Mode = ACQ/SEARCH Config = COS/NUV

This mode invokes a procedure in the COS onboard flight software that searches for the target and locates 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.

Details on the target acquisition sequence and limiting magnitudes for the PSA and BOA can be found in the COS Instrument Handbook.

This method in the NUV uses undispersed or dispersed light from the object to be observed. In dispersed light, the total counts from all three stripes are used to maximize the photon counting statistics. There are no appreciably bright airglow lines in the NUV portion of the spectrum, but subarrays are still used to exclude portions of the detector which are not illuminated by the input spectrum or image; this improves accuracy by reducing the contribution from background counts. The subarray dimensions depend on the selected spectral element; see the  COS Instrument Handbook for details.

Aperture or FOV

The following apertures are allowed:

• PSA Primary Science Aperture
• BOA Bright Object Aperture

Use of the BOA is recommended on 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 11.5: COS Spectral Elements and Central Wavelengths for the COS/NUV configuration. If a grating is selected, this will generally be the same as the one used for the subsequent TIME-TAG or ACCUM science observation. If the MIRRORA or MIRRORB spectral element is selected, the search for the target will be done in undispersed light. Use of the MIRRORB spectral element (possibly in conjunction with the BOA aperture) will attenuate the flux and allow undispersed-light acquisitions to be done on targets that would otherwise trigger bright object checks.

Wavelength

Enter the value of the central wavelength in Angstroms. Table 11.5: COS Spectral Elements and Central Wavelengths gives the allowed values of the central wavelength for each grating.

Note: No wavelength should be specified if MIRRORA or MIRRORB is the spectral element.

Optional Parameters

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

This is a required parameter that 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.

Note: 2 × 2 dispersed light searches with the NUV channel should be avoided in most cases, as simulations show significantly degraded accuracy; a 2 × 2 search can even worsen the alignment of a target that is initially well centered. For more details on target acquisition strategies, please 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: (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. 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).

Because of the higher detector background of the NUV detector, use of the floored threshold is considered necessary to achieve acceptable accuracy with the flux-weighted centroiding algorithm. Therefore, the default is to use FLUX-WT-FLR with a SCAN-SIZE of 3 or larger. See the COS Instrument Handbook for recommendations on using 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 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/IMAGE Config = COS/NUV

ACQ/IMAGE Mode invokes a procedure in the COS onboard flight software that takes an image in undispersed light using the NUV mirror to center the target within the selected aperture. This procedure consists of several steps.

1. The position of the science aperture on the detector (which may drift with time) is located by flashing the calibration lamp (also in undispersed light) and measuring the location of the calibration image.
2. The target acquisition image is then taken within a 4 × 4 arcsec subarray centered at the expected center of the science aperture.
3. Next, a 9 × 9 pixel checkbox is passed across the subarray to identify the region with the maximum counts. Within the checkbox, a flux-weighted centroiding algorithm is used to calculate the expected target position.
4. Finally, HST is repositioned to place that location at the center of the science aperture, and a subsequent exposure is commanded and downlinked to verify that the target ended up in the preferred location.

Use of ACQ/IMAGE Mode requires that the science aperture (1.25 arcsec radius) contain appreciable flux from the target. This operation is theoretically limited to sources within 2 arcsec of the center of the aperture; an initial pointing error less than 1.0 arcsec is recommended. If the uncertainty in the initial target position is greater than this, the use of ACQ/IMAGE should be preceded by an exposure in ACQ/SEARCH mode to locate the target. Details on the target acquisition sequence and limiting magnitudes for the PSA and BOA, as well as subarray dimensions for the calibration and acquisition images, can be found in the  COS Instrument Handbook.

Aperture or FOV

The following apertures are allowed:

• PSA Primary Science Aperture
• BOA Bright Object Aperture

 Spectral Element

Enter either MIRRORA or MIRRORB. The gratings are not allowed in this Mode. Use of the MIRRORB spectral element and/or the BOA aperture will attenuate the flux and allow an image acquisition to be done on targets that would otherwise trigger bright object violations.

Wavelength

The Wavelength parameter does not apply to this Mode and should be left blank.

Optional Parameters

There are no Optional Parameters for this Mode.

Number of Iterations

The Number_Of_Iterations must be 1 in this Mode.

Time Per Exposure

Enter the total 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. This time will be used both for the acquisition image and the subsequent confirmation image.

The procedures to determine the exposure time for the ACQ/IMAGE exposure are given in the COS Instrument Handbook.

Special Requirements

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

Mode = ACQ/PEAKXD Config = COS/NUV

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. This procedure consists of several steps.

1. The optimum cross-dispersion position of the science aperture on the detector is located by flashing the calibration lamp and measuring the mean cross-dispersion location of the calibration spectrum.
2. Then the target spectrum is recorded in dispersed light for the specified period of time, and its mean cross-dispersion position is also computed.
3. Finally, HST is repositioned so that the target location coincides with the optimum cross-dispersion position for the target spectrum within the aperture.

If the target coordinates are not accurate to 0.4", an ACQ/PEAKXD should be preceded by an ACQ/SEARCH to ensure that the target is positioned in the 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 can be found in the COS Instrument Handbook.

