Viewing Jupiter's Impact Site

So you want to see the impact site on Jupiter that was recently discovered by Australian amateur astronomer Anthony Wesley and are wondering how. You need a suitable telescope and to be looking at the correct time when the impact site is facing us.

Let's start with the when can I see it question:

This weekend there will be a couple of good opportunities to see the Jupiter impact site from the West coast with amateur telescopes. Jupiter will both be up and the impact site will be facing towards us.

The impact site is at longitude 216 deg (CMII longitude system) and the iceinspace site has conveniently calculated transit times for the next few days. A transit time is when the impact site is directly facing Earth. AS Jupiter takes ~10 hours to complete one rotation there are several times per day:

2009 Jul 24 05:16 ( 216°) 15:12 ( 216°)
2009 Jul 25 01:08 ( 216°) 11:03 ( 216°) 20:59 ( 216°)
2009 Jul 26 06:54 ( 216°) 16:50 ( 216°)
2009 Jul 27 02:45 ( 216°) 12:41 ( 216°) 22:37 ( 216°)
2009 Jul 28 08:32 ( 216°) 18:28 ( 216°)
2009 Jul 29 04:23 ( 216°) 14:19 ( 216°)
2009 Jul 30 00:15 ( 216°) 10:10 ( 216°) 20:06 ( 216°)
2009 Jul 31 06:01 ( 216°) 15:57 ( 216°)

These are UTC so have to be converted to Pacific Time by subtracting 7 hours. The next few opportunities together with the altitude of Jupiter (how high it is in the sky) are:

Friday night: the best time is 4:03AM Sat morning (altitude 29 deg)
Saturday night: the best time is 11:54 PM (altitude 37 deg)

Saturday night is the best option as the altitude is higher. Jupiter has a ~10 hour day so viewing is good ~1.5 hours either side of these times, provided Jupiter is above the horizon.

So now we know when to look but what do we need to look with?

The impact site on Jupiter is ~1/100th the diameter of Jupiter or <1 arcsec across. This is about 1/6000th the size of the moon.

To see this you need a telescope with a resolution of 1 arcsec or better and suitable atmospheric viewing conditions - the strong color contrast between the dark mark and the planet will help however. A telescope of 5 1/2 in (138 mm) diameter or larger has enough resolving power. You will also need to have enough magnification.

The rule of thumb is that a magnification of 50X per inch of aperture is typically the maximum possible. This means that for a 6 in diameter telescope the maximum usable magnification for a well collimated telescope under good atmospheric seeing conditions is ~300X. The eyepiece needed to get this magnification is found by dividing the telescope focal length by the magnification. Thus for my telescopes I get these results:

Takahashi FC76 refractor
Diameter: 76 mm
Focal Length: 600 mm
Resolution comment: too small to reoslve impact site
Max Magnification: ~150x
Eyepiece focal length for max.mag: 4 mm


Celestron C8 Schmidt Cassegrain
Diameter: 203 mm (8in)
Focal Length: 2032 mm
Resolution comment: Has sufficient resolution
Max Magnification: ~400X
Eyepiece focal length for max.mag: 5mm

Meade 12in Lightbridge
Diameter:304.8 mm (12 in)
Focal Length: 1524 mm
Resolution comment: Has sufficient resolution
Max Magnification: ~600x
Eyepiece focal length for max.mag: 2.5 mm

So I will be using my SCT to try to view (image actually) the impact site as the combination of aperture and focal length is best for this object.

Where to look for Jupiter?
If you need help finding Jupiter there are several resources:
Sky and Telescope interactive sky chart
Space.com monthly star chart


and many others online.

Good luck and Clear Skies.