Step 4: Preparing a list of observation requests (continued)
Specifying mosaics of CCD fields
Mosaics of CCD fields are useful in various kinds of observing programs, such as sky surveys and recoveries of Near-Earth Objects (NEOs) and comets. Sky surveys would normally employ mosaics consisting of rows of fields-of-view aligned with the east-west direction. NEO and comet recoveries normally involve searching along a narrow line in the sky, whose orientation may be arbitrary. To minimize the telescope time required to perform the search, TAO allows one to specify mosaics whose rows are aligned at an arbitrary position angle. It is often the case that a single row of partially overlapping fields-of-view is enough to cover the recovery search line. If necessary, a mosaic consisting of various parallel rows of fields-of-view oriented at an arbitrary position angle may also be specified. The TAO scheduler automatically takes care of minimizing the overlap between the fields in the various rows of the mosaic, so as to maximize the sky coverage per unit time. Mosaics of CCD fields are also very useful when trying to confirm objects listed in the NEO Confirmation Page (NEOCP). In this case the observation requests specifying the search mosaics will be automatically written by script NEOCPMon to file TAO\targets\NEOCP.stg.
Minor planet recoveries usually start by determining the length and orientation of the ephemeris uncertainty line (which in reality is often a very elongated uncertainty ellipse). For a subset of the known minor planets, this information may be obtained from the following sources:
As an example, let us consider a hypothetical attempt to recover the Amor object 2002 XH4 at Valinhos (MPC code 860) on 2004 August 21 UT:
The parameters specifying the search mosaic appear on continuation lines below the initial line
The & character at the end of all lines but the last one indicates that the observation request continues on the next line.
The first continuation line specifies the mosaic geometry parameters through the following five keywords:
Each of the remaining continuation lines specifies one field-of-view to be imaged. Each field is identified by a pair of integers (i, j). The central field of the mosaic (0,0) is centered at the center of the mosaic (blue dot in the above figure). Field (1,0) is obtained by shifting the central field by a certain distance D toward the position angle specified in the PA keyword. This distance D depends on the values of PA, width, overlap, and the dimensions of the field-of-view. Fields (2,0), (3,0),... are obtained by shifting field (0,0) by distances 2D, 3D,... toward the same position angle. Fields (-1,0), (-2,0),... are directly opposite to (1,0), (2,0),..., respectively, with respect to the central field. The fields labeled (i,0), where i is an integer, form the central row of the mosaic. In the above example, the narrow uncertainty ellipse can be covered with the five fields (0,0), (1,0), (-1,0), (2,0), (-2,0) belonging to the central row. These are the five fields requested in the last five continuation lines of the observation request.
If one needs to cover a wider strip around the central row, one can request observations of fields located along rows which are parallel to the central row but displaced from it by multiples of a certain distance L. The two rows adjacent to the central row (one on each side) have fields labeled (i,1) and (i,-1), where i is an integer. Field (0,1) is obtained by displacing the central field by a certain distance towards a certain position angle which differs from the position angle specified by the keyword PA. In the above figure, the center of field (0,1) would be located at the end of the horizontal line segment emanating from the blue dot. Fields (1,1), (2,1),... are obtained by displacing (0,1) by distances D, 2D,... toward the position angle specified with the keyword PA. Fields (-1,1), (-2,1),... are directly opposite to (1,1), (2,1),..., respectively, with respect to (0,1). A field (i,-1) on the opposite row may be obtained by reflecting field (-i,1) with respect to the central field.
In general, the offset of the center of field (i,j) with respect to the center of field (0,0) is given by a vector
where u is a vector of length D pointing toward the position angle specified by keyword PA and v is another suitable vector which is not parallel to u. The centers of the mosaic fields form a regular lattice near the center of the mosaic, with basis vectors u and v.