Space Telescope Science Institute |

ACS Instrument Handbook Cycle 23 |

help@stsci.edu |

Advanced Camera for Surveys Instrument Handbook for Cycle 23 > Chapter 8: Overheads and Orbit-Time Determination > 8.3 Orbit Use Determination Examples

• Example in Section 8.3.1 is a simple WFC image in one filter, using dither.

• 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.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

Table 8.4: Orbit Calculation for Example 2

2 × 3.0 = 6.0 4 × 8.0 = 32.0 2 × 3.0 = 6.0 2 × 3.0 = 6.0 4 × 8.0 = 32.0 Table 8.5: Orbit calculation for example 3.

2 × 15.0 = 30.0 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:

3 × 1.0 2 × 13.3 = 26.6 2 × 8.3 = 16.6 4 × 1.0 2 × 13.3 = 26.6 2 × 8.3 = 16.6

Advanced Camera for Surveys Instrument Handbook for Cycle 23 > Chapter 8: Overheads and Orbit-Time Determination > 8.3 Orbit Use Determination Examples