Chapter 10:
Imaging Reference MaterialIn this chapter. . .
In this Chapter, we provide imaging reference material, in support of the information presented in Chapter 9.
10.1 Introduction
This chapter provides reference material to help you select your filter and detector configuration, and determine your observing plan (e.g., total required exposure time, and number of exposures). This chapter is, for the most part, organized by filter and detector. For each imaging mode the following are provided:
- Plots of integrated system throughput as a function of wavelength.
- Plots of the time needed to achieve a desired signal-to-noise ratio vs. magnitude for all filters for a point source and a one arcseconds2 extended source.
- Tables of color corrections ABn to go from Johnson V magnitude to AB magnitude.
10.2 Using the Information in this Chapter
10.2.1 Sensitivity Units and Conversions
This chapter contains plots of throughputs for each imaging mode. "Determining Count Rates from Sensitivities" in Section 9.2 explains how to use these throughputs to calculate expected count rates from your source.
The first figure for each imaging mode gives the integrated system throughput. This is the combination of the efficiencies of the detector and of the optical elements in the light path. The throughputs in this handbook are based in part on ground test data. Although, at the time of writing the overall detector efficiency curve and most filter throughputs have been adjusted based on in-flight data. The throughput is defined as the number of detected counts/second/cm2 of telescope area relative to the incident flux in photons/cm2/second. For the CCD, “counts” is the number of electrons detected. For the MAMA, “counts” is the number of valid events processed by the detector electronics after passing through the various pulse-shape and anti-coincidence filters. In both cases the detected counts obey Poisson statistics. The throughput includes all obscuration effects in the optical train (e.g., due to the HST secondary).
To recalculate the throughput with the most recent CCD QE tables in synphot, you can create total-system-throughput tables (instrument plus OTA) using the synphot calcband task. calcband takes any valid obsmode command string as input and produces an STSDAS table with two columns of data called “wavelength” and “throughput” as its output. For example, to evaluate the throughput for the F475W filter and the WFC detector, chip 1, you would use the command calcband acs,wfc1,f475w sdssg_thpt. The resulting throughput table is stored in sdssg_thpt.
The ramp filters are not included in this chapter because the passband will change depending on the chosen central wavelength. The width of the passband and available range of central wavelengths for each ramp segment are listed in Table 5.2. Additionally, the passband for a ramp segment can be plotted with synphot using the following command calcband acs,wfc1,fr388n#3880 sdssg_thpt where the #3880 is the desired central wavelength in Angstroms.
10.2.2 Signal-to-Noise
For each imaging mode, plots are provided to estimate the signal-to-noise ratio (S/N) for a representative source. The first figure shows S/N for point sources (GAIN=1). The second figure shows S/N for uniform extended sources of area 1 arcseconds2.
The different line styles in the S/N figures delineate regions where different sources of noise dominate. A particular source of noise (readnoise for example) is presumed to dominate if it contributes more than half the total noise in the observations.
The point- and extended-source S/N figures are shown for average and low sky levels. For point sources, an aperture size of 5 x 5 pixels has been used for the WFC, 9 x 9 pixels for HRC, and 15 x 15 pixels for the SBC S/N evaluation. For extended sources, a 1 arcseconds2 aperture was used. For the CCD the readnoise has been computed assuming a number of readouts NREAD= integer (t / 1000 seconds), where t is the exposure time, with a minimum NREAD=2. That is, each exposure has a minimum CR-SPLIT=2. Different line styles in the figures are used to indicate which source of noise dominates.
To the left of the vertical line in the SBC S/N plots, the count rate from the source exceeds the 150 counts/second/pixel local count rate limit. This is computed from the model PSF, which gives 14% to 22% of the flux in the central pixel.
In situations requiring more detailed calculations (non-stellar spectra, extended sources, other sky background levels, unknown target V magnitude, etc.), the ACS Exposure-Time Calculator should be used.
Follow these steps to use the signal-to-noise plots:
- Determine the AB magnitude of your source at the wavelength of interest. There are several ways to do this.
- Examine Table 10.1, 10.2, or 10.3 (the data in these tables are for Side 1) and find ABn for the desired spectral type and filter. Sum the V magnitude of the target and ABn derived from the table.
