Astrophotography by Dennis Isaacs
Making
a star layer and High Pass filtering. |
A problem often found with nebulae is that they have relatively large scale features such as shock fronts that sometimes do not look satisfactorily sharp. If you sharpen the picture in the conventional way either the shocks are not affected or the stars are completely ruined. The following is a method of getting the best of both worlds, respectable detail in the nebula and stars that have a pleasing stellar profile rather than looking like blobs of white paint. I have used a picture of the California Nebula to illustrate the technique. The pictures used are crops of the actual picture which was taken with the FSQ 106 and ST10. The real picture would be 640 x 425mm at the magnification shown here. The technique I have used for this example is quite simple, stretch the original until the nebula detail is starting to show nicely, don't fully stretch it at this stage. Duplicate the layer and then go to Image - Adjust - Threshold. The Layer will be converted to pure black and white. Everything above level 128 will be white and everything below 128 will be black. You can adjust the effect of this transition in the picture with the slider. Moving it to the left will pick up more and smaller stars but possibly some nebula as well. (because it was above level 128 originally). Once satisfied that as many stars as possible are selected use the Magic Wand set to a low tolerance and (I usually) then click in the background. All of the background will be picked up immediately, you then invert the selection and the job is done. If you end up picking up bits of nebula as well you can address that problem with Quick Mask. Switch to Quick Mask (letter Q) and using a suitable brush with the foreground colour set to white simply paint out the parts of the selection you do not want. Then hit letter Q again to switch back to normal edit mode. At this stage do not forget to delete the Threshold layer, it is no longer needed. Once selected, go to Selection, Modify and Expand the selection by an amount that looks right. It will vary with the camera resolution, the sky quality and anything else you can think of. The point is there is no absolute value for the expansion, it is something you have to get a feel for. I find 1-4 pixels usually does it but you can have a problem with stars of widely differing sizes, if so revert to Quick Mask and tidy up. Once the selection is complete go to Select - Feather and feather by 1-3 pixels. The amount tends to be in proportion to the original expansion. Stars vary in size and sharpness so there is no fixed rule for this expansion/feather. I have been using 1/1 for some time without a problem. The idea behind this expansion is simple. You need the stars on the star layer to be bigger than the stars on the underlying layer or they will not be properly covered up and sharpening artefacts may show. Beware of doing any work on the star layer by itself, ie, prior to flattening. It can go dreadfully wrong. Now simply hit Ctrl J to put the selection onto its own layer, you should be copying the background layer. You should now duplicate the background layer so you have three layers, topmost is the star layer, then the duplicate of the background and at the bottom is the background. Make the centre layer active and set its blend mode to Overlay. Go to Filter - Other - High Pass and set a value which will again depend on what you are trying to achieve and what qualities you have in the picture. For the example I used 15 pix and I think that is really quite high. For fine detail a value of 2-6 might be more usual but for large scale nebula detail a larger radius is required. The two top layers can be alternately switched on and off to see the effect of the sharpening and the masking of the stars (now suffering from unwanted sharpening) by the star layer. Once done, I flatten the image and finish the stretching. Apply a careful degree of Neat Image and finally use Unsharp Mask sharpening to brighten up the smaller stars. USM needs to be applied very carefully to avoid the blob-of-white-paint look. Keep the radius small to limit the effect to the smaller stars, set the Amount to around 60-100% and Threshold to about 10-15. All the filter effects can be moderated by Edit Fade or by filtering on a duplicate layer and then reducing layer opacity. The control you have is virtually infinite. If I was doing this on an RGB image I would do exactly the same. Many people use colour range as the selection tool but it is nowhere near as efficient. Especially if your stars are in colour. |
| Shown below is the original, cropped for display purposes and partly stretched. I never fully stretch at this stage as it runs the risk of saturating the stars, once that is done there is no hope. |
| Here is the three layer picture with the stars selected and then copied onto their own layer. The other two layers are switched off to show the star layer and the effect of it's having been expanded and feathered prior to copying. The halos are perfectly normal as they are actually a copy of the background layer. You should not try star reduction at this stage as they will uncover the stars on the underlying layers that are about to be sharpened. |
| Basically the same screen grab as above but with all the layers switched on and the middle layer set to Overlay. This increases the contrast considerably but that contrast increase goes away when the High Pass filter is invoked. The halos around the stars on the star layer are now quite prominent as they still represent the background layer. |
| It is not immediately obvious what is going on below. The High Pass filter has been applied and the middle layer set to Overlay as normal. The top layer is the star layer that has been copied from the background and not sharpened in any way. The middle layer is now showing the effects of the sharpening and the black outline is a 'hole' in the top layer that allows the sharpened stars to show through. You have to look hard to see the difference, the stars are brighter and have a sharper edge than before. An effect that we do not want. This unwanted star sharpening, although subtle here, can be diabolical if the filter values are not chosen carefully. You may need to go back and forth a number of times before you get it right. With High Pass you have to apply the full effect in order to see it properly. If it is not right, click back in the history palette and do it again. |
| Here is another screen grab showing the before and after views. This is still at the initial stage of stretching so is a bit soft. I deliberately keep the contrast softer than wanted at this stage so as to reduce the risk of saturating the stars with the filtering. The nebula on the left hand side is beginning to look very soft now compared to the High Pass sharpened version on the right. Remember this was one HP pass of 15 pixels. There is no reason why you should not use a much smaller radius and apply it several times or start with a medium radius and then apply a smaller one. Keep a close eye on the 'graininess' in the nebula, the smooth tones should still be smooth. Sharpening will make it noisier but it has to be kept under control. |
| Here is the finished version. The contrast has been turned up a bit in Curves and Neat Image applied using a pre made noise profile. The final action was to use Unsharp Mask sharpening to brighten up the smaller stars. Typically Amount 60-100%, Radius 0.7-1.5 pixels and Threshold 10-15 to avoid sharpening the smooth tones. |
A before and after |
The finished picture |
There are three other methods of selecting stars that spring to mind, I shall call them False Layer, Overlay, and High Pass. The different methods sometimes lend themselves to doing the job when others fail. High Pass for example, might be used to select stars when they are in many colours as the filter turns everything into grey. The four methods are described briefly below without the benefit of pictures as they all essentially follow the technique described above, ie, select the stars and put them on their own layer.
False Layer Overlay High Pass Always make sure that an un-adjusted layer (the background) is the active layer before doing Ctrl J to make a star layer. remember for the main technique described you end up with three layers, bottom is the background, then the Overlay blended layer and on top is the star layer. |
