What would cause systematics in fitting?
• GALFIT causes for systematic errors?: GALFIT allows for a considerable amount flexibility in dealing with galaxy imaging data, and the algorithm has been tested by a number of people over the years. The tests ranged from basic validations that the code can create/fit single component models correctly, to fitting multiple components to a galaxy, to exploring the effects of neighboring contamination. To my knowledge, when used to its full capacity, GALFIT can recover known input simulation parameters accurate to within the Poisson scatter, without significant systematic errors in the parameter estimates. The last statement does not mean that systematic errors cannot arise, because they can and often do, regardless of which algorithm one uses to do the analysis. However, to understand why and what can be done about it, it is important to differentiate between systematic errors that are caused by things in the data, as opposed to errors that are caused by the software: whereas the former
