3.1 Fixed Targets

The Fixed Targets form is used to specify fixed 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.


The Targets list tells us where you wish to point HST and so must be filled out with care, precision, and accuracy. For proposals with a large number of fixed targets, please note that there is a capability to ingest a comma-separated text file with the appropriate target information.

Target Identification

The following information is required to identify and classify each target.

Target Number [Target_Number]

Each target in your program will be assigned its own unique number (which can be changed by the user) by APT (they are base 10 and go from 1 to 999). A different target must be defined whenever different coordinates or a different target description are required. Separate targets should be defined and listed if you plan to take observations at several points within an extended object. For example, if you were to take spectra at three different locations in the Crab Nebula, each point must have its own target number, name, and coordinates, such as CRAB1, CRAB2, and CRAB3.

For text file

If you are using the Text Proposal File, all target numbers and names within a proposal must be unique.

Target Name [Target_Name and Alternate_Name]

Target names provide unique designations for the targets that will be used throughout the proposal. These names will also be used to designate targets in the HST data archive. Prospective proposers and archival researchers use these names to determine whether HST has observed a particular object. This facility will be most useful if consistent naming conventions are used.

The following conventions must be followed in naming targets:

  • A new target name must be defined for each (celestial) target. For example, for several pointings within a galaxy, one might define target names such as NGC4486-NUC, NGC4486-JET, NGC4486-POS1, and NGC4486-POS2.
  • The length of a target name must be anywhere from 2 to 31 characters.
  • No blanks are permitted in target names. Blanks between a letter and a numeral must be suppressed (e.g., HD140283, NGC4378), but a hyphen (and not an underscore) must replace blanks between two letters or two numerals (e.g., ALPHA-CEN, NGC224-0040+4058). Also, a hyphen should be used where required for clarity (e.g., NGC4486-POS1).
  • Only letters, numerals, hyphens, periods (.), and + or are allowed in target names; other punctuation is not permitted (e.g., BARNARDS-STAR is valid, but BARNARD’S-STAR is not). Greek letters must be spelled out (e.g., ALPHA-ORI). Letters may be upper-case or lower-case, but will always be treated as if they are upper case (e.g. Alpha-Cen will be treated as if written ALPHA-CEN).
  • Degree signs must be represented by an upper-case “D” (e.g., CD-42°14462 becomes CD-42D14462).
  • Some special target names are reserved for calibrations and other purposes and may not be used for names of external pointings; see Table 3.1: Designations of Special Targets .

If desired, you may give one or two “common names” for your target, which will be entered in the Alternate_Names. For example, HD124897 could have Alternate_Names of ALPHA-BOO and ARCTURUS.

Only Target_Name is used when the target name is repeated in the Visit and Exposure Specifications.

Catalogued Targets

If your target is in a well-known catalog (e.g. SDSS, NGC, PG), then use that catalog designation for the target name. This is the name your object will have in the Archive, so please try and select the most common name for the target to make it easier for archive researchers to find your target.

Uncatalogued Targets

Objects that have not been cataloged or named must be assigned one of the following designations:

  1. Isolated objects must be designated by a code name (the allowed codes are STAR, NEB, GAL, STAR-CLUS, GAL-CLUS, QSO, SKY, FIELD, and OBJ), followed by a hyphen and the object’s J2000 equatorial coordinates, if possible, rounded to seconds of time and seconds of arc (e.g., for a star at J2000 coordinates RA: 1H 34M 28S, DEC: –15D 31' 38", the designation would be STAR-013428-153138).
  2. Uncataloged objects within star clusters, nebulae, or galaxies must be designated by the name of the parent body followed by a hyphen and the rounded J2000 coordinates, if possible, of the object (e.g., for a target within NGC 224 with J2000 coordinates  RA: 0H 40M 12S, DEC: +40D 58' 48", the designation would be NGC224-004012+405848).
  3. Positions within nebulae or galaxies may also be designated by the name of the parent object followed by a hyphen and a qualifier. The qualifier should be brief, but informative (e.g., the jet in NGC 4486 could be designated NGC4486-JET). Other examples are: NGC5139-ROA24, LMC-R136A, ABELL30-CENTRAL-STAR, NGC205-NUC.

Alternate Names

In addition to the catalog name, a target could be assigned at most two alternate “common names,” or aliases, if they exist. Examples of common names are the following: BARNARDS-STAR, OMEGA-CEN and M31.

