SAA OBSERVING: My first Caroline Herschel object!
- From: Margo Schulter <mschulter@xxxxxxxxxxxxxxx>
- Date: 30 Jan 2009 07:30:51 GMT
Hello, everyone.
The following is a report I posted to the Sacramento Valley
Astronomical Society (SVAS), while I'll repost here with a
few explanations as to context.
One aspect of celebrating IYA 2009 within this local group is
the Hodierna Project, an endeavor to observe and share with
others the objects catalogued or mapped as part of an incomplete
star atlas project by the great 17th-century Sicilian astronomer
Giovanni Battista Hodernia, whose treatise on nebulae is a
fascinating chapter in a saga which might be called "Galileo:
The Next Generation."
The Hodierna objects, described in some excellent sources
available on the Web including a germinal article from the
_Journal for the History of Astronomy_ and a fine summary
by SEDS, include some of the bright and attractive deep sky
objects in the sky, many of them in the southern celestial
hemisphere. Here's a quick list of some of these southern
objects that I've observed from around latitude 38d34' --
Hodierna at his Sicilian site was a bit more than a degree
further to the south. Of course, this means not only that
these objects are visible in Sicily and Central California,
but that they will be visible through most of the Southern
Hemisphere, and often more advantageously situated:
<http://www.bestII.com/~mschulter/HodiernaSouthernBinocular100.txt>
Anyway, what happened the night before this was that in the
process of getting a different perspective on some of these
objects with my 200mm f/6 Dob, having viewed them with 15X70
binoculars more comparable to what Hodierna had available
around 1654 (but likely with a dramatically greater field of
view!), I happened upon M48/NGC 2548.
This, as I was well aware, was my first Caroline Herschel
object -- something I've been waiting for over the last
18 months and a bit more. While NGC 253 and NGC 2360 have
been difficult to see due to urban light pollution, her
independently rediscovered NGC 2548 was easy, with the
identification being the main test of skill (a sketch
of surrounding star patterns served this task well, as
I describe below).
Anyway, here's the story as I wrote it for the SVAS, with
special IYA greetings to all.
* * *
Last night, while starting with a focus mostly on the Hodierna
objects, I found that my observing session with a 200mm f/6 Dob
entered a new territory: my first object of the set of 14 now regarded
as discoveries or independent rediscoveries of Caroline Herschel
(1750-1848), astronomer and partner in research with her brother
William Herschel.
Curiously, two fortuitous events may have helped to make this result
possible: Charles Messier's error in charting or mathematics as to the
declination of his open cluster M48; and the installation in my
apartment building a bit more than a year ago of new and improved
windows.
Before getting into my story, I should quickly explain that the new
Hodierna Project of the SVAS for the International Year of Astronomy
2009 focuses on the deep sky observations of Giovanni Battista
Hodierna, a 17th-century Sicilian astronomy and younger contemporary
of Galilei who saw and studied many of the nebulae later viewed by
Messier, the Herschels, and others. Hodierna's treatise on nebulae
published in 1654, and some star maps from an incomplete atlas
project, include many of most bright and beautiful deep sky objects.
However, his writings were unknown to the astronomers of the 18th
century, whose "discoveries" of Hodierna objects are now understood to
be in fact independment rediscoveries.
For many Hodierna objects like M42, M41, and M47, or the more
southerly open clusters Collinder (Cr) 135 and 140 and NGC 2451 (all
visible now in the evening hours), 15X70 binoculars may be ideal.
The aperture and magnification are comparable to those likely for a
telescope in 1654, while also sufficing to cut through urban light
pollution (at least in part!).
The last couple of nights, however, I've been trying my 200mm or 7.9"
Dob on some of these objects, an instrument more comparable to
Caroline Herschel's largest Newtonian "sweeper" at around 9.2". The
greater light-gathering power, which I use at a magnification of 40X
with an ultra-wide-angle 30mm eyepiece, has its own charms.
