Target Acquisition
COS Target Acquisition ETC
The COS acquisition exposure time calculators are used to help observers determine the exposure times required for the different kinds of target acquisition. There are four acquisition modes that can be used with COS: ACQ/IMAGE, ACQ/SEARCH, ACQ/PEAKXD, and ACQ/PEAKD. These are described in detail in Chapter 8 of the COS IHB. Note that the acquisition ETCs do not include overhead times associated with slews, grating changes, etc. These must be determined separately by the user from the information provided in Chapter 9 of the IHB or, in Phase II, with APT.
The acquisition ETCs are very similar to the normal imaging and spectroscopic ETCs, but there are some differences, as described below. Note that in all cases, a S/N of 40 is recommended for COS acquisition exposures.
ACQ/SEARCH
This mode uses a spiral search to locate the target. It can be done in either imaging or dispersed light modes, so the user selects the appropriate acquisition ETC to calculate exposure times. If done in dispersed light mode, then the calculation is the same as for ACQ/PEAKD.
ACQ/IMAGE
This ETC is almost identical to the imaging ETC. The only difference is the size of the subarray used when calculating the signal to noise ratio (SNR). In the normal imaging ETC the S/N is computed in a 170x170 pixel subarray, but for acquisitions the subarray size is 345x816 pixels. A larger subarray is used to account for the slight wobble associated with the Optics Select Mechanisms. The NUV detector dark background for this imaging acquisition subarray is approximately 60 counts per second, about the same as one gets from a point source with a flat flux distribution of fλ=2.3x10-16 erg cm-2 sec-1 Å-1.
ACQ/PEAKXD and ACQ/PEAKD
These dispersed light acquisition ETCs are very similar to the normal spectroscopic ETCs, with two important exceptions.
First, the geocoronal airglow lines reduce the SNR for acquisitions because they fill the aperture regardless of the pointing direction. To minimize this effect, the spectral region around the two strongest geocoronal lines, Lyα λ0=1215.7 and O I λ0=1302, are removed before the counts are summed by the flight software. Both of these lines appear in the FUV. The excluded region must be wide enough to account for the full width of the entrance aperture, 2.5 arcsec, with a little extra margin. The excluded region is approximately 350 pixels for G130M and G160M, and 1027 pixels for G140L.
The two remaining geocoronal lines, O I λ0=1356 and O II λ0=2471, are both weak enough that no exclusion is done. Since the latter line is the only geocoronal line in the NUV, no lines are excluded for NUV dispersed light target acquisition.
The second difference is the number of stripes used for the NUV modes. For ACQ/PEAKD and ACQ/SEARCH it is most beneficial to use the light from all three stripes, and the ETC computes the SNR on this basis. For ACQ/PEAKXD one wants to accurately locate the spectrum in the cross-dispersion direction, and the position of a single stripe is measured. The default stripe is B, but the user may select either stripe A or C instead, if there is significantly more light in one of these. For the FUV, no such complications exist. Counts from segments A and B are added together, except for the excised geocoronal lines, to determine the SNR.
STIS Target Acquisition ETC
The STIS ACQ ETC allows calculations to be done for STIS ACQ observations and for imaging mode STIS ACQ/PEAK observations. (Results for spectroscopic ACQ/PEAK observations must be estimated using the ordinary STIS spectroscopic ETC with CCDGAIN=4, CR-SPLIT=1, and 1x1 binning). STIS ACQ and ACQ/PEAK observations always use the STIS CCD with CCDGAIN=4. The user must select either a STIS CCD imaging aperture in order to calculate an ACQ observation, or a narrow STIS slit to calculate an ACQ/PEAK observation. For ACQ observations of extended targets, the user may also specify the checkbox size over which the S/N is calculated. The user sets CHECKBOX=n, where n must be an odd number between 3 and 105: the checkbox will then have dimension n × 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 arcsecond) / (0.05 arcsecond/pixel) + 1]. The maximum checkbox is 105 pixels on a side, or ~5 × 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. Other parameters in the STIS Acquisition ETC are specified in the same way as standard imaging observations, except that there are no additional CCD parameters.