2.4 COS Data Products

The following sections discuss the COS raw science data files, intermediate calibration products, final calibration products, and auxiliary data files. Uncalibrated science data include all raw science data generated during Generic Conversion that have not been processed through the calibration pipeline. These raw files are the input files to the calcos pipeline, usually as part of an association file (see "Association Tables (ASN)" in Section 2.4.4). The pipeline produces both individual calibrated exposure files and, when appropriate, a final combined product file.

2.4.1 Uncalibrated Science Data Files

Raw ACCUM Images (rawaccum)

For ACCUM data, the raw files contain a set of images, as shown in Figure 2.1, and have filenames with the suffix rawaccum for NUV data, or rawaccum_a and rawaccum_b for the two segments of the FUV detector. The SCI extension contains an image of the total accumulated counts during an exposure. For NUV data the ERR and DQ extensions have only a header with no data. For FUV data the ERR extension has only a header with no data, and the DQ extension is populated with data quality information only for pixels that are outside the subarray boundaries (defined below). The DQ extensions will be populated in the flt files, after calibration pipeline processing. Even though FUV rawaccum_a[b] data are 16384 × 1024 images, only portions of them contain actual data. These portions are called subarrays. Typically, three subarrays are used for each segment of an FUV ACCUM image. Two are centered on the stim pulse positions and the third is a stripe 128 pixels high which is centered on the wavecal spectrum of the object. Figure 2.2 shows these spectral region subarrays superimposed on two FUV rawtag images. As Figure 2.2 shows, the wavecal spectrum falls outside of the subarray. Consequently, wavecals must be taken separately for ACCUM data.

Figure 2.2: Overlay of FUV ACCUM Subarrays on FUV TIME-TAG Data.



The above figures show FUV TAGFLASH data for both segments with the corresponding ACCUM subarrays noted by the dark lines. The data plotted here are the raw event locations prior to calibration processing. The distortion in the data, particularly for segment A, is very noticeable and discussed further in Section 3.4.5. Note that the full active area in Y is not shown.

Raw TIME-TAG Events Lists (rawtag)

Raw events tables contain the locations and arrival times of individual photon events collected in TIME-TAG mode. These files have the suffix rawtag for NUV or rawtag_a[b] for the two FUV segments. Figure 2.3 shows the format of a rawtag table. The first extension contains the events list, in which each row of the table corresponds to a single event in the data stream and the columns of the table contain scalar quantities that describe the event. The second extension contains the good time intervals (GTI) table, where an uninterrupted period of time is considered as one good time interval. Interruptions in the data taking due to memory overflow could result in more than one GTI. Table 2.2 shows the columns of a rawtag table.

Figure 2.3: FITS File Format for Raw and corrected TIME-TAG Tables.



Table 2.2: Columns of a Raw TIME-TAG Data Table.

Extension  1
Column NameUnitsData TypeDescription

TIME

sec

float

Elapsed time in seconds since the exposure start time

RAWX

pixel

integer

Pixel coordinate along the dispersion axis

RAWY

pixel

integer

Pixel coordinate along the cross-dispersion axis

PHA1


 

byte

Pulse height amplitude (0–31)

Extension 2

Column Name

Units

Data Type

Description

START

sec

float

Start good time interval since exposure start

STOP

sec

float

End good time interval

 1 The PHA column is present in the NUV data only for symmetry with the FUV data columns. For NUV data the values in this column are set to 0, since no pulse height amplitudes are available.

For more information on working with TIME-TAG data see Section 5.4.

Pulse Height Amplitude Files (pha)

For FUV ACCUM data only, a 7-bit pulse height amplitude histogram is accumulated in the onboard detector electronics. This information is placed in a file with the suffix pha. The pulse-height histogram files contain a primary header with no data and a single FITS image SCI extension containing a histogram of the pulse-height distribution during the exposure. The pulse height amplitude files do not contain an ERR or DQ extension, as shown in Figure 2.4. The pulse height distribution is an image array of length 128, corresponding to the number of photons with pulse height values from 0 to 127, corresponding to the pulse heights of 0–31 available in TIME-TAG data.

Figure 2.4: FITS Array Extension File for COS.



2.4.2 Intermediate Science Data Files


Corrected Events Lists (corrtag)

The COS pipeline produces corrected TIME-TAG events lists and stores them in binary tables with suffix corrtag. These files have a main header and three extensions: a corrected events list extension, a good time interval extension, and a timeline table extension, with a format similar to the one shown in Figure 2.3. The first extension of the corrtag file is the events table (see Table 2.3) which includes X and Y event locations that have been corrected for thermal and geometric distortions and for walk (see Section 3.4), Doppler shift, and offsets due to OSM motions in both the dispersion and cross-dispersion directions. It also includes wavelengths associated with events that occur within the active area of the detectors and a data quality (DQ) flag for each event (see Table 2.18). The second extension gives the start and stop times of the good time intervals (as in the rawtag file), and the third extension is the timeline table. The timeline table includes second by second values for spacecraft position, solar and target altitude above the horizon, and count rates for the most prominent airglow lines and the background. These observed rates might include counts from other external sources in addition to the ones from the airglow line itself. The data in this extension can be useful for reprocessing TIME-TAG data to exclude, for example, daytime data using the Python tool costools.timefilter, described in Section 5.4.2.

