Space Telescope Science Institute
Cycle 21 Phase II Proposal Instructions
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Chapter 3: Fixed and Generic Targets > 3.2 Target Coordinates

3.2 Target Coordinates
The following information is required to allow for HST to be properly pointed at the target.
3.2.1 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 0.15" (1 sigma), while the absolute positions on the International Celestial Reference System (ICRS) have errors of 0.25" (1 sigma). 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 3.11; 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 3.11, 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. Please contact your PC if you need additional information. 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.
Note: 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.
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.
3.2.2 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; or
By specifying a region (area) of the sky.
Text Proposal File
If you are using the Text Proposal File, target position items must be separated by commas.
3.2.3 Equatorial Coordinates
If you specify the target position directly in terms of equatorial coordinates (as opposed to specifying an offset or a region), then the right ascension and declination <values> must be provided:
RA: <value> DEC: <value>
RA: +/– <uncertainty> DEC: +/– <uncertainty>.
The uncertainties should represent the accuracy (1 sigma) of the target coordinates, not the region within which a target could be observed (e.g., for a sky measurement). SeeSection 3.2.5 for instructions on how to designate regions as targets.
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 given in the table below.
Note: 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.
Text Proposal 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.
3.2.4 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).
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.
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 arc-sec, and the value for DEC offset may be in units of arcsec ("). See example below.
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:
-2.34S DEC: 1.6"
Text Proposal 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).
3.2.5 Region of Sky (Extended Targets)
Sometimes it is necessary to define a region of sky rather than a specific point. Examples are extended targets (such as emission nebulae and galaxies) and blank-sky regions for background measurements (if it is acceptable to make the observation anywhere within a region). As with Equatorial Coordinates, the Equinox must be J2000.
The units used for regions should be used in the same way as for coordinates; see Section 3.2.3 on page 35. You can choose either a rectangular or a circular region.
Specify the equatorial coordinates and the sides of a rectangle (for RA in arcsec, arcmin, minutes of time or seconds of time; for DEC in arcsec, arcmin or degrees)
Specify the equatorial coordinates and a radius (in arcsec, arcmin or degrees)
Text Proposal File
For a rectangular region, the format for equatorial coordinates must be used followed by a comma and the word REGION; the values following +/– will then be interpreted as one-half the lengths of the sides of the rectangular area, rather than as uncertainties in the coordinates.
In the following example, a region 4 arcmin wide in right ascension by 2 arcmin high in declination is specified:
RA = 3H 51M 27S +/– 2', DEC = –37D 13' 25" +/– 1', REGION
For a circular region, REGION must be followed by another comma and the radius of the region in the format R = <radius>; in this case, no uncertainties should be attached to the RA and DEC. Here is an example of a circular region with a radius of 2 arcmin:
RA = 3H 51M 27S, DEC = –37D 13' 25", REGION, R = 2'
Note that the units of R must be specified.
3.2.6 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. 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.
General Guidelines
3.2.7 Getting Coordinates from the GSC2 or DSS
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-Convertor’ 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 Section 3.2.1).
3.2.8 Early Acquisitions
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 (seeSection 7.3.1). In that case, enter coordinates as accurate as possible on the Target List.
Text Proposal File
3.2.9 Equinox for Coordinates [Equinox]
Enter the Equinox as J2000.
3.2.10 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 coordinate reference frame is tied to the HST Guide Star Catalog 1. The Plate ID of the special catalogs (those with an ID ZZZx, where x=0-9, a-z) must be included with your coordinates.
To be used if the coordinates are not GSC1 or ICRS.
3.2.11 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 Section 3.2.4 ).
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. However, some catalogs contain coordinates already adjusted to an epoch of J2000: the Hipparcos Input Catalogue (in cases where the star was known to have a proper motion at the time the catalog was generated; often used in the GSC for stars brighter than m(V) ~ 9) and the PPM Star Catalog. When these catalogs are being used, it is appropriate to specify an epoch of 2000. (These remarks do not apply to the Hipparcos Output Catalog, which was adjusted to an epoch of 1991.25.)
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.
20yy.y or 19yy.y
For some notes on proper motions and units, see Section 3.2.11.


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