5) Creating a star layer

[MartinB]

Creating a Star Layer

After adjusting the histogram using curves and levels the next task in your processing workflow should be to create a star layer. This layer will allow you to make changes to the main image without messing up the stars vice versa.

There are different ways of going about it. Here’s the way I find most effective.

1) Create a duplicate layer (see selections and layers primer)

2) Make the duplicate layer active and select the dust and scratches filter from the filters menu under noise. Make sure the threshold is zero then pull the radius slider over to the right. The image will blur and the stars start to disappear. Just go far enough to make the stars disappear. It is ok if one or two very bright stars are still discernable. You don’t want to go any further than you have to or you will start to include bits of the main target in your selection. A value of 7-15 will normally do the trick.

3) Now for the fun bit. Select the blend mode (set by default as “normal”) as “difference”. What you should now see is the stars from your image and very little else – maybe a few smudges of your target. The difference mode compares each pixel value of the duplicate image with the background. It only allows through from the layer below those pixel values which are different – in this case the stars.

4) Choose “Colour range” from the select menu. Click on the image radio button and select quick mask from the drop down list. Now click on the brightest star in the main image (you can click on the image in the colour select window but this is obviously less precise because it is smaller. Change the dropper now to the middle one which has a +symbol beside it. By clicking on the range of stars with different brightnesses you will select all the stars. You can see what you have selected since these stars will no longer be covered by the pink mask. You may need to zoom your image by pressing ctrl +. Be careful to click precisely on the star. If you miss and click on the background you will select the whole image. If you do this by mistake select the right hand minus dropper and click on the same spot to remove the dark selection. Once you are happy that you have a good star selection click ok.

5) You will now see selection crawling ant lines around your stars. We need to create a border around the stars and then feather the selection to avoid visible transitions. Return to the select menu, click on modify then expand. The enlargement required can vary but a range of between 4 and 7 is usually about right. Go for a larger value if you are dealing with bloated, fuzzy stars. Click OK and you will now see bigger circles surrounding the stars. Return to the select menu and choose feather. The feather value should be 50-75% the expand value. If you chose to expand by 5 pixels 3 would be a good feather value. Click ok

6) Click to the left of the background copy layer to remove the eyeball. This will make this layer invisible. Make sure you also make the bottom, original layer the active one by clicking on it. Now you will see the selection on your image proper. You may see non star areas selected, especially bright portions within a galaxy. These need to be deselected by using the polygon or free hand lasso tool whilst holding down the alt key. When you do this you will see a tiny little – sign appear besides the lasso symbol. Draw round the selection you wish to remove and release the left mouse button whilst continuing to hold down the alt key. You may also find that one or two large bright stars haven’t been selected. Again use a lasso tool but this time hold down the shift key to add to the selection. Make sure you leave a border containing a bit of the background. Redo your feather selection. Alternatively you can set the feather value prior to adding a selection.

7) You should now have a good star selection. Once you are happy with it delete the duplicate layer, it has done it’s job – right click on this layer in the layers window and select delete layer. Don’t worry, you won’t loose the selection.

8) You need to save your star selection. Click once more on the select and click on “save selection” at the bottom. Call the selection Stars. Having done this select layers from the menu bar, choose new layer, layer via copy. This will make a new layer of your star selection.

You should now find that you can sharpen, blur or do whatever you want with the main image and the stars will be unaffected. The next primer will show you how to improve the appearance of your stars. You can flatten the image at any time and easily recreate the star layer – just click on select, chose load selection and choose Stars. This will bring back your saved selection. It is a good idea to do this if you are making global image changes such as removing a colour cast or making contrast adjustments to keep your star borders identical to the main image background.

If you remove the eyeball symbol from the background layer you can see your star selection. Don’t worry that the stars look grey and blobby , the light grey is indicating the feathering rather than the actual star colour.

So, no more dark halos when you sharpen your images and no more fuzzy stars when you gaussian blur!