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9 Image Reduction and Processing
by hand before combining them into a single colour image. You should be aware that
Photoshop does not handle FITS files and GIMP does not always handle them well. If
you are using an image processing package, I suggest therefore that you convert your
files from FITS to an uncompressed standard format such as TIFF. FITS Liberator
is the ideal tool to undertake this conversion and to perform your initial stretch.
In some cases, such as when a target has a lot of fine structure, it may be possible
to add a fourth, luminance, channel. A luminance is a low signal-to-noise frame,
often taken without a filter. Adding this as a luminance channel to create an LRGB
image enhances fine detail lost in lower SNR filtered images. Many amateurs spend
as much time on the luminance frame as on the rest of the image. If you do not
intend to use the images for science, then a luminance is strongly recommended. If
you do intend to use the images for science, the time spent on the luminance can be
best used for improving the SNR of the science frame. You can imagine that in an
RGB image each pixel has three values assigned to it, representing the mix of red,
blue, and green. In an LRGB image, there is a fourth value, which tells the display
how bright a pixel shall be. As the human eye is much more sensitive to changes in
brightness than it is to colour, the effect of adding the luminance frame is to enhance
the differences in brightness between regions.
Be aware that when processing LRGB images it is good practice to work on
the combined RGB frame and the luminance frames separately in order to get the
colour balance and stretching correct. You then break up the RGB frame into separate
channels and then recombine everything into an LRGB.
There are almost as many approaches to creating an RGB (or LRGB if we have
a luminance frame) as there are software packages. For example, MaximDL makes
the transition from single images to colour image seamless in a one-step approach.
Users select the image combination they wish, for example, RGB, LRGB, or CYMK
(cyan, yellow, magenta and black for litho printing) and then assign a channel to each
one. Once the data have been accepted, Maxim will create the image, which can be
saved in most standard formats.
AstroImageJ takes a rather different approach. In AstroImageJ (which cannot
handle luminance data yet), the individual frames are loaded into a stack. We can
consider a stack to be a cube, with the x- and y-axes representing the pixel position
and the z-axis the frame number, or slice number in AstroImageJ terminology. The
user assigns a colour to each slice, and the cube is merged vertically to form a colour
image.
Producing a colour image in Photoshop or GIMP is a little more complicated than
using an astronomical imaging package. Hence, I will walk through the process for
both applications.
I will start with Photoshop. I assume you will be using Photoshop CS6,
2 and
we will be producing an LRGB image from four frames that have been previously
stacked, aligned, and converted to TIFF or JPEG. Open your four frames and then
add a fifth new frame. The setting for that frame should be the same as one of your
open frames—it does not matter which. Set Background Contents to Background
2 Although this instruction should work with most previous versions.
9 Image Reduction and Processing
by hand before combining them into a single colour image. You should be aware that
Photoshop does not handle FITS files and GIMP does not always handle them well. If
you are using an image processing package, I suggest therefore that you convert your
files from FITS to an uncompressed standard format such as TIFF. FITS Liberator
is the ideal tool to undertake this conversion and to perform your initial stretch.
In some cases, such as when a target has a lot of fine structure, it may be possible
to add a fourth, luminance, channel. A luminance is a low signal-to-noise frame,
often taken without a filter. Adding this as a luminance channel to create an LRGB
image enhances fine detail lost in lower SNR filtered images. Many amateurs spend
as much time on the luminance frame as on the rest of the image. If you do not
intend to use the images for science, then a luminance is strongly recommended. If
you do intend to use the images for science, the time spent on the luminance can be
best used for improving the SNR of the science frame. You can imagine that in an
RGB image each pixel has three values assigned to it, representing the mix of red,
blue, and green. In an LRGB image, there is a fourth value, which tells the display
how bright a pixel shall be. As the human eye is much more sensitive to changes in
brightness than it is to colour, the effect of adding the luminance frame is to enhance
the differences in brightness between regions.
Be aware that when processing LRGB images it is good practice to work on
the combined RGB frame and the luminance frames separately in order to get the
colour balance and stretching correct. You then break up the RGB frame into separate
channels and then recombine everything into an LRGB.
There are almost as many approaches to creating an RGB (or LRGB if we have
a luminance frame) as there are software packages. For example, MaximDL makes
the transition from single images to colour image seamless in a one-step approach.
Users select the image combination they wish, for example, RGB, LRGB, or CYMK
(cyan, yellow, magenta and black for litho printing) and then assign a channel to each
one. Once the data have been accepted, Maxim will create the image, which can be
saved in most standard formats.
AstroImageJ takes a rather different approach. In AstroImageJ (which cannot
handle luminance data yet), the individual frames are loaded into a stack. We can
consider a stack to be a cube, with the x- and y-axes representing the pixel position
and the z-axis the frame number, or slice number in AstroImageJ terminology. The
user assigns a colour to each slice, and the cube is merged vertically to form a colour
image.
Producing a colour image in Photoshop or GIMP is a little more complicated than
using an astronomical imaging package. Hence, I will walk through the process for
both applications.
I will start with Photoshop. I assume you will be using Photoshop CS6,
2 and
we will be producing an LRGB image from four frames that have been previously
stacked, aligned, and converted to TIFF or JPEG. Open your four frames and then
add a fifth new frame. The setting for that frame should be the same as one of your
open frames—it does not matter which. Set Background Contents to Background
2 Although this instruction should work with most previous versions.