Examples in determining the length of an observation with HST.

Please note: APT should be used to get the most accurate determination of orbit calculations.  This information is provided for estimates.

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 -10^o. 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

Table of COS Example 1 Incurred Overheads

Table of COS Example 1 Orbit Planning

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

Table of COS Example 2 Incurred Overheads 

Table of COS Example 2 Orbit Planning 

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 

FGS Example: Incurred Overheads 

FGS Example: Orbit Planning 

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 

STIS Example: Incurred Overheads¹

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 

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

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

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.

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