Onboard Target-Acquisition Peakups (ACQ/PEAK)

When slits less than or equal to 0.1 arcsecond wide or the narrow coronographic bars are used, an acquisition-peakup exposure should be performed following the acquisition exposure to center the target in the slit or coronographic bar. You should also consider performing a peakup if you have acquired an offset star, rather than your target, to compensate for any additional uncertainties in your knowledge of the offsets. We recommend that for a long series of exposures taken through slits which are less than or equal to 0.1 arcsecond in either dimension, a peakup be performed every 4-5 orbits. This will ensure that drifts (see Drift Rates) do not cause the target to move out of the slit.

Figure 8.9 illustrates the basic peakup sequence. When a peakup exposure is performed, the telescope is moved to step the target across the slit or bar. At each step (or dwell point), an image1 of the sky is taken and the total flux in a specified subarray is determined. To allow for a more accurate calculation, the minimum flux value in the peakup (the PEDESTAL) is subtracted from each step. The flight software (FSW) then selects the position of maximum flux, using a flux-weighted centroiding technique to determine the optimum position to a fraction of a dwell step. At the conclusion of the ACQ/PEAK exposure, the FSW moves the telescope to position the target at the derived optimal position within the aperture.

Figure 8.9: Schematic of Peakup Sequence

Peakup exposures can be taken with either a mirror (to peak up in undispersed white light) or a grating (to peak up in dispersed light) and with the CCD detector only. Subarrays can be specified to limit the region of the detector (sky) over which the flux is determined at each dwell point. The default subarray sizes, 32 by 32 for white-light (mirror) peakups and 32 (perpendicular to the dispersion) by 1022 (in the dispersion direction) for dispersed-light peakups, are appropriate for peakups on point-sources. They should be changed only if you are performing diffuse-source peakups or if you wish to isolate a single line in dispersed-light peakups, and only upon consultation with an Instrument Scientist.

We recommend performing all CCD peakups using the mirror unless your target is too bright.

You do not specify the parameters of the stepping sequence employed during the peakup; it is predetermined, based on the aperture you have chosen. Table 8.5 below shows the scan sequence employed for all of the long and echelle slits. The scan sequence for a peakup may include a linear scan in the dispersion direction (SEARCH=LINEARAXIS1), a linear scan perpendicular to the dispersion axis (SEARCH=LINEARAXIS2), or a spiral search pattern (SEARCH=SPIRAL). Additional parameters are the number of steps (NUMSTEPS) and the step intervals between each dwell point (STEPSIZE). Note that all ACQ/PEAKs are single-stage peakups, except for the smallest slit (0.1X0.03), which requires a 2-stage peakup.

Selecting Peakup Parameters

To plan your acquisition peakup, you must specify:

• The optical element.
• The APERTURE (program slit) upon which to peak up.
• The exposure time for the peakup image.

Selecting the Optical Element

Peakups can be performed either with a dispersive element in a spectroscopic configuration with any of the allowed grating/detector combinations, or in undispersed white light in an imaging configuration. Most peakup exposures should be performed using the CCD in imaging mode (white light).

If your target is otherwise too bright to perform a CCD peakup with a camera mirror in place, you can use the echelle slits 0.2X0.05ND (which has an ND filter with a factor of 100 attenuation or the 0.3x0.05ND, with attenuation by a factor of 1000) or use a dispersed-light peakup. Also note that if you wish to peak up in a particular line for which there is no imaging filter, a dispersed-light peakup using a grating should be used. Observers should generally perform dispersed light peakups with the same gratings and apertures they intend to use for their scientific observations.

If extremely high target acquisition centering accuracy is required, observers should consider peaking up in a smaller slit than the program slit; the slit-to-slit positioning accuracy is 0.005 arcsecond.

