Thebe and Amalthea are the two largest of Jupiter's innermost moons, although both are small and irregularly shaped. Amalthea measures approximately 250 x 150 x 130 km, and Thebe just 120 x 100 x 80 km. Both are dark and reddish in colour, with albedos of 9% (Amalthea) and just 4.7% for Thebe (similar to coal!) 

After a first successful attempt at Amalthea a few nights ago I decided to push a bit further to try and snag Thebe as well.  With its smaller size and lower albedo, Thebe at mag 15.4 is significantly fainter than Amalthea (mag 13.6), although it does orbit at a slightly larger distance and hence gets further from Jupiter's overwhelming glare. Thebe was only detected in 1979 using images taken by the Voyager 1 space probe as it flew past Jupiter and as far as I can determine has only been rarely imaged from Earth, making it a challenging target for a 4" scope.  

The orbital periods of the two moons are almost exactly 12 h (Amalthea) and 16 h (Thebe) , which means that every 24 hours Amalthea reaches almost exactly the same point in its orbit after completing two rotations, and Thebe swaps between opposite orbital positions after completing 1.5 rotations. From my location in Australia, at the moment both moons are pretty much at their maximum distance from Jupiter every evening around 9-10 pm - very convenient!

Following the process I used for Amalthea, I collected images for 2 hours using a red filter to reduce atmospheric scattering and increase the contrast between Jupiter and the red-coloured moons. A 2 sec exposure time was used, resulting in 3600 images. These were binned in lots of 400 (800 s total exposure) using a custom-written Matlab script that aligned frames on the over-exposed Jupiter. With Thebe very close to its maximum distance from Jupiter it appears virtually stationary with respect to the planet's centre, obviating the need to correct for its motion. Amalthea in contrast 'dives' into Jupiter and hence appears as a short streak. 

The glare from Jupiter shows a complex octagonal structure which seems to result from blooming in the CMOS array of the camera. Some residue of this pattern can be seen in the images above. To reduce the scattered light, a low-pass filtered copy of each frame was produced and subtracted from the original frame. This removes coarse structures - the scattered light pattern - and allows low-contrast fine details to be seen. At Thebe's location, the intensity of the scattered light is approximately 8000 ADC counts, versus just 80 counts for Thebe itself, meaning that the moon contributes just 1% additional intensity over the background. After the background subtraction, a simple linear intensity stretch was applied. No further image processing was used.

The background removal process results in a ring around the over-exposed part of the image that corresponds to Jupiter. A separate image of Jupiter (50% selection of 2000 frames, 2 ms exposure) acquired using the same equipment just prior to the main imaging session has been superimposed to indicate the location, scale and orientation of the planet. 

To confirm the identities of the two moons, I downloaded RA and DEC offsets for all of the inner and Galilean moons from the Jupiter Ephemeris Generator . The positions of Io and Europe with respect to Jupiter's centre were used to confirm the image scale (0.7" per pixel) and orientation. The expected positions of Thebe and Amalthea in the image were then calculated and are indicated with the broken horizontal yellow lines. Excellent agreement is observed, confirming the identity of the two moons. Io and Europa, both heavily overexposed, can be seen in the lower part of the image. Two stars of 11th and 12th magnitude can also be seen, streaked by the 800 s motion of Jupiter against the sky.

The final image presented above corresponds to the second 800 s integration period when Thebe lies at its maximum distance from Jupiter and can be most clearly seen. The original image is an animation of the first 8 x 800 s frames which shows Amalthea and then Thebe disappearing into the glare as they approach Jupiter.