8.2 Target Acquisition Modes
Observations with HST's STIS instrument generally require a target acquisition before obtaining science data, and the Astronomer’s Proposal Tool (APT) has parameters for specifying these observations. |
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Format definitions
Boldface type indicates the name of an APT parameter or a value for a parameter.
Black text indicates an important note.
Magenta text indicates available but unsupported parameters (requires prior approval from STScI).
Red text indicates restricted parameters (for STScI use only).
Brown text indicates text file parameters.
Items in brackets - <value> - are required values.
Items in square brackets - [<value>] - are optional.
Introduction
HST pointing now uses the GSC2 catalog to determine the position of guide stars. Following the initial guide star acquisition for your visit, the target location in the aperture plane should now be known to an accuracy of 0.2 to 0.5 arcseconds. This is a considerable improvement over the 1-2 arcsecond accuracy achieved using GSC1. However, for STIS spectroscopic observations of an external target, any uncertainty in the target position along the dispersion direction will directly translate into an uncertainty in the zero point of the wavelength scale. Therefore, for most STIS spectroscopic science observations, even those using large apertures, and for any science observations using the coronagraphic bars, it is strongly recommended that a target acquisition (ACQ Mode) exposure and possibly one or more acquisition peakup exposures (ACQ/PEAK Mode) be done to center your target. The CCD detector may be used for acquisitions of targets that will be observed in the CCD and MAMA ACCUM and TIME-TAG modes. Details on STIS target acquisition modes are in the STIS Instrument Handbook.
 The STIS Target Acquisition Exposure Time Calculator (ETC) and Target Acquisition Simulator are tools provided by STScI to help plan acquisitions. You can find these tools at: http://www.stsci.edu/hst/instrumentation/stis/data-analysis-and-software-tools. |
Mode = ACQ Config = STIS/CCD
The STIS onboard flight software locates the target in the STIS FOV for subsequent science exposures. At the end of the onboard acquisition process, HST is repositioned to place the target in the aperture specified on the first science exposure in the series.
Onboard acquisitions of two different types of targets are supported: point and extended sources. The total flux within a sequence of overlapping “checkboxes” within an acquisition subarray is determined, and the target position is located within the brightest checkbox.
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 acquisition aperture is appropriate for CCD target acquisition. Details on the CCD target acquisition sequence and limiting magnitudes for the acquisition apertures and spectroscopic slits can be found in the STIS Instrument Handbook.
Aperture or FOV
See Table 8.7: Supported STIS Apertures.
Enter one of the following apertures:
| Entry | Description |
| F28X50LP | Optical longpass; preferred target acquisition aperture |
| F28X50OII | [OII] 3740 Å; use for bright sources or emission-line objects |
| F28X50OIII | [OIII] 5007 Å; use for bright sources or emission-line objects[1] |
| F25ND3 | Neutral density filter for overly bright targets; ND = ~10−3 |
| F25ND5 | Neutral density filter for overly bright targets; ND = ~10−5 |
| 50CCD | Clear aperture for very faint sources |
1 Please note that this filter has a substantial red leak; see the STIS Instrument Handbook.
Spectral Element
MIRROR is the only valid spectral element for this Mode.
Optional Parameters
See Table 8.1 Supported Optional Parameters for STIS Instrument Modes for a listing by operating mode of all Optional Parameters. Here are descriptions for those Optional Parameters for ACQ mode.
ACQTYPE
=POINT (default), DIFFUSE
Specifies the type of onboard target acquisition algorithm used in locating the target. Onboard algorithms are available for acquiring a point or extended (diffuse) source.
With the default value of POINT, an acquisition subarray of 100 × 100 CCD pixels (5 × 5 arcsec) and a checkbox of 3 × 3 CCD pixels are always used. The target position is the flux-weighted centroid of the brightest checkbox within the subarray.
With the DIFFUSE acquisition type, the acquisition subarray is a square array. The number of pixels on each side is 101 + CHECKBOX (see below). The target position is either the geometric center or the flux-weighted centroid of the brightest checkbox, depending on the DIFFUSE–CENTER parameter.
DIFFUSE–CENTER
=FLUX–CENTROID (default), GEOMETRIC–CENTER
Specifies whether the target position of a extended source is to be located as the flux weighted centroid (FLUX–CENTROID) or the geometric center (GEOMETRIC–CENTER) of the brightest checkbox. DIFFUSE– CENTER is only permitted if ACQTYPE=DIFFUSE.
