Last touched 2001 December 4
Originally published in M-111, the Newsletter of the Richland (Ohio) Astronomical Society
Reflections in the Eyepiece
By Robert Bunge
I knew I had the right field, but the galaxy was no where to be seen. I knew it had to be there. Information from various sources suggested that it should be visible in my 10-inch. It had a photographic magnitude of 11. I figured if I added about an eighth of a magnitude, I would have a rough estimate of the galaxy's visual mag. Ten point two is normally within reach of my ten, and it was a pretty good night.
But a second look at the references showed that the galaxy was listed as being small - about two arcminutes across. Hmmm, maybe I need more power. I was using a 28mm eyepiece, about 48x. I popped in a 12mm (120x), and looked again. Ahhh, a faint glow surrounding what had appeared as star at the lower power!
I walked over to Biff Smooter and said, "Man, I'm spoiled. I had to WORK to find that galaxy. In the 31-inch, it would have jumped out of the eyepiece and onto my drawing board."
Biff, who had been listening to me mumble to myself during the search, said, "Well, with the 31-inch you'd be observing at 225x, right?"
"Sure, that's the power I use to look for objects with the 31. That's pretty low power for the blue beast," I replied.
"Think about it. At 225x, you have a lot more image scale then you did at 48x. That's the best thing about the 31, the image scale... it's huge," he said. "Really, most objects in the night sky are very small. Not only does your telescope collect light, but another very important function is that it enlarges a tiny section of sky," he added.
So I thought about it. Normally, amateurs tend to think of galaxies as big objects. Sure some are, but off the top of your head rattle the big ones off.
M-31, M-101, M-33, M-106, NGC 4546 and perhaps M-51.
About the sametime as the above story took place, I recieved a call from Arizona amateur and astronomy writer Brian Skiff. He had seen some graphs I had done using a Macintosh computer and wanted to know if I could do a couple for an article he was working on. I agreed, and listened to what he wanted.
A couple of years ago, I started collecting astronomical databases in electronic format. These days, I've got megabytes of them. At one point, I even wrote off to NASA and got a tape full of them in return (Aren't public record grand?). I also found it was pretty easy to talk a statistical package on one of the Ohio State University mainframe computers into reading the tape and coming up with mounds of data about the databases. Pretty soon I had databases about databases.
Oh my how my hard drives ache!
Though out all of this, I've never been much into the recent craze of personal computer programs that simulate the night sky or look at the solar system from above, below and inside out. Well, they're neat once, maybe twice, but what's the use? I'd rather be out under the stars. I've had a hard time finding one of these programs that would help me with my observing. Finder charts? Who needs them with the likes of Uranometria? I had, on the other hand managed to get the mainframe to spit out lists of objects that meant certain parameters. Large galaxies. What about galaxies with a negative radial vecocities -- or in other words, blue shifted galaxies? This sort of thing is useful. But, perhaps there are other ways but these datbases to work for me at the eyepiece.
Brian had asked me to make a chart of the number of galaxies for each size listed in the Uppsala General Catalogue (UGC). The UGC is a great catalogue, it's only problem is that it only covers the northern hemisphere. But, it lists major axis and minor axis for all but six of it's 12,940 objects in both red and blue light. Brian asked me to look at the major axis in blue light.
First, I got the computer to come up with a list of the number of galaxies for each size the UGC lists. The sizes range from .15 arcminute to 200 arcminutes (M-31). About every size in between was included. Brian suspected what Biff had learned from years of observing.
Most galaxies are small.
The UGC is a pretty complete catalogue of galaxies bigger than one arcminute and brighter than photographic magnitude of 14.5. Galaxies smaller than one arcminute are included for some special reason - perhaps they appear to be interacting or are close to a bigger galaxy, or maybe they are brighter than 14.5.
The result of the computer search said there are 2,122 galaxies listed in the UGC with a size of one arcminute. There are also 2,014 listed with a size of 1.1 arcminutes, and 1,382 UGC galaxies had a size of 1.3 arcminutes. That's 5,518 galaxies, or a wopping 42 percent of the total catalogue.
Lesson learned. Almost half of all galaxies brighter than a photographic magnitude of 14.5 are smaller than two arcminutes. In a conservation with Brian, he commented that this sort of blows away the myth that all deep sky observing should be done at low powers. I think he's right. At one point, Brian said, "Most of the galaxies in the UGC are smaller than Jupiter!"
I bet a lot of those fainter galaxies are visible in smaller scopes, if enough power is used to show them (which requires good optics and good seeing, but those are other stories). The numbers show it. So before you give up on that galaxy plotted on Uranometria, try a power play. You might be surprised!
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