HST Primer: Observing Considerations

Additional considerations in planing observations.

Bright-Object Constraints

Some scientific instruments must be protected against over-illumination. Observations that violate these protections cannot be executed and should not be proposed. The constraints discussed below are safety constraints. 

Data may also be affected by bright objects at limits that are substantially fainter than the safety limits discussed in the following sections. Such bright object-related effects include non-linearity, saturation, and residual image effects. Please consult the appropriate instrument handbook for the instrument being used.


ACS and STIS CCDs have no safety-related brightness limits.

The MAMA detectors in the ACS/SBC, COS (NUV), and STIS, as well as the COS/XDL (FUV) detector, can be damaged by excessive levels of illumination and are therefore protected by hardware safety mechanisms. In order to avoid triggering their safety mechanisms, successful proposers using these detectors are required to perform a detailed check of the field surrounding their targets for excessively bright sources (this must be done by the Phase II deadline). Bright object checks can be done using tools in APT. Exceptions to Phase II bright object checks are moving targets; these must be cleared after scheduling windows have been established. Due to the complexity of bright object checks for moving targets, no further changes to the observing setup are permitted once such targets have been cleared.

Bright object count rate limits are mode dependent. Specific values are given in the instrument handbooks, including examples of magnitude screening limits for astronomical objects observed in the most commonly used modes. In addition, the Exposure Time Calculators (ETCs), accessible from the HST Instruments webpage, can be used to determine if a particular target and instrument configuration combination exceeds the global or local count rate screening limits.

Objects with strong UV fluxes (e.g., early-type stars) can have a screening limit as faint as V = 19. Therefore, proposers using any of these health and safety instrument modes must refer to the relevant instrument handbooks for instructions on performing a detailed analysis for their specific sources and discuss the results in the Description of the Observations section of the Phase I proposal. Both the targets and other objects in the FOV will have to be cleared during Phase II, but if the field is particularly crowded or if any object in the FOV is known to pose a brightness concern, observers are asked to explain, in the “Description of Observations” section of the Phase I program, how they propose to clear them during Phase II. 

For the STIS MAMAs, these limits are given in Section 7.7 of the STIS Instrument Handbook. For the COS XDL and MAMA, screening limits are given in Chapter 10 of the COS Instrument Handbook. For the SBC, the V magnitude screening limits are quoted in Section 7.2 of the ACS Instrument Handbook.  Note that the SBC broadband imaging sensitivities have been revised upwards by 30% from original estimates (see ACS ISR 2019-05).  Note also that for SBC prism spectroscopy, a direct image must be added manually to provide the wavelength calibration, and it will drive the safety issue since the direct filters are more sensitive than the prisms. This image must be included in the Observing Summary and the safety discussion.

In the case of irregular variables that are either known to undergo unpredictable outbursts, or belong to classes of objects that are subject to outbursts, the proposer must determine whether the target will violate the bright object limits during an outburst. If a violation is possible, the proposer must outline a strategy that will ensure that the target is safe to observe with COS, STIS/MAMA or ACS/SBC. The observing strategy might include additional observations, obtained over a timescale appropriate to the particular type of variable object, with either HST or ground-based telescopes. If HST data are to be used for this purpose, the required orbits must be requested in Phase I. STScI reserves the right to limit the number of visits requiring quiescence verification observations within 20 days or less of an HST observation to no more than 12 such visits per Cycle. Further details about these procedures are presented in ACS ISR 2006-04. The general policies described there apply to the STIS/MAMA and COS detectors as well, with suitable scaling for the differences in the exact Bright Object Protection (BOP) limits for each detector and mode. These limits are described in the individual instrument handbooks.


Objects as bright as V = 3.0 may be observed if the 5-magnitude neutral-density filter (F5ND) is used. Observations of all objects brighter than V = 8.0 should be performed with this filter. A hardware limitation prevents the FGS target acquisition from succeeding for any target brighter than V = 8.0 (3.0 with F5ND.)


There are no safety-related brightness limits for WFC3. Furthermore, overexposure of UVIS images does not leave persistent signals in subsequent exposures. Signals in IR images that are greater than approximately half full well do persist and have significant implications for the design of observing strategies and data analysis. See the WFC3 Instrument Handbook for more information.

Target Acquisitions

Target acquisition is the procedure used to ensure that the target is in the field of view of the requested aperture to the level of accuracy required by the observer. There are several distinct methods of target acquisition; each method has a different approach and different accuracy and will take different amounts of time and resources to complete. The required level of accuracy depends on the size of the aperture used to obtain the science data and on the nature of the science program.

Target Acquisition without the Ground System

Blind acquisition

For blind acquisition, guide stars are acquired and the FGSs are used for pointing control. The pointing is accurate to the guide star position uncertainty, which is approximately 0.3" RMS, plus the instrument-to-FGS alignment error.

Onboard acquisition

For onboard acquisition, software specific to the scientific instrument centers the fiducial point onto the target. Onboard target acquisitions are needed for COS and STIS spectroscopic observations (except slitless) and all coronagraphic observations with STIS. WFC3 does not have onboard acquisition capabilities, which means that all WFC3 target acquisitions are blind acquisitions. For specific information on methods and expected pointing accuracies, see the instrument handbook for the instrument being used.