Special Targets

The names of certain types of targets must be designated by appending a code to the target name. For example, -CALIB should be appended to the name of a target that is being observed only as a calibration standard for other observations. These designations will assist in planning of the observing schedule. The possible codes are listed in Table 3.1: Designations of Special Targets .

Table 3.1: Designations of Special Targets

Target TypeCodeDescription

External calibration target

-CALIBAn astronomical target used for calibration (e.g., BD+28D4211-CALIB). Internal calibration sources (e.g., WAVE) and calibrations using the Earth must not be included in the Target List.

Offset acquisition target


A target that will be used for an offset acquisition; it is the object that will be acquired first, from which an offset will be applied to move to the target of interest (e.g., 3C273-OFFSET).

Two separate exposures must be defined on the Visit and Exposure Specifications; an acquisition of the -OFFSET target, and a science exposure of the (target of interest) program target. The location of the latter target may be specified either by equatorial coordinates or by an offset (see Target Position Type [Position]).

For example: to observe the JET in 3C273, first acquire “stellar-like” source 3C273-OFFSET, then offset to program target 3C273-JET.

Special designations

These are reserved designations and may not be used as the names of external pointing in a target list:


Target Category and 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.

Target Extent [Extended]

This field is required for COS observations only. Both COS FUV and NUV observations of targets with FWHM larger than about 0.6 arcsec or radius larger than 0.35 arcsec should be considered as extended. “YES” should be selected for such targets and “NO” for targets of smaller angular extent. The field should be left blank if the target is not being observed by COS.

This definition is intended to provide an indication of whether or not the source extent might be large enough to affect the reliability of the default pipeline extraction algorithm. See COS ISR 2015-03 and Section 5.9 of the COS Instrument Handbook for further details.There are a number of other issues that should be considered when observing extended targets with COS, including the effects on spectral resolution and target acquisition. Please note that the exposure-level parameter EXTENDED has been deprecated and should not be selected for any Cycle 23 proposals or beyond.

Target Coordinates

The following information is required to allow for HST to be properly pointed at the target.

Required Accuracies of Target Positions

The HST Scientific Instruments (SIs) typically have very small apertures and fields of view. Target-acquisition apertures for several of the SIs are only a few seconds of arc in size. Since the HST has no video acquisition cameras, it is essential to have accurate coordinates for targets. In many cases targets will be placed in the final observing aperture after a sequence of target-acquisition observations. This will only work, however, if the target coordinates are sufficiently accurate and precise to place the target in the first of these acquisition apertures.

HST uses two guide stars to stabilize the pointing of the telescope and to place the target in the desired aperture. The fundamental problem, then, is to determine the position of the target relative to the guide stars in the surrounding area with sufficient accuracy to place the target in the aperture. The specific pair of guide stars to be used cannot be determined in advance of the observation; several possible pairs will often be available for each target. The guide stars are chosen from the Guide Star Catalog 2 (GSC2). Over the HST FOV, the relative position errors between guide stars is now (using Gaia astrometry) on the order of a few mas – i.e. better than HST can actually point. Note that these errors are derived at the epoch of the position measurement and will increase slowly in time due to proper motion.

The accuracies of positions typically needed for target acquisition with each of the SIs are shown in Table 3.3: Required Coordinate Accuracies, these are predicated upon the positions being in the ICRS frame, which is the reference frame of the GSC2 catalog. Inaccurate target coordinates can result in failed target acquisitions and can therefore waste valuable HST observing time. As indicated in Table 3.3: Required Coordinate Accuracies, it is the observer’s responsibility to provide accurate coordinates in all cases, but in particular they must be in the ICRS reference frame when using STIS or COS. Although ICRS frame-based coordinates are not required for FGS observations, it is still prudent to check the accuracy of your coordinates. All observers can generate target confirmation charts in APT to help them verify the target coordinates in the ICRS reference frame.

(red star) HST proposals executed before July 1991, as well as engineering proposals of type OV, SV, SMOV, and CAL, should not be used to derive target coordinates. Coordinates from such proposals may be unreliable owing to poor calibration and/or engineering-related pointing changes made during the observations.

Table 3.3: Required Coordinate Accuracies

Instrument ConfigurationAccuracy Required (1 sigma, arcsec)ICRS Coordinates Required?

