Introduction

Three target-acquisition (TA) modes are available with the COS FUV detector.  

• ACQ/SEARCH - Uses a spiral search to locate the target.
• ACQ/PEAKXD - Uses a linear search to center the target in the cross-dispersion direction.
• ACQ/PEAKD - Uses a linear search to center the target in the dispersion direction.

All of these modes use the dispersed spectrum to center the target in the aperture.  Imaging TAs are available only with the NUV detector.

Mode = ACQ/SEARCH Config = COS/FUV

ACQ/SEARCH invokes a procedure in the COS onboard flight software that searches for the target and centers it in the selected COS aperture for subsequent science exposures. A series of exposures are taken at different HST pointings in a spiral pattern from the initial pointing, and then the HST is repositioned to maximize the light intensity through the COS aperture.

This mode is required for targets whose coordinates are not known to at least 0.4". Most COS observation sequences that use ACQ/SEARCH mode will also need to use other acquisition exposures (ACQ/IMAGE, ACQ/PEAKD, and/or ACQ/PEAKXD) to achieve proper centering of the target in the science aperture.

Information from the Target List, along with the exposure time, will be used to verify that the integrated target flux through the selected aperture is appropriate for target acquisition. Details on the target acquisition sequence and limiting magnitudes for the PSA and BOA apertures can be found in the COS Instrument Handbook.

This method uses dispersed light from the object to be observed. Subarrays are used to avoid geocoronal airglow lines and detector features, which could bias the target acquisition calculations, as well as portions of the detector that are not illuminated by the target’s spectrum. The subarray dimensions depend on the selected grating; see the COS Instrument Handbook for details.

Aperture or FOV

The aperture used for the ACQ/SEARCH Mode exposure does not need to be the same as the aperture used for the subsequent TIME-TAG or ACCUM science exposures. The following apertures are allowed:

• PSA Primary Science Aperture
• BOA Bright Object Aperture

Use of the BOA is recommended for bright targets to attenuate the flux and allow the acquisition to proceed without triggering bright-object violations.

Spectral Element

Enter a spectral element from Table 11.7: COS Spectral Elements and Central Wavelengths for the COS/FUV configuration. In order to minimize overheads, this will generally be the same as the one used for the subsequent target acquisition exposures and TIME-TAG or ACCUM science observation. Target acquisition with the COS FUV detector is in dispersed light only.

Wavelength

Enter the value of the central wavelength in Ångstroms. Table 11.7: COS Spectral Elements and Central Wavelengths gives the allowed values of the central wavelength for each grating.

Optional Parameters

LIFETIME-POS

= LP1, LP2, LP3, LP4, LP5, LP6, LP7, LP10, LP11

Specifies which FUV lifetime position (LP) to use for this exposure.  Supported values are given in Section 11.1 Introduction to COS.

SEGMENT

= BOTH (default for all G130M cenwaves < 1300 A, all G160M cenwaves, and G140L/1280)

= A (default and only allowed value for these cases: G140L, λ=1105 Å, and G130M, λ=1300, 1309, 1318, 1327 Å at LIFETIME-POS=LP5)

= B (restricted for G140L, λ=1280 Å)

Indicates which segment of the FUV detector to use for an acquisition. A value of BOTH will activate both segments. Segments A and B record the long and short wavelength portions of the spectrum respectively. If A is selected, only segment A of the detector will be activated for photon detection, and the spectrum will contain data from only that half of the detector. If B is selected, only segment B of the detector will be activated and used to generate data. Use of a single segment may be warranted for sources that are too bright to observe safely over the entire detector. Target acquisitions at LP5 with G130M, λ=1300, 1309, 1318, 1327 Å will use SEGMENT=A only, in order to protect the SEGMENT=B from illumination by geocoronal Lyα emission. See the COS Instrument Handbook for more information.

If you use the G140L grating at the 1105 Angstrom wavelength setting, SEGMENT defaults to A as zeroth-order light from the target falls onto segment B and becomes a bright-object concern.

Note: No target acquisition subarrays are defined for segment B when the G140L grating is used, because the count rate after excluding areas contaminated by geocoronal airglow lines is expected to be too low to support target acquisition. However, as increasing the high voltage on FUVB in preparations for observations takes a finite amount of time, overheads can be minimized by obtaining a TA with segment=BOTH for science observations with segment=BOTH, increasing the amount of time for G140L science observations.

