1. Introduction

NEMO is a collection of programs, running under a standard UNIX shell^1.1, capable of running various stellar dynamical codes and related utilities (initialization, analysis, gridding, orbits). It can be thought of as a collection of various ``groups'' (packages) of programs, a group being defined by their common file structure. In addition, a common user interface is defined with which the user communicates with a program. User interfaces will be described in much more detail in the next chapter and Appendix .

In order to run NEMO programs, your UNIX environment has to be modified slightly. This is normally done in the form of a few additions to a startup file (.cshrc or .login if you use the C-shell). Appendix  gives a full description.

Let us first give you an overview of the various ``groups'' of programs, as they clearly show the structure of NEMO to a first time user:

The N-body group is defined by a common file structure of ``snapshots''. In this group we find various programs to create N-body systems (spherical, disk), methods to compute the gravitational field (softened Newtonian, hierarchical, Fourier expansion), and time-integrators (leapfrog, Runge-Kutta). Many utilities exist to manipulate, analyze and display these data.

The orbit group is defined by a common file structure of ``orbits'' . It is mainly intended to calculate the path of an individual orbit in a static potential and then analyze it. This group is closely related to the before mentioned N-body group, and utilities in both groups can interact with each other. For example, it is possible to freeze the potential of an N-body snapshot, and calculate the path of a specific star in it, now conserving energy exactly. Or to extract the path of a selected star, and extract an orbit from it.

The image group is defined by a common file structure of ``images'' , i.e. two dimensional rectangular pixel arrays with a 'value' defined for every pixel. Actually an image may also have a third axis, although this axis often has a slightly different meaning (e.g. Doppler velocity ). It is possible to generate arbitrary two-(and three-) dimensional images from snapshots, FITS files of such images can be created, which can then be exported to other familiar astronomical data reduction packages. There exists a variety of programs in the astronomical community to manipulate data through FITS format.

The table group appears quite commonly among application programs in all of the above mentioned groups. Most of the time it is a simple ASCII file in the form of a matrix of numbers (like a spreadsheet). A few programs in NEMO can manipulate, display and plot such table files, although there are many superior programs and packages outside of NEMO available with similar functionality. It is mostly through these table files that we leave the NEMO environment, and persue analysis in a different environment/package. The obvious advantage of storing tables in binary form is the self-documenting nature of NEMO's binary files. For historical reasons, most tables are displayed and created in ASCII, though you will find a few binary tables in NEMO.

More groups and intermediate file structures are readily defined, as NEMO is also an excellent development system. We encourage users to define their own (or extend existing) data structures as the need arises. In Chapter we will detail some 'rules' how to incorporate/add new software into the package, and extend your NEMO environment.

The remaining chapters of this first part of the manual outline various concepts that you will find necessary to work with NEMO. Chapter  outlines the user interface (commandline, shells etc.), details are deferred to Appendix . Chapter  explains how data is stored on disk and can be manipulated, including the concept of function descriptors in NEMO. Chapter  details how data can be graphically displayed, either using NEMO itself or external programs.

The second part of the manual is a cookbook: Chapter  gives a variety of examples of use.

The third part of the manual is the programmers manual: Chapter  is for the more adventurous user who wants to modify or extend NEMO.

The last part of the manual are Appendices with a large variety of reference information.