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 five 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

Action

Time (Minutes)

Explanation

Orbit 1

Initial guide-star acquisition

6.0

Needed at start of observation of a new target

WFC overhead for the first exposure

4.0

Includes filter change, camera set-up, and readout

Spacecraft slew0.5
First science exposure20.0

WFC overhead for the subsequent science exposure in the series

2.5

Includes readout

The next science exposure in the series20.0
Total time for science exposures40.0
Total used time in the orbit53.0


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

Action

Time (Minutes)

Explanation

Orbit 1

Initial guide-star acquisition

6.0

Needed at start of observation of a new target

WFC overhead for the first exposures

4.0

Includes camera set-up, and readout

WFC overhead for the second exposures

3.0

Includes filter change, and readout

Spacecraft slew0.5

WFC overhead for the third and fourth exposures

× 3.0 = 6.0

Includes filter change and readout

Four science exposures× 8.0 = 32.0
Total used time in the orbit51.5

Orbit 2

Initial guide-star acquisition

4.0

Needed at start of observation

WFC overhead for the first and second exposures

× 3.0 = 6.0

Includes camera set-up and readout

Spacecraft slew0.5

WFC overhead for the third and fourth exposures

× 3.0 = 6.0

Includes filter change and readout

Four science exposures× 8.0 = 32.0
Total used time in the orbit48.5

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.

Action

Time (Minutes)

Explanation

Orbit 1

Initial guide-star acquisition

6.0

Needed at start of observation of a new target

Predefined imaging exposure for grism spectroscopy

7.0

Needed to co-locate the targets and their spectra in the FOV

WFC overhead for the first science exposure in the series

4.0

Includes filter change, camera set-up, and readout

Spacecraft slew

0.5


 

WFC overhead for the subsequent science exposure in the series

2.5

Includes readout

Two science exposures

× 15.0 = 30.0


 

Total science exposure time

30.0


 

Total used time in the orbit

50.0


 


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.

Action

Time (Minutes)

Explanation

Orbit 1

Initial guide-star acquisition

6.0

Needed at start of observation of a new target

SBC overhead for the first exposure

1.4

Includes filter change and camera setup

SBC overhead for each of the following three exposures

× 1.0

Includes filter change (3 × 0.4 minutes)

Spacecraft slew

0.3

Slew performed between the second and third exposures in this orbit

Exposures with filter F150LP

× 13.3 = 26.6

Exposures at the first two pointings of the dither pattern

Exposures with filter F165LP

× 8.3 = 16.6

Exposures at the first two pointings of the dither pattern

Total time used in the orbit53.9

Orbit 2

Guide-star re-acquisition

~4.0

Needed at start of new orbit to observe the same target

SBC overhead for each of the following three exposures

× 1.0

Includes filter change (4 × 0.4 minutes)

Spacecraft slew

0.3

Slew performed between the second and third exposures in this orbit

Exposures with filter F150LP

× 13.3 = 26.6

Exposures at the first two pointings of the dither pattern

Exposures with filter F165LP

× 8.3 = 16.6

Exposures at the first two pointings of the dither pattern

Total time used in the orbit51.5


The total time is slightly shorter in the second orbit because of the shorter time required for guide star re-acquisition and 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.