OBS builds a plot that indicates when an asteroid is observable. Observability depends on declination, solar elongation, ephemeris uncertainty, magnitude, and galactic latitude. Each of these five parameters is plotted separately. At the bottom of the plot, windows of observing opportunity are indicated.
Using the various parameter plots, the user can see why an asteroid is not observable at a particular time. Armed with this information, the user can sometimes devise a way to get a required observation.
Cookies are used to store default values for the user. Defaults are stored for the following parameters:
The observatory code can generally be selected from the Observatory Selection List. If your observatory does not have an observatory code, select an observatory near you. If you do not know of observatories near you, just take the default (LONEOS at Flagstaff, Arizona, USA). The error introduced by this approximation should not be noticeable except in the case of an asteroid near the Earth.
The Chart Start Date is the UTC date when the plot begins. It should be in the format indicated on the web page, that is, yyyy mm dd, where y, m and d are digits indicating, respectively, year, month and day.
The timespan is the number of years for which you want observability information.
This value is the maximum distance from the zenith that you can point your instrument. It is used to set your declination limit.
This value is the minimum distance from the galactic plane that you can effectively observe. It is an average value and does not account for regions of the milky way in which there are dark patches. You should be aggressive with this parameter and select limits that go a few degrees closer to the milky way than you normally like to go.
The limiting V magnitude is the faintest V magnitude your instrument can image. As the magnitudes of many asteroids are not well known, you should be aggressive with this parameter, say by adding as much as a magnitude to what you think your telescope can detect.
The probability of imaging is the probability that the asteroid will be in your field of view (FOV) when your instrument is pointed at the nominal position of the asteroid. The probability depends on your FOV and on the ephemeris uncertainty, expressed in standard deviations, or sigma, of the asteroid. The value that you enter is the minimum acceptable probability that you will find the asteroid in your FOV. The value must lie between zero and one. Typically, one uses a value of 0.5 or greater.
Sigma is the standard deviation described in all statistics texts. In the context of asteroid orbits, the position of the asteroid is uncertain because the orbit depends on astrometric measurements that contain errors. Thus, the orbit calculation, almost always a least-squares fit, will reflect some of the measurement errors. Almost all the uncertainty lies along the line of variation (LOV). the orbital arc of the asteroid as seen from Earth.
Because we know where the LOV crosses your FOV, we can use Gaussian statistics to calculate the probability that you will see the asteroid. For example, if your FOV extends one sigma on each side of the nominal position of an asteroid, the probability of finding the asteroid is 68%. If it extends three sigma on each side of the asteroid, the probability becomes 99.7%
The software assumes a rectangular FOV, aligned in the RA and Decl directions, and requests both a width and height of the FOV. Note that the units are in arcseconds. A circular FOV can be handled by entering the FOV diameter in both the width and height field.
The graph produced by the web page will have time plotted on the horizontal axis. The vertical axes pertain to five separate panels: Declination, Ephemeris Uncertainty, Solar Elongation, V Magnitude, and Galactic Latitude.
Each panel will have a horizontal line indicating the limit you specified for your instrument or location. A dashed line indicates that the asteroid is not observable because it does not meet your specified criterion. A solid line indicates that that the parameter meets your criterion.
At the bottom of the graph is a panel that indicates windows of observing opportunity; that is, when the asteroid meets all your observing criteria. The windows are denoted by grey areas.
Suppose you find that an asteroid meets all your observing criteria except galactic latitude. It might still be possible to observe the asteroid but it would require that you take a few more images than normal so that you can find the asteroid separated from the surrounding stars. It might also require that you study the images longer to find the moving object.
If the object in need of observations is observable except that it is too far north or south, you would probably request that someone in the other hemisphere make the observation for you. Likewise, if only the object's faint magnitude is preventing a required observation, then you can request that someone with a more powerful instrument make the required observation.
When the Solar elongation is too small, there is generally nothing that can be done but wait.
Finally, if the uncertainty is too large, you can request observations from someone with an instrument having a larger FOV. Or you can go on a search campaign and take lots of images covering lots of sky.
If there are multiple reasons preventing observations, it will be much more difficult to get a required observation. For example, a very faint object with a large uncertainty would require an instrument with a large FOV and a large aperture. Such an instrument might not be available. These situations have to be evaluated carefully, considering the effort required for the observation and the value of the observation.