Advanced Camera for Surveys Instrument Handbook for Cycle 14
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| Advanced Camera for Surveys Instrument Handbook for Cycle 14 | |||||
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10.1 IntroductionChapter 10:
Imaging Reference Material
10.2 Using the Information in this Chapter
10.2.1 Sensitivity Units and Conversions
10.2.2 Signal-To-Noise
10.2.3 Point Spread Functions
WFC/F435W
WFC/F475W
WFC/F502N
WFC/F550M
WFC/F555W
WFC/F606W
WFC/F625W
WFC/F658N
WFC/F660N
WFC/F775W
WFC/F814W
WFC/F850LP
WFC/G800L
WFC/CLEAR
HRC/F220W
HRC/F250W
HRC/F330W
HRC/F344N
HRC/F435W
HRC/F475W
HRC/F502N
HRC/F550M
HRC/F555W
HRC/F606W
HRC/F625W
HRC/F658N
HRC/F660N
HRC/F775W
HRC/F814W
HRC/F850LP
HRC/F892N
HRC/G800L
HRC/PR200L
HRC/CLEAR
SBC/F115LP
SBC/F122M
SBC/F125LP
SBC/F140LP
SBC/F150LP
SBC/F165LP
SBC/PR110L
SBC/PR130L
10.3 Distortion in the ACS
10.3.1 WFC
10.3.2 HRC
10.3.3 SBC
10.3.4 Summary
In this Chapter, we provide imaging reference material, in support of the information presented in Chapter 6.
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 arcsec2 extended source.
- Tables of color corrections AB
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" on page 102 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 per second per cm2 of telescope area relative to the incident flux in photons per cm2 per 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). Comparisons of the throughput between ACS and the other instruments can be found in Which Instrument to Use?.
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.
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 arcsec2.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 5x5 pixels has been used for the WFC, 9x9 pixels for HRC and 15x15 pixels for the SBC S/N evaluation. For extended sources a 1 arcsec2 aperture was used. For the CCD the read noise has been computed assuming a number of readouts
NREAD= integer (t / 1000 s), where t is the exposure time, with a minimumNREAD=2. That is, each exposure has a minimumCR-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 sec-1 pix-1 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 Calculatorshould 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 and find AB
- Alternatively, compute ABMAG (=V+AB
, or
.
- Find the appropriate plot for the filter in question, and locate V+AB
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 0.1s, although the minimum exposure time for the WFC CCD channel is 0.5s.
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.
WFC/F435W
Description
Figure 10.1:Integrated System Throughput for WFC/F435W
Figure 10.2: Point Source S/N vs. V+AB
Figure 10.3: Extended Source S/N vs. V+AB
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+AB
Figure 10.6: Extended Source S/N vs. V+AB
WFC/F502N
Description
Figure 10.7: Integrated System Throughput for WFC/F502N
Figure 10.8: Point Source S/N vs. V+AB
Figure 10.9: Extended Source S/N vs. V+AB
WFC/F550M
Description
Figure 10.10: Integrated System Throughput for WFC/F550M
Figure 10.11: Point Source S/N vs. V+AB
Figure 10.12: Extended Source S/N vs. V+AB
WFC/F555W
Description
Figure 10.13: Integrated System Throughput for WFC/F555W
Figure 10.14: Point Source S/N vs. V+AB
Figure 10.15: Extended Source S/N vs. V+AB
WFC/F606W
Description
Figure 10.16: Integrated System Throughput for WFC/F606W
Figure 10.17: Point Source S/N vs. V+AB
Figure 10.18: Extended Source S/N vs. V+AB
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+AB
Figure 10.21: Extended Source S/N vs. V+AB
WFC/F658N
Description
Figure 10.22: Integrated System Throughput for WFC/F658N
Figure 10.23: Point Source S/N vs. V+AB
Figure 10.24: Extended Source S/N vs. V+AB
WFC/F660N
Description
Figure 10.25: Integrated System Throughput for WFC/F660N
Figure 10.26: Point Source S/N vs. V+AB
Figure 10.27: Extended Source S/N vs. V+AB
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+AB
Figure 10.30: Extended Source S/N vs. V+AB
WFC/F814W
Description
Figure 10.31: Integrated System Throughput for WFC/F814W
Figure 10.32: Point Source S/N vs. V+AB
Figure 10.33: Extended Source S/N vs. V+AB
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+AB
