What do these images show?

Observing surface detail on the Galilean moons has been possible for decades. Bernard Lyot compiled amongst the best maps of the moons using the 38 and 60 cm refractors at Pic du Midi in the French Pyrenees. For the last few years, some amateur astronomers have produced images with surface detail visible on Ganymede. What do these features represent and how can you take them with your telescope?

It is one thing to take an image of the Galilean moons which purportedly shows surface detail on Ganymede, Io or Callisto. It is quite another to be able to prove the reality of these features. I have not yet seen a detailed examination, by amateurs astronomers, of the reality of these features, or of how much of the detail is real. How can this be done?

Synchronous Rotation - It is important to be able to repeat the observations. Repeatability lends weight to the reality of the features being observed. Since the Galilean moons are locked into synchronous rotation due to tidal forces, they always keep the same face towards Jupiter. This means that after every revolution around Jupiter, the same face of a moon is presented to the Earth making repeated observations of the same face of the moon possible. For example, Ganymede rotates (and revolves around Jupiter) in 7d 3h 45m Earth days. So if you photograph Ganymede after slightly more than a week, you will be observing the same side of the moon. Comparison of the images is thus possible and invaluable - any features represented on both such images is more likely to be real. Also, try to take images several minutes apart. Ganymede rotates slowly, so if you take 2 images separated by 1 hour, they should show the same side of Ganymede. Look at my March 6 image of Ganymede below. You will see that the detail is reproduced after 42 minutes AND is in agreement with the JPL simulation view showing Galileo regio and Osiris (Osiris is the white patch at the 1 o'clock position in my image).

Comparison with Maps - You should compare your images with prediction-software generated images of the moons. One of these is available from JPL (JPL Solar System Simulator). You should be able to identify features in your image with features shown on the predicted image. Although JPL generated maps were not made with visible-light filters (and so will not be identical to webcam images), they should be in rough agreement.

Apparent Sizes and Albedo Features - The apparent sizes of the moons makes photography of surface detail difficult. At favourable oppositions, the sizes of the moons ranges from 1.72" for Ganymede down to 1.03" for Europa. Ganymede does possess several large dark areas such as Galileo regio and Nicholson regio, and some small bright crater ray systems such as Osiris. Europa and Callisto do not have any large albedo features. Io has several very large-scale albedo features such as Pele, a large volcano. My image of Io, below, shows it when Pele was visible from Earth. In my image, Pele is visible as a darkening just south of the equator (compare to JPL view, where Pele is visible as a prominent orange-ringed area).

Filters - If you image Jupiter and Ganymede when they are less than 50 degrees above the horizon, you may need to use an infra-red (IR) blocking filter to prevent IR light from Jupiter reaching your CCD detector. This is because at altitudes below 50 degrees, differential atmospheric refraction of light will cause IR light to appear offset from red, green and blue light to such an extent that focussing is difficult, and getting a sharp image is impossible. In fact, at low altitudes, red, green and blue light images will need to be shifted back into a single image using programs such as Registax or Photoshop. However since the IR light will be included in the red light image, it is almost impossible to shift its image back into correct registration, hence the need to block it out with an IR light blocking filter. The best thing to do, however, is to image Jupiter when it is at least 50 degrees above the horizon since, by excluding any filters, you reduce optical abberations introduced by extra optical elements.

Image Scale Required - Anyone wishing to image surface detail on Ganymede needs to provide an image scale which shows Ganymede as an image with a diameter of many pixels. I used 0.13"/pixel which gives an image of Ganymede about 12 pixels across. Anything smaller than this would make the appearance of surface detail more difficult. A larger image scale would be desirable, if you can achieve it. For a 31-cm aperture telescope, use f/28-f/35.

Sharpening Filters - Sharpening filters have an annoying habit of introducing artefacts. This is due to the fact that, by their nature, they emphasize differences in brightness value between adjacent or near-adjacent pixels. Although we hope that these differences arise due to the surface detail we are trying to image, they more commonly arise due to noise in the images. Consequently, aggressive sharpening with unsharp masking filters (such as those in Registax or Photoshop) can lead to the present of "surface detail" which is actually image noise. Be very careful about fooling yourself into thinking that features on Galilean moons in your images represent surface detail.

Ganymede on Feb 20

Io on Feb 22

Ganymede on March 6

Ganymede on March 14

Ganymede on April 18

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Last updated February 2016