I run Adobe Photoshop CS version 8. What a program! You could continue to learn about the nuances until the cows come home. There is such an enormous volume of tools to extract that extra miniscule piece of latent information from the image you have taken that the learning curve is endless. All I propose to do is to run through the basic routine that I adopt in processing an image. There is so very much more that the expert will do in fine tuning his or her image. If you want to get technical I recommend Photoshop Astronomy by R. Scott Ireland. This is an in-depth look at the application of Photoshop tools for astronomical imaging. Now back to mere mortals:
Since I acquire images in FITS format the first thing to do is to download the European Space Agency's free FITS Liberator plug in for Adobe. This will enable the stacked FITS images to be opened. I have also purchased Neat Noise Reduction plug in, which can often help to smooth an image taken in less than good conditions. The color image will frequently benefit from the application of Neat.
Another plug in that I have purchased and use regularly is Russell Croman's Gradient Terminator. Casts over filtered color images caused by the moon or light pollution can be quickly rectified in the simple procedure offered by this program. One thing remains clear; however, no matter how many rectification systems you have the best images are obtained on moonless nights of good seeing and transparency with minimal light pollution and excellent guiding.
Having opened the unfiltered CCD sharp processed image I first, in "levels," adjust the white and black points to bring out some of the detail. I am careful not to be too heavy handed and never attempt to stretch the image "all the way" in this configuration. Now to "curves," where the possibilities are endless. I like to make quite a number of small adjustments, gradually stretching the image until I like what I see. It is a good idea to go a step too far because the "history" pallet gives you the opportunity to step back to most previous levels. Trial and error is certainly the message here. You may find that brighter parts of the image become overexposed; in this case, pick up the "history brush tool," designate the appropriate brush size and apply it as appropriate. Sometimes this will result in a dark "blob." in this event go to "edit" and fade the selection until you have the right balance. "Brightness and contrast" is a tool that is frowned upon by some, but I find that modest adjustment here can help an image. If imaging in the Milky Way with huge swathes of dust and nebulosity it can help to adjust the density of black in "selective color." To do this, you will have to convert the image to color mode, select black, and advance the density—trial and error once more. For the nebulosity go to "shadows/highlights" and play around. There are many options here.
Can you improve the image further? I like to try a modest "unsharp mask" filter. The history pallet gives the opportunity to rapidly check whether this step has made an improvement; often you will have a degradation. While in "filters," if stars are a little bloated, you might try the "minimum" application and fade the result as appropriate. A last step might be to overlay the image with a duplicate of itself, call up the "high pass" filter at around four-eight pixels, blend in "overlay," and reduce opacity if required. Then, in "layer," go to "layer mask—hide all," and with the "paintbrush" tool reveal the extent of the image that benefits from the treatment.
To demonstrate the effect of various tools, Fig. 8.5 shows a wide field image centered on NGC 7822 in Cassiopeia, downloaded into Photoshop and adjusted in "levels." Figures 8.6-8.8 show further adjustments.
Now that the luminance image is complete the hard work starts. I usually assemble the filtered color images in both Maxim and Photoshop and work on the better result. In Maxim the function is available to incorporate the luminance for an LRGB, but I prefer to work with separate color and clear filter images. The color image may look a little "blocky." If so, I try a "Gaussian blur" at around one or two pixels radius. If combining in Photoshop you need to convert the images to 8 bit and use the "merge" option to create the color. I find it usually helps to boost the "saturation," since the combine will be at around 50% and color tends to get washed out. The combination can be done through "layers" in Photoshop but is probably easier to accomplish in Registar, especially if the images are a little rotated due to being taken on different
Figure 8.8. "Shadows" adjustment.
nights. Invariably the quality of the combined LRGB will be lower than the clear filter image. If combining in Photoshop there is a whole raft of "blending" options in "layers," where endless experimentation is possible. Photoshop provides a myriad of tools and systems to get the best from an image. I use just a few basic ones.
Is the color balance right? The human eye, well mine anyway, cannot detect color in deep space, but there are technical ways to assess color, and top imagers will spend much time honing color.
Search the Web for your target and see the high-class images that are available. Maybe light clouds happened to roll across each time you exposed the blue filter, requiring you to boost it a little. If the image suffers from a gradient this would be the time to apply "Gradient Terminator." Try some of the "auto adjust" buttons and see what you get; a click in History will instantly take you back a step if the process is retrograde. You will most likely need to go to "saturation" again for a further boost. Go to "Adjustments, match color—density," and perhaps a boost here will help. In "curves" experiment a little; also, if the target is a galaxy or other item with an isolated position, pick up the left-hand eyedropper and apply it in different parts of the background, observing the result. With Milky Way nebulae the color is often too pink. Try "Adjustments—replace color," click on the color you want to change, adjust the fuzziness to suit, and adjust the saturation of that color. "Selective color" gives the opportunity to manipulate the hue with which you are unhappy and "Shadows-Highlights" controls overall balance.
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