3.4 Target Position Type[Position]

A position type is required for each fixed target. It 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.

It is also possible to specify that the coordinates were obtained using the Guide Star Selection System (GSSS; see 3.4.5 Determining Coordinates in the Guide Star Selection System (GSSS) Reference System), or that they are currently uncertain or unknown, and that more accurate coordinates will be provided by the observer after an early acquisition exposure is taken, or in real time during the HST observations.

Text Proposal File

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

3.4.1 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 analog to the video acquisition cameras common on many ground-based telescopes, 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 ICRS have errors of 0.25" (1 sigma). Note that these errors are derived at the epoch of the GSC plate 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 International Celestial Reference System (ICRS), which is the reference frame of the GSC2 catalog. Note that several of the SIs have multiple acquisition apertures of different sizes that may be used. Be sure when selecting acquisition apertures to keep the coordinate uncertainties in mind. Furthermore, be sure to provide one sigma uncertainties with your positions so that STScI may check the appropriateness of your acquisition exposures. 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 with the NIC1 and NIC2 detectors. Please contact your PC if you need additional information. Although ICRS frame-based coordinates are not required for FGS and WFPC2 observations, it is still prudent to check the accuracy of your coordinates. All observers will be provided target confirmation charts by their PC to help them verify the target coordinates in the ICRS reference frame. The Principal Investigator of a program is responsible for ensuring that target coordinates are accurate, both at the time of program submission, and later when target confirmation charts are provided. The following address has pertinent information on target confirmation charts:

http://www.stsci.edu/public/confirmation-chart.html
 

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.

Table 3.11: Required Coordinate Accuracies 

Instrument Configuration	Accuracy Required (1 sigma, arcsec)	ICRS Coordinates Required?
WFPC2 (WFC)	10	No
WFPC2 (PC)	5	No
FGS	1	No
ACS/SBC	5	No
NIC11	3	Yes
NIC2 (2)	5	Yes
NIC3 (2)	10	No
COS/FUV

1If multiple NICMOS detectors are being used in parallel, the primary detector (the detector used for the exposures in the <primary-exp-list> of the PARallel WITH Special Requirement; see PARallel <parallel-exp-list> WITH <primary-exp-list> (replaced by Coordinated Parallel Containers in the APT User Interface)) determines the required coordinate accuracy for the observation and whether GSC2 frame-based coordinates are required.

3.4.2 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 default to 0.1". They 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). See 3.4.4 Region of Sky (Extended Targets) 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).
• The declination value must be expressed in degrees(D), minutes ('), and seconds (") of arc.
• The units must be selected (from a pull down list) for both the value and its uncertainty. The uncertainty must be expressed in one and only one of the units used to express the related RA and/or DEC, with the additional units of minutes (') of arc or seconds (") of arc being allowed for right ascension. (In other words, the RA may be expressed as a combination of three units (H M S), but its uncertainty must be in terms of a single unit such as S or ".) To clarify:

  Quantity and units specified	Acceptable units for uncertainty
  RA: H-M-S	timemin, timesec, arcmin, arcsec
  DEC: D-M-S	degrees, arcmin, arcsec

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.

• 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")
• Units must be provided for both a value and its uncertainty (see " Acceptable units for uncertainty")

3.4.3 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 offset-reference-coordinates. A polar offset is expressed as an angular separation and position angle of a line drawn from the offset reference target to the target. 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 on the Visit and Exposure Specifications via Special Requirements (see Chapter 7: Special Requirements [Visit and Exposure Special_Requirements]).

Warning: If your object has significant proper motion no correction may be applied. See 3.8 Is Proper Motion or Parallax Relevant?, where it notes that the proper motion for the target is taken to be the same as for the offset object.

You specify the offset as a difference in EQUATORIAL coordinates from a target <name>:
Position Type: Offset
Offset: RA: <value> DEC: <value>
Uncertainty: RA: <value> DEC: <value>
From Target: <target name>

3. The value for RA offset may be in units of seconds of time or in decimal degrees, and the value for DEC offset may be in units of arcmin (') or arcsec ("), or in decimal degrees (see example below). The uncertainty must be expressed in one and only one of the units used to express the related RA and/or DEC. Offsets in RECTANGULAR coordinates are interpreted as displacements in the tangent plane whose origin is located at the reference target. For displacements of less than about one degree, they may be interpreted as the offsets that are measured on a photographic plate, without making errors larger than about 0.5 arcsec. The format for the specification of an offset in rectangular coordinates is:

   XI-OFF = <value> +/- <uncertainty>, ETA-OFF = <value> +/- <uncertainty>, FROM <target number>

   Note: North and east displacements are considered positive (+), south and west are negative (-).

