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A description is provided of the overall performance of the STIS CCD after the HST Servicing Mission #4. Most aspects of CCD performance are found to be consistent with extrapolations of the trends seen prior to the failure of STIS in August 2004. However, over the 4.5 years since STIS was last operational, some CCD parameters have changed significantly. The gain values for the CCDGAIN = 4 setting has been re-determined; the new value of 4.016 e-/DN is about 0.5% lower than the value that was previously adopted. The Read Noise through amp D, as determined from unbinned bias images, has increased; it now has a value of 5.6 e- for gain = 1 and 8.3 e- for gain = 4, up from previous values of 5.4 e- and 7.7 e-, respectively. The dark current has increased significantly, as was expected due to the on-orbit radiation environment. The dark current has increased from 0.0070 to 0.0134 e-/pixel/s (for a reference temperature of 22 C) in the center of the chip - roughly a factor of 2 increase. Additionally, the slope in the dark current along CCD columns has increased by a similar factor, resulting in a dark current near the detector readout amplifier that is about 30% lower than the median over the whole detector. The charge transfer inefficiency (CTI) has increased by a factor of about 2 as well, although the increased dark current will significantly moderate the actual CTI losses for typical observations. Currently measured values for
CTI as well as for time-dependent spectroscopic sensitivity are in excellent agreement with a simple extrapolation of trends seen prior to the 2004 failure.
As a result of the STIS CCD being exposed to radiation damage the dark current has increased significantly - by roughly a factor of 2. Since there is no thermistor for STIS Side-2 electronics all darks are scaled to a reference temperature. The reference temperature used for normalizing the dark current values reported here is 22 C. The dark current is calculated by taking the median within a 200x200 pixel area near the top and center of the CCD chip. The dark current values can be found in Table 1. Note that the center of the chip dark current (which is consistent with that of the full CCD area) is increasing faster than at the top of the chip i.e., there is a slope associated with the dark current along the CCD columns that is increasing with time. Evidence of this slope can be found in Figure 1. Note that only superdarks for
Side-2 electronics are used in estimating the dark current and that only the data after 2004 is used in the linear fits with the exception of SM4 data. Note that the linear extrapolation for the top of the chip fits the post-SM4 data reasonably well, while on the other hand the center of the chip fall below the post-SM4 data.
Figure 4: The evolution of the read noise for gain = 1 and gain = 4 is shown. The read noise is calculated from unbinned pairs of bias images.
Date
Figure 1: This represents the growth of the median dark current as a function of time. The diamonds are for the median dark current in 200x200 pixel area near the top of the chip, while the asterisks represent the same except in the center of the chip. The dark current has been normalized to a CCD housing temperature of 22 C.
Annealing for the STIS CCD is done approximately every month and measurements are made at 5 different count rate levels. As shown in Figure 2, the
Side-1 hot pixel growth rate is higher than for Side-2 electronics. The likely reason for this is that the CCD is being kept at a colder temperature for Side-2 electronics on average, since the Side-2 electronics feature a higher
(constant) current than the average current for Side-1 electronics. The cooler CCD after the switch to the Side-
2 electronics also caused the number of hot pixels to decrease for a short period of time. However, for post-
SM4 data the number of hot pixels is higher than predicted if a linear trend is extrapolated from prefailure Side-2 data. It should be noted that appropriate dithering of CCD observations are increasingly important as the number of constantly hot pixels with intensities
0.1 e – /s is currently 3.4% of all pixels.
Figure 2: Represented here is the number of post-anneal hot pixels as a function of time. The arrows in the plot delineate the number of hot pixels associated with Side-1 and Side-2 electronics. Note that for Side-2 the growth rate of hot pixels has declined as compared to Side-1. Additionally, the SMOV data show that the number of hot pixels is higher than expected when using a linear extrapolation from Side-2 data.
Spurious charge is a feature of the STIS CCD that can be found along columns (i.e., in the parallel clocking direction; Goudfrooij & Walsh 1997). This feature is essentially a position-dependent addition to the background and is inserted during pre-flush and readout when a small amount of charge is left behind on the detector during each clocking. Due to radiation damage the amplitude and slope of this structure has increased with time: More charge traps are accumulating in the CCD, thereby injecting extra amounts of spurious charge. This effect causes a ramp along the columns whose slope increases with the number and intensity of hot pixels. Figure 3 shows the increase in spurious charge and Table 1 contains approximate values derived from the plot. The spurious charge is plotted as a function of time with values derived from all (weekly) superbias reference files. The spurious charge values are quoted for the center and top of the STIS CCD.
Figure 3: Shown here is the spurious charge for gain =
1 and for two positions on the detector. See the legend in the upper left in the figure.
The gain for the gain = 1 setting has stayed consistent with pre-failure data and with the ground calibration. However, the gain for gain = 4 has been remeasured and the new value is different by about 0.5% from 4.036 e-/DN to 4.016 e-/DN. This was derived using spectra of the flux standard star AGK+81D266 in both gain settings. The ratio of the gains (gain4/gain1) is plotted versus pixel number along the dispersion after correction for CTE (see below) and from the normalization constant the value for gain = 4 is determined. These values can also be found in Table 1.
As the STIS CCD ages, radiation damage produces an increased number of charge traps which decrease Charge Transfer Efficiency (CTE). It is often more useful to think in terms of Charge Transfer
efficiency (CTI), which is defined as CTI
1 - CTE.
This effect manifests itself as a loss of signal upon read out and this loss of signal varies with the distance from the read-out amplifier. The more charge traps that are encountered by the signal during read-out, the more signal loss occurs. Two CTI tests were incorporated in determining the CTI: The Extended Pixel Edge Test
Response (EPER) and the Internal Sparse Field Test.
