Pre-requisite: To follow this lecture, you will need a web browser connected to internet, a shell to launch scripts (e.g., bash, csh), Aladin, and TOPCAT. I. SkyBoT: Sky Body Tracker We will first train ourselves with the Sky Body Tracker (SkyBoT) tool. SkyBoT can list all Solar System Objects (SSOs) present within a given field of view at a given epoch. Alike all the services host at IMCCE (VO Solar System Portal, on vo.imcce.fr), SkyBoT requests can be submitted in different ways (URL, PhP, SOAP), and results can be provided in different format (votable, ascii, html). Please refer to the online help for more details at any moments: http://vo.imcce.fr/webservices/skybot/?documentation I.1 Request SkyBoT from Aladin - Launch Aladin - Retrieve the POSSII image (DSS2-red) at coordinates RA 03 22 33 DEC +12 58 48 (height/width of 15 arcminutes) NB: several ways for downloading this image: - Click on “open folder” icon at the top left corner of the Aladin window. Select the DSS tab (left hand side), and select ESO database. - In the “File” main menu, select “Load an astronomical image” / “DSS” and choose the ESO database. - As you can see, there are several trails present on the image. They correspond to moving objects, i.e., solar system objects. - In the Aladin Image Server (Ctrl-I), select SkyBoT in the list of servers (right column) - There are 7 fields, we will discuss hereafter each of them Target Radius Epoch Observer location Search For Max. uncertainty Display filter - First, let's see the native output of such a request. Click on the submit button. - Aladin created a new layer, interpreting the VO Table sent by SkyBoT in response to your request (this request uses the coneSearch utility: http://vo.imcce.fr/webservices/skybot/?conesearch). For each SSOs, its position, apparent motion, and name is displayed in a new layer by Aladin. - The 4 fields required for this results were Target, Radius, Epoch, and Observer location. Target specify the center coordinates of the field of view, here the coordinates of the POSS image. Radius is the size of the field of view on sky. Epoch is the mid-observing time, which is of course critical in our case of *moving* targets. Finally, the observer location gives the summit of the cone used for the search, i.e., the position of the observer, here geocenter (code 500). This location is generally given as a 3 characters code, called IAU observatory code: http://www.minorplanetcenter.net/iau/lists/ObsCodesF.html. If the observatory has no IAU code, you can simply enter its longitude and latitude. - This last parameter is critical owing to the finite (and small) distance of solar system objects compared to the distant stars. Hence, two different places on Earth would observe the same SSO at different coordinates at the exact same time. - Two other parameters restrict the cone search to categories of SSOs (asteroids only, asteroids and planets...), to objects with accurate ephemeris (with cuts on the maximum position uncertainty). The last parameter list the different quantities to be displayed (apparent motion, position uncertainty...). - As you surely saw, the predicted positions of the SSOs do not correspond to the trails. This is due to an incorrect datation of the photographic POSS plate. Change the epoch to 1991-10-06T09:47:13 (i.e., 10h before) and request SkyBoT. - The predicted positions of the SSOs now correspond to the trails. - Load the images from the STScl and CDS databases and check the observation dates. Two options for this: - [1] dropdown menu on the image (right hand stack) => “properties” - [2] select image layer (click on name) and then Main Menu Edit => FITS header. Depending on the database you selected, you get following the dates: - STScl: [1] none [2] DATE-OBS= '1991-10-05T09:72:00' /Observation: Date/Time - ESO: [1] 1991-10-06T19:47:13 (1991.762695313) [2] DATE-OBS= '1991/10/06 ' /UT date of Observation UT = '##################' /UT time of observation EPOCH = 1.9917626953130E+03 /Epoch of plate - CDS: [1] 1991-10-06T09:54:00 (1991.76156741958) [2] MJD-OBS = 48535.4125 / Modified Julian Date at start of observation EPOCH = 1991.76156741958 / Epoch in Julian Year at start of observation DATE-OBS= '1991/10/07 ' /UT date of Observation UT = '##################' /UT time of observation I.2 Request SkyBoT from HTTP - Let's now try the same request using another interface. We will not review here the query form (http://vo.imcce.fr/webservices/skybot/?forms) but concentrated