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1024 × 1024 illuminated pixels
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∼ 20% @ 3000 Å ∼ 67% @ 6000 Å ∼ 29% @ 9000 Å
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0.016 e−/s/ pix (but varies with detector T)
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5.62 e− rms at GAIN=1 (1 e − of which is pattern noise) 8.0 e − rms at GAIN=4 (0.2 e − of which is pattern noise)
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144,000 e− over the inner portion of the detector 120,000 e − over the outer portion of the detector
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First, the Side-2 electronics do not have a working CCD temperature controller, and the detector can no longer be held at a fixed temperature. As a result, the CCD dark current now fluctuates with the detector temperature. Fortunately, the dark current variation correlates well with the CCD housing temperature, with the dark current varying by ~ 7%/°C. Further details are given in STIS ISR 2001-03 and references therein. All reference file dark images prepared for use with STIS/CCD data taken during Side-2 operations are now scaled to a standard housing temperature of 18°C before being delivered. Before subtraction of the appropriate dark file from Side-2 CCD data, the
calstis package rescales the dark using the CCD housing temperature value in the
OCCDHTAV keyword which is found in the extension header of each sub-exposure.
The second change is an increase in the read noise by about 1 electron when CCDGAIN=1 and by 0.2 electrons when
CCDGAIN=4. This extra read noise appears in the form of coherent pattern noise, and under some circumstances it may be possible to ameliorate this noise by using Fourier filtering techniques. See
STIS ISR 2001-05 for additional discussion.
The coherent pattern noise appears to be unchanged in its amplitude and behavior, however, the overall readnoise of the STIS CCD has increased after the repair. During the period of Side-2 operations, the mean readnoise measured using the default readout amplifier with CCDGAIN=1 was 5.44 electrons; in the months after SM4 and during Cycle 17 calibrations the mean was 5.62 electrons. For
CCDGAIN=4, the readnoise has increased from 7.6 electrons to 8.0 electrons. Similar increases in readnoise were seen in the three other STIS CCD amplifiers. As Amp D continues to show the lowest readnoise, it remains the default amplifier and is used for all GO science exposures.
The spectral response of the unfiltered CCD is shown in Figure 5.1 (labeled as
50CCD). This figure illustrates the extremely wide bandpass over which this CCD can operate. The wide wavelength coverage is an advantage for deep optical imaging (although the Advanced Camera for Surveys and the Wide Field Camera 3 are better suited to most optical imaging programs). The NUV sensitivity of the CCD makes it a good alternative to the
NUV-MAMA for low- and intermediate-resolution spectroscopy from ~1700 to 3100 Å using the
G230LB and
G230MB grating modes (
Table 4.1).
Sensitivity variations in CCD spectroscopic configurations have been determined to be due primarily to increasing charge transfer efficiency (CTE) losses (see Section 7.3.7), temperature fluctuations since the switch to the Side-2 electronics (see
Section 7.2.2), and actual time-dependent changes in sensitivity. For a more detailed analysis of the STIS Sensitivity Monitor observations from 1997 through March 2004 please refer to
STIS ISR 2004-04. Sensitivity monitor measurements collected between March 2004 and STIS failure in August 2004 are consistent with the trends reported in this ISR.
Trends for time-dependent sensitivity (TDS) for the CCD low-resolution (L) modes are shown in Figure 7.1. Sensitivities measured after SM4 are consistent with an extrapolation of the trends seen before the 2004 failure. Consistent with pre-failure trends, the sensitivity demonstrates a temperature dependence of +0.30, +0.26, and +0.08%/°C for G230LB, G430L, and G750L, respectively (see
STIS ISR 2009-02). Selected wavelength settings of the medium-resolution (M) gratings
G230MB,
G430M, and
G750M have also been monitored. Sensitivity trends measured for the limited M-mode wavelength coverage are similar to those observed in the L-modes at corresponding wavelengths. The
G230LB and
G230MB CCD configurations exhibit behavior similar to that found for the
NUV-MAMA G230L mode (see
Section 7.4.3), featuring an increase in sensitivity during the first 1.5 years of STIS operations, followed by decreasing sensitivity, with a slow-down in the decline beginning in early 2002 (see
Figure 7.15).
