I’ve mentioned it before that you don’t need a fancy telescope and tracking equatorial mount to get into astrophotography.  Simply a camera on a tripod with a short focal lens can do wonders, especially with the high ISO settings in new cameras. A single 10 seconds exposition can reveal lots of stars, however to capture more photons a longer exposure is not better as the stars will become streaks.  But one can easily improve the image and get better signal/noise ratio by stacking multiple images.

However, there is one drawback to stacking multiple exposures if you decide to also capture the landscape: Earth rotates, therefore the sky moves while the landscape stays still.  If you align the images using the stars, then the landscape becomes a blur.  Not the end result that we want.  Luckily a quick composition with two layers and a mask solves everything.

Below is a single 10 seconds exposure at ISO 800 with a 17mm F4 lens; you have the landscape with city lights and the stars above.  Yes that is Orion…

Single 10sec exposure (ISO 800)

In order to improve my signal, I worked with IRIS to align and stack 5 frames, this reveals many more stars, but also amplified the light pollution.

Aligning and stacking 5 images. More stars appear.

Luckily within IRIS there is a function to remove sky gradient.  The algorithm takes a series of sample points and attempts to make the sky uniform.  Not bad, the images are not a hopeless case.

Removing the sky gradient with IRIS

As mentioned above, the alignment was performed with the stars, hence the background is now blurring.  Below is a close-up.

But when aligning on stars, the landscape blurs.

That is just 5 images, stack a much larger quantity or with more time between frames and it will only get worse.  It becomes pointless to shoot with the landscape if the end result is blurry.  Luckily working with layers in a photo editor can easily solve the issue.  We want to keep the stars from the stacked image, but the landscape from a single frame.  Follow these easy steps:

1. Load into your base layer one of you single frames.  This is what will be used for the landscape.
2. Load into a new layer your stacked image.  As your stacked image contains more and brighter stars select to Lighten Only instead of normally adding both layers.  You can play with the brightness of the stacked layer, and/or darken the base layer to get the desired blending.
3. Create a mask to the stacked layer such that the blurred landscape is not permitted to show through.  See image below, I simply grabbed the airbrush and blackened the landscape area in the mask such that it will not show through the layer.  Note that the I only edited the mask, not the image itself.

Creating a mask for my layer: white is transparent, black will block

The end result, is improved image of the sky, and a landscape that is still sharp.

Both layers added with the mask

Below is a comparison the composition with stack and layer (left) and a single shot (right).  We are able to achieve both of our goals of getting more stars (more signal) while keeping the landscape from becoming a blur.

Comparing the composition with layers (left) and single shot (right)

And why not take some time to identify some key features in the image.

Constellations Orion and Taurus above the landscape. (Click to open)