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 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 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

Acquistion

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.

  • Planned Reference Stars: A table containing information on the planned reference stars. Detailed contents are listed below.

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.

  • Planned Reference Stars: A table containing information on the planned reference stars. Detailed contents are listed below.

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.

  • Planned Reference Stars: A table containing information on the planned reference stars. Detailed contents are listed below.

  • 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.

  • Planned Reference Stars: A table containing information on the planned reference stars. Detailed contents are listed below.

  • 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 “Acquistion” 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 for details of the contents.

  • FGS Centroid Packet: A table containing guide star centroiding information. See below for details of the contents.

  • Track subarray table: A table containing subarray information over the duration of the product. See below 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 for details of the contents.

  • FGS Centroid Packet: A table containing guide star centroiding information. See below for details of the contents.

  • Track subarray table: A table containing subarray information over the duration of the product. See below 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

\(sqrt(delta\_ddc\_ra^2 + delta\_ddc\_dec^2)\)

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 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 for details of the contents.

  • FGS Centroid Packet: A table containing guide star centroiding information. See above 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 for details of the contents.

  • FGS Centroid Packet: A table containing guide star centroiding information. See above for details of the contents.

  • ADSF: The data model meta data.