Early acquisition

For early acquisition, an image is taken in an earlier visit and analyzed by the PI. The PI may then update the target coordinates in the Phase II proposal for use with subsequent visits. 

Target Acquisition with Ground System Support

Target acquisitions that cannot be accomplished successfully or efficiently via one of the above-mentioned methods may still be possible with STScI ground system support. This is accomplished by analyzing certain data, calculating and uplinking appropriate pointing corrections to the telescope. 

One example is a technique to avoid repeating multistage target acquisitions for an object that will be observed in several visits with the same instrument configuration, using the same guide stars. Multistage target acquisitions to center a target in a small aperture (as in STIS) consume a large fraction of orbital visibility in an orbit. However, if the offsets are determined from engineering and image data obtained from the initial successful target acquisition, those offsets can then be applied to subsequent visits. Request for this support is implicit in the specification of the “Save Offset” and “Use Offset” special requirements in the Phase II proposal.

Solar System Targets

Objects within the Solar System move with respect to the fixed stars. HST has the capability to point at, and track moving targets, including planets, their satellites and surface features on them, with sub-arcsecond accuracy. However, there are a variety of practical limitations on the use of these capabilities that must be considered before addressing the feasibility of any particular investigation. Proposals to observe the Moon will be permitted in Cycle 31. Please consult the Lunar Observations User Information Report (UIR-2007-01) for more information.

HST is capable of tracking moving targets with the same precision achieved for fixed targets. This is accomplished by maintaining FGS Fine Lock on guide stars and driving the FGS star sensors in the appropriate path, thus moving the telescope to track the target. Tracking under FGS control is technically possible for apparent target motions up to 5 arcsec/s. In practice, however, FGS tracking of a fast moving target may be able to accommodate a full HST orbit’s worth of observing even if the target motion forces the guide stars out of the FGS apertures before completion of the orbit; in such cases, it is possible to continue observing under (less accurate) gyro guiding.

The track for a moving target is derived from its orbital elements. Orbital elements for all planets and their satellites are available at STScI. For other objects, the Principal Investigator (PI) must provide orbital elements for the target in the Phase II proposal. The “Requires Ephemeris Correction” special requirement, described in the Phase II Proposal Instructions can be used to insert an offset shortly before the observation is executed to eliminate zero-point errors due to an inaccurate ephemeris. See HST Observation Types

Offsets, Patterns, and Dithering

Offsets are routinely used to reposition a target in the instrument field of view. The size of the offset is limited by the requirement that both guide stars remain within the respective fields of view of their FGSs. Offsets within single detectors (the most common type) can be performed to within ≈ 0.003". Offsets that continue across separate visits (when executed with the same guide stars) will typically have an accuracy of ~0.05".

Patterns are used to place the telescope at multiple positions to allow for dithered or mosaiced observations. Patterns can define a linear, spiral, or parallelogram series of observation points. Patterns can also be combined to produce a more complex series of observation points. In addition, “convenience patterns” have been predefined to represent typical dither and mosaic strategies; for details see the Phase II Instructions available at the Phase II Program Preparation webpage. The possible pattern area is limited by the requirement that the same guide stars be used throughout the pattern. This implies a maximum of about 120 arcseconds of linear motion.

For most small- or medium-sized imaging programs (i.e., up to a few orbits per target/field combination), dither patterns are designed to provide half-pixel subsampling and to move bad pixels and inter-chip gaps to different locations on the sky. Larger programs may benefit from more complex dithering strategies, to provide, for example, even finer subsampling of the detector pixels. The data can be combined using the DrizzlePac software.

In general, undithered observations with the ACS CCDs, and WFC3’s CCD and IR detectors will not be approved without strong justification for why it is required for the scientific objectives. Otherwise, hot pixels and other detector artifacts will compromise the program and the archival value of the data. Further details about the options and advantages of ACS patterns can be found in the ACS Instrument Handbook, the Phase II Proposal Instructions, and the ACS Dither webpage. Information on dithering for WFC3 observations is found in Appendix C of the WFC3 Instrument Handbook, the Phase II Proposal InstructionsWFC3 ISR 2010-09, and WFC3 ISR 2016-14, and WFC3 ISR 2020-07.

STIS CCD observations may normally benefit from dithering to eliminate hot pixels and improve PSF sampling, although, for spectroscopic observations, this may complicate data reduction. See the STIS Instrument Handbook and the Phase II Proposal Instructions for further details.

Spatial Scans

Spatial scan capability, available primarily for WFC3, facilitates cutting-edge photometric and astrometric observations of very bright targets. This mode supports observations that otherwise would result in detector saturation, and it provides the potential to expand the use of WFC3 to new areas of scientific discovery. For some science applications, this mode can also result in greater scientific return using fewer HST orbits. 

The use of spatial scans is not permitted with coordinated or pure parallels, moving targets, and internal targets. Additionally, scan exposures may not use the optional parameter CR-SPLIT and may not have special requirements that force them to occur at the same spacecraft position as another exposure. Please see the Phase II Proposal Instructions, the WFC3 Instrument Handbook for more details on this mode.

STIS is also capable of spatial scanning as an Available-but-Unsupported mode.  See the STIS Instrument Handbook for details.

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