Thebe, or Jupiter XIV


Thebe was discovered in 1973 in images taken from the Voyager 1 spacecraft and given its official name in 1983. It is the second largest of the inner satellites of Jupiter, though nowhere spherical in shape — it is a 116 × 98 ×84 km tri-axial ellipsoid.

When this image was created it was at magnitude 15.7, and so would be very easy to image if it were not so close to Jupiter. However, Jupiter is only 30 arc-seconds or so from Thebe at most (which is the case for this observation), and the planet is around 15 million times brighter than the satellite. The scattered light from Jupiter almost completely drowns out Thebe and very heavy image processing is needed to remove the background and to stretch the contrast of what is left.

Stacking the sub-images on the stars indicated that the limiting magnitude was at least 16.7, a full magnitude fainter than Thebe. The subs were then stacked on the predicted motion of Thebe but nothing could be seen near the expected position, other than a track of a passing cosmic ray a little to the north and west. An estimate of the background was made by blurring that image with a Gaussian kernel with a 20-pixel sigma. All fine detail, such as stars and cosmic ray hits, were lost in the blurring. That background was then subtracted from the original image. The subtracted image shows a faint circular object at precisely the predicted position for Thebe. It also shows a number of trailed stars and a scattering of artifacts, principally cosmic rays and imperfectly removed background. The over-exposed and distorted blobs are Jupiter and a coupe of Galilean satellites.

As far as I know, this is the only image of Thebe ever taken by an amateur astronomer. I have not been able to find any other images taken from a terrestrial observatory. The image is displayed negated (i.e. black stars on a light background) to help make the satellite more visible.

Image information
Date and time of observation  2020-08-05 21:25 UT
Telescope 0.4m f/6.5 Dilworth-Relay
Camera Starlight Xpress Trius-PRO SX814 CCD
Filter None
Exposure 1×20s and 62×5s sum-stacked on predicted motion of the satellite
Centre of image RA 19h23m11.08s  Dec -22°25'33"
Image dimensions 11.8 arcmin × 3.2 arcmin