The NIRSpec fixed slits spectroscopy mode has several dither and nod pattern options for improving spatial or spectral sampling and removing astrophysical background flux.
Dither and nod options for NIRSpec fixed slits
The NIRSpec fixed slits (FS) spectroscopy mode has several dither and nod options available. Dithers along the slit are also referred to as "nods". Designated in APT as Primary Dither Positions 1, these are larger offsets used to subtract nearby background flux. As a result, the nod options are best used for targets that are not significantly spatially extended on the scale of the offset. The FS mode also offers small-scale sub-pixel (<0.1") offsets of the target position called Sub-Pixel Dithers, which will be used to mitigate detector effects, help remove cosmic rays, and particularly improve spectral and/or spatial sampling. The FS mode nods provide the primary offset positions along the spectral slits, and the dither options provide optional subpixel offsets to improve sampling. The nod positions and sub-pixel dither offsets can be used separately or together. Table 1 and Figure 1 show a summary view of these nod and dither options and the ways they can be combined.
Table 1. Options for the NIRSpec FS offsets
2-, 3-, or 5-position nods are available.
The Primary Dither Positions option may take the following values:
There will be one primary position along the slit.
|2||There will be 2 primary positions along the slit.|
|3||There will be 3 primary positions along the slit.|
|5||There will be 5 primary positions along the slit.|
The primary nod positions will be defined to optimize throughput based on the FS hardware characteristics. The precise positions for each nod are slit-dependent.
Small-scale offsets in the spectral (dispersion) direction, spatial (cross-dispersion) direction, or both the spectral and spatial directions are available. These small scale offsets are the same for all slits, and are fixed offsets from the primary nod positions.
The Sub-Pixel Pattern dither option may take the following values:
|No sub-pixel dithering is performed.|
|SPECTRAL||Three spectral dither positions are defined for each primary nod position along the slit.|
|SPATIAL||Two spatial dither positions are defined for each primary nod position along the slit.|
|BOTH||Four dither positions are defined for each primary nod position along the slit.|
Note that spectral offsets will decenter the source in the slit in the dispersion direction and therefore incur a change in slit losses, which are wavelength dependent. All offered dither options will be calibrated, but this calibration will be more challenging for SPECTRAL and BOTH dither patterns.
Options selected for dithers and nods determine the number of exposures that are acquired, as shown in Figure 1. Any combination of primary nod positions and sub-pixel dithers can be selected. Table 2 lists the specific slit-dependent primary nod positions. Table 3 lists the relative offsets with respect to the primary nod positions for the subpixel dithers.
The "A" fixed slits (S200A1, S200A2, S400A1, and S1600A1) on the left side of detector NRS1 can have wavelengths lost to the detector gap in the NIRSpec high spectral resolution mode (R ~ 2700). However, the wavelength gap can be filled and the full range of high resolution data can be acquired if the S200A1 and S200A2 slits are used together. When the S200A1 and S200A2 option is specified, the nod/dither pattern executes twice. The target is placed in the S200A1 slit and the requested exposures are executed at that position. Next, a move is made to the S200A2 slit and an identical set of exposures are executed.
Table 2. NIRSpec fixed slit primary nod positions
|Slit||Primary Y position (arcsec)|
Primary dither positions ("nods") for the 5 fixed slit apertures in the ideal coordinate frame in cross dispersion (Y) direction in arcseconds. The X position (dispersion direction) is zero arcsec in all cases. All dither positions are relative to the reference point for that aperture.
Table 3. NIRSpec fixed slit sub-pixel dither relative offsets
The relative offset (with respect to the primary dither positions) of the available sub-pixel dither patterns in NIRSpec ideal coordinates in arcseconds.
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