Acquisition exposures are controlled by the Flight Software (FSW). Figure 8.3 highlights the basic steps in the acquisition process. The sequence comprises two discrete stages: the coarse-locate phase (steps 1-2) and the fine-locate phase (steps 3-5); a sixth step centers the target in the science aperture. The coarse-locate phase is performed to place the target as close as possible to the aperture center prior to the final telescope move. This step ensures that the final slew needed to move the target into the aperture is a small one, and minimizes uncertainties in the calculation of the required slew caused by detector or optical distortions.
0.2X0.2 slit.
0.2X0.2 slit is determined relative to the target. The external shutter is closed (to prevent a possible overlight condition), the 0.2X0.2 slit is rotated into position by the slit wheel, and an image is obtained with the slit illuminated by the HITM line lamps. The slit image is processed and a finding algorithm is then used to determine the coordinates of the center of the slit.
0.2X0.2 slit, and performs a small-angle maneuver of HST to place the target in the center of the aperture.
An acquisition exposure produces scientific data, which include the images of the target produced in steps 1 and 3, and the image of the 0.2X0.2 slit produced in step 4. These data will be returned to you with your scientific data as part of the pipeline products, and they can be analyzed with the tastis tool in STSDAS.
STIS supports two basic types of acquisitions: point-source acquisitions (ACQTYPE=POINT) and diffuse-source acquisitions (ACQTYPE=DIFFUSE). Diffuse-source acquisitions are appropriate for sources that exhibit smooth or peaked surface brightness on some size scale, such as centers of galaxies, some planets and planetary satellites (see Solar-System Acquisitions), or nebulae.
To locate the target, the flight software first passes a square checkbox over the image and determines the flux contained within the checkbox at each pixel in the subarray. The flight software then selects the checkbox with the maximum flux and determines the target center within that checkbox according to the type of acquisition specified.
For point-source acquisitions (ACQTYPE=POINT), the checkbox size is fixed at 3 x 3 pixels (0.15x0.15 arcsecond) and the flight software determines the target location by finding the flux-weighted centroid of the pixels in the brightest checkbox (see Figure 8.4).
For diffuse acquisitions (ACQTYPE=DIFFUSE), the flight software determines the target location either by finding the flux-weighted centroid of the pixels in the brightest checkbox or by determining the geometric center of the brightest checkbox (see Figure 8.5).
For DIffuse acquisitions, the user must specify both the target acquisition centering method (Diffuse-center=geometric-CENTER or FLUX-centroid) and the checkbox size. The user sets Checkbox=n, where n must be an odd number less than or equal to 105: the checkbox will then have dimension n x n pixels. CHECKBOX should be set to the minimum size which ensures that the brightest checkbox will be the one centered on the region of interest (i.e., if your object is peaked within a region of 1 arcsecond, set CHECKBOX=21 (= 1 + (1 arcsecond/0.05 arcsecond pixel-1)). The maximum checkbox is 105 pixels on a side, or ~5 x 5 arcseconds. The subarray used for a diffuse-source acquisition target image is CHECKBOX+101 pixels on a side. The STIS Target-Acquisition Simulator can be used to determine the optimal CHECKBOX size.
Figure 8.6 shows a simulated example of a diffuse source, the nucleus of the galaxy M86, acquired using a diffuse-source acquisition.
Figure 8.6: Simulated Diffuse Acquisition of Elliptical Galaxy M86. Created by running the flight-software algorithm on a STIS image. CHECKBOX=25 produced good centering. Smaller values caused checkbox to center on local brightness enhancements offset from galaxy center.
To plan your acquisition, you must select:
APERTURE to be used for target imaging during the acquisition.
Figure 8.2 shows the flow of specifying a target-acquisition scheme.
