It has been about a month since I finally got to start using my Nikon D700 for serious astrophotography, namely prime focus shots through a telescope. So, after playing around with the camera for hours, it is pretty safe to offer commentary on it.
For astro, the D700's biggest advantage is the live view screen, a massive 3-inch, 920,000 dot (that's almost 1Mp there) model that allows one to zoom in up to 13x magnification in the focus mode. For anyone who's got a camera that uses the old, 230,000 dot-ish LCD screens, this is like going from a rabbit ears to plasma TV. When in live view mode and zoomed in at the full 13x, the most surprising thing is that you shouldn't focus on a bright star like you would try to do with an optical viewfinder. As good as the screen is, it doesn't replace the eye, which meas that a bright, 0 magnitude star ill produce too much glare for accurate focusing. Instead, go for something in the 2nd to 3rd magnitude range. Star found, simply focus your scope until the star is at its smallest, with the time of color shift (on my refractor, a star will go from purple to green) being another giveaway to the point of perfect focus. That done, take the camera out of live view and you're set for an all-night shoot.
The next major advantage of the D700 is that it is much more forgiving in terms of accurate polar alignment than an APS-C or a Four-Thirds camera thanks to its lack of a crop factor. Believe it or not, the most difficult part of astrophotography is getting an accurate polar alignment. Think about it: the Earth is constantly rotating and you are attempting to shoot through a telescope with probably around 15-20x power where the stars will move out of the field of view in probably a minute or so. Now, for AP, not only do you want those stars to stay in the field of view, but stay perfectly centered all night so that there is no movement whatsoever so as to prevent oval stars and shifting stellar positions over the course of hundreds of photos taken over the course of hours. That's quite a feat, but it must be done! Now, with its wide (and thus less magnified) field of view, the D700 (or any FF camera) is much more forgiving in this respect than one with a sub-frame sensor, which means less time spent on fine-tuning alignment and more time for shooting.
The next great thing about the D700 is the built-in intervalometer, or programmable remote function. Coming from camp Canon, one had to buy an expensive programmable remote (or a cheap knock-off direct from China) in order to do astrophotography all night. This was undesirable for two reasons: first, the extra cost, and second, the extra cords. Anyone who does telescopic AP knows that there are a lot of cords required and that these can be a tripping hazard in the dark. Second, unless you have a holder for your remote on the mount, a gust of wind or simply the movement of the camera/scope as it tracks can pull the remote off of the spreader (or wherever else you put it) and out of the socket just enough to break the electrical connection, which means no more pictures. This has happened to me a few times and I was none too pleased about it, either!
So, is all rosy with the D700? Unfortunately, no.
The first thing that one will notice with the D700 is less power thanks to the camera's FF sensor. In effect, using the D700 will cut your scope's effective focal length by a third if using an APS-C camera and by half is using a Four-Thirds model. On the other hand, you can just crop your pictures in order to get perfect framing. Me? I'll take less power and more forgiving polar alignment any day.
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