Don Wade's Astronomical Image Processing Tutorials
Submitted: Tuesday, 8th August 2006 by Doug McEachern
Sometimes imaging sessions at the telescope result in image data that does not produce an image that lives up to our expectations. There might have been some guiding issues that caused out of round stars, or light pollution might have upset colour balance, introduced unattractive gradients or a host of other negative factors. Rather than dumping all that data, or settling for a scrappy image, some time spent with post processing routines of varying complexity can usually salvage an image.
Most people have a rudimentary knowledge of Photoshop. However when special post processing technique is required to fix an image to look just how we want, many people are under equipped.
Recently I became aware of a DVD containing several tutorials on Astro image post processing using mostly Photoshop with some use of Paint Shop Pro, Maxim/dl and CCDsoft.
The DVD contains tutorials grouped into two categories.
The tutorials in this first group are:
The second group;
General Topics and Other Program Tutorials
Tutorials 1 & 2 work through setting up PhotoShop for efficient and convenient workflow, through to the many uses of the Brush Tool. These two tutorials are a must for the novice PhotoShop user, and serve as a good refresher for those more familiar users.
Basic Levels and Curves, demonstrates how, by the use of these two tools an image can be stretched without loosing any data to bring out a lot of image detail that would not otherwise be displayed.
Noise Reduction Using the Color Range Tool. In this tutorial Don demonstrates by examples how best to treat a noisy background without affecting the subject data in any way. This tutorial contains a lot of useful additional instruction, depending on your basic Photoshop knowledge and skills.
Assembling an LRGB Image. Although more immediately applicable to monochrome imaging with colour filters, this tutorial demonstrates one of several techniques for assembling individual colour frames into an LRGB final image. Even people with DSLRs and other ‘one shot’ colour imagers can benefit from this tutorial because sometimes we use software to split a colour image into separate R, G, and B channels. With some techniques including Don’s method, these channels can be recombined and a luminance channel added, giving greater definition to the final image or even without separating an RGB image, a luminance channel could be added, again increasing definition. (See also the tutorial on Digital Development Processing in MaxIm DL for luminance processing)
Selecting Stars to a Separate Layer. This tutorial demonstrates a method of moving star images to their own layer, leaving the background and nebulosity intact on the background layer. This is useful when we want to apply various tools and filters selectively to one layer and not the other. An example might be to sharpen the nebulosity yet leave the star images unsharpened. There are other examples within the demonstration.
Background Smoothing using a Difference Layer. In this tutorial Don demonstrates how a poor background can be smoothed out and flattened without affecting the image.
Using Russell Croman’s GradientXTerminator plug-in. This tutorial uses a PhotoShop Plug-in to remove a rather nasty gradient from the background of an image. It looks like it will remove amplifier glow and similar artifacts that have not been removed with reduction by dark frame subtraction, and many other unwanted gradients.
Selective Sharpening Using a High Pass Filter Mask. In this Tutorial Don demonstrates a technique that allows the selective sharpening of Nebula detail, without sharpening star images. Don’s example highlights dust lane detail in the arms of a spiral galaxy.
Using a Variable Brightness Mask. Don demonstrates another method of sharpening some nebulosity without creating dark halos around foreground stars.
Bringing out Star Color. Don demonstrates one of a number of methods of enhancing star colours. The results of Don’s method are really quite impressive. This particular tutorial is available for viewing at Don’s website.
Star Elongation Repair Using the Offset Filter. This is my favorite of all Don’s demonstrations. Here Don details a method of correcting star elongation that might be due to guide issues or balance issues or even field curvature. The seldom mentioned deficit of an image with elongated stars is the fact that any nebulosity or galaxy image within, will invariably suffer elongation too. This tends to escape comment because of the very ‘nebulous’ nature of most non stellar objects, though it must be there masking or blurring some of the finer details. If for no other tutorial, in my opinion this one justifies the acquisition of the DVD.
Star Repair Using the Smudge Tool. This tutorial demonstrates a useful method of repairing a limited number of deformed star images that have been caused by any one of a multitude of optical/mechanical defects.
Damaged or Deformed Star Repair. Don demonstrates a technique useful for repairing a star image that has been deformed by the careless use of a ‘deblooming ‘tool. This is something that really only applies to users of certain cooled CCD imagers. However this procedure is not limited to Don’s example; really it is a way of restoring symmetry to any star image and could be used to correct certain colour aberrations as well as physical deformation. A highly useful tool.
Tutorials in this last section are based on MaxIm DL, Paint Shop Pro or CCDSoft. I know that some DSLR imagers have MaxDSLR (for the purpose of this review; it is the same as MaxIm DL)
About Histograms. In this tutorial Don talks about histograms and bases his demonstration on the ‘screen stretch window’ in MaxIm DL. Even for those without this program, this tutorial can strengthen understanding of histograms.
Digital Development in Maxim DL. This tutorial begins in MaxIm DL and concludes in PhotoShop. It is a useful way of preparing a Luminance frame for later use in the compiling of an LRGB image.
Star Bloom removal using MaxIm DL. This tutorial is very interesting in its own right, however it applies only to image data acquired using Non Anti Blooming Gate ccd imagers such as some SBIG cameras; ST7,8 &10.
Flat and Dark Reduction in CCDSoft. In this tutorial, Don demonstrates the use of dark frame subtraction and flat fielding of light frames. This is a very necessary process for those using cooled CCD cameras and Meade DSIs etc. Even DSLR images benefit from reduction, especially flat fielding.
Align and Combine in CCDSoft. This useful tutorial demonstrates how a batch of images are aligned and combined. Aligning registers each image to a master image and combining uses one of several possible algorisms to produce one image that is the combination of all images in a set. This produces a least noisy image and could remove random artifacts such as cosmic ray strikes and satellite trails that might be present in any one of the component images.
Layer Mask Sharpening in Paint Shop Pro. In this tutorial, Don uses Paint Shop pro to enhance the dust lanes of a galaxy image. This tutorial has the same end goal as the procedure used with PhotoShop (Selective Sharpening Using a High Pass Filter Mask) but uses Paint Shop Pro.
Using The Lasso Selection Tool in Paint Shop Pro. In this tutorial, Don shows how to set up the Paint Shop Pro screen for a convenient work flow and demonstrates a number of features.
I have enjoyed and benefited from the tutorials in this DVD and recommend it to all people who wish to improve their image processing skills.
The DVD can be ordered by visiting http://www.waid-observatory.com/DVD-Tutorials.html