SCAN-SIZE
= 2, 3, 4, 5 (points)

A required parameter which specifies the size of the square spiral search region in terms of the number of dwell points on a side (e.g., 2 × 2, 3 × 3, 4 × 4, or 5 × 5). A larger search pattern samples a larger area, but the time required scales as the square of SCAN-SIZE. In some cases it may be advisable to use two ACQ exposures, such as a 3 × 3 search followed by a 2 × 2 search; this strategy uses less time than a single 4 × 4 search. For more details on target acquisition strategies, consult the COS Instrument Handbook.

STEP-SIZE
= 1.767 (default); 0.2 - 2.0 (arcsec)

Specifies the size in arcseconds of each step in the spiral search. Both the PSA and BOA are 2.5 arcsec in diameter. The recommended offset of 1.767 arcsec is the offset at which diagonal dwell points just overlap, and the maximum spacing that provides continuous coverage. Simulations show that offsets larger than 1.767 arcsec will introduce errors due to unsampled areas within the search pattern.

CENTER
= DEF (default), FLUX-WT, FLUX-WT-FLR, BRIGHTEST

Specifies the method used for locating the target within the search pattern. Two methods are available: a flux-weighted centroiding algorithm (FLUX-WT or FLUX-WT-FLR) and a return to the brightest dwell point (BRIGHTEST).

If FLUX-WT-FLR is used, the minimum number of counts measured from any of the dwell points (the "floor") will be subtracted from the number at other dwell points before computing the centroid. The idea behind this threshold is to reduce the contribution of background counts. Note that this has the effect of eliminating the dwell point with the minimum counts from the centroid calculation. This drastically reduces the centering accuracy for SCAN-SIZE=2; therefore FLUX-WT-FLR is not allowed if SCAN-SIZE=2 (FLUX-WT is recommended).

The default is to use FLUX-WT-FLR with a SCAN-SIZE of 3 or larger. See the COS Instrument Handbook for recommendations on when to use these different methods.

Number of Iterations

The Number_Of_Iterations must be 1 in this Mode.

Time Per Exposure

Enter the total time of data collection at each dwell point as Time_Per_Exposure. Time_Per_Exposure must be an integral multiple of 0.1 seconds. If it is not, its value will be rounded down to the next lower integral multiple of 0.1 sec, or set to 0.1 seconds if a smaller value is specified.

The procedures to determine the exposure time for the ACQ/SEARCH Mode exposure are given in the COS Instrument Handbook. The COS Exposure Time Calculator should be used to estimate the exposure time based on the nature of the source. The exposure time will be repeated at every dwell point in the search.

Special Requirements

The special requirement "POSition TARGet <X-value>,<Y-value>" is not permitted on ACQ/SEARCH exposures.

Mode = ACQ/PEAKXD Config = COS/FUV

ACQ/PEAKXD mode invokes a procedure in the COS onboard flight software that places the target near an optimal position within the selected aperture in the cross-dispersion direction on the detector.

For Cycle 25 and onward, the ACQ/PEAKXD centering algorithm has been enhanced in the COS flight software to employ the ACQ/PEAKD algorithm, but executed in the cross-dispersion direction. Three optional parameters (NUM-POS, STEP-SIZE, and CENTER) have been added to ACQ/PEAKXD in APT to support this enhancement. A linear scan is performed in the cross-dispersion direction, with the number of dwell points corresponding to the value of NUM-POS. The optional parameters NUM-POS, STEP-SIZE and CENTER have default values that are appropriate for centering a point source with target coordinates accurate to 0.4".

The optional parameters, LIFETIME-POS and LOCAL-THRESHOLD, have been implemented in engineering mode.

If the target coordinates are not accurate to 0.4", an ACQ/PEAKXD should normally be preceded by an ACQ/SEARCH to ensure that the target is positioned in the aperture. Details on the target acquisition sequence and limiting magnitudes for the PSA and BOA can be found in the COS Instrument Handbook.

Subarrays are used during target acquisition to avoid undesirable detector features (e.g., hot spots) and geocoronal airglow lines, which could bias the target acquisition calculations, as well as portions of the detector which are not illuminated by the input spectrum. The subarray dimensions depend on the selected grating and central wavelength; see the COS Instrument Handbook for details.

Aperture or FOV

The aperture used for the ACQ/PEAKXD Mode exposure need not be the same as the aperture used for the subsequent TIME-TAG or ACCUM science exposures. The following apertures are allowed:

• PSA Primary Science Aperture
• BOA Bright Object Aperture

As with spectroscopic observations, use of the BOA is recommended on relatively bright targets to attenuate the flux and allow the acquisition to proceed without triggering bright object violations.

