DSLRs

The next step up from webcams are DSLRs, "digital single lens reflex" cameras. These cameras are much like their film-using ancestors in that they feature interchangeable lenses and through-the-lens viewing and composing. They are different in that they replace film with an electronic chip. When DSLRs first came out, amateurs were anxious to try them on the night sky. Being self-contained (no computer) and looking just like the 35mm SLRs of yore, they seemed a natural for astrophotography. Not only would there be the attraction of a sensitive CCD chip (actually, for most DSLRS, then and now, a CMOS chip), they could be used at the telescope in much the same way as a film SLR using many of the same accessories. Before long, cutting-edge amateurs were shooting the night sky with their DSLRs with varying results.

Early digital single lens reflexes turned out to be limited for astrophotography. The biggest problem was they couldn't expose for very long—30 seconds was usually the max—and the resulting images were quite noisy. Also, the chips in the initial cameras were fairly small and low-resolution affairs. Talented amateurs found workarounds such as stacking multiple images, but the biggest help was that DSLRs began evolving rapidly. Chips started getting bigger, and sensors as large as a 35mm frame were soon available for costs that were at least manageable by serious but not wealthy astrophotographers. Even better, camera makers began giving their DSLRs the capability of exposing for as long as desired via a "bulb" setting, like that on film SLRs. Canon and Nikon, especially, also worked to make long exposures less noisy. Canon actually acknowledged that its cameras were used for astrophotography and even released a model, the 20Da (no longer available) aimed at the astro market.

It wasn't all sweetness and light, though. Despite these advances, DSLRs are less sensitive than (monochrome) astronomical CCD cameras. Longer exposures are required, and guiding and mount sturdiness has become more critical. Also, our dreams of using DSLRs just like SLRs—no computer to lug into the field, just mount on scope and shoot—came to naught. Some astrophotographers do use DSLRs that way, but the best results are obtained by connecting the cameras to laptops. If nothing else, this makes focusing much easier. Squinting through the dim and small finders of these cameras or at the small "live" video displays a few DSLRs now feature, it's hard to see anything, much less focus a star field sharply. The biggest drawback to the DSLR, however, is that camera manufacturers add a strong IR block filter to the cameras' innards. This is done to make lifelike color easy to obtain in terrestrial images, but it harms DSLR sensitivity to dim red nebulas.

Despite these caveats and a few others, amateurs are doing remarkable work with these cameras, producing some incredibly beautiful color sky shots. Most of this has been a matter of adequate processing software; amateurs have helped here, designing hardware interfaces that make the cameras easy to control from PCs and writing the software needed to maximize the DSLR's astronomical potential. Some astronomy entrepreneurs are even selling "modified" DSLRs that have had their IR block filters removed (as Canon did with its 20Da). One thing makes a DSLR a great choice for many folks: it can be used for general picture taking as well as astro-use, which makes the price a little easier for many families to bear. Let's go so far as to say that for imagers who want large and easy color pictures, a DSLR, not an astro CCD cam, is the way to go. One thing is for sure: the color images you can get with one of these (Plate 57) is better than anything you could have gotten back in the "good old" film days.

What about specific DSLRs? Look through the photo magazines, and it appears not much has changed in SLR photography in the last thirty years. The top dogs are still Canon, Nikon, Pentax, and Olympus. When it comes to astrophotography, however, things have changed. Canon has surpassed Olympus, the former favorite of celestial imagers, and for good reason: the Canon DSLRs' low noise characteristics. Which Canon? The Digital Rebel is probably the most popular DSLR for astro-imaging. This reasonably priced camera—about $750 with a lens—and now boasting a 12.2 megapixel sensor in its latest (XTsi/450D) version is the closest thing we've got to a digital OM1 (the film SLR so beloved of astrophotographers). This is not to suggest a Nikon, Pentax, or Olympus can't take decent sky pictures—they can. But the astrophotos done by today's Canons are better. Is there any reason to invest in a non-Canon DSLR? Perhaps, if you've got a large collection of Pentax, Nikon, or Olympus lenses left over from the film days.

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