HTML automatically generated with rman

## 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 used here is VERY slow, since it evaluates the integral exactly as given in Eq. 2-3 of e.g. Galactic Dynamics by Binney and Tremaine (1987, 2008). 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
report=N
Give a rolling percentage progress report every N pixels computed. Default: 0 (meaning no progress report)
nbench=1
Set this to how often the integral is computed, in order for fast versions (small maps) to get accurate timings on fast machines. [Default: 1]

## Units

As an example, if units are 1e10Msun and kpc, Gravc = 43007.1 (see units(1NEMO) , the potential

potential(GIPSY), potfft(MIRIAD), rotcurves(1NEMO) , units(1NEMO) , 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