NIRSpec has five Fixed Slits (FSs) cut into the metal that separates the four quadrants of the micro-shutter assembly (MSA). All FS apertures can be used in the NIRSpec FS spectroscopy observing mode, but the 1.6 arcsec x 1.6 arcsec wide aperture alone is used for the Bright object time series (BOTS) mode. All of these apertures are always open, even if the Multi-Object Spectroscopy (MOS) mode or Integral Field Unit (IFU) mode is being used. By construction, the fixed slits' spectra project onto a different part of the detector than the MSA or IFU spectra, and therefore there is no overlap or contamination from failed open MSA shutters. The FSs can provide the most sensitive, highest contrast spectroscopy available with NIRSpec. Additional information is available on the Fixed Slits hardware and the Fixed Slits observing mode.
This page deals with the preparation of a NIRSpec Fixed Slits observation using the Astronomer's Proposal Tool (APT) template.
All 5 NIRSpec fixed slit (FS) apertures can be used in the NIRSpec FS spectroscopy observing mode. These apertures are always open, even if the Multi-Object Spectroscopy (MOS) mode or Integral Field Unit (IFU) mode is being used. In order to design a fixed slit observation, you have to complete a template in the Astronomer's Proposal Tool (APT). In this article, we describe how to fill out the NIRSpec fixed slits Template.
Step-by-Step APT instructions
We assume the observer has already defined Target(s)1 to be observed, created an observation folder, and loaded the NIRSpec FS template. Instructions for doing this are presented in the JWST Astronomers Proposal Tool Overview.
The Fixed Slit template can be divided into 4 sections, as shown in Figure 1.
- Section 1: This section of the APT form shows that the NIRSpec Fixed Slit Spectroscopy template has been selected, along with a Target that was previously defined by the user.
- Section 2: The area contains parameters that are for the proposer's information. The observer cannot edit this section.
- Section 3: In this section the observer enters target acquisition parameters.
- Section 4: In this section the user defines the science parameters that determine the specifics of the observation.
Sections 3 and 4 are part of the 'NIRSpec Fixed Slit Spectroscopy' tab. There are three additional tabs: 'Mosaic Properties', 'Special Requirements', and 'Comments', which are discussed at the end of this article.
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Figure 1. The NIRSpec Fixed Slit Spectroscopy template
The NIRSpec Fixed Slit Spectroscopy template tab in APT. Other tabs are also available: Mosaic Properties, Special Requirements, and Comments.
FS Template section 1: Generic
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FS Template section 2: Observation information
The following is included for information only and is generic to all templates. The article JWST APT Observations contains more detailed information on the topics Visit Splitting, Duration, and Data volume.
FS Template section 3: Target Acquisition Parameters
For NIRSpec Fixed Slit observations there are three options for target acquisition (TA): the Wide Aperture Target Acquisition (WATA), the procedure using MSA reference stars (called MSATA), and NONE. A complete description of the target acquisition procedures used to point the telescope with different JWST NIRSpec observing modes are addressed in the NIRSpec Target Acquisition article.
Section 3 in Figure 1 is where the user specifies the TA Parameters. For NIRSpec FS observations, the TA method called WATA (the default) is recommended. Suggested absolute coordinate accuracies for the acquisition target are on the order of 150 milli-arcsec or better to ensure that the acquisition target falls into the S1600A1 acquisition aperture. The option TA method = NONE is not recommended in most cases. The resulting pointing accuracy will be that delivered by the GS acquisition at the start of the Observation. For reference, the absolute pointing accuracy of JWST is expected to be 0.45 to 0.3 arcseconds (one sigma, per axis), compared to the 1.6" square aperture. A three-sigma excursion in the blind pointing could place the source outside the aperture.
The WATA procedure typically starts with placing the science target in the S1600A1 aperture and centering it with onboard software. In this case, the Acq Target is the science target. It is also possible to use WATA with an offset target. In that case, the offset target is defined in the Target folder of the APT proposal, and can then be selected as the Acq Target in APT. When using an offset target, it is important to ensure that the relative astrometric accuracy of the two targets is sufficient to place the science target in the S1600A1 aperture. In either case, WATA is appropriate for targets that are point sources or nearly point sources. A detailed description of this TA method is found in the NIRSpec Wide Aperture Target Acquisition article.
