Sky & Telescope - February 2019 - 34

Crab Nebula

The Visual Crab
As the only supernova remnant in the Messier list, the
Crab seems like it should be an amazing telescopic sight.
For instance, its fairly substantial size of 6′ × 4′ makes the
8.4-magnitude Crab Nebula bright and large enough to be an
obvious fuzz ball even in my 80-mm ﬁnderscope. I ﬁrst saw
the Crab in 1982 through my 12.5-inch telescope, and it was
bright enough that it was easy to see with direct vision from
my light-polluted backyard. It seemed that perhaps with a
little averted vison I might observe some details.
No such luck! It didn't take long to realize that none of
the famous ﬁlaments could be resolved. It didn't look like
the few telescopic comets I'd already seen either - they
had tails - but I took it on faith that that's how they often
looked to Messier.
But in the 1990s with a 20-inch
scope I was able to detect the Crab as
a chubby, fuzzy S. No internal details,
just an amorphous nebula with a funny
shape that was dimmer on the ends
than in the middle. It stood out quite
well on the best nights, and I was able
to see the neutron star along with the
slightly brighter ﬁeld star that appears
right next to it. Both of these stars are
rather faint at approximately 16th magnitude, so it took steady seeing conditions, high magniﬁcation, and averted
vision to see them well.
And that is so incredibly cool! After
all, how many pulsars can you see in
an amateur scope? Only one, the Crab
Pulsar - PSR B0531+21.

ments are low-contrast, difﬁcult-to-see features under even
the best observing conditions with a large amateur telescope.
I've tried many times to see more, but the O III sketch shows
my best effort with my own scopes.
However, I once saw considerably more ﬁlaments in Jimi
Lowrey's 48-inch scope but didn't have time to make a representative sketch. You can imagine how awesome the Crab
Nebula looked, though.

A Dead-Solid Awesome Sight
Even better - way better in fact - in April of 2010 I was
part of a group of ten observers that rented the 90-inch Bok
telescope on Kitt Peak for one night of visual observations.
The Crab was our second object of the night, and although
ﬁlaments were everywhere, they were surprisingly subtle.
Okay, now I don't feel so bad about seeing only a few ﬁlaments in my own scopes, and even though they were still a
spectacular sight I had hoped for a brighter view. We used
a 41-mm Panoptic eyepiece, which gave a magniﬁcation of
502×, along with a Lumicon O III ﬁlter. From my notes:

The Crab and the O III Filter
An O III ﬁlter is even more essential
to seeing ﬁlaments in the Crab than
the hydrogen-beta ﬁlter is to seeing the
Horsehead Nebula (S&T: Jan. 2019, p.
58). When using an O III ﬁlter with
my 20- and 28-inch scopes, the Crab's
brightest ﬁlaments become visible, and
the overall shape of the nebula becomes
oval. I get excited each time I see this
transformation, and my sketches illustrate the difference as seen through my
28-inch scope.
You'll also notice I've seen only suggestions of the fainter ﬁlaments and
hints of a ragged perimeter. The ﬁla34

FE B RUA RY 2 019 * SK Y & TELESCOPE

p TELESCOPE AND CHOPPER The image
at top shows the 90-inch Bok telescope at
Kitt Peak in Arizona. Below, Dan Gray's rotating shutter - which he called a "chopper" -
and the eyepiece were attached to the bottom
end of the telescope.

"I can't sketch all the ﬁlaments that jump
out with the O III ﬁlter - a unique sight!
I'd need a good hour to sketch all the detail,
and that would cause a riot."
However, seeing ﬁlaments was of secondary importance because our primary
aim was to blink the Crab pulsar.
Dan Gray of Sidereal Technology was
one of the observers, and he'd made a
programmable rotating shutter, which
he called a "chopper," to speciﬁcally use
on the Crab pulsar. It was designed to
ﬁt on the eyepiece end of the 90-inch
Bok scope and was set to rotate a little
more slowly (29.431 rotations/second)
than the frequency of the pulsar (30.23
rotations/second).
Dan removed the 41-mm eyepiece
with the O III ﬁlter and quickly installed
the shutter, much to our excitement. We
used a 17-mm Nagler eyepiece with the
shutter, producing a magniﬁcation of
1210×, which narrowed the ﬁeld of view
to just the central portion of the Crab.
The pulsar and the nearby ﬁeld star
were the two most obvious things in the
eyepiece and were set against a background of bright nebulosity.
Everyone was eager for a look, but I
was ﬁrst in line. Man was I excited! But
would it work? Would the pulsar blink?
Oh my goodness . . . OH MY GOODNESS! Even though we had hoped to see

TELESCOPE: HOWA RD BA NICH; CHOPPER: DA N G R AY

The Crab is approximately 6,500 light-years away, stretches
13 light-years along its major axis, and is expanding at a rate
of 1,500 kilometers per second. In other words, it's an absolutely fantastic object.

Sky & Telescope - February 2019

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