Introduction

A Fixed Target specification defines where to point JWST for an observation and so must be filled out with care, precision, and accuracy.   For proposals with a large number of fixed targets, please note that there is a capability to ingest a comma-separated text file with the appropriate target information.

Target Identification 

The following information is used to identify and classify each target. A different target must be defined whenever different coordinates are required. For instance, separate targets should be defined and listed if you plan to take observations at several points within an extended object. (If you want to take images at three different locations in the Crab Nebula, each point must have its own target number, name, and coordinates.)

Number 

APT assigns each target a number from 1 to 999. This number is unique within the target list but may be changed by the user.

Name in the Proposal 

NAME IN THE PROPOSAL provides a unique designation for a target for use throughout the proposal.  For example, for three locations in the Crab Nebula, you could use the names: CRAB1, CRAB2, and CRAB3.

 The following restrictions should be noted for the target name in the proposal:

• The length of a target name is limited to 2 to 31 characters.
• Only upper and lower-case letters, numerals, '.', '+', '_' and '-' are allowed. Blank spaces are not allowed.
• Two targets may not have the same name, or the same name with different cases.

Name for the Archive 

Using a common name for the NAME IN THE PROPOSAL (e.g. CRABNEBULA instead of NGC1952) can make a proposal more readable. Annotating the name (e.g. CRAB1, CRAB2) may be useful when sampling parts of an extended object. But these names are not as helpful for people searching the Archive. So, there is an optional second target name – the NAME FOR THE ARCHIVE. Proposers and archival researchers use these names to determine whether JWST has observed a particular object. This facility will be most useful if the names supplied are resolvable with standard name resolvers such as NED or SIMBAD. This name is not required, but if used is limited to 2 to 31 characters in length.

Target Description 

One to five Target Descriptions must be selected for each target. The Target Descriptions will be one of the fields used by archival researchers in searching through the JWST data archive; thus it is extremely important that the information be supplied by the observer who knows the intention for each target.

Each target must be assigned a single primary category from Table 1 , and at least one descriptive keyword, chosen from the appropriate  tables (see Table 1 for which table is appropriate for each category).

Table 1. Target Categories 

Target Coordinates 

The following information is required to allow for JWST to be properly pointed at the target.

Required Accuracy of Target Positions 

JWST uses one guide star to stabilize the pointing of the telescope and to place the target in the desired aperture. The fundamental problem, then, is to determine the position of the target relative to the guide star in the surrounding area with sufficient accuracy to place the target in the aperture. The specific guide star to be used cannot be determined in advance of the observation; several possible stars will often be available for each target. The guide stars are chosen from the Guide Star Catalog 2 (GSC2). The absolute pointing accuracy of JWST, without a science target acquisition, is  0.10" (1-σ, radial).  Errors in the proposer specified ICRS coordinates of the science target will contribute additional pointing errors. Targets that need to be placed within small apertures (coronagraphs, spectroscopic slits, etc) will require onboard target acquisition.

Inaccurate target coordinates can result in failed target acquisitions and can therefore waste valuable JWST observing time.  Please contact the JWST Help Desk at jwsthelp.stsci.edu if you need additional information. 

The Principal Investigator of a proposal is responsible for ensuring that target coordinates are accurate.

Targets near the Celestial Poles: Be very careful if your target lies near a celestial pole. Many precession routines break down in this regime, and uncertainties in position are exacerbated in this region. Also, patterns that you may execute with an instrument could cross the pole, leading to confusion in position. All these issues can be resolved, but careful attention is needed.

ICRS Coordinates, Epoch, and Uncertainty 

The right ascension and declination of the target must be provided.  These coordinates should only be supplied in the International Celestial Reference System (ICRS).

• Right ascension can be entered in decimal degrees (e.g. 15.0), and APT will immediately reformat as HMS (e.g. 01H 00M 00.0S). Note that it is not possible to input RA in decimal hours.
• Declination can be entered in decimal degrees (e.g. -20.5), and APT will immediately reformat as DMS (e.g. -20D 30’ 00.0”).
• Epoch is optional unless Proper Motion is supplied. (See section below on Proper Motion.)

If the sign of the declination is not indicated, a positive declination is assumed, but we urge you to always include the sign as a way of reducing errors.

Coordinate uncertainties are optional. Multiple units for uncertainty are available:

• The uncertainties should represent the accuracy (1 sigma) of the target coordinates, not the region within which a tar­get could be observed (e.g., for a sky measurement).

Determining Coordinates in the Reference Frame Appropriate for JWST Observations 

The JWST reference frame is effectively defined by the positions of the Guide Star used for each pointing. JWST uses the Guide Star Catalog (GSC2), which is an all-sky catalog of stars calibrated to be in the International Celestial Reference System (ICRS), which has been adopted by the IAU as the new fundamental reference frame.

For observations it is vital that you provide positions derived in the ICRS reference frame.

General Guidelines

• If your target has a position that is in a catalog using the ICRS you may use the coordinates directly. These include GSC2, Hipparcos, Tycho, SDSS, 2MASS and FIRST.
• If your target is an extended object where the observation position does not correspond to the catalog coordinates, we recommend that you obtain an image of the field and measure your target coordinates in the ICRS reference frame.
• If your target has non-negligible proper motion, you must provide the epoch of the coordinates as well as the proper motion values. 

Access to the GSC2, the Digitized Sky Survey (DSS) and other cata­logs/surveys is built into the Aladin interface in APT (Note: for the DSS use the POSS2UKSTU-Red plates that were used to build GSC2).

