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Guide star data products

The FGS guiding capabilities are provided by 4 guide star functions: Identification, Acquisition, Track, and Fine Guide. Data downlinked by these functions is processed by DMS to provide uncalibrated and calibrated guide star data products. The uncalibrated products consist of raw pixel value data arrays, as well as different kinds of tabular information related to the guide stars and centroid locations of the guide star as computed by the on-board FGS Flight Software (FSW). Calibrated guide star products are created by the calwebb_guider <calwebb_guider> pipeline. Briefly, the processing performed applies bad pixel masks and flat-fields the science data, as well as computing countrate images from the multiple groups within each integration contained in a given product. The countrate images, which are computed by the guider_cds <guider_cds_step> step, are computed for most modes by simply differencing groups 1 and 2 in each integration and dividing by the group time.

File naming

Guide star product file names contain identifiers related to the function in use and the time at which the data were obtained. The table below lists the file name syntax used for each of the guiding functions and the related value of the EXP_TYPE keyword.

+----------------+---------------+-----------------------------------------------------+ | Function | EXP_TYPE | File name | +================+===============+=====================================================+ | Identification | FGS_ID-IMAGE | jw<pppppooovvv>_gs-id<m>_image-uncal.fits | + +---------------+-----------------------------------------------------+ | | FGS_ID-STACK | jw<pppppooovvv>_gs-id<m>_stacked-uncal.fits | +----------------+---------------+-----------------------------------------------------+ | Acquisition | FGS_ACQ1 | jw<pppppooovvv>_gs-acq1<yyyydddhhmmss>_uncal.fits | + +---------------+-----------------------------------------------------+ | | FGS_ACQ2 | jw<pppppooovvv>_gs-acq2<yyyydddhhmmss>_uncal.fits | +----------------+---------------+-----------------------------------------------------+ | Track | FGS_TRACK | jw<pppppooovvv>_gs-track<yyyydddhhmmss>_uncal.fits | +----------------+---------------+-----------------------------------------------------+ | Fine Guide | FGS_FINEGUIDE | jw<pppppooovvv>_gs-fg<yyyydddhhmmss>_uncal.fits | +----------------+---------------+-----------------------------------------------------+

where the file name fields are:

jw

mission identifier

ppppp

program id

ooo

observation number

vvv

visit number

m

ID attempt counter (1-8)

yyyydddhhmmss

time stamp at the end of the data in the file

Uncalibrated products use the "uncal" file name suffix as shown above, while calibrated products use a "cal" suffix. The relevance of the "image" and "stacked" designations for the Identification mode products is described below.

ID mode

The "Identification" guiding function images the field of view by reading the detector in a series subarray "strips" that, collectively, cover most of the field. A total of 36 subarray strips are read out, each of which is 64 x 2048 pixels in size. Each strip has 8 pixels of overlap with its adjoining strips, resulting in a total of 2024 unique detector rows that've been read out. ID mode uses 2 groups per integration and 2 integrations, resulting in a total of 4 reads. Each subarray strip has its 4 reads performed before moving on to the next subarray.

DMS creates 2 different forms of products for ID mode data: one in which an image is constructed by simply stacking or butting the data from adjacent subarray strips against one another and the other in which the overlap regions of the strips are taken into account by averaging the pixel values. The first form is referred to as a "stacked" product and the second as an "image" product.

The FITS file structure for uncalibrated ID "image" products is as follows:

HDU EXTNAME HDU Type Data Type Dimensions

0

 N/A primary N/A N/A

1

 SCI IMAGE uint16 2024 x 2048 x 2 x 2

2

 Flight Reference Stars BINTABLE N/A 4 cols x nstars

3

 Planned Reference Stars BINTABLE N/A 10 cols x nstars
  • SCI: 4-D data array containing the raw pixel values. The subarray overlaps have been accounted for, resulting in image dimensions of 2024 x 2048 pixels, with the 2 groups and 2 integrations stacked along the 3rd and 4th array axes.
  • Flight Reference Stars: A table containing information on the actual reference stars used by the FSW. Detailed contents are listed below <flight_ref_stars>.
  • Planned Reference Stars: A table containing information on the planned reference stars. Detailed contents are listed below <planned_ref_stars>.

