Observation by Paul Leyland: Neptune, Triton and Nereid

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Paul Leyland


Paul Leyland


2018 Nov 27 - 20:40


2019 Jan 22 - 15:48



  • 0.4m Dilworth-Relay
  • SBIG-8 with AO-7 autoguider
  • No filter
  • 2018-11-27T20:40:40 -- 22:46:50
  • 2018-11-27T20:44:28 -- 20:44:31

22 subs median combined 400s for Nereid, single 3s for Neptune & Triton


Tacande Observatory

Target name

Neptune, Triton and Nereid


Neptune, Triton and Nereid

About this image

The second in my series of imaging smaller satellites in the outer solar system, Nereid was markedly easier than Sycorax.  Neptune and Triton were overexposed even at 3s but that's the best I could put into the composite.  Nereid is circled in the main image which is made from 22 subs median-combined with a total exposure time of 400 seconds.  According to the MPC ephemeris Nereid was magnitude 19.4 at the time, though I've not (yet) made any attempt at either photometry or astrometry.

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Paul Leyland
Paul Leyland, 2019 Jan 24 - 23:49 UTC

Now done the astrometry using GAIA-DR2 positions of 49 stars within 10 arcmin of Nereid converted to J2000.  SExtractor was used to locate the centroids and IRAF/CCMAP to set the WCS (with RMS errors of 0.7as in each coordinate).  XY2SKY then returned the following position for Nereid: 23:00:46.425 -07:22:55.94

The MPC ephemeris for the mid-point of the exposure is computed as 23:00:46.3 -07:22:59 which differs from my result by 0.1s in RA and 3" in Dec.  I'm happy enough with that, given the faintness of the image and that the plate scale is 1.4 arcsec per pixel.

Lesson learned: 10 minute exposure isn't really long enough for accurate astrometry on a 19.4 mag object

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