The Fixed Targets form is used to specify fixed targets in some JWST observations. The Astronomer's Proposal Tool (APT) is used to enter the targets into the proposal.
The Targets list tells us where you wish to point JWST 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 with the appropriate target information.
The following information is required to identify and classify each target.
Each target in your proposal will be assigned its own unique number, ranging from 1 to 999, by APT; note that these can be changed by the user.. A different target must be defined whenever different coordinates are required. Separate targets should be defined and listed if you plan to take observations at several points within an extended object. For example, if you were to take images at three different locations in the Crab Nebula, each point must have its own target number, name, and coordinates, such as CRAB1, CRAB2, and CRAB3.
Name in the Proposal
NAME IN THE PROPOSAL provides a unique designation for a target for use throughout the proposal. This target name is required, must be 2 to 31 characters in length and only use letters, numbers, “.”, “+”, and “-”. No other characters are allowed including blanks.
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 must be 2 to 31 characters in length.
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
|Cluster of Galaxies||Clusters of Galaxies table|
|Solar System||Solar System table|
|Stellar Cluster||Stellar Cluster table|
The following information is required to allow for JWST to be properly pointed at the target.
Required Accuracies 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 anticipated pointing accuracy of the telescope after acquisition of the guide star is required to be 1" (1σ, 1 axis), but actual performance achieved should be about 0.25" (1σ, 1 axis), as will be determined from inflight data. Errors in the proposer specified ICRS coordinates of the science target will contribute additional pointing errors. For 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.|
J2000 Coordinates and Uncertainty
The right ascension and declination <values> must be provided; the uncertainties are optional.
|RA: <value>||Dec: <value>|
Uncertainty RA: +/– <value>
|Uncertainty Dec: +/– <value>|
- The year of the equator and equinox to which the coordinates are referred must be J2000 (Julian). It is not necessary to apply precession corrections to coordinates from positional catalogs. The Guide Star Catalog and the Hipparcos Input Catalogue both use the J2000 equinox. (Note, however, that the Hipparcos Output Catalogue is epoch 1991.25, which means proper motions can have significant effects if you are not careful.)
- 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).
- The right ascension value must be expressed in hours (H), minutes (M), and seconds of time (S). Note that you can enter the right ascension in decimal degrees (e.g. 15.0), and APT will automatically converted it to HMS format (01H 00M 00.0S). Note that it is not possible to input RA in decimal hours.
- The declination value must be expressed in degrees (D), minutes ('), and seconds (") of arc. Note that you can enter the declination in decimal degrees (e.g. -20.5), and APT will automatically converted it to DMS format (e.g. -20D 30’ 00.0”).
- The units for the uncertainty must be selected from the following values:
|Quantity and units specified||Acceptable units for uncertainty|
|RA: H-M-S||timemin, timesec, arcmin, arcsec|
|Dec: D-M-S||degrees, arcmin, arcsec|
|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.|
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. We are using the Guide Star Catalog (GSC2), which is an all-sky catalog of stars calibrated to be on 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.|
- 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 a relevant proper motion, you must provide the epoch of the coordinate 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 your coordinates. 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 Confirmation Charts which show the target coordinates (as entered in the proposal) overlaid on the field from the DSS.
|You are responsible for verifying that the coordinates are correct.|
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.).
Note for developer: Add the line "Recommended for spectroscopy (for advice to data reduction pipeline)" after the parameter.
If it is impossible to obtain adequate plate material to measure coordinates to the required accuracy (for example, a very crowded field which cannot be resolved using ground-based observations, NIRSpec MSA observations), it may be necessary to obtain an early acquisition image with JWST. In that case, enter coordinates as accurate as possible on the Target List.
For science observations which require contemporaneous background observations, you need to associate the science and background observations so the background subtraction can be done during pipeline processing. The mechanism for associating the observations is to associate the targets. The following templates have been set up to support linked background observations: MIRI LRS, MIRI MRS, NIRSpec Fixed Slit, and NIRSpec IFU.
Observations of this target require companion background observation(s)
Choosing this option indicates that this target will be used in science observations which will require background observations. If this option is chosen you will be prompted for the target(s) that will serve as background.
Target(s) to be used as background observation(s)
If this target will require background observations, you must select the other target(s) in the proposal which will be used for those observations. Note that targets chosen to be background targets will not be able to also be science targets. If your proposal has targets which will serve both as science targets and background targets you will need to create the target twice with slightly different names.
Note to developer: Ensure that appropriate background observations have been created and will be executed at the same time, by issuing an error unless: Observations of a target marked as requiring a background observation are in a Sequence Non-Interruptible group and for each science exposure one of the background exposures in that sequence must be similar. A similar exposure has the same instrument, template, grating, filter, subarray, and aperture, but not necessarily the same exposure time parameters.
Is Proper Motion of 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.
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 for the epoch 1995.0, and that will be observed in 2020.0, 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 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.|
- RA: For Proper Motion in RA, the value can be in units of mas/year, arcsec/year, or seconds of time/year. The selection of units is especially critical for RA, as there is a large difference between 15 seconds of time/year and 15 arscec/year, so the observation will fail if improper units are provided.
- DEC: For Proper Motion in Declination, the value can be in units of mas/year or arcsec/year.
- EPOCH: The “Epoch of position” is the date of the the data from which the position of a star with proper motion was measured, or to which it has been corrected. Any epoch may be given but it must be correctly specified. SIMBAD coordinates are usually corrected to epoch 2000 regardless of the original source epoch.
- The “Epoch of position” may or may not be the same as the date of “Equinox for Coordinates” (J2000). Remember that the “Epoch of position” is the date the target position is referred to, whereas the “Equinox of Coordinates” is the date of the coordinate frame, which changes because of the precession of the Earth's axis. For example, a star with a large proper motion may have its coordinates given in the J2000 system, but the numbers themselves are for epoch 1984, meaning that the star was at the specified position on January 1, 1984. Epoch should be of the form 20yy.y or 19yy.y. (Note: An epoch is purely a time, and one of the form "J1991.25" is nonsensical.)
- Ordinarily the epoch of position is earlier than the present date. In the Guide Star Catalog2 (GSC2), the equinox is J2000 while the epoch depends on the individual plate. It is not necessary to adjust your coordinates to be those that would be measured if the plate were taken in the year 2000. However, some catalogs contain coordinates already adjusted to an epoch of 2000: the Hipparcos Input Catalogue (in cases where the star was known to have a proper motion at the time the catalog was generated; often used in the GSC2 for stars brighter than m(V) ~= 9) and the PPM Star Catalog. When these catalogs are being used, it is appropriate to specify an epoch of J2000. (These remarks do not apply to the Hipparcos Output Catalog.)
- ANNUAL PARALLAX: The unit for parallax is arcsec/year.
The example below is for the object DM–9D697 (Epsilon Eridani), where the proper motion data are taken from the SAO Catalog.
Proper Motion in RA
PROPER MOTION DEC
Proper Motion in Dec
Epoch of Position
20yy.y or 19yy.y
Developer note: The units for RA PM and Dec PM should be arcseconds/year in the fixed target table of the database.
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.
Version 7 (April 16, 2019)
- Editorial change (moved change log to main article)
Version 6 (April 3, 2018)
- Editorial change (fixed bad links)
Version 5 (December 28, 2017)
Version 4 (October 9, 2017)
- PR 83610 - added Extended parameter
Version 3 (May 11, 2017)
- Updated Target Description to only support 5 values.
Version 2 (October 6, 2016)
- PR 85913 and PR 63771 - rewrote the Target Description section
- PR 85608 - remove text on target flux and ETC Run number
Version 1 (June 16, 2016)
This is the converted Word File from Chapter 3.
Fixed Targets description