Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/documents/dhb/web/c26_stisdataanal.fm5.html
Дата изменения: Wed Jul 1 20:35:22 1998
Дата индексирования: Sat Dec 22 17:00:17 2007
Кодировка:
Поисковые слова: arp 220

[Top] [Prev] [Next] [Bottom]

23.5 Target Acquisition Data

Target Acquisition Basics

There are two types of STIS target acquisition: ACQ and ACQ/PEAK; for more details on target acquisition, see STIS ISR 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 23.1a), 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 23.1b, 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 23.1c 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 0.5 pixels (0.025 arcseconds). A final slew to center the target in the science aperture is performed at the start of the following science observation.

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 star in the aperture, and a confirmation image (a 32 x 32 grid) is obtained; the accuracy of the ACQ/PEAK is 5% of the slit width. Note that 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 star is present (see Figure 23.2; image_raw.fits[sci,1]). To confirm the presence of a star, use the imexamine task and make certain that the FHWM is small. The measured values for the images in Figure 23.2 are given in Table 23.4.

Measured Brightness for STIS ACQ Image

Type

Enclosed

Gaussian

Direct

Star

1.59

1.57

1.49

Sky

15.22

142.84

4.81

**Measured Brightness for STIS ACQ Image**
Type	Enclosed	Gaussian	Direct
Star	1.59	1.57	1.49
Sky	15.22	142.84	4.81

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, if retrieving data via StarView, you must request uncalibrated data to receive target acquisition observations. 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.

Users of STIS data must request "uncalibrated" data when retrieving ACQ and ACQ/PEAK observations through StarView. Since ACQ-like data do not get -calibrated there are no calibrated data products.

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.

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:

cl> listarea image_raw.fits[4]

Sample         1         2         3

Line

      1     4707.     260769.        0.

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 23.3.

Figure 23.3: Flux Values at Each Stage in Peakup

For a 9-step spiral pattern, pixels [1,1] through [1,3] are the lower row of the pattern, [2,1] through [2,3] are the middle row, and [3,1] 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:

cl> imstat image_raw.fits[1]fields="image,npix,mean"

This will give you an output like the following:

cl> imstat o4de01jdq_raw.fits[1]

#               IMAGE    NPIX   MEAN

 o4de01jdq_raw.fits[1]   32704  8.241

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; to display the value on the screen, do the following in IRAF:

cl> listpix image_spt.fits[1] | grep 712

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.

[Top] [Prev] [Next] [Bottom]

stevens@stsci.edu