My first Magazine Sky & Telescope - 02.2019

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 ﬁ nderscope. I ﬁ rst 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 ﬁ laments 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 ﬁ eld star that appears right next to it. Both of these stars are rather faint at approximately 16th mag- nitude, so it took steady seeing condi- tions, high magniﬁ cation, 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, difﬁ cult-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 ﬁ laments in Jimi Lowrey’s 48-inch scope but didn’t have time to make a rep- resentative 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 ﬁ laments were everywhere, they were surprisingly subtle. Okay, now I don’t feel so bad about seeing only a few ﬁ la- ments 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 magniﬁ cation of 502×, along with a Lumicon O III ﬁ lter. From my notes: The Crab and the O III Filter An O III ﬁ lter is even more essential to seeing ﬁ laments in the Crab than the hydrogen-beta ﬁ lter is to seeing the Horsehead Nebula (S&T: Jan. 2019, p. 58). When using an O III ﬁ lter with my 20- and 28-inch scopes, the Crab’s brightest ﬁ laments become visible, and the overall shape of the nebula becomes oval. I get excited each time I see this transformation, and my sketches illus- trate the difference as seen through my 28-inch scope. You’ll also notice I’ve seen only sug- gestions of the fainter ﬁ laments and hints of a ragged perimeter. The ﬁ la- 34 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 rotat- ing shutter — which he called a “chopper” — and the eyepiece were attached to the bottom end of the telescope. “I can’t sketch all the ﬁ laments that jump out with the O III ﬁ lter — a unique sight! I’d need a good hour to sketch all the detail, and that would cause a riot.” However, seeing ﬁ laments was of sec- ondary 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 speciﬁ cally use on the Crab pulsar. It was designed to ﬁ t 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 ﬁ lter and quickly installed the shutter, much to our excitement. We used a 17-mm Nagler eyepiece with the shutter, producing a magniﬁ cation of 1210×, which narrowed the ﬁ eld of view to just the central portion of the Crab. The pulsar and the nearby ﬁ eld star were the two most obvious things in the eyepiece and were set against a back- ground of bright nebulosity. Everyone was eager for a look, but I was ﬁ rst in line. Man was I excited! But would it work? Would the pulsar blink? Oh my goodness . . . OH MY GOOD- NESS! Even though we had hoped to see 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 abso- lutely fantastic object.

My first Magazine Sky & Telescope - 02.2019 | Page 36