Early Autumn – The Perfect Time to See the Andromeda Galaxy!
Observing and Imaging the Andromeda Galaxy M31
The nights are drawing in, and there is a nip in the air. Early autumn with its darker evenings, is the ideal time to get into deep sky astronomy, and view one of the most spectacular sights in the night sky.
The Andromeda Galaxy, also known as Messier 31 or M31, is our cosmic neighbour as far as galaxies go. It is the largest member of the Local Group of galaxies, of which our galaxy the Milky Way is also a member. In many ways, it is a spiral galaxy twin to our own Milky Way but is around 10% larger. Lying 2 million light years away, the photons we see from Andromeda have been travelling since early humans first migrated out of Africa.
M31 also holds a special place in the history of astronomy - it was the galaxy that Edwin Hubble observed and helped him prove the existence of galaxies. This evidence helped overturn the prevailing view at the time that ‘Spiral Nebulae’, as galaxies like Andromeda were once called, were actually separate island universes. Hubble used observations of a particular type of variable star known as a Cepheid Variable to calculate the distance. The distance proved to be so remote that the stars and therefore the nebulae must be separate and independent from our own Milky Way.
How to see the Andromeda Galaxy
M31 is visible throughout the year in northern hemisphere, but the easiest times to view are autumn nights when it is high in the sky in the evening.
Naked Eye – From a dark sky location, M31 is visible to most people as a fuzzy blob. Averted vision works best, where rather than looking directly at the object, instead you look slightly to the side of the target. This uses the more sensitive rod cells in your eye and enables fainter objects to be seen.
The full spiral of Andromeda occupies about three diameters of the Moon, or around the size of a thumb width held at arm’s length. The Moon is also the bane of deep sky astronomy, and even though it is bright as galaxies go, M31 can get swamped by the full Moon, and benefits from a completely moonless night.
Binoculars – Binoculars can offer a good view of the full galaxy, although without a completely dark sky, making out more than the fuzzy central core is difficult.
How to Image M31
Almost since the beginning of photographic astronomy, the Andromeda Galaxy has been a prime target for impressive images. This has been the case right up to today where M31 was the inspiration for the recent winner of the ‘Astronomy Photographer of the Year’ award. An image of Andromeda, which was processed in a particular way to give the illusion of closeness, won the prize in September 2020. (See https://www.bbc.co.uk/news/in-pictures-54105085)
When imaging galaxies such as Andromeda, it is definitely one of those cases where patience is a virtue. Telescopes are basically light buckets for collecting photons, so the longer spent collecting that light the better. To get anything more than an image of the core and a fuzzy disk around it, a number of long exposures need to be ‘stacked’. A reasonable result which would show the dust-lanes circling about the centre of the galaxy needs at least 15 minutes or so of cumulative exposure time. 15x1 minute exposure images stacked together is preferable to a single 15 minute exposure. But of course the more the better, and the best results are gained from hours of exposure time.
To obtain the requisite long exposures, it is key to have an accurately set up mount. This would mean either a motor drive, or in-built computer-controlled tracking. And even with a well set up motorised mount, the long exposure times may show up star trails. The best way of solving the star trails problem is to use a small guide scope and automated tracking. Plenty of software such as PHD2 is now available to assist with this.
DSLR Cameras - Connecting up a digital single lens reflex (DSLR) camera to telescope with a T adapter is a relatively simple and inexpensive way of successfully imaging galaxies (if you own a DSLR already). Modern cameras and the chips they contain are well suited to astrophotography, and with the addition of a cheap remote control to trigger for the shutter, long manual exposures can be taken. The benefit of a remote control is that there is no resultant blurring when the cameras shutter-release button is pressed. There is a balance to strike between increasing the ISO rating to increase sensitivity, but which will also increase noise and the length of exposure.
CCD/CMOS Cameras – Dedicated digital cameras (in particular high-end CMOS) are becoming increasingly affordable and also well suited to deep sky imaging. It used to be the case that expensive CCDs were the technology of choice for imaging faint objects but recently this has begun to change and CMOS cameras are closing in with regard to performance.
In a similar manner to increasing the ISO value for DSLRs, increasing gain on a CMOS camera increases the sensitivity but with the detriment of a more ‘noisy’ image. For some of the common cameras based on a Sony chipset, a gain of between 200-300 is probably optimum.
Stacking Software - Once the stack of images of M31 are collected, there is a range of free software that can be used for example DSS (Deep Sky Stacking). A free Windows application for image stacking of deep sky objects such as galaxies.
Whatever your method of viewing or imaging the Andromeda Galaxy, it presents an accessible and enjoyable challenge for anyone. Clear skies!