Spectral Element

Enter a spectral element from Table 11.7: COS Spectral Elements and Central Wavelengths for the COS/FUV configuration. In order to minimize overheads, this will generally be the same as the one used for the subsequent ACQ/PEAKD and TIME-TAG or ACCUM science observation.

Wavelength

Enter the value of the central wavelength in Ångstroms. Table 11.7: COS Spectral Elements and Central Wavelengths gives the allowed values of the central wavelength for each grating.

Optional Parameters

LIFETIME-POS

= LP1, LP2, LP3, LP4, LP5, LP6, LP7, LP10, LP11

Specifies which FUV lifetime position (LP) to use for this exposure.  Supported values are given in Section 11.1 Introduction to COS.

SEGMENT

= BOTH (default for G130M cenwaves < 1300 A, all G160M cenwaves, and G140L/1280)

= A (default and only allowed value for these cases: G140L, λ=1105 Å, and G130M, λ=1300, 1309, 1318, 1327 Å at LIFETIME-POS=LP5)

= B (restricted for G140L, λ= 1280 Å)

For a description see Optional Parameters.

NUM-POS
1, 3 (default), 5, 7, & 9 (points)

Specifies the number of dwell positions (including the initial target position) in a ACQ/PEAKXD scan. The time required for the scan scales with the number of positions. NUM-POS values of 3 should be used with the flux-weighted centroid algorithm without the "flooring" option (CENTER=FLUX-WT), while values greater than 3 should always use the "flooring" centering option (CENTER=FLUX-WT-FLR). In most cases, NUM-POS equals 3 (the default) is recommended for FUV PEAKXD acquisitions.

STEP-SIZE
= DEF (default), 0.01 - 2.0 (arcsec)

This parameter specifies the size in arcseconds of each step in the linear scan. This parameter defaults to 1.30" when NUM-POS=3, 1.0" when NUM-POS=5, 0.80" when NUM-POS=7, and 0.65" when NUM-POS=9. These default values are tuned to point-sources with initial centering accuracies of 0.4" or better. Note that both the PSA and BOA are 2.5 arcsec in diameter. Assuming the target is already positioned in the aperture, a scan that greatly exceeds the aperture diameter is not likely to be useful.

CENTER
= DEF (default), FLUX-WT, FLUX-WT-FLR, BRIGHTEST

Specifies the method used for locating the target within the search pattern. Two methods are available: (1) a flux-weighted centroiding algorithm (FLUX-WT or FLUX-WT-FLR) or (2) a return to the brightest dwell point (BRIGHTEST). If FLUX-WT-FLR is used, the minimum number of counts measured from any of the dwell points (the "floor") will be subtracted from the number at each of the other dwell points before computing the centroid. The idea behind this threshold is to reduce the contribution of background counts and extreme points at the edges of the scan. The defaults are to use the flux-weighted centroid algorithm without the "flooring" option (FLUX-WT) when NUM-POS=3, and with the "flooring" option (FLUX-WT-FLR) when NUM-POS>3.

Number of Iterations

The Number_Of_Iterations must be 1.

Time Per Exposure

Enter the time of data collection as Time_Per_Exposure. Time_Per_Exposure must be an integral multiple of 0.1 seconds. If it is not, its value will be rounded down to the next lower integral multiple of 0.1 sec, or set to 0.1 seconds if a smaller value is specified. The procedures to determine the exposure time for the ACQ/PEAKXD exposure are given in the COS Instrument Handbook.

Beginning with Cycle 25, FUV ACQ/PEAKXD exposures with NUM-POS > 1 will use both FUV segments, so the combined count rate for both segments should be used to calculate the exposure times. The exceptions to this are when SEGMENT = A or B is specified as an optional parameter, or when acquiring with the G140L grating. In these cases, only the single segment count rate should be considered.

Special Requirements

The special requirement "POSition TARGet <X-value>,<Y-value>" is not permitted on ACQ/PEAKXD exposures.

Mode = ACQ/PEAKD Config = COS/FUV

ACQ/PEAKD Mode invokes a procedure in the COS onboard flight software that takes a series of exposures at different HST pointings offset in the dispersion direction, and HST is repositioned to maximize (peak up) the light intensity through the COS aperture. Details on the target acquisition sequence and limiting magnitudes for the PSA and BOA can be found in the COS Instrument Handbook.