For ACCUM data, the corrtag files are somewhat different. All of the time stamps in the first extension are set to the median value of the observation. Each count in the rawaccum file becomes an event so, for example, a pixel in the rawaccum that had 100 counts would have 100 entries in the corrtag file. The RAWX, XCORR and XDOPP entries are all the same for NUV data, but can be different for FUV. In addition, RAWY and YCORR entries will have the same values. However, XFULL and YFULL can be different. In the timeline extension, the SHIFT1, airglow and DARKRATE entries are fixed, but all others are time dependent.

Table 2.3: Columns of a COS corrtag Table.

Column NameUnitsData TypeDescription

Extension 1

TIME

sec

float

Elapsed time in seconds since the exposure start time

RAWX

pixel

integer

Pixel coordinate along dispersion axis (same as in rawtag file)

RAWY

pixel

integer

Pixel coordinate along cross-dispersion axis (same as in rawtag file)

XCORR1

pixel

float

RAWX corrected for thermal and geometric distortion and for walk1

XDOPP

pixel

float

XCORR corrected for Doppler shift and for FUV only distortion

YCORR1

pixel

float

RAWY corrected for thermal and geometric distortion and for walk1

XFULL

pixel

float

XDOPP corrected for offset in the dispersion direction, based on the wavecal spectrum

YFULL2

pixel

float

YCORR corrected for offset in the cross-dispersion direction, based on the wavecal spectrum

WAVELENGTH

Angstrom

float

Only events in the active area are assigned wavelengths

EPSILON


float

Event weight based on flat field and deadtime

DQ


integer

Data quality flag

PHA3


byte

Pulse height amplitude

Extension 2

START

sec

float

Start good time interval since exposure start

STOP

sec

float

End good time interval

Extension 3

TIME

sec

float

Time in 1 sec intervals from first entry

LONGITUDE

degrees

float

Earth based longitude

LATITUDE

degrees

float

Earth based latitude

SUN_ALT

degrees

float

Altitude of the sun above the geometric horizon

SUN_ZD

degrees

float

Angle between HST and the Sun, seen from the center of Earth

TARGET_ALT

degrees

float

Altitude of the target above the geometric horizon

RADIAL_VEL

km/s

float

Instantaneous HST radial velocity toward the target

SHIFT1

pixels

float

Instantaneous dispersion direction shift (stripe B for NUV)

LY_ALPHA

counts/s

float

Total counts/sec in a box across the aperture at Ly-alpha

OI_1304

counts/s

float

Total counts/sec in a box across the aperture at O I 1304

OI_1356

counts/s

float

Total counts/sec in a box across the aperture at O I 1356

DARKRATE

counts/s

float

Counts/sec/pixel averaged over both background regions

1 The XCORR and YCORR columns are present in the NUV data only for symmetry with FUV data. Currently no distortion correction is applied to NUV data, so for NUV data the XCORR and YCORR columns are identical to the RAWX and RAWY columns.
2 For FUV data taken at LP3 and LP4, YFULL is now also corrected for the spectrum trace and offset from the template profile (see Section 3.4.14).
3 The PHA column is present in the NUV data only for symmetry with the FUV data columns. For NUV data this column is set to a default value of 0, since no pulse height amplitudes are available for NUV.

Lampflash Files (lampflash)

For TAGFLASH data, calcos produces an events list with suffix lampflash, that contains the extracted wavecal lamp flashes. Each row in the events list corresponds to a different segment or stripe and flash number (the first flash is number 1, the second is number 2, etc.). The lampflash files have the format shown in Figure 2.5. The contents of the columns in a lampflash events list are listed in Table 2.4. Columns TIME, LAMP_ON, and LAMP_OFF have the same temporal zero point as the TIME column of the rawtag and corrtag tables and the same unit (seconds). The shifts contained in the SHIFT_DISP and SHIFT_XDISP columns of the lampflash table are applied to the XDOPP and YCORR columns of the corrtag file to produce the X[Y]FULL entries. When multiple TAGFLASHES are present, the shifts are interpolated in time for events occurring between each set of flashes. Events occurring before the first flash are shifted by a value extrapolated using the slope defined by the first two flashes; events beyond the last flash are given the shift determined by the last flash. As a result, the difference between the X[Y]FULL and X[Y]CORR entries in the corrtag file can be a function of time.

Figure 2.5: FITS File Format for Lampflash Table.



Table 2.4: Columns of a COS Lampflash Table.

Column Name

Units

Data Type

Description

SEGMENT


String

FUV segment(s) or NUV stripe(s) corresponding to the extracted tagflash wavecal

TIME

sec

double

Median time of each flash

EXPTIME

sec

double

Duration of each flash in seconds

LAMP_ON

sec

double

Lamp turn on time for each flash, counting from start of exposure

LAMP_OFF

sec

double

Lamp turn off time for each flash, counting from start of exposure

NELEM


integer

Length of the WAVELENGTH, GROSS, NET, BACKGROUND, DQ, DQ_WGT, and ERROR arrays

WAVELENGTH

Å

double[nelem]

Wavelengths of each extracted tagflash wavecal spectrum(s)

GROSS

counts s–1

float[nelem]

Gross count rate of each extracted tagflash wavecal spectrum(s)

NET

counts s–1

float[nelem]

