calibration results by item detector hrc build1 cte AUTHOR : Michael R. Jones
DETECTOR : HRC Build 1 CCD detector (non-flight)
PURPOSE :
To monitor the CTE performance of the CCD detector as a function of temperature and signal level. The signal range and signal resolution of the CTE monitor test is less comprehensive than that of the thorough CTE test.
DATA ACQUISITION AND REDUCTION :
Parallel and serial CTE were measured as a function of signal level and temperature using the First Pixel Response (FPR) method. Data were obtained at five signal levels using the internal Tungsten 3 flat lamp, the F502N filter and the F625W filter. CTE monitor data were acquired at eight detector temperature set points. All data was read out through amplifier C at a gain setting of 1. The actual system GAIN=1.15 for amplifier C (in electrons/count) is reported in the HRC1 gain page. However in this report (written before those results were in - ed.) a system gain of 1 electron/count was assumed for signal level calibration. All signal levels in this report are therefore approximate rather than exact.
CTE monitor data sets were obtained in pairs at six of the eight detector temperature set points. One CTE monitor data set was acquired at the -67 °C and -90 °C set points. Individual FPR images were bias corrected and smoothed with a one-dimensional median filter to reject hot pixels, remove cosmic ray hits and reduce random noise. Data sets taken in pairs were combined by computing the straight pixel-by-pixel average of the individual median filtered frames. Neither flat field normalization to correct for pixel responsivity non-uniformity nor dark current subtraction to remove hot pixels was performed. It is anticipated that both corrections, when applied to data acquired on-orbit, will improve the quality of the FPR CTE measurement.
Some of the FPR images exhibited short, low level, row-aligned streaking noise similar to that observed in the WFC Build 1 detector. A number of data points, particularly in the lowest signal level data set, were rejected because of poor S/N. The poor S/N was partially the result of contamination by the streaking noise. Data points that yielded a CTE value of greater than 1 were rejected. No attempt was made to estimate random errors in CTE from only two measurements. For future CTE monitor data sets, random errors will be estimated at signal levels and temperature set points where at least three repeat measurements are available.
Throughout the T/V test, it was observed that the actual equilibrium temperature at the CCD focal plane was colder than the TEC set point. The same trend was also observed in the CTE monitor data. The actual focal plane temperature at each set point was derived by computing the mean of the CCDTEMP1 sensor reading recorded in the headers of the individual FITS images. Relative to the set point, the mean CCDTEMP1 temperature deviated by -0.8 °C to -2.2 °C with a mean deviation of -1.4 C. The one-sigma uncertainty in the CCDTEMP1 mean temperature, to the nearest tenth of a degree, was 0.0 °C to 0.9 °C with a mean one-sigma uncertainty of 0.4 °C. The overall range in temperature for the eight data sets, -67.8 °C to -93.2 °C, encompassed the currently planned temperature range of -80 °C to -90 °C for nominal on-orbit operation of the HRC detector.
SOFTWARE :
Reduction of the FPR data was accomplished using IDL algorithms written specifically for ACS. The results are compiled in Table 1. The two IDL routines, fpr_par.pro and fpr_ser.pro, used for the reduction of the parallel and serial FPR CTE data, respectively, are appended to this report for future reference. The input FPR images must be bias corrected. Serial overscan pixels and overclocked rows, if any, must also be culled from the images. Comment lines are embedded in the routines to provide the user with information necessary to properly use the software. The same IDL software, possibly with minor improvements, will be employed for reduction of the on-orbit data.
PARALLEL CTE RESULTS :
Figure 1 shows the measured parallel FPR CTE as a function of signal level and temperature. Assuming a system gain of 1 electron/count, the actual signal levels achieved in the CTE monitor test were somewhat lower than the target signal levels specified in the T/V test plan. The measured mean signal levels for the data taken at the eight temperature set points were combined to derive the global mean signal level and one-sigma uncertainty for each of the five curves shown in Figure 1. The one-sigma uncertainty was less than 5% in all cases. This uncertainty quantifies the error due to flat lamp repeatability and drift. It does not, however, include the error due to non-uniformity in the HRC flat lamp image.
One low temperature data point at -92.0 °C in the 46.2 count signal level curve was rejected because the CTE measurement yielded a value greater than 1. The data point at -67.8 °C has probably been corrupted by noise. This data point has nevertheless been shown for completeness.
