For imaging programs, STScI generally recommends that observers employ dithering patterns. Dithering refers to the procedure of moving the telescope by small angle offsets between individual exposures on a target. The resulting images are subsequently combined in the pipeline or by the observer using software such as
MultiDrizzle (see the
MultiDrizzle Handbook).
Dithering is used to improve image quality and resolution. By combining multiple images of a target at slightly different positions on the detector, one can compensate for detector artifacts (blemishes, dead pixels, hot pixels, transient bad pixels, and plate-scale irregularities) that may not be completely corrected by application of the calibration reference files. Combining images, whether dithered or not, can also remove any residual cosmic ray flux that has not been well removed by the up-the-ramp fitting procedure used to produce flt images (see
Section 7.7.2 and
Appendix E). Effective resolution can be improved by combining images made with sub-pixel offsets designed to better sample the PSF. This is especially important for WFC3/IR, because the PSF is undersampled by about a factor of 2 (see
Table 7.5).
Larger offsets are used to mosaic a region of sky larger than the detector field of view. (Large offsets can also be used for “chopping” to sample the thermal background. This has been recommended for NICMOS exposures at wavelengths longer than 1.7 microns, where the telescope thermal background becomes increasingly dominant, but the thermal background is not a problem for WFC3/IR). In WFC3, all offsets must be accomplished by moving the telescope (whereas in NICMOS it was also possible to move the Field Offset Mirror). Dithers must be contained within a diameter ~130 arcsec or less (depending on the availability of guide stars in the region) to use the same guide stars for all exposures. The rms pointing repeatability is significantly less accurate if different guide stars are used for some exposures (see Section 4.1 of the
MultiDrizzle Handbook). Mosaic steps and small dither steps are often combined to increase the sky coverage while also increasing resolution and removing artifacts. (See
Section 6.10.1 for a discussion of the effect of geometric distortion on PSF sampling for mosaic steps).
The set of Pattern Parameters in the observing proposal provides a convenient means for specifying the desired pattern of offsets. The pre-defined mosaic and dither patterns that have been implemented in APT to meet many of the needs outlined above are described in detail in the
Phase II Proposal Instructions. The WFC3 patterns in effect in APT at the time of publication of this Handbook are summarized in
Appendix C. Observers can define their own patters to tailor them to the amount of allocated observing time and the desired science goals of the program. Alternatively, they can use POS TARGs to implement dither steps (
Section 7.4.3), but the exposures are then not associated and hence not automatically combined in the OPUS pipeline.
Parallel observations, i.e., the simultaneous use of WFC3 with one or more other HST instruments, are the same for the IR channel as for the UVIS channel, previously described in
Section 6.10.2.
Given the variety of requirements of the scientific programs that are being executed with WFC3/IR, it is impossible to establish a single optimum observing strategy. In this section we therefore provide a few examples after guiding the reader through the main constraints that should be taken into account when designing an observation:
These constraints put contradictory requirements on the ideal observing strategy. It is clear that, given a certain amount of total observing time, the requirement of long integrations for background limited performance is incompatible with a large number of dithering positions. Also, to split ramps for readout noise suppression decreases the observing efficiency, with a negative impact on the signal to noise ratio. Because the background seen by each pixel depends on the filter (
Section 7.9.5), the optimal compromise must be determined on a case-by-case basis.
In this regard, it is useful to consider Table 7.11, which summarizes the total background seen by a pixel, including sky, telescope, and nominal dark current, and the time needed to reach 400 e
–/pixel of accumulated signal, corresponding to 20 e
–/pixel of Poisson-distributed background noise. This last value, higher than the expected readout noise of ~12 electrons after 16 reads, is used here to set the threshold for background-limited performance. The passage from readout-limited performance to background-limited performance can be regarded as the optimal exposure time for that given filter, in the sense that it allows for the largest number of dithered images without significant loss of S/N ratio (for a given total exposure time, i.e., neglecting overheads). For faint sources, the optimal integration time strongly depends on the background (zodiacal, Earth-shine thermal, and dark current) in each filter, ranging from just 220 s for the F110W filter to 2700 s for some of the narrow-band filters.
The optimal integration time needed to reach background-limited performance (see
Table 7.11) can be compared with the integration times of the sampling sequences from
Table 7.8.
Table 7.12 synthesizes the results, showing for each filter which ramp (SPARS, STEP) most closely matches the optimal integration times for NSAMP=15.
Table 7.11: Background (e–/pix/s) levels at the WFC3/IR detector. The columns show, from left to right: a) filter name; b) thermal background from the telescope and instrument; c) zodiacal background; d) earth-shine background; e) dark current; f) total background; g) integration time needed to reach background-limited performance, set at an equivalent readout noise of 20 electrons.
The selection of which sample sequence type (RAPID, SPARS, STEP; Section 7.7.3) must take into account the science goals and the restrictions placed on their use. Here are some factors to consider when selecting a sample sequence:
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STEP ramps are preferable where large dynamic range is needed; e.g., for photometry of stellar clusters. These ramps begin with a sequence of four uniform (RAPID) reads and end with a sequence of much longer uniform reads. The transition between the two uniform read rates is provided by a short sequence of logarithmically increasing read times. This design provides for correction of any non-linearities early in the exposure and allows for increased dynamic range for both bright and faint targets.
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Finally, the selection of a given sample sequence type should also be made in conjunction with the number of samples (nsamp) that will be used to achieve the desired total exposure time for the observation. Long exposures should in general use a minimum of 5-6 samples in order to allow for reliable CR rejection and to allow for at least a few unsaturated samples of bright targets in the field. For very faint targets in read-noise limited exposures, a larger number of samples will result in greater reduction of the net read noise and a more reliable fit to sources with low signal. Short exposures of bright targets, on the other hand, can get by with fewer samples. This is especially true, for example, for the direct images that accompany grism observations. Because the purpose of the direct image is to simply measure the location of sources - as opposed to accurate photometry - they can reliably use an nsamp of only 2 or 3.
The observing technique of spatial scanning was introduced for WFC3 in cycle 19. (See
http://www.stsci.edu/hst/wfc3/documents/newsletters/STAN_01_06_2011.) This capability can be used to turn stars into well-defined streaks on the detector or to spread a stellar spectrum perpendicular to its dispersion. There are at least two motivations for implementing such a capability for WFC3/IR: 1) reducing the fraction of overhead in observations of very bright stars such as those suitable for spectral characterization of transiting planets, and 2) enabling observations of very bright primary calibrators that would otherwise saturate the IR detector. Results of engineering tests in which a star was imaged with WFC3/IR under various parameterizations of HST's scanning speed and orientation, with and without a grism in place, have been reported by
McCullough & MacKenty (2011).
