Generic parameters

MIRI Medium Resolution Spectroscopy tab

Target Acquisition Parameters

The target acquisition parameters Acq Target, Acq Filter, and Acq ExposureTime are found under the MIRI Medium Resolution Spectroscopy tab, in the Target Acquisition Parameters panel.

Target ACQ 

See also: MIRI MRS Target Acquisition

For MRS spectroscopy, choose Acq Target from the pull down menu. Filters available are F560W, F1000W, F1500W, and FND. As of APT 25.4.2, NONE may also be selected for target acquisition, which will disable target acquisition for the observation.

Acq Exposure Time 

The Acq Exposure Time fields must be completed by selecting a MULTIACCUM exposure configuration. Each exposure is specified by setting the readout pattern and characteristics parameters: Acq Readout Pattern and Acq Groups/Int. 

Acq Readout Pattern

See also: MIRI Detector Readout Overview

Choices for the Acq Readout Pattern are 

• FAST (the default)
• FASTGRPAVG
• FASTGRPAVG8
• FASTGRPAVG16
• FASTGRPAVG32
• FASTGRPAVG64

Number of groups and integrations

See also: Understanding JWST Exposure Times

Acq Groups/Int

The MIRI readout timing pattern for the TA exposure is defined by only one of the MULTIACCUM parameters: Acq Groups/Int, the number of groups per integration, where a group is the product of cycling through all the pixels.

Acq Integrations/Exp, Acq Total Integrations and Acq Total Exposure Time cannot be changed by the user.

MRS Parameters

The Primary Channel parameter, as well as Dithers, Simultaneous Imaging, and Exposure Parameter dialog boxes are found under the MIRI Medium Resolution Spectroscopy tab, in the MRS Parameters panel.

Primary Channel 

See also: MRS Channels

The Primary Channel defines the pointing origin (i.e., where the science target would be centered in the absence of dithering), available dither patterns, and field boundaries (for mosaic overlap calculations). Options are:

• ALL MRS^*
• CHANNEL1 
• CHANNEL2 
• CHANNEL3
• CHANNEL4 
  
• IMAGER

The IMAGER option should only be used to place the science target in the simultaneous imaging field while the MRS is observing a dedicated background.

Dithers 

See also: MIRI MRS PSF and Dithering

The Dithers dialog box handles the creation of dither patterns, which must be created before defining your exposure sequence. Each DITHER is specified by setting several parameters: Dither Type, Optimized For, and Direction.

Dither Type

• 2-Point dithering allows for basic image separation in order to use one exposure as the "background image" for another, and to detect the source with redundant detector pixels. It does not achieve optimal half-integer spatial sampling throughout much of the field of view.
• 4-Point adds an additional offset compared to 2-Point dithering and achieves optimal half-integer spatial sampling throughout the entire field of view of all 4 channels.

The default choice should be 4-Point. Dithering may be turned off in order to create zero-offset exposures in the Exposure Parameters table.

Optimized For

• POINT SOURCE
• EXTENDED SOURCE
• BACKGROUND

The POINT SOURCE dithering schemes provide the maximum possible separation between the dither locations. The EXTENDED SOURCE dithering scheme provides the minimum possible separation that still achieves ideal spatial sampling and use of redundant detector locations. When observing point sources or otherwise small objects, the POINT SOURCE dithering scheme should generally be selected. When observing extended sources that fill much of the field of view, or when mosaicing together multiple pointings, some science cases may wish to use the EXTENDED SOURCE scheme. Note that programs using the EXTENDED SOURCE pattern should also include a separate dedicated background exposure at an off-source location since the image separation achieved in this dither scheme will be insufficient to achieve proper background subtraction.  The BACKGROUND dithering scheme provides large offsets suitable for optimizing MIRI simultaneous imaging data while the MRS is observing an empty dedicated background field.

Direction

• POSITIVE
• NEGATIVE

This choice defines the orientation of the dither offsets in the JWST focal plane; they are mirror symmetric with respect to the IFU slice orientation and achieve identical sampling. This option is provided to allow for flexibility in dithering direction if the telescope orientation is to be fixed for a given observation. The default is NEGATIVE, and is unimportant for the majority of science cases.

Simultaneous Imaging 

See also: MRS Simultaneous Imaging

MIRI offers the ability to obtain simultaneous imaging with the MIRI imager while obtaining MRS spectroscopy.  Choose:

• YES
• NO

Grating Wheel Direction

For engineering purposes only, the direction of the grating wheels can be controlled with this limited access parameter.

Imager Subarray 

See also: MIRI Detector Subarrays

If you choose simultaneous imaging, MIRI imaging supports the use of a pre-defined set of Imager Subarrays for observing targets bright enough to saturate the image in full frame readout. Each subarray is associated with a brightness limit, depending on the filter used, above which a point source will saturate in the shortest 2-group integration. A MIRI simultaneous imaging observation can only support a single subarray. If the target must be imaged using another subarray, it is necessary to create another MIRI simultaneous imaging observation.

Exposure Parameters 

An MRS sequence is specified by first setting the Wavelength Range desired for each exposure. There are 3 available settings: SHORT (A), MEDIUM (B), and LONG (C). Each setting covers one-third of the wavelength range of each channel; a full observation from 4.9 to 27.9 μm  would thus require an exposure at each of these 3 settings.

Each sequence must then be completed by selecting a MULTIACCUM exposure configuration for each detector being used (in the Exposure Parameters dialog box). Each exposure is configured by setting the readout pattern and characteristics parameters: Readout Pattern, Groups/Int, and Integrations/Exp. If you choose simultaneous imaging, you must select an imaging Filter for each MRS sequence. Each MRS sequence can support only a single imaging Filter. Finally select a Dither pattern from the list of Dithers already defined for the observation (or None for no dithering). The exposures will be repeated at each dither position following an offset of the telescope to the new position. The number of exposures at each dither position is set by Exposures/Dith, but this input must be the same for all detectors.  

Readout Pattern

See also: MIRI Detector Readout Overview

MIRI offers 2 readout modes: 

• FASTR1 (default)
• SLOWR1 (useful for deep MRS observations)

Number of groups and Integrations

See also: Understanding JWST Exposure Times

The MIRI timing pattern per exposure is defined by only 2 MULTIACCUM parameters: 

• Groups/Int: the number of groups during an integration, where a group is the product of cycling through all the pixels
• Integrations/Exp: the number of integrations during an exposure, where integration is defined as the time between resets. 

Other tabs

Mosaic Properties

See also: JWST Mosaic Overview, MIRI MRS Mosaics

See also:  Specifying Mosaics in APT 

The MIRI MRS may be used to obtain data for a region larger than their size by creating a MIRI MRS mosaic pattern under the Mosaic Properties tab.

Special Requirements

A variety of observatory level Special Requirements may be chosen under the Special Requirements tab.

Programs taking dedicated backgrounds should add the Timing special requirement of SEQUENCE NON-INTERRUPTIBLE between the science observation and dedicated background.

Comments

The Comments field (under the Comments tab) should be used for observing notes.