HSpot deals with two fundamental types of target: fixed targets and solar system objects.
A fixed target is any object that does not require a differential tracking rate. This can be a star, a galaxy, an AGN, etc. Herschel works with Equatorial J2000 coordinates and only target entry in Equatorial J2000 will be accepted (this is to facilitate checks for duplicate pointings [two or more users requesting similar observations of the same object, or region of the sky], which are extremely complicated if many coordinate systems are used for target entry). If the source is known to NED or SIMBAD these coordinates are used, if not, the user must enter a J2000 R.A. and Dec. On some occasions, for nearby stars, the proper motion of the target may become important; this can be entered in HSpot if necessary, once again, the epoch must be in 2000 coordinates. All fields can be edited after name resolution.
Some types of observation can be very unforgiving with the coordinates, particularly PACS spectroscopy and HIFI observations in the high frequency bands (Bands 6 and 7) are sensitive to poor pointing accuracy and may be affected if the quality of the position of a fixed target is low. Many faint far infrared targets do not have good coordinates and the optical position and far infrared position of a particular target may not necessarily coincide.
Typical problems found when entering user-defined coordinates for targets are missing "minus" signs in declination and, where several positions are available for a target, picking an inaccurate one.
|Caveat Emptor! The PI is responsible for the accuracy of submitted coordinates and for ensuring that the coordinates supplied are good enough to obtain the requested data. Observations that fail because of bad coordinates may only be repeated if the time required comes out of the proposal's HOTAC allocation.|
A moving target is a solar system object that requires a differential tracking rate to be programmed. On target entry the user should select the "Moving" tab and resolve the NAIF ID of the target name. The Herschel Observations Planning System will use the NAIF ID to calculate coordinates for the time of observation and to calculate the differential tracking rate required, which should be less than 10 arcsec/minute at the date of observation (this limits the capability of Herschel to see objects passing very close to the Earth, although faster rates up to 30 arcsec/min may be permitted, on a case-by-case basis, if scientifically justified). User entry of target coordinates is not permitted, as any solar system object with a reliable enough orbit to be observed by Herschel will have a NAIF ID.
Around 800 moving targets (satellites, comets, asteroids and TNOs) are in the HSpot database. More than a million have been catalogued, but it is obviously impractical to store all of them in HSpot as most are not observable by Herschel. If you wish to observe a solar system object that is not in HSpot you should send a Helpdesk ticket requesting that it be added. Allow a minimum of two or three working days for it to be included and the ephemeris to be linked to HSpot.
NAIF is NASA's Navigation and Ancilliary Information Facility. This offers an information system called SPICE for spacecraft navigation. SPICE uses a unique 7 digit identification code for all natural solar system bodies, while spacecraft are identified with a negative integer code. Because of the simplicity for this system of ID codes and given the increasing possibility of confusion of objects (for example, there are both planetary satellites and asteroids named Io, Ganymede and Dione and increasing numbers of asteroids are later found to show cometary activity and may receive multiple designations), it is increasingly used for telescope scheduling. A short summary of NAIF IDs is given in the relevant secion of the HSpot Users' Manual on the Standard Ephemeris for moving target entry.
When a Solar System Object has a well-controlled orbit of high accuracy (for a periodic comet this means two returns for which a successful linkage has been made, for a asteroid or minor body it usually means observations at a minimum of 6 or 7 oppositions, apart from Earth-crossing objects for which the criterion is typically 3) it will receive a number from the Minor Planet Center. A numbered comet has a designation such as 190P/Name, while an asteroid receives just a number. An unnumbered asteroid has a NAIF ID starting with a 3. Objects with such a designation have a relatively low accuracy ephemeris that may be considerably in error when extrapolated even a short time into the future. As an example, even an object with three oppositions may have a position that has a 3-sigma error of more than 60 arcseconds when extrapolated 5 years into the future. If the spread of observations is unfavourable, or there are few astrometric observations, it may not even be possible to obtain a good ephemeris extrapolation with a 3-opposition orbit. With 4 oppositions the 3-sigma error in the extrapolated position may still be greater than 20 arcseconds over 4 years. This means that faint objects that have not been observed recently may be difficult to locate and identify with Herschel and thus are high-risk observations. It also means that an object may not be centred on the detector so the resultant data quality may be defficient.
For comets, non-gravitational forces can make predictions quite inaccurate for returning objects, even when they are well-studied. Experience has shown that close approach comets always need ground-based astrometric support campaigns and late-time ephemeris updates to make observations possible with Herschel and that even updating the ephemeris with the latest data 4 days before the observations are to be executed (this is the latest possible time for update) and re-planning the observations may not be enough to avoid positional errors of several arceseconds between the expected and the actual position at the time of observation if no high-precision radar observations are available to tie-down the position with exactitude.