Targets near the Celestial Poles: Be very careful if your target lies near a celestial pole. In this regime many precession routines break down and uncertainties in position are exacerbated. Also, patterns that you may execute with an instrument could cross the pole, leading to confusion in position. All these issues can be resolved, but careful attention is needed.

Target Position Type [Position]

A position type is required for each fixed target. The positions may be expressed in any one of three different ways:

  • By specifying the equatorial coordinates (RA and DEC) of the target;
  • By specifying a positional offset from another target.

For text file

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

Equatorial Coordinates

If you specify the target position directly in terms of equatorial coordinates (as opposed to specifying an offset), then the right ascension and declination <values> must be provided:

RA: <value>DEC: <value>
RA: +/– <uncertainty>DEC: +/– <uncertainty>
  • The coordinates must be in Equinox J2000.
  • The uncertainties should represent the accuracy (1 sigma) of the target coordinates.
  • The right-ascension value must be expressed in hours (H), minutes (M), and seconds of time (S). If expressed as decimal degrees (e.g. 15.5), APT will automatically convert to HMS format (e.g. 01H 02M 00.0S). Note that it is not possible to enter RA in decimal hours.
  • The declination value must be expressed in degrees (D), minutes ('), and seconds (") of arc. If expressed as decimal degrees (e.g. -20.5), APT will automatically convert to DMS format (e.g. -20D 30’ 00.0”).

The units must be selected (from a pull down list). The allowed values are:

Quantity and units specified
RA: H-M-Stimemin, timesec, arcmin, arcsec
DEC: D-M-Sdegrees, arcmin, arcsec

(red star) If the sign of the declination is not indicated, a positive declination is assumed, but we urge you to always include the sign as a way of reducing errors.

For text file

In the Text Proposal File you must use the following format for RA and DEC (note the comma delimiters):

RA = <value> +/– <uncertainty>, DEC = <value> +/– <uncertainty>

The comma following the right-ascension uncertainty is required.

  • The right-ascension value must be expressed in hours (H), minutes (M), and seconds (S) of time. The declination value must be expressed in degrees (D), minutes ('), and seconds (") of arc (For example: RA = 12H 7M 13.33S +/– 0.15S, DEC = +27D 3' 8.0" +/– 0.1"). Note that while APT accepts decimal RA/DEC, as does the target import capability, the.prop file still requires H, M, and S units for RA, and D,' and '' for Dec.
  • Units must be provided for both a value and its uncertainty (see Acceptable units for uncertainty).

For engineering parameters

IMPORTANT: Both SAVE OFFSET and USE OFFSET rely on accurate coordinate uncertainties in order to calculate slew times properly. Unspecified target coordinate uncertainties may lead to a failure in execution of USE OFFSET. Program Coordinators should ensure that target coordinate uncertainties have been provided by the observer and that they are accurate.

Positional Offsets

The position of a target may alternatively be specified as an offset from a reference target. Note, however, that offsets larger than 30 arcsec may complicate the target acquisition procedure. If larger offsets are desired, please contact your Program Coordinator.

Offsets are always in the sense offset = target-coordinates minus reference-coordinates.

As with other similar quantities, we urge you to include the sign of the offset, even when it is positive, as a means of removing ambiguity.

Note that you select the <target name> which has the equatorial coordinates of the reference target, and that reference-target names have –OFFSET appended to them (see Table 3.1: Designations of Special Targets).

Positional offsets are only a convenient method of specifying target coordinates, and do not automatically imply a particular method of target acquisition; observers must explicitly specify any target acquisitions in their visits.

(red star) If your object has significant proper motion and the reference target does not have the same proper motion, do not use positional offsets to specify your object. The assumption for positional offsets is that both objects have the same proper motion. The proper motion for both objects is specified once on the reference target.

You specify the offset as a difference in EQUATORIAL coordinates from a target <name>.


Position Type: Offset     From Target: <target_name> Offset: RA: <value> DEC: <value>

The value for RA offset may be in units of seconds of time or decimal degrees, and the value for DEC offset may be in units of arcmin (') or arcsec ("), or in decimal degrees.

Example: NGC2654’s right ascension is 2.34 seconds of time less than the reference target (NGC2654-OFFSET), and its declination is 1.6 arcsec greater than NGC2654-OFFSET. The specifications for NGC2654 would be:

Position Type: Offset From Target: NGC2654-OFFSET
Offset:RA: -2.34SDEC: 1.6"

For text file

The format for an offset specification as a difference in equatorial coordinates is:

RA-OFF =<value>, DEC-OFF = <value>, FROM <target number>

Note the commas separating the items. The value for RA-OFF may be in units of seconds (S) of time, or in decimal degrees (D), and the value for DEC-OFF may be in units of arcmin (') or arcsec ("), or in decimal degrees (D).

