Beyond the stars
disruption can be fascinating to capture but do tend to require longer focal
lengths to provide enough image scale to pick them up convincingly.
Unlike when imaging the planets, the faintness of these objects means
that it’s necessary to grasp as much light as possible. Consequently, large
apertures will fare better than smaller ones. However, where clusters of
galaxies are concerned, one of their most fascinating aspects is the shells-on-
a-seashore appearance all of the different galaxies can take on.
Where there’s a mix of face-on spirals, edge-on spirals and ellipticals, a
wide field capture which shows the shape of each member can make for
an amazing group photograph. Here, the equipment requirements are less
demanding and such shots are well suited to small wide-field refracting
telescopes. Again, the relative faintness of the objects does warrant longer
exposure though, so autoguiding is once again recommended.low-power telescope is just the core of the galaxy. The real detail lies in the
outer extremities of the object. Here you’ll find the spiral arms littered with
bunches of star clouds.
To capture everything the galaxy has to offer, it’s necessary to take a
long exposure to capture the outer arms. This can sometimes lead to over-
exposure of a brightly lit core region and one technique to overcome this is
to take two sets of images – one for the faint outer regions and one for the
core. Both shots need to be taken following the same calibration process as
described in the previous chapter. When done, you should end up with two
images of the same object. One will show a bright core with barely any spiral
arms visible. The other will show the spiral arms well but the core will be
washed-out.
Both images should then be loaded into a layer-based graphics editor
such as Photoshop so that the over-exposed-core version is on the top of
the stack. It’s important at this point to make sure both layers are aligned
together in respect of the stars they contain. This can be done using the
move tool and using the keyboard’s cursor controls to gently nudge the
upper layer’s position. Toggling the visibility of the upper layer off and on
will reveal any misalignments. Rotational misalignments can be avoided by
making sure that both sets of images are taken during the same session
without moving the camera in between.
Once aligned, make sure the upper layer is visible and use a magic wand
tool to select the bright core. It’s not that crucial to make sure the selection
is right to the very edge of the core – the selection can be approximate. The
next thing to do is to create what’s known as a layer mask for the upper
layer. In Photoshop this is done by first copying the selection to the clipboard
and then holding down the Alt key (on a PC or some Macs) or option key (on
a Mac) while pressing the layer mask button shown below the list of layers
(normally a grey rectangle with a hole in the middle).
This creates a layer mask with the copied selection. A layer mask is
essentially a map of what should and shouldn’t be shown from the layer
being masked. Anything which is white in the mask itself indicates that the
layer should be opaque. Anything which is black in the mask indicates that
the layer should be transparent. Shades of grey indicate partial transparency
- the darker the grey the greater the amount of transparency applied.
The layer mask created will show the properly exposed core from the
bottom layer through the transparent hole that’s been created in the upper
layer. The hole will, at this stage, have sharp edges and look rather awkward.
The magic happens by selecting the layer mask and applying a Gaussian
blur to it – a common blurring function found in most graphics editors.
The degree of blur applied should be adjusted to allow the two layers to
merge naturally. It’s possible to adjust the appearance of the lower, properly
exposed core independently of the upper, properly exposed arms.
Photographing Galaxy Clusters
The process required to capture images of faint galaxy clusters is no
different to that described in chapter 8 for deep-sky photography. These
distant and consequently rather faint objects may require long exposures of
anywhere between 5 and 30 minutes to capture well so an autoguiding
set-up is highly recommended. The delicate streams of starlight that
can occur between galaxies which are undergoing mutual gravitational
6[6] The Space Telescope's Ulta Deep Field image at highest resolution reveals around
10,000 distant galaxies, some dating from 400 million years after the Big Bang.