When selecting WATA as the TA Method, Section 3 looks as it does in Figure 1. In this section, observers can define the remaining TA parameters for WATA: the Acq Subarray, Acq Filter, and the Acq Readout Pattern. The options for Subarray are SUB32, SUB2048, and FULL. The options for Filter are F110W, F140X, and CLEAR. The available Readout Patterns are NRS, and NRSRAPID. These selections will be used to calculate the TA exposure time.
The WATA procedure will always acquire a three-group exposure using the Acq Filter and Acq Readout Pattern selected by the user. The JWST Exposure Time Calculator (ETC) in target acquisition imaging mode should be used to estimate the exposure parameters. The desire is to produce a suitably strong signal (minimum of S/N = 20) in each group of the three group image without saturating the exposure. For quick reference, approximate magnitude ranges for S/N = 20 to saturation for WATA are shown in Table 2 of the article NIRSpec Wide Aperture Target Acquisition.
Figure 2. WATA target acquisition method
Detail of the NIRSpec FS Spectroscopy Template showing the Target Acquisition Parameters under the WATA method.
When selecting the MSATA option, the target acquisition parameters will be specified at the visit level of the Observation, not directly in the observation template. MSATA uses reference stars to accurately correct the pointing. MSATA should only be used when WATA is not possible, as it incurs larger overhead and requires a more complex planning process with an APA assigned by STScI.
Observers do not need to define reference stars in order to propose for NIRSpec FS science. That can be deferred until the final program update. The planning parameters will be specified for accepted programs after the MSA-based Target Acquisition is assigned an orient by the long range planning system. A detailed description of this TA mode is available in the NIRSpec Target Acquisition - MSATA article.
FS Template section 4: Science Parameters
The Science Parameters are defined in section 4 of the FS Spectroscopy Template, as shown in Figure 3. The Science Parameters include: the Slit, Subarray, FS Dither Parameters, and the science exposure specification(s): the spectral configuration used for science, and the detector and exposure configuration options to specify the exposure time.
Slit and Subarray
Figure 3. Science Parameters section
Detail of the NIRSpec FS Spectroscopy Template showing the Science Parameters section.
Figure 4 shows the options under 'Slit' and 'Subarray'. Once the slit is chosen, the possible subarrays are narrowed down to the options given in Table 1.
Figure 4. Selecting a Slit and Subarray
Detail of the FS Template showing the available fixed slit options (top). Once a Slit is selected (in this case S400A1) the Subarray field defaults to the matching Subarray (SUBS400A1). Other options are shown in the menu (bottom). Table 1 lists all legal values for the Subarray for each Slit.
Table 1. Available subarray options for each NIRSpec fixed slit
|S200A1||SUBS200A1, ALLSLITS, FULL|
|S200A2||SUBS200A2, ALLSLITS, FULL|
|S200B1||SUBS200B1, ALLSLITS, FULL|
|S400A1||SUBS400A1, ALLSLITS, FULL|
SUB512, SUB1024A, SUB1024B, SUB2048, ALLSLITS, FULL
S200A1 and S200A2
Note that the "S200A1 and S200A2" option is restricted for use with the high-resolution dispersers. This option provides exposures using both slits to get complete wavelength coverage. The high resolution spectra map to both detectors. Using the S200A1 alone would otherwise result in wavelength gaps due to the space between detectors. With all other dispersers, the full wavelength coverage is achieved in a single exposure with the S200A1 slit.
APT will default to a matched Subarray once a Slit is selected. The default is recommended unless the proposed science requires an alternative. Some considerations for choosing the subarray for use with fixed slit observations:
- If S200A1 and S200A2 is selected, the Subarray options are ALLSLITS and FULL.
