MIRI MRS Dithering
The MIRI medium-resolution spectrometer (MRS) is spatially undersampled at all wavelengths, particularly at the shortest wavelengths within each channel (see Figure 1). Ideal sampling of a point spread function (PSF) should provide at least two samples per spatial resolution element in order to avoid loss of information. Dithering is therefore necessary to (1) improve this spatial sampling, (2) mitigate bad pixels by sampling the image with redundant detector locations, and (3) allow for sufficient PSF separation that pairs of exposures can be used as background exposures for each other.
The MRS consists of four separate IFUs, each with their own pixel size and slice width (see Table 1 on the MRS main page), that observe a scene simultaneously. These sizes have been chosen so that specific offsets can be applied to the telescope pointing that will simultaneously produce half-integer changes in the sampling for all four channels. An offset in the across-slice direction of 0.968" for instance corresponds to a move of 5.5, 3.5, 2.5, and 1.5 slices, respectively, in channels 1–4. Similarly, offsets in the along-slice direction of 2.058" correspond to nearly half-integer offsets of 10.50, 10.50, 8.43, and 7.54 pixels, respectively, in channels 1–4.
Available dither patterns
JWST dithering allows for moves specific to MIRI MRS. Dither patterns for the observation can be implemented in the Astronomers Proposal Tool (APT) with the JWST APT MIRI MRS template. A dither pattern is defined as a sequence of small (less than 5" in the case of the MRS) moves from the starting position near the center of the MRS field of view. The sequence is fully specified in APT by choosing:
- Either a 2-Point1 or a 4-Point pattern
- Whether the pattern is optimized for a specific wavelength channel or for all channels (specified by the Primary Channel parameter), and whether the optimization is for point sources (Channel 1, Channel 2, Channel 3, Channel 4, or ALL) or Extended Sources
- The Direction of the pattern on the sky
There are two kinds of dither patterns available for MRS; patterns optimized for point source observations, and patterns optimized for extended source (or mosaicked) observations. These two kinds of patterns differ in the size and purpose of their dither offsets; point source optimized patterns maximize the offset distance to provide large point source separation at the cost of decreased common field of view, whereas extended source optimized patterns minimize the offset distance to provide the greatest common field of view at the cost of decreased point source separation. Both sets of patterns provide improved spatial sampling and detector pixel redundancy.
Dither patterns optimized for point sources
The default dither pattern for MRS is a 4-Point pattern that is optimized for point source observations at all wavelengths. As such, it provides an offset, which is large enough to separate channel 4 images, for accurate background subtraction (offset ~3 times the FWHM of a point source in channel 4) while keeping the channel 1 images comfortably within the field of view (channel 4 has larger field of view than channel 1). This pattern is illustrated in Figure 2 (top left panel).
For observers who wish to increase the separation between the point source locations in successive exposures, variations on this basic pattern are provided that further increase the separation distance. These variations are channel-specific, in the sense that they achieve the greatest separation in a particular wavelength channel at the cost of poorer spatial sampling and smaller common field of view in other channels. The pattern optimized for channel 1 is identical to the default pattern. In contrast, the pattern optimized for channel 4, for instance, achieves a point source separation of eight times the FWHM in channel 4 at the cost of moving the target entirely out of the field of view in channel 1 (see Figure 2, top right panel).
Additionally, point source dither patterns can be specified with either of two Directions on the sky. The "positive" orientation repeats the pattern of the default "negative" orientation but with the offset directions swapped in order to rotate the movement on the sky by 43º, giving some flexibility in accommodating source geometry.
Dither patterns optimized for extended sources
In cases where the science target is larger than about an arcsecond in size (or when mosaicking large areas of sky), the point source optimized pattern may be undesirable since a portion of the target will fall outside the common field of view of the dithered observations. The all-wavelength extended source dither pattern therefore provides an offset that is large enough to provide half-integer sampling in all wavelength bands, but small enough to maximize the common field of view of each band (see Figure 2, lower left panel).
Note that when using an extended source optimized pattern, a dedicated sky exposure must be declared within APT in order to provide a reference background image free of source contamination.
The largest possible common fields of view for dithered MRS observations can only be achieved at specific wavelength channels by sacrificing sampling and detector pixel redundancy in the other channels. Such channel-specific extended source patterns are therefore available for science cases that wish to maximize areal coverage at specific wavelength of interest at the expense of data quality at other wavelengths. As an example, the extended source pattern optimized for channel 1 provides a ~12 arcsec2 common field and good sampling for channel 1 at the expense of poor sampling and pixel redundancy in channel 4 (see Figure 2, lower right panel).
Since the offsets of the extended source patterns are substantially smaller than the point source optimized patterns, the Direction of these patterns need not be specified as both are identical.
2-Point vs 4-Point dithers
As described above, the MRS slice widths and pixel scales are designed such that a simple 2-Point dither pattern will nominally allow the MRS to achieve half-integer sampling in all four channels. In practice, however, optical distortions and discontinuities in the mapping of adjacent optical slices to detector pixels (see Figure 1 on the MRS main page) means that this half-integer sampling is not achieved for all locations within the MRS field.
The final image will therefore be both better and more uniformly sampled if the 2-Point pattern is repeated with a small (~0.1") offset to minimize the impact of field-dependent distortion. This 4-Point variation is available for both the point source and extended source optimized dither patterns.
The level of impact on the resulting image quality of poor spatial sampling due to 2- vs 4-point dithering is still under study, as are the impacts of no dithering (which may be desirable for certain science cases). In the worst case, (e.g., an undithered observations of a point source that falls at the boundary between adjacent slices), however, the reconstructed profile of a point source may be distorted by as much as 50% in the along-slice direction, the across-slice direction, or both.
Which pattern should I use?
The best dither pattern to use for a given set of observations depends strongly on the science case. Figure 3 summarizes the advantages and disadvantages of each of the available MRS dither patterns as they affect the point source separation, common field of view, and image sampling quality (Figure 4) in each wavelength band.
In the majority of cases, programs observing either point sources or compact sources (less than about an arcsec in extent) should use the point source optimized, 4-Point ALL dither pattern. This provides robust performance at all wavelengths and adequate point source separation in all channels such that dedicated background observations are not required. In cases where additional PSF separation is desired at longer wavelengths (due to a particularly bright source, or some extended structure surrounding the source), channel-specific options may be used at the cost of no longer having the source in the field at short wavelengths. If image quality is not a priority, a 2-Point pattern may be used instead of a 4-Point pattern in order to reduce observing overheads (particularly for deep exposures where long individual exposure times are necessary in order to reduce detector noise).
Similarly, most programs observing extended sources or using the MRS to mosaic large areas of sky should use the extended source optimized, 4-Point ALL dither pattern. Although this has the least common field of view of any of the extended source patterns, it is the only pattern that simultaneously achieves ideal half-integer sampling at all wavelengths. If a particular science program wishes to prioritize field of view at one wavelength at the expense of other wavelengths (e.g., mapping an emission line region with a specific spectral line), channel-specific options may be used at the cost of spatial sampling and/or detector pixel redundancy at other wavelengths. Likewise, if image quality is not a priority, a 2-Point pattern may be used instead of a 4-Point pattern in order to reduce observing overheads.
Gordon , K. et al. 2015, PASP, 127, 953
The Mid-Infrared Instrument for the James Webb Space Telescope, X: Operations and Data Reduction