Date: November 10, 2010

Title: Howling at the Moon, Part 2

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Podcaster: Richard Wright

Link: Evening Show – http://www.eveningshow.org
Software Bisque – http://www.bisque.com

Description: Richard provides tips for observing the Moon. Part one of this series aired on Sept. 23, 2010.

Bio: Richard S. Wright Jr. is a Sr. Software Engineer for Software Bisque, and an avid amateur astronomer. When not moonlighting with one of his six telescopes, he also teaches a night class on computer graphics programming at Full Sail University near Orlando, FL, and is the lead author of the OpenGL SuperBible. He can be contacted at opengl@bellsouth.net.

Today’s sponsor: This episode of “365 Days of Astronomy” is dedicated to PARI, the Pisgah Astronomical Research Institute, home of “Smiley”, the worlds most beloved radio telescope. Visit PARI on-line at www.pari.edu.

Transcript:

Howling at the Moon – Part II
Photographing the full moon, with and without a telescope, and why you’d want to do so.

Welcome to the 365 Days of Astronomy pod cast for November 10th, 2010. My name is Richard S. Wright Jr. I’m an amateur astronomer and photographer, and a software engineer for Software Bisque, where I get to help make some of the worlds best astronomy software products.

Today’s podcast is a continuation of the September 23rd podcast called “Howling at the Moon, Part I”. I mean that metaphorically of course. In that podcast I talked about taking pictures of the full moon with a hand held camera or preferably on a tripod. Outdoors scenes featuring the moon can be striking and dramatic, and allow for a great deal of artistic expression. If your goal however is to just capture the moon by itself, in all its full and high-resolution glory, there is no substitute for connecting your camera to the back end of a telescope. The full moon through a telescope is fascinating, and there are things you can only see or measure well during the full moon.

The first is the size of the moon. Yes that’s right, I said the size of the moon. The moon actually grows and shrinks over the course of a month. Okay, well it doesn’t really physically get larger or smaller, but it does come closer and get farther from the earth during its orbit. You see, the moons orbit around the earth is not a circle, but an ellipse… really a flattened circle or an oval. At some point on the path around us, the moon is closest to the earth; we call this point Perigee. Correspondingly, when the moon is at a position on its orbit that is furthest from the earth, we call that Apogee. The difference between these two points varies slightly from month to month, but basically it’s on the order of about 50,000 kilometers (or about 30,000 miles). These changes in distance have a measureable effect on earth’s tides, how bright the moon appears, and how large it is in the sky.

These changes are especially dramatic when the full moon falls near one of these two points. The full moon near Apogee vs. Perigee can appear as much as 14 percent larger, and be up to 30% brighter.*

If you take a picture of the moon at Apogee and again at Perigee with your camera set to the same zoom factor (or use the same telescope set up), the size of the lunar disks can actually be compared. There is a picture on my web site that shows this between two successive full moons.

Another lunar phenomenon is libration. No, not libation, but actually if the moon where to become intoxicated this term might apply to the same observed effect. The moon actually appears to wobble unsteadily as it goes around the earth. The moon itself is not actually wobbling, but again because of that elliptical or oval shaped orbit, we are able to see more of either side … it only looks likes it’s wobbling.

Features along the opposite edges appear and vanish, and we can actually see 59% of the total lunar surface thanks to this wobbling effect. Features along the limb (the edges) will disappear the next month, and new features appear on the other side. This effect isn’t just side-to-side either; it also appears to tilt up and down as well. These differences are most dramatically demonstrated when the moon is full.

When the moon is full, the sun is shining directly down on the moon from our perspective, eliminating many of the striking shadows and surface details that many observers cherish. However, out further from the center of the disk of the full moon, the sun is actually striking at an angle, not dead on, and you can always see these tell tale craters and surface features along the very edge. This is where the effects of libration are most easily seen month-to-month. No two full moon images are really exactly the same.

Only during Full Moon do you see crater ray systems in all their glory. When an object hits the moon and digs a crater, what do you think happens to all that “stuff” that was in the hole before hand? Well, it got blown everywhere! In fact, you can see the debris field, thrown out from the craters. This lighter colored material is newer than the darker surface underneath, and it shows up most vividly during the full moon. The crater Tycho has an especially dramatic ray system that extends to about a third of the visible lunar surface. I often describe this as the bullet hole in the moon when showing the moon to kids through a telescope. All I have to say is “do you see the bullet hole?”, and they know which crater I’m referring too. Other craters have ray systems too, one especially bright but small, extends from the tiny crater Proclus, and it stands out best only during the full moon.

