#1. "RE: When do you need a tracking device?" In response to Reply # 0 Mon 28-Jan-13 03:52 AM by RABaker
By "tracking device" I assume you mean devices such as an equatorial telescope mount, or even a "barn door" tracker. These devices are meant to counteract the apparent movement of heavenly bodies across the sky (although for only a short distance in the case of a barn door tracker). If you meant something else, then my responses below are probably in error...
"Do you need a tracking device to stop stars 'streaking'?" It depends - the longer the focal length of the lens the faster the apparent motion of the stars across the sensor/film. You may be able to shoot for several seconds on a fixed mount at wide angles (short focal lengths), but will have to increase the shutter speed as focal lengths increase. For a better and more complete understanding I suggest you read the "pinned" message at the top of this forum: "The Rate of Movement of Celestial Objects." How much movement is acceptable in an image will vary for individual photographers and different celestial scenes. The best way to find out what is acceptable or objectionable to you is to do some testing for yourself.
"Is a tracking device desirable for the moon?" In general, a tracking mount is not necessary for moon pictures. Since the moon is illuminated very brightly by sunlight most exposures are relatively short and the apparent motion of the moon is not a significant issue. Under most "normal" conditions, lack of accurate focus and atmospheric conditions often contribute to fuzzy moon pictures much more than the movement of the moon. As mentioned above, Neil Rothchild's message "The Rate of Movement of Celestial Objects" will help.
"I am also concerned about long exposures and possible sensor damage?" I and thousands of others have engaged in astrophotography sessions with our DSLRs that lasted several hours. These long sessions usually consist of a series of sub-exposures that are then stacked to create the final image. The sub-expposures are often between 30 seconds to 10 minutes (sometimes longer, sometimes shorter) followed by a brief "off" period to record the image to the card, repeated over and over again for several hours. As a result the DSLR is active for the full period and the sensor is on and collecting data for the full period minus only a few seconds every once in a while to record images. As far as I am aware, nobody has ever reported sensor damage from these activities.
#3. "RE: When do you need a tracking device?" In response to Reply # 0 Mon 28-Jan-13 12:13 PM by nrothschild
I can think of only three situations where you might need a tracking device for the moon:
1) A total lunar eclipse, where the optimum exposure will be 2-4 seconds or more. In fact, I am not convinced a truly high quality image can be shot without one, even with the latest cameras.
2) Earth shine (the unlit portion of a crescent moon) where the exposures are very similar to total lunar eclipse exposures. This can be done on a D700 (or later FX) with a fixed mount, but an optimum image would require long enough exposures to want a tracking mount.
3) For thin crescents at very long focal lengths (700mm or more), it bumps the edge of speeds that might result in a tiny bit of motion (1/20s to 1/30s, somewhere in that neighborhood). I just shoot at 500mm because the moon is also rather low on the horizon and I'm not convinced that atmospheric conditions warrant much more than 500mm or so. But for the truly obsessed...
The moon tends to be very noisy and my experience is that it tends to bring out noise even at modest bumps of ISO. I generally shoot the moon with my D300 and there I try to keep it at base ISO. So I could see the desire to shoot at ISO 100 on the later cameras that have that lower base ISO. And in that case it would cut the shutter speed in half when shooting thinner crescents so even more reason to use a tracking mount for *optimum* results.
There is a 4th reason, and that would be an attempt to stack multiple images with something like the freeware Registax app. That might help cut down softness due to seeing.
I have never been successful in my attempts to stack lunar images shot from a fixed mount. The reason is that the images are generally rotated somewhat on a rather random basis. This due to the fact that the moon is moving in two different axis (unless, perhaps, it is shot exactly due south).
My sense of Registax is that it is able to adjust for minor changes in declination due to drift from imperfect mount alignment but it does not deal well with the "random rotation" I mentioned. Or perhaps I don;t do it right. It is also possible, I guess, to manually align each image after the fact, and then send the images to Registax. But that is too much like work work
For more than a few quick snaps, it is far far easier to shoot the moon with a motorized mount. The idea of having the moon just sit there in the field of view, without having to chase it across the sky, is quite a novel experience .
However, these mounts and tripods are much more work to set up, and can be quite heavy, so then the matter comes down to the path of least resistance. It's usually easier to chase the moon from a fixed tripod than to haul an equatorial mount out to the back yard, set it up, and then tear it down and haul it back in and store it away. And if you are shooting from a remote site things can get even more like work!
In principle a motorized mount makes star fields easier to shoot, with much more flexibility in terms of optimum low noise exposure. However, the best Milky Way shots usually include a terrestrial foreground. In that case either the stars will blur from a fixed mount or the ground will blur from a motorized mount . The end result is you need to figure out a way to do it from a fixed mount, or somehow blend two images or two sets of images, one fixed the other moving.
#4. "RE: When do you need a tracking device?" In response to Reply # 3
South Australia, AU
Wow! Much appreciated Neil, A lot more to this than I originally thought, Guess I'll "learn to walk before I can run as they say", Haven't got 500 but I guess a 1.7TC will get me there, We have a shop near here that I think sells tracking gear, if its too cost prohibitive, may have to just go with current gear Regards, Gary