136
9 Image Reduction and Processing
duction of colour images, although a full treatise on the matter is beyond the scope
of this book.
The basic workflow of image reduction and processing is calibration, alignment,
stacking, combining, and enhancement, the last two of which apply only to image
processing. Combining is simply the process of turning a greyscale (i.e., black and
white) image into a colour one. Enhancement covers a number of processes, from
enhancing faint detail to making stars smaller and less dominant and even to adding
diffraction spikes!
There are a number of general and astronomy-specific software packages for
image reduction. Some, for example MaxImDL, are both image acquisition and
image processing packages, while others, such as StarTools, are for image processing
only. Packages such as Photoshop and GIMP are nonspecialist image processing
software, and you will need some further software prior to processing your images
in those packages. Some packages, for example DeepSkyStacked (DSS) and FIT
Liberator, perform very specific reduction tasks, such as image format conversion.
9.1.1 Calibration
Calibration of images must take place before processing, i.e., the dark bias and flat
frames must be applied. Figure 8.2 shows the pathway needed to create and apply
those frames. If you intend to stack images, you must apply the calibration frames
before stacking. As stated previously, most camera control software allows you to
apply calibration frames (for example MaxImDL), as do many of the post-imaging
packages, such as AstroImageJ and GIMP.
1
The order of calibration is the subtraction of dark and bias frames from the light
frame followed by the division of the resultant light frame by the normalised flat
frame. The resultant frame is the science frame. If you have only single images, you
may wish to perform hot and cold pixel removal at this point. Bad pixels are not so
much of an issue if you intend to undertake frame stacking.
The calibrated image should be visually checked for quality. Checking the images
might be time-consuming if there is a large number of them. However, stacking a
poor image adds nothing to the overall image quality, and flaws such as vignetting
and aircraft trails are difficult to remove except by exclusion. Images may be checked
for quality before calibration. However, a poor or misapplied calibration frame might
go undetected if images are not checked post calibration.
1 GIMP requires a plugin in order to undertake calibration.
9 Image Reduction and Processing
duction of colour images, although a full treatise on the matter is beyond the scope
of this book.
The basic workflow of image reduction and processing is calibration, alignment,
stacking, combining, and enhancement, the last two of which apply only to image
processing. Combining is simply the process of turning a greyscale (i.e., black and
white) image into a colour one. Enhancement covers a number of processes, from
enhancing faint detail to making stars smaller and less dominant and even to adding
diffraction spikes!
There are a number of general and astronomy-specific software packages for
image reduction. Some, for example MaxImDL, are both image acquisition and
image processing packages, while others, such as StarTools, are for image processing
only. Packages such as Photoshop and GIMP are nonspecialist image processing
software, and you will need some further software prior to processing your images
in those packages. Some packages, for example DeepSkyStacked (DSS) and FIT
Liberator, perform very specific reduction tasks, such as image format conversion.
9.1.1 Calibration
Calibration of images must take place before processing, i.e., the dark bias and flat
frames must be applied. Figure 8.2 shows the pathway needed to create and apply
those frames. If you intend to stack images, you must apply the calibration frames
before stacking. As stated previously, most camera control software allows you to
apply calibration frames (for example MaxImDL), as do many of the post-imaging
packages, such as AstroImageJ and GIMP.
1
The order of calibration is the subtraction of dark and bias frames from the light
frame followed by the division of the resultant light frame by the normalised flat
frame. The resultant frame is the science frame. If you have only single images, you
may wish to perform hot and cold pixel removal at this point. Bad pixels are not so
much of an issue if you intend to undertake frame stacking.
The calibrated image should be visually checked for quality. Checking the images
might be time-consuming if there is a large number of them. However, stacking a
poor image adds nothing to the overall image quality, and flaws such as vignetting
and aircraft trails are difficult to remove except by exclusion. Images may be checked
for quality before calibration. However, a poor or misapplied calibration frame might
go undetected if images are not checked post calibration.
1 GIMP requires a plugin in order to undertake calibration.