Luminosity Masks are available in Photoshop and Gimp. They are a tool I have used in Landscape Photography processing many times. I have been looking for an astronomy object image where they can be useful and with the Iris Nebula I may have found a candidate.
What are Luminosity Masks?
As the name suggests a luminosity mask is one based on the luminosity i.e brightness of pixels in an image. While masks generally are for selecting an object or the sky, luminosity masks only select brightness values. This can be useful when an image has a range of tones where detail is lost in the shadows or highlights are too bright. Selections are made in the Channels menu of the layers panel and converted to masks that can be used in the layers panels as masks.
(I have included a link below to a Photoshop Cafe video that describes how to generate and use the masks)
The luminosity masks are generated to separate the White areas, light areas, mid tones, shadows and blacks. This enables different parts of the image to be developed using adjustment layers, without affecting the other areas of the image.
An example of luminosity masking is shown below: Each of the channels masks a particular luminosity level within the image.
The image below shows the areas masked (in red) by the “Lights” luminosity channel. The mask selects the lights and masks anything darker.
NGC7023 The Iris Nebula
The Iris Nebula is located in Cephus around 1300 light years from earth. It is a bright reflection nebula lit by a magnitude +7.4 star designated HD200775. The nebula is surrounded by a region of H2 that is irradiated and glows red.
The nebula is imaged in RGB in order to accentuate the blue reflection component. This is bright and the central star also adds to the overall brightness of the central portion of the nebula.
The detail in the central area can be lost by being “blown out” by the brightness of the central star.
RGB Image Processed in Pixinsight
Below is a starless image processed in Pixinsight and stretched using the MAS (Multiscale Adaptive Stretch), as this produces the best dynamic range. The image is processed from six hours of data. Two hours each of Red, Green and Blue.
The image is a good representation of this nebula. However, as a photograph to be displayed it can be improved in the following ways:
• The image is a little flat and needs more contrast.
• The outer regions of H2 can be enhanced to be more prominent.
• The centre portion of the reflection nebula can be enhanced to bring out more detail.
• The image needs more “punch” and this can be achieved with an overall contrast difference between the various bright and dark areas.
• Saturation boost to the H2 red regions.
The image enhancements are made by creating adjustment layers from the various luminosity mask channels. Below is an extract from the Layers Panel in PS showing that four of the channels have been used. The adjustments were made using the levels tools. However, any adjustment available can be made using the luminosity masks.
In addition global adjustments were made to Hue/Saturation and Colour Balance.
The final image is shown below:
I am satisfied that I have achieved my initial goals. The image shows better overall contrast, the H2 regions have more prominence, and the centre region has better detail. The delicate structures in the centre are clearer without being overly sharpened.
This can be a difficult object to image due to the brightness of the star that generates the reflection nebula. With careful processing the full beauty can be brought out.
Centre Portion Detail.
Image captured over two nights in April 2026 from my backyard in Pembroke.
Scope: APM152/1200 f8; Camera ASI183MM; Baader CMOS R:G:B Filters; EQ6r-Pro Mount
Link to video: Insane CONTROL, luminosity masks in Photoshop
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