My Hunt for Pluto!

On the evening of Friday, August 25th, 2017, I decided to take my homemade 10-inch Dobsonian out to my back deck.  The Moon was still young, so the sky would be fairly dark a few hours after sunset.  Paired with surprisingly cool August temperatures, it looked to have the makings of a great stargazing night.

It had been weeks since I last used the big “light cannon.”  A combination of summer temperatures and humidity, all sorts of nighttime bugs, and the pre-eclipse fervor put my normal telescope usage on hiatus.  I planned out what I wanted to see, leveraging Sky Map.  The star clusters in Sagittarius seemed like good targets, followed by other deep sky objects like the Ring Nebula.  And oh yeah, Pluto is in the sky, so for kicks I put it on the list as well.

I started around 10:40 p.m., after the Moon was fully set for the night.  I turned my attention first to the South and Sagittarius.  Its remaining time in the 2017 nighttime skies is fading, and it may not be practically viewable by next month from my location.  Unfortunately, I had no luck in pinpointing the several star clusters in the heart of this constellation, due to them being already low in the sky and overtaken by my local light pollution glow.

So I scratched the South star clusters off my list and decided to try Pluto next, since it was in the vicinity of Sagittarius.  I held little hope of finding Pluto, but felt the need to try anyway, as I have been wanting to for a while.  Locating a 1,400-mile long object over three billion miles away is not easy, to put it mildly.  This would be unexplored territory for me, requiring all my rudimentary stargazing experience to date as well as the power of my 10-inch reflector.

I started by locating the “teapot handle” in Sagittarius.  It is barely visible from my yard, but leveraging the brightest star in the area, Nunki, makes for finding the handle quickly.  Nunki’s apparent magnitude is 2.05, a little less than Polaris’s, to it is still within easy viewing at my location.

As of mid-2017, Pluto is above the Sagittarius teapot asterism when looking from Northern locations on Earth.  The closet bright star to Pluto is Albaldah, with an apparent magnitude of 2.89, so still easy to find.  Albaldah is directly above the teapot, as shown in Figure 1 (above top).

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Albaldah is more officially known as Pi-Sagittarii, and it forms a triangle with two other “Sgr” stars in the area, Omicron-Sagittarii and Epsilon-Sagittarii.  This triangle provides a visual cue to where Pluto should be in the August 2017 sky, to the left of the triangle, as shown in Figure 2:

Assuming you are looking at the full image, you should see one brighter star within the orange circle along with two dimmer stars.  This brighter star is called HIP 94372, and with an apparent magnitude of 6.35, it is not visible to the eye.  So here is the leap from naked eye observing of the three Sagittarii stars to telescope viewing of HIP 94372.  Figure 3 below gives the Stellarium details on HIP 94372 along with an even closer view, now showing Pluto’s location on the evening of August 25th:

At this point in the observation session, I was heavily consulting Stellarium on my iPad, as there was no way to see the following-discussed stars unaided.  I leveraged my best-quality two-inch eyepiece, the 32mm Orion Q70 Wide-Field.  I post the name and link here not as an ad for Orion, but so you get a sense of the equipment used for this difficult exercise.  The Q70 is better than average as I have found that it significantly reduces the coma effect (blurriness around the edges) common in many 2″ eyepieces. In hindsight and for next time, I should also have had at-the-ready a high-powered 1.25″ eyepiece.

To get a sense of the field-of-view through the Q70, I was able to see both Albaldah and Omicron-Stagittarii at the same time in the same eyepiece field, with each star near the edge on opposite sides.

It is important to note here that we are discussing the limits of common star map apps.  We are getting down to 10th and 14th magnitude objects, so the overall accuracy of the maps may start to get fuzzy.  I am not saying Sky Map or Stellarium are wrong, only that this exercise approaches the limits of their usefulness.  Because as I discovered, it becomes very difficult at these magnitudes to align the computer map with what you see in your telescope.

HIP 94372, at 6.35 apparent magnitude, is easily seen through a 10-inch reflector telescope.  The second-brightest star in this area is unnamed with an apparent magnitude of 9.80 (see Figure 4).  This was still very visible via the telescope but much fainter than HIP 94372.

And so we come to the task of actually identifying Pluto.  At an apparent magnitude of over 14, is it visible at all from my Western Chicago suburban skies?  I could see, near HIP 94372, the ever-so-tiniest dot, which I assumed to be Pluto!  “Assumed to be” is key here as I cannot say for sure.  When you look at the Moon, Venus, Jupiter, Saturn, or Mars, you can say with 100% confidence what you are looking at.  But with Pluto, I am relying on approximations of a nearby bright star (Albaldah) to make even more approximations of faint star patterns seen only with a telescope.

I wanted to confirm my finding as best as I could, so I started hunting for noticeable star patterns in the area of HIP 94372 that I could recognize with the help of Stellarium.  Below and to the East I found one small set (see Figure 6).  But the “Rosetta Stone” was the pattern a little farther to the East still.  It is easily seen as a faint pattern in the telescope.  I call it “k-lambda” as I imagine it is the fusion of the letter k and Greek letter lambda in a Star Trek transporter accident.

All of the stars in k-lambda are in the apparent magnitude range of 8.2-8.3, which make them faint but still easily seen in my telescope. Figure 7 shows the details of one of these stars, called HIP 94784:

Finding these stars seems easy with the hindsight of a few days.  It involved a lot of “feeling around” past Albalduh to get my bearings at the scope.  Even moderately bright stars are easy through a telescope, but going past that 6-7 apparent magnitude threshold was like walking through a forest at night with little-to-no light.

So by leveraging these two small and faint star patterns, mapped towards the “bright” faint star HIP 94372 and anchored to the naked-eye star Albaldah, I can safely say that I found the location of Pluto that evening, even if I cannot say for 100% certain that I saw Pluto itself. See Figure 8 below for Albaldah, Pluto, and my k-lamba all in the same Stellarium image.

I really want to try to find Pluto again one more time this season with the experience I now have.  Unfortunately the Moon is growing towards Full each night, which will make the evening sky too bright for such fine work over the next week and more.  I estimate the Moon should be out of the way again around September 13th.  By then, Pluto will have nudged a bit towards the West, as shown below in Figure 9.  If I do get a chance to try for Pluto one more time this year, I will post a follow-up next month.

Thanks for reading to the end!

(And yes, I did find the Ring Nebula after my Pluto trek.)

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If you would like to follow more of my Pluto adventures, please visit the sequel post.