See asteroid Iris at its brightest with binoculars

One of the larger asteroids, Iris, has become bright enough to be seen easily with binoculars. It is also in a part of the sky where you can find it without difficulty.

A chart showing the path that Iris follows from early October to late November, 2017. Chart by Skymania, using Cartes du Ciel software

That is because it will spend the next few weeks close to the distinctive asterism made up by the three brightest stars in the constellation of Aries.

Iris is shining at magnitude 7.5 in early October, which is a little brighter than its typical brightness at opposition. But it will become noticeably brighter as it approaches opposition this year, on October 31, when it will each magnitude 6.9.

This is close to the brightest it can ever be, so it will be easy to see asteroid Iris with binoculars or a small telescope. It will not become bright enough to be seen with the naked eye.

Of its companions in the asteroid belt between Mars and Jupiter, only dwarf planet Ceres, Pallas and Vesta can become as bright. Currently they are all much fainter because of their distance.

The word asteroid comes from the Greek for “star-like” and that fits Iris well because it resembles just another star though binoculars or a telescope. Don’t expect to see anything more. However, it will be fascinating to see it brighten and then fade again over the coming weeks.

Iris is heading towards the asterism formed by the stars alpha (α) beta (β) and gamma (γ) Arietis. It will travel approximately parallel to and just south of an imaginary line joining α and β. Then on November 14, it will lie very close to γ, a star that reveals itself to be a double star when observed through a telescope.

Iris will reach its brightest in the third week of October, and remain around that magnitude until the second week of November when it will begin gradually to fade. By the end of November, it will be down to magnitude 7.6.

Iris was discovered by John Russell Hind, from Regent’s Park, London, on August 13, 1847, shining at 8.9.

How to photograph Iris

You can easily take a photograph of Iris, providing you have a camera that is able to take exposures lasting more than a few seconds. You must see the camera steady, so a tripod will also be useful.

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.

It is Full Moon on October 5, 2017, so the sky background will be rather bright, and so not allow long exposures at too high ISO ratings then. However during the rest of October, the Moon will become much less of a problem, giving you more flexibility.

You will need to be able to set your camera’s focus manually rather than use autofocus as starlight is usually 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). If your camera has “live view” then use that and zoom in on a star to help get the focus sharp.

Use a cable release, if possible, to avoid shaking the camera. Another way is to set the camera’s shutter-delay feature (usually between 2 and 10 seconds) between pushing the button and the photo being taken. It will allow 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, due to the Earth’s rotation, becomes obvious.

Longer exposures will record the stars as trails rather than points of light, which is still a pleasing effect if that is what you want! 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. Afterwards, you can use Photoshop or similar software on your computer to adjust brightness and contrast to enhance your photo if you need to.

If you take photos over several nights, you will be able to see how Iris has moved for yourself. You could again use Photoshop to combine your images and show the gradual movement of the asteroid.

Related: See what else is in the sky this month.


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