The FITS file structure for uncalibrated ID "stacked" products is as follows:

HDU EXTNAME HDU Type Data Type Dimensions

0

 N/A primary N/A N/A

1

 SCI IMAGE uint16 2304 x 2048 x 2 x 2

2

 Flight Reference Stars BINTABLE N/A 4 cols x nstars

3

 Planned Reference Stars BINTABLE N/A 10 cols x nstars
  • SCI: 4-D data array containing the raw pixel values. The subarray data are butted against one another, resulting in image dimensions of 2304 x 2048 pixels, with the 2 groups and 2 integrations stacked along the 3rd and 4th array axes.
  • Flight Reference Stars: A table containing information on the actual reference stars used by the FSW. Detailed contents are listed below <flight_ref_stars>.
  • Planned Reference Stars: A table containing information on the planned reference stars. Detailed contents are listed below <planned_ref_stars>.

The FITS file structure for calibrated ID "image" products is as follows:

HDU EXTNAME HDU Type Data Type Dimensions

0

 N/A primary N/A N/A

1

 SCI IMAGE float32 2024 x 2048 x 1

2

 ERR IMAGE float32 2024 x 2048 x 1

3

 DQ IMAGE uint32 2024 x 2048

4

 Flight Reference Stars BINTABLE N/A 4 cols x nstars

5

 Planned Reference Stars BINTABLE N/A 10 cols x nstars

6

 ASDF BINTABLE N/A variable
  • SCI: 3-D data array containing the pixel values, in units of DN/s. The data for the 2 integrations has been combined into a single image, as is done by the on-board FSW, resulting in a data array with NAXIS3 = 1.
  • ERR: 3-D data array containing uncertainty estimates for each pixel.
  • DQ: 2-D data array containing DQ flags for each pixel.
  • Flight Reference Stars: A table containing information on the actual reference stars used by the FSW. Detailed contents are listed below <flight_ref_stars>.
  • Planned Reference Stars: A table containing information on the planned reference stars. Detailed contents are listed below <planned_ref_stars>.
  • ADSF: The data model meta data.

The FITS file structure for calibrated ID "stacked" products is as follows:

HDU EXTNAME HDU Type Data Type Dimensions

0

 N/A primary N/A N/A

1

 SCI IMAGE float32 2304 x 2048 x 1

2

 ERR IMAGE float32 2304 x 2048 x 1

3

 DQ IMAGE uint32 2304 x 2048

4

 Flight Reference Stars BINTABLE N/A 4 cols x nstars

5

 Planned Reference Stars BINTABLE N/A 10 cols x nstars

6

 ASDF BINTABLE N/A variable
  • SCI: 3-D data array containing the pixel values, in units of DN/s. The data for the 2 integrations has been combined into a single image, as is done by the on-board FSW, resulting in a data array with NAXIS3=1.
  • ERR: 3-D data array containing uncertainty estimates for each pixel.
  • DQ: 2-D data array containing DQ flags for each pixel.
  • Flight Reference Stars: A table containing information on the actual reference stars used by the FSW. Detailed contents are listed below <flight_ref_stars>.
  • Planned Reference Stars: A table containing information on the planned reference stars. Detailed contents are listed below <planned_ref_stars>.
  • ADSF: The data model meta data.

Flight reference stars table

The structure and content of the Flight Reference Stars table is as follows.

Column Name Data Type Description
reference_star_id char*2 Reference star index
id_x float64 x position in FGS Ideal frame
id_y float64 y position in FGS Ideal frame
count_rate float64 count rate

Planned reference stars table

The structure and content of the Planned Reference Stars table is as follows.

Column Name Data Type Description
guide_star_order int32 Guide star index within list
reference_star_id char*12 GSC II identifier
ra float64 ICRS RA of the star
dec float64 ICRS Dec of the star
id_x float64 x position in FGS Ideal frame
id_y float64 y position in FGS Ideal frame
fgs_mag float64 magnitude
fgs_mag_uncert float64 magnitude uncertainty
count_rate float64 count rate
count_rate_uncert float64 count rate uncertainty

ACQ1 mode

The "Acquisition" guiding function ACQ1 performs 128 x 128 pixel subarray readouts of the detector, using 2 groups per integration and a total of 6 integrations. The FITS file structure for ACQ1 uncalibrated products is as follows:

HDU EXTNAME HDU Type Data Type Dimensions

0

 N/A primary N/A N/A

1

 SCI IMAGE uint16 128 x 128 x 2 x 6
  • SCI: 4-D data array containing the raw pixel values.

