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Digital SLRs for Astrophotography

Posted by Michael Covington   11/25/2004 12:00AM

Digital SLRs for Astrophotography
Long ago, I said that when somebody made a digital SLR camera (DSLR) that cost less than $1000 and was usable
for long-exposure astrophotography, I'd buy one.

Now they're doing it.

This year's leading contenders for astro-DSLRs are the Canon Digital Rebel (with its big brothers 10D and 20D)and the Nikon D70.

All of these will let you do 5- to 15-minute (or longer)exposures of deep-sky objects, and because they have no reciprocity failure, it's amazing what you can pick up.

The accompanying picture of the Pleiades is a 10-minute exposure through a 400-mm f/6.3 lens, in town, under a hazy sky. Note that nebulosity is visible in two places.

Hot pixels aren't as much of a problem as you might guess, but for best results, they have to be subtracted out. This isn't hard.

Immediately after taking each picture, take a dark frame
(with the lenscap on) with the same camera settings and temperature. Then, working with TIFF files, or with camera RAW files that you've converted to TIFF using the camera's software, subtract the dark frame from the actual picture. This can be done with utilities such as BlackFrame NR (free from, various astronomical image processing packages, or even Photoshop invert one image and put it on top of the other with 50% transparency.

The Nikon D70 does automatic dark frame subtraction, but whether or not you opt for this, the D70 applies a rather aggressive noise reduction algorithm that tends to eat star images.

I'm told you can prevent this by choosing noise reduction mode 2 (auto dark frame enabled) and turning camera power off while the dark frame is being taken.

At this point the D70 has already stored a raw image, which it will replace with a noise-reduced image if you don't interrupt it.

Even if you're a Nikonian, you can use a Canon body. Several machinists make, and sell on eBay, adapters that enable the Canon to take Nikon lenses. There is no autofocus and no aperture coupling, but you don't need those features for astrophotography anyhow.

There are also adapters for M42 (Pentax-Praktica screw mount) lenses, as well as of course the ubiquitous T-mount.

Manual focusing is much easier if you use a magnifying right-angle finder. For Canon, this is the Angle Finder C, which works like the classic Olympus Varimagni Finder (with two switch-selected magnifications) but gives a brighter image and longer eye relief.

One last note. If lunar and planetary work is your main interest, you may be better off with a digital camera that is not an SLR, because even if the SLR has mirror lock, there is still vibration from the shutter. Either couple a non-SLR digital camera afocally to your eyepiece, or best of all, use a Meade Lunar-Planetary Imager, Celestron NexImage, or modified webcam.

-Michael Covington

Click here for more about the Canon EOS Digital Rebel. -Ed.

Click here for more about the Nikon D70. -Ed.