Mars is getting closer, but can you photograph its dim moons, Phobos and Deimos?
As Mars approaches opposition on 8 December, its brilliance and position in the night sky will mean it will certainly be the centre of attention. Reaching opposition every 2.1 years, Mars is a tricky planet to get on with. Just as you’re starting to advance in terms of observing its features, it moves away from Earth again, until the next opposition when you need to relearn your techniques! One skill to learn is how to capture the two dim Martian moons, Phobos and Deimos, the subjects of this month’s challenge.
Mars oppositions are special and a time to make the effort to push the limits of your observing and imaging skills. Some oppositions are better than others and 2022’s, although not the best, is good for those of us in the Northern Hemisphere, because the planet will be high in our sky. Its closest approach falls on 1 December.
Phobos and Deimos are hard to see and challenging to image, but cameras stand a better chance of recording them. It is generally recommended that attempts are made when Mars is 20 or more arcseconds across through the eyepiece. At opposition on 8 December, Mars will appear 17 arcseconds across, which is less than ideal, but its higher altitude will play a positive part too.
In order to image the moons, you’ll need a planetary imaging setup that is capable of recording Mars as a disc. The best time to catch either moon is around elongation, their point of greatest distance, either east or west, from the planet. As they orbit Mars rapidly – Phobos every 7.7h hours, Deimos every 30.3 hours – the recommended strategy is to use a program such as WinJupos (freeware that you can download from jupos.org/gh/download.htm) to plan your observation attempts.
As the moons are fairly dim, Phobos being mag. +14.9 and Deimos +16.0, exposures of several seconds or tens of seconds may be required to record them, depending on your setup. If conditions are moist, light scatter on telescope optics or from the atmosphere can enlarge an over-exposed image of Mars to the point where it expands to cover and hide the moons. The best strategy here is to bracket your exposures and record your settings. When you end up processing the results, if you’re successful and reveal the dim moons, you’ll then be able to reuse the successful settings and refine your results next time.
One word of caution if using reflecting telescopes or any instrument that has front mirror supports: diffraction spikes will do a good job of obscuring the moons if the spikes are orientated to lie where the Moons appear.