### 7 Model data generation — PISAGEN & ADDNOISE

PISAGEN generates model data using the PISA profile fitting function (appendix B). The objects are formed using the positions and integrated intensities output by PISAFIND in its profile fitting mode (or any other data of the same form for the first four columns):-

• Object number
• X position
• Y position
• Integrated intensity.

Extra parameters not available in the other PISA routines allow PISAGEN output objects to be elliptical. The image output from PISAGEN can have noise added of either gaussian form or of a pseudo-poissonian type. The noise levels are recorded in the variance component of the output NDF. The resultant model image can then be subtracted from the original data and the difference frame may be inspected, for missed objects, the goodness of the profile fit etc.

PISAGEN can also perform the generation of model data for test purposes, the program accepts integrated intensities and can generate very accurate model objects by integrating the flux within each pixel. It is also possible to generate mixed frame data, i.e. data with objects of differing model parameters (and ellipticities). This is done by running PISAGEN multiply with the different parameters (with a zero background, except for one frame) and then combining the frames by addition. Noise can be added to this ‘mixed’ frame (which could for instance mimic a frame of stars and elliptical objects with extended wings (galaxies! although not ${R}^{1/4}$ ones)) by an ancillary program ADDNOISE, which is available with the PISA package.