7) How to make stars smaller and sharper

[MartinB]

I need to re upload the images but for now you can find the primers with images on my web site www.astropixels.co.uk Go to the resources section

Tidying up Stars

A common problem for our images is bloated or fuzzy stars. We know that stars are point sources of light so big blobby stars all over our image is bound to detract from it.

Bloated stars can be the result of a number of factors including optical defects, poor seeing and the characteristics of the ccd.

Here are some techniques to help improve the problem. You will need to start by creating a star layer as described in the Star Layer primer. You may need to experiment with the size of your expansion. You need to keep it to a minimum that will prevent artefacts developing. This is a particular problem in the example I have chosen here of the Rosette Nebula since there are so many stars in the “target area”. If the border around the stars is too large it can effect the overall image slightly.

Reduce star size

It isn’t essential to create a star layer to do this, you can do a quick job just by using the colour range selection tool however since we have created a star layer we might as well use it. Make sure the star layer is selected then select filter from the file menu, select “other” then minimum.

Leave the radius at 1. Don’t be alarmed if you see that most of your stars have disappeared. It is only necessary to select more than 1 pixel radius when dealing with very large bloated stars. When dealing with these you should select them manually so as not to obliterate the smaller stars.

Click OK on the minimum box and then go to the edit menu and select fade minimum. Now you can reduce the minimum effect to a sub pixel value. On the rosette image I have gone down to 50%. You normally need to choose somewhere between 50 and 70%. Slide right over to the left and you have taken away all the effect of the minimum filter. It is therefore very easy to guage the impact.

Sharpening the stars

There are a couple of things you can do in photoshop to enhance star appearance. The effects are subtle and if you over do it your stars can end up looking unnatural with very sharp borders.

Bring up the curves dialogue box. You want to place an anchor about a quarter of a way up the curve so that the dim areas of the star selection remain unchanged. This ensures you won’t develop halos. In the Rosette example I had to go a little higher on the curve. Then apply a couple more points, the top one is pulled to the left to raise the brightness, the lower one is pulled over to the right to preserve the mid tone thus enhancing contrast within the star. Make sure the top of the curve doesn’t hit the top before the corner as this will lead to an unnatural burnt out appearance. The above curve is probably a little too aggressive.

If you still have some fuzzy looking stars you can also do some unsharp masking. Alternatively you could skip the curves and just use unsharp masking. I have learnt that it is safer working on curves and to go very lightly on the unsharp masking since it is very easy to deform the stars and make them look blocky. This is especially prone happen when sharpening after application of a minimum filter. Keep zooming in on the stars to make sure they stay nice and round.

Go to the filter menu and select sharpen, unsharp mask. You are looking at large feature sharpening rather that tiny details so it is best to select a threshold above zero. 3-4 is a good value for a threshold and then set the radius slightly higher than this. For the Rosette I used a threshold of three and a radius of 4. You can use sub pixel values..

So that is it. You can compare the before and after. The change is subtle since the stars in the image weren’t horrendous. The ideal adjustments are very subjective so have a practice trying different values and methods. Just watch out for the tendancy to overdo things and mess up the stars.

[geppetto]

Excellent

Going to have a play later

Thanks Martin

[Saso]

Thanks for teaching use this method. I certainly try.

Saso