So, now you know why you might want to shoot a higher resolution image of the full moon. How do we go about this? Well, you need a telescope. Just about any telescope can be made to suite the purpose, and you don’t even need one of those fancy tracking telescope mounts. The full moon is pretty bright, and your exposure times are going to be on the order of hundredths of a second.

One approach is to simply hold your camera up to the eyepiece of the telescope. I’m really amazed that people can make this work at all. I’ve managed to do this myself, with of all things, an iPhone, but it’s pretty tricky. You have to have a very steady hand and be very patient. A better approach is to get an adapter that will clamp to and hold your camera over the eyepiece for you. Many telescope and accessory vendors sell these, they are not very expensive, and can be made to work with just about any point and shoot camera. To capture the full moon, you actually want a lower powered eyepiece, the moon is pretty big and you don’t need a great deal of magnification.

For truly the best results, you want to use an SLR or DSLR camera, and attach it directly to the telescope. Essentially, the telescope becomes a telephoto lens in this configuration. Telescope accessory vendors will sell you what’s called a T-Adapter for your particular camera model. Your lens comes off, and the T-Adapter goes on as if it were a lens. The T-Adapter then slides into the back of your telescope like it was an eyepiece. We call this “prime focus” photography.

An alternative to this is called “eyepiece projection”. This is where an eyepiece is included in the optical path, usually inside the T-Adapter, and this provides some additional magnification. I have never had any real success with this method, as the additional glass always creates some scattering of the bright moonlight, creating ghost images and flares. I have had better success with a high quality 2X or 3X Barlow however, as these have fewer surfaces inside to reflect the light. These produce much larger lunar images, and you may not even be able to fit the entire moon in a single exposure. Photo editing tools such as Photoshop can then be employed to create a high-resolution mosaic of the entire lunar disk.

Back to prime focus. Now you just have a giant telephoto lens on your camera. Shoot at a low ISO to reduce grain. You can afford too, even a small telescope will yield a very bright image of the full moon. You’ll need very little exposure time, typically hundredths of a second. You can use your image preview or the histogram feature to adjust the exposure time if you have a digital camera. Here the histogram is probably the single most important tool. Sometimes it’s hard to accurately judge if you’ve over exposed the moon using the image preview. With the histogram, you want a nice lump in the middle of the graph, with nothing spilling off to the left or the right.

For best results aim to underexpose ever so slightly. You can more easily brighten a darker image with photo processing software than darken it. Using a low ISO to begin with helps with this, as you can also introduce noise in your image if you try stretching it too much. If you overexpose and blow out any areas (making them all white), there is no fixing or recovering from that. Focus is very important. Trust your eyes, but only so much. I always take five or six images at a time, each time slightly tweaking the focus knob ever so slightly. After some practice I can now on a good night end up with two images that are good and so close together I can’t tell them apart. You will always get images that looked good in the viewfinder or preview, but not as good on the computer screen. More is better; just throw away the bad ones.

Here as with the terrestrial moon shots, a cable release is important; in fact even more important. Now, not only are you moving the camera when you press the shutter, but you are moving the telescope. It can take several seconds for the vibrations to dampen out too. I always count to ten at least between when I focus and make my exposure to eliminate this effect. Sometimes I tend to get in a hurry, and have to remember the telescope can’t hear me. Counting quickly will not make the scope hurry up and settle down. Don’t count like 1.2.3.4.5…. it’s more like 1…2…3…4…

Another technique that I’ve picked up only recently is to use your mirror lock. If you have a DSLR with this feature, it further reduces the vibration of your camera and telescope. What this does is lock your camera’s internal mirror in the up position before taking the image, further reducing vibration when the shutter is fired. Mirror lock doesn’t really do much for long exposure astrophotography, but for a bright moon image, I have found it dramatically, and consistently has improved the sharpness of my images. When using mirror lock, you have to fire the shutter twice. The first time pulls up and locks the mirror out of the way. The second time actually opens the shutter and takes the exposure. Don’t forget to still count to 10, but this time between cable releases.

If you have even a small telescope, I hope these tips help you out a bit, and get you going on your own lunar masterpiece! A small web gallery of images taken using some of the techniques I’ve to talked about is available at www.eveningshow.org.

So what are you waiting for? Go out there and howl at the moon… with your camera of course!
*http://science.nasa.gov/science-news/science-at-nasa/2008/09dec_fullmoon/

End of podcast:

365 Days of Astronomy
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