Monday, December 7, 2015

OR 20141125 bulges at tips of M31

Earlier this week I was on a vacation to Yosemite. Not having more room in the minivan to pack any larger aperture, I brought a 6" f/4 dob which I enjoy for wider field views. Skies were pristine. As the hotel was in a narrow canyon it turned out my best observing site was on the balcony of our room, away from the parking lot lights, where I had a narrow view of the sky from Daneb to Cassiopea to Andromeda. I loved panning this rich section of the Milky Way, and had a nice view of dark nebula Le Gentil 3. But I found myself most drawn to M31 Andromeda Galaxy, where I observed something unusual.

M31 was naked eye and nearly at zenith when I observed from 9-10pm on two successive nights. Its nucleus was quite bright and the halo was large and extended. M32 was brighter than I remember seeing before, and looked like a twin of Andromeda's nucleus. M110 was ragged and torn, clear but almost nebulous. I was using a 20mm erfle eyepiece which gave a 2 degree TFOV at 30x and a 5mm exit pupil.

While trying to see how far out Andromeda's halo extended, I noticed bulges at each tip of the halo, like dumbbell weights, but each hanging from the tips in opposite directions. The bulge to the southwest hung to the south, and the bulge to the northeast hung toward the north. See attached for my awkward notebook sketch. These appeared part of the halo, and not some intervening nebulosity (or fogged eyepiece, I checked!). The overall halo length I could detect was 2.5-2.75 degrees.

So I wonder, what did I see? I had not noticed this before using my 12.5", albeit not from sites as dark as Yosemite. Perhaps larger aperture, while it gathers more light, also narrows the field of view; maybe one can "see" these bulges as a greying of the sky but one assumes they are part of the halo. So is it only with the wider field providing the context of the rest of the galaxy can one notice the bulges as distinct features? It's puzzling though that the wide field images I find on the internet don't show these bulges, only an oval shape. Have any of the large aperture binocular owners out there observed this?

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