I am delighted to have recorded the motion trail of the magnitude 18.72 moon Nereid, over a period of 3 hours, straddling the midnight hours of 17th/18th July 2010. Until 2002, Nereid was the outermost known moon of Neptune from its discovery by G. Kuiper in 1949.
Nereid details
The mean radius of Nereid is 170km and it has a mean magnitude of 18.72. The orbital period of Nereid is listed as 360.13619 days. Nereid was too far away from Voyager 2 to be properly imaged when the spacecraft visited the Neptune system in 1989. Photos sent back show only its highly irregular shape, and no surface features could be seen at the resolution available. The parent planet Neptune (diameter 49,528.0 km) lies some 2.796 billion miles from the Sun and has a rather chilly surface temperature of -209°C.
Finding Nereid
Using the NASA (JPL) Horizons system, I generated an ephemeris for the 17/18th July 2010 at 1 hour intervals and then slewed the ‘scope to the coordinates generated by Horizons. Neptune can currently be found in the constellation of Aquarius.
The data has been extensively edited to produce a smoother looking animation as follows:
The 9x20 min sub-frames were combined to improve the image quality of the background and I removed the grossly over exposed disc of Neptune using the “Content Aware Fill” function in CS5.
An RGB disc of Neptune was generated from a separate set of 4 sec exposures through RGB filters and then blended in with the positional data in each of the 9x20 min sub frames.
The composite sub-frames were then blended in with the background field to generate 9 files for the animation. Some positional integrity (± 1 arcsec) and brightness accuracy will have been compromised through these techniques.
Notes:
The animation frames are a series of 800x600 crops from the original 1600x1200 pixel frames.
In the animation, Nereid is shown (dimly) approx 2 cms to the left of frame centre.
Using CCDStack, the central brightness of the Nereid “clump” was typically some 300-500 ADU above the average background of around 16,900 ADU.
Using CS5, I zoomed in on the footprint of Nereid and it appeared as a central brighter pixel within an array of 9 pixels, with the outer 8 being dimmer.
Some thin, high cloud affected a few of the frames.
I have included a single frame showing the motion of Nereid, but with a stationary Neptune disc rather than a trailed disc.
Imaging details:
Brisbane, QLD, Australia
17th July 11:24pm to 18th July 2:11am AEST
Celestron C9.25 F10 SCT with x0.63 Reducer.
SBIG ST2000, 9x20 min exposures.
Image scale 1.03 arcsec/pixel
Stunning result Dennis, congratulations Nice and comprehensive write up too. You are really giving that ST2000 a workout!
Is Nereid showing orbital movement relative to Neptune in these frames? I ask because Triton seems to be static with respect to Neptune so I wouldn't expect Nereid to move either during the same time, but Nereid and Neptune/Triton seem to follow slightly different paths in the animation?
Dennis I love your work on the outer planets as you push the boundaries of whats achievable for amateurs.
Just a tiny observation. Usually when I see an animation there is normally some flickering or variance with time of other static objects. The fact you have made those objects totally static makes the presentation seem synthetic and clinical rather than real.
Just my impression and I hope you take the small criticism in the spirit intended.
Thank you everyone for your comments and feedback, they are much appreciated. Been away on an at times somewhat wet holiday on Moreton Island, so apologies for the late reply!
@Rolf – I’ll revisit my processing steps in light of your comments and see if I have introduced any anomalies here.
@ Geoff – with animations I noticed that variations of “constant” data such as the sky background and fixed stars tends to blow out the file size, so I have been experimenting with standardising the background and then adding the moving objects as Layers in Photoshop and I agree – it does look quite artificial compared to seeing the variations across the original real sky frames.
Great going Dennis. You've made it look deceptively easy again, when in reality I'm sure data collection and image processing was a very exacting process. Congrats!
Nice and comprehensive write up too. You are really giving that ST2000 a workout!
on Moreton Island, so apologies for the late reply!