Net count rate of each extracted tagflash wavecal spectrum(s)

BACKGROUND

counts s–1

float[nelem]

Background count rate of each extracted tagflash wavecal spectrum(s)

SHIFT_DISP

pixel

float

Dispersion direction shift(s) determined by comparing each tagflash wavecal with a wavecal template

SHIFT_XDISP

pixel

float

Cross-dispersion direction shift(s) determined by comparing each tagflash wavecal with a wavecal template

CHI_SQUARE


float

Chi square of comparison between tagflash wavecal and wavecal template

N_DEG_FREEDOM


integer

Number of degrees of freedom in chi square comparison

SPEC_FOUND


boolean

T (true) or F (false), if each tagflash wavecal spectrum was found or not

Counts Files (counts)

The counts images are an intermediate calibrated output product for both imaging and spectroscopic data with suffix counts. These files contain three extensions (SCI, ERR, and DQ) as shown in Figure 2.1. These files are constructed by summing up the events from each pixel using the XFULL and YFULL coordinates. The data are in units of counts per pixel. For FUV data the images are 16384 columns in the x (dispersion) direction by 1024 rows in the y (cross-dispersion) direction. The NUV images are 1274 columns in the x direction by 1024 rows in the cross-dispersion direction for spectroscopic data, and 1024 × 1024 for data obtained in imaging mode. The NUV spectroscopic files have more pixels in the dispersion direction than the actual NUV detector. This is because the counts files (and flt files) have been corrected for Doppler shift and OSM shift (including FP-POS offset), so the width was increased to accommodate those shifts. The FUV images are not extended since the active area is less than the size of the detector, so these effects can be incorporated into the images without the need to extend them. The FUV data are also corrected for walk and geometric distortions.

Flat-Fielded Image Files (flt)

For spectroscopic data a flat-fielded image is an intermediate calibrated data file. These files have a suffix, flt, and contain three extensions (SCI, ERR, and DQ) as shown in Figure 2.1. These files are constructed by summing up the values in the EPSILON column for each pixel using the XFULL and YFULL coordinates. The data are in units of the count rate. For FUV data the images are 16384 × 1024, and, like the counts images, the NUV images are 1274 × 1024 for spectroscopic data and 1024 ×1024 for data obtained in imaging mode. The flt images are corrected for deadtime effects. The NUV images are corrected for all flat-field effects and the FUV data are currently corrected for only the largest fixed-pattern features; the XDL grid-wire shadows, low-order flat-field variations (L-flats), and large geometric distortion artifacts.

2.4.3 Final Science Data Files (and Product Files)

The initial input files to calcos are the association tables with suffix asn. These files provide the calibration pipeline with information about how the data files are associated. In general, only exposures taken in sequence with the same spectral element, central wavelength (if applicable), and aperture at any FP-POS will be associated. For more information on COS association files see the "Association Tables (ASN)" portion of Section 2.4.4.

Processing of each individual exposure in the association produces a final calibrated result named with exposure rootname and suffix x1d (spectroscopy) or flt (imaging).

Next, for each FP-POS position <n> (where <n>=1, 2, 3, or 4), if there are multiple spectroscopic exposures in the association that use the same FP-POS position, calcos will combine them into a file named with the association rootname and suffix x1dsum<n>, where <n> is the integer FP-POS value. If there is a single exposure with a given FP-POS value in the association, the x1dsum<n> file contains the x1d spectrum to which the DQ_WGT is applied (see Section 3.4.22).

 Lastly, a final association product file is produced with association rootname and suffix x1dsum (spectroscopy) or fltsum (imaging) by combining all science exposures in the association.

One-Dimensional Extracted Spectra (x1d, x1dsum)

The COS pipeline produces extracted one-dimensional spectra and stores them in binary tables with suffix x1d, x1dsum<n>, or x1dsum. Figure 2.6 shows the format of the 1-D extracted spectra table.

Figure 2.6: FITS File Format for 1-D Extracted Spectrum Table.



These COS extracted spectra tables can be 1- to 3-Dimensional, with one row for each unique segment or stripe. For FUV data there are typically two rows which correspond to segments A and B distinguished by "FUVA" and "FUVB" in the SEGMENT column respectively. For NUV data there are three rows, "NUVA," "NUVB," and "NUVC" corresponding to stripes A, B, and C, respectively. Each table column can contain either a scalar value or an array of values, such as WAVELENGTH or FLUX. For example, NELEM will contain a scalar number, while the WAVELENGTH column will contain an array of wavelengths. Table 2.5 shows the contents of the different columns in an extracted spectrum table. A discussion of the data in COS extracted spectra is provided in Section 3.4.18.


Table 2.5: Columns of a COS Extracted Spectrum Table.

