Many astrophotography images contain very faint structures. With a focused stretching for the luminance channel, we can brighten these structures slickly so are not lost in the background haze.

Once we’ve completed a basic stretch on the luminance channel, we dan direct our attention to the more subtle areas of the object.

Aggressive Stretch

In order to bring out the subtle details, we need to do a much more aggressive stretch than we would normally do. The stretch is done on a cloned version of our luminance image.

Aggressive Histogram Transformation
Aggressive Histogram Transformation
Aggressive Luminance Stretch
Aggressive Luminance Stretch

With an aggressive stretch, we can more easily see the faint areas coming off of the arms of the galaxy. Those faint areas are what we need to bring out in our luminance image. To do this, we need to create a range mask.

Range Mask

Because we only want to brighten the very faint areas of the galaxy, we need to create a range mask that targets only those areas. The very bright and very dark areas must be ignored.

Range Mask
Range Mask
Luminance Range Mask
Luminance Range Mask

To do this effectively, you will want to use live view.

  • Enable Live View
  • Screening: Enabled
  • Smoothness: Set to 100
  • Lower Limit: Adjust to the point where the faint areas are still visible
  • Upper Limit: Adjust so the bright areas just reach the edges of the faint area
  • Fuzziness: Set between .2 and .3

This might take a few cycles to get the right levels. After the mask is created, we can apply to our luminance.

Luminance Enhancement

The range mask, once applied to our original, stretched image, will protect the bright and dark areas from modification. Any changes we make to our image will only impact the faint wispy areas.

Faint Histogram Transformation
Enhanced Histogram Transformation
Enhanced Luminance Stretch
Enhanced Luminance Stretch

As we move the middle Histogram Transformation slider to the left, the faint areas will gradually become brighter. Avoid going too far as the faint areas need to gradually merge with the brighter areas. We need to avoid sharp boundaries between the masked and unmasked areas.

Once the adjustments are complete. Save the file.

Luminance Combination

I’ve tried the rest of the luminance processing workflow with a luminance image containing stars and one without. The one with stars turns out more crisp as the additional luminance processing steps improve sharpness, which impacts the stars.

This is the point where we need to reintegrate the stars into our luminance image. The luminance should be stretched and have a star mask applied (inverted) to protect the stars.

Original Luminance with Star Mask
Original Luminance with Star Mask
Pixel Math Luminance Integration
Pixel Math Luminance Integration

We can now use Pixel Math to integrate the luminance (with stars) and the enhanced luminance (without stars).

  • Formula: $T+LumStarless
  • Rescale result: Enabled
  • Replace target image: Enabled
  • Drag the triangle onto the Luminance image with stars

Once applied, you should see the fainter areas get gradually brighter. This should be a subtle improvement. This is faint areas. If they are made too bright, it will look heavily processed.

Next Steps

Our next step for the Luminance Workflow is to add definition to the bright areas with Luminance HDR Transformation.