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Lunar Astrophotography With Small Refractor


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Lunar Photography with a small refractor and a DSLR

Lunar photography with a small refractor and a DSLR


By Mark Kilner, May 2005

Disclaimer

As is the custom, I should point out that I have no interest in Vixen, Tele Vue, Canon or any other brands mentioned in the text below other than as a customer. This article is an overview of methods and processes, all of which may be achieved with similar equipment from other manufacturers.

Any opinions expressed are purely those of the author.



Copernicus region, 10 Dec 2005. Stack of 31 images at ISO 100, 1/60 sec.

Introduction


Even newcomers to astronomy cannot fail to have noticed that the hobby is in the midst of a digital revolution. Many amateurs' images of the Moon and the planets are so good, it seems like an injustice to use the word "amateur". And now the rest of us want to get in on the act.

"But I need at least an 8-inch telescope and a specialised camera if I want to take detailed photos of the Moon."

Well, no, that's not true. It is possible to obtain very respectable images of the Moon with a small telescope, and hopefully this article will go some way towards proving that. With patience and a little care, you may achieve more than you thought possibleÉ

Equipment and Accessories


My telescope, which I've owned for sixteen years, is a trusty old Vixen 102mm f9.8 achromatic refractor, mounted on an equally trusty old Super Polaris equatorial mount. Of course, back in the spring of 1990, I never dreamed that I would one day use it to photograph fine detail on the Moon, but it's advances like that which keep this hobby so endlessly fascinating.

My camera, which I've owned for one year, is a Canon EOS 350D; I'll refer to it as the Rebel XT for the purposes of this article (it sounds cooler, too). When I made the decision to get a digital camera, I chose a DSLR so I'd have the option of attaching different lenses (and telescopes of course). I also wanted to leave the way clear for any future attempts at deep-sky photography. Owners of simpler point-and-shoot cameras can still get results using eyepiece projection, and ? when it comes to processing - they may have an advantage over DSLR owners if their camera is capable of capturing AVI files.

The Rebel XT camera body is small and light, not much heavier than a typical diagonal and 1.25 inch eyepiece arrangement, which means you won't have to re-balance your scope.

If you're shooting at prime focus, you'll need a T-Ring adapter and nosepiece. You'll also need a cable release: I use the Canon RS-60E3. A red light torch is advisable too; those little buttons can seem awfully similar in the dark.

In its highest quality (RAW) mode, the Rebel XT captures images of 3456 by 2304 pixels. With the f9.8 Vixen attached, the Moon fits neatly into the frame, allowing me to capture large images of the whole lunar disc. If I want to go in close to capture fine detail, I use a Tele Vue 2.5x Powermate. (Bear in mind you'll need to adjust exposure times or ISO settings accordingly if you increase the effective focal length.)

Examples of the different image scales are shown below:



Unprocessed RAW image (10% original size)



RAW image with 2.5x Powermate in place (10% original size)


When using a Barlow or a Powermate, you may notice that the image softens quite dramatically towards the edge of the frame, in which case you'll need to keep your object of interest (usually near the terminator) as close to the centre as possible.

Preparation


When shooting the Moon (especially in close-up), it's useful to know in advance where the most interesting features are along the terminator. There are several digital atlases available; I use the Virtual Moon Atlas (reviewed elsewhere on this site), and found it meets all my requirements. It even corrects for libration, if you're feeling ambitions and want to go after elusive limb targets like Mare Orientale.

It's a free download too, which helps.

Also, you might want to prepare your camera for night-time use before you go outside:

Camera Settings: (Rebel XT specific)
Quality: Raw

Parameters: 2 (neutral)

LCD brightness: change this to the lowest possible setting. Lunar images which seem dazzlingly bright to your dark-adapted eyes will often appear disappointingly underexposed when you're back at your computer.

Auto-rotate: I usually turn this off.

Drive mode: continuous

ISO: I find 100 or 200 is usually sufficient.

White balance: I usually leave this on Auto and correct it later on if necessary, although I've seen other sites which recommend the "daylight" setting. (I convert most of my lunar images to black-and-white anyway, unless I'm attempting an enhancement to bring out the subtle colours of the Moon.)