- Alternatively, compute ABMAG (=V+ABn) from the source flux, using the relation
, or
.
- Find the appropriate plot for the filter in question, and locate V+ABn on the horizontal axis. Then read off the signal-to-noise ratio for the desired exposure time, or vice-versa.
The “x” characters at the top of each plot indicate the onset of saturation, in the case of the CCD. The “x” shows where the total number of counts exceeds the 16 bit buffer size of 65,535.
Note that the plots show the S/N as a function of source magnitude for exposure times as short as 0.1 seconds, although the minimum exposure time for the WFC CCD channel is 0.5 seconds.
10.2.3 Point Spread Functions
All information about the PSF are based on the modeled encircled energy data presented in ACS Point Spread Functions in Section 5.6
WFC/F435W
Description
Johnson B filter.
Figure 10.1: Integrated system throughput for WFC/F435W.Figure 10.2: Point source S/N vs. V+ABn for the WFC/F435W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.3: Extended source S/N vs. V+ABn for the WFC/F435W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.WFC/F475W
Description
Sloan Digital Sky Survey g filter.
Figure 10.4: Integrated system throughput for WFC/F475W.Figure 10.5: Point source S/N vs. V+ABn for the WFC/F475W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.6: Extended source S/N vs. V+ABn for the WFC/F475W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.WFC/F502N
Description
OIII filter.
Figure 10.7: Integrated system throughput for WFC/F502N.Figure 10.8: Point source S/N vs. V+ABn for the WFC/F502N filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.9: Extended source S/N vs. V+ABn for the WFC/F502N filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.WFC/F550M
Description
Narrow V filter.
Figure 10.10: Integrated system throughput for WFC/F550M.Figure 10.11: Point source S/N vs. V+ABn for the WFC/F550M filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.12: Extended source S/N vs. V+ABn for the WFC/F550M filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.WFC/F555W
Description
Johnson V filter.
Figure 10.13: Integrated system throughput for WFC/F555W.Figure 10.14: Point source S/N vs. V+ABn for the WFC/F555W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.15: Extended source S/N vs. V+ABn for the WFC/F555W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.WFC/F606W
Description
Broad V filter.
Figure 10.16: Integrated system throughput for WFC/F606W.Figure 10.17: Point source S/N vs. V+ABn for the WFC/F606W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.18: Extended source S/N vs. V+ABn for the WFC/F606W. Top curves are for low sky and bottom curves are for average sky for a 1arcsec2 area.WFC/F625W
Description
Sloan Digital Sky Survey r filter.
Figure 10.19: Integrated system throughput for WFC/F625W.Figure 10.20: Point source S/N vs. V+ABn for the WFC/F625W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.21: Extended source S/N vs. V+ABn for the WFC/F625W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.WFC/F658N
Description
Ha filter.
Figure 10.22: Integrated system throughput for WFC/F658N.Figure 10.23: Point source S/N vs. V+ABn for the WFC/F658N filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.24: Extended source S/N vs. V+ABn for the WFC/F658N filter. Top curves are for low sky and bottom curves are for average sky for 1 arcsec2 area.WFC/F660N
Description
NII filter.
Figure 10.25: Integrated system throughput for WFC/F660N.Figure 10.26: Point source S/N vs. V+ABn for the WFC/F660N filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.27: Extended source S/N vs. V+ABn for the WFC/F660N filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.WFC/F775W
Description
Sloan Digital Sky Survey i filter
Figure 10.28: Integrated system throughput for WFC/F775W.Figure 10.29: Point source S/N vs. V+ABn for the WFC/F775W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.30: Extended source S/N vs. V+ABn for the WFC/F775W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.WFC/F814W
Description
Broad I filter.
Figure 10.31: Integrated system throughput for WFC/F814W.Figure 10.32: Point source S/N vs. V+ABn for the WFC/F814W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.33: Extended source S/N vs. V+ABn for the WFC/F814W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.WFC/F850LP
Description
Sloan Digital Sky Survey z filter.
