Introduction

A Fixed Target specification defines where to point HST for an exposure 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 used to identify and classify each target. A different target must be defined whenever different coordinates are required. For instance, separate targets should be defined and listed if you plan to take observations at several points within an extended object. (If you want to take images at three different locations in the Crab Nebula, each point must have its own target number, name, and coordinates.)

Target Number [Target_Number]

APT assigns each target a number from 1 to 999 (base 10). This number is unique within the target list but may be changed by the user.

Target Name [Target_Name and Alternate_Names]

Target names provide unique designations for the targets that will be used throughout the proposal. For example, for three locations in the Crab Nebula, you could use the names: CRAB1, CRAB2, and CRAB3. 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 is limited to 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 .

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 can be assigned at most two alternate “common names,” or aliases, if they exist. For example, HD124897 could have Alternate_Names of ALPHA-BOO and ARCTURUS.

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 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

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

Some HST Scientific Instruments (SIs) have very small apertures and fields of view. Target-acquisition apertures for several of the SIs are only a few arcseconds in size. Since the HST has no acquisition cameras, it is essential to have accurate coordinates for targets. In many cases, particularly with the spectrographs, 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. (APT always interprets equatorial coordinates as being in the ICRS frame.) It is the observer’s responsibility to provide accurate coordinates in all cases. Inaccurate target coordinates can result in failed observations and can therefore waste valuable HST observing time. APT requires observers to state whether or not the coordinates are in the ICRS frame (see below). Due to their smaller apertures, entering coordinates not in the ICRS frame will generate an error for COS and STIS. Non-ICRS coordinates may be entered in APT for the other instruments, but APT will treat them as ICRS coordinates and they will not be converted. Although ICRS frame-based coordinates are not required for FGS observations, ICRS is highly recommended. All observers must generate target confirmation charts in APT to help them verify the supplied target coordinates are sufficiently accurate when interpreted as being in the ICRS reference frame.

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

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 two different ways:

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

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. These coordinates should be supplied in the International Celestial Reference System (ICRS).

• Right ascension can be entered in decimal degrees (e.g. 15.0), and APT will immediately reformat as HMS (e.g. 01H 00M 00.0S). Note that it is not possible to input RA in decimal hours.
• Declination can be entered in decimal degrees (e.g. -20.5), and APT will immediately reformat as DMS (e.g. -20D 30’ 00.0”).
• Epoch is optional unless Proper Motion is supplied. (See section below on Proper Motion.)

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.

Coordinate uncertainties are optional. Multiple units for uncertainty are available:

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.

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>.

Equatorial

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:

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.

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 non-negligible 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.

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 the coordinates of your target. 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. In that case, enter coordinates that are as accurate as possible on the Target List.

Are coordinates ICRS? [Reference_Frame]

Select one of the following values for each target:

Yes

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

No

To be used to note that coordinates are not ICRS. Note that the coordinates will be treated the same way by HST - all coordinates will be observed as though they are ICRS. 

Source of Coordinates

Document the source of the target coordinates. This is optional for ICRS coordinates, but required for non ICRS coordinates. This is for documentation purposes only and will not affect how the coordinates are used for pointing HST.

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. In order to enforce that proper motion has been considered there is a checkbox to indicate that proper motion is NEGLIGIBLE OR NOT APPLICABLE. Unless this check box is checked, observations that require very accurate coordinates will generate an error for the target if proper motion is not supplied.

Note 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 1999.0, and that will be observed in 2025.0, 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 Annual Parallax Data  [RA_PM, Dec_PM, Annual_Parallax]

The following additional information is required for targets where proper motion and parallax are “relevant”; note that uncertainties for Proper Motion and Annual Parallax are not required.

If a sign is not given for Proper Motion, a positive value will be assumed, but it is better to be explicit.

• When specifying PROPER MOTION RA and DEC in APT it is especially critical to select the same units as those given in the catalog referenced. (APT will do the conversion of units if needed.) The observation could fail if the wrong units are selected.

• The “EPOCH of position” is the date of the data from which the coordinates of the object were measured (or to which they have been corrected).  For example, coordinates from the SIMBAD Astronomical Database are usually corrected to epoch 2000 regardless of the original source epoch. But in the Guide Star Catalog2 (GSC2) the epoch depends on the individual plate or source catalog. It is not necessary to adjust your coordinates to be those that would be measured if the plate were taken in the year 2000 (you just have to properly report the epoch of the coordinates you are using).

Here is an example of a fully specified set of equatorial fixed target pointing parameters:

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.

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.

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.

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,
   or
   Line flux (point source), line surface flux (extended source), and line width of brightest emission line in the wavelength range of observation.

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.

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

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

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.

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