Seeing Pluto

July 16th, 2015

All the excitement over the flyby of the New Horizons probe past Pluto recalled to mind my own experience with Pluto. I am one of the tiny number of people who have actually laid eyes on Pluto. It’s quite difficult, and the effort is an entertaining tale.

Pluto is too faint to be seen in most telescopes. It ranges in brightness from about 14 to about 16, depending on where it is in its elliptical orbit. Now, theoretically, you should be able to see Pluto with a 12-inch telescope, and I suppose that a few people have actually pulled this off, but my experience suggests that this requires a considerable exercise of imagination. 

In 1974, I was a graduate student at the University of Missouri at Columbia. The Physics department had a 16-inch telescope in a dome on the roof of the Physics building. Theoretically, Pluto should have been easy to see in such an instrument. In practice, however, there are always factors that reduce the performance of a telescope. Among these:

The angular altitude of the object. The atmosphere absorbs some of the light from an object; the lower you look, the fainter the star appears. Pluto at the time was, if I recall correctly, in the constellation Ophiuchus, at a declination of about -15º. This means that, at best, it would be at an angular altitude of only about 35º. That cost about half a magnitude.

Next comes atmospheric clarity. The biggest obstacle here is water vapor. The greater the absolute humidity, the more light is blocked. The single most important factor here is temperature; colder is better. Unfortunately for us, Pluto’s location in Ophiuchus meant that it was best observed in summer, when it’s hot. However, we realized that we could look for Pluto in March well after 1:00 AM, and it might be pretty cold. Accordingly, we waited until a cold front swept through and showed up at the observatory at 1:00 AM to make our effort. 

A third factor is background light. This comes from city lights. The ideal observing location is far from any artificial lights. Unfortunately, we had no control over the amount of light pollution; we had to do the best we could in Columbia, Missouri. Fortunately, the town was not too large and many of the lights were out in the early morning hours. 

When we arrived at the observatory, we opened the slit, and pointed the telescope to the place in the sky where we thought Pluto would be.

This process was tricky. We had a number of star maps of different resolutions. We did not use the best star map to guide us; we did not even open that star map prior to looking for Pluto. Instead, we used the next-best star map and plotted the position of Pluto on that map. This got us close enough that we knew that Pluto was somewhere in the field of view we were looking at. We memorized the pattern of stars in that field of view.

The fourth factor in limiting magnitude is your eye sensitivity. When the eye is exposed to light, the photoreceptors lose sensitivity. We therefore had to give our eyes time to dark adapt. So we sat in the map room underneath the telescope, with the door closed, in total darkness. We sat there for half an hour, getting dark adapted. Then we covered our heads with towels and groped our way up to the observing deck, completely blind. We did not want to lose our dark adaptation by exposing our eyes to the “glare” of the ambient light inside the unlit dome.

I struggled up the observing ladder and extended the towel to cover the eyepiece. Then I stared long and hard at the field of view, using averted vision (looking out the side of the eye to bring to bear the most sensitive rods in the retina). I built up a mental image of the field of view, making a mental map of every star I could see. I didn’t know which one was Pluto — that was part of the plan. When I was certain that I had a solid mental map of the field of view, I went back to the map room. When my colleague joined me, we used faint red flashlights (red doesn’t affect the rods much) and separately drew our maps of the star fields. 

Now we had two hand-drawn maps of the stars in the field of view. We compared them; they matched closely. At this point, we broke out the detailed map of the area and compared our hand-drawn maps with the correct map. Sure enough, we both showed a star that didn’t show up on the map, and we both showed it in the same location. We had found Pluto!

We again memorized the star fields, turned off our red flashlights, and again sat for 30 minutes. Again we went up to the telescope with towels over our heads, and again we inspected the star field. This time, knowing which of the stars was Pluto, we were able to see it knowing what we were seeing. Even then, it was not visible directly; we had to use averted vision to make it out. But it was definitely there.