Determining Coordinates in the Reference Frame Appropriate for HST Observations

The HST reference frame is effectively defined by the positions of the Guide Stars that are selected for each pointing. At launch, we used the Guide Star Catalog (GSC1) which was an all-sky catalog of stars down to 15th magnitude built from Schmidt Sky Survey plates. That catalog was updated (GSC2) and calibrated to be on the International Celestial Reference System (ICRS), which has been adopted by the IAU as the fundamental reference frame. (In Cycle 28 the upgrade to GSC 2.4.2 incorporated guide star positions, proper motions and magnitudes from Gaia Data Release 2.) This simplifies the procedure for providing HST coordinates since it removes the necessity to tie the object coordinates back to the GSC1 and the plates used to construct it.

(red star) For observations that require accurate coordinates, such as those listed as ICRS Coordinates Required in the table of Required Coordinate Accuracies, it is vital that you provide positions derived in the ICRS reference frame.

General Guidelines

  • If your target has a position that is in a catalog using the ICRS you may use the coordinates directly. These include GSC2, Hipparcos, Tycho, SDSS, 2MASS, FIRST, and Gaia.
  • If your target is an extended object where the observation position does not correspond to the catalog coordinates, we recommend that you obtain an image of the field and measure your target coordinates in the ICRS reference frame.
  • If your target has a relevant proper motion, you must provide the epoch of the coordinate as well as the proper motion values.
  • Access to the GSC2, the Digitized Sky Survey (DSS) and other catalogs/surveys is built into the Aladin interface in APT (Note: for the DSS use the POS- S2UKSTU-Red plates that were used to build GSC2). Alternatively, you can also find links to query the GSC2 catalog and retrieve POSS2UKSTU-Red DSS images on the HST Support page. Do not use the data contained in the 102-volume set of CD ROMs distributed as the Digitized Sky Survey (DSS-I).

Getting Coordinates from the GSC2 or DSS

  • If your target is a star brighter than m(V)~20 then it typically will be visible on the DSS images and present in the GSC2 catalog. Using the GSC2 position will ensure that the target is in the same reference frame as the selected guide stars.
  • For extended sources visible on the photographic survey plates, we strongly recommend that you examine the DSS image and check your coordinates. Depending on the brightness, morphology and structure of the galaxy the GSC2 coordinate may not correspond to the aperture location you require for your observation. The DSS headers downloaded from STScI contain ICRS-based FITS WCS information to allow you to measure the image using various image analysis tools.
  • Please note that the GSC2 coordinates for bright stars come from various sources which are more accurate than positions measured from the Schmidt plates.
  • If you have used HST to observe a target in an earlier cycle and already have GSC1 based coordinates,  you also have the option of using a ’Coordinate-Converter’ that is available at the HST Support page. This is a simple web-based tool that allows one to enter either a GSC1 ID or coordinate. In the case of an ID it will directly look up the GSC2 coordinate for that object. If you enter a coordinate, it will derive a mean offset between GSC1 and GSC2 over the HST FOV and apply that correction to the position.

As a final check that the coordinates are correct, you must prepare a Confirmation Chart (in APT) showing the target coordinates (as entered in the proposal) overlaid on the field from the DSS. Ultimately, you are responsible for verifying that the coordinates are correct (see Required Accuracies of Target Positions).

Early Acquisition

If it is impossible to obtain adequate plate material to measure coordinates to the required accuracy (for example, a very crowded field which cannot be resolved using ground-based observations), it may be necessary to obtain an early acquisition image or to perform an acquisition that involves real-time interaction with the telescope (see General Exposure-level Special Requirements). In that case, enter coordinates as accurate as possible on the Target List.

For text file

Equinox for Coordinates [Equinox]

Enter the Equinox as J2000.

Coordinate Reference Frame [Reference_Frame]

Select one of the following Reference_Frame values for each target.


To be used if the coordinate reference frame is tied to the International Celestial Reference System (ICRS).


To be used if the coordinates are not ICRS.

Is Proper Motion or Parallax Relevant?

