There are no hard bright-object limits to worry about for CCD observations, since the CCD cannot be damaged by observations of bright sources. However, the CCD pixels do saturate at high accumulated count levels, due to the finite depth of the CCD full well. The CCD saturates at ~144,000 electrons pixel-1 in most of the effective area of the chip; however, over the outermost (serial=x) portion the CCD saturates at 120,000 electrons pixel-1. The variation of the CCD full well over the chip occurs because of nonuniformity in the process of boron implantation, which creates the potential wells in this type of CCD. Accumulations up to the full well limit can be observed only in the CCDGAIN=4 setting, as the gain amplifier already saturates at ~33,000 electrons pixel-1 in the CCDGAIN=1 setting (see Analog-to-Digital Conversion: Selecting the CCDGAIN).
Saturation imposes a limit on the product of the count rate and the integration time. Keep the total counts in the pixels of interest below the saturation level, either by keeping the exposure time short enough that the limit is not violated in any single integration or by choosing a more appropriate configuration. You can allow saturation to occur in regions of the image over which you do not wish to extract information (e.g., you can allow a star or single emission line to saturate if you are interested in other features). Remember, however, that once the CCD full well is over full, charge will bleed along the columns of the CCD so that neighboring pixels (along the slit for spectroscopic observations) will also be affected. Saturation cannot be corrected for in post-observation data processing.
An interesting exception to this is in Gilliland, Goudfrooij & Kimble, 1999, PASP, 111, 1009-for CCDGAIN=4 the response remains linear up to, and even far beyond saturation if one integrates over the pixel bled into. For point sources saturation does not compromise spectral purity. Signal to noise values of ~10,000 have been demonstrated for saturated data-see STIS ISR 99-05 for a time series application.
In Determining Count Rates from Sensitivities, we explained how to determine the peak counts sec-1 pixel-1 expected for your observation. In Chapter 13 for each grating mode and in Chapter 14 for each imaging mode, we provide, for spectroscopy and imaging, respectively, plots of exposure time to fill the CCD full well versus source flux for each STIS configuration. Lastly, exposure-time calculators are available on the STScI STIS web site. Use one of these sources to ensure that your observations will not saturate sources of interest.
The minimum CCD exposure time is 0.1 seconds, providing a true limit to the brightest source that can be observed without saturating.
The STIS CCD camera features a high-speed shutter which eliminates the need for a shutter illumination correction, even at the shortest commandable exposure time of 0.1 seconds. The only two minor drawbacks of using this shortest exposure time are the following: (i) a non-reproducible large-scale variation in intensity of a very low amplitude (~0.2%) which is due to a slight non-uniformity of the shutter speed, and (ii) a mean count rate which is ~3% lower than those of longer exposures, which is due to an inaccuracy of the shutter timing at this setting. These minor effects occur only for the shortest exposure times, and disappear completely for exposure times of 0.3 seconds and longer.
All CCD exposures are affected by cosmic rays. The rate of cosmic ray hits in orbit is very high compared to ground-based observations. The current rate at which pixels are affected by cosmic-ray hits is 30.0 (± 3.7) pixels per second for the STIS CCD. To allow removal of cosmic rays in post-observation data processing we recommend that whenever possible, given signal-to-noise constraints, you take two or more exposures in any given CCD configuration (see also "CR-SPLIT" on page 225). The greater the number of independent exposures, the more robust is the removal of cosmic rays and for very long integrations it is convenient to split the exposure into more than two separate images to avoid coincident cosmic-ray hits. As an example, for two 1200 sec exposures, about 1250 CCD pixels will be hit in both images and will therefore be unrecoverable. Moreover, since cosmic ray hits typically affect ~5 pixels per event, these pixels will not be independently placed, but rather will frequently be adjacent to other unrecoverable pixels. In general, we recommend that individual exposures should not exceed ~1000 sec duration to avoid excessive amounts of uncorrectable cosmic rays in the images. However, observers must balance the benefit of removing cosmic rays against the loss in signal-to-noise which results from the splitting of exposures when in the read-noise-limited regime.
