ELIMINATING DARK RINGS AROUND STARS IN LRGB IMAGES.

Luminance images typically have smaller star sizes compared to RGB images. This is particularly prevalent when the luminance images are taken through a Ha filter. Due to the size mismatch, the LRGB image will invariably show dark rings around stars on a bright background such as nebulae.   

Here is a typical example.

In this case the luminance images were taken through a red filter and processed to reduce star size.

The green and blue filtered images were taken on a different night in bad seeing conditions. The stars are therefore bloated in the images.

When the L image is combined with the RGB image, the characteristic dark rings are conspicuous due to the mismatch.

The technique for eliminating halos involves reducing the size of the stars in the RGB image using the minimum filter in Photoshop prior to combining with the L image.

The procedure is as follows:

(1)   Construct the colour image by using only the R, G and B images. The luminance image will be combined later on.

(2)   Open the RGB image in Photoshop and select the magic wand tool. Make sure the add option is also selected in toolbar.

(3)   Click on the image. The magic wand tool will automatically select stars. If portions of the object are selected continue to click on these areas until only stars are in the selection.

At this stage any processing of the image will occur outside the selected areas. Since we only want to apply the minimum filter inside the selected areas, right click on the image and “Select Inverse”.

Select Filter ---> Other -----> Minimum

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Select a radius of 2 and click OK. The resultant RGB image will not look very attractive and be full of processing artefacts. This will not be a problem however as the luminance image will provide the detail to the image.

Process the luminance image as you would normally do. The luminance image will be layered onto the RGB image. To help combine the images you may want to register the images using software such as Registar before layering the images.

When combining the images use the luminosity option.

 The LRGB image is free from star halos.