The FITS file structure for ACQ1 calibrated products is as follows:

HDU EXTNAME HDU Type Data Type Dimensions

0

 N/A primary N/A N/A

1

 SCI IMAGE float32 128 x 128 x 6

2

 ERR IMAGE float32 128 x 128 x 6

3

 DQ IMAGE uint32 128 x 128

4

 ASDF BINTABLE N/A variable
  • SCI: 3-D data array containing the pixel values, in units of DN/s. Count rate images have been computed for each of the 6 integrations by differencing the 2 groups of each integration.
  • ERR: 3-D data array containing uncertainty estimates for each pixel.
  • DQ: 2-D data array containing DQ flags for each pixel.
  • ADSF: The data model meta data.

ACQ2 mode

The "Acquisition" guiding function ACQ2 performs 32 x 32 pixel subarray readouts of the detector, using 2 groups per integration and a total of 5 integrations. The FITS file structure for ACQ2 uncalibrated products is as follows:

HDU EXTNAME HDU Type Data Type Dimensions

0

 N/A primary N/A N/A

1

 SCI IMAGE uint16 32 x 32 x 2 x 5
  • SCI: 4-D data array containing the raw pixel values.

The FITS file structure for ACQ2 calibrated products is as follows:

HDU EXTNAME HDU Type Data Type Dimensions

0

 N/A primary N/A N/A

1

 SCI IMAGE float32 32 x 32 x 5

2

 ERR IMAGE float32 32 x 32 x 5

3

 DQ IMAGE uint32 32 x 32

4

 ASDF BINTABLE N/A variable
  • SCI: 3-D data array containing the pixel values, in units of DN/s. Count rate images have been computed for each of the 5 integrations by differencing the 2 groups of each integration.
  • ERR: 3-D data array containing uncertainty estimates for each pixel.
  • DQ: 2-D data array containing DQ flags for each pixel.
  • ADSF: The data model meta data.

Track mode

The "Track" guiding function performs 32 x 32 pixel subarray readouts, the location of which can move on the detector as the FGS FSW tracks the position of the guide star. The subarray readouts are performed with a cadence of 16 Hz. Each integration consists of 2 groups, and the total number of integrations (NINTS) can be very large (in the thousands). The FITS file structure for TRACK uncalibrated products is as follows:

HDU EXTNAME HDU Type Data Type Dimensions

0

 N/A primary N/A N/A

1

 SCI IMAGE uint16 32 x 32 x 2 x nints

2

 Pointing BINTABLE N/A 12 cols x nrows

3

 FGS Centroid Packet BINTABLE N/A 17 cols x nrows

4

 Track subarray table BINTABLE N/A 5 cols x nrows
  • SCI: 4-D data array containing the raw pixel values.
  • Pointing: A table containing guide star position and jitter information. See below <pointing_table> for details of the contents.
  • FGS Centroid Packet: A table containing guide star centroiding information. See below <centroid_table> for details of the contents.
  • Track subarray table: A table containing subarray information over the duration of the product. See below <subarray_table> for details of the contents.

The FITS file structure for TRACK calibrated products is as follows:

HDU EXTNAME HDU Type Data Type Dimensions

0

 N/A primary N/A N/A

1

 SCI IMAGE float32 32 x 32 x nints

2

 ERR IMAGE float32 32 x 32 x nints

3

 DQ IMAGE uint32 32 x 32

4

 POINTING BINTABLE N/A 12 cols x nrows

5

 FGS CENTROID PACKET BINTABLE N/A 17 cols x nrows

6

 TRACK SUBARRAY TABLE BINTABLE N/A 5 cols x nrows

7

 ASDF BINTABLE N/A variable
  • SCI: 3-D data array containing the pixel values, in units of DN/s. Count rate images for each integration have been computed by differencing the 2 groups in each integration.
  • ERR: 3-D data array containing uncertainty estimates for each pixel.
  • DQ: 2-D data array containing DQ flags for each pixel.
  • Pointing: A table containing guide star position and jitter information. See below <pointing_table> for details of the contents.
  • FGS Centroid Packet: A table containing guide star centroiding information. See below <centroid_table> for details of the contents.
  • Track subarray table: A table containing subarray information over the duration of the product. See below <subarray_table> for details of the contents.
  • ADSF: The data model meta data.

Pointing table

The structure and content of the Pointing table is as follows.