CHECKBOX =3–105 (pixels; odd only)
Specifies the length of a side of a square pixel array over which the light intensity is summed to locate the brightest region of the acquisition. This parameter is required for extended targets (ACQTYPE=DIFFUSE). It is not allowed for point sources (ACQTYPE=POINT), for which a fixed checkbox size based on the PSF will be used. The value of CHECKBOX should equal the size of the brightest area of the source the observation is attempting to locate. The dimension of the acquisition subarray will be 101 pixels plus the CHECKBOX parameter. The value of CHECKBOX must be an odd number.
For available but unsupported parameters
Number of Iterations
The Number_of_Iterations must be 1 in this Mode.
Time Per Exposure
The procedures to determine the exposure time for the ACQ exposures are given in the STIS Instrument Handbook.
The exposure time for ACQ exposures on GO proposals must be less than 5 minutes for ACQTYPE=POINT acquisitions, and for ACQTYPE=DIFFUSE acquisitions must be less than 500/(101+CHECKBOX) minutes.
Special Requirements
None.
Mode = ACQ/PEAK Config = STIS/CCD
In ACQ/PEAK Mode, a series of exposures is taken at several HST pointings offset along the detector axes, and the HST is repositioned to maximize (peak up) or minimize (peak down) the light intensity within a default or proposer-specified detector subarray. AXIS1 and AXIS2 are in the dispersion and cross-dispersion (or spatial) directions respectively. For more information, including when peakups are required, see the STIS Instrument Handbook.
Mode ACQ/PEAK should always be preceded by an ACQuisition.
Aperture or FOV
The aperture used for the ACQ/PEAK exposure will generally be identical to the aperture used for the subsequent science exposure. For maximum centering accuracy, a peakup aperture smaller than the science exposure may be chosen. Peakups are required whenever an aperture dimension is less than or equal to 0.1 arcsec. Two peakups are recommended for the 0.1X0.03 aperture: one with the 0.2X0.09 aperture followed by another with the 0.1X0.03 aperture.
See Table 8.7: Supported STIS Apertures for the allowed ACQ/PEAK apertures. Note that you cannot perform a peakup with an E2 aperture (i.e., 52X... apertures ending with the E2 suffix), which are designed for use in conjunction with CCD Fringe Flats. If you need an ACQ/PEAK exposure prior to using an E2 aperture, perform the peakup using the 52X0.1E1 aperture. Then select the appropriate E2 aperture for the following science exposures.
When an ACQ/PEAK exposure using the occulting bar aperture 52X0.2F1 is specified, a peakdown rather than a peakup will automatically be performed. A peakup will be performed otherwise. To peak up on the width of a slit with an occulting bar before peaking down on the bar itself, the occulting bar aperture name with –R appended to it should be used. Such exposures will peak up on a reference point offset from the occulting bar along the slit’s length.
For available but unsupported parameters
Spectral Element
MIRROR is the recommended spectral element for this mode unless the object is too bright. See the STIS Instrument Handbook for additional guidance on selecting the spectral element. See Table 8.2: Supported Central Wavelengths for STIS Gratings Spectral.
Wavelength
Enter the value of the central wavelength in Angstroms. Table 8.2: Supported Central Wavelengths for STIS Gratings Spectral gives the allowed values of the central wavelength for each grating. Consult the STIS Instrument Handbook section 13.3 Gratings for the associated minimum and maximum wavelength and other information pertaining to the gratings.
| No wavelength should be specified if MIRROR is the spectral element. |
Optional Parameters
See Table 8.1 Supported Optional Parameters for STIS Instrument Modes for a listing by operating mode of all Optional Parameters. Here are descriptions for those Optional Parameters for ACQ/PEAK mode.
SIZEAXIS2 =DEF (default); 30–1022 (pixels, dispersive elements), 16–1022 (pixels, MIRROR)
Specifies the size of a CCD subarray in detector coordinates along the AXIS2 (cross-dispersion or spatial) direction.
The default subarray with SIZEAXIS2=DEF depends on the aperture and spectral element. With the MIRROR element, the default subarray will generally be 32 × 32 CCD pixels centered on the aperture. However, the F1 fiducial slits use a 32 × 64 subarray. A subarray of 16 × 16 is also allowed when MIRROR is used.