In observations of faint sources, particularly for dispersed light exposures, the intrinsic count rates can be very low. The exposure time needed to reach a break-even between the read-out noise and the Poisson noise per pixel associated with the minimal sky background is ~15 minutes for imaging in 50CCD mode, and ~36 minutes for slitless spectroscopy with G750L. With a dark current of ~0.004 e- sec-1 it takes ~80 minutes of integration for the Poisson statistics on the detector background to equal the read noise. Therefore, repeated short exposures of faint sources can significantly increase the total noise from added readouts. Selecting the correct number and length of repeated integrations requires a consideration of the trade-off between increased read noise and more robust cosmic-ray elimination. The STIS Exposure Time Calculators, or the S/N plots in Chapter 13 and Chapter 14, can help you determine whether your observations are in the read-noise dominated regime.
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Be sure to take at least two identical CCD exposures in each configuration to allow removal of cosmic rays in post-observation data processing. |
Hot pixels, caused by radiation damage, occur in the STIS CCD. Dark frames are currently obtained twice a day in order to maintain a master list of hot pixels and to update the pipeline superdark reference files on a weekly basis. On a monthly time scale, the CCD is raised to ambient temperature, from its normal operating temperature of ~ -83° C, in order to permit annealing of hot pixels.
Analysis of on-orbit data has shown that the annealing process is successful in that ~75% of transient hot pixels (hotter than 0.1 electron sec-1 pix-1) are currently annealed away each month. Apart from the transient hot pixels, there is a substantial number of hot pixels that stay persistently hot after anneals. At present, ~1.6% of the pixels of the STIS CCD are persistently hot. The total number of hot (>0.1 electron sec-1 pix-1) pixels is ~17,000 after an anneal, as of March 2001 (see Figure 7.7). The different points in Figure 7.7 represent pixels with dark current above each listed threshold. Note the monotonic increase in hot pixels with time. This increase is especially important for the cases of binned or spectral data, as there is an increasing probability for hot pixels to lie within a given bin or extraction box, respectively. A detailed description of the variation in hot pixel numbers since launch can be found in STIS ISR 98-06.
Note that both binned and spectral data will increasingly suffer from the effects of hot pixels as the percentage of non-annealed pixels increases. Just prior to an anneal, up to 1.8% of all CCD pixels are hot, i.e., both persistent and ``annealable'' hot pixels. In the case of spectral data, with a normal extraction box height of 7 pixels, this means that 12 - 13% of the extracted pixels will be affected by a hot pixel. For imaging data involving rectification, the rectification process interpolates unremoved hot pixels into the four adjacent pixels. For the case of MxN binning, therefore, 4xMxN pixels will be affected by a combination of the binning and rectification process.
While post-pipeline calibration using appropriate STIS reference superdarks allows one to subtract most hot pixels correctly (to within the accuracy set by Poisson statistics), the best way to eliminate all hot pixels is by dithering (making pixel-scale positional offsets between individual exposures). Dithering as a method of data taking is described in detail in Chapter 11. An HST handbook on dither strategies and advantages, together with example data is available on-line at:
http://www.stsci.edu/instruments/wfpc2/Wfpc2_driz/dither_handbook.html.
Figure 7.7: Hot Pixels Remaining after each Anneal
Analysis of CCD images taken during ground calibration and in Cycle 7 has revealed low-level changes in the bias pattern (at the tenths of a DN level) and a low-level amplifier nonlinearity. This non-linearity ("amplifier ringing") was uncovered during the analysis of the overscan region on flat-field images (reported in STIS Instrument Science Report 97-09). The bias value of a given row in the serial overscan region of flat-field images is depressed with respect to the nominal bias value by an amount proportional to the mean signal in that row. However, the small proportionality factors and low DN levels at which the nonlinearity occurs render the problem negligible for most STIS scientific applications. Instances of data that may be slightly affected by this problem (at the <1% level) are aperture photometry of faint sources (in imaging mode), especially in the case of a crowded region with nearby bright sources which would cause a local depression of the bias value, and photometry of diffuse extended objects which cover a large number of pixels. The brightest hot pixels (see Hot Pixels) also cause a measurable local depression in the bias value, but their effect is corrected for by using the appropriate superdark reference file (or daily dark file) during CCD calibration.