Figure 10.34: Integrated system throughput for WFC/F850LP.Figure 10.35: Point source S/N vs. V+ABn for the WFC/F850LP filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.36: Extended source S/N vs. V+ABn for the WFC/F850LP filter. Top curves are for low sky and bottom curves are for average sky for a 1arcsec2 area.WFC/G800L
Description
Grism.
Figure 10.37: Integrated system throughput for WFC/G800L.Figure 10.38: Point source S/N vs. V+ABn for the WFC/G800L filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.39: Extended source S/N vs. V+ABn for the WFC/G800L filter. Top curves are for low sky and bottom curves are for average sky for a 1arcsec2 area.WFC/CLEAR
Description
Clear filter.
Figure 10.40: Integrated system throughput for WFC/Clear.Figure 10.41: Point source S/N vs. V+ABn for the WFC/Clear filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.42: Extended source S/N vs. V+ABn for the WFC/Clear filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F220W
Description
Near-UV filter.
Figure 10.43: Integrated system throughput for HRC/F220W.Figure 10.44: Point source S/N vs. V+ABn for the HRC/F220W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.45: Extended source S/N vs. V+ABn for the HRC/F220W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F250W
Description
Near-UV filter.
Figure 10.46: Integrated system throughput for HRC/F250W.Figure 10.47: Point Source S/N vs. V+ABn for the HRC/F250W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.48: Extended Source S/N vs. V+ABn for the HRC/F250W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F330W
Description
HRC u filter.
Figure 10.49: Integrated system throughput for HRC/F330W.Figure 10.50: Point source S/N vs. V+ABn for the HRC/F330W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.51: Extended source S/N vs. V+ABn for the HRC/F330W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F344N
Description
NeV filter.
Figure 10.52: Integrated system throughput for HRC/F344N.Figure 10.53: Point source S/N vs. V+ABn for the HRC/F344N filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.54: Extended source S/N vs. V+ABn for the HRC/F344N filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F435W
Description
Johnson B filter.
Figure 10.55: Integrated system throughput for HRC/F435W.Figure 10.56: Point source S/N vs. V+ABn for the HRC/F435W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.57: Extended source S/N vs. V+ABn for the HRC/F435W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F475W
Description
Sloan Digital Sky Survey g filter.
Figure 10.58: Integrated system throughput for HRC/F475W.Figure 10.59: Point source S/N vs. V+ABn for the HRC/F475W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.60: Extended source S/N vs. V+ABn for the HRC/F475W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F502N
Description
OIII filter.
Figure 10.61: Integrated system throughput for HRC/F502N.Figure 10.62: Point source S/N vs. V+ABn for the HRC/F502N filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.63: Extended source S/N vs. V+ABn for the HRC/F502N filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F550M
Description
Narrow V filter.
Figure 10.64: Integrated system throughput for HRC/F550M.Figure 10.65: Point source S/N vs. V+ABn for the HRC/F550M filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.66: Extended source S/N vs. V+ABn for the HRC/F550M filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F555W
Description
Johnson V filter.
Figure 10.67: Integrated system throughput for HRC/F555W.Figure 10.68: Point source S/N vs. V+ABn for the HRC/F555W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.69: Extended source S/N vs. V+ABn for the HRC/F555W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F606W
Description
Broad V filter.
Figure 10.70: Integrated system throughput for HRC/F606W.Figure 10.71: Point source S/N vs. V+ABn for the HRC/F606W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.72: Extended source S/N vs. V+ABn for the HRC/F606W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F625W
Description
Sloan Digital Sky Survey r filter.
Figure 10.73: Integrated system throughput for HRC/F625W.Figure 10.74: Point source S/N vs. V+ABn for the HRC/F625W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.75: Extended source S/N vs. V+ABn for the HRC/F625W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F658N
Description
Ha filter.
Figure 10.76: Integrated system throughput for HRC/F658N.Figure 10.77: Point source S/N vs. V+ABn for the HRC/F658N filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.78: Extended source S/N vs. V+ABn for the HRC/F658N filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F660N
Description
NII filter.
Figure 10.79: Integrated system throughput for HRC/F660N.Figure 10.80: Point source S/N vs. V+ABn for the HRC/F660N filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.81: Extended source S/N vs. V+ABn for the HRC/F660N filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F775W
Description
Sloan Digital Sky Survey i filter.
