Phase 2 Proposal Instructions for Cycle 10
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Phase 2 Proposal Instructions for Cycle 10 | |||
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3.4 Target Position [Position]
A position is required for each fixed target. The position 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; see3.4.5Determining 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.
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 (GSC). The uncertainty of the relative positions of a group of potential guide stars in the GSC is 0.33 arcsec (1 sigma) in the north and about 0.5 arcsec in the south.
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 coordinate system of GSC, which is based upon the AGK3 and SAO reference systems. 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 1 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 observerÕs responsibility to provide accurate coordinates in all cases, but in particular they must be in the GSC reference frame when using STISwith the NIC1 and NIC2 detectors. Please contact your PC if you need additional information. Although GSC 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 GSC 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:
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
WFPC2 (WFC) 10 No WFPC2 (PC) 5 No STIS 1 Yes FGS 1 No ACS/WFC 10 No ACS/HRC 4 No ACS/SBC 5 No ACS/HRC-OCCULT 1 Yes ACS/HRC-CORON 1 Yes NIC11 3 Yes NIC2 (2) 5 Yes NIC3 (2) 10 No
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>) determines the required coordinate accuracy for the observation and whether GSC 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 and their uncertainties are required, using the following format:
RA = <value> +/Ð <uncertainty>, DEC = <value> +/Ð <uncertainty>
- The comma following the right-ascension uncertainty is required.
- The uncertainties 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). See3.4.4Region of Sky (Extended Targets) for instructions on how to designate regions as targets.
- The right-ascension value must be expressed either in hours (H), minutes (M), and seconds (S) of time; or in decimal degrees (D). For example, RA = 108.14375D and RA = 7H 12M 34.5S are equivalent.
- The declination value must be expressed either in degrees (D), minutes ('), and seconds (") of arc; or in decimal degrees (D). Thus DEC = -12D 55M 33.3S and DEC = -12.92592D are equivalent.
- Units must be provided for both a 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:
RA: H-M-S timemin, timesec, arcmin, arcsec RA: Degrees degrees DEC: D-M-S degrees, arcmin, arcsec DEC: Degrees degrees
RA = 12H 7M 13.33S +/Ð 0.15S, DEC = +27D 3' 8.0" +/Ð 0.1" and RA = 181.80554D +/- 0.00063D, DEC = +27.05222D +/- 0.00003D are equivalent.
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.
3.4.3 Positional Offsets
The position of a target may alternatively be specified as an offset from a reference target. It is possible to specify offsets in equatorial coordinates, in rectangular coordinates, or in polar coordinates, all of which have their origin at the 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.
Equatorial and rectangular offsets are always in the sense offset = target-coordinates minus offset-reference-coordinates, while 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 in the examples below, FROM 6, 3, and 4 are the target numbers which contain the equatorial coordinates of the reference targets, and that the reference-target names have ÐOFFSET appended (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 Chapter7:Special Requirements [Visit_Requirements and Special_Requirements]).
Warning: If your object has significant proper motion no correction may be applied. See 3.8Is 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.
- The format for the specification of an offset as a difference in EQUATORIAL coordinates is:
RA-OFF =<value> +/Ð <uncertainty>, DEC-OFF = <value> +/Ð <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.
In the following example, a targetÕs right ascension is 1.1 seconds of time greater than, and its declination is 10 arcsec less than, the right ascension and declination of target number 6:
RAÐOFF = +1.1S +/Ð 0.2S, DECÐOFF = Ð10" +/Ð 1", FROM 6- 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 (Ð).
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:
- 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:
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). It is possible to specify rectangular or circular regions. An Equinox value should be specified with the region coordinates (see3.5Equinox for Coordinates [Equinox]).
The units used for regions should be used in the same way as for coordinates; see3.4.2Equatorial Coordinates.
Rectangular Region
For a rectangular region, the format for equatorial coordinates must be used, followed by a comma and the word REGION; the values following +/Ð 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 +/Ð 2', DEC = Ð37D 13' 25" +/Ð 1', REGION
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 = Ð37D 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 defined by the HST Guide Star Catalog (GSC) that STScI has created. For observations that require accurate coordinates, such as those listed as ÒGSC Coordinates RequiredÓ in Table 3.11, it is vital that you provide positions derived in the same reference frame as the original GSC plate material.