TDS corrections for all CCD modes have been implemented into the STIS pipeline as new TDSTAB reference files (see Section 15.1) and will correct fluxes of extracted spectra for sensitivity changes to a typical accuracy of 1% or better. CTE corrections have also been implemented (see
Section 7.3.7). These new TDS trends have also been incorporated into reference files used by
synphot, pysynphot and the
STIS ETCs. This enables count rate predictions to take the sensitivity changes into account. The default TDS throughputs for
synphot, pysynphot, and
STIS ETC calculations are extrapolated to their values for a date of April 2014.
The amplitude of the fringes is a strong function of wavelength and spectral resolution. Table 7.2 lists the observed percentile peak-to-peak and rms amplitudes of the fringes as a function of central wavelength for the
G750M and
G750L gratings. The listed “peak-to-peak” amplitudes are the best measure of the impact of the fringing on your data. The rms values at wavelengths < 7000 Å give a good indication of the counting statistics in the flat-field images used for this analysis.
Table 7.3 compares the estimated peak-to-peak fringe amplitudes
after flat-fielding by the library flat and those after flat fielding with an appropriately processed contemporaneous flat. These estimates are based upon actual measurements of spectra of both point sources and extended sources made during Cycle 7 (the results for point sources and extended sources were essentially the same).
Figure 7.2 shows such a comparison for a
G750L spectrum of a white dwarf; in this figure, the top panel shows white dwarf GD153 (central wavelength 7751 Å) with no flat-field correction, the second spectrum shows the result of de-fringing with the standard pipeline flat field, and the third spectrum shows the result of de-fringing with a contemporaneous flat (all spectra were divided by a smooth spline fit to the stellar continuum). It is clear that a contemporaneous flat provides a great improvement over the use of a library flat. Therefore, if you are observing in the far red (> 7500 Å) and using grating
G750L or
G750M, you should take a contemporaneous flat field along with your scientific observations. More detailed information and analysis on fringe correction for STIS long-wavelength spectra can be found in
STIS ISR 1998-19,
STIS ISR 1998-29, and the references therein.
Examination of long slit observations in the CCD spectroscopic modes has revealed periodic variations of intensity along the slit when highly monochromatic, calibration lamp sources are used. An example of such ‘chevron-pattern’ variations is shown in Figure 7.3 and
Figure 7.4. These variations are thought to be the result of transmission variations through the highly parallel faces of the order sorting filters that are situated next to the gratings in the optical path. In the cross dispersion direction, the modulation amplitude depends on the line width with a maximum of 13% for a monochromatic source in
G430L and
G430M modes, and 4.5% in
G750L and
G750M modes. Periods range from 40 - 80 pix/cycle. In the dispersion direction, there is a small, residual, high frequency modulation with a peak amplitude of about 1.5% in
G430M at the 5471Å setting and with smaller amplitudes in all other modes and settings. No such modulation has been observed in any of the MAMA modes. Some modulation is also apparent in the CCD
G230LB and
G230MB modes, however the amplitudes of these modulations are much smaller.
Verification testing has shown that STIS meets its image-quality specifications. While the optics provide fine images at the focal plane, the detected point spread functions (PSFs) are degraded somewhat more than expected by the CCD at wavelengths longward of about 7500 Å, where a broad halo appears, surrounding the PSF core. This halo is believed to be due to scatter within the CCD mounting substrate, which becomes more pronounced as the silicon transparency increases at long wavelengths. The effects of the red halo (see Figure 7.5), which extend to radii greater than 100 pixels (5 arcseconds), are not included in the encircled energies as a function of observing wavelength that are described for the CCD spectroscopic and imaging modes in
Chapter 13 and
14, respectively. However, estimates of the encircled energy vs. radius that include the halo are shown in
Table 7.4. The integrated energy in the halo amounts to approximately 20% of the total at 8050 Å and 30% at 9050 Å (see also
STIS ISR 1997-13 for the implication for long-slit spectroscopic observations at long wavelengths). Note that the ACS WFC CCDs have a front-side metallization that reduces the large angle long wavelength halo problem in those detectors. This problem has been eliminated from the WFC3 CCD.
