Reference Frames Relevant to FGS Operations and Data Products
Various reference frames relevant to Fine Guidance Sensor (FGS) operations (FGS ideal, FGS detector, science, V, J, and World Coordinate System) and data products are discussed in this article.
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Reference frames
As described below, the FGS, science instruments, and the Attitude Control System (ACS) use various reference frames to provide/exchange information. Users may encounter these frames in data products or technical documentation about guiding issues. A brief overview of each is given below.
FGS ideal frame
Most FGS onboard activities occur in this frame. The ideal coordinate system is a distortion-removed frame with its origin at the center of detector (X, Y = 1024.5, 1024.5), its Y axis aligned with that of the detector, and positions indicated in units of arcsec. See JWST Instrument Ideal Coordinate Systems for details.
FGS detector frame
This is the frame in which detectors are read out and pixels packetized. Stream data products (jwpppppvvvooo_gs-*_stream.fits, see Guiding Data Products Contents) remain in the detector frame, and should remain in the detector frame. Stream files are not typically used for science. The stream data files capture the stream of readouts in the detector frame. The origins and direction of the axes of the detector frames differ from guider 1 to guider 2, and also differ from the ideal and science frames.
Science frame
All guiding uncalibrated and calibrated file types are presented in this frame. The science frame and ideal coordinate frame are similar except that the science coordinate frame is not corrected for distortion. This frame is the representation normally displayed by science analysis software such as SAOImage DS9 and Ginga. Positions are indicated in units of pixels.
V-frame
The V-frame is mainly used for representing science instrument (SI) positions (and sources detected on them) on the focal plane. The origin is at the vertex of the primary mirror parent conic which is located near the entry point of light reflected from the Fine Steering Mirror (FSM). +V1 is the boresight of the telescope and points toward the secondary mirror, +V3 points away from the sunshield, and +V2 is orthogonal to both of these, forming the "thumb" of a right-handed coordinate system. All SI and FGS2 FOVs are calibrated with respect to FGS1’s reference location (center), which is assumed fixed in this frame. See JWST Target Observability, Observatory Coordinate System and JWST Field of Regard (FoR) for details.
J-frame
The J-frame is the spacecraft’s fundamental coordinate system and has its origin at the Observatory to Launch Vehicle Interface Ring (LVIR) which is located near the central bottom of the spacecraft. J1, J2, and J3 are roughly parallel to V1, V2, and V3, respectively. The J-frame is tied to both the Star Tracker Assemblies (STAs) and the local FGS 1 axes. See Spacecraft Bus for more details.
WCS (World Coordinate System)
See also: Using the Engineering Data Portal (documentation external to JDox)
The World Coordinate System (WCS) enables the conversion from pixel coordinates to RA and Dec, and may be reliably determined only for TRACK and fine guide (FG) data, when the Observatory is in closed-loop guiding.
Determining the World Coordinate System (WCS) for guide star pipeline data is challenging because there are no apertures specified in the Science Instrument Aperture File (SIAF) for the FGS guiding functions, and the V frame to reference pixel conversion is not available. To set a WCS for TRACK and FG guiding images, one may determine the position of the guide star on the detector from engineering telemetry (see below), with the corresponding RA and Dec of the guide star from the Guide Star Catalog (GSC). A user may determine what guide star was used for a particular science observation from the GDSTARID keyword in the primary header of the science FITS file. The status of the visit is available in the VISITSTA keyword in the primary science header, but keep in mind that occasionally, a visit will "succeed" on the wrong guide star. WCS parameters are listed in Table 2 below.
Table 2. WCS parameter keywords
| WCS Keyword | Description | Source |
|---|---|---|
WCSAXES | Number of WCS axes | Default value = '2' |
CRPIX1 | Guide star x position on the detector in the ideal coordinate frame | Guide star x position on the detector |
CRPIX2 | Guide star y position on the detector in the ideal coordinate frame | Guide star y position on the detector |
CRVAL1 | RA of the guide star (degrees) | PPSDB: candidate_guide_star: ra |
CRVAL2 | Dec of the guide star (degrees) | PPSDB: candidate_guide_star: dec |
CUNIT1 | Units of CRVAL1 | Default value = 'degrees' |
CUNIT2 | Units of CRVAL2 | Default value = 'degrees' |
CTYPE1 | First axis coordinate type | Default value = 'RA---TAN' |
CTYPE2 | Second axis coordinate type | Default value = 'DEC--TAN' |
CDELT1 | Increment per pixel, increasing eastward (degrees) | Read from SIAF for guider in use |
CDELT2 | Increment per pixel, increasing northward (degrees) | Read from SIAF for guider in use |
PC1_1 | Linear transformation matrix element | cos(theta) |
PC1_2 | Linear transformation matrix element | -sin(theta) |
PC2_1 | Linear transformation matrix element | sin(theta) |
PC2_2 | Linear transformation matrix element | cos(theta) |
Use the following engineering telemetry parameters to determine the guide star position on the detector for each guiding function. These are in units of arcsec and must be converted to pixels.
- For acquisition failures, use the commanded x and y position of the guide star for the WCS, SA_ZFGGSCMDX and SA_ZFGGSCMDY, respectively.
- For ID mode use IFGS_ID_XPOSG and IFGS_ID_YPOSG.
- For ACQ mode use IFGS_ACQ_XPOSG and IFGS_ACQ_YPOSG.
- For TRACK & FG modes use IFGS_CTDGS_X and IFGS_CTDGS_Y.
The WCS reference pixel (i.e., the origin of the WCS frame) is the center of the guider.
An example Python script to retrieve engineering telemetry from MAST is shown on the JWST Time-Series Observations Noise Sources article.