Advanced Piggybacking

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The next step up from piggybacking is shooting through either a long focal length camera lens or a small refractor, something with a focal length in the range of 400 to 500mm, perhaps. This extra focal length will deliver finer details in larger deep sky objects and permit the capture of some medium-sized DSOs. Galaxies M81 and M82, for example, are minute smudges at 50mm, but begin to come alive with detail at 500mm.

Small refractors are attractive to piggybackers, since they are more versatile than long camera lenses. Also, a small refractor can be used for guiding and visual observing as well as imaging. A little achromat like Synta's much-loved Short Tube 80 may also be a hair better on sharpness than a DSLR "kit" zoom lens. Unfortunately, the refractor's stars will be bloated and will display plenty of chromatic aberration (false color) due to the simple achromatic objective. A nice alternative to an ST80 is one of the inexpensive "ED" apochromat refractors from China that are now available from outfits such as William Optics.

Although not as color free or sharp as top-of-the-line camera lenses or "true" APO refractors (with three-lenses element and/or a fluorite element), the EDs can do nearly as good a job, especially if equipped with one of the field flattener lenses available for many of them. A nice choice for the new astro-imager is one of the ubiquitous 66mm f/6 EDs that can often be bought for less than $400. The 66mm doesn't sound like much aperture, but it's utterly amazing what can be captured by one using a sensitive camera (Plate 77). At the longer focal lengths produced by even a 66mm ED scope, the imager's life does become more complicated; guiding the telescope, for example, becomes a necessity if perfect stars are desired. The upside is that guiding is still relatively non-critical at 300 to 600mm. See the Prime Focus discussion and the "Demon" section below for tips.

Prime Focus

This is the Holy Grail for most aspiring imagers: taking long exposure shots through the CAT, beautiful pictures like those produced by that icon of amateur astrophotog-raphers, Jack Newton. We won't all be able to make sky pictures like Newton's; most of us don't have the equipment, the time, the talent, or the skies. It is relatively easy, however, for most of us to produce at least recognizable deep sky images these days. With sensitive CCD cameras, a 3- to 5-minute exposure through a C8 delivers a huge amount of detail, more than an hour of exposure did during the film days. Shorter exposure also means there is less time for things to go wrong.

What's needed in addition to a camera and a CAT for prime focus imaging? A very desirable accessory for most imagers is a focal reducer (see Chapter 6). A non-reduced f/10 C8 has a focal length of 2000mm, and it is insanely difficult for even experienced astrophotographers to get perfect results with a set up like that. All that

Plate 77. (Rosette Nebula) Today's sensitive CCD cameras make it possible for even a 66mm aperture telescope to bring home the faint wonders of deep space, such as the Rosette Nebula. Piggybacked William Optics 66SD refractor and SBIG ST2000 camera. Credit: Author.

focal length makes guiding hyper-critical. Any small errors in keeping the guide star centered will be very obvious in images. Exposures must be relatively long, too, even with sensitive CCD cameras. An f/10 system is, in photographer's parlance, "slow;" which means it takes a long time to build up a well-exposed image. Celestron sells an f/6.3 reducer/corrector and Meade sells both an f/6.3 reducer/corrector and an f/3.3 reducer. Large chip CCD cameras and DSLRs do best with the f/6.3, since stars at the edge of the field tend to be badly distorted in f/3.3 reducers. A camera with a smaller imaging chip, like the DSI, may do well with an f/3.3. If the chip is not large enough to register the ugly stars at the f/3.3's field edge, its wider field and faster optics are real advantages, especially for beginning astrophotographers. Other manufacturers produce both cheaper and more expensive reducers, but I have not seen one yet that's worlds better (or worse) than Meade's and Celestron's bread and butter reducers and reducer/correctors.

How is a camera attached to a CAT? A CCD is usually furnished with a 1.25- or 2-inch "nosepiece" that allows it to be inserted into a visual back of the telescope. A DSLR will need a T-adapter and a prime focus adapter. A T-adapter is a metal ring that takes the place of the camera's lens. One end features a mount appropriate for a particular camera model—Canon, Nikon, etc.—the other end is equipped with T (universal mount) threads, which can be screwed onto a prime focus adapter. The scope end of an SCT style prime focus adapter features a threaded ring that lets the whole shebang thread onto the rear port (or a focal reducer). Note that most integrating cameras' nosepieces can be unscrewed to reveal T threads that will allow the camera to be mounted on a prime focus adapter, which is usually a more secure arrangement than using the nosepiece and a visual back. Prime focus adapters are available from most astro-dealers.

If the intent is to take exposures longer than one minute, some kind of guiding system will be needed. In the past many astrophotographers used a device called an Off Axis Guider (OAG), which used a small prism to intercept a small amount of light before it reached the camera. This tiny prism was able to deliver star images from the edge of the telescope's field, allowing the scope to be guided during the exposure with the aid of a crosshair reticle eyepiece. Watch the star, and when it drifts off the crosshair, push a button on the hand control. Continue to watch and make corrections for the duration of the exposure. Today a few imagers still use OAGs (with guide cameras, usually, not "manually"), but most often with guide cameras (see the Guiding Demon section below) rather than with eyepieces.

Given the current unpopularity of OAGs, most CAT users guide with a small piggyback scope on the main tube. This does add the problem of flexure to the mix; if its mounting bracket is not stiff enough, the guide scope may move independently of the main scope as it changes position while tracking across the sky. Even a small bit of flexure will cause trailed stars in images. Thankfully, the short exposures required by CCD and DSLR cameras make this a minor problem for most imagers. The mount doesn't move far enough across the sky over the course of a 5- to 15-minute exposure to cause the guide scope to flex much. Guide scopes are certainly easier to use than OAGs, as they are not limited to the tiny selection of stars provided by the OAG's ridiculously tiny "pickoff" prism.

Many SBIG users have almost returned to the days of OAGs—in a sense. Some of the company's cameras, as mentioned earlier, are equipped with guide chips. Like the OAG, the guide chip is limited to stars at the periphery of the field. Due to the sensitivity of SBIG guide chips, however, you can almost always find a suitable star.

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