Comet 17/P Holmes

Christopher Hendrie

Whoa! That's no star! Although I'd estimate it at similar magnitude to Sigma, the comet is clearly an extended object under just 10x magnification.

I walked back home, gathered my scope bag and mount, and hauled them back out to the park. Strangely, it's the first time I've done that, even though "walkable to the park in one trip without heroics" was one of the criteria that guided me towards my 100mm refractor. With Jupiter visible from my south-facing patio and deep sky a short drive away, the park just didn't end up as a site of choice. But tonight everything aligned and the envisioned walk-to-observe worked perfectly; even the sprinklers were merciful.

At 100x magnification, the comet is like nothing else I've ever seen. (Probably because I've never seen a comet with magnification before!) There's a bright core which is just a bit too big to be stellar, and then an extended halo shading smoothly from perhaps 20% subjective core brightness to 5% core brightness, extending out perhaps one arcminute or so (I didn't measure, I'm just guesstimating). At lower magnifications, the halo appears slightly displaced to the east, so the core isn't exactly centered. At 100x, there seemed to me to be a hint of a wedge of brightness coming off the core to the west, so that perhaps the brightest part of the core (the mode?) was centered in the halo but the center of luminosity (the mean? the centroid?) was offset.

Incidental questions: what are the correct astronomical terms for the brightest point in an extended object and the luminosity-weighted centroid?

I noticed that the halo was particularly sensitive to changes in my eye position, especially with the Lanthanum Superwide 22mm eyepiece (which I've noticed in daylight to have extensive kidneybean vignetting before it obviously blacks out). I suspect that the uniformity of the halo made small changes in brightness obvious, or maybe it's the my eye trying to mistakenly subtract out "blur". Whatever the cause, it was an odd observing experience, unlike any other object I've seen. I guess there aren't too many objects which are both bright and nebulous... no planetary nebula is going to reach mag 3 unless we're uncomfortably close to it.

I'm very glad I took the time and trouble to take a close look at Holmes. I suspect there were more details to see if I but had more patience (ie. a chair and some confidence that the sprinklers weren't about to attack). I've finally seen a comet up close! (Yes, somehow I missed McNaught, and the last comet I remember before that was Hyakutake).

Oh yeah, I also noticed that big bold satellite of ours, impossible to miss tonight. When it's time to come in, it's an awe-inspiring way to blind myself temporarily and end the session definitively. :-)

Regards,
Christopher