Quote:
Originally Posted by markids77
Please explain what setup you have to so accurately track the movement of these things across the sky. After a three hour static exposure I would expect to see only a pattern of smears across the image. Whatever you use synchs damn near perfectly!
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Good catch! There's a rule called the 500 rule where you divide 500 into the effective focal length of your lens/telescope. [Edit: Got the backwards... Divide the focal length into 500] The result is approximately the number of seconds you can expose before the stars start to streak. So a 500mm lens on a full frame camera can only take about a 1 second exposure without some kind of tracking mount.
I use an equatorial mount that tracks the target at a rate to compensate for the earth's rotation. The mount must be carefully aligned with the North Celestial Pole - almost Polaris, but not quite. The stars appear to move in a circle with the center being the celestial pole. With the mount aligned the camera and scope move in an arc identical to the apparent movement of the stars. I can frame and focus an object and track it visually all night, though not accurately enough for really long exposures. With just the mount, 1-2 minute exposures are possible. That's limited by the mechanical integrity of the mount. Hey, it comes from China - enough said. To improve this, I have added a separate guiding scope and camera that locks on a single star and sends small corrections to the equatorial mount every few seconds to correct for gear lash etc. in the mechanism. With a guide scope (connector to a computer) it's pretty easy to get 5-10 minute exposures. 20-30 minute exposures are possible with higher end hardware.
The trick is to take many exposures and stack them with a software package that examines each frame and determines what is noise from the camera sensor and what is good signal. The software uses "dark frames", exposures taken with the lens cap on, from the camera to subtract the camera sensor noise from the actual exposures.
The processing is considerably more time consuming than the image capture.
It's really cool to point your telescope at something you can't see with the naked eye and see an image pop up on the camera LCD or computer screen. Especially when it took the light from that object 2.7 millions years to get here.
The camera, guide scope/camera, and the mount are all controlled by a laptop via USB. Once I set up the session, I can kick back and just look at the stars or go to the car for a nap.
Here's a pic of the setup.
