DB_PLACO

COMMAND TYPE : SELECTION

PURPOSE:

To select objects inside a sky area defined by its center and size.

DESCRIPTION:

This command searches, on a catalogue, the objects that are inside sky region defined by the center field and the size. 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 the distance in arcsec between the retrieved objects and the center field and the distance X-Y evaluated on the tangent plane to the center field of the objects can be added to the output records. The routines for the evaluation of the tangent plane comes from SLALIB library, distributed by STARLINK. The calculation of the distance between two sky coordinates 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 declination. This formula is used when the distances are grater than one arcmin.

The user can define how to search the coincidence between the center field 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 picked up in the Personal Data Base for future action.

When the target catalogue is sorted with respect to 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 catalogue.

The DB_CROSS is an extension of this command and works on a list of center fields to get data on more fields.

INPUT FIELDS of the command:

From: PDB ?

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

Example : Y

Catalogue Name

The name of the target catalogue.

Example : SAO

Select:

The user must enter the name of the fields of the target catalogue that can be extracted and written in the output file. The fields names must be divided by space or by comma. The following special keywords can be used:

%ALL Returns all the fields of the target catalogue

%REC Adds the record number of the target catalogue

%DIS Adds distance of the object from the center field

%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 insert the constraints on the fields of the target catalogue. Only the records that comply with this filter will be picked-up in the output file. Refer to Query Language 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.

1Coordinate

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

Center

The user must enter here the first coordinate value of the center field. A message on the screen helps to define the units of the input values (hours or degrees) according to the type of coordinate in use.

Example : 12 30 35.21

Size in arcsec

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

Example : 30.0

2Coordinate

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

Center

The user must enter here the second coordinate value of the center field.

Example : -41 30 45.21

Size in arcsec

When the user choices to select objects 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

Sky Box, Circle ?

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 entering here the character N only the nearest object is returned, if almost an object exists inside the defined circular region.

Example : C

Output: File Name

The name of the output file that contains the fields selected from the target 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 contains 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:

Field NameExplanationKey
POInameRecord number of the catalogue%REC
DISTname Distance between object and center filed%DIS
XDISname X-Distance on the tangent plane%DXY
YDISnameY-Distance on the tangent plane%DXY

Where the name of the output field is build using a prefix and four or five characters of the name of the catalogue (name).



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