The moon in my sights

With the weather cooling off, the astronomy bug has bitten me again, as it tends to do every year at this time. Driving hone this past week I've been treated to some exceptionally clear skies, so I decided to take out my telescope and play around a bit. I had it out last week, but most of that time was spent struggling with collimation--which is the technical term for making sure all the mirrors line up in my Newtonian reflector telescope. If they don't, then distortion degrades the image of whatever you happen to be looking at. So I used a compass to get a passably accurate polar alignment, then monkeyed with the mirror settings for half an hour before I finally was satisfied that the scope was as collimated as good as I was going to get it. Collimation is a very basic task for owners of Newtonian telescopes, but man, I struggle with it.

With collimation more or less achieved and a pretty half-moon beckoning in the sky above, I was inspired to break out my Canon 7D and see how it handled for astrophotography. The 7D has live view, which my XTi does not, which should make accurate focusing much easier--at least in theory. The moon is an easy target, so I gave it a shot. There are two way to take astrophotos through a telescope--using the scope itself as the lens, which is called "prime photography," or using an eyepiece between the telescope and camera for increased magnification, which is called "eyepiece projection photography." Of the two, eyepiece projection is the more challenging, but I tend to do it most often because I like to mix my astrophotography with visual observing. With prime focus, although the image is brighter and sharper, the rear mirror must be moved inside the telescope tube which renders it unsuitable for visual observing. Plus, it would have to be re-collimated, and you know where I stand on that.

The image above shows the southeast of the moon. In the center of the image, the overlapping craters are, from lower left to upper right, are Janssen, Fabricus and Metius. Just to the right of them is a shallow, diagonal gash that is the Rheita Valley. Pretty cool, huh? I shot this image, and the one below, using a 12mm GSO Plössl eyepiece. The image at the top of the page of the entire moon was taken using a 20mm Plössl for a lower magnification and a wider field of view.

The image above shows the moon from the lunar equator southward. The smooth, grayis areas are the Sea of Fertility (right) and the Sea of Nectar (left). The Sea of Tranquility joins them at the top of the image, with the Apollo 11 landing site in the upper left-hand corner. The prominent crater with the central peak on the western edge of the Sea of Nectar is Theophilus. Notice the loss of sharp definition in the lower left of the image--I'll back to that shortly.

With the 12mm Plössl images turning out so nicely, I thought I'd push my telescope's capabilities to the max, and traded the 12mm for a 4mm Plössl, which is the highest magnification eyepiece I own. Turns out the 4mm is a bust for astrophotography--I could not bring it to focus with my camera. There simply wasn't enough inward focus on the telescope's focuser, and the image stayed blurry. So I tried my next highest magnification eyepiece, a 6mm Plössl. This one did work, as evidenced by the image above. The crater Theophilus and the Sea of Nectar are visible upper right hand corner. The view was certainly much closer, but it was also much dimmer. Not only that, but turbulence in the atmosphere was more obvious, distorting and degrading the view. Despite my best efforts at focusing precisely, the rather soft image above is the clearest I could manage. For practical purporses, 9mm is probably the highest magnification eyepiece I can use under normal sky conditions for astrophotography.

In case you were wondering, the image above shows how the eyepieces connect with the camera in the telescope adapter. Different eyepieces are swapped out for narrower or wider fields of view. What suprised me the most was the poor imaging results I got from my wider eyepieces, such as the 20mm Plössl. The full view of the moon at the top of this post was taken with the 20mm, as was the image below. Notice the halation and distortion that becomes more apparent the farther away from the center you get. I'm really not sure what this is--I'd have thought a lower-power eyepiece would minimize distortion, but the opposite appears to be the case. Because the higher magnification eyepieces restrict the field of view to a very narrow portion of the telescope's mirror reflection, I suspect the distortion is the result of either the natural "coma" distortion inherent in the Newtonian mirrored telescope design, or errors from poor collimation. I'm not at all confident my collimation is good, but that's an awful lot of distortion. The same goes for coma. I'll be asking people more knowledgeable than myself in the future to try and figure this one out.

As I was wrapping up my photo session, three middle school kids who'd been walking up and down the block all evening stopped and asked "can you see the moon through that?" I removed the camera and re-balanced the mount, then invited them to look. There were gasps of of amazement and much marveling all around. Then I showed them Jupiter and its four Galilean satellites. One exclaimed "I like science now," which gave me a chuckle. They thanked me then headed home, and as they were walking away, I heard one say, "No wonder Monkey Girl is so smart!" Monkey Girl being my eldest daughter. That made me smile--two compliments for the price of one.

I'm going to try and set up my telescope in the front yard more often during these mild autumn evenings. Even though light pollution keeps me from viewing any deep space object--galaxies and nebulas and clusters--the moon and planets are still gorgeous and more than impressive for a bit of astronomical outreach among the neighbors. I just wish I could figure out how to control that fuzzy distortion in my images...

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