Figure 10.82: Integrated system throughput for HRC/F775W.Figure 10.83: Point source S/N vs. V+ABn for the HRC/F775W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.84: Extended source S/N vs. V+ABn for the HRC/F775W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F814W
Description
Broad I filter.
Figure 10.85: Integrated system throughput for HRC/F814W.Figure 10.86: Point source S/N vs. V+ABn for the HRC/F814W filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.87: Extended source S/N vs. V+ABn for the HRC/F814W filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F850LP
Description
Sloan Digital Sky Survey z filter.
Figure 10.88: Integrated system throughput for HRC/F850LP.Figure 10.89: Point source S/N vs. V+ABn for the HRC/F850LP filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.90: Extended source S/N vs. V+ABn for the HRC/F850LP filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/F892N
Description
Methane filter.
Figure 10.91: Integrated system throughput for HRC/F892N.Figure 10.92: Point source S/N vs. V+ABn for the HRC/F892N filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.93: Extended source S/N vs. V+ABn for the HRC/F892N filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/G800L
Description
Grism.
Figure 10.94: Integrated system throughput for HRC/G800L.Figure 10.95: Point source S/N vs. V+ABn for the HRC/G800L filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.96: Extended source S/N vs. V+ABn for the HRC/G800L filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/PR200L
Description
HRC Prism.
Figure 10.97: Integrated system throughput for HRC/PR200L.Figure 10.98: Point source S/N vs. V+ABn for the HRC/PR200L filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.99: Extended source S/N vs. V+ABn for the HRC/PR200L filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.HRC/CLEAR
Description
HRC Clear Filter.
Figure 10.100: Integrated system throughput for HRC/ClearFigure 10.101: Point source S/N vs. V+ABn for the HRC/Clear filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.102: Extended source S/N vs. V+ABn for the HRC/Clear filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.SBC/F115LP
Description
MgF2 filter.
Figure 10.103: Integrated system throughput for SBC/F115LP.Figure 10.104: Point source S/N vs. V+ABn for the SBC/F115LP filter.Top curves are for low sky; bottom curves are for average sky.Figure 10.105: Extended source S/N vs. V+ABn for the SBC/F115LP filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.SBC/F122M
Description
Lyman a filter.
Figure 10.106: Integrated system throughput for SBC/F122M.Figure 10.107: Point source S/N vs. V+ABn for the SBC/F122M filter.Top curves are for low sky; bottom curves are for average sky.Figure 10.108: Extended source S/N vs. V+ABn for the SBC/F122M filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.SBC/F125LP
Description
CaF2 filter.
Figure 10.109: Integrated system throughput for SBC/F125LP.Figure 10.110: Point source S/N vs. V+ABn for the SBC/F125LP filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.111: Extended source S/N vs. V+ABn for the SBC/F125LP filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.SBC/F140LP
Description
BaF2 filter.
Figure 10.112: Integrated system throughput for SBC/F140LP.Figure 10.113: Point source S/N vs. V+ABn for the SBC/F140LP filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.114: Extended source S/N vs. V+ABn for the SBC/F140LP filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.SBC/F150LP
Description
Crystal Quartz filter.
Figure 10.115: Integrated system throughput for SBC/F165LP.Figure 10.116: Point source S/N vs. V+ABn for the SBC/F150LP filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.117: Extended source S/N vs. V+ABn for the SBC/F150LP filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.SBC/F165LP
Description
Dynasil filter.
Figure 10.118: Integrated system throughput for SBC/F165LP.Figure 10.119: Point source S/N vs. V+ABn for the SBC/F165LP filter.Top curves are for low sky; bottom curves are for average sky.Figure 10.120: Extended source S/N vs. V+ABn for the SBC/F165LP filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.SBC/PR110L
Description
LiF2 Prism.
Figure 10.121: Integrated system throughput for SBC/PR110LP.Figure 10.122: Point source S/N vs. V+ABn for the SBC/PR110LP filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.123: Extended source S/N vs. V+ABn for the SBC/PR110LP filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.SBC/PR130L
Description
CaF2 Prism.