- For very bright sources, using ALLSLITS may saturate the exposure in the first group. To work around this, use S200A1 with the SUBS200A1 subarray in one exposure and S200A2 and the SUBS200A2 in a separate exposure. Use the ETC to determine exposure settings and saturation. The NRSRAPID readout pattern is recommended in all sciences cases that have no constraints on data volume (which will be flagged in APT). This pattern helps to avoid saturation when using subarrays.
- For the S1600A1 aperture, several Subarrays are available to allow for observing very bright targets. It is recommended to choose the largest subarray possible without saturating. Note that the smallest subarrays, SUB512 and SUB512S, only capture the full wavelength coverage for the Prism.
- For the remaining Fixed Slits, each one has a preferred (matched) Subarray of the same name, but it is also possible to select ALLSLITS or FULL frame detector readout. The ALLSLITS option reads the detectors for the whole region illuminated by all the Fixed Slits, and FULL reads the entire detector area. Selection of the FULL subarray option enables use of the readout . These patterns suppress 1/frequency noise and improve detector performance. NRSIRS2RAPID is the recommended Readout Pattern unless there are data volume issues in the program (these issues will be flagged by APT).
Following the selection of the Slit and Subarray, the user must select a Dither Parameters for the observation. The Primary Dither Positions may be chosen to be NONE, 2, 3, or 5 positions. The Sub-Pixel Pattern options are NONE, SPECTRAL, SPATIAL, or BOTH as shown in Figure 5. These patterns are illustrated and fully explained in the article NIRSpec FS Dither and Nod Patterns. Dithering in the spatial and spectral directions can improve signal-to-noise and sampling of the PSF. Recommendations for dithering FS observations are given in the article on Dithering Recommended Strategies.
Figure 5. Primary Dither Positions and optional Sub-Pixel Patterns
Detail of the FS Template where we show the available Primary Dither Positions and the optional Sub-Pixel Patterns.
Create an exposure configuration
An exposure specification is created by using the 'ADD' button located at the bottom of section 4 of the FS Template, as shown in Figure 3. Each line in this field represents an exposure specification - a set of exposures acquired with the specified dither option. The science exposure parameters that the observer needs to define for each row in this field are listed in Table 2. The Exposure Time Calculator (ETC) should be used to determine the best values of these parameters to optimize the signal-to-noise of your observation.
Table 2: Description of Science Parameters for a given exposure specification.
|Grating/Filter||Select a grating/filter combination from the pull down menu. The article NIRSpec Dispersers and Filters describes all the available combinations for NIRSpec.|
|Readout Pattern||The default value is NRS, but the available values are NRS, NRSRAPID, NRSIRS2, and NRSIRS2RAPID. These patterns are described in full detail in the article NIRSpec Detector Readout Patterns. Select the pattern that best suits your observation.|
|Groups/Int||The number of groups during an integration, where a group is the product of reading the detectors with a specific readout pattern. This number is determined from the ETC. |
|Integrations/Exp||The number of integrations during an exposure, where integration is defined as the time between resets. This number is determined from the ETC. |
This option is available to automatically add calibration exposures to a science exposure. For the FS template, the only options are NONE or WAVECAL. NONE is the default and is recommended. Autocals can add significant overhead to an observation. The instrument model wavelength calibration is expected to meet the wavelength calibration accuracy requirement, making Autocals unnecessary. Additionally, the grating wheel sensor removesany zero point wavelength shift (in post-processing) due to slight variations in the position of the grating wheel.
Note that if the S200A1 and S200A2 option is chosen then the observation is restricted to the high-resolution dispersers (G140H, G235H, and G395H) and their allowed Filters. In this case, an Execution Order table will appear below the Gratings/Filters input table in the APT GUI. This table is not editable and shows the order of execution of exposure specifications of both slits.
NIRSpec's fixed slits may be used to obtain data for a region larger than their size by creating a mosaic pattern. The use of APT's mosaic tool is described in the article NIRSpec FS and IFU Mosaic APT Guide.
A variety of observatory level special requirements may be chosen.
The comments field should be used only to record observing notes.