Figure 10.36: Extended Source S/N vs. V+AB
WFC/G800L
Description
Figure 10.37: Integrated System Throughput for WFC/G800L
Figure 10.38: Point Source S/N vs. V+AB
Figure 10.39: Extended Source S/N vs. V+AB
WFC/CLEAR
Description
Figure 10.40: Integrated System Throughput for WFC/Clear
Figure 10.41: Point source S/N vs. V+AB
Figure 10.42: Extended source S/N vs. V+AB
HRC/F220W
Description
Figure 10.43: Integrated System Throughput for HRC/F220W
Figure 10.44: Point source S/N vs. V+AB
Figure 10.45: Extended source S/N vs. V+AB
HRC/F250W
Description
Figure 10.46: Integrated System Throughput for HRC/F250W
Figure 10.47: Point Source S/N vs. V+AB
Figure 10.48: Extended Source S/N vs. V+AB
HRC/F330W
Description
Figure 10.49: Integrated System Throughput for HRC/F330W
Figure 10.50: Point Source S/N vs. V+AB
Figure 10.51: Extended Source S/N vs. V+AB
HRC/F344N
Description
Figure 10.52: Integrated System Throughput for HRC/F344N
Figure 10.53: Point Source S/N vs. V+AB
Figure 10.54: Extended Source S/N vs. V+AB
HRC/F435W
Description
Figure 10.55: Integrated System Throughput for HRC/F435W
Figure 10.56: Point Source S/N vs. V+AB
Figure 10.57: Extended Source S/N vs. V+AB
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+AB
Figure 10.60: Extended Source S/N vs. V+AB
HRC/F502N
Description
Figure 10.61: Integrated System Throughput for HRC/F502N
Figure 10.62: Point Source S/N vs. V+AB
Figure 10.63: Extended Source S/N vs. V+AB
HRC/F550M
Description
Figure 10.64: Integrated System Throughput for HRC/F550M
Figure 10.65: Point Source S/N vs. V+AB
Figure 10.66: Extended Source S/N vs. V+AB
HRC/F555W
Description
Figure 10.67: Integrated System Throughput for HRC/F555W
Figure 10.68: Point Source S/N vs. V+AB
Figure 10.69: Extended Source S/N vs. V+AB
HRC/F606W
Description
Figure 10.70: Integrated System Throughput for HRC/F606W
Figure 10.71: Point Source S/N vs. V+AB
Figure 10.72: Extended Source S/N vs. V+AB
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+AB
Figure 10.75: Extended Source S/N vs. V+AB
HRC/F658N
Description
Figure 10.76: Integrated System Throughput for HRC/F658N
Figure 10.77: Point Source S/N vs. V+AB
Figure 10.78: Extended Source S/N vs. V+AB
HRC/F660N
Description
Figure 10.79: Integrated System Throughput for HRC/F660N
Figure 10.80: Point Source S/N vs. V+AB
Figure 10.81: Extended Source S/N vs. V+AB
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+AB
Figure 10.84: Extended Source S/N vs. V+AB
HRC/F814W
Description
Figure 10.85: Integrated System Throughput for HRC/F814W
Figure 10.86: Point Source S/N vs. V+AB
Figure 10.87: Extended Source S/N vs. V+AB
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+AB
Figure 10.90: Extended Source S/N vs. V+AB
HRC/F892N
Description
Figure 10.91: Integrated System Throughput for HRC/F892N
Figure 10.92: Point Source S/N vs. V+AB
Figure 10.93: Extended Source S/N vs. V+AB
HRC/G800L
Description
Figure 10.94: Integrated System Throughput for HRC/G800L
Figure 10.95: Point Source S/N vs. V+AB
Figure 10.96: Extended Source S/N vs. V+AB
HRC/PR200L
Description
Figure 10.97: Integrated System Throughput for HRC/PR200L
Figure 10.98: Point Source S/N vs. V+AB
Figure 10.99: Extended Source S/N vs. V+AB
HRC/CLEAR
Description
Figure 10.100: Integrated System Throughput for HRC/Clear
Figure 10.101: Point Source S/N vs. V+AB
Figure 10.102: Extended Source S/N vs. V+AB
SBC/F115LP
Description
Figure 10.103: Integrated System Throughput for SBC/F115LP
Figure 10.104: Point Source S/N vs. V+AB
Figure 10.105: Extended Source S/N vs. V+AB
SBC/F122M
Description
Figure 10.106: Integrated System Throughput for SBC/F122M
Figure 10.107: Point Source S/N vs. V+AB
Figure 10.108: Extended Source S/N vs. V+AB
SBC/F125LP
Description
Figure 10.109: Integrated System Throughput for SBC/F125LP
Figure 10.110: Point Source S/N vs. V+AB
Figure 10.111: Extended Source S/N vs. V+AB
SBC/F140LP
Description
Figure 10.112: Integrated System Throughput for SBC/F140LP
Figure 10.113: Point Source S/N vs. V+AB
Figure 10.114: Extended Source S/N vs. V+AB
SBC/F150LP
Description
Figure 10.115: Integrated System Throughput for SBC/F165LP
Figure 10.116: Point Source S/N vs. V+AB
Figure 10.117: Extended Source S/N vs. V+AB
.