Column Name

Units

Data Type

Description

SEGMENT


string

FUV segments or NUV stripe names

EXPTIME

seconds

float

Corrected exposure times for each segment

NELEM


integer

Length of the array fields, such as the WAVELENGTH and GROSS arrays

WAVELENGTH

Å

double[nelem]

Wavelengths corresponding to fluxes

FLUX

erg s–1 cm–2 Å–1

float[nelem]

Flux calibrated NET spectrum

ERROR

erg s–1 cm–2 Å–1

float[nelem]

Internal error estimate

GROSS

counts s–1

float[nelem]

Gross extracted spectrum count rate

NET

counts s–1

float[nelem]

Difference of GROSS and BACKGROUND arrays

BACKGROUND

counts s–1

float[nelem]

Background count rate

GCOUNTS

counts

float[nelem]

Gross counts

DQ_WGT


float[nelem]

Weight (0 or 1) depending on DQ

DQ


short[nelem]

Logical OR of data quality flags in extraction region

DQ_OUTER


short[nelem]

Data quality flag in outer extraction region

BACKGROUND_PER_PIXEL

counts s–1

float[nelem]

Average background per pixel

NUM_EXTRACT_ROWS


integer

Number of extracted rows

ACTUAL_EE


double[nelem]

Actual energy enclosed between outer zone boundaries

Y_LOWER_OUTER


double[nelem]

Index of lower outer extraction zone boundary

Y_LOWER_INNER


double[nelem]

Index of lower inner extraction aperture boundary

Y_UPPER_OUTER


double[nelem]

Index of upper outer extraction zone boundary

Y_UPPER_INNER


double[nelem]

Index of upper inner extraction zone boundary

Flat-Fielded Image Files (flt, fltsum)

For NUV imaging observations, the flt and fltsum images are the final data products, with the latter being a simple sum of the individuals when several exposures are processed together. They are fully linearized and flat-field corrected images. Unlike the flt files produced for the spectroscopic data (which are intermediate data products with a format of 1274 × 1024, see Section 2.4.2), the formats of the flt and fltsum files for imaging data are 1024 × 1024, since Doppler and OSM motions are not applied.

2.4.4 Auxiliary Data Files


Association Tables (ASN)

An association file is created for all COS observation sets, and has the suffix asn (e.g., lcwj01010_asn.fits). This file holds a single binary table extension, which can be displayed with the astropy.table.Table module.

Calcos calibrates raw data from multiple science exposures and any contemporaneously obtained line lamp calibration exposures through the pipeline as an associated unit. Each individual science exposure in an association is fully calibrated in the process. The information within an association table shows how a set of exposures are related, and informs the COS calibration pipeline how to process the data.

An example association table is shown below. Note that all related COS exposures will be listed in an association table, with the exception of acquisitions, darks, and flats. It is possible to have an association which contains only one exposure. The association file lists the rootnames of the associated exposures as well as their membership role in the association. The exposures listed in an association table directly correspond to individual raw FITS files. For example, the association table can describe how wavecal exposures are linked to science exposures. Table 2.6 summarizes the different exposure membership types (MEMTYPES) used for COS association tables.

Table 2.6: Member Types in COS Associations.

MEMTYPE

Description

EXP-AWAVE

Input automatic wavelength calibration exposure

EXP-FP

Input science exposure

EXP-GWAVE

Input GO wavelength calibration exposure

PROD-FP

Output science product


The table below illustrates the contents of the association table for a sequence of spectroscopic exposures for four FP-POS positions.

Sample Association Table l9v221010_asn. To display the association table for ldel05050_asn.fits:

> from astropy.io import fits
> fits.info('ldel05050_asn.fits')
Filename: ldel05050_asn.fits
No. Name  Ver Type     Cards Dimensions Format
  0 PRIMARY 1 PrimaryHDU  43 ()
  1 ASN     1 BinTableHDU 25 5R × 3C    [14A, 14A, L]
> from astropy.table import Table
> t = Table.read('ldel05050_asn.fits', hdu=1)
> print(t)

MEMNAME   MEMTYPE MEMPRSNT
--------- ------- --------
LDEL05JYQ EXP-FP  True
LDEL05K0Q EXP-FP  True
LDEL05K2Q EXP-FP  True
LDEL05K4Q EXP-FP  True
LDEL05050 PROD-FP True


In the above example, MEMTYPE describes the exposure membership type or role in the association. The column MEMPRSNT lists whether the member is present or not. The association file can be modified to not include a member during processing by changing the MEMPRSNT to ‘false.’

The association table above lists the names of the four associated exposures that are calibrated and combined to create the various association products which will have a rootname of ldel05050. This particular association is created from a single TIME-TAG spectroscopic APT specification with FP-POS=ALL specified in the Phase II file, which leads to a science exposure taken at each FP-POS location. For example, the first entry in the table, ldel05jyq, is the rootname of a single external science exposure taken with FP-POS=1. This exposure corresponds to the following rawtag files: ldel05jyq_rawtag_a.fits, ldel05jyq_rawtag_b.fits. The memtype of this exposure is EXP-FP which shows that it is an external exposure. Similar files correspond to the remaining three entries in the association file for data taken with the remaining three FP-POS positions. The pipeline will calibrate the members of an association as a unit, producing the calibrated data products for each individual exposure as well as the final combined association data product. For this particular association, the pipeline will produce a final combined association product, ldel05050_x1dsum.fits, which contains the final FP-POS combined, calibrated spectrum.

Trailer Files (TRL)

When COS data are processed in the HDA, the output messages from generic conversion and the different calibration steps are stored in a FITS ASCII table known as the trailer file, with suffix trl. Each time the archive processes data, the old trailer file is erased and a new one created using the results of the most recent processing performed. The archive will produce a trailer file for each individual exposure and association product. Association product trailer files contain the appended information from all the exposures in the association, in order of processing. The order of processing is the same as the order of exposures in the association table, with the exception of AUTO or GO wavecals which are always processed first.

