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Non-Sidereal Targets

Non-sidereal objects can be specified in a number of ways, depending on the type of object and whether non-sidereal tracking is required. Although the guidelines below should cover most cases, PIs of non-sidereal objects are encouraged to communicate with their National Gemini Office (NGO) representative and their Contact Scientist (CS) early in the Phase II process to discuss the best way to support their observations. In all cases, the minimum non-sidereal target definition should include dummy coordinates for the object midway through the semester for queue planning purposes as well as an entry for the guide probe of interest. Upon observation, the observer will update the object coordinates and choose a guide star based on the current coordinates. Nearly all non-sidereal objects can be supported --- we have observed Near-Earth Asteroids closer than the moon.

Sidereally Tracked Objects:

If observations of non-sidereal objects are desired with tracking at the sidereal rate, then the target component should be set up as a standard J2000 sidereal target. An ephemeris for the target should be included in a note to the observer including UT date/time, J2000 RA, Declination and any other information relevant to the observer (for instance magnitude for objects with large flux variations). The observer will use this note to determine the object position and select a guide star at the time of observation. If the object is recognized by Horizons, then a dummy target may be created for the observer using the "Orbital Elements" method.

Orbital Elements Method (Horizons-Recognized Objects):

Non-sidereal objects with orbital elements supplied by Horizons can be queried by the OT through an interface with JPL/Horizons if an internet connection is active. Examples of such targets are numbered objects or objects with provisional designations. Note that orbital elements are not recommended for Near Earth Asteroids because the algorithm used by the telescope to determine object position is not sufficiently high precision to observe targets passing extremely close to the Earth. Objects should be referred by name rather than number if possible to reduce the chance of improper number input (causing observation of the wrong target) and to reduce confusion with low-numbered objects such as (1), which can alternately describe both Ceres and Mercury. PIs should also update the RA and Dec to a position near the middle of the semester for queue planning purposes.

To define a non-sidereal target, use the Target "Type" pull-down menu (which defaults to "Sidereal Target") to select either "JPL minor body (Comet)" or "MPC minor planet (Asteroid)". Enter the target name or number into the "Name" field and hit <enter> or click the magnifying glass to query the JPL/Horizons database. This will return orbital elements and the coordinates of the target at the specified date and time. It is up to the PI to double-check orbital elements retrieved by the query tool and to ensure the object's ephemeris is sufficiently well known for observation. Tracking will occur at the non-sidereal rate, although please be sure to look below for non-sidereal tracking details for your guide probe, particularly limitations of the GMOS OIWFS. An example of non-sidereal tracking of a numbered object appears below for the asteroid Ceres using the MPC minor planet (Asteroid) format:

OT Target Environment Orbital Elements Entry

Ephemeris Method (Non-Sidereal Objects Not in the Horizons System or Moons):

Objects not yet cataloged by JPL/Horizons, or objects not in Heliocentric orbit, are currently not supported by the above query tool. Sidereal tracking (see above) is the favored method for observing these objects if that is scientifically viable. If non-sidereal tracking is required, then we will need a machine-readable ephemeris which needs to be produced in a very specific format and should be tested by the observatory prior to observation. This method requires additional operational resources for testing so please contact your NGO and CS if you suspect that you will need this functionality, as it is a mode not yet well supported by the OT. We prefer the use of the machine-readable ephemeris to user-computed orbital elements because we do not have the ability to test third-party fitting methods nor the ability to translate between different orbital element formats (such as Barycentric versus Heliocentric). In addition, the machine-readable format allows the tracking of any object path on the sky.

The ephemeris format is: Date(UT) HR:MN JD(UT) R.A. DEC dRA/dt*cosD d(DEC)/dt, where R.A. and DEC are J2000 astrometric right ascension and declination of the target center adjusted for light-time, and dRA/dt*cosD and d(DEC)/dt are the rate of change of target center apparent RA and DEC (airless) in arcseconds per hour. If generating the ephemeris file from Horizons be sure to specify high precision output. It is also recommended to skip output during daylight to minimize the size of the file. Note that Horizons occasionally produces 60.000 in the RA or DEC seconds column. This is not allowed by our telescope control software, and lines with seconds = 60.000 must be removed. Our software also requires that there are no blank lines or lines with text between $$SOE and $$EOE (e.g. ">..... Daylight Cut-off Requested .....<"). Finally, the maximum ephemeris length is 1440 lines. An an abbreviated version is displayed below, and a full-length example ephemeris file may be downloaded here: Titan.eph.

