Thursday, April 26, 2012

The Moon Miranda (A Note)

(I’ve wanted to do this post for a few days, but I’ve been trying to come up with more to say. I haven’t really been successful at coming up with a lot, so I’m just going to do a short post.)

I’ve been interested in amateur astronomy for a long time. I mean, a long time. Normally, when some astronomy question occurs to me, I can almost always make a reasonable guess at what the answer probably will be. Every now and then, however, some question will pop up that I realize I can’t even guess at.

Last week that happened right here at the blog.

When I wrote the post, “Miranda And Miranda And Miranda,” I included a line saying that maybe someday I’ll own a larger telescope, and I meant that maybe someday I’ll be able to see Miranda.

But to be absolutely honest I wasn’t exactly sure when I wrote that line if Miranda even could be seen by amateur equipment. Amateur equipment is so extraordinarily powerful these days that I just took it for granted that Miranda would be visible with a suitably large telescope.

But when I thought about it I wasn’t sure.

So I’ve been looking around on the net. And it turns out that the moons of Uranus are intriguing targets for amateurs, and Miranda is the most difficult of the five large moons of Uranus.

First of all, it is possible for amateurs to photograph Miranda. It’s difficult, but as far back as 2002 amateurs were getting photos of Miranda. Here is one from an amateur named Ed Grafton:

In that photo, Miranda is the dimmest moon visible, just below Uranus. The details of the photo, and discussion of the difficulties involved are here: Sky and Telescope: The Elusive Moons of Uranus

But that photo raises some interesting issues on this topic.

First of all, that photograph was heavily processed. Miranda is so dim and comparatively so close to Uranus that the planet’s brightness can “wash-out” the image of Miranda in a normal astrophotograph.

However, astrophotography has progressed a lot in the last decade. Instead of depending on very long exposures where a bright object can wash-out nearby dim objects, nowadays most amateur astrophotographs are created by taking a great many medium-length photographs and using software to “stack” the images together. So Miranda might be easier to photograph with current equipment.

Second, that photograph was captured with a 14 inch telescope. Although that is a reasonably large telescope, nowadays amateurs have access to telescopes much larger. Meade Instruments has a sixteen inch model available off-the-shelf, and their top-of-the-line ‘semi-pro’ scope is twenty inches.

More to the point, however, there are Dobsonian scopes available now that get up into the mid-twenties. I did a post about Dobsonian scopes, because one amateur even built a thirty-six inch [!] telescope: Dobsonian

Obsession Telescopes has three models of twenty inches or more.

And optics are very well made these days. A twenty inch telescope gathers much more light than a fourteen inch telescope.

So it’s a really interesting question: Can an amateur astronomer see Miranda?

Certainly an amateur astronomer can photograph Miranda.

I strongly suspect that with high-quality modern optics and a twenty-something inch Dobsonian telescope an amateur could see Miranda.

But I don’t know for sure.

If I ever have a backyard again I will return to this question.

The technical issues of photographing Miranda don’t really seem all that interesting to me. I wrote about astrophotography a little in Dumbbell And Gobbledygook. I’m sure that is possible and with modern equipment I bet it isn’t even really all that hard.

I love photography, but I’ve never been too drawn to astrophotography.

But the very cool thing about Dobsonian telescopes is that they put an unbelievable amount of light-gathering power in front of a visual observer. Things that seem unthinkable become possible. I love those kinds of things. If I ever have a backyard again, I will return to this question with a Dobsonian telescope and find out for sure.

That will be fun.


Anonymous said...

My guess is that Miranda would still be out of reach. With my 6" scope, I can see down to mag 10.5 (I can just barely see Saturn's moon Rhea).

The visual magnitudes are a logarithmic scale- 5 magnitudes is equivalent to 100 times dimmer. Thinking of it the other way, increasing the area of your objective mirror by 100 times will let you see 5 magnitudes dimmer.

Now, a 20" mirror has 10 times the area of a 6" mirror. Therefore, it would let me see 2.5 magnitudes dimmer- I'd be able to see down to 13th mag. If I went out into the desert, instead of my suburban neighborhood, I might get down to 14 or 15.

Miranda, however, is mag 16.3. Just out of reach, unless you take a photo. Maybe the 36" scope could do it.

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