To produce aesthetically pleasing
astroimages requires a substantial amount of processing after the image
is collected at the telescope. The basic process used on most of my images
is as follows:
- Calibration. The
image from the camera is far from perfect. It contains a fixed variation
between pixels caused by thermal electrons, pixels with a non-uniform
response, and the shadows of dust in the optical path and vignetting
from the optics itself. Fortunately, nearly all of this can be removed
by subtracting out a dark frame, made with no light hitting the
sensor, and dividing by a flat frame, made by imaging a uniform
- Stacking. I seldom
take a single exposure of an object. Instead I take lots of images and
combine them. An image listed as being a 60-minute exposure may be the
sum of twenty 3-minute exposures. Because the images are probably all
very slightly misaligned with respect to each other, co-adding them
requires registering them first. This is accomplished by carefully locating
the centroid of one or more reference stars in each image, then shifting
with a resolution of much less than a single pixel. Finally, the images
are added or averaged.
- Scaling. The camera
captures over 60,000 different levels of gray, but the eye can only
see a few hundred. So the brightness scale of the camera needs to be
matched to that of the eye. This could be a simple linear compression,
but doing that will tend to produce images like those seen on film-
the bright areas will look saturated, and the dim areas will not show
much detail. So instead, I apply a non-linear stretch to the data, usually
a variation of a logarithm, to allow the final image to display much
more information in both the bright and dim areas of the object.
- Sharpening. Every
image is made looking through the atmosphere, and this results in some
blurring. It is often desirable to sharpen up the image a little, either
using a simple kernel filter, a more complex operation called
unsharp masking, or a very sophisticated process called deconvolution,
which attempts to mathematically undo the different sources of distortion
that blur the final image.
- Special processing.
A wide variety of special processing algorithms are available to deal
with different situations. One interesting technique, which I call shift
processing, is described here.