Photoshop Masks Series: Highlights, Objects, and Alpha Channels

Masks are one of Photoshop’s most central tools in astro post-processing. This article organizes several ways to build the masks I use most often: highlight-selection masks, object masks, and how to save a finished mask into an Alpha channel for reuse. Honestly, I’ve always been a bit lazy about seriously learning Photoshop, but since I was going to bring it into my workflow, I still had to shore up these fundamentals.

Highlight-Selection Mask

A highlight-selection mask is used to lock onto the bright parts of an image, making it easy to apply targeted processing to the highlight range. The steps are as follows:

Select → Color Range → Highlights, then adjust to the range you want
Select → Modify → Expand, to appropriately enlarge the selection
Select → Modify → Feather, to feather the enlarged selection
Press the “Add layer mask” button
The highlight-selection mask for that layer is done

Screen showing the steps to build a highlight-selection mask in Photoshop

The purpose of this mask is to make adjustments within the highlight range, such as sharpening, boosting saturation, or limiting the range where deconvolution acts. Conversely, if you invert this mask (Ctrl+I), you can protect the highlight areas and instead process the low-light parts like the background, for example noise reduction or lowering saturation.

Object Mask

The purpose of an object mask is to denoise and smooth the background while protecting galaxies and stars from being affected. The steps:

Duplicate the background layer
Ctrl+A to select all of the background, then Ctrl+C to copy it
Add a layer mask (create the mask)
Hold Alt + left-click to enter mask editing mode
Ctrl+V to paste the background copied in step 2 (it now appears in grayscale)
Ctrl+I to invert the mask
Use Levels to tune the mask to a suitable state

Object mask step 1: duplicate the background layer, producing a “Background copy” layer

Object mask step 2: Ctrl+A to select all of the background; a selection border appears around the whole frame

Object mask step 3: add a white layer mask to the “Background copy” layer

Object mask step 4: hold Alt + left-click to enter mask editing mode; the mask is fully white at this point

Object mask step 5: Ctrl+V to paste the copied background into the mask, appearing in grayscale (the galaxy is the bright part)

Object mask step 6: Ctrl+I to invert the mask; the background turns white, the galaxy and stars turn black

Object mask step 7: use Levels to tune the mask to a suitable state

Once the object mask is done, you can denoise the background without affecting the galaxy or the stars. This is especially handy when processing galaxy images: the background gets clean while the subject remains completely untouched.

Saving a Mask with an Alpha Channel

Anyone who has used Photoshop knows that selections and masks can be converted back and forth. But if I’ve painstakingly built a mask and want to reuse it later, where should I put it?

PixInsight is fairly straightforward on this front: a mask is itself an image file, so you just save it as a new file or bundle it inside a Project. And Photoshop? The answer is put it into the “Channels.” Beyond the three RGB channels, a file can hold quite a few extra channels, and we can store a mask as a channel. When we need it later, we convert it straight into a selection and turn it back into a mask for use.

Screen showing a mask saved as an Alpha channel in Photoshop

Note: Tutorial Videos Moved to YouTube

In the past I tried posting mask-related demo videos on Facebook, but the videos kept getting stuck in the review stage and would never publish. So for related tutorial videos going forward, I’ve moved them all to YouTube; fellow hobbyists who want them can find them on the channel: Evan Tsai’s YouTube channel.