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Name

ccdpot - potential of an infinitesimally thin disk

Synopsis

ccdpot [parameter=value]

Description

Computes the potential in the plane of an infinitesimally thin disk. The method here is VERY slow, since it evaluates the integral exactly given as Eq. 2-3 in e.g. Galactic Dynamics by Binney and Tremaine (1987). The integral is replaced by a sum over the pixel values of the input image of
                             Sigma(p)
   Pot(P) = -Gravc * SUM   ---------- dx dy 
                            | p - P |

P, and p are vector positions, Sigma(p) is the surface density at position p. dx and dy are pixel sizes and the distance |p-P| is measured in pixels.

A faster way is to use FFT’s, as described by Hockney & Eastwood (1978), and implemented in MIRIAD’s potfft program.

Parameters

The following parameters are recognized in any order if the keyword is also given:
in=
Input image file, in image(5NEMO) format. No default.
out=
Output image file. No default.
gravc=
Gravitional constant. Normally taken as 1, but this allows you to more easily convert to your own units already. See also units(5NEMO) . Default: 1

See Also

potential(GIPSY), potfft(MIRIAD), image(5NEMO)

Timing

Since for each of the N^2 pixels, all other N^2 pixels will be interrogated, this algorithm is O(N^4). In practice you will find it to be more like O(N^5). The code precomputes a kernel, which is simplified if we can assume the pixel size in X and Y are the same. If not, the program will currently probably compute it terribly wrong.

Author

Peter Teuben (loosely based on Roelof Bottema’s POTENTIAL code)

Update History


26-Jul-02    V0.1 Created to check potfft   PJT
22-oct-02    V0.2 correct kernel at (0,0)    PJT
28-feb-03    V0.3 added gravc=    PJT


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