Astrophotography
Master twilight photography.
There are some great opportunities to catch Mercury, Venus and the Moon in evening twilight this month. With the Northern Hemipshere’s spring equinox occurring on 20 March, this is a great time of year to view evening planets and the earlier phases of the Moon, in the west after sunset. This is because the ecliptic plane is starting to make a steep angle with the western horizon at sunset. As the planets and the Moon tend to remain close to the projection of this plane across the sky, a great circle known as the ecliptic, they tend to remain higher in the western sky after sunset than at other times of the year.
Mercury and Venus appear to dance with one another throughout the month, joined by a thin crescent Moon on 18 March. Dazzling though these objects can seem – especially Venus – they can be tricky to photograph well due to the background twilight sky.
By its very nature, an evening twilight sky is unevenly illuminated, brightening towards the region where the Sun has recently set. Variations in the quality of the atmosphere as well as cloud cover mean that settings used for one twilight shoot won’t necessarily be appropriate for another. An incorrectly set camera may overexpose the background sky causing you to lose the Moon or planets. Underexposure will create a scene that looks too dark and moody.
Then there’s the question of image scale. Use too wide a lens and bright planets that look so stunning to the naked eye will appear as tiny, hard-to-see dots against a bright sky. Use too high an image scale and the context becomes lost because it’ll be difficult to frame objects with the horizon. For evening twilight photography, it’s possible to wait for your targets to lose altitude as they approach setting. But wait too long and the thicker layer of atmosphere their light has to pass through will cause them to appear overly dim, removing the effect that inspired you to photograph them in the first place.
As should now be becoming clear, there’s a lot more to twilight photography than you might think. As well as the considerations previously mentioned, correct settings may not last long because as the Sun dips ever lower below the horizon, so the brightness of the sky continues to change.
The end of astronomical twilight occurs when the Sun reaches an altitude of 18° below the horizon and no longer affects sky brightness. Between astronomical twilight ending and starting again, the sky reaches a steady state of true darkness. The trouble is, by the time this condition has begun, the western planets will have set.
The brightness of the twilight sky often facilitates the use of basic automatic cameras, such as those built into smartphones. There is generally enough light present to allow such devices to both focus and determine settings. Often automatic settings aren’t conducive to capturing planets or the Moon and here the use of a third party app that will give you more control is recommended. Using such an app, it should be possible to emulate the settings recommended in the step-by-step guide.
Also, in the step by step guide we show you how to approach twilight photography and give you some tips on how to capture this month’s close conjunctions between Mercury, Venus and the Moon. Find out more about this conjunction on pages 52 and 56.