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Non-Sidereal Targets

Starting with the 2016B OT all nonsidereally tracked targets must be specified using ephmerides.

Non-sidereal objects may be specified in three ways, depending on whether the object is listed in the JPL/Horizons database, and whether non-sidereal tracking is desired. In all cases, the minimum non-sidereal target definition should include approximate coordinates for the object midway through the semester for queue planning purposes as well as an entry for the guide probe of interest. The nighttime observer will then 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.

Nonsidereally Tracked Objects in the JPL/Horizons database:

Non-sidereal objects listed in the JPL/Horizons database may be queried by the OT if an internet connection is active. In the "General" section of the Target component select "Nonsidereal Target" as the type, enter the object name, and click the magnifying glass to resolve the target name (objects should be referred by name rather than number if possible to minimize confusion). This will pop up a menu to select whether the target type is a Comet, Asteroid, or Major body (planet or satellite). If the target name matches multiple known objects then you will need to choose the correct one. The OT will remember the unique target identifier (listed below the name) for future coordinate updates. When the target has been uniquely identified the OT will download and store a low-resolution ephemeris from JPL/Horizons for queue planning purposes. The OT will automatically set the "Scheduling" time to the middle of the semester. If the observation has a single timing window you may want to set the Scheduling time to be inside that window, otherwise the nighttime observer will update the Scheduling time a few minutes before slewing the telescope, allowing the OT to automatically select a guide star. 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 a nonsidereally tracked observation of Titan appears below.

Target Environment nonsidereal target

Nonsidereally Tracked Objects not in the JPL/Horizons database:

Objects not yet cataloged by JPL/Horizons require a user-supplied 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.

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. The header shown below is required to tell the Telescope Control System (TCS) the frame of reference. The TCS expects the JD, RA, Dec and track rates to start at (zero base) columns 19, 40, 54 and 69. The RA hours and Dec degrees should be zero-padded ("% 02d"). There must be no "60.000" in the RA or DEC seconds columns, and there must be no blank lines or lines with text between $$SOE and $$EOE (e.g. ">..... Daylight Cut-off Requested .....<"). Ephemeris files may skip daylight hours to minimize the size of the file, which has a maximum length of 1440 lines. An abbreviated example is displayed below, and a full-length example ephemeris file may be downloaded here: Titan.eph.

***************************************************************************************
 Date__(UT)__HR:MN Date_________JDUT     R.A.___(ICRF/J2000.0)___DEC dRA*cosD d(DEC)/dt
***************************************************************************************
$$SOE
 2013-Jan-01 16:00 2456294.166666667 Am  14 30 58.5670 -12 25 00.360 8.861123  -2.58933
 2013-Jan-02 15:00 2456295.125000000  m  14 31 13.2425 -12 25 56.391 9.525960  -2.34342
 2013-Jan-02 16:00 2456295.166666667 Am  14 31 13.8926 -12 25 58.733 9.522656  -2.32523
 2013-Jan-03 15:00 2456296.125000000  m  14 31 29.7369 -12 26 49.967 10.32543  -2.19019
 2013-Jan-03 16:00 2456296.166666667 Am  14 31 30.4417 -12 26 52.159 10.32590  -2.17690
 2013-Jan-04 15:00 2456297.125000000  m  14 31 47.5724 -12 27 41.351 11.13377  -2.15827
 2013-Jan-04 16:00 2456297.166666667 Am  14 31 48.3324 -12 27 43.514 11.13236  -2.14981
 2013-Jan-05 15:00 2456298.125000000  m  14 32 06.6554 -12 28 33.383 11.82130  -2.23958
 2013-Jan-05 16:00 2456298.166666667 Am  14 32 07.4622 -12 28 35.631 11.81295  -2.23534
 2013-Jan-06 15:00 2456299.125000000  m  14 32 26.6930 -12 29 28.560 12.27522  -2.41416
 2013-Jan-06 16:00 2456299.166666667 Am  14 32 27.5303 -12 29 30.984 12.25568  -2.41308
 2013-Jan-07 15:00 2456300.125000000  m  14 32 47.2249 -12 30 28.771 12.40569  -2.65160
 2013-Jan-07 16:00 2456300.166666667 Am  14 32 48.0707 -12 30 31.433 12.37181  -2.65221
 2013-Jan-08 15:00 2456301.125000000  m  14 33 07.6681 -12 31 35.057 12.15404  -2.91218
 2013-Jan-08 16:00 2456301.166666667 Am  14 33 08.4963 -12 31 37.980 12.10425  -2.91263
 2013-Jan-09 15:00 2456302.125000000  m  14 33 27.3769 -12 32 47.412 11.50484  -3.14873
 2013-Jan-09 16:00 2456302.166666667 Am  14 33 28.1603 -12 32 50.570 11.43964  -3.14695
 2013-Jan-10 15:00 2456303.125000000     14 33 45.7250 -12 34 04.650 10.49977  -3.31122
 2013-Jan-10 16:00 2456303.166666667 Am  14 33 46.4393 -12 34 07.967 10.42232  -3.30518
$$EOE
***************************************************************************************

To define a nonsidereal target with a user-supplied ephemeris select "Sidereal Target" in the Target "Type" pull-down menu, and enter the name of the ephemeris file as the target (e.g. "Uncataloged.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 with a user-supplied ephemeris file appears below for Titan.

OT Target Environment Ephemeris Entry

Sidereally Tracked Objects:

If observations of nonsidereal objects are desired with tracking at the sidereal rate, then the target component must 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 and time, J2000 RA and 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 "User" target may be created for the observer to provide the correct coordinates.

Peripheral wavefront sensor guiding with nonsidereal tracking:

For non-sidereal tracking and guiding with the peripheral wavefront sensors, the target should be specified as discussed above, and PWFS2 must be selected in the "Auto Guide Search" drop-down menu. If the target is specified with a user-supplied ephemeris please make sure that the RA and Dec are updated for a point in the middle of the semester. The nighttime observer will update the coordinates prior to the observation and AGS will select a new guide star. When using the peripheral wavefront sensors with GMOS, it is likely that some portion of the field will be vignetted by the probe arm, so please specify the clear aperture required in a note to the observer.

Altair guiding with nonsidereal tracking:

For non-sidereal tracking and guiding with the peripheral wavefront sensors, the target should be specified as discussed above, and AOWFS must be selected in the "Auto Guide Search" drop-down menu. If the target is specified with a user-supplied ephemeris please make sure that the RA and Dec are updated for a point in the middle of the semester. The nighttime observer will update the coordinates prior to the observation and AGS will select a new guide star. If guiding on a nonsidereal object the guide star must be defined manually. Use the green "+" to add an "Altair AOWFS" target and configure it as described above. If guiding on the science target, the AOWFS target should be the same as the Base target.

GMOS & Flamingos-2 OIWFS guiding with nonsidereal tracking:

The GMOS & Flamingos-2 OIWFS have severe limitations for tracking non-sidereal objects, and its use is not appropriate for fast-moving objects. Electronic non-sidereal tracking with the 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.

GeMS with nonsidereal tracking:

GeMS does not currently support nonsidereal tracking.