8.3 Orbit Use Determination Examples

The easiest way to learn to compute total orbit time requests is to work through a few examples. Below we provide four different examples:

• Example in Section 8.3.1 is a simple WFC image in one filter, using dithers.
• Example in Section 8.3.2 is a two-orbit multi-filter WFC observation using dithering.
  
• Example in Section 8.3.3 is a one-orbit WFC grism spectroscopic observation.
  
• Example in Section 8.3.4 is a two-orbit SBC observation.
  

These examples represent fairly typical uses of ACS.

8.3.1 Sample Orbit Calculation 1

Consider a target to be imaged with WFC in a given filter in one orbit. Using the ETC, we find that we need 2400 seconds of exposure time to reach the desired level of signal-to-noise ratio. Given that the observation is split into a series of two dithers, we map the overheads and the science exposure times onto the orbit as follows:

Table 8.3: Orbit Calculation for Example 1

Thus, the two WFC exposures totaling 2400 seconds make full use of the typically available time in one orbit. The exposure times can be adjusted if the actual target visibility time differs from the derived total used time.

8.3.2 Sample Orbit Calculation 2

This example illustrates the orbit calculation for a WFC observation with the ACS box pattern, which implements imaging at four offset pointings. The goal of the observation is to obtain a dithered image of a field in such a way that would allow us to bridge the 50 pixel interchip gap between the WFC CCDs in the combined image. Given the WFC plate scale 0.05 arcseconds/pixel, this requires that the offsets in the dithering pattern are larger than 2.5 arcseconds. Each offset will then take 0.5 minutes to move the spacecraft from one pointing in the pattern to another. We have determined that the exposure time necessary to reach the desired signal-to-noise ratio is 80 minutes. The orbit calculation will involve a series of 8 exposures (two exposures at each of the four pointings in the dithering pattern) split across two orbits. Slew time below is only applicable for slews <10 arc seconds. For longer slews, overhead time may increase. If the exposure time of each image is less than 337 seconds, extra time for the buffer dump will be needed:

Table 8.4: Orbit Calculation for Example 2

8.3.3 Sample Orbit Calculation 3

This example illustrates the orbit calculation for a simple 30 minute WFC grism spectroscopic observation broken with a series of two dithered exposures.

Table 8.5: Orbit calculation for example 3.

Unlike similar imaging exposures, here we have to take into account an additional imaging exposure before the sequence of spectroscopic exposures, which takes 10 minutes off the available orbit time.

8.3.4 Sample Orbit Calculation 4

This example deals with the orbit calculation for an observation of a relatively faint extended object using the SBC. The target has to be observed using two filters, F150LP and F165LP. The ETC shows that the required S/N for the observations are achieved in 3200 seconds and 2000 seconds for the F150LP and the F165LP filters, respectively. There is no readout noise associated with SBC exposures; therefore, the observations can be split into four equally long dither pointings. Since the average visibility time for the target is ~55 minutes, the images can be taken in two orbits, as shown in Table 8.6. The standard ACS-SBC-DITHER-BOX pattern, which allows for the rejection of most artifacts, is suitable for these observations. Here are the details of the orbit calculation:

Table 8.6: Orbit calculation for example 4.

The total time is slightly shorter in the second orbit because of the shorter time required for SBC overheads. There is also ~1 min of visibility available in the first orbit that can be used for the observations. However, we recommend dithered observations in each filter using the same exposure time for each pointing.