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Imaging Thebe (Jupiter-XIV)

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Xilman's picture
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Joined: 24/03/2018 - 15:17
Imaging Thebe (Jupiter-XIV)

Is anyone aware of any images of Thebe taken by amateurs?  I have yet to find anything on the interweb thingy.

I believe I have succeeded.  Conditions were particularly good last night with fairly transparent skies (much of the calima dust has blown over) and reasonable seeing. I managed to catch the satellite when it was within an arc-second of elongation.

Thebe is notorious because although it is fairly bright, at V=15.7, its maximum Joviocentric separation is only 70 arcseconds. An exposure of only a few seconds is sufficient to detect a 15.7 mag star on my 0.4m Dilworth.

For this observation a total of 62 exposures of 5 seconds each and one of 20 seconds. They were co-added (by summing the pixel values to maximize the dynamic range) after compensation for the mean motion of the satellite during the exposures. An image stacked on the stars has a limiting magnitude of at least Gaia-g = 16.7, a full magnitude fainter than Thebe.

Nothing but a very over-exposed Jupiter, Gallilean satellites and a fewer relatively bright stars were visible at first. Even after adjusting the contrast range, nothing could be seen in the predicted position because of the overpowering background light from Jupiter. As with my earlier observation of globular clusters in M81, I blurred the image with a Gaussian kernel (with sigma set to 20 pixels) and subtracted it from the original. All the fine detail, such as stars and a few cosmic ray hits, were lost in the blurring, leaving only a relatively good approximation to the sky background.

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.

Some representative images will be posted to my personal page shortly.