A morning crescent Moon is a great target to photograph. The morning crescent represents a time when we’re seeing the illuminated portion of the Moon’s globe apparently diminishing in size: a waning crescent Moon.
An evening lunar crescent represents a time when we’re seeing the Moon’s phase growing: a waxing crescent Moon.
The term ‘young’ or ‘old’ when used to describe the Moon’s age refers to where the Moon sits in its 29.5-day synodic month – the complete cycle of phase of the Moon as seen from Earth.
This period is defined to reset at new Moon (age equal to 0 days), and grow thereafter, resetting to zero again at the next new Moon.
A young lunar crescent is one that’s less than a few days old. An old lunar crescent is one which is further in the synodic month than a few days before new Moon.
Photographing the Moon under dark-sky conditions is fairly straightforward. Such a Moon is easy to locate and focus on because it’s bright against the dark sky.
Old or young crescents present as thin slivers of illuminated lunar surface and, for this reason, appear less bright than they would in their fuller phases.
Thin crescents occur when the Moon is near to the Sun in the sky and this also hinders visibility, the crescent being easily lost against the bright twilight backdrop. As avid Moon-watchers will know, the Moon is often visible during the day.
Here we have a problem: in order to photograph such a Moon well, it’s essential to achieve sharp focus.
Being so thin, even a small deviation from the true focus position of your camera lens or scope may cause the Moon’s light to spread into a blur that is lost against the background sky.
One way around this is to pre-focus at infinity the night before. For a camera lens this requires it to be set to manual focus before carefully focusing on a distant object, such as a bright star or planet.
A piece of low tack electrical tape can then be used to carefully ‘lock’ the lens’s focuser ring in place, hopefully holding it secure in readiness for your next thin Moon attempt.
This technique doesn’t always work. Variations in temperature during the day or inadvertently knocking the focus ring, even while stuck down, may cause a change in focus.
It’s better to focus on something a long way off before your attempt on the Moon. For waning crescents that appear in the morning sky, this will mean getting up early to take advantage of a dark pre-dawn twilight sky using a bright star or planet for focus.
For waxing crescents, the situation is trickier. Focusing on the Sun is the better option, but this requires the use of a white light solar safety filter on the lens or scope to keep things safe.
As ever, remember to follow solar safety precautions and never look at the Sun directly with the naked eye.
Once focused, the hard part is done and by following our guide steps below, you’ll be able to grab yourself an ultra-thin Moon. Once you’ve caught one, you’ll be hooked to try for more!
Equipment for photographing a thin crescent Moon
- DSLR camera
- Shutter-release cable
- Lens with a focal length between 200-1,000mm
Photographing a crescent Moon, step-by-step
Homework is important for thin Moon attempts. Determine where the Moon is relative to the Sun and work out where it will be relative to the horizon after sunset. Choose a shooting location where the horizon appears low and unobstructed. A clear sky is best, as haze or low cloud will hide the crescent from view.
If you plan to use a lens, choose one which will show a good crescent. This typically means something with a focal length starting around the 200-300mm mark. A 1000mm setup (typically achieved using a telescope) will give you a good crescent size. A focal ratio between f/9 and f/16 is ideal.
Keep ISO in a low range of 100-1600. Start low and increase for evening sessions, as the sky darkens over time. For morning sessions, start high and lower the ISO as sunrise approaches. See opposite for how to focus accurately. If you can see the Moon on the back of the camera, use that to ensure the focus is sharp.
Line your camera up so the Moon is in view. There’s a chance you may not be able to see the crescent visually if it’s very thin. If so, you’ll need to judge where to point your camera relative to the horizon and rely on it being able to record the crescent. Take an image using a test exposure of 1 second.
Examine the result using your camera’s histogram display. If the graph is stacked hard to the left or right, you’ll need to adjust exposure. If it’s too dark, you’ll need to increase the exposure and/or ISO; if it’s too light, you’ll need to decrease the exposure and/or ISO. Aim to get the peak within the left-right limits of the graph.
Load the results into your image editing software. A tweak to the brightness and contrast settings may help bring out the faint lunar crescent. Alternatively, open the curves tool and place an anchor in the centre of the adjustment line, then adjust the curve’s shape to become more of an ‘S’ to improve its visibility.
Pete Lawrence is an expert astro imager and a presenter on The Sky at Night. This guide originally appeared in the February 2021 issue of BBC Sky at Night Magazine.