And the keyhole

by Bob Jardine

The Keyhole Nebula was one of the trip's mysteries for me. I hadn't done enough preparation to know exactly what to look for, although almost all of the descriptions of the Eta Carina region mention it, so I was looking for it. But it wasn't at all obvious at the eyepiece. Let me say that differently. There was a lot of dark stuff that was obvious, but no keyhole shape jumped out at me. So I also did a sketch.

During the days between observations, I kept searching for a good description among the resources that I had with me (not much). Surprisingly, Hartung was not helpful at all; not in the index, and no mention whatsoever in the description of NGC 3372 (although the Homunculus is pretty-well described). The other descriptions that I had from various sources also didn't mention the keyhole (or equated it with the whole of NGC 3372). Finally, I looked at The Astro League's brief description, which was reasonably direct. This matched my sketch pretty well.

"...a small, curved, dark dust lane crossing the brightest portion of the Eta Carina nebula. It is located West of the star Eta Carina. The keyhole shape has become less apparent over the years because of changes in Eta Carin and the associated nebulosity...." (A.L. Southern Sky Telescopic Club)

When I returned home, I hoped to find another clear description. S. J. O'Meara's book "The Caldwell Objects" (sorry about using the C word, folks) wasn't very helpful, which was disappointing, because I thought its goal was to provide good descriptions and sketches; neither one satisfied me, although he does try to describe it. His photo is next to worthless -- it even has the keyhole labeled, although there is no indication what the label is attached to or how big it is, and the sketch doesn't call out or label the Keyhole (and it is on much too big a scale).

Next I went to the NGC/IC website to get Steve Gottlieb's description. That was helpful (more helpful than O'Meara's) and was consistent with the A.L. description quoted above. However, I still couldn't pick out the keyhole from the many photos I had seen.

The description and side-by-side sketch and photo that you (David) provided was the clincher. I can now see the keyhole (or what remains of it) in the left photo, and also in the photo in Hartung's book. My little sketch, as poor as it is, at least labels the little arc of dark stuff as "keyhole ?" with clarity. Now I can "erase" the question mark.

(BTW: from Herschel's sketch, I would have called this the "guitar-case" nebula, not "keyhole")

Posted on sf-bay-tac Apr 17, 2005 19:17:11 PT
Converted by report.pm 1.2 Apr 23, 2005 21:27:17 PT