on the http request (http://vo.imcce.fr/webservices/skybot/conesearch?) that allows an easy interface with any code. - As you can see in the online help (http://vo.imcce.fr/webservices/skybot/?conesearch), the syntax is straightforward, with each parameter provided in the url following: -parametrer1=value1&-parametrer1=value2&... - The results are then displayed in the browser. A simple way to interface the service with any of your code is to dump the result locally, e.g., using curl or wget commands: curl "request" or wget "request" - Try to construct the request url corresponding to the latest result you had in Aladin hereabove ----Solution---- RA = 03 22 33 = 50.595833 DEC = +12 58 48 = 12.980000 epoch = 1991-10-06T09:47:13 http://vo.imcce.fr/webservices/skybot/conesearch?-ep=1991-10-06T09:47:13&-ra=50.5958&-dec=12.98&-rm=14 - The native output of our services are VO Table. You can specify different output with the mime parameter, e.g., mime=text to output an ascii file. II. Miriade: Solar System Object Ephemeris Generator Miriade is the core of all ephemeris generated at IMCCE. It computes and provide general position and orientation ephemeris with the ephemcc and ephemph methods. Second order information such as rise, transit and set times of target can be requested (rts method), and visibility graphics produced (ViSION). A typical usage of our services would include using ViSION to find the best night to observe a given set of objects. Position with a fine time step would then be computed with ephemcc. The images acquired being then compared with SkyBoT & VizieR star catalogs. For objects with known spin axis and shape, ephemph describe their orientation on the plane of the sky at any epoch. We will concentrate here on the ephemcc method, using the http request. For the exercice, it may be easier to use the html output to examine the results, while many "real-life" usages would use the votable or ascii outputs and dump the results for local codes. For all this exercice, we highly recommend to seek information on the discussed parameters in the online documentation: http://vo.imcce.fr/webservices/miriade/?ephemcc As a good practice, please always fill the "from" parameter (e.g., &-from=B.Carry) for our statistics purpose. If you encounter an issue with our services, this flag will also help us in traking the request that failed. II.1 Where is Saturn? Find present coordinates of Saturn. Although SkyBoT may be more appropriate for such a request, it is the simpliest we can construct with ephemcc (see exercice I and more particularly SkyBoT Resolver method not discussed here). Only two or three parameters should be necessary: name, type, and ep. Answer http://vo.imcce.fr/webservices/miriade/ephemcc/?-name=6&-type=planet&-ep=now&-mime=html Note that Miriade recalls us the basic settings we implicitely used when making this simple request. The coordinates are given in the equatorial astrometric J2000 reference frame, from the geocenter. If you are not familiar with these definitions, see for instance (in French): https://media4.obspm.fr/public/AMC/pages_ephemerides/mctc-systemes-coordonnees.html II.2 Where will be Saturn? Can you now predict the coordinates of Saturn every hour for the next two days? Answer http://vo.imcce.fr/webservices/miriade/ephemcc/?-name=6&-type=planet&-ep=now&-nbd=48&-step=1h&-mime=html II.3 What time is it? The oldest clocks were all based on the apparent position of the Sun in the sky: the sundials. If you are familiar with them, you should know there is a difference between the time indicated by the Sun and that of our clocks (based on atomic vibrations) due to the non-circular orbit of the Earth and the tilt between the ecliptic plane and Earth equator. We will visualize this using TopCat. You will there dump the results locally to open them subsequently with topcat. Request the position of the Sun for a whole year, at a fix time every day, starting from January the first (e.g., at 2 pm) as seen from Paris. We want here the coordinates of the Sun in a local reference frame (azimuth/Elevation or hour angle/elevation). Answer http://vo.imcce.fr/webservices/miriade/ephemcc/?-name=Sun&-type=planet&-ep=2014-01-01T14:00:00&-nbd=365&-step=1d&-observer=007&-tcoor=3&-mime=votable II.4 The situation in 3-D We have seen how to generate the coordinates of objects on the plane of the sky. Miriade natively compute the cartesian coordinates in 3-D in the solar system and then converts them in local spherical reference frames. Miriade can therefore directly export the 3-D coordinates. Compute and display the close encounter between asteroid (99942) Apophis and the Earth in April 2029. For the same epochs, display (on the same graph) the position of the Moon. Answer: http://vo.imcce.fr/webservices/miriade/ephemcc/?