If a small aperture or occulting spot is to be used, even a relatively small proper motion or parallax may cause difficulties in acquiring the target. In such cases, the Proper Motion/Parallax data must be provided. Note, however, that proper motion and parallax values may not be specified for a target that is specified by a positional offset. Such targets will be taken to have the same proper motion and parallax as the reference target (see Positional Offsets).

The observer must determine whether or not proper motion or parallax is relevant. In general, this will depend on the size of the acquisition aperture of the SI that will be used and the epoch of the coordinates that have been provided. For example, the STIS uses a target acquisition area of 5 arcsec square. For a star whose coordinates are given for the epoch 1987.0, and that will be observed in 2013.0, a proper motion of approximately 0.05 "/year yields a total motion of 1.3", which is just greater than half the minimum center-to-edge distance (1.25") of the aperture, and therefore is relevant.

Proper Motion and Parallax Data

The following information is required for targets where proper motion and parallax are “relevant”; note that uncertainties for RA_PM, Dec_PM, and Annual_Parallax are not required. If a sign is not given for RA_PM or Dec_PM, a positive value will be assumed, but it is better to be explicit.


For proper motion in RA, the value can be in units of mas/year, arcsec/year or seconds of time/year. The selection of units is especially critical for RA_PM, as there is a large difference between 15 seconds of time/year and 15 arscec/year, so the observation will fail if improper units are provided.


For proper motion in DEC, the value can be in units of mas/year or arcsec/year.


The “Epoch of position” is the date of the data from which the position of a star with proper motion was measured, or to which it has been corrected. Any epoch may be given but it must be correctly specified. SIMBAD coordinates are usually corrected to epoch 2000 regardless of the original source epoch.

The “Epoch of position” may or may not be the same as the date of “Equinox for Coordinates” (required to be J2000 for HST). Remember that the “Epoch of position” is the date the target position is referred to, whereas the “Equinox of Coordinates” is the date of the coordinate frame, which changes because of the precession of the Earth's axis. For example, a star with a large proper motion may have its coordinates given in the J2000 system, but the numbers themselves are for epoch 1984, meaning that the star was at the specified position on January 1, 1984. Epoch should be of the form 20yy.y or 19yy.y. (Note: An epoch is purely a time, and one of the form "J1991.25" is invalid.)

Ordinarily the epoch of position is earlier than the present date. In the Guide Star Catalog (GSC), the equinox is J2000 while the epoch depends on the individual plate. It is not necessary to adjust your coordinates to be those that would be measured if the plate were taken in the year 2000.   Note that some catalogs contain coordinates already adjusted to a common epoch. For instance Gaia Data Release 2 has a common epoch of 2015.5 while Gaia Data Release 3 has a common epoch of 2016.0.   


The unit for parallax is arcsec.

The example in the table below is for the object DM–9D697 (Epsilon Eridani), where the proper motion data are taken from the SAO Catalog.

RA_PMProper Motion in RAmas/yr, arcsec/yr or sec /yr

–0.0662 sec/yr

DEC_PMProper Motion in DECmas/yr or arcsec/yr19 mas/yr
EpochEpoch of Position20yy.y or 19yy.y1984.5
Annual_ParallaxAnnual parallaxarcsec0.30488

For some notes on proper motions and units, see Is Proper Motion or Parallax Relevant?.

Target Brightness and Bright Object Checking

Flux information is required to support bright object (ACS/SBC, COS, FGS, and STIS/MAMA) and persistence (WFC3/IR) checking on the target.

(red star) All flux information relevant for bright object checking must be provided.

Flux Data [Flux and Other_Fluxes]

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. Note that all objects in the field need to be checked, and there is a Bright Object Tool in APT to support that checking.

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), or J magnitude (IR 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.

Radial Velocity or Redshift [RV_or_Z]

Give, if known, the heliocentric radial velocity or redshift of the target. The format is <velocity in km/sec> or <redshift>; examples are +1198 (Radial Velocity) and 1.95 (Z). The units must not be specified.

For text file

In the Text Proposal File the format is V = +1198 and Z = 1.95.

Bright-Object Constraints

Several of the Scientific Instruments must be protected against over-illumination. The Table 3.4: Bright Object Constraints summarizes the safety restrictions  by instrument. You should not propose observations which violate these guidelines. Non-linearity, saturation, or other temporary effects which may occur at substantially fainter limits than those identified below are described in Section 7.7.2 in the STIS Instrument Handbooks.