In ACQ/PEAKXD Mode, acquisition subarrays are limited to a single stripe. The subarray dimensions may also depend on the selected grating; see the COS Instrument Handbook for details.

Aperture or FOV

The aperture used for the ACQ/PEAKXD Mode exposure will generally 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 one of the gratings from Table 11.5: COS Spectral Elements and Central Wavelengths for the COS/NUV configuration. This will generally be the same as the one used for the subsequent TIME-TAG or ACCUM science observation. This Mode only uses dispersed light, so the mirror may not be used.

Wavelength

Enter the value of the central wavelength in Angstroms. Table 11.5: COS Spectral Elements and Central Wavelengths gives the allowed values of the central wavelength for each grating. Due to increased NUV detector background and mechanism position uncertainties, certain NUV central wavelength settings have proven to be unreliable in the NUV ACQ/PEAKXD target acquisition phase. The NUV central wavelengths settings listed in bold in Table 11.5: COS Spectral Elements and Central Wavelengths have been proven reliable for ACQ/PEAKXD using STRIPE=DEF. NUV ACQ/PEAKXD exposures should use one of these settings. See the COS Instrument Handbook for further details. Users who wish to use other central wavelengths, or non-default STRIPE settings, should contact the COS team via the Help Desk.

Optional Parameters

STRIPE
= DEF (default), SHORT, MEDIUM, LONG

Identifies the spectral stripe to use for the acquisition subarray. The shortest wavelengths of the spectrum are projected onto the stripe designated by SHORT, wavelengths in the middle of the range onto the stripe designated by MEDIUM, and the longest wavelengths onto the stripe designated by LONG. To determine the mapping between the selected spectral wavelength and the wavelength coverage in each stripe, please see the COS Instrument Handbook.

There are special restrictions with the G230L grating, which positions first-order light on the detector only for certain stripes. The MEDIUM stripe is required for the 2635 central wavelength, the SHORT stripe is required for the 3360 wavelength, and either SHORT or MEDIUM may be used for the 2950 and 3000 wavelengths. The LONG stripe may not be used with G230L at all.

Note: A value of DEF is normally the same as MEDIUM. The one exception is with the G230L grating and the 3360 wavelength, where only the SHORT stripe is allowed. STRIPE=DEF is preferred because the flight software only stores the cross-dispersion offset between the location of the calibration spectrum and the optimal location of the science spectrum for the DEF stripe. The true offset varies by +/− 2.5 pixels between stripes, so there will be a slight error in the calculated "optimal" position of the target spectrum if SHORT or LONG is used. This might cause a small drop in flux, but is not expected to affect the success of target acquisition.

Number of Iterations

The Number_Of_Iterations must be 1 in this Mode.

Time Per Exposure

Enter the total 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.

Special Requirements

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

Mode = ACQ/PEAKD Config = COS/NUV

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 the HST is repositioned to maximize (peak up) the light intensity through the COS aperture.

The visual magnitude 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 can be found in the COS Instrument Handbook.

An ACQ/PEAKD 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 ACQ/SEARCH and ACQ/PEAKXD exposures to ensure that the target is positioned in the aperture and well centered in the cross-dispersion direction.

There are no appreciably bright airglow lines in the NUV portion of the spectrum, but subarrays are still used to exclude portions of the detector which are not illuminated by the input spectrum or image; this improves accuracy by reducing the contribution from background counts. The subarray dimensions depend on the selected grating; see the COS Instrument Handbook for details. For this Mode, there is no need to specify the spectral stripe as in ACQ/PEAKXD Mode because the subarrays include counts from all stripes to maximize the photon counting statistics.

Aperture or FOV

The aperture used for the ACQ/PEAKD Mode exposure will generally 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 on relatively bright targets to attenuate the flux and allow the acquisition to proceed without triggering bright object violations.

Spectral Element

Enter one of the gratings from Table 11.5: COS Spectral Elements and Central Wavelengths for the COS/NUV configuration. This will generally be the same as the one used for the subsequent TIME-TAG or ACCUM science observation. This Mode only uses dispersed light, so the mirror may not be used.

Wavelength

Enter the value of the central wavelength in Angstroms. Table 11.5: COS Spectral Elements and Central Wavelengths gives the allowed values of the central wavelength for each grating.

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. When the flux-weighted centroid centering method with the "flooring" option is used (FLUX-WT-FLR), increasing the number of positions will improve the accuracy, but simulations show that value of a 5 does almost as well as 9. For more details consult the COS Instrument Handbook.

STEP-SIZE
= 0.01 - 2.0 (arcsec)

This is 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 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 when NUM-POS=3, and to use the "flooring" option when NUM-POS > 3. See the COS Instrument Handbook for more details on these recommendations.

Number of Iterations

The Number_Of_Iterations must be 1 in this Mode.

Time Per Exposure

Enter the total 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/PEAKD exposure are given in the COS Instrument Handbook.

Special Requirements

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

Related Links

11.2.1 COS FUV Target Acquisition Modes

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