Figure 10.124: Integrated system throughput for SBC/PR130LP.Figure 10.125: Point source S/N vs. V+ABn for the SBC/PR130LP filter. Top curves are for low sky; bottom curves are for average sky.Figure 10.126: Extended source S/N vs. V+ABn for the SBC/PR130LP filter. Top curves are for low sky and bottom curves are for average sky for a 1 arcsec2 area.Table 10.1: Color corrections ABn to go from Johnson V magnitude to AB magnitude for the WFC.Table 10.2: Color corrections ABn to go from Johnson V magnitude to AB magnitude for the HRC.Table 10.3: Color corrections ABn to go from Johnson V magnitude to AB magnitude for the SBC.10.3 Distortion in the ACS
The ACS detectors exhibit more distortion than previous HST instruments. The principal reason for this is that the optics have been designed with a minimum number of components, consistent with correcting for the spherical aberration induced by the OTA, without introducing coma. The result is a high throughput, but focal surfaces far from normal to the principal rays. The WFC detector is tilted at 22 deg giving an elongation of 8% while the HRC and SBC have a 25× tilt leading to an elongation of 12%. In each case, the scales in arcseconds per pixel are smaller along the radial direction of the OTA (Optical Telescope Assembly) field of view than along the tangential direction.
The orientations of the ACS detector edges are approximately in line with the V2 and V3 coordinate axes of the telescope. Consequently, the eigenaxes of the scale transformation are along the diagonals for WFC, and the apertures and pixels appear non-rectangular in the sky projection. For the HRC and SBC the situation is even more irregular because the aperture diagonals do not lie along a radius of the HST field of view. Figure 7.8 shows the ACS apertures in the telescope’s V2V3 reference frame. For a telescope roll angle of zero this would correspond to an on-sky view with the V3 axis aligned with North and the V2 with East.
There is not only a strong geometric distortion of ACS detectors but a significant variation of the scale across each detector. For the WFC the scale is changing in amount of 10% from corner to corner. For the HRC and SBC this variation is only about 1% as they cover much smaller fields of view. The area on the sky covered by a WFC pixel varies by about 18% from corner to corner, corrections for which must be made in photometry of extended objects. This variation of scale creates a problematic effect in combining ACS images by the fact that an integral pixel shift near the center of the detector will translate into a non-integral displacement for pixels near the edges. This will imply some computational complexity in accurate alignment in order to combine images and will depend on the accuracy of the geometric distortion model.
Accurate geometric distortion corrections for the WFC and HRC detectors by Anderson & King (Anderson & King, 2002,2004,2006) were derived from observations of the globular cluster 47 Tuc, with multiple pointings and orientations, and through the F475W filter. The geometric distortion models for each of the WFC chips and the HRC detector are expressed in a 4th order polynomial and filter dependent look-up tables. The solution for both cameras is accurate to 0.01 pixels. The coefficients of the 4th order polynomial the filter dependent look-up tables as well as correction images are installed in the ACS on-the-fly re-calibration (OTFR) pipeline and could be used independently in IRAF task CALACS.
Additionally, an area of the open cluster NGC188, for which accurate astrometry is available, was used to establish the exact location and orientation of the aperture in telescope coordinates. At the same time, the scale factors were confirmed.
For the SBC, the geometric distortion was derived using the observation of globular cluster NGC6681 taken through F125LP. The alignment was established by observing the same globular cluster with the HRC and SBC consecutively to establish the relative locations. The SBC position was derived from the HRC position.
10.3.1 WFC
The rhombus shape of the WFC is evident in Figure 7.8. The angle between the X and Y axes is 84.9× for WFC1 and 86.1× for WFC2. The geometric distortion map for WFC1 and WFC2 is illustrated in Figure 10.127, a vector diagram shows the contribution of the non-linear part of a quadratic fit only. The size of the residuals are scaled by a factor of 5 relative to the sky coordinates and could reach the residuals at about 4.1 arcseconds or 82 ACS/WFC pixels. At the center of chip WFC1 the scale in the X direction is 0.0493 arcseconds per pixel, and 0.0486 arcseconds per pixel in the Y direction. In the case of WFC2, the scale is 0.0498 arcseconds per pixel, and 0.0503 arcseconds per pixel in X and Y direction respectively. Between the corner of WFC nearest to the V1 axis and the diagonally opposite corner, the scale increases by 10%. Because of that WFC1 forms a slightly distorted rectangle 201 by 100 arcseconds in size, while WFC2 is 203 by 103 arcseconds. There is a 2.5 arcsecond gap between the two chips.
