Eye adaptation and direct light pollution
A couple of months ago, I moved with my family to a smaller apartment in the same city, just a dozen blocks further north than my last location. As there are two apartments in the building’s 3rd floor, we chose the apartment in the back, which has two rooms facing to the south providing a clear view of the southern sky over the city. We also have the privilege of free access to the 4th floor terrace with a metallic roof. One part of the north facing roof, about 30 square feet, can be rolled open. With both places, the apartment and the terrace, I have access to the north and the south sky. I think this is more sky than in my last location.
Unfortunately, not all is perfect and the best of the open view in the north is also the Achilles heel: I get a lot of direct light pollution that comes from the neighborhoods on the mountain and billboards from the north of the city. There is also an overpass and highway (as seen in the right picture) and of course, I get the light coming from those street lights.
But every clear night over the city is an opportunity to stargaze and on the last May 4, I had one of those nights: moonless with a transparency of 2/3; there was a very thin layer of clouds, almost imperceptible on the sky. Scorpius was at a good angle above the horizon (about 60°) to avoid a considerable amount of light pollution so I thought that it was the moment to take the telescope out and have some fun. I decided to look for M80, an apparent medium difficult target that could be seen from my LPS according others DSO of the same type I had seen before (e.g. M9, M10, M12 and M4).
Two or three times before, I had tried to spot M80 either with Binoculars or telescope, but I did not success, so it was the challenge for that night.
I started from Antares and by using some Stellarium guidance I hoped to get M80. I was able to jump from Antares to ι Sco (HIP80815) and then to ρ Oph (HIP80473), which seemed like Mickey Mouse facing upside down because of the inverted view in the eyepiece, but finally I got lost just about 1 degree northern east from this last star. The stars in the 25 mm eyepiece seemed too weak to recognize as a pattern of stars dimmer than 7.0 of apparent magnitude. I tried also the 10mm eyepiece to see if I could get a better view, but at that moment. I felt that I was forcing my eyes and it was kind of painful for them.
About 30 minutes after I started searching, I realized that I needed a better environment without direct light pollution. I decided to place a blanket as a blackout so I could reduce the direct light by about 70%. It allowed me to see more, but I was still in the wrong spot of the sky and trying to resolve stars of >9 Mag, I was forcing my eyes too much.
Finally, after I almost surrendered, I rechecked in Stellarium and by comparing it with the eyepiece view, I found a remarkable shape: an obtuse triangle formed by the stars HIP80126, HIP80238A and HIP79897 (The brightest star in the big circle). Just less than a half degree from HIP79897, there was a patter of 4 stars that formed another triangle just below the “big one”. One of those stars seemed to be fuzzy and just slightly fatter than the others so immediately I changed the eyepiece for the 10 mm and Voilà! There it was M80.
Even though M80 has an apparent magnitude of 7.20, it shined as bright as its closest neighbors: 3 stars of about 8.5 of apparent magnitude forming an obtuse triangle. Compared to my previous globular cluster observations, I think this one is the faintest of all. Whatever, it was worth to spend some time looking at it.
After this observation night, I noticed that despite that this DSO was close to its zenith and therefore less affected by the city LP, it was not easy to recognize during the first hour of observation even though there was a remarkable pattern of stars around it for “easy” location, for an intermediate stargazer.
In conclusion, I learned that even though light pollution affects the observation of the sky, direct light pollution and lack of eye adaptation are the strongest enemies of locating faint DSOs.
Please enjoy this marvelous creation of God.
Edited by Jennifer Steinberg (editor in chief)