Three problems are present when there is uncertainty in the ephemeris. In approximate order of increasing importance these are:
Possible errors in the required tracking rate.
In general the tracking errors should be kept below 1 arcsecond during the observation. This can be a problem with long observations on objects moving at high velovity on a strongly curved track for which the interpolation of the position may not be good enough.
Difficulties with photometry
To be carried out successfully, the target must be centred in the array to within a certain level of accuracy. If this is not achieved, photometry may be difficult or impossible to obtain.
Problems with target identification
Not all Solar System Objects have suitably accurate ephemerids; occasionally there may be errors of tens of seconds of arc, minutes or even, for a few objects, degrees in the ephemeris position. In the HSpot Users' Manual a list of solar system objects included HSpot is given in which flags objects with deficient ephemerides at the time of writing. Always check though to see if a better orbit is now available.
In detail, the issues that users may find when ephemeris information is uncertain are:
In general this should not be a problem with distant objects, it may become a serious problem with more nearby ones, particularly Near Earth Objects where it may be difficult to keep the target accurately centred.
For PACS photometry, the source position must be known with high enough precision that it should fall within a bolometer matrix of 52x52 arcseconds. In practical terms this means that the following criteria of positional accuracy should be fulfilled.
-- For aperture photometry: 15 arcseconds.
-- For PSF fitting: < 10 arcseconds
For SPIRE the main consideration is that the FWHM of the detectors is 18 arcseconds and the jiggle amplitude 6 arcseconds: if the positional error is greater than the jiggle amplitude there will be light losses.
For HIFI it should be remembered that the smallest aperture (that of Band 7b) is 13 arcseconds, thus necesitating centering at the arcsecond level to avoid light losses.
Target identification problems
For numbered asteroids the ephemeris should be of sufficient precision in almost all cases.
For unnumbered asteroids and minor bodies it may be essential to take astrometry to refine the orbit before observations can be attempted with Herschel.
For numbered and ToO comets, recent astrometry may be essential, depending on the case. A numbered comet will almost invariably require post-recovery astrometry to refine the orbit before observation can be attempted and any close-approach object will need late-time astrometry. Recently discovered comets with a short orbital arc will also almost invariably require pre-Herschel observation astrometry to refine their ephemeris.
Standard practice at the HSC is to download the ephemerids for Solar System Objects from the JPL Horizons database every 4 weeks, as our planning is done in cycles of 2 weeks. This means that in an extreme case the ephemeris information for an object may be as much as 2 months out of date. Normally this does not matter, as the errors will be too small to be significant. If the orbit is well-defined, the error is usually almost entirely in the direction of motion, so a Solar System Object (asteroid or comet) will reach a given point in its orbit slightly advanced or slightly delayed with respect to the ephemeris prediction. For Main Belt Asteroids (MBAs) advances or delays of 10 or more minutes are not unknown; at a typical distance of observation of 2AU this translates into a very small error in the actual observed position on the sky.
For objects that come closer to the Earth and move more rapidly, the error in the ephemeris may be much larger, even though the absolute precision of knowledge the object's position may be an order of magnitude better than for an MBA if it is intensely observed. In one recent case an SSO was found to be more than 20 arcseconds different from the ephemeris prediction published only 2 months previously. For Near Earth Asteroids and comets an ephemeris may become effectively completely unusable in a week or less. Observers should be aware that it is their responsibility to ensure that there is sufficient knowledge of the ephemeris of a target for effective scheduling and warn HSC to take the necessary measures to schedule with the most up-to-date available information.
Normally observations are planned and sent to MOC for uplink to the satellite a minimum of 2 weeks in advance and more usually at least 3 weeks in advance. If knowledge of the object's position is likely to be insufficient at that time, the observer should request - in advance - that the observations be re-planned closer to the date of execution, taking advantage of a better ephemeris.
It is the responsibility of the observer to warn the HSC, via a Helpdesk ticket, of potential ephemeris problems that might affect scheduling of a Solar System Object. This should be done far enough in advance to be taken into account in the standard planning cycle. However, no re-planning of observations can be contemplated less than 4 days in advance of their execution; if the ephemeris is potentially not robust for 96 hours in advance of execution, the observations must be designed to be robust enough to compensate for any positional errors that may occur.
It is not sufficient to assume that the HSC will automatically spot all potential conflicts with Solar System Objects in advance.