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Non-Sidereal Targets

Content owned by Tom Seccull

Phase II: Requesting & Monitoring Observations

This web page is a guide for Principal Investigators (PIs), Contact Scientists (CSs), and NGOs supporting programs at Gemini with non-sidereal targets. Here you’ll find notes, tips, and advice that should help you to maximize the odds of success for your non-sidereal programs.

Useful Tools, Tips, and Websites

The JPL Horizons Ephemeris Service

Horizons is the ephemeris service offered by the Solar System Dynamics group at NASA’s Jet Propulsion Laboratory, and is used by the OT to fetch ephemerides for non-sidereal targets in Gemini programs. Horizons is capable of much more, however, and there are a number of parameters that may come in useful when designing observations and building observing programs at phase II. At several points this page will refer to numbered observer quantities that can be selected as outputs by Horizons (see screenshot below); on the Horizons webform they are listed when you click the link to change Table Settings. Note that descriptions of each output parameter printed from Horizons, along with the units used, are listed in the footer of an ephemeris when it is calculated.

Target Environment nonsidereal target

IAU Minor Planet Center (MPC) Observatory Codes

568 is the Maunakea MPC observatory code.
I11 is the Cerro Pachón MPC observatory code.

You can use these to designate Observer Location in Horizons instead of typing out Observer Location names in full. Useful if you’re in a hurry.

NASA Small Body Database

The Small Body Database is another service offered by NASA JPL that provides access to data related to asteroids, comets, centaurs, Trans-Neptunian Objects (TNOs), and interstellar objects. Contact scientists and NGOs may be most interested in orbital elements, orbit diagrams, and alternate designations.

Targets, Guiding, and Ephemerides

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 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 non-sidereal 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

Ephemeris Uncertainty

The fact that a non-sidereal target is in JPL Horizons, or that it is numbered (i.e. it has a 123456 MPC number in front of its provisional 2021 AB12 designation), doesn’t have much bearing on the quality of its ephemeris. When a minor planet is numbered by the MPC, the ephemeris is deemed good enough that the target is unlikely to be completely lost any time soon, but doesn’t guarantee that it will be easy to acquire or track, especially for spectroscopy. If an object isn’t numbered it is a very good idea to check its ephemeris uncertainty to make sure it will be where it is expected to be when observations are executed. This can be done by using Horizons to generate an ephemeris for the target covering the relevant timeframe (i.e. current semester or Fast Turnaround cycle) while selecting option 36 (3σ RA & DEC uncertainty in arcsec) in the Horizons Table Settings. Even if an object is numbered, it’s usually a good idea to check anyway, just to be sure.

Ephemeris uncertainty below 1" in RA and DEC is great, and uncertainty below 2" should be fine for spectroscopic acquisitions. Acquisition and maintaining tracking for spectroscopic observations may be more difficult for targets with larger uncertainties; these targets may require finding charts that show the uncertainty ellipse of the ephemeris to give the night crew a better idea of where to look when acquiring, especially if the target is faint. For an example see below. This is a finder chart for a slow moving Trans-Neptunian Object, marked with its changing position, a reference coordinate, and ±3σ position error ellipses taken from the output from Horizons. Something like this may be useful to the night crew when acquiring a non-siderel target with an uncertain ephemeris, especially if the field is crowded

OT Target Environment Ephemeris Entry

Maintaining precise tracking during long spectroscopic sequences targeting non-sidereal objects with high ephemeris uncertainy can be an issue. In this case the night crew should be reminded to monitor the flux levels in spectroscopic exposures as they come in. If the flux drops in a way that doesn’t correlate with changes in IQ or CC then the target may be drifting out of the slit, and a reacquisition may be needed. PIs of spectroscopic observations at risk of being affected by this should be encouraged to allow extra time for reacquisitions in their program, and notes should be put in the OT to inform the night crew whether, and how often, reacquisitions should be performed. If poor tracking is a possibility for their targets, PIs should be made aware of the risks that slit losses pose to the accuracy and quality of their data.