-name=Apophis&-type=aster&-ep=2029-04-01T12:00:00&-nbd=500&-step=1h&-observer=500&-tcoor=2&-mime=votable http://vo.imcce.fr/webservices/miriade/ephemcc/?-name=Moon&-type=satel&-ep=2029-04-01T12:00:00&-nbd=500&-step=1h&-observer=500&-tcoor=2&-mime=votable II.5 From a certain point of view Until now, we mainly request ephemeris from the geocenter. To obtain accurate ephemeris, you will however need to specify your exact location on Eart. Objects in the solar system are much closer than the background stars, and parallaxe effects can be tremendous. What were the coordinates of asteroid Duende (aka 2012 DA14) during its close encouter with Earth (take 2013, February the 15th, at 11pm) as seen from two observatories in Europe (e.g., Paris and La Palma)? Answer: http://vo.imcce.fr/webservices/miriade/ephemcc/?-name=Duende&-type=aster&-ep=2013-02-15T23:00:00&-nbd=1&-step=1h&-observer=007&-tcoor=1&-mime=html http://vo.imcce.fr/webservices/miriade/ephemcc/?-name=Duende&-type=aster&-ep=2013-02-15T23:00:00&-nbd=1&-step=1h&-observer=950&-tcoor=1&-mime=html II.7 Observers and reference plane Ephemeris deals with dynamics. Because everything moves fast in the solar system (compared to most astrophyical objects), the choice of the center of the reference frame has strong implication. We will illustrate this here, together with the choice of reference frame itself. Compare the cartesian coordinates of Uranus in ecliptic and equatorial reference frame, as seen from the Sun and the Earth. You can use the 3-D plot utility of topcat for that. Answer http://vo.imcce.fr/webservices/miriade/ephemcc/?-name=7&-type=planet&-ep=now&-nbd=1000&-step=5d&-tcoor=2&-rplane=1&-mime=votable http://vo.imcce.fr/webservices/miriade/ephemcc/?-name=7&-type=planet&-ep=now&-nbd=1000&-step=5d&-tcoor=2&-rplane=2&-mime=votable http://vo.imcce.fr/webservices/miriade/ephemcc/?-name=7&-type=planet&-ep=now&-nbd=1000&-step=5d&-tcoor=2&-rplane=1&-observer=@sun&-mime=votable http://vo.imcce.fr/webservices/miriade/ephemcc/?-name=7&-type=planet&-ep=now&-nbd=1000&-step=5d&-tcoor=2&-rplane=2&-observer=@sun&-mime=votable III. Other services and Interoperability III.1 SkyBoT 3-D The SkyBoT service (see exercice I) is efficient because the ephemeris of all the solar system objects are pre-computed weekly, for the period 1899-2050 (see the Status method for more details: http://vo.imcce.fr/webservices/skybot/?status). The SkyBoT 3-D service is still under development, but you can already retrieve any epoch present in SkyBoT database, using the getAsterFile method: http://vo.imcce.fr/webservices/skybot3d/?getAsterFile Let's dump locally the current position on the plane the sky of all near-Earth asteroids using this service. We will manipulate the data in next section. Answer: http://vo.imcce.fr/webservices/skybot3d/getAster_query.php?-ep=now&-family=NEA&-limit=0&-mime=votable&-coord=spherical&-from=BCarry&-getFile=1 III.2 Interoperability We can now open the file containing the NEAs we generated above with TOPCAT. Such manipulation can be useful to display the histogram of their apparent magnitude for instance. Open Aladin and charge DSS (or any of the available all-sky colored catalog). In TOPCAT, select the NEAs table and broadcast it to Aladin. You should see a large band of the sky covered with all the known NEAs (~10,000). This layer is similar to the result of a SkyBoT request (see exercice I). III.3 ViSION When preparing proposals for observing time, it is always necessary to find the optimum date given our target list. Later, when preparing the night of observation, the relative position of the objects on the celestial sphere, their respective time of transit will dictate the pace of the night. The ViSION method has been designed for that purpose, from a list of target, a location on Earth, and a specified time span, the visibility of the targets is computed and displayed based on a series of threshold (e.g., elevation, magnitude). Use ViSION query form to find the planets observable tonight: http://vo.imcce.fr/webservices/miriade/?forms Search now a better epoch for the rest of the year to see several planet during the same night (use a time step of 2 weeks approximately).