My goal last night was to take a closer look at NGC 2362, a very
compact open cluster around Tau Canis Majoris which I had seemed
unable to distinguish from this bright star with my 15X70's. Here I
ran into an evident difficulty of local architecture: while I could
locate Tau CMa in my Dob's finder scope, the window sill or wall of my
bedroom interposed itself between the desired object and the Dob's
mirror.
However, while confirming an observation of M47 I had made the
previous night (my first with the Dob), which in the larger instrument
had a resolved clarity comparing with the more romantic "fuzzy patch"
in the 15X70's (each with its attractions), I happened to sweep
considerably to the east. There I found what looked like an unfamiliar
object, and set myself to sketch it out for later identification using
the Millennium Star Atlas or the fchart program for Linux. Groping
pen, logbook, and red LED flashlight, I strived for a crude but
somehat recognizable drawing of the cluster, and especially of the
surrounding patterns of stars (often the key to a confident
identification).
The cluster itself was uncommonly beautiful: a beauty of detail within
a yet rather wide and encompassing field of a tad more than 2 degrees.
What struck me especially was a thin "Y" of stars within the cluster,
with a star seeming to flicker somewhat at point near where the
strokes of the "Y" would have joined were this an actual letter. This
was in part sheer enjoyment, as I tried a bit of averted vision, and
in part an attempt to "organize" the exquisite jumble of the cluster
into geometric patterns or symbols that I could more easily sketch for
comparison against an atlas page or software-generated chart.
That night, after the session, I was quickly able to verify M48 as an
object fitting my sketch -- and thus realized that I had seen my first
Caroline Herschel object! For a summary regarding this "matriach of
astronomy" and her deep sky discoveries, including some interesting
questions about certain of her observations raised from an amateur
astronomer's perspective by Tony Flanders, please see this page and
its links to some of the many valuable sources on the Web:
<http://www.bestII.com/~mschulter/CHAT001.txt>
Now for those two fortuitous factors helping to make my observation
possible. The first was the installation, as I mentioned, of new
windows in my apartment building. While I'm not sure they would be
described as of optical quality, they amazingly permit a quite
pleasant _closed-window_ view of many deep sky objects not only with
the 15X70 binoculars, but with the 200mm Dob at 40X!
This means that I can conveniently observe objects in the eastern half
of the window, the part that always remains closed and thus is without
a screen. Apart from avoiding the apodozing effect of the screen, this
means that with an object approaching the celestial equator such as
M42 or M48, I can observe it a couple of hours before it reaches
transit or culmination, when its elevation might place it above the
viewing window. Still thrilling from my first view with my Dob the
previous evening of M42 (of course it's great also with the 15X70's),
I was delighted to learn that my "eastern strategy" had brought M48
within my Dob's reach also.
Now for Messier's part in this scenario: when describing M48 in the
edition of his famous catalogue published in 1781 and used by the
Herschels, he happened to get the declination wrong -- possibly, it
has been suggested, because he may have placed it one grid line too
far north when he sought to record the coordinates. Thus M48 became a
"missing Messier" until Oswald Thomas in 1934 and T. F. Morris more
definitively in 1959 linked Messier's description to the Herschel
object known as NGC 2548, thus solving the mystery.
Thus when Caroline Herschel observed this cluster on March 8, 1783,
she could quite accurately conclude that she had discovered something
at a location not mentioned in Messier's catalogue -- or, as her log
sometimes reads: "Messier has it not."
What she had more precisely made was an independent rediscovery --
published as early as 1789, but only connected about half a century
later with Messier's "missing" M48 and its stray coordinates.
The rediscovery of Hodierna likewise puts Messier, the Herschels, and
others in a new and richer perspective.
Finally, I should note that Caroline Herschel did not yet have
available in early 1783 the fine Newtonian sweepers she would later
acquire: rather, for her independent rediscovery of what would
ultimately prove to be M48 she used a modest refractor or monocle
having "optics such as are commonly used in a finder," with an
unrecorded aperture and a magnification of 14.5X. This simplicity of
means, harking back to the era of Galileo and Hodierna, may add a note
of due humility to my celebration of stumbling upon the same object
with a modern 200mm/7.9" Dob.
With many thanks,
Margo Schulter
mschulter@xxxxxxxxxx
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