True colours of Uranus and Neptune revealed

Forums Imaging True colours of Uranus and Neptune revealed

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  • #621156
    Mr Jack Martin
    Participant
    #621162

    I recall this very point coming up at a lecture at last year’s Winchester weekend. Of course, it is well known that these camera images are enhanced. And some quite odd-looking colours can result from the combination of certain narrowband filters. Because news media are often uncritical, enhanced images can easily appear in print or on websites without any technical explanation of which filters have been combined. And members of the public think that the planet really looks like that. Sometimes the eyepiece view can help, and as I have been observing these planets since the 1970s I make a few comments.

    The colours of Uranus and Neptune are similar when examined in the eyepiece. I have always described Uranus as slightly bluish-green, in reflectors of up to 1 metre aperture. That was again my experience several times late in 2023 using a 254 mm reflector. To my eye Neptune is just a little more bluish, but not much more so than Uranus. Of course, without a close conjunction, one cannot compare them in the same field of view!

    I wonder if someone can work out when such an opportunity will occur? Doubtless it has already been done!

    #621163
    Dominic Ford
    Keymaster

    Dear Richard,

    I fear we’ve got a while to wait for such a conjunction. By my calculations, the next will be in 2164-6 (0.8-degree separation). A graph of the angular separation of the pair around this time is here: https://in-the-sky.org/graphs.php?gtype=8&startday=7&startmonth=1&startyear=2162&duration=8&obj1type=1&obj1txt=Uranus&obj2type=0&obj2txt=Neptune

    However, my calculations also suggest there was a recent conjunction of the pair (less close; 1 deg separation) in 1993: https://in-the-sky.org/graphs.php?gtype=8&startday=7&startmonth=1&startyear=1992&duration=7&obj1type=1&obj1txt=Uranus&obj2type=0&obj2txt=Neptune . I’m a few years too young to have observed that, but I’m sure there are plenty of others here who would have done!

    Cheers,

    Dominic

    #621165
    Dr Paul Leyland
    Participant

    1993 was my fallow period.

    Marriage tends to do that to people.

    #621166
    Grant Privett
    Participant

    Surely comparing the low dispersion spectrum of Uranus with that of Neptune would seal the discussion pretty well?

    #621169
    Robin Leadbeater
    Participant

    We have Uranus spectra in the BAA database but Neptune only in the IR

    I remember a comparison of the spectra by Christophe Pellier using a Star Analyser
    https://www.cloudynights.com/topic/645833-neptune-full-spectrum-comparison-with-uranus/
    (Very similar but Uranus has slight red/green bias compared with Neptune )

    Cheers
    Robin

    #621181
    Grant Privett
    Participant

    This might be worth a read: MNRAS 527, 11521–11538 (2024)

    18 fun packed pages… https://academic.oup.com/mnras/article/527/4/11521/7511973

    Figure 1 is interesting.

    • This reply was modified 10 months, 2 weeks ago by Grant Privett.
    • This reply was modified 10 months, 2 weeks ago by Grant Privett.
    #621184
    Robin Leadbeater
    Participant

    Figure 1 is interesting

    That pretty much matches what Christophe saw with the Star Analyser. If you normalise the spectra in the red/green then Neptune shows a slight blue excess and both planets have similar depth Methane absorption bands

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