The first step is to determine what object you are going to use for your target acquisition. Note that the STIS software will always acquire the brightest object in the 5x5 arcsecond search area. If your target is isolated, or if there are no brighter objects in the STIS bandpass you have selected (see below) within 5 arcseconds of your target, then you can acquire your target directly. If there are brighter targets nearby, then you will need to acquire an offset target (generally the brighter nearby star), and then perform a slew to the scientific target. The offset technique is also recommended for precise pointing to specific surface or atmospheric features for large (>~4 arcseconds) planetary targets such as Mars, Jupiter, and Saturn, with the offset target being a planetary satellite (see Solar-System Acquisitions for further details). Note that HST performs small-angle maneuvers (SAM) quite accurately, with a 3 arcsecond maneuver having an error of 0.003 arcsecond, while a 2 arcminute maneuver (the maximum to ensure that a single set of guide stars can be used for both targets) yields a 0.02 arcsecond error. The offset should not significantly affect target acquisition centering accuracy even in the smallest echelle slits, and your target acquisition centering uncertainty will generally be dominated by your knowledge of the absolute offset between the acquisition star and your target. If you are uncertain whether a nearby object is brighter than your target in the STIS bandpass, it is safest to select an isolated object and perform your acquisition on it.
If you are observing a diffuse source, you should first check to see if there is a suitable star which you can use as an acquisition target; an offset can then be used to move to the desired position, as needed. If you wish to acquire a diffuse object directly, then it is important to know your source structure as seen at ~0.1 arcsecond resolution to properly plan your acquisition strategy if you need accurate (a few tenths of an arcsecond) centering. We recommend that you first check the HST archive to determine if your target has been observed by HST with WF/PC-1, WFPC2, FOC, or STIS. If it has, that exposure can be used to determine the optimal acquisition strategy using the Target-Acquisition Simulator. If it has not yet been observed with HST, we suggest you take an early acquisition image, either with the STIS or with WFPC2, which you can use to determine your optimal acquisition strategy. This is particularly important if your program requires placement of a narrow slit accurately on a diffuse object.
Once you have selected your acquisition object, you need to measure its coordinates (when possible, i.e. not for moving targets) in the Guide Star reference frame; information on measuring coordinates can be found on the following web page:
http://www-gsss.stsci.edu/support/phase2.html
You will need to include the PLATE-ID along with your coordinates so that we can be certain to select the guide stars for your observation from the same plate on which your measurement was based. If you are deriving coordinates from WFPC2 data (i.e., from the metric task) or STIS data (i.e., from the xy2rd task), you still need to provide a PLATE-ID; see Determining the PLATE-ID of HST Observations for instructions on determining the PLATE-ID from HST data.
If you are acquiring a point-like object, you should select the point-source acquisition (ACQTYPE=POINT). This selection will find the flux-weighted centroid of the object using a 3 x 3 checkbox. If you are acquiring a diffuse object, you should select the diffuse source acquisition (ACQTYPE=DIFFUSE). A diffuse acquisition will also require selecting the target acquisition centering algorithm (DIFFUSE-CENTER=FLUX-CENTROID or GEOMETRIC-CENTER) and a checkbox size (CHECKBOX=3-105). Note that selecting a DIFFUSE acquisition with CHECKBOX=3 and DIFFUSE-CENTER=FLUX-CENTROID is equivalent to selecting the POINT source algorithm. A Target-Acquisition Simulator is available to assist you in selecting the best checkbox size if you have an image at a similar resolution to STIS and in a similar bandpass.
The apertures available for target acquisitions are the same set that can be used for CCD imaging and are listed in Table 8.2 below. They include the visible long pass filter, the clear unfiltered aperture, the [O III] narrowband filter, the [O II] narrowband filter, and the neutral-density filters which provide attenuations for bright sources of 10-3 and 10-5. F28X50LP is the preferred target-acquisition aperture. The longpass filter is the preferred (compared to the clear 50CCD aperture) because it blocks the ultraviolet photons, which can otherwise elevate the detector dark count in the subsequent scientific exposures (see UV Light and the STIS CCD). For bright sources which saturate the CCD in 0.1 second with the longpass filter (see Table 8.3), you can use either the narrowband [O III] (F28X50OIII) or [O II] (F28X50OII) filters as the acquisition aperture, or one of the neutral density filters. The [O II] and [O III] filters can also be used to locate the target in the light of an emission line. Note that the [O III] filter has a large red leak at 
> 10,000Å (see Chapter 14), so it should be used with caution; the [O II] filter has no measurable red leak.
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We recommend the longpass |
To achieve robust target location:
ACQ exposure.
CCDGAIN of 4 must be used for all acquisition images.
