# HST Primer: Orbit Calculation Examples

Examples in determining the length of an observation with HST.

# ACS Examples

Examples of ACS usage may be found in Chapter 8 of the ACS Instrument Handbook.

# COS Examples

This section illustrates two examples of COS Phase I observations. Additional details are available in the COS Instrument Handbook.

Example 1: COS NUV Imaging and Spectroscopy in One Orbit

The COS NUV channel is used to observe an object at declination -10o. First, an image is taken, followed by obtaining a TIME-TAG spectrum with two exposures using different gratings. The specific observations, as requested by the user, are summarized in the table below.

• There are 54 minutes of orbital visibility per orbit.
• Overheads for the observations are compiled and listed.

Therefore, a preliminary observing strategy can be created by the user to fit the observations in one orbit, as shown. In this example, we assume that the coordinates of the object are known to an accuracy of 0.4 arcsec or better. See the COS Instrument Handbook for details.

Table of COS Example 1 Planned Exposures

Config

Mode

Spectral element

Number of exposures

Time per exposure (seconds)

Notes

COS/NUV

ACQ/IMAGE

MIRRORA

1

20

target acquisition

COS/NUV

TIME-TAG

G185M

1

1200

central wavelength 1850 Å

COS/NUV

TIME-TAG

G225M

1

600

central wavelength 2250 Å

Table of COS Example 1 Incurred Overheads

Notes

Guide star acquisition

6.5

First orbit in new visit

ACQ/IMAGE

3

Target acquisition

NUV spectroscopy

5

Single spectroscopic exposure

Table of COS Example 1 Orbit Planning

Activity

Duration (minutes)

Total elapsed time (minutes)

Orbit 1

Guide star acquisition

6.5

6.5

ACQ/IMAGE

3

9.5

5

14.5

G185M Exp time

20

34.5

3

37.5

G225M Exp time

10

47.5

Unused

7

54.5

### Example 2: COS NUV Imaging and FUV Spectroscopy in Two Orbits

The COS NUV channel is used to acquire the same object as in Example 1, in imaging mode, then uses the FUV channel to obtain a TIME-TAG spectrum with two exposures using the same grating and central wavelength. The planned exposures are summarized in the table below.

A preliminary observing strategy can be created by the user to fit the observations in two orbits, as shown. In this example, we assume that the coordinates of the object are known to an accuracy of 0.4 arcsec or better. See the COS Instrument Handbook for details.

Table of COS Example 2 Planned Exposures

Config

Mode

Spectral element

Number of exposures

Time per exposures (seconds)

Notes

COS/NUV

ACQ/IMAGE

MIRRORA

1

30

Target acquisition

COS/FUV

TIME-TAG

G160M

1

2100

Central wavelength 1600 Å

COS/FUV

TIME-TAG

G160M

1

2700

Central wavelength 1600 Å

Table of COS Example 2 Incurred Overheads

Notes

Guide star acquisition

6.5

First orbit in new visit

Guide star reacquisition

6.5

Per orbit after first orbit

ACQ/IMAGE

3

Target acquisition

FUV spectroscopy

5

First spectroscopic exposure in a series of identical exposures

FUV spectroscopy

2

Subsequent spectroscopic exposure in a series of identical exposures

Table of COS Example 2 Orbit Planning

Activity

Duration (minutes)

Total elapsed time (minutes)

Orbit 1

Guide star acquisition

6.5

6.5

ACQ/IMAGE

3

9.5

5

14.5

G160M Exp time

35

49.5

Unused

5

54.5

Orbit 2

Guide star reacquisition

6.5

6.5

2

8.5

G160M Exp time

45

53.5

Unused

1

54.5

# FGS Example

The FGS is used to observe a binary star named Binary01, as well as five reference stars (ref1, ref2, ref3, ref4, ref5). All stars are in the same FGS field of view, and can, therefore, be observed in one visit. Stars ref4 and ref5 have magnitude V = 14.6, and all the other targets have 13.0 < V < 14.0. To enable the removal of drift and jitter in the post-observation analysis of the data, the Binary01 is observed in POS mode several times (twice in this example) and the reference stars are each observed twice. The desired exposures are listed.

• The targets have a declination of +42°, which gives 57 minutes of orbital visibility per orbit.
• The associated overheads are listed

A preliminary observing strategy can be created by the user to fit the observations in one orbit, as shown.  Proposers for the FGS are urged to contact the HST help desk to get up-to-date information.

FGS Example: Planned Exposures

Config

Mode

Spectral element

Number of exposures

Time per exposure (minutes)

Notes

FGS

POS

F583W

8

0.6

Two exposures of target Binary01, and two exposures for each of the targets ref1, ref2, and ref3

FGS

POS

F583W

4

1.1

Two exposures for each of the targets ref4 and ref5

FGS

TRANS

F583W

1

13.4

20 scans of 40 sec each for Binary01

Notes

see Table

Guide star acquisition

6.5

First orbit in new visit

6.2

POS

1

Per exposure, V < 14

6.8

POS

2

Per exposure, 14 < V < 15

6.8

TRANS

1

Per target

6.8

TRANS

0.2

Per scan

6.8

Instrument Setup

4

Once every orbit

6.9

Instrument Shutdown

3

Once every orbit

6.9

FGS Example: Orbit Planning

Activity

Duration (minutes)

Elapsed time (minutes)

Orbit 1

Guide star acquisition

6.5

6.5

Instrument Setup

4

10.5

Binary01/POS mode Exp Time

0.6

11.1

1

12.1

ref1/POS mode Exp Time

0.6

12.7

1

13.7

ref2/POS mode Exp Time

0.6

13.8

1

14.8

ref3/POS mode Exp Time

0.6

15.4

1

16.4

ref4/POS mode Exp Time

1.1

17.5

2

19.5

ref5/POS mode Exp Time

1.1

20.6

2

22.6

Binary01/POS mode Exp Time

0.6

23.2

1

24.2

ref1/POS mode Exp Time

0.6

24.8

1

25.8

ref2/POS mode Exp Time

0.6

26.4

1

27.4

ref3/POS mode Exp Time

0.6

28

1

29

ref4/POS mode Exp Time

1.1

30.1

2

32.1

ref5/POS mode Exp Time

1.1

33.2

2

35.2

Binary01/TRANS mode Exp Time

13.4

48.6

1+(20x0.2)

53.6

Instrument Shutdown

3

56.6

Unused

0.4

57.0

# STIS Example

STIS is used to observe a target at a declination of +15° degrees according to an observing strategy outlined. A preliminary observing strategy can be created by the user to fit the observations in two orbits, as shown.

STIS Example: Planned Exposures

Config

Mode

Spectral

element

Number of exposures

Time per exposures (minutes)

Notes

STIS/CCD

ACQ

0.1

Target acquisition

STIS/CCD

ACQ/PEAK

1

Peakup acquisition (necessary because of use of 0.1" wide slit for subsequent spectra)

STIS/CCD

IMAGING

F28X50OII

2

2

STIS/NUV

IMAGING

F25QTZ

1

21

STIS/NUV

SPECTROSCOPIC

G230L

1

13

with 52X0.1 slit

STIS/FUV

SPECTROSCOPIC

G140L

1

20

with 52X0.1 slit

Notes

see Table

Guide star acquisition

6.5

First orbit in new visit

6.2

Guide star reacquisition

6.5

Per orbit after first orbit

6.2

ACQ

6

Target acquisition

6.4

ACQ/PEAK

6

Peakup target acquisition

6.4

CCD imaging (first)

5

First exposure in a series of identical exposures

6.10

CCD imaging (not first)

1

Per exposure after the first exposure in a series

6.10

MAMA imaging

5

First exposure

6.10

MAMA spectroscopy

8

Only 1 minute if there is no change from first exposure. Add 4 minutes (6 minutes for modes G140M and G230M) for each exposure time interval that exceeds 2300 seconds at the same grating position.

6.10

1These overheads are for budgeting purposes. The actual overhead incurred depends upon the particular spectral element selected since the overhead to move the grating is position-dependent and the time for an auto-wavecal is mode-dependent. Additional efficiencies, such as executing auto-wavecals during guide star reacquisitions or occultation, that may be realized from fine-tuning of the actual exposures (but cannot be assumed a priori), are not included.

STIS Example: Orbit Planning

Activity

Duration (minutes)

Elapsed time (minutes)

Orbit 1

Guide star acquisition

6.5

6.5

ACQ

6

12.5

ACQ/PEAK

6

18.5

F28X50OII Exp Time

2x2

22.5

5+1

28.5

F25QTZ Exp Time

21

49.5

5

54.5

Unused

0

54.5

Orbit 2

Guide star reacquisition

6.5

6.5

G230L Exp Time

13

18.5

8

26.5

G140L Exp Time

20

46.5

8

54.5

Unused

1

55.5

# WFC3 Examples

## Example 1: IR, 1 orbit, 2 filters

The WFC3 IR channel is used to obtain images of a target in filters F110W and F160W. Assume that the ETC has shown that exposure times of 10 minutes for F110W and 20 minutes for F160W are adequate for the science goals. These exposure times can be achieved with the up-the-ramp MULTIACCUM sequences SPARS50 (11.7 min) and SPARS100 (23.4 min), respectively. Also, assume an orbital visibility1 of 54 minutes. A summary of the orbit calculations is shown.

Orbit Calculation for WFC3 Example 1

Action

Time (minutes)

Explanation

Guide-star acquisition

6.5

Needed at start of observation of new target

2 × 1.0 = 2.0

Includes filter changes, camera set-ups, and readouts

Science exposure in F110W

11.7

Science exposure in F160W

23.4

Total time used

43.1

The total time used in the orbit shows that the target can be imaged in the selected filters within one orbit. Furthermore, the first exposure can be dumped from the buffer during the second exposure. The approximately nine minutes of unused time could be used for an additional exposure, during which the second exposure would be dumped.

## Example 2: UVIS, Dithering, 2 Orbits, 1 Filter

This example illustrates the orbit calculation for a UVIS observation with a WFC3 UVIS box dithering pattern, which implements imaging at four pointings. The goal of the observation is to obtain a dithered image of a field such that it bridges the ~1 arcsec inter-chip gap between the UVIS CCDs when the images are combined.

For the purposes of this example, the following conditions are adopted: (1) the exposure time necessary to reach the desired signal-to-noise ratio is 80 minutes. (2) The orbital visibility for each orbit is 58 minutes. (3) Assume that cosmic ray removal will be done using the drizzlepac package; therefore the exposure will not be broken into sub-exposures. (4) One 20 minute exposure is obtained at each pointing in a four-point dither, taken over two orbits.

The orbit calculation for this visit is shown. Note that for a two arcsec offset maneuver, the three dither offsets will take 0.5 minutes each.

Orbit Calculation for WFC3 Example 2

ActionTime (minutes)Explanation

Orbit 1

Guide-star acquisition

6.5

Needed at start of observation of new target

2.6

Includes filter change, camera set-up, and readout

2.1

Spacecraft maneuver

0.5

To offset from first to second dither pointing

Two science exposures

2 × 20 = 40.0

Exposures at the first two pointings in the dither pattern

Total time used in orbit 1

51.2

Orbit 2

Guide star reacquisition

6.5

Needed at start of new orbit to observe the same target

UVIS overheads for 3rd and 4th exposures

2 × 2.1 = 4.2

Spacecraft maneuvers

2 × 0.5 = 1.0

To offset to the 3rd and 4th dither pointings

Two science exposures

2 × 20 = 40.0

Exposures at the final two pointings in the dither pattern

Total time used in orbit 2

50.2

No overhead is incurred to dump the exposures because they are all longer than 339 seconds. Thus the desired exposures can be accomplished within the two orbits, and there are about seven to eight minutes of unused orbital visibility per orbit that could be used to increase the exposure times.

Next: HST Primer: Data Processing and the HST Data Archive