Disregarding the anomalous high temperature data point in the 46.2 count curve, the parallel CTE improved with decreasing temperature from -67.8 °C to -89.3 °C. There is also evidence for a correlation between the magnitude of the improvement in performance and signal level. The magnitude of the improvement in CTE appears to be greatest at the lowest signal levels. Cooling the detector below -89.3 °C yielded no appreciable improvement in performance at any signal level. Indeed, the 46.2 count CTE curve appears to sharply peak at -89.3 °C. The reality of the peak is questionable, however, without the -92.0 °C data point to confirm the decrease in CTE below -89.3 °C.
SERIAL CTE RESULTS :
Figure 2 shows the measured serial FPR CTI (=1-CTE) as a function of signal level and temperature. As was found in the parallel monitor test, the actual signal levels obtained in the serial monitor test were lower than the T/V test plan target levels. The one-sigma uncertainties in the global mean signal levels were again less than 5%, indicating good flat lamp stability and repeatability.
One data point was rejected in the 48.0 count and 4161.6 count data sets and four points were rejected in the 8342.0 count data set. The data points were rejected because the CTE measurements yielded values greater than 1. There is no obvious trend in the temperatures of the rejected points and therefore no reason to suspect a problem either in the test procedure or in the serial FPR clock patterns (the images visually appeared normal).
Unlike the parallel CTE, there is no readily obvious trend in the serial CTE as a function of temperature. At signal levels above ~700 counts, there is a possible minimum in the serial CTE at a temperature of -85.4 °C. However, the two low signal level curves below 700 counts do not exhibit minima at the same temperature. The data is of insufficient quality and resolution to firmly conclude that an operating temperature of -85 °C will degrade serial CTE performance. Because of the flatness of the serial CTE curves at ~360 counts and ~720, the serial CTE becomes progressively worse compared to the parallel CTE at temperatures below ~-80 °C. At signal levels above ~4100 counts, the serial CTE is superior to the parallel CTE over the entire temperature range.
The qualitative difference in parallel and serial CTE with temperature is likely to be due to the fact that the serial pixel transfer time (~22 µsec/pixel) is much shorter than the parallel row transfer time ( >23 msec/row). CTE performance depends critically upon the relationship between the pixel transfer time and the electron trap emission time constant. The electron trap emission time constant is a sensitive function of temperature. The physical mechanism that leads to quantitatively superior serial CTE performance at higher signal levels is less clear. One possibility may be that the transfer of a row of high signal level data through the serial register leaves behind a sufficient amount of deferred charge to effectively produce a fat zero for the next row of data.
CONCLUSION :
The ACS Contract End Item (CEI) Specification [Reference 1, Section 4.4.3.1] for the CTE performance of the HRC detector is >0.99999 and >0.999996 at -80 °C at signal levels of 1620 electrons/pixel and 100,000 electrons/pixel, respectively. No distinction is made in the CEI Specification between parallel and serial CTE. At -80 °C and at a signal level of 1620 counts/pixel, the CTE monitor results indicate that the non-flight HRC Build 1 detector meets the CEI requirement for both parallel and serial transfer. No conclusion can be drawn regarding the 100,000 electron/pixel requirement because the highest signal level in the CTE monitor test was ~8200 counts.
The parallel CTE improves with decreasing temperature from ~-68 °C to ~-89 °C. No parallel CTE performance advantage was achieved by cooling the detector below ~-89 °C at any signal level from ~50 counts to ~8200 counts. At temperatures between ~-89 °C and ~-93 °C, the parallel CTE appears to be flat at signal levels above ~350 counts and may actually degrade at lower signal levels. There is no obvious trend in the serial CTE as a function of temperature.
It is emphasized that the preceding conclusions may or may not be valid for the flight Build 2 HRC detector. Compliance with the CEI CTE Specification should be verified by test for the Build 2 detector when it is integrated with the instrument and driven by its matched flight CEB and flight clock patterns. The serial and parallel CTE monitor tests should also, if at all possible, be repeated to verify the temperature trends in the Build 1 data. The temperature trends in the Build 1 parallel CTE data, if confirmed for Build 2, may be an important factor in determining the best operating point for HRC science observations.
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