The maximum possible exposure time for a point-source (ACQTYPE=POINT) acquisition exposure is 5 minutes; this limit restricts acquisitions to sources brighter than 24.5 magnitudes in V. This limit is imposed because, for longer exposure times, the target acquisitions become compromised by coincident cosmic-ray impacts, which will lead to acquisition failures. The maximum possible exposure time for a diffuse acquisition (ACQTYPE=DIFFUSE) depends on the checkbox and is given by:
The minimum exposure time allowed for an acquisition is 0.1 second. Table 8.3 gives the limiting magnitudes at which the CCD will saturate in a 0.1 second exposure; sources brighter than these limits cannot be acquired with the CCD using these filters. Remember that the ND filters can also be used to acquire targets; these filters provide attenuations of 10-3 (7.5 magnitudes) and 10-5 (12.5 magnitudes) relative to the clear (50CCD)filter. Note that the ND filters are contained in the slit wheel with other slits and apertures, and so cannot be used in conjunction with other filters. The table illustrates that it is possible to image any star using a filter from the suite, including the neutral-density filters, available for STIS.
Figure 8.7 can help you estimate exposure times-it plots exposure time versus V magnitude to achieve a signal-to-noise ratio of 40 for stars having a range of spectral types, for the clear, longpass, [O III], and [O II] filters. To determine the exact exposure time for your target, you should use the STIS Target-Acquisition Exposure Time Calculator (see Examples). Note that the overheads in target acquisitions are substantially longer than most exposure times, so as long as you do not saturate (i.e., come within 30% of the full well) you should increase your exposure time by a factor of 2-5 above the minimum required (e.g., if the exposure time to obtain a S/N of 40 is 0.3 second, then you should lengthen it to 1 second if no saturation occurs). The exposure time entered into your Phase II template is the time for each of the two exposures in the ACQ sequence, unlike the case of CR-SPLIT observations where it is the total time.
As part of your Phase II submission, you will need to specify the V magnitude of the object you are acquiring (the scientific target or the offset target). The RPS2 software will perform a check to make certain your target will not saturate the CCD (which would cause the acquisition to fail). Note that the test assumes the worst case (the 50CCD aperture and an M6V spectral type), which may not be appropriate for your target and configuration. If RPS2 gives you a warning about the acquisition exposure, please simply double check to make certain you will not saturate.
Figure 8.7: Time to Achieve a Signal-to-Noise ratio of 40 for CCD Acquisitions
STIS acquisition techniques for solar-system targets are identical to those described in the preceding sections, and no special strategies are required. The one exception is that the offset target is almost always a satellite instead of a nearby star, and it is not necessary to measure the coordinates of the acquisition targets (planets or satellites) in the Guide Star reference frame.
Acquisition exposure times can be accurately estimated using the STIS Target Acquisition Exposure-Time Calculator: use the Kurucz model G2 V (solar spectrum), and normalize it using the V magnitude for point-like (
0.1 arcsec) objects or the V magnitude per arcsec2 and the appropriate target size in arcseconds for more extended targets.
For convenience, we summarize the types of acquisitions recommended for the most common solar-system targets in Table 8.4 below. If precise pointing to a specific feature is not required, then blind pointing (which is accurate to ~1-2") can be used for the larger targets such as Mars, Jupiter, and Saturn.
Acquisition exposures must be specified during Phase II as individual exposure-logsheet lines which precede the scientific exposures for which they are intended. The user requests a target-acquisition exposure by specifying MODE=ACQ on the proposal logsheet, and setting the optional parameter ACQTYPE=POINT or ACQTYPE=DIFFUSE. If ACQTYPE=DIFFUSE is selected, the observer must also specify DIFFUSE-CENTER and CHECKBOX.
In addition, you are required to provide the V magnitude and spectral type of the acquisition target. This information will allow for checks of your target-acquisition exposure times, should they be needed.
Exposure-Time Calculator and RPS2 examples are provided in Examples.
1 The processing done by the FSW is rudimentary; a single bias number is subtracted, bad pixels are set to the average of the 4 adjacent pixels, negative-valued pixels are set to zero, and each pixel is assigned the minimum from the two images (as a form of cosmic-ray and hot pixel rejection).
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Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |