Observation by Paul Leyland: Amalthea (Jupiter-V)

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Paul Leyland


Paul Leyland


2020 Sep 03 - 21:50


2020 Sep 06 - 12:16



  • 0.4m Dilworth-Relay
  • SX Trius Pro 814
  • No filter

630s in 21 x 30s subs. Average stacked on satellite mean motion.


Tacande Observatory, MPC J22

Target name

Amalthea or Jupiter-V


Amalthea (Jupiter-V)

About this image

Amalthea is of 14th magnitude and so would be very easy to image if it were not for the overpowering brightness of Jupiter --- which was over 16 magnitudes brighter and its limb  less than 35 seconds of arc distant. The image shown is the result of stacking on the predicted motion of Amalthea. The moon is in a close orbit and moves rapidly so the background stars are all trailed.

In the raw images and their stacks, Amalthea is only just barely visible in the background glow and then only if one knows precisely where to look. To remove much of the background and so increase the contrast (though at the cost of introducing artefacts) AstroImageJ was used for its rolling-ball background subtraction feature with a ball diameter of 30 pixels. Amalthea is marked as the circular dot whereas the stars appear as dark lines, generally at the start of one of the artefacts mentioned above.  The Galilean satellites are also indicated; from left to right they are Ganymede, Europa, Io and Callisto.

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Martin Lewis
Martin Lewis, 2020 Sep 08 - 09:48 UTC

Good work Paul on a very challenging target. Wondered how much Amalthea moves in 10mins?


Paul Leyland
Paul Leyland, 2020 Sep 08 - 13:42 UTC

That question is fairly easy to answer with the aid of the JPL Horizons site. Here is a snippet around the time of observation

Date__(UT)__HR:MN R.A._____(ICRF)_____DEC dRA*cosD d(DEC)/dt APmag S-brt ang-sep/v Ang-diam
2020-Sep-03 21:40 m 19 15 05.44 -22 43 50.7 -14.6825 -1.48978 14.211 8.368 52.3272/* 0.076895
2020-Sep-03 21:50 m 19 15 05.25 -22 43 51.0 -17.0342 -1.85940 14.211 8.368 50.4730/* 0.076895
2020-Sep-03 22:00 m 19 15 05.03 -22 43 51.4 -19.2889 -2.21977 14.211 8.368 48.2291/* 0.076895

The significant columns are those which read "-17.0342 -1.85940" at the time 21:50. They give the motion in RA and Dec in seconds of arc per hour. Pythagorus tells us that the motion was 17.14 arcsec per hour, or 2.9 arcsec in 10 minutes, at 21:50. Note that angular velocity changes very rapidly. It was 14.76 as/h at 21:40 and 19.42 as/h at 22:00.

Incidentally, the raw image scale is 0.29 as/px on my equipment. The image was taken in 2x2 binning mode, so 2.9" corresponds to 5 pixels in the image which was subsequently processed.

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