Access to the GSC, as well as lightly compressed versions of the plates used to construct the GSC, is available on the CASB WWW server. Begin at
http://www-gsss.stsci.edu/support/phase2.htmland follow the links under ÒGeneral Guidelines.Ó Coordinates produced using these facilities will be in the same reference frame as the GSC.Here are some more detailed guidelines for different categories of brightness and type:
- Stars with m(V) between about 9 and 14 (at high galactic latitude) or between about 9 and 13 (at low galactic latitude): Most of these are contained in the GSC. You can retrieve GSC coordinates using a form available on the CASB Web server listed above. When you have the GSC coordinates, enter them in the Position field of the Target List, followed by the plate identification number (not the region number) from the GSC, using the format PLATEÐID = <plate identifier>. This identifies the target coordinates as coming from the GSC. Also enter the GSC name as a target alias in Alternate_Names.
RA = 12H 13M 14.27S +/Ð 0.3", DEC = Ð13D 11' 03.3" +/Ð 0.3", PLATEÐID=00TB
For stars brighter than about m(V) = 9, coordinates from the Hipparcos Input Catalogue have been added to the GSC; these coordinates can be recognized as having a plate designation of +056. Enter the GSC name as a target alias, but do not include the PLATEÐID for coordinates derived from the Hipparcos Input Catalogue (but do indicate in the target-level Coordinate_Source field that the coordinates are from Hipparcos). Note that coordinates from the Hipparcos Output Catalog are on a different system from the GSC; but the differences are negligible in almost all instances.
- Stars fainter than about m(V) = 13 Ð 14, and extended sources: The plate material that was used to construct the GSC will generally allow identification of targets to magnitudes of V = 19 in the northern- and J = 22 in the southern hemisphere. Use the facility and instructions available on the CASB Web server listed above to determine coordinates of targets on the GSC plates.
Once coordinates have been obtained, Position should again contain the plate ID from GSC, using the PLATEÐID = <number> format. Contact your Program Coordinator if you need assistance.
- Stars fainter than about m(V) = 13 Ð 14, and extended sources (for which special plates or ground-based CCD images were obtained): You must transfer the GSC reference frame to your special plates or CCD images. You will need about ten stars from GSC as your reference stars when carrying out plate reductions. Enter the coordinates in Position, again with the PLATEÐID = <number> format, to indicate which GS plate provided the coordinates used for the plate reduction.
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 plate material used to construct the Guide Star Catalog. You are responsible for verifying that the coordinates are correct. (See3.4.1Required 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 GSC is NOT the same one that is contained in the 102-volume set of CD ROMs distributed as the Digitized Sky Survey (DSS-I) and that is available elsewhere at the stsci.edu Web site. The DSS-I uses the 1950 epoch POSS-E plates for the north, whereas the GSC in the north is based on the 1982 epoch Palomar Quick V. Similarly, in a few cases, the southern plates used in the GSC are not the ones in the DSS-I. HST users may use the DSS-I CD ROMs to prepare their Phase II submissions only if they verify that the DSS-I plates are the same as those used in the GSC, i.e., the plate ID field must be identical.
3.4.7 Prohibition on the Use of GSC Version 1.2
Preliminary access to an improved (astrometrically recalibrated) GSC, version 1.2, has been provided via a WWW server. As this revision has significantly different astrometry from the GSC currently used by HST operations (version 1.1), GSC version 1.2 must not be used for Phase II target determinations. Note that all GSC CD ROMs in circulation are from version 1.1 or lower.
Uncertain or Unknown Coordinates
If it is impossible to obtain adequate plate material to measure coordinates to the required accuracy (e.g., 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 (see7.3.1Target Acquisition). In that case, coordinates as accurate as possible must be entered on the Target List.
3.4.8 Special Catalogs
The option PLATE-ID can be used to specify one of the special guide star catalogs which have been created by the STScI to accommodate special guide star requirements. These catalogs are primarily used for selecting guide stars from a subset of stars in the region which have certain properties or more accurate relative positions. This option is most commonly used when the relative accuracy of guide stars and target stars must be known with great precision.
The Special Catalogs currently available are:
ZZZZ NGC188 ZZZX Iota Carinae ZZZW HR6636 ZZZV HR6850 ZZZU HR6945 ZZZT NGC5617 ZZZR Omega Centauri ZZZQ Eta Carinae
Only a modest number of specific objects are listed in these catalogs; contact your PC. Note that any proper motion for objects on these plates has been applied through the year 2000, and so an Epoch of 2000 should be specified
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Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |
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