DB_CROSS

COMMAND TYPE : JUNCTION

PURPOSE:

Spatial cross correlation between a data set containing center field coordinates and a catalogue.

DESCRIPTION:

This command searches, on a catalogue, objects that are inside sky regions defined by the relative center fields. When an user has a list of astronomical objects, in his PDB, he can search for positional coincidences with a target catalogue. He can get a new file in which coordinates, parameters extracted from target catalogue and a pointer to the sequential position of the center field list are present.

Moreover at the output records can be added the distance in arcsec, between the found objects and the center fields, and the distance X-Y evaluated on the tangent plane at the center field of the objects. The routines for the evaluation of the tangent plane come from SLALIB library, distributed from STARLINK. The calculation of the distance between two celestial positions uses the formula:

DIST = acos( abs( sin(D1) * sin(D2) + cos(A2-A1) * cos(D1) * cos(D2) ) )

Where A1 and A2 are the Right Ascension and D1, D2 the declinations. This formula is used when the distance is greater than one arcmin.

The user can define how to search the coincidence between the center fields and the catalogue objects. He can use a rectangular box on the tangent plane or retrieve all the objects that are closer than a defined distance, or he can have the nearest object only. The output file can be inserted in the Personal Data Base for future action. Using the DB_MERGE command the user can append, to the records of this new record catalogue, fields coming from original center field catalogue.

When target catalogue is sorted on a coordinate, this routine use a dichotomy search strategy, and the retrieval is fast. The user can use the DB_SORT command to sort large target catalogues.

This command is an extension of DB_PLACO that works on a single center field.

INPUT FIELDS of the command:

Original: PDB Catalogue

The user must enter the name of a PDB catalogue that contains the coordinates of the center fields.

Example : CENTERFL

1Coord.

The user must enter here the name that defines the first coordinate of the center fields in the open catalogue, that is the longitude in a generic longitude/latitude coordinate system. First coordinates are: Right Ascension, ecliptic longitude, galactic longitude and so on.

Example : RA

2Coord.

The user must enter here the name that defines the second coordinate of the center field in the opened catalogue, that is the latitude in a generic longitude/latitude coordinate system. Second coordinates are declination, ecliptic latitude, galactic latitude and so on.

Example : DEC

From: Target: PDB ?

The target catalogue can be a public catalogue or a personal one. The user must enter Y when the target catalogue is in Personal Data Base.

Example : Y

Catalogue

The name of the target catalogue.

Example : SAO

1Coord.

The user must enter here the name that defines the first coordinate in the target catalogue, that is the longitude in a generic longitude/latitude coordinate system. First coordinates are: Right Ascension, ecliptic longitude, galactic longitude and so on.

Example : RA

2Coord.

The user must enter here the name that defines the second coordinate in the target catalogue, that is the latitude in a generic longitude/latitude coordinate system. Second coordinates are declination, ecliptic latitude, galactic latitude and so on.

Example : DEC

Select:

User must enter the name of the fields of the target catalogue to be written in the output file. The field names must be divided by space or by comma. The following special keywords can be used:

%ALL Returns all the fields of the input cat.

%REC Adds the record number of the target cat.

%DIS Adds distance of the object from center

%DXY Adds X-Y distance on the tangent plane

Example : RA DEC MAG %REC %DIS

The CTRL-L key provides more rows to write field names.

Where

The user can define the constraints on the fields of the target catalogue. Only records satisfeing the constraints will be written in the output file. Refer to chapter to have explanations about the statements and the available functions. The user can set constraints on all the fields of the target catalogue and on function of these.

Example : MAG < 13 AND (MAG-MAGV) < 1.5

The CTRL-L key provides more rows to write constraints.

Sky Box, Circle, or nearest object ?

The user must enter here a code that defines the method used in the search. He can search the objects inside a rectangular box on the tangent plane entering here the character B. He can search all the objects inside a circular region entering here the character C. Or only the nearest object, is returned, if almost an object exists inside the defined circular region, entering here the character N.

Example : C

1Coord. Size in arcsec

When the user decides to select objects of the target catalogue on a sky box, he must enter here the half width size of the first coordinate of the box in arcsec. Otherwise this field defines the radius of the circular region where to search. The radius limits also the region where search the nearest object.

Example : 30.0

2Coord. Size in arcsec

When the user decides to select object of the target catalogue on a sky box, he must enter here the half width size of the second coordinate of the box in arcsec. Otherwise this field is not used.

Example : 30.0

Output: File Name

The name of the output file that contains the fields selected from target catalogue and the pointer to the center fields catalogue.

Example : COINCID.DAT

PDB Catalogue Name

The name of the new catalogue in the Personal Data Base. When this field is filled an entry in the PDB for this new catalogue will be created and the user can continue to work on these new data with DIRA. Otherwise only the output file will be written. Valid catalogue names are strings of 12 or less characters that do not contain any special symbols.

Example : COINCID

NOTE:

This task writes in the output file, beyond the selected fields, pointers and distances. The sequence of these fields, in the output catalogue is fixed, without regards to the order in which the key was inserted in the 'Select' field. After the selected fields you can have in this order:

Where the name of the output field is build using a prefix and four or five characters of the name of the target catalogue (tnam) or of original catalogue (onam). The pointer to record number of the original catalogue is always written, the other fields can be selected by keys.



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