All CCD images contain noise. Much of this noise comes from the
electronics of the camera itself and can be removed by
calibrating
the image. Noise also results from the statistical nature of light.
But the mathematics of imaging can be used to your advantage: noise is
random, whereas the signal from the object being imaged is not. Put
another way, noise changes from image to image but the object does
not. By adding two images, the signal from the object is doubled
but the noise increases only 40%. The fancy term for this is increasing
the signal to noise ratio. The more images you combine, the more you
increase the signal to noise ratio (although there is a point of diminishing
returns).
First we will look at the results of combining images and then see how to
actually do it.
The Results of Combining
Taking another look at our single, calibrated exposure of the
Sculptor Galaxy from earlier, you can see some noise, or graininess, in the fainter portions
of the image. Below is a crop of the outer spiral arm at full resolution,
so the noise can be seen.
Below is the result, again at full resolution, of combining six exposures.
Notice how much less noise (and more detail) is now visible.
Aligning Images
Note: Programs such as CCDSoft, Maxim
DL, and RegiStar will automatically rotate and scale images to align them. This
technique is recommended as a great time saver. However, it is not always
flawless in the event of images which have been rescaled or significantly
rotated. In most cases this method works fine.
In order to combine multiple images each image must be aligned with the
others so they will all line up when composited.
Note: In CCDOPS, the images are
automatically aligned and combined when you use the Track & Accumulate
function. See the section on Basics of Taking CCD Images
for more details
on Track & Accumulate. If you have taken individual images and wish to
combine them you will need a program like MaxIm DL or CCDSoft. The
following alignment steps are demonstrated using MaxIm, but other programs work in a very similar way.
First, select the files to be combined. In MaxIm DL the Combine Files window
looks like the following:
Select the files you wish to combine by highlighting them in the list of
files. Above, the files ngc253_L001, ngc253_L002, etc., are selected. File details
are given at the bottom right of the window--a handy way to make sure you are
selecting the right files. Once the files are selected, clicking Combine
will open the image files along with a new Combine window.
Many of these features are more advanced, so for now just make sure that
"Auto - star matching" is selected as the Align Mode and Median is
selected for Output at the bottom. Click OK and the imaged should
be summed together to produce something like this:
Now that you have a nice, noise-free image, what's
next? Image processing filters!
Next, Image Processing Filters....
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