Shooting mode: Tv (Shutter-Priority AE). If you use Av (Aperture-Priority AE), the camera will invariably over-expose the image, so unfortunately you're going to have to figure out the correct exposure times for your aperture yourself. And remember ? settings which work for a thin crescent Moon are not going to work for a nearly-full Moon. Also, the difference in brightness between the lunar highlands and the dark maria is much more apparent in photos than it is at the eyepiece.

With the 2.5x Powermate in place, I find that 1/30 sec is about as slow as I can go and still get a usable image. If the image is too dim then I increase the ISO rating to compensate. Incidentally, the Rebel XT does include a mirror lock-up custom function, the usefulness of which has been much debated across astronomy forums. I won't go into it here, suffice to say that in the few tests I've conducted I couldn't see any difference at the short exposure times I typically use.

One other thing: make sure your camera battery is charged up. Nothing runs it down faster than constantly zooming in and out of each image to check the focus.

In the field


This may sound obvious, but always look at your target first through a high-power eyepiece. Not only is this a good way of comparing what your eye sees with what the camera sees, it's also a chance to evaluate the seeing conditions and decide whether it's worth taking any photos. Looking first might save you a waste of time and disk space, and, after all, it is why you bought the telescope in the first place, isn't it?

Is your target at a high elevation or is it low, just above some rooftops? Is your telescope on a grassy lawn or on a hard surface which may still be radiating heat into the air? All these factors are going to affect the sharpness of your images.

If your telescope is on an equatorial mount, rough polar alignment is usually adequate, provided you capture all your images within a relatively short space of time. Image-stacking software like RegiStax can correct for field rotation, but this adds a lot of processing time, and it may take a few attempts before you get it right.

Perhaps the hardest part of the process is focusing - and this is one area where the Rebel XT's compact size does count against it, because the viewfinder is tiny. Expect to take some time over this; focusing, taking a photo, magnifying it on the LCD screen, and readjusting the focus until you get the sharpest possible image.

In terms of coverage, I typically capture anywhere up to about 30 shots for lunar images, and as many as I can fit on the memory card for planetary images (more on that later).

Processing


TIF images are big: a full-frame image from the Rebel XT is typically 46 MB in size. Now imagine trying to stack a few dozen of those when your PC only has 512 MB or 1 GB of RAM. That's why I manually crop each RAW image first using Canon's Digital Photo Professional before converting them to TIF. Converting lunar images to black-and-white also reduces file-size and saves processing time. If you're really pushed for memory, you can convert them to JPG instead of TIF and accept the resulting loss of quality.

I use RegiStax V3 for stacking and processing, with final touch-ups in Paint Shop Pro if required. Whole websites have been devoted to the intricacies of image stacking ? which I won't get into here - other to say that I use trial-and-error until I get something that works. Wavelet settings can be saved, so you can instantly apply them to other images, although bear in mind that what works for the Moon may not work for other objects.

If you've never used image-stacking software before, you may be pleasantly surprised at just how much detail you can extract, as illustrated below:



Typical unprocessed RAW image at 100%, cropped to show detail in and around Petavius.



Stack of 31 images at 100%, histogram stretch, no additional processing.



Processed image, after wavelet sharpening.

Conclusion


When it comes to capturing detailed expanses of the Moon, a small telescope and a DSLR are a winning combination. Sure, there are easier ways to image the Moon. Using a DSLR can be awkward, tedious, and sometimes downright painful (high target + low viewfinder = bad back), but you know what? It's worth it.

And, in case you're wondering, I was mad enough to have a go at the planets too:





Mars - stack of 94 images

Jupiter ? stack of 127 images

Saturn - stack of 117 images


If you've got a digital camera, give it go; you may surprise yourself!

Warning: Taking photos of the Moon and planets through a small telescope may leave you susceptible to aperture fever. The symptoms usually begin with the following train of thought:
If I can get images like this with a 4-inch telescope, imagine what I could get with an 8-inch, or a 10-inch, orÉ



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