Uncertainty maps
Newly discovered
fast moving objects that are unconfirmed (i.e. those only reported by the discoverer) may have been
observed for a short period of time, in many cases for less than an hour. The
longer they go unobserved after discovery, the more uncertain the current prediction
gets.
When such objects are posted on the
NEO Confirmation Page they have extra information provided to help the observer
gain an idea of the likely area on the sky that the object may lie in. This
is given in the form of an
uncertainty map and also the RA & Dec offsets used to draw the uncertainty map.
Fig.
1 Example Uncertainty Map
With this map an
observer can try finding the object, often by starting at the most likely place
where the RA and Dec offsets are both zero (the nominal position) and then
working outwards until the object is found.
However, each point on
the uncertainty map is the representation of an orbit that has been calculated
by the Minor Planet Center and each of these orbits fits the original discovery
positions within the expected accuracy of the discovery observations. The spread of positions shows that the
different orbits result in predicted positions (ephemerides) having different
rates of motion on the sky.
If the new object is
faint then the observer with a small telescope needs to take into account the
differing rates of motion of the object, using track & stack techniques, so that the
image of the new object is strengthened enough to be easily visible. The
ephemeris provided directly from the NEO Confirmation Page are calculated for
the nominal position only and the rate of motion at the extremities of the
uncertainty map can be significantly different (faster or slower motion) and can
make the detection very difficult or
even impossible if the object is far from the nominal position.
Difficulties using Uncertainty Maps
An observer searching
for an object using an uncertainty map faces a number of issues:
-
The area to be
searched may be
much larger than the field of view of the CCD camera, careful planning of
the search area may be needed.
-
The object may be
moving fast, making it difficult to ensure accurate coverage of
the whole area. Offsetting from the nominal position can be prone to
mistakes, especially as the distance away from the nominal position increases.
-
The uncertainty
area expands with time, sometimes appreciably in just a few hours
-
The difference in
apparent speed from one side of the uncertainty area to the other may be
significant, ideally the observer needs to take this into account when
stacking if
searching for faint fast moving objects.
| Here is a 24h animated
uncertainty map of an object discovered by LINEAR that appeared on the NEO
Confirmation page in May 2004.
Discovered at about 05h UT on May 24th at mag +18.8,
moving at 34"/min it was added to the NEOCP at 18:30 UT as a mag
+19.3 object. By 21:00 UT the speed at the nominal position was
24"/min and the animation covers the 24 hours from then onwards.
The North-South spread of the uncertainty area at 21:00
UT on 24th is about 1.5° and 24h later has expanded to more than 7°
long.
Each square in the grid represents a 20'x20' area,
slightly smaller than the field of view of the CCD camera at Great
Shefford (the size used during searches to ensure some overlap between
adjacent search areas).
As can be seen, the area grows rapidly in size and
demonstrates some of the issues described above. This particular object
was lost, no other observatories managed to locate it before it faded from
view. |
Fig 2.
|
Variant Orbits
In January 2004 the
Minor Planet Center started to provide ephemerides (including the speed and
direction of motion) for each point on the uncertainty map, not just for the
nominal position. These ephemerides, calculated for each of the 'variant' orbits used to
create the uncertainty map are accessed by clicking on the Offsets link under
the Uncertainty column of the NEOCP ephemeris (see Fig.
3). Note that if an object has been
picked up and reported after the discovery then the orbit will be far better
known and the uncertainty column will be removed by the Minor Planet Center, so
not all objects on the NEO Confirmation Page will have uncertainty maps and
offsets given.
As an example, the
uncertainty map in Fig. 1 was provided by the MPC for an object
assigned a temporary designation of AQ97025, discovered by LINEAR on 2004
Oct 12 at about 9h40m UT. This was first posted on the NEO Confirmation Page just
over 12 hours later at 21h51m UT and the uncertainty map shows the likely
range of positional uncertainty at midnight just over two hours after that.
As can be seen, the extent of the uncertainty area is about
2,500" in RA and 3,500" in Declination (or approximately 2/3° x 1°),
already quite a large area.
With the equipment at Great Shefford giving a field of view of
slightly less than 1/2° square this area would already involve taking three or
four different fields to cover the whole area adequately.
| As mentioned earlier, each dot
on the map represents an independent orbit and has its own associated
speed and direction of motion. When stacking
images these different speeds need to be taken into account. Depending on
how quickly the numbers vary it may even be required to use different
values to stack different parts of the image within a single field of view. |
Note: A position at coordinates 0,0
on the map is regarded as the most likely place to find the object (the
nominal position), but there is a possibility that the object could be
found anywhere on the map. Indeed, sometimes objects with unusual motion
or with uncharacteristically poor discovery positions do lie completely
outside the uncertainty map. See Major
News About Minor Objects issued 2004 March 17th for an example when
2004 FH was found to be well outside its uncertainty area, 44 hours
before passing by Earth just 4 Earth diameters away. |
|
