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.
LG.
Edited by Jennifer Steinberg (editor in chief)
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