An attractive feature of mosaics is conservation of resolution. For a given CCD chip, a shorter focal length will result in a dropoff in resolution as each pixel covers a larger area of sky. Creating a mosaic conserves the resolution by maintaining a longer focal length. Also due to the increased number of pixels the larger file sizes of mosaics generally give higher quality prints, sometimes up to poster size. Mosaics can be made with any number of images, but to keep things simple we will discuss the creation of a mosaic from four individual images.
Proper planning is critical in the pursuit of a successful mosaic.
Individual frames need to be overlapped slightly. In other words each frame needs to have a common reference point, usually a bright star that will be present in the corner of each image. It is desirable to overlap only slightly to match up the frames but not too much so as to maximize the effective field size. Exposure values need to be consistent for all images. Different exposure times will make it difficult to match contrast and brightness of each frame later on.