From the May 2015 issue

The storied sky

Astroimager Adam Block gives readers a little insight as to how he chooses the processing techniques to use on each of his images.
By | Published: May 25, 2015 | Last updated on May 18, 2023
People often ask how I choose processing techniques. Beyond the rigorous steps of calibration, what remains are artistic choices that blend style and editing. Asking which technique to use next, in a cookbook fashion, is approaching image processing backward. Instead, ask: What do I want the image to communicate that is thought-provoking? In this column, I’ll give two examples of pictures that tell fascinating stories and the processing decisions that gave them voice.

Globular clusters NGC 6522 and NGC 6528 (left image) float amid the seemingly uncountable stars toward the center of our galaxy. Here, we look through Baade’s Window and see more stars than normal because of a break in the dust clouds that pervade the area. In addition, two globular clusters, each representing a hyperbole of a stellar swarm, scream that stars and their sheer number are part of the story here.

What did Johannes Kepler, Heinrich Olbers, and even Edgar Allan Poe see in their mind’s eye when they considered an infinite universe of stars while trying to reckon with the darkness of the night sky? This stellar field could represent something close. And I wanted the picture to communicate the story of Olbers’ paradox, which asks why, if the universe is infinite, we don’t see stars covering the sky.

NGC 6522 (upper right) and NGC 6528 (lower left)
NGC 6522 (upper right), NGC 6528 (lower left), and the multitude of stars within this region inspired the author to create a scene illustrating Olbers’ paradox.
All images: Adam Block/Mount Lemmon Skycenter/University of Arizona
When processing the image, I applied a few more iterations to the deconvolution than I might otherwise do for a large diffuse object. I also masked this sharpened image less when blending it with the original. (See my January 2015 column about masks and deconvolution online at I then used an unsharp mask on the entire image at a value that was less than the average profile of stars. This acts as an edge enhancement and prevents stars from looking “connected” and indistinct.

I also brightened the image aggressively. Normally, stars cause visual confusion (see June 2015’s column about getting the maximum out of the minimum filter), but in this case we want the stars emphasized. Finally, when applying the mask for noise reduction, I was careful that “Smoothing” only act on the darkest pixels so the faintest stars didn’t dim.

My second example is the Orion Nebula (M42), which, even for all of its fame, holds untold stories. The stellar winds of embedded stars blow bubbles within the clouds of gas. I wished to communicate the motion of the gases in the nebula, so, compared to the above example, I needed to take a gentler approach.

For this image of the Orion Nebula (M42), the author wanted to show how already formed stars within the cloud create bubbles of thicker material.
Note that some of the bubbles shown are not centered on the stars. This is because the winds from the central stars are so strong that structures are blown back radially. Indeed, a star near the center has developed a beautiful bow shock due to the onslaught. Any high-contrast processing adjustments, such as high-pass filters and unsharp masks, reduce the translucent edges of the bubbles to stark boundaries that appear as nothing more than texture of the nebula. So, I monitor these structures at each processing step knowing that certain adjustments will greatly impact their appearance.

In these examples, my background in astronomy helped me find inspiration to highlight elements in the images. I encourage you to find compelling attributes to your own astrophotographic subjects. It may be that a single feature in the image is the starting point, or it could be that the subject embodies a more conceptual theme. With this in mind, the road to processing your images will be more direct because you will have a clear destination.

In my next column, I will show how to create field-of-view indicators without specialized commercial software.