Stacking Techniques - Searching using NEOCP Variant Orbits

Keeping Track

The Minor Planet Center (MPC) posts details of newly discovered Near Earth Objects (NEOs) on the NEO Confirmation Page, so that other observers can attempt to confirm the existence of the objects and to help to refine their orbits. If only the discovery observations have been reported, or it has been seen for less than 24 hours, then the MPC will normally refer to the object as a '1 nighter' and will provide extra information to indicate the likely uncertainty in their prediction so that observers trying to find the object can search the right area of the sky.

The normal details given by the MPC consist of an ephemeris (a list of dates and corresponding positions) for the most likely place a new object is expected to be. The extra details for a 1-nighter consist of a map of the possible area in the sky that the object might be found (Fig. 1). Each dot on the uncertainty map is the result of a separate prediction by the MPC - a range of likely orbits is calculated, all of which fit the available observations within the limits of their expected errors and each separate orbit (or variant orbit) results in a dot on the uncertainty map. Using the NEO Confirmation page, ephemerides for any of the dots on the map can be generated to assist the observer trying to find the object. Each ephemeris will give a slightly different speed and direction of motion for the object. Sometimes there can be large differences in motion from one side of an uncertainty map to the other (see here).

Most NEOCP objects are relatively slow moving and often close to their predicted positions while others are fast moving with correspondingly larger uncertainties. Generally, only those with uncertainties larger than your field of view require special attention as described below.

Issues to overcome

Trying to find a fast moving object in an area of sky much larger than the field of view of your telescope & camera requires some care to ensure that the entire search area (the uncertainty map) is properly covered. In practice it is very easy to cover the same area more than once and to miss other areas completely, as the uncertainty area is moving with respect to the background stars and is also expanding as the hours go by (sometimes rapidly, see an animation here).

If the object itself is bright enough to be easily recorded by your equipment in single exposures, then the main benefit of the uncertainty maps is to help keep track of areas that have been searched. When the object is finally found, then the nearest variant orbit can be used to determine the speed and direction of motion reasonably close to reality so that stacking can be used to get a stronger image to report positions to the Minor Planet Center.

However, if the object is faint, either so that it would be difficult to notice on individual exposures, or so faint that it would only be recorded by stacking exposures together, then as well as helping keep track of the areas searched, the variant orbits also provide the likely speed and direction information critical to successful stacking. Sometimes there will be only small variations across the entire uncertainty map, but quite often there will be substantial differences, the speed varying by a factor of 2 or 3 or even more from one side to the other. Stacking with the appropriate values may make the difference between finding and not finding the object.

The method used at Great Shefford to conduct a search of a large uncertainty map is described below. Some elements are suitable for software automation but the basics are described here, using the NEO Confirmation page facilities and an Excel spreadsheet. 

Image capture, stacking (if required) and searching of the images is best done in real time, with the goal of trying to locate the object before the entire area has been searched.

Fig. 1 Example Uncertainty Map

Method

Getting the raw uncertainty map data:

• Generate the basic NEOCP ephemeris for the object to be searched for. Either enter your MPC observatory code or your observatory longitude/latitude/altitude and press the Get Ephemerides button. If present, the uncertainty information is available via links on the right hand of the page (Map/Offsets)

• Display the NEOCP Uncertainty Offsets in a web browser for a time at or just before when the search is to start. This will be the starting time of any variant ephemerides generated.

• Save the displayed list of offsets as a text file (e.g. File/Save As and Save as type 'Text File (*.txt)' in MS Internet Explorer). The file will be named similar to Uncertainty Offsets for object (yyyy mm dd_ddd).txt (where object is the temporary designation used on the NEOCP and yyyy mm dd_ddd is the date and time of the generated list of offsets)

• Open a spreadsheet and import the text file (Data/Get External Data/Import text file from within MS Excel). The columns imported are the RA and Dec offsets (in seconds of arc), the variant orbit number and the distance indicator character (! = between 0.05-0.01 AU from Earth, !! = 0.01-0.0027 AU and *** = nearer than 0.0027 AU (= the distance of the Moon).