Observers taking full-frame CCD images obtain both physical overscan (i.e., actual CCD pixels; columns 1-19 on the raw image) and virtual overscan (i.e., added electronically to the image; rows 1-20 on the raw image) on their frames; the virtual overscan is not subject to the amplifier nonlinearity problem and can be used to estimate the importance of this effect in the images. Observers using subarrays (e.g., to reduce the time interval between reads and limit the data volume when performing variability observations in the optical; see also Chapter 11) will obtain only the physical overscan.
The STIS CCD suffers from imperfect charge transfer efficiency (CTE), which causes some signal to be lost when charge is transferred through the chip during readout. As the nominal read-out amplifier is situated at the top right corner of the STIS CCD, the CTE problem has two possible observational consequences: (1) making objects at lower row numbers (more pixel-to-pixel charge transfers) appear fainter than they would if they were at high row numbers. As this loss is suffered along the parallel clocking direction, we refer to this effect as parallel CTE loss; and (2) making objects on the left side of the chip appear fainter than on the right side, hereafter referred to as serial CTE loss.
The current lack of a comprehensive theoretical understanding of CTE effects introduces an uncertainty for STIS photometry. The CTE problems are caused by electron traps in the CCD which are filled as charge passes through pixels. However, not all traps are accessible to all electrons passing through. Some traps are only accessible if there is significant charge involved. This model suggests that there will not be significant CTE losses in the presence of background, particularly for faint stars, because background electrons fill the traps before such stars pass through. For brighter stars with background there will still be some loss because their charge may access traps that are unaffected by the background that previously clocked through. Faint stars in areas with little background may suffer from larger losses.
Our annual determination of the CTE of the STIS CCD has indicated a significant deterioration of the parallel-register Charge Transfer Inefficiency (CTI = 1 - CTE). The results reported here represent measurements performed on Oct. 13, 2000, i.e., after 3.6 years on-orbit. It should be noted at the outset that CTE effects have not been incorporated into the STIS Exposure Time Calculators as yet. Thus, should you believe the CTE losses described herein may impact your program, you will need to provide longer exposure times in your Cycle 11 proposal to compensate for the anticipated losses. In particular, Cycle 11 observers using the STIS CCD to observe faint targets (especially spectroscopy of point sources or imaging of faint point sources) with less than a few hundred counts (integrated over an aperture of 5 pixels diameter) above a low background are advised to adjust their exposure times appropriately. Furthermore, observers using the CCD for spectroscopy of sources having a spatial extent less than a few arcsec are urged to use the pseudo-apertures located near row 900 of the CCD. (See the section on Mitigation of CTI Effects)
The CTE test employed imaging observations of a sparse star field and was executed with the STIS CCD in Clear mode (50CCD) and using CCDGAIN = 1. The observations were taken in the CVZ and using two exposure times (20 s and 100 s) in order to study the dependence of CTE on the sky background. All images were alternatively read out by two amplifiers, located on opposite sides of the CCD. The subsequent analysis consisted of aperture photometry of all sufficiently uncrowded stars in the field with intensities higher than 4 
above the background. A circular aperture with a radius of 2 pixels was used, centered on the stars. The CTI follows from the slope of the relation between the ratio of the count rates of stars in images read out by the two amplifiers and the Y coordinate (i.e., the CCD row number). The results of the analysis are provided in Table 7.4 where the CTI values (in units of relative charge lost per pixel) are listed as a function of the sky background and the object intensity within the aperture. The CTI results are listed for two epochs: 2.5 and 3.6 years after launch. Comparing the results of the two epochs, it can be seen that the CTI is increasing by ~15% per year on average. This can be used as a rule of thumb for estimating exposure times in Cycle 11.
Table 7.4 also shows that the CTI is improved significantly with increasing background, especially for faint objects. This is mostly advantageous for CCD imaging programs, for which a sky background of 10 electrons/pixel is achieved in an exposure time of only ~10 - 15 seconds (with ``average'' Earthshine and using the 50CCD filter). During the remainder of Cycle 9 and 10, more calibration data will be analyzed to determine a full functional description of the dependence of the CTI on time, object intensity, and sky background. Further analyses of CTI effects will be reported as STIS Instrument Science Reports on the web.