Selecting the Aperture

A peakup can be done using any of the long or echelle slits in Table 8.5 as the APERTURE.You will (typically) want to specify the peakup aperture as the aperture used for the subsequent scientific observations, although it is possible to specify a smaller aperture than your program aperture if you require higher target acquisition centering accuracy. Instances in which you may wish to utilize a smaller aperture for the acquisition are coronographic observations, observations requiring accurate photometry (where the source should be properly centered in a large slit), and bright-source acquisitions. Note that peakups using the NX0.2 apertures (those with widths of 0.2" in the dispersion direction) are no longer recommended as they provide no refinement in pointing over that routinely achieved in a normal ACQ.

For acquisitions under the narrow occulting fiducial (52X0.2F1), a peakup acquisition using a small slit should be used. The bar and wedge positions on the 50CORON aperture (imaging) are all large enough that a peakup is not required. However, if you require accurate target acquisition centering (for example, to place a calibration star at the same position under the bar or wedge to measure the scattered-light profile), then a peakup is required. Note that a peakdown acquisition is no longer recommended. See Coronographic Spectroscopy and the examples on pages 187 and  189.

Determining the Peakup Exposure Time

The required exposure time for CCD imaging (mirror) peakups is the time to obtain a minimum of 5000 electrons (1250 DN) from a point-source, or equivalently, 5000 electrons from the peak of a diffuse source which is contained in a 4 x 4 pixel region. For CCD dispersive (grating) peakups, the exposure time is the time to obtain a minimum of 80,000 electrons (20,000 DN) integrated across the spectrum from a point-source (or equivalently, 80,000 electrons from the peak of a diffuse source integrated over 4 pixels perpendicular to the dispersion axis). For CCD dispersive peakups on a single emission line, the exposure time is the time to obtain a minimum of 5000 electrons in the chosen line; a small subarray is selected to isolate the line.

To determine the exact exposure time, you should use the STIS Target-Acquisition Exposure-Time Calculator (for imaging peakups) or the STIS Spectroscopic Exposure-Time Calculator (for dispersive peakups). Be sure to include the effect of your chosen slit throughput (see Chapter 13) in your calculation (e.g., for imaging peakups, if the Exposure-Time Calculator does not account for slit loss). For CCD peakups you must be sure not to saturate the CCD during your exposure. Table 8.3 lists the brightest magnitude star for a range of spectral types on which a CCD peakup exposure can be performed in white light, assuming zero slit losses. Note that the overheads in target acquisition are substantially longer than most exposure times, so as long as you do not saturate (within 30% of the full well) your target, you should increase your exposure time by a factor of 2-5 above the minimum required (e.g., if the exposure time to obtain the requisite number of electrons is 0.3 second, then you can lengthen it to 1 second if no saturation occurs).

There is a limit on the maximum exposure time allowed for CCD peakups, which is imposed to ensure that multiple coincident cosmic rays do not affect the target acquisition centering accuracy. Table 8.6 lists the maximum CCD exposure time for point-source white-light and dispersed-light peakups for each aperture.2

Table 8.6: Maximum Allowed Exposure Times for CCD Peakups

Slit (APERTURE)	Imaging Maximum Exposure Time for Dwell (minutes)	Spectroscopic Maximum Exposure time for Dwell (minutes)
52X0.1	7.6	1.3
52X0.2, 52X0.2F1-R	9.8	1.7
6X0.2	9.8	1.7
0.2X0.06	7.6	1.3
0.2X0.2	9.8	1.7
0.2X0.09	7.6	1.3
0.2X0.05ND	6.4	1.1
0.3X0.05ND	6.4	1.1

Specifying Acquisition Peakups in Phase II

The user requests a peakup acquisition exposure during Phase II by specifying MODE=ACQ/PEAK on the proposal logsheet. The default setting for the scan (SEARCH, NUMSTEPS, STEPSIZE) for your chosen APERTURE is then automatically selected from the lookup table.

2 More generally, the maximum allowed exposure time for CCD ACQ/PEAKs is: minutes.