In the trailer files from the HDA, the output messages from generic conversion appear first in the file. This section contains information relevant to the selection of the best reference files and the population of some of the header keywords. The second part of this file contains information from calcos processing. Each task in the calcos pipeline creates messages during processing which describe the progress of the calibration, and appear in the order in which each step was performed. These messages are relevant to understand how the data were calibrated, and in some cases, to determine the accuracy of the products.

It is highly recommended to always examine the trailer files.

In the last section of the _trl file, the calcos steps are indicated by their module name. The calcos messages provide information on the input and output files for each step, the corrections performed, information regarding the reference files used, and in the case of FUV data, messages about the location of the stim pulses, or shift correction applied to the data. Calcos also gives warnings when the appropriate correction to the data could not be applied. For more detailed information on the calibration steps and structure of calcos, please refer to Chapter 3.

Calcos Trailer Files (TRA)

When calcos is run on a personal machine, calcos redirects the output of its steps to the STDOUT and an ASCII file with name rootname.tra. Note, the level of detail included in the output messages can be modified when running calcos (see "Run calcos"). So, when run on a personal machine, calcos will not overwrite the trl file but rather will direct the output to STDOUT and an ASCII tra file. The tra file is formatted like the trl file but with two exceptions: the tra file will not contain the output messages from generic conversion, and the tra file is not converted to FITS format. Each time calcos is run on a file, the STDOUT messages will be appended to the tra file if it already exists. Also, when running calcos on a personal machine there will be no tra created for the association products (ASN files). Instead, the calcos messages for association products will be sent only to STDOUT.

Support Files (SPT)

The support files contain information about the observation and engineering data from the instrument and spacecraft that were recorded at the time of the observation. A COS support file contains a primary header and a variable number of image extensions. Depending on the length of the exposure, the support file will contain one or more "imsets," each of which includes a support extension (EXTNAME = 'SUPPORT") and two snap extensions (EXTNAME = 'SNAP1' and 'SNAP2'):

The SUPPORT extension contains a header with the proposal information and an (16-bit) image array containing the data which populate the SPT header keyword values. The image array element values are used to populate the header keywords.

Following the support extension in each imset, there are two engineering snapshot extensions. These extensions contain samples of many instrument and telescope parameters from telemetry data during the course of the exposure. The SNAP1 extension in the first imset will always contain telemetry information collected immediately before the exposure begins. Subsequent SNAP1 extensions in all imsets will repeat the information contained in the first one. The SNAP2 extension in each imset will be populated with telemetry data taken during the course of the exposure. The final SNAP2 extension will be populated with data taken immediately after the completion of the science exposure.

Figure 2.7 depicts the structure of an N extension COS support file. With several snapshots of COS telemetry values, one may track the instrument status periodically throughout an exposure. For a schematic listing of the spt headers with detailed information about the spt header keywords, see:
http://stdatu.stsci.edu/keyword/cgi-bin/kdct-header.cgi?i=COS&s=20.1&db=Operational.

Acquisition Files (RAWACQ)

All COS acquisition exposures will produce a single raw data file with suffix rawacq. Almost all COS spectroscopic science exposures are preceded by an acquisition sequence or exposure to center the target in the aperture. Keywords in the header of COS science data identify the exposure names of relevant acquisition exposures in each visit. In addition, there are several other useful keywords in the COS acquisition exposures that describe the acquisition parameters used, as well as the calculated centroid positions and slew offsets. Table 2.7 lists all the relevant acquisition keywords.

Figure 2.7: COS Support File.



COS support file with N extensions. The initial imset contains telemetry values at the start of the exposure. For subsequent imsets, only the second snap extension contains valid data.
Table 2.7: ACQ/IMAGE Header Keywords.

Keyword Name

Description

ACQSNAME1

Rootname of first acquisition search exposure

ACQINAME1

Rootname of first acquisition image exposure

PEAKXNAM1

Rootname of first cross-dispersion (XD) peakup exposure

PEAKDNAM1

Rootname of first along-dispersion (AD) peakup exposure

ACQ_NUM1

Total number of exposures in acquisition sequence

LAMPSTAT

Status of Wavecal lamp exposure (LTAIMCAL)

LAMPTIME

Lamp exposure integration time(s)

LAMPMXCR

Measured centroid of lamp exposure in X (AD)

LAMPMYCR

Measured centroid of lamp exposure in Y (XD)

LAMPEVNT

Number of events in the lamp exposure

LAMPCNTR

Lamp Centering method

LMPSUBX1

X coordinate of the left of the lamp subarray (pixels)

LMPSUBX2

X coordinate of the right of the lamp subarray (pixels)

LMPSUBY1

Y coordinate of the top of the lamp subarray (pixels)

LMPSUBY2

Y coordinate of the bottom of the lamp subarray (pixels)

ACQSTAT

Status of the acquisition exposure (LTAIMAGE)

TARGTIME

Acquisition exposure integration time(s)

ACQCENTX

Measured target centroid in X (AD) direction

ACQCENTY

Measured target centroid in Y (XD) direction

WCA2SCIX

WCA to science Aperture offset in X (AD)

WCA2SCIY

WCA to Science aperture offset in Y (XD)

ACQPREFX

Desired target X (AD) position

ACQPREFY

Desired target Y (XD) position

ACQSLEWX

Slew offset in X (AD) (arcseconds)

ACQSLEWY

Slew offset in Y (XD) (arcseconds)

TRGSUBX1

X coordinate of the left of the target subarray (pixels)

TRGSUBX2

X coordinate of the right of the target subarray (pixels)

TRGSUBY1

Y coordinate of the left of the target subarray (pixels)

TRGSUBY2

Y coordinate of the right of the target subarray (pixels)

1 These keywords are also found in the COS science headers in addition to being in the acquisition headers.


Table 2.8: ACQ/SEARCH Header Keywords.

