The GHS Tutorials

Dave Payne, the man who first proposed the use of generalised hyperbolic equations for stretching astronomical data, has prepared this series of instructional videos.

Tutorial 1: Introducing GHS Version 2 In this tutorial video Dave introduces the GHS script, including the main features of the interface.  He explores how the inputs are used to control the shape of the stretching transform function and its impact on an image.  He also explains how to use the histogram, including using the log histogram, and the image inspection dialog to help choose the stretch parameters.

Tutorial 2: More advanced stretching

In this tutorial, an emission nebula image that fills the frame and spans a large degree of dynamic range is stretched. Starting with the initial stretch of a linear image and continuing with intermediate stretches, Dave shows how to zero in on the parameters needed for the transform equations. Along the way, the importance of contrast placement is emphasized and how to display dim nebulosity while protecting stars is illustrated. Finally, the ability of the script to stretch saturation to restore or enhance a colour image is shown.

Tutorial 3: More options, colour stretch and image blending

In this tutorial, M51, a spiral galaxy with interesting features is used as the subject matter. The first half of the video describes the properties of the hyperbolic equations used in the script, why they are appropriate for stretching and how they encompass existing transforms, and even extend their utility. The video also demonstrates how the stretching script, can be applied to avoid artifacts, preseve data integrity, explore the features of the data, and even invert previous stretch results. The second half of the video shows how the “arcsinh stretch process” has been incorporated into the GHS stript and can be used to enhance colour saturation and avoid “bleaching” while stretching. This allows for the benefits of both the GHS transform benefits, together with the colour enhancing benefits of the arcsinh process. In addition, the blending option is described where the results of a stretch can be back-blended with previous or other versions of an image to mitigate changes.