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Non-Sidereal Targets

Content owned by Tom Seccull

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
 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 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:

  • The telescope pointing must be correct at the time of observation. This should go without saying, but remember that you cannot rely on the OT to point the telescope based on the Horizons ephemeris. The night crew must manually enter the coordinates listed in an ephemeris note (provided by the PI) into the observation’s target component before slewing.

  • The target must not be allowed to drift out of frame. To ensure the target isn’t lost the PI and CS should check how long the target is expected to be in frame when pointing at a fixed position in the sky. If this time is longer than he observation then all is well. If not, the CS should work with the PI to adapt the observing strategy. One possible solution would be to test whether slewing to the position of the target at the median time of the observation will keep it in frame for longer than if the target is centered in the field of view at the start. Alternatively, slewing to follow the target in steps could be done, but this will increase overheads which will eat into program time. For GMOS imaging observations the CS should also determine whether the PI would prefer to keep the target on the central CCD (this is often the case). For faster moving targets it may be possible to keep them on CCD2 for longer by rotating the PA of the observation to align with the direction of the target’s motion. A minor planet’s speed and direction of motion is output by Horizons as output parameter 47 (Sky motion: rate and angles, in units of "/minute and degrees counter-clockwise from North).

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:

  • How fast is the target moving? This is relatively easy to determine. A minor planet’s speed and direction of motion is output by Horizons as output parameter 47 (Sky motion: rate and angles, in units of "/minute and degrees counter-clockwise from North). Obviously it will not be possible to guide on faster targets for as long as those with lower rates of motion.

  • How long does guiding need to be maintained? In other words, what is the minimum amount of time required in the observation sequence between pauses to allow for adjustments of the guiding and selection of new guide stars? In practice the absolute minimum is the time needed to integrate and read out a single frame, but choosing a new guide star between every frame of a sequence is rarely practical. The time needed for the minimum sequence should be worked out by the PI and CS. The overhead time required for re-establishing guiding should also be considered when doing this.

  • How far can the target move before the guide star falls outside the guide probe patrol field? For non-sidereal targets it is very difficult to pre-emptively choose a guide star, so typically this distance needs to be estimated for each guide probe. For PWFS1 and PWFS2, a good average value is ∼3', but this distance may vary between 1'-10' depending on the location of the guide star within the patrol field. For GMOS OIWFS, non-sidereal tracking is acheived by electronic offsetting, which means that a star can move only up to 1” before the guide star begins to drift out of view of the guide probe.

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.


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:

  • You are imaging a target with an uncertain ephemeris with GMOS. - If the ephemeris of a non-sidereal target is uncertain it may be advisable to minimise the chance that the target will fall in the GMOS chip gaps by changing the PA. In these cases the PA setting may depend on whether the ephemeris is less certain in RA or DEC. If RA is less certain the PA should be aligned to either 90◦ or 270◦ from North. If the ephemeris is less certain in DEC a PA of 0◦ or 180◦ will be better.

  • You are sidereally imaging a rapidly moving target with GMOS. - If a rapidly moving target is being imaged with GMOS while tracking sidereally, it will be possible to keep the target on the central CCD for longer if the PA is aligned to the direction of motion of the target. The required angle is pro- vided by Horizons when observer quantity 47 (Sky motion: rate & angles) is selected. Adding these angles to the ephemeris note required for this observing mode may be beneficial.

  • You are targeting a specific part of an extended object for spectroscopy. - As with sidereal extended objects, the PI may want to ensure a certain part of an extended object (like a comet or planet) is in the slit. If a nonsidereal target is extended and it isn’t already obvious from the program’s proposal, it’s a good idea to check if the PI wants a specific PA.

  • You are spectroscopically observing a satellite of one of the planets. - If you’re getting a spectrum of a moon of one of the planets, it’s worth remembering that the planets are very bright and can have a significant halo of scattered light around them, so when a planet’s satellite is being tracked it is likely to be moving through this background (which varies as a function of distance to the planet on the sky); this means that the level, gradient, and curvature of the background along the slit (in the spatial direction) may vary from one frame to the next in the spectroscopic sequence. In turn this variation may make it difficult to subtract the sky using simple A-B dither pairs. This problem is most pronounced for satellites that are very close to their parent planet, as they move quickly through regions with a lot of scattered light. It’s possible to mitigate some of the background variation by trying to ensure that the slit is roughly aligned parallel to a line drawn between the satellite and the center of the planet’s disk (i.e. perpendicular to the edge of the planet’s disk). By doing this the variation in the background will not be eliminated completely, but it will be somewhat reduced, and should enable a cleaner sky subtraction later on.

Order Blocking Filters for GMOS Spectroscopy

  • If you’re observing a comet with GMOS at wavelengths longer than ∼750 nm, use of an order blocking filter should be considered in order to block out aliases of the strong CN, C2, and C3 emission lines that often appear in the spectra of comets in the range 380 < λ < 550 nm.

