Advanced Image Processing

Optimum Exposures

Capturing faint detail in deep-sky astronomical images is all about overcoming noise.  Noise comes from a variety of sources: Heat (dark current) CCD readout Light pollution/sky background Cosmic rays Pixel defects Optimal images will reduce these noise sources as much as possible.  CCDs are designed to generate low dark current, and cameras are cooled to… More »

True Color Imaging

Color in astronomical images has long been the subject of much debate.  The basic ideas behind “true color” imaging are outlined below, and tips are presented for capturing better color CCD images. Color in Astroimaging The real problem with color in astronomy comes from the fact that color is so subjective.  Everyone has a different… More »

Harold Nyquist

Harold Nyquist is at the center of a raging CCD debate.  Nyquist died, by the way, about the time CCD cameras were first being used by professional astronomers, and nearly two decades before they became popular with amateur astronomers.  So how did poor Harry get involved in all this? In the 1920s, while working at… More »

Super-Unsharp Masking

Unsharp masking can be used to significantly enhance the detail visible in certain objects.  It can also easily make an image hideous.  The process is often overdone for no apparent gain.  However, on some objects, the technique of super-unsharp masking can go beyond mere novelty and actually show off an incredible level of structure which… More »

Creating A Dynamic-Range Mask

The following is a simple procedure which can be used to enhance faint detail in an image while preserving detail within bright regions of an image.  This is particularly useful for bright nebulae (such as Orion or the Swan) or galaxies with bright cores. How it Works Many astronomical targets have an extreme dynamic range,… More »

Color Imaging Techniques

well as other color image techniques such as LRGB and CMYK imaging. Advanced RGB Techniques The basic idea behind RGB imaging is to combine three images taken through red, green, and blue filters.  This creates a full-color image.  Since color filters let less light through than a clear filter would, an individual exposure through a… More »

Image Processing Filters

We touched on high-pass and low-pass filters in the Image Processing Basics section.  Here we go into more detail on more advanced filters such as unsharp masking, low-pass filtering for noise removal, and filters for reducing image artifacts. Unsharp Mask Filter Unsharp masking is a technique originally used in the photographic darkroom.  It was popularized… More »

Combining Images

Combining images was covered briefly in the Image Processing Basics section.  Here, more detail is given on methods for combining multiple images to increase detail and to reduce noise. Methods for Combining Images The simplest way to combine images is to add them together.  This is called summing the images.  Summing is, for example, what… More »

Calibrating Images

In the Image Processing Basics section on Calibrating Images we discussed only one type of image calibration — dark subtraction.  This is the most important calibration step, but not the only one that can be performed.  Other techniques include using flat fields and bias frames to calibrate a raw CCD image. Flat Fields Flat fields… More »