3.3 Generic Targets

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The Generic Targets form is used to specify generic targets in some HST observations. The Astronomer's Proposal Tool (APT) is used to enter the targets into the proposal.

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Format definitions

Boldface type indicates the name of an APT parameter or a value for a parameter.

(red star) Black text indicates an important note.

Magenta text indicates available but unsupported parameters (requires prior approval from STScI).

Red text indicates restricted parameters (for STScI use only).

Brown text indicates text file parameters.

Items in brackets - <value> - are required values.

Items in square brackets - [<value>] - are optional.


Generic targets are those that can only be described in terms of astronomical characteristics or a general location in the sky, and are used when the target will be specified at a later date (for example, Targets of Opportunity).

Visits using Generic targets must have either an ON HOLD or a TARGET OF OPPORTUNITY special requirement. The Generic target should be replaced by a fully specified Fixed or Solar System target when the ON HOLD is removed from the visit.

Target Number(s) [Target_Number]

The target number will be assigned by the APT software. This can be changed by the user.

For text file

If you are using the Text Proposal File, generic targets should be given individual names and numbers just like fixed targets.

Target Name [Target_Name]

A descriptive name must be provided for each target. If a name cannot be specified in advance, a provisional name should be supplied. When the actual observation is made, a more specific name will be added to the target designation. Either the provisional name or the updated name can then be used for archival searches (e.g., SN might be the provisional name, while SN-1995D might be the updated name). A unique target name must be assigned to each generic target.

Target Description [Description]

A target description must be selected for each target. The Target Description will be one of the key fields used by archival researchers in searching through the HST data archive; thus it is extremely important that the information be filled out completely and accurately for each target.

Each target must be assigned a single primary category from Table 3.2: Target Categories, and at least one descriptive keyword, chosen from the appropriate table. The discrete features and descriptors may be used as descriptive keywords for any category. A maximum of five descriptive keywords may be selected.

The categories in Table 3.2: Target Categories, and some of the descriptive keywords in the tables, are followed by explanatory text in parentheses. This text is provided only for explanatory purposes and is not part of the category or keyword itself.

Table 3.2: Target Categories

CategoryDescriptive Keywords
SOLAR SYSTEM (Solar System Object)Table 3.5 Solar System
STAR (Galactic Stellar Object)Table 3.6 Star
EXT-STAR (Star in an External Galaxy)Table 3.6 Star
STELLAR CLUSTER (Galactic Star Cluster, Group, or Association)Table 3.7 Stellar Cluster
EXT-CLUSTER (Star Cluster in an External Galaxy)Table 3.7 Stellar Cluster
GALAXY (Galaxy or AGN)Table 3.8 Galaxy
CLUSTER OF GALAXIES (Galaxy Groupings, Clusters, Large-scale StructureTable 3.9 Clusters of Galaxies
ISM (Interstellar Medium of the Galaxy)Table 3.10 ISM
EXT-MEDIUM (Interstellar Medium of an External Galaxy)Table 3.10 ISM
UNIDENTIFIED (Unidentified Objects)Table 3.11 Unidentified 
CALIBRATION (Calibration Observations)Table 3.12 Calibration

For text file

If you are using the Text Proposal File, target description items must be separated by commas.

Flux Data [Flux]

Flux information must be provided for all targets, and there can be more than one entry for a given target. STScI uses flux information to check for over-illumination of sensitive detectors. All entries are values as observed at the Earth, rather than intrinsic values.

COS, ACS/SBC and STIS/MAMA proposals cannot be implemented without flux information for all targets because of the critical requirements to protect the detectors from damage by excessively bright objects.

The flux information is provided in two separate fields:

  • Flux in V Magnitude with an uncertainty. This is required for targets observed by the FGS, STIS/FUV-MAMA, STIS/NUV-MAMA, COS and ACS/SBC. For all other instrument configurations, it’s optional.
  • Other Fluxes (separated by commas), which is entered in free text.

In the “Other Fluxes” field, the spectral type and reddening could be provided if you think it’s important. As many additional flux values as appropriate for the requested exposures should be provided. For example, ultraviolet or emission-line fluxes should be given if the target is to be observed in the ultraviolet or through a narrow-band filter, or several magnitudes might be provided if the target is a variable star to be observed at various brightness levels. In some cases (Targets of Opportunity, variable objects, etc.) the estimated flux data may be very uncertain, but the best available estimates should nevertheless be given, along with appropriate uncertainties and comments.

It may be important to specify the flux of a background source as well as the target flux. For example, a globular cluster in M87 may be seen against the bright background of the galaxy. The suffix –BKG should be appended to a background flux specification in this case (e.g. SURF-BKG(B) = 20 +/– 0.2 mag/arcsec2). Use a comma to separate entries if more than one flux value is given.

For text file

If you are using the Text Proposal File, flux items in a list must be separated by commas.

General Guidelines on What Flux Data to Include

The following summary provides general guidelines for what flux information could be included in three general areas.

Target Flux

  1. Magnitudes: V magnitude (point source), V surface brightness (extended source), J magnitude (IR source), or GALEX NUV magnitude (UV source).
  2. Flux: flux at specified wavelength.
  3. Color: B-V, U-B, J-K, etc.
  4. Reddening: E(B-V). If no entry for E(B-V) is given, E(B-V) = 0 will be assumed.
  5. Spectral type (point source).

Background Flux

  1. Non-dispersive spectral element: Broad-band surface brightness or surface brightness at specified wavelength; BKG must be specified in the name of the flux parameter. For IR sources, this refers to the astronomical background and not the thermal background.
  2. Dispersive spectral element: Surface brightness of continuum; -BKG must be specified in the name of the flux parameter. For IR sources, this refers to the astronomical background and not the thermal background.

Surface Flux

  1. Non-dispersive  spectral  element:  Flux  (point  source)  or  surface  flux (extended source) in wavelength range of observation.
  2. Dispersive spectral element: Continuum (point source) or surface (extended source) flux at wavelength of observation and size of the region specified,
    Line flux (point source), line surface flux (extended source), and line width of brightest emission line in the wavelength range of observation.
(red star) Details of how the above flux information was derived should be given in the Observing Description or Target Comment, as appropriate. If any of the required flux data cannot be provided or are deemed to be unnecessary, these points must also be explained in that section. Incomplete flux information may delay the implementation of your proposal, especially in the case of ACS/SBC, COS and STIS/MAMA observations.

Comments [Comments]

Any additional information you wish may be entered in “Comments” area. Comments are not interpreted by the software, but are maintained in the data base and do appear on the formatted outputs.

Criteria [Criteria]

The "Criteria" field is before you describe your Generic Target. Examples are “Next SN in M31”, “Next bright, galactic nova”, or “GRB at z>5”.

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Related Links

3.1 Fixed Targets
3.2 Solar System Targets

Table of Contents

Change Log

Version Cycle 31 June - Aug 2023

        PROPINST-91401Document change in ON HOLD behavior for Generic targets

        Also removed bullet point: "The exposure is SAME POS AS another exposure (so that the pointing is determined by the other exposure)."  because it is no longer the case.