This past week, I have had my telescope out almost every night, snapping pictures of Jupiter. More articles will be forthcoming with new images of our largest planet. But before Jupiter and its current area of sky fades into the Southwest for the Winter, I wanted to search for another planet in its proximity: Neptune.
Neptune is the farthest known planet from our Sun in the Solar System (we shall leave the Pluto matter aside for now). For a brief moment this week, I fancied the notion of attempting a Neptune photographing session from my Dobsonian. I quickly realized, though, that Neptune is far too distant, small, and dim to attempt an astrophotography pinpoint without the aid of an electronic guided mount. Plan B involved binocular observation followed by a sketch at the Dobsonian, leveraging an eyepiece intended for naked-eye viewing.
I searched for, found, and sketched Neptune and its surrounding sky over the course of back-to-back nights. In this article, I detail my methodology for finding the eighth planet via star maps, star hops, and final location through binoculars and telescope.
Night One – Planning and Binoculars
Let’s start our narrative by looking at my South night sky, several hours after Sunset. Most of my planning and posted screenshots here are from Stellarium, my go-to application on PC, iPad, iPhone, and even web for locating the faintest of sky objects.
This may be a little confusing, but I wanted to show the Constellation outlines for reference. It is important to realize that, especially through light pollution, this is a horribly “empty” area of the sky, with few bright stars. Pisces and Aquarius, the Constellations Neptune is between, are two of the faintest, especially in the context of the Zodiac. The implication is that faint stars make “star hopping” difficult.
Fortunately for this year, we have a very bright “star” in the area, in the planet Jupiter. It is from Jupiter that I was able to star hop via binoculars to get to the area of Neptune. So even with zero naked-eye-viewable stars around Neptune, leveraging Jupiter as a starting point made it possible, through binoculars this night and telescope the following, to track stars over to Neptune.
Here is better-labeled view of my South sky, showing the locations of Jupiter, Neptune, and Saturn, in the Southwest at this time. Even though Saturn is of little importance to Neptune’s location, it did provide me with a relative bearing against Jupiter to guess the approximate location of Neptune.
In my planning methodology, I wanted to zoom-in on that area of sky with Jupiter and Neptune, again assuming Jupiter would be my starting point for eventually locating Neptune. In Stellarium, I zoomed in:
In this closeup, we start to see a series of stars to the right of and below Jupiter which may “launch” us into at least Neptune’s general proximity. Here is the same screenshot, with how I imagined the useful stars to perform a star hop:
A star hop from Jupiter’s current position to Neptune is not easy, but there is at least a jumping-off point if planned correctly. To the 4:30 position below Jupiter, there are four stars, part of Pisces, that roughly form a rectangle. They were fairly easy to locate through binoculars. From the rectangle, two other (relatively) bright stars form a line that start the push into the general direction of Neptune.
The next step, for finding Neptune with binoculars, is a bit of a leap of faith, since the last “bright” star in the hop above, 20 Pcs a.k.a. HIP 117375, is magnitude 5.45, and the brightest star near Neptune is going to be about 6.25, at best (lower numbers indicate brighter sky objects). As reference, Neptune’s magnitude is 7.82, which I soon learned, is barely at the visible limit even through binoculars.
So we need to “jump off” from star 20 Psc, to the right, and try to land in the general location of Neptune, which has a much fainter group of stars. Here is a more detailed view in Stellarium from the start of the top of the “rectangle” to the right, to 20 Psc, and then to the general location of Neptune:
And for guidance, here is the same picture showing the hop flow:
In the above image, notice the upside-down cross that I marked. These four stars, along with a few below them, are the best guide to the current location of Neptune. As you can see, Neptune is just to the left of the cross. So from 20 Pcs, we can bypass Neptune a bit to land on the cross, which as I learned was easy to spot with my binoculars.
Once at the cross, knowing the approximate location of Neptune should be easy, but remember, Neptune is very faint. Let’s focus on the upside-down cross and Neptune:
The star of the cross closest to Neptune is HIP 116402 at magnitude 7.20, so slightly brighter. Through my binoculars, I was able to see HIP 116402 fairly easily. However, I had trouble seeing Neptune.
Enter the concept of averted vision, which is a phenomenon where you can see faint objects by not looking directly at them. It was only through averted vision that I could see the spec that is Neptune, through binoculars. The technique takes a little practice through an eyepiece, but as I kept looking above Neptune, I was eventually able to see it.
So on the night of September 29th, 2022, I can safely say that I observed Neptune for the first time.
Night Two – Sketching through the Telescope
I considered the find through binoculars as the first step, since I really wanted to see Neptune, without averted vision, through my big Dobsonian. On the next evening, September 30th, I set up my 254mm telescope for observation. I also set up Procreate on my iPad Pro to sketch, which I have not done for some time.
Unfortunately, clouds rolled in just as I wanted to get started before midnight. Thankfully, as a weekend night, I could spare staying up even later. The clouds finally cleared after 1 a.m.
At the telescope, using my telescope’s two view finders, I was able to replicate the star hop method from the night prior, to find the cross. The only tangible challenge was wrapping my mind around the upside-down view of Newtonian refactors like my Dobsonian. Essentially, I was looking at a backwards image, and had to realize I needed to find Neptune to the right of the cross and not the left, as seen through binoculars the night before.
For reference, again the closest bright star to Neptune is HIP 116402:
You see a few stars above HIP 116402 and Neptune, forming a small arch between the two. It was this pattern that I found looking through my Q70 32mm (my best 2″ eyepiece). I have marked HIP 116402 and Neptune in my sketch which is at the very top of this article. Here is the raw sketch without the markers, so you can compare the “dots” with what was shown in Stellarium:
These dots/stars are really the only objects I saw within this view, nothing much past magnitude 8.
Through the Dobsonian telescope, I had no trouble locating Neptune, once I acclimated my viewing to the upside-down image. Neptune is far, far brighter than Pluto, which is barely at the edge of my Dobsonian vision, on a good night (Pluto being +14 magnitude). Magnitudes under 8, like Neptune, light up very nicely through a large Dob.
In 2023, I hope to apply this same technique to see Uranus again, when it returns to the night sky.