8.3 Onboard Target Acquisition Peakups - ACQ PEAK

When slits of width less than or equal to 0.1 arcsecond are used, an acquisition-peakup exposure should be performed (following the acquisition exposure) to center the target in the slit. You should also consider performing a peakup on the target if you have initially acquired an offset star (rather than your target), to compensate for any additional uncertainties in your knowledge of the offsets. We also 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 at least every 4–5 orbits. This will ensure that drifts (see Section 8.1.4) do not cause the target to move out of the slit. Programs with more stringent requirements on position or wavelength stability will need more frequent peakups.

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. 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 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. A "confirmation image" is then taken through the aperture on the subarray and is included in the ACQ/PEAK data set.

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 × 32 for white-light (mirror) peakups and 32 (perpendicular to the dispersion) × 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.

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). When using the 0.1X0.03 slit, users are advised to add two ACQ/PEAKs as shown in Table 8.5.

Table 8.5: Peakup Scan Sequences and Parameters for Supported Spectroscopic Slits.

¹ Peakup acquisitions are not recommended for apertures wider than 0.1 arcsec.
² The 0.2X0.05ND or the 0.3X0.05ND slits can be used in place of the 0.2X0.09 slit.

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

If your target is otherwise too bright to perform a peakup with the CCD camera mirror in place, you can use the echelle slit 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 a dispersed light peakup will be performed with a grating other than that used for the scientific observation, an additional overhead of ~3 min should be included to account for movement of the grating wheel.

Selecting the Aperture

A peakup can be done using any of the long or echelle slits listed 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 in wider slits (which normally do not require peak-ups). The slit-to-slit positioning accuracy is 0.005 arcsecond. Instances in which you may wish to utilize a smaller aperture for the acquisition are observations requiring accurate photometry (where the source should be properly centered in a wide slit) and bright-source acquisitions. Note that peakups using the NX0.2 apertures (those with widths of 0.2 arcseconds in the dispersion direction) are no longer recommended as they provide no refinement in pointing over that routinely achieved in a normal ACQ. If an ACQ/PEAK is needed for an NX0.2 science exposure (e.g., after an ACQ on an offset target or to re-center after a few orbits), better positioning accuracy can be achieved with a narrower aperture, such as NX0.1.

The suffix E1 on an aperture name (e.g., 52X0.2E1) indicates that the target will be positioned high on the CCD detector, at about row 900, in that aperture. This is useful for greatly reducing CTE effects, since that location is close to the default readout amplifier. If an E1 aperture is used for the science exposure, it is best to use an E1 aperture for the peakup, to eliminate small errors in slewing and possible small errors in the definitions of the E1 positions.

For coronagraphic imaging, the bar and wedge positions on the 50CORON aperture are all large enough that a peakup is not required. However, if you require especially 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 may be useful. Note that a peakdown acquisition is not recommended (see Section 12.10).

Determining the Peakup Exposure Time

The minimum 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 × 4 pixel region. For CCD dispersive (grating) peakups, the minimum 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 TA ETC (for acquisitions and imaging peakups) or the Spectroscopic ETC (for dispersive peakups). For acquisitions and peakups you must be sure not to saturate the CCD during your exposure. Table 8.3 lists, for a range of spectral types, the brightest magnitude at 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 approach saturation (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). This is especially important for peakups, where low signal-to-noise is the leading cause of poor centering.

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

Table 8.6: Maximum Allowed Exposure Times for CCD Peakups.

8.3.2 Specifying Acquisition Peakups in Phase II 

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

¹ For CCD ACQ/PEAKs the same type of processing is applied as in acquisitions by the FSW to remove the bias and cosmic rays, with the only difference being that there is no offset performed between the two images taken at each pointing, for obvious reasons.