5.7 Target Acquisition Data
5.7.1 Target Acquisition Basics
There are two types of STIS target acquisition: ACQ and ACQ/PEAK; for more details on target acquisition, see ISR STIS 97-03. For ACQ observations, there are three parts to the target acquisition data that you receive. The first is an image of the target in the target acquisition sub-array (100 x 100 pixels for POINT sources, user-defined for DIFFUSE sources) based on the initial pointing (see Figure 5.1a, image_raw.fits[sci,1]). The software then determines the position of the target with a flux-weighted pointing algorithm and calculates the slew needed to place the target at a reference point in the target acquisition sub-array; for DIFFUSE sources, an option to perform a geometric centroiding is available. An image of the target at this corrected position is then obtained (see Figure 5.1b, image_raw.fits[sci,2])-this is the coarse centering. To perform the fine centering (i.e., to place the object precisely in a slit), a 32 x 32 pixel image of the reference 0.2X0.2 aperture is obtained (see Figure 5.1c image_raw.fits[sci,3]), and the location of the aperture determined. A fine slew is then performed to center the target in the reference aperture, which should be accurate to 0.2 pixels (0.01 arcsec). A final slew to center the target in the science aperture is performed at the start of the following science observation.
Table 5.4: Measured Brightness for STIS ACQ Image
| Type |
Enclosed |
Gaussian |
Direct |
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If a narrow slit is used for the science, an ACQ/PEAK acquisition may have been performed. The slit is scanned across the object with a pattern determined by the aperture selected. The telescope is then slewed to center the target in the aperture, and a confirmation image (a 32 x 32 pixel grid) is obtained; the accuracy of the ACQ/PEAK is 5% of the slit width. There is only one image in the ACQ/PEAK data file, showing the confirmation image through the slit, and associated with the extension [sci,1]. The last extension in the file (image_raw.fits[4]) contains the values in the individual steps of the ACQ/PEAK (use listpix to view these values).
When examining the confirmation image, note that the slit will be illuminated by the sky even if no target is present (see Figure 5.2; image_raw.fits[sci,1]). To confirm the presence of a compact target, use the imexamine task and make certain that the FWHM is small. The measured values for the images in Figure 5.2 are given in Table 5.4.
Figure 5.1: Three Stages of an ACQ Observation
Table 5.5: ACQ/PEAK Confirmation Images
5.7.2 ACQ Data
An examination of the target acquisition data (either from the raw data or the paper products) will allow you to detect gross errors in the centering of your target. Note that target acquisition data are always uncalibrated. A comparison of the initial [sci,1] and post-coarse slew [sci,2] images should show the object moving close to the center of the acquisition sub-array.
You can also verify that the fluxes in both images, which are found in the science header under the keyword MAXCHCNT, are consistent by performing the following steps in IRAF:
cl> imheader image_raw.fits[1] long+ | grep MAXCHCNT
cl> imheader image_raw.fits[4] long+ | grep MAXCHCNT |
The first value will be the target flux in the maximum checkbox (3x3 for POINT sources, user-defined for DIFFUSE sources) in the initial image, while the second is the maximum checkbox in the post-coarse slew image. If the fluxes are not consistent, or if the object did not move closer to the center of the array, there is likely a problem with your acquisition.
5.7.3 ACQ/Peak Data
To verify that the ACQ/PEAK worked, examine the flux values at each stage of the peakup (listed in the paper products or in the data file). The fluxes can be found by looking at the fourth extension (image_raw.fits[4]) of the peakup data via the IRAF command:
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cl> listarea image_raw.fits[4]
Sample 1 2 3
Line
1 4707. 260769. 0.
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For a 3-step linear peakup, the pixel [1,1] is the leftmost scan position, [2,1] is the middle position, and [3,1] is the rightmost position. See Figure 5.2.
Figure 5.2: Flux Values at Each Stage in Peakup
For a 9-step spiral pattern, pixels [1,1] through [3,1] are the lower row of the pattern, [1,2] through [3,2] are the middle row, and [1,3] through [3,3] are the upper row. Note that one of the steps will always have a value of zero.
To determine the flux in the confirmation image, do the following in IRAF:
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cl> imstat image_raw.fits[1] fields="image,npix,mean"
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This will give you an output like the following:
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cl> imstat o4de01jdq_raw.fits[1]
# IMAGE NPIX MEAN
o4de01jdq_raw.fits[1] 32704 8.241
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The total counts in the image is the product of the number of pixels (NPIX) and the mean value (MEAN), or 269,514 in this example.
Note that you will need to perform one correction to the mean value prior to your comparison. The flux values in the peakup scan have been adjusted to subtract the minimum flux value in the peakup data (which is why one value in the peakup is always zero). This value needs to be subtracted from the counts in the confirmation to do a proper comparison. The value can be found in pixel 712 of the _spt image (i.e., support, planning and telemetry file). To display the value on the screen, type the following in IRAF:
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cl> listpix image_spt.fits[1] | grep 712
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In the example, the value was 6008, which means the corrected number of counts in the confirmation image is 263,506.
Comparison of the maximum flux value during the peakup (260,769) with the flux in the post-ACQ/PEAK confirmation image (263,506) should show that the flux in the confirmation image was greater than or equal to the maximum flux in the peakup grid. If this is not the case, then there is likely a problem with your peakup acquisition.
5.7.4 The tastis Task
The stsdas.hst_calib.stis package contains a task, tastis, that will print general information about each input target acquisition image, and will analyze both types of STIS target acquisitions: ACQs and ACQ/PEAKs.
For an ACQ, target positions, in global and local (subarray) coordinates, and the total flux of the target in the maximum checkbox during both acquisition phases (coarse and fine) are displayed; the location of the reference aperture (used during the fine locate phase) is also displayed. For an ACQ/PEAK, the flux values at each step during the peakup, the total flux in the post-slew confirmation image, and the pedestal value subtracted from each dwell point are displayed.
The task will calculate the slews performed during the acquisition, and provide warnings of possible problems that could indicate a failure of the acquisition:
ACQ:
- If the fine slew (to center the target in the reference aperture) is greater than 4 pixels, it may indicate that the target found in the coarse locate phase was different than that found in the fine locate phase.
- If the ratio of the flux in the maximum checkbox in the coarse and fine stages differs by more than 10%, it may indicate different targets were found in each phase.
ACQ/PEAK:
- If the flux in the confirmation image (minus the pedestal value) is less than 90% of the maximum flux in the peakup, it may indicate that the source is not well centered.
- If the flux test failed because no point in the peakup scan has a flux that is at least 30% higher than any other point, it may indicate no target was found.
- If the first or last (left and right) flux value in the scan is not 0, it may indicate contamination from a nearby object.
If any acquisition in the input list did not succeed according to the above criteria, then a message to that effect will be printed.
