Archive for January 5th, 2010

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Observing and photographing the moon with binoculars

January 5, 2010

I’m always saying that you can see craters on the moon with binoculars, but I suspect that many people don’t believe me. So here’s the proof.

Here’s a raw, completely unretouched image I took on the evening of January 2nd.

The same image, cropped and lightly sharpened using “unsharp mask” in GIMP.

Then converted to grayscale, which gets rid of the annoying coloration on the limb. That’s chromatic aberration or CA, which is present in any optical system that uses lenses to collect light. The problem is that different wavelengths of light have different refractive properties, so a lens can never bring all of the wavelengths to focus at the same point. In telescopes and binoculars, the out-of-focus wavelengths at either end of the spectrum make yellowish and purplish halos around bright objects, even in daytime. You can knock down the CA to unnoticeable levels by using combinations of very expensive glass in the lenses, as in apochromatic refractors or APOs, or with anti-fringing filters, but it can never be completely eliminated.

Here’s the final version of the image, in which I tweaked the brightness and contrast using the “Curves” function in GIMP. This lets you selectively brighten and darken pixels of different values, and I use it on almost everything.

So what have we got? Well, first of all, there are dozens of craters in view. Now here I have to confess that looking at these photographs is cheating, a bit. The digital images are magnified by the camera and blown up to a convenient size on your screen, so you can pick out a LOT more detail from these pictures than you would out in the dark with the binoculars alone, even if they were mounted.

Nevertheless, the camera couldn’t capture detail that wasn’t there, so all of this was at the eyepiece, and how much you might get would depend on your visual acuity and level of experience. Experience counts, and the more experience you have, the more you realize that it counts. A big part of one’s growth as an observer is learning to see, which largely means cultivating the patience that it takes for your eyes to suss out the subtle details present in whatever you’re observing.

I decided to take this picture because I was really blown away by the sharpness of the features along the terminator, especially Mare Crisium and the nearby craters. Like all of the maria or lunar seas, Mare Crisium is an impact basin that was flooded with basalt; unlike most of the other maria, Crisium actually looks like a giant, flooded crater. Just north of Mare Crisium is the ancient crater Cleomedes, which you might easily pass over when it is less dramatically lit. Farther north along the terminator, the flat-floored crater Endymion is a black pool of shadow.

A final confession. Despite the title of this post, I didn’t take this photo through binoculars. I took it through the 9×50 finderscope on my big telescope. A pair of commonly available 10×50 binoculars would offer the same angular resolution and slightly more magnification, and would therefore show you even more–especially if they were solidly mounted. Here is a much better picture from a couple of years ago that I really did take through 10×50 binoculars.

Okay, so you can get serviceable pictures of the moon using a point-and-shoot digital camera and cheap binoculars. But how?

First, mount the binoculars on a tripod to keep them steady. Ideally, once you get them aimed and focused you won’t have to touch them at all while you’re taking pictures, except to periodically re-aim them as the moon crawls across the sky.

Second, use a digital camera that offers optical zoom instead of electronic zoom, and use as much optical zoom as the camera will give. I get the camera lined up behind the eyepiece first, get the moon on the little screen at back, and then start zooming. Once the camera is zoomed, its field of view is so small that if you lose your target, you may have to start all over again.

Third, I turn off the flash and set the camera to macro (“flower”) mode. I know that other people have gotten good results focusing the camera at infinity, and more power to ’em, but I get my best results in macro mode.

Fourth, it is really super-handy if you have a camera that allows you to manually set the exposure time. My old Nikon Coolpix 4500 does this  easily. If the feature is available on my much newer Coolpix L19, I haven’t found it yet. One of these days I need to rant about how most newer point-and-shoot digital cameras suck compared to the 4500, but not today.

Fifth, take tons of pictures. Seriously. For every photo of mine you see posted here, there are on average 99 others that I took and discarded. I’m not kidding, and I’m not exaggerating. If I come off as a half-decent photographer, it’s because digital cameras allow me to take zillions of pictures and present the handful that worked out. Storage space is effectively free these days, so take as many pictures as you can at the eyepiece and sort ’em out later. It’s worth fiddling with the focus of the binoculars or scope a bit between blocks of photos, just in case the camera’s sweet spot is slightly different from your eyes’ (for example, because you’re farsighted or astigmatic or whatever).

Sixth, download GIMP, which is free, fairly easy to use, and will allow you do just about everything that Photoshop does. Then make a new folder with copies of your best images and start experimenting with Unsharp Mask, Curves, and the rest (do not experiment on your original files).

Seventh, read up on how other people get their shots. What astronomers call ‘afocal projection photography’ is more widely known as ‘digiscoping‘, especially amongst birders, and there are tons of sites out there with advice and examples.

Good luck!