query_disc

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/query_disc.pro 

FORMAT

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

QUALIFIERS

Nside

HEALPix resolution parameter used to index the pixel list (scalar integer)

Vector0

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

radius of the disc (in radians, unless DEG is set), (scalar real)

Listpix

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)

Nlist

on output: number of pixels in Listpix (=0 if no pixel is found).

KEYWORDS

/DEG

if set Radius is in degrees instead of radians

/NESTED

if set, the output list uses the NESTED numbering scheme instead of the default RING

/INCLUSIVE

if set, all the pixels overlapping (even partially) with the disc are listed, otherwise only those whose center lies within the disc are listed

DESCRIPTION

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

RELATED ROUTINES

This section lists the routines related to query_disc

idl

version 6.4 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

EXAMPLE:

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.