As it has been shown by Anderson (2007), the linear terms of the ACS/WFC geometric distortion are changing over time and distortion-corrected positions could be off by up to 0.3 pixels from the beginning of ACS observation.
Geometric distortion affects not only the astrometry but the photometry as well, since it induces an apparent variation in surface brightness across the field of view. In order to preserve the photometric accuracy, an additional correction to the photometry is required, by multiplying the ACS/WFC flat-fielded images by the pixel area map. The effective area of each pixel is shown in Figure 10.128 as a contour plot. The range of area is from 0.89 to 1.08 time to the central value.
Figure 10.127: The geometric distortion map for the ACS/WFC, which shows only the non-linear component to the solution. Note that this figure is rotated 180× with respect to the pipeline calibration products, where WFC2 is the lower half of the detector.10.3.2 HRC
The High Resolution Channel has its edges aligned approximately along the V2 and V3 axes. In this case, the center of the aperture lies on a line passing through the V2V3 origin and making an angle of 22× with the V3 axis. The diagonal of the aperture does not correspond to a radius of the HST field of view. So the distortion has no particular symmetry with respect to the detector axes. The focal plane of HRC is also 25× away from the plane normal to the light path, and because of this the scales along the axes differ by 14%. The full field of view of the HRC is less than 30 arcseconds, therefore the scale variation over the field is much less than for the WFC and it is about 1%. At the center, the X and Y scales are 0.0284 and 0.0248 arcseconds/pixel respectively. The average scales across the middle of the detector are 0.02842 and 0.02485 arcseconds/pixel making the X and Y widths 29.1 and 25.4 arcseconds. The slightly non-square projected aperture shape is evident in Figure 7.8. The angle between the X and Y axes on the sky is 84×.2. The geometric distortion map for HRC is given in Figure 10.129, where the residuals from the non-linearity are scaled by a factor of 10 relative to the sky coordinates and could reach the residuals at about 0.14 arcseconds, or 4.9 ACS/HRC pixels.
The same as for the WFC, geometric distortion affects not only the astrometry but the photometry as well, and a correction for the pixel area is required to restore the proper total counts of the target. The effective area of each pixel is shown in Figure 10.130 as a contour plot. The maximum deviation from the central value is about 3%.
Figure 10.128: The map of the effective pixel areas of the ACS/WFC chips.10.3.3 SBC
The Solar Blind Channel contains the MAMA detector. It is centered close to the HRC position in the V2V3 plane and has a slightly larger field of view, about 35 by 31 arcseconds. The scales and distortions have now been measured directly. The maximum distortion displacement is about 2 pixels or 0.06 arcseconds. Figure 10.131 shows the distortion map for the SBC detector.
The HRC and SBC both have much smaller areas than the WFC. In the X direction the scale is 0.0338 arcseconds/pixel while in the Y direction it is 0.0301 arcseconds/pixel. Like the HRC, the SBC exhibits a 13% difference between X and Y scales with a variation across the aperture of a little over 2%. The same as for the other cameras, due to geometric distortion, the photometry is also affected by a variation in pixel size across the SBC. The effective area for each pixel is shown in Figure 10.131 as a contour plot. The maximum deviation from the central value is just over 2%. (Gilliland et.al. 2007)
Figure 10.131: The map of the effective pixel areas of the SBC. The areas are normalized to 0.025 arcsecond square pixels.
Figure 10.129: The geometric distortion map for the HRC.Figure 10.130: The map of the effective pixel areas of the HRC.Figure 10.131: The geometric distortion map for the ACS/SBCFigure 10.132: The map of the effective pixel areas of the ACS/SBC
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