The advent of CCD imaging seems to have breathed new life into the classic reflector versus refractor debate. And, as always been the case, no one seems to have a definitive solution because there is, in fact, no definitive solution. Reflectors, refractors, and a variety of compound optical systems each have a place in CCD imaging. In this article, we'll take a look at what the classic optical systems offer, and offer some tips on how you can decide what type of telescope is best for your type of CCD observing.
We'll begin our survey with the classic achromatic refractor which, as it happens, was the first type of telescope that I tried for CCD imaging. The setup was this: I placed an SBIG ST4 camera at the focus of my 6-inch f/15 refractor, the same one described in Build Your Own Telescope, and set out to make some images of the gibbous Moon. This lens is a truly fine performer, so I expected success. But it turned into a very frustrating experience: the lunar craters never got sharp no matter how I focused. It took me several nights and a lot of experimenting to determine exactly what was happening.
All achromatic refractors suffer from considerable secondary chromatic aberration, but the lenses are designed to minimize the visual impact. The shortest focus of the lens occurs in yellow-green, and both blue and deep red focus farther from the lens. This works well for visual observing because they eye is sensitive to yellow-green, and the out-of-focus red and blue make only a faint purple halo around the star image. But CCDs are most sensitive at a wavelengths around 7500 Angstroms, in the near-infrared part of the spectrum, so when you focus for a CCD, you're focusing some distance beyond normal. What you see is a sharp image formed in near-infrared light and an out-of-focus blur of red, orange, yellow, and green light. Even though blue and violet might focus with the deep red and near infrared, the defocused light from the center of the visible spectrum washes away almost all image contrast.
The importance of the chromatic blur depends on the aperture and focal length of the lens. You can, for example, get acceptable images through an achromatic finder telescope with a 50 millimeter f/5 lens, but somewhere not far above 3 inches aperture, classic doublets simply don't form images good enough for satisfactory CCD imaging. Star images taken at the focus of my 6-inch f/15 achromatic refractor have haloes more about 30 pixels in diameter.
With apochromats, the story is more complicated. Most apos are optimized for the visible part of the spectrum with little concern what happens in the near infrared, which, of course, is fully justified because you cannot see light in these wavelengths. But apos vary enormously in design. Some bring the near-infrared to the same focus as visible, and other do not. As a result, some apos form sharp images with CCDs while others do not. Given the wide variety of possible designs, the only way to know how a given apo will perform is to shoot some test image of stars and examine what you get. The most sensitive test is a through-focus star test: a lens that forms sharp star images will give identical intrafocal and extrafocal star images.
I have seen numerous CCD images made with a variety of three-element apos made by AstroPhysics, and they seem to be exceptionally well corrected because stars images are tight and crisp. My own 4-inch f/5 TeleVue Genesis gives crisp and acceptable images with slight haloes approximately 4 pixels in diameter. I have not seen enough images from other refractors to form any reliable conclusions, and so advise testing any apo before committing serious dollars to using it for CCD imaging.
Well-corrected telephoto lenses often perform extremely well for imaging fields up to several degrees across, but you need to check out the individual lens because telephoto designs vary. To give good images, the lens must be reasonably well corrected when it's focused for the near infrared, and the yellow-green light must still focus reasonably close to the near-infrared focus. If you shoot a star field with a telephoto and see sharp star images enveloped in soft haloes of light, you're probably looking at an out-of-focus visible-light problem. If this is indeed the case, when you place a red filter over the lens, the haloes will disappear and you'll be left with crisp, sharp star images. For imaging the big H-alpha nebulosities, a red-filtered lens is great, but if you want to do galaxies, it's hardly ideal. At swap meets you can often pick up a used telephoto lens, especially one in the 135 to 250 millimeter range, for a pretty good price. Among brand-name lenses, I have been consistently impressed with the performance of those made by Nikon. Avoid zooms; they seldom perform as well as fixed-focus telephotos and the mechanisms tend to "drift" when you least expect it.
After refractors come the reflectors. Reflectors are completely color-free, of course. Furthermore, unless you plan to use a CCD that is large (i.e., more than 10 millimeters across the diagonal) or one that has small pixels (i.e., smaller than 10 micrometers), or have a Newtonian that is faster than f/4, the coma blur simply doesn't get large enough to be visible. But reflectors do suffer from a variety of ills that refractors seldom exhibit. Newtonians especially are prone to field flooding because light can reach the CCD in a variety of ways. For example, light from the ground can enter the bottom of the tube around the mirror. Light may also scatter to the CCD from a short tube, or enter around the base of the focuser. Such stray light can seriously degrade the performance of your CCD camera, especially in urban and suburban observing sites. Fortunately, these sources of unwanted light are easy to eliminate with a tube-bottom baffle, a "snoot" at the front of the tube twice the diameter of the mirror, and the addition black sealant, caulking compound, or tape around the focuser. Once sealed, you should be able to shine a bright flashlight all over the outside of the telescope without increasing the background sky brightness.
In addition to admitting stray light, many reflectors introduce spikes around bright stars due to diffraction from secondary mirror supports, but this seldom poses much of a problem. Some people even find diffraction spikes aesthetically pleasing! Properly made Newtonians are great for CCD imaging.
Naturally, it makes sense to check the optical performance of a reflector
by taking extrafocal and intrafocal star images. The resulting "donut-o-gram"
images should be the same on either side of focus. If you see bright rims
on one side and bright centers on the other side, you're looking at evidence
for spherical aberration. In focus, this aberration will show up as a bright
core enveloped in a soft halo. Study some of the Hubble's images to see
whether you want this particular aberration in your telescope. If you see
signs of astigmatism or other figure irregularities with one of today's
large, thin mirrors, check that the mirror cell is working properly.
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