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DPChallenge Forums >> General Discussion >> Astro Star/Sky-Tracking
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05/29/2021 05:58:47 PM · #1
Not sure if this is discussed if so then pointing me to that would be helpful :)

if I pick any sky-tracker it rotates at the axis and the rotation measurement happens based on something

Questions:

1. How do the rotation speed is calculated?
2. How do rotation speed changes with say 14mm or 24mm or 200mm lens?

05/30/2021 11:59:11 AM · #2
In order to get accurate tracking, the rotation axis of the tracker must be aligned parallel to the Earth's axis of rotation. That's done by doing a "polar alignment." Once aligned, if the rotation rate of the tracker is the same as, but opposite to the rotation rate of the Earth, then a camera mounted to it will always point to the same spot in the sky. So to directly answer your questions:
1.) There is one correct rate for star tracking, called the "sidereal rate" (there are others, such as solar or lunar)
2.) The rate does not change based on focal length.
05/30/2021 01:08:57 PM · #3
What Kirbic said. Equatorial tracking mounts are typically used in conjunction with a ballhead, so the camera can be pointed in any direction without disturbing the polar alignment. Some models offer additional tracking speeds for special purposes like time lapse photography.
05/30/2021 07:03:51 PM · #4
The earth rotates at the same rate regardless of the focal length of you lens. So tracking rate does not depend on the focal length. But focal length does impact what you can get away with. The focal length affects the size of your field of view. The dimness of the sky object affects the duration of exposure (shutter speed) required. These two factors combine to affect how much accuracy you need from your tracking mount. Without the right accuracy, stars will appear as smudges rather than as round points of light.

A wide lens looks at a lot of sky. The motion of an individual star as a percentage of that field of view per second may be relatively small. With a 14 mm lens and a shutter speed 20 seconds or less, you may get acceptably round stars across a wide expanse of sky. If you are in a dark sky location and use a moderately high ISO, you may have enough exposure to show the brighter constellations well, or even the Andromeda galaxy as an oval smudge using just your tripod and no tracking.

On the other hand, a 200 mm lens looks at a much smaller part of the sky. Focusing well becomes a more critical task and better tacking becomes important. The motion of an individual star as a percent of a smaller field of view per second may be very large. For most objects, there is not enough light to support a fast enough shutter speed to freeze the motion. So, to keep things still enough on your camera sensor to capture without smearing the stars, you need a good tracking mount. Possible exception: a bright moon often reflects enough light to demand a short shutter speed, and you may be able to get a sharp picture of the craters at 100 to 200 mm even without a tracking device. Experiment and see what works for you.

The dimmer the object the longer exposure you need (increasing the need for good tracking) and the smaller the field of view, the more accuracy you need. For especially dim objects, very long exposures (and good tracking devices) may be mandatory to detect them at all. Even then you may need to stack multiple long exposures to detect dim objects. Tracking devices have accuracy limits. Some mounts routinely get good exposures with no motion distortion over one or two minute exposures (if set up level with good polar alignment). Excellent mounts with a rock solid support, exceptionally smooth mechanisms, and fantastic polar alignment may get round stars even during five minute exposures. Very expensive high-end equipment handled with amazing expertise may be able to handle much longer exposures (which still require stacking for many objects).

Search for the "Cloudy Nights" discussion forum for more detailed information than you ever expected possible.
05/30/2021 09:51:32 PM · #5
Thank you so much for the information, these are definitely valuable information that I was looking for, I have done Milkyway and all but when someone said they open the shutter for 2 minutes and during that time stars/milkyway will move so they said they are using tracker and hence the question came to my mind, on how would they work, there must be some formula or something :)

Once again thank you so much for valuable information :)
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