The CCD plate scale is 0.05078 arcsec/pix for imaging observations (see STIS ISR 2001-02), and in the spatial (across the dispersion) direction for spectroscopic observations. Due to the effect of anamorphic magnification, for spectroscopic observations the plate scale in the dispersion direction is slightly different and it depends on the grating used and its tilt. The plate scale in the dispersion direction ranges from 0.0512 to 0.0581 arcsec/pix (see
STIS ISR 1998-23).
The CCD detector produces a relatively faint, out-of-focus, ring-shaped “ghost” image, due to specular reflection from the CCD surface and window. The ring contains about 1% of the total energy in the image and is very stable. Additional rings of similar size can be seen at other locations in the field in grossly saturated images, but these contain only of order 10−5 of the total energy and are thus not likely to be detected in normal scientific images. Lines drawn from stars in images through their respective ghosts are found to converge at a “radiant point” located to the lower right of the image center. This effect is illustrated in
Figure 7.6 where the line segments are drawn from pixel coordinates (528, 342) (in 1024
× 1024 user coordinates) through the centroids of the brightest stars in the image. Note that these line segments intercept the centers of the ring-like ghosts very well. Observers who wish to avoid placing very faint objects within the range of the ghosts may want to take this geometry into account when writing Phase II submissions.
A full detector readout is 1062 × 1044 pixels including physical and virtual overscans. Scientific data are obtained on 1024
× 1024 pixels, each projecting to ~0.05
× 0.05 arcseconds on the sky. For spectroscopic observations, the dispersion axis runs along
AXIS1 (image X or along a row of the CCD), and the spatial axis of the slits runs along
AXIS2 (image Y or along a column of the CCD). The CCD supports the use of subarrays to read out only a portion of the detector, and on-chip binning. For more details see
Section 11.1.1.
Electrons that accumulate in the CCD wells are read out and converted to data numbers (DN, the format of the output image) by the analog-to-digital converter at a default CCDGAIN of 1 e
−/DN (i.e., every electron registers 1 DN). The CCD is also capable of operating at a gain of 4 e
−/DN
1. The analog-to-digital converter operates at 16 bits, producing a maximum of 65,536 DN/pix. This is not a limitation at either gain setting, because other factors set the maximum observable DN to lower levels in each case (
Section 7.3 below).
The CCDGAIN=1 setting has the lower read noise (
Table 7.1) and digitization noise. Although the read noise has increased since the switch to the Side-2 electronics in July 2001 (see
Section 7.2.2),
CCDGAIN=1 is still the most appropriate setting for observations of faint sources. However, saturation occurs at about 33,000 e
− at the
CCDGAIN=1 setting (as described in
Section 7.3 below).
The CCDGAIN=4 setting allows use of the entire CCD full well of 144,000 e
−, and use of the
CCDGAIN=4 setting is therefore recommended for imaging photometry of objects whenever more than 33,000 e
− might be obtained in a single pixel of an individual sub-exposure. However, short exposures taken in
CCDGAIN=4 show a large-scale pattern noise (“ripple”) that is not removed by the standard bias images. This pattern noise is in addition to the usual coherent noise visible since STIS switched to using the backup Side-2 electronics.
Figure 7.7 (a 0.2 second exposure of a lamp-illuminated small slit) shows an example of the
CCDGAIN=4 ripple. The peak-to-peak intensities of these ripples vary from near zero to about 1 DN, and there is a large amount of coherence in the noise pattern. This coherence makes background determination difficult and limits the precision of photometry of faint objects in shallow exposures taken using
CCDGAIN=4.
The CCD response when using CCDGAIN=4 remains linear even beyond the 144,000 e
− full well limit if one integrates over the pixels bled into (
Section 7.3.2), and for specialized observations needing extremely high S/N, this property may be useful.
e−/DN.