  • Almost all longslit observations of minor planets should use order blocking filters as needed to remove second order contamination at longer wavelengths. In very specific cases where the spectrum of the target is expected to be featureless AND neutral in color (i.e. not red), it is possible to get a reliable reflectance spectrum across a wide optical bandpass without an order blocking filter. In all other cases, however, use of an order blocking filter is strongly recommended to ensure that the shape of the final spectrum is accurate at wavelengths above 800 nm.

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:

  • Echeclus - Most minor planets are not named like this, but many prominent ones are.

  • 2000 EC98 - All minor planets have a provisional designation like this. They are assigned at discovery based on the date of discovery. See this Wikipedia page for details on how the designation system works, if you’re interested. Note that sometimes minor planet provisional desingations can be stylised as 2000EC98 or 2000 EC98. (Technically speaking the latter of these is the correct format). When checking the GOA its a good idea to search for the designation both with and without the space.

  • 60558 - These numbers are assigned to minor planets roughly sequentially by the MPC when their orbits are secured well enough that they are not likely to be lost any time soon.

  • 174P - Periodic comets (short period ones with origins in the Kuiper Belt, and orbital periods <200 yrs) are assigned this kind of designation (Echeclus has sporadic outbursts of cometary activity).

  • 2002 GJ27 - Some minor planets may have more than one provisional designation (those that do will have a primary provisional designation). This typically happens when a minor planet is lost and then rediscovered, or if it is independently discovered a second time before being determined to be an object that is already known. Usually it isn’t necessary to check these in the GOA as they are rarely used.

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

  • 2I - Interstellar objects have an I designation similar to the P used for periodic comets.

  • C/2019 Q4 - Long period comets (usually from the Oort Cloud) have provisional designations prepended with a "C/". When checking the GOA for these you may need to swap the / for "%2F". Again, make sure to try searching this designation both with and without the space.

  • (Borisov) - Comets are named after their discoverers, or the survey that discovered them. This means that many comets have the same survey name attached to them, so they have to be differentiated by their provisional designation.

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.

  • Official Designation - All natural satellites, after their orbits are well defined, are given an official designation which is the first letter of the name of their parent planet followed by a Roman numeral (because Galileo was a trendsetter like that). Today these numbers are incremented for every new satellite discovered around a planet, but in the past they designated the order of their orbits starting from the planet and moving outwards. This means that the order of discovery may not necessarily match the number a moon has if it was discovered before around 1900. Mercury has no known Moons, but if one is ever discovered its official designation will begin with "H" (for Hermes) to avoid confusion with the moons of Mars.

  • Name - Until 2009 it was the policy of the IAU to name all natural satellites, but now we’re getting very good at discovering the huge numbers of tiny ones orbiting the giant planets, so these days many moons are not (yet) named.

  • Provisional Designation - Like asteroids, comets, and TNOs, natural satellites are first given a provisional designation until their orbit is constrained. These have the form S/2021 U 1. The "S/" designation tells us it’s a satellite, and is immediately followed by the year of discovery, the first letter of the name of the planet that it orbits, and a number that increments each time a new moon is discovered at that planet in that year. Therefore S/2021 U 1 would be the first new moon of Uranus discovered in 2021.

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

  • Does the observation require a non-standard position angle? (i.e. not 0 or aligned to the average parallactic angle)

  • If the observation is GMOS longslit spectroscopy, is an order blocking filter defined?

  • Are timing windows needed to avoid observing the target when it is blended with background sidereal sources? If so, have these timing windows been defined?

  • Is the target in Horizons? If so, does the ephemeris stored in the target component of the OT span the relevant semester?

  • If the target’s ephemeris is not in Horizons, has the PI provded an ephemeris file? Has the ephemeris file been tested during daytime operations?

  • Is there any doubt about the uncertainty of the target’s ephemeris? If the target has an MPC number it is probably fine, but if it doesn’t this should definitely be checked and accounted for in the observing strategy.

  • If the observation is spectroscopy of a target with an uncertain ephemeris, does the PI want to periodically reacquire to keep the target in the slit? If so, leave a note in the OT to tell the night crew how frequently they should reacquire. If not, leave a note in the OT to tell the night crew that reacquisitions are not necessary.

  • Will it be possible to guide while tracking the target for the full duration of the observation? If not, does the observing strategy need to be adjusted? Have you placed notes to the night crew into the OT to inform them of how often they should reacquire with a new guide star?

  • If the observation is sidereally tracked imaging of a non-sidereal target, has the PI provided a coordinates note for the night crew? Has the speed of the target’s non-sidereal motion been accounted for in the observing strategy?

  • If solar calibrator stars are being observed in a spectroscopic program, do they have a good airmass match to the target? If observing in the NIR, and if the calibrator stars won’t take longer than a normal telluric standard would, they can be charged to partner time. If they will take longer, or if observing at optical wavelengths, they need to be charged to program time.