   The values of XI-OFF and ETA-OFF are required to both be expressed in arcmin (') or arcsec ("), and the uncertainties must be expressed in the same units (in other words, all four angular quantities must be in the same units). In the following example, a target has been measured on a sky-survey plate, with a scale of 67 arcsec/mm, to lie 0.5 mm west of, and 1 mm north of, target number 3, with an accuracy of 1 arcsec:

   XI-OFF = -33" +/- 1", ETA-OFF = +67" +/- 1", FROM 3

4. The format for a POLAR offset specification is:

   R = <value>, PA = <value>, FROM <target number>

   Note that uncertainties must not be included in this type of specification. The separation is expressed in units of minutes (') or seconds (") of arc. The position angle of the target with respect to the reference target is measured east of north, and the unit of measurement (degrees) is required. In the following example, a target lies 10 arcsec from target number 4, at a position angle, measured at target 4 from north through east, of 60 degrees:

   R = 10", PA = 60D, FROM 4

By default APT provides a value of 0.1" for the uncertainties.

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
Offset: RA: -2.34S DEC: 1.6"
Uncertainty: RA: 0.01S DEC: 0.1"
From Target: NGC2654-OFFSET

Text Proposal File

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

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

Note the uncertainties and the commas separating the three 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). The uncertainty must be expressed in one and only one of the units used to express the related RA and/or DEC.

3.4.4 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). An Equinox value should be specified with the region coordinates (see Section 3.5 ).

The units used for regions should be used in the same way as for coordinates; see Section 3.4.2 . You can choose either a rectangular or a circular region.

• Rectangular: 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)
• Circular: 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.

Circular Region

If it is desired to specify 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.

3.4.5 Determining Coordinates in the Guide Star Selection System (GSSS) Reference System

The HST reference frame is effectively defined by the positions of the Guide Stars that are selected for each pointing. Since launch, we have used the Guide Star Catalog (GSC1) which was an all-sky catalog of stars down to 15th magnitude built from Schmidt Sky Survey plates. This catalog has been updated (GSC2) with more recent epoch plates and calibrated to be on the International Celestial Reference System (ICRS), which has been adopted by the IAU as the new 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.

For observations that require accurate coordinates, such as those listed as "ICRS Coordinates Required" in Table 3.11, 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 and FIRST.
• 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. Typically, this is done by measuring a sufficient number of reference stars (for example, from GSC2, Tycho, Hipparcos, 2MASS, SDSS - any catalog that uses the ICRS) to derive your astrometric transformation.
• If your target has a relevant proper motion, you must provide the epoch of the coordinate as well as the proper motion values (see Section 3.8, "Is Proper Motion or Parallax Relevant?" .)
• 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 POSS2UKSTU-Red plates that were used to build GSC2). You can also find links to query the GSC2 catalog and retrieve POSS2UKSTU-Red DSS images on the HST Support web page at
  http://www-gsss.stsci.edu/HSTsupport/HSTsupport.htm.

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.
• Please enter the GSC2 name as a target alias in Alternate_Names.
• 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 Aladin (or other image analysis tool).
• Please note that the GSC2 coordinates for bright stars come from the Tycho2 catalog as these are more accurate than positions measured from the Schmidt plates.
• Using coordinates from GSC 1.1 associated special catalogs will be permitted only under exceptional circumstances (contact your Program Coordinator). Other uses of GSC 1.1 are not allowed. Coordinates from the development version GSC 1.2 cannot be used at all as it is not available in the HST ground system.
• 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 part of preparing your observations your Program Coordinator will provide, as a final check that the coordinates are correct, a Confirmation Chart 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 3.4.1, "Required Accuracies of Target Positions" ).

3.4.6 A Caution on Astrometry Prepared from STScI Plate Scans

Note that the set of plates used to construct the GSC2 coordinates is NOT the same one that is contained in the 102-volume set of CD ROMs distributed as the Digitized Sky Survey (DSS-I). The GSC2 coordinates are primarily derived from the POSS-II Red survey in the northern hemisphere and the AAO-SES/ER Red surveys in the south. These images are only available on-line. If you wish to measure your target coordinates from these images, please download the images using the links from the CASG Web server listed above.

3.4.7 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 (see Section 7.3.1 ). In that case, enter coordinates as accurate as possible on the Target List.