Catch Mercury and the Pleiades
How to image the pair low in the sky, and widen your targets to Aldebaran and the Hyades
Mercury is well-positioned in May, joining its brighter neighbour Venus in the evening twilight above the west-northwest horizon. Together, the pair are easy to photograph due to their brightness. On 1 May, Mercury shines at mag. –1.0, Venus at –3.8.
There’s an added bonus because Mercury will pass close to the Pleiades open cluster in Taurus at the month’s start, which is an opportunity to image it with the cluster in the same field of view. However, there is a catch as both objects will appear low as the sky darkens enough for them to become visible. This presents a challenge for honing your astro-imaging skills, planning and executing the capture, and some post-capture processing to reveal everything clearly.
Whether you succeed will be down to several factors, the most important being the weather, the clarity of the horizon and how prepared you are. The weather does what it wants of course, but with Mercury close to the Pleiades for six evenings, at least we’re not reliant on just one date being clear. The clarity of the horizon is down to you – and how easy it will be to travel in May. However, being prepared is something that can be practised beforehand, because the Pleiades will be visible in the correct part of the sky, in dark twilight, in the run up to May. Attempting to image them beforehand will allow you to gain experience in the processes involved.
Take time to experiment
A background sky that’s not dark means there will be a limit to your exposure settings: set the ISO too high or exposure time too long and you’ll get an overexposed sky in the background. Again, this is something that you can experiment with. Begin with a lowish ISO value, say something in the 200–800 range; a lower range will give you better tonal quality in the image.
A fixed tripod should suffice, although a driven equatorial mount will work too and make life easier. If using a driven mount, set the ISO low, use an f-stop number in the mid-range and aim for an exposure of several seconds. The low ISO will keep the image quality high while the stopped down aperture of a mid-range f-stop keeps stars and planets sharp.
As ever, select a lens that will give you optimal coverage. If you’re after the Pleiades and Mercury alone, a long frame dimension of 8–10˚ is best. For a non-full frame (APS-C) camera, a lens with a focal length less than 125mm is ideal. On a full-frame camera, a 200mm lens will achieve the same coverage. If you want Aldebaran and the Hyades in shot, a long-dimension of 18˚ is recommended. Here, the lens requirements are 70mm (non-full frame) and 110mm (full frame).
The magic here comes from post-processing. If you get a shot showing Mercury with a barely discernible view of the Pleiades, it should be possible to tweak the final image into something looking reasonable.
Recommended equipment: DSLR, lenses, tripod or a driven equatorial mount, remote shutter release
Send your images to: gallery@skyatnightmagazine.com