APT contains a Bright Object Tool (BOT) that should be used to check on these constraints. Proposers should check the entire field using the Aladin interface provided in APT.

Table 3.4: Bright Object Constraints


The STIS MAMA (UV) detectors are subject to strict bright-object controls to prevent potentially fatal damage. Maximum permissible local and global count rates have been established for the STIS MAMA detectors, both for imaging and spectroscopy, and are given in the STIS Instrument Handbook. These limits have been translated into limiting magnitudes and fluxes for the various configurations and modes (see Table 13.45 for spectroscopy and Table 14.40 for imaging). Some of these limits are quite faint.

All potential targets and field objects within a certain field of view, which depends on the configuration selected, should be checked for safety with the STIS Exposure Time Calculator (ETC available at http://etc.stsci.edu/) and with the Bright Object tool in APT[1]. Measured UV fluxes must be provided for any object within 1 magnitude of the brightness limits; if not previously available, they must be first observed with a safe STIS mode. Unless they can be screened with GSC2/DSS, magnitudes and colors, or a UV image, other fluxes must be provided for all objects in the fields of proposed STIS MAMA observations, including the background fields of solar-system targets. ORIENTs and/or POS TARGs may be used to avoid potentially problematic objects.

There are no safety-related brightness limits for the STIS CCD. See the STIS Instrument Handbook for a description of saturation levels, residual charge, and other effects.


The ACS/SBC MAMA detector is subject to strict bright-object controls to prevent potentially fatal damage. Maximum permissible local and global count rates have been established for both imaging and spectroscopy; see the ACS Instrument Handbook (Section 7.2). These limits have been translated into a table of magnitudes as a function of spectral type for the various configurations and modes (ACS Instrument Handbook, Table 7.4). Some of these limits are quite faint.

All potential targets should be checked for safety with the ACS Exposure Time Calculator (ETC available at http://etc.stsci.edu/etc/input/acs/imaging) and with the Bright Object tool in APT. Measured UV fluxes must be provided for any objects within one magnitude of the spectroscopic limits. This requirement extends to both targets and field objects on the detector. ORIENTs and/or POS TARGs may be used to avoid the latter. Unless they can be screened with GSC2/DSS, magnitudes and colors, or a UV image, other fluxes must be provided for all objects in the fields of proposed SBC observations, including the background fields of solar-system targets.

There are no safety-related brightness limits for the ACS/WFC CCD cameras. See the ACS Instrument Handbook for a description of saturation levels, residual charge, and other effects.


The FGS may not be used to view objects brighter than mV = 1.8.

The FGS may view objects brighter than mV = 8.0 only if the neutral-density filter is in place.


The detectors used in the COS/FUV and COS/NUV channels are both subject to strict bright-object controls to prevent potentially fatal damage. The maximum permissible local and global count rates have been established and are given in the COS Instrument Handbook. These limits have been translated into equivalent limiting magnitudes and fluxes. Some of these limits are quite faint.

All potential targets and field objects within a certain field of view must be checked for safety with the COS Exposure Time Calculator (ETC available at http://etc.stsci.edu/etc/input/cos/spectroscopic/) and with the Bright Object tool in APT[1]. Measured UV fluxes must be provided for any object within one magnitude of the brightness limits. Unless they can be screened with GSC2/DSS, magnitudes and colors, or a UV image, other fluxes must be provided for all objects in the fields of proposed COS observations, including the background fields of solar-system targets. ORIENTs and/or POS TARGs may be used to avoid potentially problematic objects.


There are no safety-related brightness limitations for either the UVIS or IR channels. Please refer to the WFC3 Instrument Handbook, Appendix D for a description of persistence of saturated images.

1 See  http://apst.stsci.edu/apt/external/help/documentation/Running-BOT.html for a description of the APT Bright Object tool.

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Related Links

3.2 Solar System Targets
3.3 Generic Targets

Table of Contents

Change Log

Version Cycle 31 July 2023

       Fixed six links that were poorly formed.

Version Cycle 30 April - May 2022

       PROPINST-91386 - Remove Reference Frame GSC1 for HST Fixed Targets

       Updated Epoch section to replace common epoch example with more modern for GAIA DR2 and DR3 catalogs.

       PROPINST-70842 (Remove HST Region targets)

Version Cycle 29 May 2021

       Changes due to enhancement of guide star catalog with GAIA coordinates.