Keyword Name

Description

ACQSNAME1

Rootname of first acquisition search exposure

ACQINAME1

Rootname of first acquisition image exposure

PEAKXNAM1

Rootname of first cross-dispersion (XD) peakup exposure

PEAKDNAM1

Rootname of first along-dispersion (AD) peakup exposure

ACQ_NUM1

Total number of exposures in acquisition sequence

ACQSTAT

Status of target exposure

TARGTIME

Integration time per dwell (s)

CENTER

Centering method used by the search

ACQFLOOR

Threshold Floor used (for FLUX-WEIGHT-FLOOR centering method)

SCANSIZE

Number of dwells per side of the square pattern

ACQNPOS

Total number of dwells

STEPSIZE

Scan step size between dwells (arcsec)

ENDSLEWX

Commanded X-direction (AD) slew from the final dwell point (arcsec)

ENDSLEWY

Commanded Y-direction (XD) slew from the final dwell point (arcsec)

ACQSLEWX

Commanded X-direction (AD) slew from the center of the search pattern (arcsec)

ACQSLEWY

Commanded Y-direction (XD) slew from the center of the search pattern (arcsec)

SEGMENT2

FUV Segment used

TRGSUBX13

X coordinate of the left of the target subarray (pixels)

TRGSUBX23

X coordinate of the right of the target subarray (pixels)

TRGSUBY13

Y coordinate of the top of the target subarray (pixels)

TRGSUBY23

Y coordinate of the bottom of the target subarray (pixels)

TRGAS1X12

X coordinate of the left of the first segment A target subarray

TRGAS1X22

X coordinate of the right of the first segment A target subarray

TRGAS1Y12

Y coordinate of the top of the first segment A target subarray

TRGAS1Y22

Y coordinate of the bottom of the first segment A target subarray

TRGBS1X12

X coordinate of the left of the first segment B target subarray

TRGBS1X22

X coordinate of the right of the first segment B target subarray

TRGBS1Y12

Y coordinate of the top of the first segment B target subarray

TRGBS1Y22

Y coordinate of the bottom of the first segment B target subarray

TRGAS2X12

X coordinate of the left of the second segment A target subarray

TRGAS2X22

X coordinate of the right of the second segment A target subarray

TRGAS2Y12

Y coordinate of the top of the second segment A target subarray

TRGAS2Y22

Y coordinate of the bottom of the second segment A target subarray

TRGBS2X12

X coordinate of the left of the second segment B target subarray

TRGBS2X22

X coordinate of the right of the second segment B target subarray

TRGBS2Y12

Y coordinate of the top of the second segment B target subarray

TRGBS2Y22

Y coordinate of the bottom of the second segment B target subarray

1 These keywords are also found in the COS science headers in addition to being in the acquisition headers.
2 FUV only. Note that two more (the third and fourth) FUV target subarrays may be used in the future.
3 NUV only.


Table 2.9: ACQ/PEAKXD Header Keywords.

Keyword Name

Description

ACQSNAME1

Rootname of first acquisition search exposure

ACQINAME1

Rootname of first acquisition image exposure

PEAKXNAM1

Rootname of first cross-dispersion (XD) peakup exposure

PEAKDNAM1

Rootname of first along-dispersion (AD) peakup exposure

ACQ_NUM1

Total number of exposures in acquisition sequence

LAMPSTAT

Status of lamp exposure (LTACAL)

LAMPTIME

Integration time of lamp exposure(s)

LAMPMYCR

Measured centroid of lamp exposure in Y (AD)

LAMPEVNT

Number of events in lamp exposure

LAMPCNTR

Lamp Centering Method

LSTRIPE2

NUV Lamp Stripe used for target acquisition

LMPSUBX12

X coordinate of the left of the lamp subarray (pixels)

LMPSUBX22

X coordinate of the right of the lamp subarray (pixels)

LMPSUBY12

Y coordinate of the top of the lamp subarray (pixels)

LMPSUBY22

Y coordinate of the bottom of the lamp subarray (pixels)

