Phase 2 Proposal Instructions for Cycle 10

10.2 Mode = ACQ
Configuration = STIS/CCD

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The STIS onboard flight software locates the target in the STIS FOV for subsequent science exposures. At the end of the onboard acquisition process, the 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 sources and diffuse fields. The total flux within a sequence of overlapping �check boxes� within an acquisition subarray is determined, and the target position is located within the brightest check box.

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.

10.2.1 Aperture or FOV

See Table10.1: Supported Optional Parameters for STIS Instrument Modes and Table10.4: Supported STIS Apertures.

Enter one of the following apertures:

Entry	Description
F28X50LP	Optical longpass; preferred target acquisition aperture
F28X50OII	[OII] 3740 �; use for bright sources or emission-line objects
F28X50OIII	[OIII] 5007 �; use for bright sources or emission-line objects1
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

1Please note that this filter has a substantial red leak; see the STIS Instrument Handbook.

 

10.2.2 Spectral Element

MIRROR is the only valid spectral element for this Mode.

10.2.3 Optional Parameters

See Table10.1: 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 source or a diffuse field.

With the default value of POINT, an acquisition subarray of 100 x 100 CCD pixels (5x5 arcsec) and a check box of 3x3 CCD pixels is always used. The target position is the flux-weighted centroid of the brightest check box 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 check box, depending on the DIFFUSE�CENTER parameter.

DIFFUSE�CENTER
=FLUX�CENTROID (default), GEOMETRIC�CENTER

Specifies whether the target position of a diffuse field is to be located as the flux weighted centroid (FLUX�CENTROID) or the geometric center (GEOMETRIC�CENTER) of the brightest check box. 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 diffuse 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.

10.2.4 Number of Iterations

The Number_Of_Iterations must be 1 in this Mode.

10.2.5 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.

10.2.6 Special Requirements

None.