This test does not require any specialized equipment (Janesick et al. 1991) and uses an evenly illuminated (flat-field) exposure. The test then measures the charge in the overscan region of the CCD after subtracting the overscan bias level. The CTI is then a measure of excess charge that remains, which is a manifestation of CTE loss. The CTI is then estimated from the equation:
CTI
EPER
= S
D
/(S
LC
N
P
) (1) where S
D
is the total deferred charge (e-) in the overscan region, S
LC
is the charge level in e- of the last illuminated column or row, and N
P
is the number of pixel transfers
(Janesick 2001). There are three main points of this analysis that can be derived from Figure 5:
1) Trends in time can be clearly determined from the high accuracy of the EPER CTI measurements.
The read noise for gain =1 and gain = 4 is derived from pairs of unbinned bias frames being readout with the default amplifier D. To eliminate the effect of the 2-D structure in the bias signal of the STIS CCD, two bias images (taken in the same observing visit) are subtracted from one another. The read noise is then calculated from the difference image. Several sub-sections of the differenced bias image are used in the calculation to avoid systematic issues. Figure 4 illustrates the time dependence of the read noise values for gains 1 and 4. Values for the read noise can be found in Table 1.
Figure 5: Shown is the parallel EPER CTI as a function of time as measured from spectral flat field images.
2) Right after SM3A (epoch 2000.0) a clear change in the slope of the time dependence for the EPER CTI occurred.
3) Finally, EPER CTI values derived from the post-SM4 data are substantially higher than the prediction based on a linear extrapolation of the 2000 – 2004 data.
The cause of the dramatic increase in CTI is likely temperature related. The operating temperature of the
STIS CCD (~ -83 C) falls within the range where there is a strong dependence of CTI on temperature (see Goudfrooij et al. 2009 and references therein). This temperature dependence is seen with cooled CCDs that have a parallel clocking period of
100
s. The STIS CCD has a parallel clocking period of 23.2 ms and studies of the STIS CCD indicate that the steepest part of the slope is thought to be at
~ -80 C. Figure 6 shows this behavior as can be seen in the sharp increase of EPER CTI in the (relatively warm) post-
SM4 data versus pre-failure data.
Figure 6: Parallel CTI taken after July 2001 plotted against the CCD housing temperature. The post-SM4 data points are shown in red. Note that a substantial temperature dependence for the CTI is indicated, especially for CCD housing temperatures
21 C.
This test measures charge loss within an extraction aperture in spectroscopic mode. This test has been implemented on the STIS CCD with on-orbit as well as ground calibration data. From this test a time dependence can be derived (for details please see the attached Goudfrooij et al. 2009 report and references therein). Equation 2 depicts the CTI time dependence:
CTI(t) = CTI
0
[1 +
(t - t
0
)] (2) where CTI(t) is the CTI at a given time, CTI
0
is the CTI extrapolated to zero background,
is the slope of the time dependence, t is the time (in years), and t
0
is 2000.6. The results from this test can be found in Figure 7. Note that the fits to the data are excellent. The linear fit for the 3450 e – level was chosen for the representative slope value (value is in Table 2) because there is pre-flight data for this intensity level (only). The results are shown for gain = 1.
Figure 7: CTI extrapolated to zero background as function of time. The legend shows which symbol/color corresponds to which signal level. Solid lines show linear fits to the data, whereas dotted lines show the prediction of the CTI formula used in the STIS pipeline for CCD spectroscopy.
The insertion of STIS into HST during SM2 in March
1997 is delineated by the dotted vertical line.
The flux standard star AGK+81D266 is used in studying the time dependence of the sensitivity of the STIS
CCD using the 52x2 arcsec slit. The loss of sensitivity can be attributed mainly to optics contamination. Figure 8 illustrates the time dependence on sensitivity after taking the post-SM4 data into account. Note that CTI and CCD temperature corrections have been accounted for in the plots. Furthermore, trends have essentially followed the pre-failure data.
Figure 8: The relative sensitivity as a function of time is shown for the three first-order CCD gratings (G230LB,
G430L, and G750L).
Table 1: The values quoted here correspond to various CCD parameters from pre- and post-failure analysis. The first column lists the parameter names, the second column lists the prefailure values, and the third column lists the post-failure values.
Dark Current
(Top of Chip)
Dark Current
(Center of Chip)
Spurious Charge
(Top of Chip)
Spurious Charge
(Center of Chip)
Read Noise GN = 1
Read Noise GN = 4
CTI Slope
GN = 1
GN = 4
≈
≈
0.0043 e-/pixel/s
0.0070 e-/pixel/s
0.55 e-/pixel
0.75 e-/pixel
5.4 e-
7.7 e-
0.218 %/year
1.0 e-/DN
4.036 e-/DN
0.0095 e-/pixel/s
0.0134 e-/pixel/s
≈
≈
0.78 e-/pixel
1.2 e-/pixel
5.6 e-
8.3 e-
0.216 %/year
1.0 e-/DN
4.016 e-/DN
Goudfrooij, P., & Walsh, J. R., 1997, STIS Instrument
Science Report 1997-09 (Baltimore: STScI)
Goudfrooij, P., et al. 2009, STIS Instrument Science
Report 2009-02 (Baltimore: STScI)
Janesick, J. R., Soli, G., Eloit, T., & Collins, S., 1991,
“The Effects of Proton Damage on Charge-
Coupled Devices”, in
1447, 87
Janesick, J.R., 2001, “Scientific Charge-Coupled
Devices” (Bellingham: SPIE Press)