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This IDL facility provides a means to find the index of all pixels within an angular distance Radius from a defined center.

Location in HEALPix directory tree: src/idl/toolkit/


IDL> query_disc , Nside, Vector0, Radius, Listpix, [Nlist, DEG=, NESTED=, INCLUSIVE=]


HEALPix resolution parameter used to index the pixel list (scalar integer)
position vector of the disc center (3 elements vector) NB : the norm of Vector0 does not have to be one, what is consider is the intersection of the sphere with the line of direction Vector0.
radius of the disc (in radians, unless DEG is set), (scalar real)
on output: list of ordered index for the pixels found within a radius Radius of the position defined by vector0. The RING numbering scheme is used unless the keyword NESTED is set. (=-1 if the radius is too small and no pixel is found)
on output: number of pixels in Listpix (=0 if no pixel is found).


if set Radius is in degrees instead of radians
if set, the output list uses the NESTED numbering scheme instead of the default RING
if set, all the pixels overlapping (even partially) with the disc are listed, otherwise only those whose center lies within the disc are listed


query_disc finds the pixels within the given disc in a selective way WITHOUT scanning all the sky pixels. The numbering scheme of the output list and the inclusiveness of the disc can be changed


This section lists the routines related to query_disc

version 6.1 or more is necessary to run query_disc .
ang2pix, pix2ang
conversion between angles and pixel index
vec2pix, pix2vec
conversion between vector and pixel index
query_disc, query_polygon,
query_strip, query_triangle
render the list of pixels enclosed respectively in a given disc, polygon, latitude strip and triangle


query_disc , 256L, [.5,.5,0.], 10., listpix, nlist, /Deg, /Nest

On return listpix contains the index of the (5982) pixels within 10 degrees from the point on the sphere having the direction [.5,.5,0.]. The pixel indices correspond to the Nested scheme with resolution 256.

Eric Hivon 2010-06-18
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