For available but unsupported parameters
With a first-order CCD grating (G230LB, G230MB, G430L, G430M, G750L, G750M), the default subarray will generally be 1022 × 32 CCD pixels. However, the F1 fiducial slits use a 1022 × 64 subarray. The F2 fiducial slits use a 1022 × 128 subarray. These subarrays have been chosen to minimize cosmic ray perturbations on the acquisition process.
For available but unsupported parameters
The SIZEAXIS2, SIZEAXIS1, SEARCH, NUM–POS, and STEP–SIZE parameters are required with the following apertures: BAR5, BAR10, WEDGEA0.6, WEDGEA1.0, WEDGEA1.8, WEDGEA2.0, WEDGEA2.5, WEDGEA2.8, WEDGEB1.0, WEDGEB1.8, WEDGEB2.0, WEDGEB2.5, WEDGEB2.8, 0.5X0.5, 6X0.5, 52X0.1B0.5, 52X0.1B1.0, 52X0.1B3.0, 52X0.5, 52X0.5F1, 52X0.5F1–R, 52X0.5F2, 52X0.5F2–R, 52X2, 52X2F1, 52X2F1–R,52X2F2, and 52X2F2–R.
SIZEAXIS1 =DEF (default); 30–1060 (pixels, dispersive elements), 16–1060 (pixels, MIRROR)
Specifies the size of a CCD subarray in detector coordinates along the AXIS1 (dispersion) direction. The default subarray with SIZEAXIS1=DEF depends on the aperture and spectral element, as given under SIZEAXIS2 above.
CENTERAXIS2 =TARGET (default); 17–1009 (pixels, dispersive elements), 10– 1016 (pixels, MIRROR)
Specifies the center of a CCD subarray in detector coordinates along the AXIS2 (cross-dispersion or spatial) direction. The default value of TARGET means that the subarray will be centered on the detector coordinates of the target (which change with POSition TARGet offsets).
CENTERAXIS1 =TARGET (default); 17–1047 (pixels, dispersive elements), 10– 1054 (pixels, MIRROR)
Specifies the center of a CCD subarray in detector coordinates along the AXIS1 (dispersion) direction. The default value of TARGET means that the subarray will be centered on the detector coordinates of the target.
SEARCH =DEF (default), SPIRAL, LINEARAXIS1, LINEARAXIS2
Specifies whether an ACQ/PEAK exposure should consist of a SPIRAL, LINEARAXIS1, or LINEARAXIS2 scan of vehicle positions (in detector coordinates) to peak up or peak down the light intensity through a spectroscopic slit. The default value will be based on the slit and scan type requested for the exposure. See the STIS Instrument Handbook for default values.
| If any of the parameters SEARCH, NUM–POS, or STEP–SIZE have non-default values, then all three parameters must have non-default values. |
NUM–POS =DEF (default); 3–49 (pixels; odd only)
Specifies the number of dwell positions (including the initial target position) in an ACQ/PEAK scan. The default value will be based on the slit and scan type requested for the exposure; see the STIS Instrument Handbook for default values. Only odd numeric values are allowed. If SEARCH=SPIRAL, the value of NUM–POS must be 9, 25, or 49.
STEP–SIZE =DEF (default); 1–1000 (milliarcsec)
Specifies the size in milli-arcseconds of each vehicle step in a centering scan. The default value will be based on the slit and scan type requested for the exposure. See the STIS Instrument Handbook for default values.
CENTROID =YES (default), NO
Specifies the algorithm to center the target. If CENTROID=YES, the flight software uses a flux-weighted centroiding technique to determine the optimum position.
If CENTROID=NO, the brightest dwell point will be used for the optimum position. A peakdown will automatically be performed if the selected aperture (see Table 8.7: Supported STIS Apertures) contains an occulting bar or wedge. For all other apertures, a peakup is performed. The occulting-bar apertures are those listed in the paragraph at the end of the “Aperture or FOV” section above.
GAIN =4 (default); 1, 2, 8 (e/DN)
Specifies the gain of the CCD electronics in e/DN.
Number of Iterations
The Number_of_Iterations must be 1 in this Mode.
Time Per Exposure
The procedures to determine the exposure time for the ACQ/PEAK exposure are given in the STIS Instrument Handbook.
Special Requirements
None.