Archive for October 7th, 2015


Sketching NGC 6633

October 7, 2015

Wedel NGC 6633 2015-10-06 - inverted and cropped

As long-time readers will recall, I have been bully on the idea of sketching DSOs for a long time. I have been inspired by the careful observations and sketches of Doug Rennie and Terry Nakazono in particular. So I made up a blank observing form (which is now up on the sidebar here), printed out some copies, and decided to finally give it a shot. I was going to do M57 first, and kick off my much-discussed, long-delayed Suburban Messier project. But I’d just been emailing with Doug and he’d recommended NGC 6633 as a rewarding open cluster for visual observation, and as I was flipping around in my Pocket Sky Atlas I noticed that it was well-placed high in the southeastern sky.

I was rolling with the XT10. I figured that whatever target I went for, I’d want to capture as many background stars as possible, and the XT10 has much better light grasp and angular resolution than anything else I own.

I started at moderate magnification with the 8-24mm zoom but kept backing out to try to get more context for the cluster, and I ended up with my trusty old 32mm Plossl. The transparency here was appalling. The sky looked clear, in that there was no naked-eye-visible haze or clouds, but it was very humid, and all of that water vapor in the air was bouncing back the city lights like crazy. The sky was about as bright as I have seen it without actual clouds up there. Here’s a measure of how humid it was – all of my exposed stuff dewed up! I don’t think that has ever happened to me here in Claremont.

Wedel NGC 6633 2015-10-06

As far as my method – I was using a 0.5mm mechanical pencil and a click eraser. I started out by trying to frame the field of view with some bright ‘anchor’ stars and then interpolate between them to flesh things out. This proved frustrating – inevitably I’d get one region ‘starred in’ to my satisfaction and then see that its geometry was off compared to a neighboring section. So I did a fair amount of erasing and repositioning. On the first pass I was mainly trying to get the positions of the stars correct.

Then while I was still at the eyepiece I went back and ‘brightened’ up some of the stars by drawing over them with slightly larger circles. I tried to sort them into about five bins, from the bright star south of the cluster, through the brightest anchor stars, the major cluster members, the minor cluster members, to the barely-theres.

Finally, when I brought the drawing inside I touched up a few stars that were noticeably out-of-round.

So the drawing you see here is the ‘rough’ drawing, but with about three layers of revision layered on top. I don’t know if this is good practice or not, it’s just what I did this time, pretty much making everything up as I went along.

As for the cluster, NGC 6633 has a fairly recent nickname: the ‘Italy cluster’. Here’s a diagram from this blog, with my sketch inverted and rotated to match:

NGC 6633 comparo

I can buy it. I wouldn’t have ever picked out that by myself, but I can see the shape in my drawing, and I didn’t know it was there when I was drawing it.

So, I have rather mixed feelings about all of this. While I was doing the sketch, all I could think about was how difficult it was, and how badly I was screwing it up. But I’m fairly happy with the result – it is at least recognizable as NGC 6633 – and I know that I know that cluster and the surrounding starfield a lot better now. Probably better than I know any other single object. I can’t think of another time that I invested so much time and energy on a single observing target.

Maybe this is the beginning of wisdom.

UPDATE October 26, 2015

Here are a couple of sketches of NGC 6633 sent along by Terry Nakazono with permission to post. Thanks, Terry!

NGC 6633 at 31X (7-19-12)

From July 9, 2012.

NGC 6633 at 44X (6-28-13)

From June 28, 2013.