So, recently I went out for a look at Neptune with my telescope and I found the general area of where Neptune was. However, when I took a see for myself, there were a ton of blue stars everywhere and I had a really hard time deciphering which one was actually Neptune, because along with all the other stars, Neptune is blue too and it blends in easily with the rest. How do I know which is the blue planet?
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1$\begingroup$ Do stars seen from your telescope have diffraction spikes? If they do, that’s an easy way to distinguish a planet from a star, since a planet won’t have them $\endgroup$– Justin TCommented Oct 11, 2022 at 23:59
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2$\begingroup$ Can you mention what kind of optics you are using to look for Neptune? Binoculars? Telescope? Focal length and aperture diameter? Also, potentially helpful: How am I supposed to locate the planets Uranus and Neptune with a 70 mm f/5.7 refractor? $\endgroup$– uhohCommented Oct 12, 2022 at 0:02
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4$\begingroup$ At about 200x power, you should see a recognizable disk. At low power, use a planetarium program (or star chart) to "star hop" and recognized it based on its position to other stars. $\endgroup$– Greg MillerCommented Oct 12, 2022 at 0:41
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$\begingroup$ d’Arrest confirmed Le Verrier’s Neptune solution with a… oh, nine inch refractor (off the top of my head), and could tell immediately it was nonstellar. I believe the remark was that it was ‘a big boy’ (except in German). I presume you are not using a world-class nine-incher? $\endgroup$– caInstrumentCommented Oct 17, 2022 at 23:27
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1$\begingroup$ Even at 125x (with 50cm aperture) one sees clearly that it is a disk compared to stars. IMHO the blueish colour is also quite distinctive, differing from blueish stars. $\endgroup$– planetmakerCommented Oct 18, 2022 at 1:15
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1 Answer
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Neptune is a tricky one: its color is not very different from a type A star, and its angular diameter is only twice that of the Airy disk of a star in a 20cm reflector. Even with good optics in good seeing conditions, the planetary disk will be a little blurry. To me it looks like a small piece of colored chalk, but that's not enough to be sure.
The keys to positive identification are a good finder chart and optics that help you match it. It's possible to locate it using a 1× sight and a 1° low-power eyepiece field, but something in between makes it easier. I use an 8×50 straight-through finder, which shows a ~5° field to magnitude ≲9.
The year-long charts at in-the-sky.org are at a useful scale. In Stellarium you might try a 10° field of view (ctrl-alt-6) and use the View > Markings panel to add a circular FOV corresponding to your finder scope. Remember that some optics will invert the view. If you can confirm that the stars you see match the ones on the chart, then the other object is Neptune.