When imaging non-sidereal targets the ephemeris uncertainty can be much larger without causing problems. The only limit is that you should be able to put the target in the desired Field of View (FoV). Some PIs will want to ensure that the target is within a certain distance of the center of the FoV, which will define the maximum ephemeris uncertainty. Others may just want the target somewhere on the detector, which sets the upper limit on ephemeris uncertainty at its maximum for the instrument in question.

If a target has only just been discovered (e.g. a new interstellar object or hazardous near-Earth asteroid), its ephemeris uncertainty will be very large until enough of an observational arc has been measured to constrain its orbit. During this time, the ephemeris uncertainty may be so high that the target may even fall outside the predicted imaging FoV. If a PI wants to oberve such a target, advise them of the risk that their target may be missed. It is possible that they may still want to attempt the observation, which is ok as long as they know that they are risking their awarded time.

Bear in mind that a target’s RA uncertainty may be very different to its DEC uncertainty, so the region in which the target is expected to be is almost always an ellipse.

Sidereally Tracked Objects:

If observations of non-sidereal 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. Two crucial things need to happen to make these observations successful:

Peripheral wavefront sensor guiding with non-sidereal 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 non-sidereal 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 non-sidereal 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 non-sidereal 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 non-sidereal tracking:

GeMS does not currently support non-sidereal tracking.

The Limits of Guiding When Tracking Non-Sidereal Targets

Most non-sidereal targets will be moving slowly enough that they can be tracked for the full duration of their observation. The limits of the telescope’s and instruments' ability to maintain guiding while tracking a non-sidereal target should only become a concern if the target is moving very quickly, or if OIWFS is being used to guide non-sidereally (see above). To check whether it will be possible/reasonable to guide while tracking a non-sidereal target you need to know three things:

With this information in hand it will be possible to check if guiding is possible for a given instrument/telescope configuration. The minimum sequence time gives the minimum time needed between changes of guide star. Combined, the rate of motion and minimum time can be used to work out the minimum motion of the telescope needed to maintain tracking of the target. If that motion does not cause the average guide star to leave the patrol field of the guide probe, then it is likely that the observation can be performed while guiding; if it does, however, the PI may have to consider observing their target unguided, which will result in a decrease in the delivered S/N due to an increase in the delivered IQ. Observing unguided does have the benefit, however, of eliminating the need for added overheads due to changing guide stars and re-establishing guiding for each of them.

Unguided:

In certain fields and conditions a guide star may not be available, and observations of rapidly moving objects (>~10"/min) with PWFS2 may only permit guiding for very short periods before it is necessary to select a new guide star (which will incur additional acquisition overheads). In these situations, one may elect to execute the observation unguided (using either sidereal or non-sidereal tracking), with the caveat that the delivered image quality will suffer. To set up an unguided observation go to the Target component and use the green "+" to add an empty "Guide Group", and then change the Guide Group Name from "Manual" to "Unguided".

Phase II Considerations

Timing Windows

To get accurate observations of non-sidereal targets, they must be observed when they are not crossing or blended with background sidereal sources like stars and galaxies (this goes for both imaging and spectroscopy). Timing windows in the Observing Conditions component of a program in the OT are the best way for PIs and CSs to inform the Queue Coordinator (QC) and observers about when a non-sidereal object can be observed. This issue becomes more important for more distant objects, as they move more slowly. For the most distant TNOs it may take several hours to cross a background star, while for asteroids it may take only minutes. In the case of rapid objects, it’s possible to simply remove frames with contaminated data and use the remaining ones that were taken when the object was clear of background sources, so for rapidly moving objects in the inner Solar System, timing windows may not be required at all.

In practice, calculating and defining timing windows for non-sidereal objects can be a long and laborious process, especially if the PI wants to carefully set up timing windows for a large sample of targets over a large time period (e.g. a whole semester). The simplest low-tech way to get timing windows is to use the position editor in the OT to check whether the ephemeris of a target comes close to background sources and define timing windows based on that. This is extremely slow, and not necessarily very accurate; it should only realistically be considered for time critical DD programs where the OT only needs to be checked a handful of nights ahead. A better method, although often still slow, is to write a script that checks the minor planet’s ephemeris against stellar catalogs like Gaia. This works much better and can also provide a pre-formatted .tw file that the OT can ingest and read automatically. It may still be necessary, however, to do some manual checks, especially in the NIR where pesky redshifted galaxies start to become visible.

