How to see brightening Comet ASASSN with binoculars

A comet is making its first approach to the inner Solar System for around 3,600 years. Comet ASASSN is expected to be visible with binoculars over the next few weeks.

This is a wide-angle view showing the track of Comet C/2017 ASASSN O1 during September and October as viewed from mid-northern latitudes. Chart by Skymania

It has been named Comet ASASSN, because it was detected by an automatic sky survey camera with that memorable name for an acronym. It is short for the All-Sky Automated Survey for Supernovae as its main task is to keep watch for exploding stars.

In the course of its regular imaging of the night sky, it picks up other transient events, including this comet.

The comet, whose full official label is Comet C/2017 O1 (ASASSN), was detected back on 19th July, when shining at a very dim 15th magnitude, by the automated survey instrument at Cerro Tololo Inter-American Observatory in Chile. Since then, it has brightened considerably to around magnitude 9.

This makes the comet still a challenge if you are looking with binoculars or a small telescope. Unlike a 9th magnitude star, which shines as a distinct point of light, the comet’s glow is spread out as a less obvious blur.

The comet is at the centre of this (cropped) photo. It is a stack of 10 images taken on the morning of 28 August using a 50 mm lens on a Fuji X-M1 camera (90 sec at ISO 1600) from Walmer, UK. Image credit: Paul Sutherland

It has already been possible, however, to record it with digital cameras. The author caught it in a series of short exposures, lasting between 30 and 90 seconds, on the morning of 28 August. It looks like just a small fuzzy object, though the centre is more condensed.

The good news is that the comet is visible against a dark sky, far from the Sun. Many comets only become bright when they are in twilight so are less easy to see.

Currently, in the early days of September, the Moon is approaching full phase, which will hamper attempts to see it. But after Full Moon on 6 September, its phase will begin to wane. Conditions will be much more favourable in the second half of the month.

Comet ASASSN is expected to continue to brighten during the coming weeks as it heads further into the Solar System. It reaches perihelion, its closest point to the Sun, on 14th October. However, it will still be further from the Sun than planet Mars. After perihelion, it will begin its long journey back into the depths of space, not returning to our vicinity until the 57th century.

How bright will Comet ASASSN become?

It is impossible to say exactly how bright Comet ASASSN will become, because comets are very unpredictable objects. Sometimes we expect a lot from them (remember ISON?) and then they just fizzle out. Other comets may flare, due to an outburst of gas from their icy cores as they are warmed by the Sun.

Comet ASASSN has already been in outburst. Shortly after its discovery, it suddenly became about 100 times brighter, reaching magnitude 10. Its present magnitude of 9 is still brighter than the figures calculated by the NASA-funded Minor Planet Center, hosted by the Smithsonian Astrophysical Observatory.

Astronomers’ best guess is that it will reach 8th magnitude, making it a nice target for binoculars, but if we are lucky it could become brighter than that.

How to find Comet ASASSN

We have produced some charts, using the free Cartes du Ciel software. Our first is a wide view of the sky, showing the comet’s track through Taurus and northwards between the familiar shapes of Auriga and Perseus. We also show more detailed views of the track in the months of September and October.

The little comet “ticks” along the track show where the comet wil be at 0h Universal Time (UT) at the start of the date numbered as it heads from the bottom of the chart towards the top.

You will need binoculars at least to see the comet. It should be distinguishable from nearby stars by its fuzzy appearance.

This is a wide-angle view showing the track of Comet C/2017 ASASSN O1 during September. Chart by Skymania
This is a wide-angle view showing the track of Comet C/2017 ASASSN O1 during October 2017. Chart by Skymania

How to photograph Comet ASASSN

You can photograph Comet ASASSN for yourself if you have a camera that is able to take manual exposures lasting more than a few seconds. You must keep the camera steady, ideally by mounting it on a tripod, and pointed towards the stars indicated for the correct date on our charts.

Open the camera lens to its widest aperture if you can and set the camera’s “film speed” (or ISO rating) to a high number of 400 or above so that it is more sensitive to faint light. If you have no light pollution, and the Moon is out of the sky, you might be able to go up to 1600 or even 3200.

The camera will need to be set to manual focus rather than autofocus as starlight is generally too dim for most cameras to be able to lock onto it. Turn the lens to infinity (the little squiggle resembling a figure 8 lying on its side). You can achieve even sharper focus if your camera has “live view” and you can see a bright star on your screen while you are focusing.

Use a cable release, if you have one, to avoid shaking the camera. Alternatively, you could set the camera’s shutter-delay feature (usually between 2 and 10 seconds) between pushing the button and the photo being taken. People usually use this facility so they can get themselves into their own photos, but it works equally well in allowing the camera to settle down once you’ve fired the shutter!

Using a standard lens, you should be able to open the shutter for 15 seconds or so before the motion of the stars across the sky becomes apparent. This is due to the rotation of the Earth. Longer exposures will record the stars as trails rather than points of ligh. With a telephoto lens, the effect is more noticeable, and your exposures will need to be shorter than 15 seconds.

Take a range of shots, using different exposure times and ISO ratings, and see which gives you the best results. Nowadays, in this digital age, you can find out almost immediately by checking your camera’s screen. Later you can use Photoshop or an alternative piece of software on your computer to adjust brightness and contrast to enhance your photo if you need to.

Related: The night sky this month

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