Observation target specification

Observation target input

Observational targets and calibrators are grouped in observational catalogues provided by the requesting astronomer or constructed in collaboration with MeerKAT astronomers.

The minimum requirement is a comma-separated file that contains a list of targets, with or without calibrators: <catalogue_name>.csv

The structure of each target specified in the input catalogues can be assumed to be:

• Target name [optional]
• Tags defining target coordinate type
• Target X location
• Target Y location
• Flux model [optional]

Note: All information provided in the catalogue file must be standard text, no unicode characters are allowed.

Target name

User free form string associated with the target. Sometimes a target may have a number of names associated and these can be listed using the '|' to separate names. *J0025-2602 | PKS 0023-26 | OB-238 The asterisk, '*', character indicate the preferred target name to use. Important to note is that the standard naming convention assumed is the J format and some auto tasks may not run if a different naming convention is selected as preferred target name.

Tags

A list of tags associated with the provided target coordinates. These are generally provided by the proposing astronomer for pointing information. The first tag is always the format of the coordinates provided: radec or azel or gal J2000 is assumed as default epoch.

The following calibrator tags may appear in submitted/existing catalogues: delaycal, gaincal, bpcal, fluxcal, polcal, pointingcal. Important to note is that these tags are used by the telescope pipelines and some MeerKAT processing software require the delaycal tag for secondary gain/phase calibrators.

Target location

Targets can be equatorial, horizontal or galactic.

• Equatorial coordinates
  • X = ra (right ascension) -- given as HH:MM:SS.f
  • Y = dec (declination) -- given as [sign]DD:MM:SS.f
• Horizontal coordinates
  • X = az (azimuth angle) -- given as an angle in degrees
  • Y = alt (altitude angle) -- given as angle above horizon in degrees
• Galactic coordinates (Heliocentric)
  • X = l (galactic longitude) -- given as an angle in degrees
  • Y = b (galactic latitude) -- given as an angle in degrees

It is important to note that the target coordinates must be astrometric. That means the coordinates for the epoch specified. Maximum northern pointing angle for MeerKAT is +33.

Specials such as planets are simply Sun, special and not discussed further. The full list of special object accepted by katpoint: specials = ['Sun', 'Moon', 'Mercury', 'Venus', 'Mars', 'Jupiter', 'Saturn', 'Uranus', 'Neptune']

For objects such as comets a heliocentric elliptical orbit is assumed with the orbital position from as specified through the XEphem EDB database. Note: since the orbital parameters are added to a comma separated string for katpoint, the commas in the ephemeris specifications are replaced with tildes, ~, when adding it as a target to a CSV catalogue or YAML observation file.

C/2002 Y1 (Juels-Holvorcem)~e~103.7816~166.2194~128.8232~242.5695~0.0002609~0.99705756~0.0000~04/13.2508/2003~2000~g  6.5~4.0

Notes on observation behaviour

Fundamentally AstroKAT assumes and schedules targets in equatorial (ra, dec) coordinates with delay tracking enabled. Thus, internally,

• galactic (l, b) coordinates will be converted to their equatorial (ICRS) coordinates when observing,
• as well as all scan type observations specifying targets in horizontal (az, el) coordinates.

The conversion of scan type targets to equatorial coordinates are a special case. In general, horizontal coordinate specifications will be submitted as is, since the anticipated behaviour is pointing at a fixed sky location and waiting for the sky to drift by. Thus, the concept of a phase centre does not exist when specifying horizontal coordinates and these pointings will not have delay tracking corrections.

An exception to the rule has been identified for the scan type observations which may be specified as horizontal target coordinates, but require delay tracking since the telescope is moving across the target. In order to accommodate this specification, all targets for scan type observations will be converted to (ra, dec) coordinates internally to ensure delay tracking corrections.

Example Targets

Targets are implemented using katpoint, based on PyEphem

target = 'J1331+3030 | *3C286, radec bpcal polcal, 13:31:08.288, +30:30:32.959'
target = katpoint.Target(target)
target.__dict__
{'aliases': ['J1331+3030'],
 'antenna': None,
 'body': <ephem.FixedBody '3C286' at 0x1088e22f0>,
 'flux_freq_MHz': None,
 'flux_model': None,
 'name': '3C286',
 'tags': ['radec', 'bpcal', 'polcal']}

target ='G328.24-0.55, radec B1950, 15:54:06.11, -53:50:47.0'
target = katpoint.Target(target)
target.__dict__
{'aliases': [],
 'antenna': None,
 'body': <ephem.FixedBody 'G328.24-0.55' at 0x1088e2450>,
 'flux_freq_MHz': None,
 'flux_model': None,
 'name': 'G328.24-0.55',
 'tags': ['radec', 'B1950']}

Known issues

Target names or coordinates can potentially have issues if hidden or special characters were captured during a copy and paste process. Simple validation is by parsing each target provided as a katpoint.Target and evaluating success as shown in the examples above.

Most important fields to verify is that a ephem FixedBody object could be created, which means that the coordinates could be read and parsed, as well as the tags has been assigned correctly.

target.body
<ephem.FixedBody '3C286' at 0x1088e25b0>

target.tags
['radec', 'bpcal', 'polcal']

Errors can be difficult to identify, but the following list provides the most common:

• Spaces can be replaced with short space, long space or tab characters
• Name(s) specification can contain the '|', '*' and '-' characters prone to be replaced with special characters by word processing software.
• Target declination in the southern hemisphere requires a negative sign,'-', and this is most common replaced by the double dash '--' character and difficult to see the difference when reading the target. Also when C&P the negative sign of the declination coordinate can be lost, meaning the target could potentially be unreachable if it now gives a far northern target declination.
• Spelling errors can be provided by selecting from a list of tags

Most common issues slip in when provided targets are copy and paste from wordprocessors or especially from PDF documents.