SBC/F165LP
Description
Figure 10.118: Integrated System Throughput for SBC/F165LP
Figure 10.119: Point Source S/N vs. V+AB
Figure 10.120: Extended Source S/N vs. V+AB
SBC/PR110L
Description
Figure 10.121: Integrated System Throughput for SBC/PR110LP
Figure 10.122: Point Source S/N vs. V+AB
Figure 10.123: Extended Source S/N vs. V+AB
SBC/PR130L
Description
Figure 10.124: Integrated System Throughput for SBC/PR130LP
Figure 10.125: Point Source S/N vs. V+AB
Figure 10.126: Extended Source S/N vs. V+AB
Table 10.1: Color Corrections AB
Table 10.2: Color Corrections AB
Table 10.3: Color Corrections AB
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 degrees giving an elongation of 8% while the HRC and SBC have a 25 degree 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 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 8.7 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.
If these were the only distortions they would not really present much difficulty. Their impact on photometry and mosaicing or dithering could be simply computed. A more problematic effect is the variation of scale across each detector. For the WFC this amounts to a change 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, allowance for which must be made in photometry of extended objects. Dithering and mosaicing are complicated 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. Even this is not a fundamental difficulty, but will imply some computational complexity in registering images and will depend on an accurate measurement of distortions.
The results presented here are derived from on-sky measurements. For WFC and HRC multiple pointings of 47 Tucanae were taken through the F475W filter, resulting in several thousand star location measurements. These were analyzed to express the distortion as quartic polynomials. The residuals of the fit to these polynomials are of order 0.2 pixels, much larger than the errors. Correction images describing these residuals are now supplied and used in CALACS to bring the final errors down to about 0.01 pixels. Additionally, an area of open cluster NGC 188, for which astrometric data are 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 distortion measurement used the target NGC 6681 and filter F125LP. The alignment was established by observing this same target with the HRC and SBC consecutively to establish the relative locations. The SBC position was thereby derived from the HRC position.
10.3.1 WFC
The rhombus shape of the WFC is evident in Figure 8.7. The angle between the x and y axes is 84.9 for WFC1 and 86.1 for WFC2. The WFC distortion is illustrated in Figure 10.127, a vector displacement diagram which shows the contribution of the non-linear part of a quadratic fit to the data. The vectors represent the displacements and have been scaled up by a factor of 5 for display. The corner displacements are about 82 pixels. The principal effect is the diagonal variation of scale.
At the center of chip WFC1 the scale in the x direction is 0.0493 arcsec/pixel and 0.0486 in the y direction. For WFC2 these figures are 0.0498 and 0.0503. Between the corner of WFC nearest to the V1 axis and the diagonally opposite corner, the overall scale increases by 10%. 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.
Figure 10.127: Non-linear component of the WFC Distortion. Note that this figure is rotated 180o with respect to the pipeline calibration products, where WFC2 is the lower half of the detector.
The resulting variation of the projected pixel area on the sky requires corrections to the photometry of point objects. A contour plot of relative pixel size across the WFC normalized to the central pixel, is shown in Figure 10.128. The range of area is from 0.89 to 1.08 times the central value.
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 degrees 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. Again, because the focal plane, and therefore the detector plane is 25 degrees away from the plane normal to the light path, the scales along the axes differ by 14%. However, since the HRC is less than 30 arcsec across, the scale variation over the field is much less than for the WFC, being about 1%. At the center the x and y scales are 0.0284 and 0.0248 arcsec/pixel respectively. The average scales across the middle of the detector are 0.02842 and 0.02485 arcsec/pixel making the x and y widths 29.1 and 25.4 arcsec. The slightly non-square projected aperture shape is evident in Figure 8.7. The angle between the x and y axes on the sky is 84.2 degrees. A vector plot of the deviation from linearity is given in Figure 10.129 in which the deviations have been magnified by a factor of 10 for illustrative purposes. The largest deviation is 4.9 pixels in the top left corner and corresponds to about 0.1 arcsec. The variation of pixel size across the HRC to be used for photometric correction of point sources is shown in Figure 10.130. The maximum deviation from the central value is just over 2%.
Figure 10.128: Variation of the WFC effective pixel area with position in detector coordinates.
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. The HRC and SBC both have much smaller areas than the WFC. In the x direction the scale is 0.0338 arcsec/pixel while in the y direction it is 0.0301. 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%.
Figure 10.129: HRC Distortion
Figure 10.130: Variation of the HRC effective pixel area with position in detector coordinates.
10.3.4 Summary
All values presented here are based on in-flight measurements taken during SMOV. Formerly they were based on Ball Aerospace optical simulator measurements. The new values are quite close to the previously published numbers in that scale factors have changed by less than 1% and the aperture locations moved by only a few arcseconds.
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