It's advisable for PIs to set up timing windows if they can, otherwise they run the risk of having their targets observed too close to a background source (potentially a very bright one) that spoils their science data. 

When observing planetary satellites, it is crucial to remember that timing windows must be set up to avoid observing the target when it is either behind, or transiting its parent planet. Option 12 in the Observer Table Settings of JPL Horizons (Satellite angular separ/vis) provides the angular separation of the center of the satellite and the center of the planet’s disk, along with a visibility code noting whether the satellite is transiting, partially eclipsed by, occulted by, or free and clear of its parent planet. Check the notes at the bottom of the output from Horizons to get details on what each specific code means.

Position Angle

There are a number of situations in which the PA of a non-sidereal observation may need to be carefully considered. In most cases, however, a PA of 0 for imaging or a PA aligned to the average parallactic angle for spectroscopy is sufficient. The following cases are ones where the PA might need to be set to something else:

Order Blocking Filters for GMOS Spectroscopy

Solar Calibrator Stars for Reflectance Spectroscopy

If a PI wants to get a reflectance spectrum of their target they will need to observe at least one sun-like calibrator star alongside their science target. If observing in the NIR, the solar calibrator star can take the place of the telluric standard (they are used in the same way) and be charged to partner time as long as the observation of the star doesn’t take any longer than a normal telluric standard would. At optical wavelengths the solar calibrator stars need to be charged to program time. Like telluric standards they should have a good match to the science target in terms of airmass. Most of the solar calibrator stars that are chosen by PIs are very bright. In the NIR this is usually not a big problem as bright telluric stds are also common. In the optical, however, the CS and PI must be careful to check that the standard star won’t saturate during the spectroscopic sequence. It’s better if the star doesn’t saturate during the acquisition either, but this can rarely be guaranteed. It’s ok to take the first step of the acquisition unbinned if observing with GMOS. Using the g filter is also a good idea for the solar calibrator acquisition as it has a lower peak transmission than most of the filters at longer optical wavelengths.

Non-Sidereal Target Conflicts

Minor Planets

Solar System minor planets often have multiple designations with different purposes and meanings, which can make it challenging to check the Gemini Observatory Archive (GOA) for target conflicts. If you need to search for a minor planet in the GOA, make sure you check each of the designations it has, as dif- ferent PIs might use different designations for the same object when defining their observations. As an example see the listing for the centaur Echeclus in the Small Body Database. Echeclus has a large number of designations, some are listed at the bottom of the Small Body Database page under the "Alternate Designations" section:

For some of the other designations the Small Body database listing for 2I/Borisov will be used.

Another letter designation you might see is "A/", which means that an object is asteroidal (as opposed to cometary). In some cases, for example if an asteroid is on a cometary orbit, it makes sense to designate a minor planet as asteroidal, even though asteroids by default are not designated this way. For example, 1I/’Oumuamua was provisionally designated A/2017 U1 because it was on a hyperbolic cometary orbit but showed no signs of cometary activity.

Natural Satellites

Natural satellites may also be referred to by multiple designations. These may include a name, an official designation, and a provisional designation, depending on when over the past few centuries it was discovered. This Wikipedia article goes into detail if you want to know more. NASA JPL records a list of all planetary satellites, their multiple designations, and their discovery circumstances.

For example the Uranian moon, Puck, was provisionally designated S/1985 U 1 and is officially designated Uranus XV, or UXV. The same system is applied to the moons of the dwarf planets, such that Haumea’s moon Hi’iaka was provisionally designated S/2005 (2003 EL61) 1 and is officially designated Haumea I. It’s also possible to designate Hi’iaka as S/2005 (136108) 1, using Haumea’s MPC number designation instead of its provisional one.

Non-Sidereal Check List