CELESTIAL
I have decided to create a separate gallery and
index for my mosaic imagery. I find this aspect of CCD imaging very
difficult but most rewarding. Nearly all the other images I have
produced are merely snap shots compared to the complexity of these mosaics.
They can require many hours at the telescope to achieve the support frames,
and many additional hours of post processing at the computer to gain a
finished product. I am increasingly regarding my mosaic work
as art.
Mosaics, or composite imagery, is a way of taking several images and combing them into a larger final image. Although is helps to have a telescope with a wide field of view to start with, this is not a prerequisite. I do not intend to go into great depth here concerning the mosaic process. A fine article, written by Robert Gendler and Joel Gelber, details the process and can be accessed at the link below.
To capture the images necessary to create a composite image, one must first choose a suitable registration point in the image. This can be a star, clump of nebulosity, etc. In a simple four frame mosaic, moving this reference point to each corner and imaging will guarantee sufficient overlap in the frame. Then one has to combine the image set into the mosaic. This is done in Adobe Photoshop. The difficult part of making a mosaic, as can be seen in the above image, is adjusting the individual frames so that they all match in brightness and contrast, thus eliminating any "stitch" lines. Objects that have a great dynamic range (such as M31 or the Great Orion Nebula) can be especially difficult to process. Another trying aspect of mosaics is that there can be a certain amount of rotation in last images of the set. This occurs during long exposure sets--the last frame may not match up with the first frame. This requires the image to be rotated in order to fit properly.