LMPAS1X13

X coordinate of the left of the first segment A lamp subarray

LMPAS1X23

X coordinate of the right of the first segment A lamp subarray

LMPAS1Y13

Y coordinate of the top of the first segment A lamp subarray

LMPAS1Y23

Y coordinate of the bottom of the first segment A lamp subarray

LMPBS1X13

X coordinate of the left of the first segment B lamp subarray

LMPBS1X23

X coordinate of the right of the first segment B lamp subarray

LMPBS1Y13

Y coordinate of the top of the first segment B lamp subarray

LMPBS1Y23

Y coordinate of the bottom of the first segment B lamp subarray

LMPAS2X13

X coordinate of the left of the second segment A lamp subarray

LMPAS2X23

X coordinate of the right of the second segment A lamp subarray

LMPAS2Y13

Y coordinate of the top of the second segment A lamp subarray

LMPAS2Y23

Y coordinate of the bottom of the second segment A lamp subarray

LMPBS2X13

X coordinate of the left of the second segment B lamp subarray

LMPBS2X23

X coordinate of the right of the second segment B lamp subarray

LMPBS2Y13

Y coordinate of the top of the second segment B lamp subarray

LMPBS2Y23

Y coordinate of the bottom of the second segment B lamp subarray

ACQSTAT

Status of target exposure (LTAPKXD)

TARGTIME

Acquisition exposure integration time(s)

ACQMEASY

Measured target centroid in Y (XD) direction

ACQPREFY

Desired computed Y position

ACQSLEWY

Slew offset in Y (XD) (arcsec)

TARGEVNT

Number of events in the acquisition exposure

STRIPE2

NUV Stripe used for target acquisition

SEGMENT3

FUV detector segment name (FUVA or FUVB or BOTH)4

TRGSUBX12

X coordinate of the left of the target subarray (pixels)

TRGSUBX22

X coordinate of the right of the target subarray (pixels)

TRGSUBY12

Y coordinate of the top of the target subarray (pixels)

TRGSUBY22

Y coordinate of the bottom of the target subarray (pixels)

TRGAS1X13

X coordinate of the left of the first segment A target subarray

TRGAS1X23

X coordinate of the right of the first segment A target subarray

TRGAS1Y13

Y coordinate of the top of the first segment A target subarray

TRGAS1Y23

Y coordinate of the bottom of the first segment A target subarray

TRGBS1X13

X coordinate of the left of the first segment B target subarray

TRGBS1X23

X coordinate of the right of the first segment B target subarray

TRGBS1Y13

Y coordinate of the top of the first segment B target subarray

TRGBS1Y23

Y coordinate of the bottom of the first segment B target subarray

TRGAS2X13

X coordinate of the left of the second segment A target subarray

TRGAS2X23

X coordinate of the right of the second segment A target subarray

TRGAS2Y13

Y coordinate of the top of the second segment A target subarray

TRGAS2Y23

Y coordinate of the bottom of the second segment A target subarray

TRGBS2X13

X coordinate of the left of the second segment B target subarray

TRGBS2X23

X coordinate of the right of the second segment B target subarray

TRGBS2Y13

Y coordinate of the top of the second segment B target subarray

TRGBS2Y23

Y coordinate of the bottom of the second segment B target subarray

1 These keywords are also found in the COS science headers in addition to being in the acquisition headers.
2 NUV only.
3 FUV only.
4 Although the keyword SEGMENT can take the value BOTH, only FUVA is used for PEAKXD.


 Table 2.10: ACQ/PEAKD Header Keywords.

Keyword Name

Description

ACQSNAME1

Rootname of first acquisition search exposure

ACQINAME1

Rootname of first acquisition image exposure

PEAKXNAM1

Rootname of first cross-dispersion (XD) peakup exposure

PEAKDNAM1

Rootname of first along-dispersion (AD) peakup exposure

ACQ_NUM1

Total number of exposures in acquisition sequence

ACQSTAT

Status of acquisition (LTAPKD)

TARGTIME

Acquisition exposure integration time(s)

CENTER

Centering method used

ACQFLOOR

Threshold floor value

ACQNPOS

Number of dwells in the acquisition

STEPSIZE

Peakup scan stepsize (arcsec)

ACQMEASX

Measured target centroid in X (AD) direction

ACQPREFX

Desired computed X (AD) position

ENDSLEWX

X (AD) slew from final dwell position (arcsec)

ACQSLEWX

Slew offset from center in X (AD) (arcsec)

SEGMENT2

FUV detector segment name (FUVA or FUVB or BOTH)

TRGSUBX13

X coordinate of the left of the target subarray (pixels)

TRGSUBX23

X coordinate of the right of the target subarray (pixels)

TRGSUBY13

Y coordinate of the top of the target subarray (pixels)

TRGSUBY23

Y coordinate of the bottom of the target subarray (pixels)

TRGAS1X12

X coordinate of the left of the first segment A target subarray

TRGAS1X22

X coordinate of the right of the first segment A target subarray

TRGAS1Y12

Y coordinate of the top of the first segment A target subarray

TRGAS1Y22

Y coordinate of the bottom of the first segment A target subarray

TRGBS1X12

X coordinate of the left of the first segment B target subarray

TRGBS1X22

X coordinate of the right of the first segment B target subarray

TRGBS1Y12

Y coordinate of the top of the first segment B target subarray

TRGBS1Y22

Y coordinate of the bottom of the first segment B target subarray

TRGAS2X12

X coordinate of the left of the second segment A target subarray

TRGAS2X22

X coordinate of the right of the second segment A target subarray

TRGAS2Y12

Y coordinate of the top of the second segment A target subarray

TRGAS2Y22

Y coordinate of the bottom of the second segment A target subarray

TRGBS2X12

X coordinate of the left of the second segment B target subarray

TRGBS2X22

X coordinate of the right of the second segment B target subarray

TRGBS2Y12

Y coordinate of the top of the second segment B target subarray

TRGBS2Y22

Y coordinate of the bottom of the second segment B target subarray

1 These keywords are also found in the COS science headers in addition to being in the acquisition headers.
2 FUV only.
3 NUV only.