Getting Coordinates from the GSC2 or DSS

• If your target is a star brighter than m(V)~20 then it typically will be visible on the DSS images and present in the GSC2 catalog. Using the GSC2 position will ensure that the target is in the same reference frame as the guide star.
• For extended sources visible on the photographic survey plates, we strongly recommend that you examine the DSS image and check the coordinates of your target. Depending on the brightness, morphology and structure of the galaxy the GSC2 coordinate may not correspond to the aperture location you require for your observation. The DSS headers downloaded from STScI contain ICRS-based FITS WCS information to allow you to measure the image using various image analysis tools.
• Please note that the GSC2 coordinates for bright stars come from the Tycho2 catalog as these are more accurate than positions measured from the Schmidt plates.

As part of preparing your fixed targets, you must create Target Confirmation Charts in APT which show the target coordinates (as entered in the proposal) overlaid on the field from the DSS.

A note on Pre Imaging

If it is impossible to obtain adequate coordinates with the required accuracy (for example, for observing a very crowded field which cannot be resolved using ground-based observations, or for spectral observations using tiny apertures as for the NIRSpec MSA, or for sources with large proper motion or proper motion uncertainties determined from observations taken more than several years prior), it may be necessary to obtain a pre image with JWST. 

You are responsible for verifying that the coordinates are correct.

Is Proper Motion or Parallax Relevant? 

If a small aperture or occulter  is to be used, even a relatively small proper motion or parallax may cause difficulties in acquiring the target. In such cases, the Proper Motion/Parallax data must be provided. In order to enforce that proper motion has been considered there is a checkbox to indicate that proper motion is NEGLIGIBLE OR NOT APPLICABLE. Unless this check box is checked, observations that require very accurate coordinates will generate an error for the target if proper motion is not supplied.

The observer must determine whether or not proper motion or parallax is relevant. In general, this will depend on the size of the acquisition aperture of the SI that will be used and the epoch of the coordinates that have been provided. For example, the NIRISS SOSS mode uses a target acquisition area of 4.2 arcsec square. For a star whose coordinates are given in Epoch 2000.0, and that will be observed in 2025.0, a proper motion of 0.05”/year yields a total motion of 1.25", which is greater than half the minimum center-to-edge distance (1.1”) of the aperture, and therefore relevant.

Proper Motion and Annual Parallax Data 

The following additional information is required for targets where proper motion and parallax are “relevant”; note that uncertainties for Proper Motion and Annual Parallax are not required.

If a sign is not given for Proper Motion, a positive value will be assumed, but it is better to be explicit.

• When specifying PROPER MOTION RA and DEC in APT it is especially critical to select the same units as those given in the catalog referenced. (APT will do the conversion of units if needed.) The observation could fail if the wrong units are selected.

• The “EPOCH of position” is the date of the data from which the coordinates of the object were measured (or to which they have been corrected).  For example, coordinates from the SIMBAD Astronomical Database are usually corrected to epoch 2000 regardless of the original source epoch. But in the Guide Star Catalog2 (GSC2) the epoch depends on the individual plate or source catalog. It is not necessary to adjust your coordinates to be those that would be measured if the plate were taken in the year 2000 (you just have to properly report the epoch of the coordinates you are using).
• The only unit for ANNUAL PARALLAX is arcsec in the geocentric reference frame, used to automatically calculate the parallax for JWST.

Here is an example of a fully specified set of target pointing parameters:

Developer note: The units for RA PM and Dec PM should be arcseconds/year in the fixed target table of the database.

Extended 

EXTENDED = UNKNOWN (default), YES, NO

This parameter is used to indicate if the target is extended to support data pipeline processing, and is recommended for spectroscopic observations. When left as UNKNOWN, the pipeline will use default values based on the instrument configuration (e.g. template, wavelength, etc.).

Background Targets 

See also: Specifying APT Background Targets; Special fixed target cases: Mosaics, multi-object spectroscopic targets, and background targets

A target may be designated as requiring one or more background observations. Background observations may be linked to other observations so that background may be subtracted during pipeline processing.

Observations of this target require companion background observation(s) 

Selecting this option indicates that observations of this target require one or more companion background observations. 

Target(s) to be used as background observation(s) 

This option is only available when "Observations of this target require companion background observation(s)" is selected.

Select one or more targets to be used in background observations linked to observations of this target. Only targets from the current proposal are available to be selected.

A background observation:

• Observes a target selected to be used in a background observation, and
• Must have an exposure specification similar to the science observation. A similar exposure specification has the same instrument, template, grating, filter, subarray, and aperture, but not necessarily the same exposure time parameters.

Note that targets selected to be used in background observations cannot also be targets of science observations. If your proposal has targets that serve as both science targets and background targets, you will need to create the target twice with slightly different names. Otherwise, the data processing pipeline will not be able to automatically apply the correct processing.

Observations of targets designated as requiring background observations

For science observations of targets designated as requiring background observations, the science observation may be linked to its background observation with either SEQUENCE OBSERVATIONS NON -INTERRUPTIBLE or a GROUP OBSERVATIONS NON -INTERRUPTIBLE special requirement. Both observations must have similar exposure specifications.

• This link is required for the MIRI LRS, MIRI MRS, NIRSpec Fixed Slit, and NIRSpec IFU templates (APT provides an error).
• For MIRI Coronagraphic Imaging, this link is optional but recommended (APT provides a warning). 
• For all other templates this link is optional (APT provides a warning). 

Comments

Any additional information that you wish to enter can be entered in COMMENTS area. Comments are not interpreted by the software, but are maintained in the data base and do appear in the PDF output.

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