Target body name: Titan (606)                     {source: SAT351}
Center body name: Earth (399)                     {source: DE405}
Center-site name: Mauna Kea
Start time      : A.D. 2013-Jan-01 00:00:00.0000 UT      
Stop  time      : A.D. 2013-Jan-10 00:00:00.0000 UT      
Step-size       : 60 minutes
Target pole/equ : IAU_TITAN                       {East-longitude -}
Target radii    : 2575.0 x 2575.0 x 2575.0 km     {Equator, meridian, pole}    
Center geodetic : 204.527800,19.8261152,4.2078485 {E-lon(deg),Lat(deg),Alt(km)}
Center cylindric: 204.527800,6006.35451,2151.0229 {E-lon(deg),Dxy(km),Dz(km)}
Center pole/equ : High-precision EOP model        {East-longitude +}
Center radii    : 6378.1 x 6378.1 x 6356.8 km     {Equator, meridian, pole}    
Target primary  : Saturn                          {source: DE405}
Interfering body: MOON (Req= 1737.400) km         {source: DE405}
Deflecting body : Sun, EARTH                      {source: DE405}
Deflecting GMs  : 1.3271E+11, 3.9860E+05 km^3/s^2                              
Atmos refraction: NO (AIRLESS)
RA format       : HMS
Time format     : BOTH
EOP file        : eop.121031.p130122                                           
EOP coverage    : DATA-BASED 1962-JAN-20 TO 2012-OCT-31. PREDICTS-> 2013-JAN-21
Units conversion: 1 AU= 149597870.691 km, c= 299792.458 km/s, 1 day= 86400.0 s 
Table cut-offs 1: Elevation (-90.0deg=NO ),Airmass (< 2.000=YES),Daylight (YES)
Table cut-offs 2: Solar Elongation (  0.0,180.0=NO )                           
 Date__(UT)__HR:MN Date_________JDUT     R.A.___(ICRF/J2000.0)___DEC dRA*cosD d(DEC)/dt
 2013-Jan-01 16:00 2456294.166666667 Am  14 30 58.5670 -12 25 00.360 8.861123  -2.58933
 2013-Jan-02 15:00 2456295.125000000  m  14 31 13.2425 -12 25 56.391 9.525960  -2.34342
 2013-Jan-02 16:00 2456295.166666667 Am  14 31 13.8926 -12 25 58.733 9.522656  -2.32523
 2013-Jan-03 15:00 2456296.125000000  m  14 31 29.7369 -12 26 49.967 10.32543  -2.19019
 2013-Jan-03 16:00 2456296.166666667 Am  14 31 30.4417 -12 26 52.159 10.32590  -2.17690
 2013-Jan-04 15:00 2456297.125000000  m  14 31 47.5724 -12 27 41.351 11.13377  -2.15827
 2013-Jan-04 16:00 2456297.166666667 Am  14 31 48.3324 -12 27 43.514 11.13236  -2.14981
 2013-Jan-05 15:00 2456298.125000000  m  14 32 06.6554 -12 28 33.383 11.82130  -2.23958
 2013-Jan-05 16:00 2456298.166666667 Am  14 32 07.4622 -12 28 35.631 11.81295  -2.23534
 2013-Jan-06 15:00 2456299.125000000  m  14 32 26.6930 -12 29 28.560 12.27522  -2.41416
 2013-Jan-06 16:00 2456299.166666667 Am  14 32 27.5303 -12 29 30.984 12.25568  -2.41308
 2013-Jan-07 15:00 2456300.125000000  m  14 32 47.2249 -12 30 28.771 12.40569  -2.65160
 2013-Jan-07 16:00 2456300.166666667 Am  14 32 48.0707 -12 30 31.433 12.37181  -2.65221
 2013-Jan-08 15:00 2456301.125000000  m  14 33 07.6681 -12 31 35.057 12.15404  -2.91218
 2013-Jan-08 16:00 2456301.166666667 Am  14 33 08.4963 -12 31 37.980 12.10425  -2.91263
 2013-Jan-09 15:00 2456302.125000000  m  14 33 27.3769 -12 32 47.412 11.50484  -3.14873
 2013-Jan-09 16:00 2456302.166666667 Am  14 33 28.1603 -12 32 50.570 11.43964  -3.14695
 2013-Jan-10 15:00 2456303.125000000     14 33 45.7250 -12 34 04.650 10.49977  -3.31122
 2013-Jan-10 16:00 2456303.166666667 Am  14 33 46.4393 -12 34 07.967 10.42232  -3.30518

To define a non-sidereal target using the "Ephemeris" method, select "Sidereal Target" in the Target "Type" pull-down menu, and enter the name of the ephemeris file as the target (e.g. "Titan.eph"). Attach the ephemeris file to your program using the File Attachment tab of the Gemini Science Program component in the Program Editor. Tracking will occur at the non-sidereal rate, although please be sure to look below for non-sidereal tracking details for your guide probe, particularly issues with the GMOS OIWFS. An example of non-sidereal tracking using the ephemeris method appears below for Titan.

OT Target Environment Ephemeris Entry

Peripheral wavefront sensor guiding with non-sidereal tracking

For non-sidereal tracking and guiding with the peripheral wavefront sensors, the target should be specified with either orbital elements or a machine-readable ephemeris as discussed above. PIs should make sure that the RA and Dec are updated for a point in the middle of the semester prior to submission. A "dummy" guide star should be chosen which will be updated by the observer when the object is actually observed. At the time of observation, the actual guide star will be selected. When using the peripheral wavefront sensors with GMOS, it is likely that some portion of the field will be vignetted by the probe arm. Thus, the user should also specify in a note to the observer the clear aperture required.

Altair guiding with non-sidereal tracking

Altair can understand orbital elements or the machine-readable ephemeris and track at any non-sidereal rate. For guiding on a non-sidereal object with Altair, define the Base and the AOWFS as the same non-sidereal target. These coordinates can be computed using the JPL/Horizons query tool for numbered targets. As with all non-sidereal objects, PIs should include a note with an ephemeris for the target for the entire semester so the observer can verify the telescope position is correct.

GMOS OIWFS guiding with non-sidereal tracking

The GMOS OIWFS has severe limitations for tracking non-sidereal objects, and its use is not appropriate for fast-moving objects. Electronic non-sidereal tracking with the GMOS OIWFS can be performed for only 1 arcsecond of total motion on the sky, so science exposures (including readout) must be chosen to complete within this range of motion. Following each exposure, a dither must be performed using the offset iterator in order to "reset" the OIWFS star to the center of the OIWFS field of view. This should be done with the offset iterator. If this 1 arcsecond of motion is not sufficient due to the high rate of motion of the object or the use of long exposure times, then one of the peripheral wavefront sensors should be used instead.