PEAKD and SEARCH Acquisitions

Acquisition peakups in the dispersion direction (ACQ/PEAKD) and acquisition spiral searches (ACQ/SEARCH) both use the flux from exposures taken at different dwell points to center the target. For more information on these types of COS acquisitions see Sections 7.6.4 and 7.6.2 respectively of the COS Instrument Handbook. Data for these acquisitions contain one binary table extension which describes the acquisition search pattern dwell point locations and counts as shown in Table 2.11 and Figure 2.8.


Table 2.11: Columns of an ACQ/SEARCH or ACQ/PEAKD Table.

Column Name

Units

Description

DWELL_POINT


Dwell point number in search pattern

DISP_OFFSET1

arcsec

Offset in dispersion direction from the initial target pointing

XDISP_OFFSET

arcsec

Offset in the cross-dispersion direction from the initial target pointing

COUNTS

counts

Raw counts value at dwell point

1 This column is only present in ACQ/SEARCH tables.

Figure 2.8: FITS File Format for ACQ/SEARCH and ACQ/PEAKD Data.



PEAKXD Acquisition

Acquisition peakups in the cross-dispersion direction (ACQ/PEAKXD) use a TIME-TAG spectrum to center the target in the cross-dispersion direction. For more information on the ACQ/PEAKXD algorithm see Section 7.6.3 of the COS Instrument Handbook. An ACQ/PEAKXD exposure includes only a primary header and extension header. There are no data downlinked for this type of acquisition.

 IMAGE Acquisition

Acquisition images (ACQ/IMAGE) use an NUV image to center the target in the aperture. For more information on the ACQ/IMAGE algorithm see Section 7.5 of the COS Instrument Handbook. An ACQ/IMAGE exposure produces a raw data file containing two science image extensions corresponding to the initial and final pointing:

  • [SCI,1] is an image of the initial target pointing.
  • [SCI,2] is a confirmation image after the acquisition procedure has been performed.

See Figure 2.9 for the FITS file format for ACQ/IMAGE data.

Figure 2.9: FITS File Format for ACQ/IMAGE Data.



Jitter Files (jit)

 The COS jitter files include engineering data that describe the performance of the Pointing Control System (PCS) including the Fine Guidance Sensors that are used to control the vehicle pointing. The jitter files report on PCS engineering data during the duration of the observation. The support files contain information about the observation and engineering data from the instrument and spacecraft that were recorded at the time of the observation. COS jitter files utilize the file format shown in Figure 2.10 for all science observations, excluding acquisitions.


Figure 2.10: FITS File Format for JITTER Data.



The jitter tables contain PCS data for each three-second interval during the observation, as listed in Table 2.12. For more information on jitter files refer to Chapter 6 of the Introduction to HST Data Handbooks.


Table 2.12: Columns of a jitter Table.

Column Name

Data Type

Units

Description

SECONDS

float

seconds

'Seconds' three-second intervals from start

V2_DOM

float

arcsec

Dominant FGS V2 Coordinate

V3_DOM

float

arcsec

Dominant FGS V3 Coordinate

V2_ROLL

float

arcsec

Roll FGS V2 Coordinate

V3_ROLL

float

arcsec

Roll FGS V3 Coordinate

SI_V2_AVG

float

arcsec

Mean jitter in V2 over 3 seconds

SI_V2_RMS

float

arcsec

Peak jitter in V2 over 3 seconds

SI_V2_P2P

float

arcsec

RMS jitter in V2 over 3 seconds

SI_V3_AVG

float

arcsec

Mean jitter in V3 over 3 seconds

SI_V3_RMS

float

arcsec

Peak jitter in V3 over 3 seconds

SI_V3_P2P

float

arcsec

RMS jitter in V3 over 3 seconds

RA

double

degrees

Right Ascension of aperture reference

DEC

double

degrees

Declination of aperture reference

ROLL

doublet

degree

Position angle between North and +V3

LIMBANG

float

degree

Position angle between V1 axis and Earth limb

TERMANG

float

degree

Angle between V1 axis and terminator

LOS_ZENITH

float

degree

Angle between HST Zenith and target

LATITUDE

float

degree

HST subpoint latitude

LONGITDUE

float

degree

HST subpoint longitude

MAG_V1

float

Gauss

Magnetic field along V1

MAG_V2

float

Gauss

Magnetic field along V2

MAG_V3

float

Gauss

Magnetic field along V3

BRIGHTLIMB

integer


Earth limb of LimbAng is bright (1 or 0) t

FGS_FLAGS

float


FGS status flags

DAYNIGHT

string


Observation taken during the day (0) or night (1)

RECENTER

string


Recentering status flag, event in progress = 1

TAKEDATA

string


Vehicle guiding status, nominal GS tracking = 1

SLEWFLAG

string


Vehicle slewing status, slewing = 1

2-D Spacecraft Pointing Histogram (jif)

The COS jif files are a 2-D histogram of the corresponding jit file (see Jitter Files) and have the file format shown in Figure 2.11 for all science observations excluding acquisitions.


Figure 2.11: FITS File Format for jif Data.