing (1/18 wave RMS). The largest focus offset is along the gap between the two CCDs. Variations in the width of the PSF core are dominated by changes in CCD charge diffusion, which is dependent on the thickness of the detector (12-17 microns for the WFC). The PSF FWHM in F550M, for example, can vary by 20% over the field (0.10"-0.13").
The PSFs in the HRC and SBC are reasonably constant over their fields. The HRC FWHM is 0.060"-0.073" in F550M. More details on ACS PSF field variations are provided in
Figure 4.12: ACS WFC PSF - F625WACS ISR 03-06. The Tiny Tim PSF simulator includes field dependent aberrations and charge diffusion and may be used to estimate the impact of these variations.
Figure 4.13: ACS HRC PSF - F625W
Figure 4.14: ACS WFC PSFs (10" x10"). FR914M images are saturated.
Figure 4.15: ACS HRC PSFs (3.25" x3.25")
4.8 Two-Gyro Guiding
At some future date HST may be operated with only two gyroscopes, hence causing additional spacecraft jitter and possible degradation of the effective PSF. Extensive tests of two-gyro guiding are planned in early 2005, where these impacts should become more clear.
We anticipate that a two-gyro version of the ACS Exposure Time Calculator will be available to help assess these impacts.
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