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Alt Az, goto's and zenith

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#1 HowardK

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Posted 22 April 2013 - 07:23 AM

Am i right in saying that with an altaz mount as you goto objects near the zenith then the goto accuracy deteriorates?

#2 CharlesW

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Posted 22 April 2013 - 09:05 AM

It's something of a moot point because as the scope gets near zenith, there is no room for your face or a camera between the base and the back of the scope. I always had to stop well before zenith. Different story with a wedge.

#3 rmollise

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Posted 22 April 2013 - 09:41 AM

Am i right in saying that with an altaz mount as you goto objects near the zenith then the goto accuracy deteriorates?


That's generally been my experience, yes. "How much" depends on the mount: NexStar 11 GPS "a little"...ETX 125 EC "a lot." ;)

#4 HowardK

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Posted 22 April 2013 - 10:42 AM

I have the skywatcher az eq6

No problems like a fork with space for face or camera

#5 JMW

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Posted 22 April 2013 - 04:06 PM

My refractors on my DM6 using a 12 inch pier extension can pivot at the zenith without problems. I have 2nd handle on my mount for pushing at near the zenith. Its hard to get leverage when the scopes straight up without it.

#6 rmollise

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Posted 22 April 2013 - 04:14 PM

My refractors on my DM6 using a 12 inch pier extension can pivot at the zenith without problems. I have 2nd handle on my mount for pushing at near the zenith. Its hard to get leverage when the scopes straight up without it.


That's cool, but the DM is not a go-to mount...

#7 Quintessence

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Posted 22 April 2013 - 04:58 PM

From a mathematical perspective, when aiming close to the zenith, small changes in celestial coordinates correspond to much larger changes in alt-azimuth coordinates. Any pointing errors get multiplied by the same factors. In addition to the usual round-off and other pointing errors, the time-dependent error also becomes extreme near the zenith. That is, in terms of alt-azimuth coordinates, you are trying to hit a rapidly moving target.

So, any coordinate/computer-guided alt-azimuth mount will become more tedious near the zenith. The issue translates to DSC-guided manual mounts as well as to GoTo Mounts -- although worse for a GoTo mount because the issue is compounded by the problem of rapidly varying the motor controls.

The underlying problem has nothing at all to do with "how easily" a mount turns on its axes. It's strictly about the fact that alt-azimuth coordinates change rapidly with respect to the rate at which celestial coordinates change near the zenith. Of course, the mathematics is reflecting the physical nature of an alt-azimuth mount.

Note also that this same problem affects alt-az TRACKING near the zenith -- not just aiming.

It's interesting to note the differences in comparing an equatorial mount near its "hole" at the celestial pole. Although perhaps awkward to aim near the pole, an equatorial mount will not have analogous aiming/tracking issues because it points in terms of celestial coordinates. Of course, there is something of a singularity at the pole itself -- where movement across the pole produces a discontinuous jump in coordinates.

Charles

#8 JMW

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Posted 22 April 2013 - 09:20 PM

My DM6 is push-to with 10K encoders connected to Argo Navis. So accuracy is still related but I provide the motive force. I think the mount being orthogonal and the scope being parallel with the mount dovetail helps with reducing cone error and improves the pointing model.

#9 SkipW

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Posted 22 April 2013 - 09:38 PM

It's called "Gimbal Lock" and happens when the optical axis of your instrument is nearly parallel to one of the axes of rotation of your mount; you essentially lose one of the degrees of freedom of motion. It's straight up for alt-az mounts and toward the poles for equatorials.






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