# Long Cylinder

## Description:

Cylinder model for long cylinders.

Background
The default numerical integration scheme in SasView leads to numerical instabilities in the calculation of the cylinder form factor when the length of the cylinders becomes too long. For practical purposes, the limit of applicability of the normal cylinder model is when $length$ > 1000 Å (which is not so long).

This Model
This model computes the same cylinder form factor as the normal cylinder model, but does so with a 501 point Gaussian integration scheme in place of the normal 76 point integration scheme. This leads to a vastly improved calculation which has been tested for $length$ = 400000 Å, but which may also slow down model-fitting.

Note: This model also requires the file gauss501.c

Limitations
The cylinder form factor is not appropriate for so-called 'infinitely thin' cylinders or rods. These would be better dealt with by models using appropriate limiting forms of the normal form factor. Some discussion of this can be found in our GitHub issues, for example, https://github.com/SasView/sasmodels/issues/109

Authorship and Verification
Author: Paul Kienzle Date:23/06/2020