ACQ/PEAKD Mode should always be preceded by an ACQ/PEAKXD exposure. In addition, if the target coordinates are not accurate to 0.4", the ACQ/PEAKD should be preceded by both an ACQ/SEARCH and an ACQ/PEAKXD exposure to ensure that the target is positioned in the aperture and well centered in the cross-dispersion direction.

Subarrays are used during target acquisition to avoid geocoronal airglow lines, which could bias the target acquisition calculations, as well as portions of the detector which are not illuminated by the input spectrum. The subarray dimensions depend on the selected grating and central wavelength; see the COS Instrument Handbook for details.

Aperture or FOV

The aperture used for the ACQ/PEAKD Mode exposure need not be the same as the aperture used for the subsequent TIME-TAG or ACCUM science exposures. The following apertures are allowed:

• PSA Primary Science Aperture
• BOA Bright Object Aperture

As with spectroscopic observations, use of the BOA is recommended on relatively bright targets to attenuate the flux and allow the acquisition to proceed without triggering bright object violations.

Spectral Element

Enter a spectral element from Table 11.7: COS Spectral Elements and Central Wavelengths for the COS/FUV configuration. In order to minimize overheads, this will generally be the same as the one used for the subsequent TIME-TAG or ACCUM science observation. Target acquisition with the COS FUV detector is in dispersed light only.

Wavelength

Enter the value of the central wavelength in Ångstroms.

Optional Parameters

LIFETIME-POS

= LP1, LP2, LP3, LP4, LP5, LP6, LP7, LP10, LP11

Specifies which FUV lifetime position (LP) to use for this exposure.  Supported values are given in Section 11.1 Introduction to COS.

SEGMENT

= BOTH (default for G130M cenwaves < 1300 A, all G160M cenwaves, and G140L/1280)
= A (default and only allowed value for these cases: G140L, λ=1105 Å, and G130M, λ=1300, 1309, 1318, 1327 Å at LIFETIME-POS=LP5)
= B (restricted for G140L, λ= 1280 Å)

This is the same as in ACQ/SEARCH Mode; see Optional Parameters.

NUM-POS
= 3, 5 (default), 7, 9 (points)

Specifies the number of dwell positions (including the initial target position) in an ACQ/PEAKD scan. The time required for the scan scales with the number of positions. The default value of 5 is recommended in combination with the flux-weighted centroid centering method with the "flooring" option (FLUX-WT-FLR). If you select NUM-POS=3, we recommend using the flux-weighted centroid algorithm without the "flooring" option (FLUX-WT). See the COS Instrument Handbook for more details on these recommendations.

STEP-SIZE
= 0.01 - 2.0 (arcsec)

A required parameter that specifies the size in arcseconds of each step in the linear scan. The optimal step size may depend on the number of positions and the centering method; see the COS Instrument Handbook for details. Note that both the PSA and BOA are 2.5 arcsec in diameter. Assuming the target is already positioned in the aperture, a scan that greatly exceeds the aperture diameter is not likely to be useful.

CENTER
= DEF (default), FLUX-WT, FLUX-WT-FLR, BRIGHTEST

Specifies the method used for locating the target within the search pattern. Two methods are available: (1) a flux-weighted centroiding algorithm (FLUX-WT or FLUX-WT-FLR) or (2) a return to the brightest dwell point (BRIGHTEST).

If FLUX-WT-FLR is used, the minimum number of counts measured from any of the dwell points (the "floor") will be subtracted from the number at other dwell points before computing the centroid. The idea behind this threshold is to reduce the contribution of background counts and extreme points at the edges of the scan.

The defaults are to use the flux-weighted centroid algorithm without the "flooring" option (FLUX-WT) when NUM-POS=3, and the "flooring" option (FLUX-WT-FLR) when NUM-POS > 3. See the COS Instrument Handbook for more details.

Number of Iterations

The Number_Of_Iterations must be 1 in this Mode.

Time Per Exposure

Enter the total time of data collection at each dwell point as Time_Per_Exposure. Time_Per_Exposure must be an integral multiple of 0.1 seconds. If it is not, its value will be rounded down to the next lower integral multiple of 0.1 seconds; it will be set to 0.1 seconds if a smaller value is specified.

The procedures to determine the exposure time for the ACQ/PEAKD exposure are given in the COS Instrument Handbook. The exposure time will be repeated at every dwell point in the search.

Special Requirements

The special requirement "POSition TARGet <X-value>,<Y-value>" is not permitted for ACQ/PEAKD exposures.

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