It sounds so simple to make a mosaic from multiple images. To make a 2x2 mosaic you need of course 4 images, and for a 4x4 mosaic you need 16. Since a normal color image requires 4 images (L,R,G,B) to produce the color, a 4x4 color mosaic would actually required 64 images. When I capture color images through the telescope I would normally collect 3 sets of the 4 color images at minimum to improve the quality by stacking the individual images then take a statistical mean. To do this level of quality would require 192 images to produce one 4x4 color mosaic. If each individual image (subframe) uses a 15 minute exposure, the one image, a 4x4 color mosaic, would require about 48 hours of open shutter camera time (over several nights) on the telescope if all imaging went perfectly. This is just the data collection.
Alignment and rotation of the images into the mosaic is the next big problem. Since it is virtually impossible to have a clean-edge matching any 2 images, some degree of overlap is required. Overlapped images then result in excessive darkness along the overlap itself. Because the earth rotates and each new image requires a new guide-star (I do not have instrument rotators), the individual images rotate from image to image. The longer the time between images the larger the degree of rotation. Images taken with days or weeks of separation have significant rotation.
Finally, the contrast and brightness of any two adjoining images is never exactly the same even if taken one after the other. Differences in seeing conditions, moisture, and pollutants cause difference in the contrast and brightness of individual images.
I have begun to learn about mosaics using the Heart Nebula, IC1805.
Here are 3 thumbnail versions of the Heart Nebula:
This image is my 1st attempt at a mosaic, taken 12.11.07 with the Takahashi FRC300. It is a 2x2 (4 images) using only a Hydrogen Alpha filter. I am happy with the large amount of detail that is visible, but even at 4x4 the entire nebula is not captured within the image.
This image was taken on 01.29.08 with the Takahashi FSQ106. With the shorter focal length and larger field of view, I was able to capture the entire nebula in just one image. However, the level of detail is noticeable reduced with lower magnification.
Today, 01.30.08, using images from the Takahashi FRC300 I completed a 4x4 mosaic (16 images), again only with an HA filter. More details are again present, but I still do not have the courage to start to attempt to build a color mosaic [SEE: Part-II Below].
It is also interesting to consider the files sizes: Each of the 16 individual images in the 4x4 mosaic is a 17.8MB file from a 1x1bin exposure of the FLI Proline09000 camera. All 16 images would yield one combined image with 285MB of data. This is a very large amount of detail. The construction/aligning process reduces the ¼+ gigabyte of image data down to a 47MB overlapping combined image like you see below. Finally, after all editing and cropping is completed, the resulting web-image is only ~300K bytes of data.
Here is an interesting work-in-process image of the 4x4 (16 images) after image reduction processing, normalization, alignment and combination but prior to final editing:
I completed work on my first 2x2 color mosaic. It is of C49 the Rosette Nebula. Using only a 2x2 framework I still used 25 hours of exposure time to produce the following single image. I processed the image at 3 different levels of zoom. Therefore the single image can be viewed as three different levels of magnification [click on the image below]:
The Second color mosaic was 3x2 and I used 37.5 hours of exposure taken over about a two week timeframe. The final image of M31, the Andromeda Galaxy, is a 40x57 inch picture with a resolution of 200px/in. The file size of the digital image is 514MB. I actually had to purchase more memory for my desktop computer to handle the file sizes involved with processing the data. Click on the image below.