ABSTRACT
This chapter discusses the various focusing aids and their reliability along with a few considerations. A long thin cone of telescope is less sensitive to focusing position than a short fat one, corresponding to a small and a large focal ratio. The large integrated applications, Maxim DL and TheSkyX, do have some automated features that can enable temperature compensated focus tracking and focus shifts with filter changes. These are open loop focus changes, determined by prior evaluation. Maxim DL has autofocus capabilities that use the approximate focus position and then measure the half flux density (HFD) values for a series of evenly stepped focus positions on either side. It then calculates the optimum focus point. There are a number of considerations that prevent a simple one-time only focus being sufficient for an imaging night. Those using monochrome CCD cameras with RGB filters may require a slightly different focus position for each of the filters.
