|In Search of Solar Nirvana
(Typical processing overview)
|The colors may be pretty, but this image is grossly saturated!
As a raw image, the above shot may look to be quite promising, but is actually grossly saturated. Precision H-a etalon and
blocking filters hold the frequency to 656.28nm with a bandpass of less than 0.07nm, which is Red. Not yellow, not orange,
not even pink, but Red... NOTHING BUT RED (whew!). The colors are a result of signal level strong enough to leak over into
the Blue and Green channels.
If we can understand the premise that only a precise frequency (Red, Red, Red!) is passed by H-a filters, the rest falls more
easily into place! As with many astro-images, H-a shots are often "stretched" to enhance the contrast of subtle details that
would otherwise be difficult to discern. Processing can also include intentional false-coloring, to further bring out details.
The processed grayscale image is then recombined into Red,
Green, Blue and luminence channels, forming the above
For more information on this particular image, and H-a imaging in general visit the 08-Mar-03 page, or for the entire
Solar section visit the Solar Imaging section.
I am often asked what a typical raw H-a image looks like and what steps are used to process it. I wish to stress that I
am not a solar guru, but merely a fellow traveller sharing some insights he has gleaned along the way. Nor is this
intended to be a tutorial, as many splendid examples already exist. Successful methods for gathering and
processing of H-a images vary greatly and are largely a procedure of trial and error, fine-tuned for your specific
equipment and needs. Your path to Solar Nirvana may be quite different from mine!
If you have not already done so, you may want to peruse the primer "Variations in H-a Solar Exposures" and "In
Search of Solar Nirvana" Part1, as they provide the basis for this page.
|The panels seen below are from an individual image at various stages of processing
This is a JPEG translation from a RAW image stored by the
camera. Potential high resolution images are kept in the RAW
mode, so as to preserve image integrity and reduce JPEG
compression degradation. JPEG images are NOT used for
processing, but as a quick reference (which also includes EXIF
data) to the actual RAW images.
This is what we work with... the RAW 16/48 bit image which
remains unstretched and untainted by any camera firmware
intervention or file compression. The darker coloration is a
result of it not having been "stretched" by the camera's
internal routines. This allows us to make the intensity level
decisions ourselves during processing rather than the use of
a canned routine!
The image is split into its components, and only the R channel
is saved, as it is the only one that should have any H-a signal
(note: some digital cameras have excessive signal leakage,
and the image may appear stronger in other channels. See
Part I for more information).
This is then "stretched" and de-convolved (a process that
attempts to reverse atmospheric effects etc that "convolves" or
rolls together nasty stuff which degrade the image). Numerous
iterations of an algorithm known as Richardson-Lucy were
performed using Images Plus software, resulting in the above
The image to the left is used to re-form a color composite,
which will darken from the effect of the constituent channels
to become similar in contrast to the above image.
The image above has been purposely compressed in
dynamic range (the distance between all black and all white)
for illustrative purposes only, to mimic the contrast a
color-composite will usually possess. If these settings were
actually used however, image intensity after recombination
would be extremely dark, requiring re-expansion which in
turn would considerably weaken contrast.
Finally, the image is composited with one that has been
processed to favor prominence details. Final polish and sizing
then take place, and appropriate JPEG compression levels
are used for the web-viewed results.
|And that my friends, is the journey an H-a image might take!