For the observer, a few strategies for minimizing the effect of CTI should be noted. First of all, one should take longer exposures whenever possible to increase the object counts and the sky background per exposure, both of which reduce CTI. Users who are thinking about dithering and shortening their exposure times to allow for, e.g., more dither positions, may want to take this into account. Another strategy is to place the target of interest near the top of the CCD. To this end, we have defined a set of spectroscopic Mitigation of CTI Effects which are located near row 900 of the STIS CCD. We recommend the use of these apertures for spectroscopy of point sources and compact objects with a spatial extent of less than about 2 arcsec in radius.
Decreasing charge transfer efficiency in the STIS CCD has a detrimental effect on faint spectra acquired at the default location at the center of the chip. For sources with fluxes less than ~1 x 10-16 erg cm-2 s-1 Å-1, less than ~100 electrons are accumulated per pixel in exposure times of 1000 s or less. (This is the longest integration time we recommend due to the deleterious impact of multiple cosmic rays in a CR-SPLIT at longer integration times.) At signal levels of 50 - 100 e-, 15% or more of the charge can be lost during readout due to charge-transfer inefficiencies. Many STIS science programs have fluxes in this range. For spectra of point sources and compact objects such as galactic nuclei, the full length of the slit is not needed. A target location closer to the read-out amplifier near the end of the slit can decrease the charge lost during parallel transfers by a factor of ~5. One could achieve this offset through the use of offset targets or appropriate POS TARG entries on the Phase II proposal, but these methods are a bit cumbersome and can be prone to error. We have therefore defined a set of pseudo-apertures that use the same physical long slits available for STIS CCD observations, but have their default target placement near row 900, ~5 arcseconds from the top of the STIS CCD. This is schematically illustrated in Figure 7.8. Observers can use these new aperture names to place their targets at this location in a rather transparent fashion.
The new aperture names and the approximate Y location of the resulting spectra are given in Table 7.5. Use of the new aperture name eliminates the need to specify an offset for the ACQ/PEAK and a POS TARG. The new apertures are also recognized by the calibration pipeline software, so spectra are extracted from the correct location using appropriate wavelength solutions, spectral traces, and background regions. For optimum throughput when using these apertures, we recommend using an ACQ/PEAK exposure to center the target in the aperture when using aperture 52X0.1E1 and 52X0.05E1. While use of these apertures will ameliorate CTE losses, we caution observers to carefully assess the potential impact on their science programs due to the decreased spatial coverage and the relative locations of the bars on the slit.
| Aperture |
Y Location |
ACQ/PEAK |
|---|---|---|
| 52X2E1 |
894 |
no |
| 52X0.5E1 |
893 |
no |
| 52X0.2E1 |
893 |
no |
| 52X0.1E1 |
898 |
yes |
| 52X0.05E1 |
898 |
yes |
In the optical, each photon generates a single electron. However, in the near UV, shortward of ~3200 Å there is a finite probability of creating more than one electron per UV photon (see Christensen, O., J. App. Phys. 47, 689, 1976). Users will need to take this into account when calculating signal-to-noise ratios and exposure times for the G230LB and G230MB gratings, as described in Special Case-Spectroscopic CCD Observations at 
<2500 Å.
Initial laboratory testing of STIS CCDs showed that excessive illumination by UV light can cause an elevation in residual dark current, due to a surface chemistry effect. However, the actual STIS flight CCD was tested for this effect during ground calibration by the STIS IDT and the effect was found to be much less than previously suspected; this effect is now a concern only for clear (50CCD) imaging of extremely UV-bright targets. Observations of fields with UV-bright objects should be dithered (i.e., positional offsets applied between readouts) to ensure that the UV tail from bright sources does not cause a residual elevation of the dark current for subsequent science observations. It is also recommended to use the longpass-filtered aperture, F28X50LP, rather than the 50CCD clear aperture, during target acquisitions (see also Selecting Target-Acquisition Parameters) when possible. The specific results of the ground testing on the effect of UV overillumination are summarized in Table 7.6.
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Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |