JWST user documentation is under development; current versions are preliminary and subject to revision.

Page tree

Versions Compared

Key

  • This line was added.
  • This line was removed.
  • Formatting was changed.

Content Layer
id277319515
Content Column
id277319517
classpdf-full
Content Block
overflowauto
nameSummary
id277319516

The target acquisition procedures used to point the telescope with different JWST NIRSpec observing modes are addressed.

Content Block
overflowauto
nameContent
id277805920

Introduction 

Following a slew of the JWST spacecraft, the JWST on-board software can execute target acquisition (TA) corrections that are designed to remove any initial pointing errors - i.e., inaccuracies in the absolute guide star and science target positions.  The purpose of the NIRSpec TA procedures is to fine-tune the JWST pointing to very accurately place a given target into one of the fixed slits, the IFU, or to precisely align a set of targets within the grid of MSA shutters.

Two methods have been designed for NIRSpec target acquisition: standard TA using the MSA (called TACQ), and the bright object TA (BOTA) through the wide FS aperture. Like all JWST observations, the NIRSpec standard target acquisition is executed autonomously.  A third option, called "VERIFY_ONLY", is available in some observing modes to only assess the field pointing without executing a target acquisition sequence or corrective small angle maneuvers of the telescope pointing.

 


Available methods for NIRSpec target acquisition

NIRSpec observers can choose between three available methods for target acquisition methods: the standard method (called TACQ), the Bright Object TA (BOTA) method and the VERIFY_ONLY option. Because different NIRSpec science modes require different degrees of pointing accuracy, not all methods are available in all modes. Table 1 shows the availability of TA methods vs. the science observing mode.


Table 1. Target acquisition methods available for each NIRSpec observing mode. 


Target Acquisition Option
NIRSpec Observing Modes or Methods
MOSIFUFSBOTSMoving Targets
NIRSpec Standard TA (TACQ)

  
NIRSpec VERIFY_ONLY

  

NIRSpec Bright Object TA (BOTA)   

 


The standard TA option, TACQ uses reference stars observed through the open MOS shutters in the MSA to accurately align science sources with their dedicated apertures.  The following table describes the various TA methods, their expected performances, and the methods used to compute the corrective small-angle maneuver.

 

Table 2. NIRSpec Target acquisition methods with accuracy and brief descriptions.
TA TypeTA Approximate AccuracyDescription
Standard target acquisition (TACQ)

Expected accuracies of approximately 20 - 60 milli-arcseconds (Depends on relative astrometric accuracy of reference targets and science sources).

TACQ will be used for most NIRSpec science observations performed with the micro shutter assembly (MSA). The TA reference objects are observed through the open micro-shutters, and centroids are calculated and used to accurately correct the initial pointing and orient for all NIRSpec observations that use TACQ. The delivered TA performance depends significantly on the astrometric accuracy of both the reference stars and the science sources in the catalog used to plan the TA and the science. The TACQ procedure is designed to work with input relative astrometric accuracy of 5 to 50 milli-arcseconds. Relative astrometric accuracy of 5 mas in the TA and science planning catalog is necessary to deliver a TA accuracy of 20 milli-arcseconds.  This is the best estimated accuracy achievable for placing science targets into the narrowest NIRSpec apertures: the 0.2" wide NIRSpec MSA slitlets and the 0.2" NIRSpec fixed slits
VERIFY_ONLYJWST Pointing Performance - 0".68, 1sigma radial.The NIRSpec VERIFY_ONLY method relies only on guide star acquisition performed by the FGS to establish the telescope pointing. This method is ideal for placing compact sources in the IFU aperture, or when using the MSA to place a long slit on extended objects. VERIFY_ONLY can also be used for placing moving targets within the IFU aperture. Presently, this option is only available in IFU and MOS science modes, the expected accuracy can be too poor for the narrow FSs.
Bright object target acquisition (BOTA)Expected accuracy of target placement in the NIRSpec Wide Aperture to 20 milli-arcseconds (Depends primarily on centroiding accuracy of the target).The NIRSpec bright object target acquisition (BOTA) is the default method used to acquire most targets for high S/N spectrophotometric observations in bright object time series (BOTS) mode, including transiting exoplanets in the brightness range 10.0 < K < 16.1 (or, alternatively, 11.9 < AB < 18.1).  This method corrects any errors in absolute telescope pointing (and absolute knowledge of the source coordinates) by executing a TA centroid of the science target and offsetting it to the center of the wide aperture.

 

 


 

Content Block
overflowauto
nameRelated links
id280628021

Related links

JWST User Documentation Home
NIRSpec Observing Modes
NIRSpec Multi-Object Spectroscopy
NIRSpec IFU Spectroscopy
NIRSpec Fixed Slits Spectroscopy
NIRSpec Bright Object Time-Series Spectroscopy
JWST Astronomers Proposal Tool, APT
JWST APT website 
JWST Exposure Time Calculator
JWST ETC website

Content Block
overflowauto
nameReferences
id281944791

References

JWST technical documents

Content Column
id277319512
classpdf-hide
Content Block
overflowauto
nameInstrument icon
id278719485

Multiexcerpt include
MultiExcerptNameNIRSpec icon and Name
PageWithExcerptMR:NIRSpec icon and name

Content Block
nameUpdates
id1074221578
HTML Wrap
padding0
margin0
idupdatesbox
HTML Wrap
tagspan
classupdatesbox-heading

Last updated

Published July 29, 2017


 

Comment

The format for 'Last updated' is shown below. Enter updates above this internal"Comment" box. Only enter major updates (not typos or formatting changes).


Updated April 05, 2017

  • Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam fermentum vestibulum est. Cras rhoncus. 
  • Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Sed quis tortor. 

Published March 02, 2017


Restrict Content
groupsconfluence-authors

Tables are only visible to JDox content providers

"Updated" refers to the updated or publish dates for the public pages.

Page properties
idcomments
UpdatedJuly 29, 2017
Open commentsNo
TeamNIRSpec
AuthorBeck, Ubeda, Wislowski
Page properties
idlayout
UpdatedJuly 29, 2017
Fix pageNo
Type-
Teameditors