Column Name Data Type Units Description
time float64 milli-sec Time since start of data file
jitter float64 milli-arcsec sqrt(delta_ddc_ra2 + delta_ddc_dec2)
delta_ddc_ra float64 milli-arcsec Initial DDC RA - Current
delta_ddc_dec float64 milli-arcsec Initial DDC Dec - Current
delta_aperture_pa float64 milli-arcsec Initial PA - Current
delta_v1_ra float64 milli-arcsec Initial V frame RA - Current
delta_v1_dec float64 milli-arcsec Initial V frame Dec - Current
delta_v3_pa float64 milli-arcsec Initial V frame PA - Current
delta_j1_ra float64 milli-arcsec Initial J frame RA - Current
delta_j1_dec float64 milli-arcsec Initial J frame Dec - Current
delta_j3_pa float64 milli-arcsec Initial J frame PA - Current
HGA_motion int32 N/A
HGA state: 0 = moving,
1 = finished, 2 = offline

FGS Centroid Packet table

The structure and content of the Centroid Packet table is as follows.

Column Name Data Type Description
observatory_time char*23 UTC time when packet was generated
centroid_time char*23 Fine guidance centroid time
guide_star_position_x float64 FGS Ideal Frame (arcsec)
guide_star_position_y float64 FGS Ideal Frame (arcsec)
guide_star_instrument_counts_per_sec float64 Instrument counts/sec
signal_to_noise_current_frame float64 For current image frame
delta_signal float64 Between current and previous frame
delta_noise float64 Between current and previous frame
psf_width_x int32 Bias from ideal guide star position (pixels)
psf_width_y int32 Bias from ideal guide star position (pixels)
data_quality int32 Centroid data quality
bad_pixel_flag char*4 Bad pixel status for current subwindow (0/1)
bad_centroid_dq_flag char*50 Bad centroid for current subwindow (0/1)
cosmic_ray_hit_flag char*5 NO/YES
sw_subwindow_loc_change_flag char*5 NO/YES
guide_star_at_detector_subwindow_boundary_flag char*5 NO/YES
subwindow_out_of_FOV_flag char*5 NO/YES

Track Subarray table

The Track Subarray table contains location and size information for the detector subarray window that is used during the track function to follow the guide star. The structure and content of the Track Subarray table is as follows.

Column Name Data Type Description
observatory_time char*23 UTC time when packet was generated
x_corner float64 Subarray x corner (pixels)
y_corner float64 Subarray y corner (pixels)
x_size int16 Subarray x size (pixels)
y_size int16 Subarray y size (pixels)

FineGuide mode

The "FineGuide" guiding function performs 8 x 8 pixel subarray readouts, at a fixed location on the detector, and with a cadence of 16 Hz, from which the FGS FSW computes centroids for the guide star. To reduce readout noise contribution to the centroid calculation, "Fowler" sampling of the readouts is employed. Each integration consists of 4 readouts at the beginning, a signal accumulation period, and 4 readouts at the end. The detector is then reset and the readout cycle repeats for the next integration. The 4 readouts at the beginning are averaged together, the 4 readouts at the end are averaged together, and then the difference of the 2 averages is computed to form a final countrate image for each integration. This approach to creating the countrate images is used both on-board and in the calwebb_guider <calwebb_guider> pipeline when the raw data are processed on the ground.

The FITS file structure for FineGuide uncalibrated products is as follows:

HDU EXTNAME HDU Type Data Type Dimensions

0

 N/A primary N/A N/A

1

 SCI IMAGE uint16 8 x 8 x 8 x nints

2

 Pointing BINTABLE N/A 12 cols x nrows

3

 FGS Centroid Packet BINTABLE N/A 17 cols x nrows
  • SCI: 4-D data array containing the raw pixel values.
  • Pointing: A table containing guide star position and jitter information. See above <pointing_table> for details of the contents.
  • FGS Centroid Packet: A table containing guide star centroiding information. See above <centroid_table> for details of the contents.

The FITS file structure for FineGuide calibrated products is as follows:

HDU EXTNAME HDU Type Data Type Dimensions

0

 N/A primary N/A N/A

1

 SCI IMAGE float32 8 x 8 x nints

2

 ERR IMAGE float32 8 x 8 x nints

3

 DQ IMAGE uint32 8 x 8

4

 POINTING BINTABLE N/A 12 cols x nrows

5

 FGS CENTROID PACKET BINTABLE N/A 17 cols x nrows

6

 ASDF BINTABLE N/A variable
  • SCI: 3-D data array containing the pixel values, in units of DN/s. Count rate images for each integration have been computed using the Fowler sampling scheme described above.
  • ERR: 3-D data array containing uncertainty estimates for each pixel.
  • DQ: 2-D data array containing DQ flags for each pixel.
  • Pointing: A table containing guide star position and jitter information. See above <pointing_table> for details of the contents.
  • FGS Centroid Packet: A table containing guide star centroiding information. See above <centroid_table> for details of the contents.
  • ADSF: The data model meta data.