<p>Note that how fast - and which direction - you need to move also depends on where you're pointing; it's not a one rate fits everything solution.</p>

<p>For example, if you're pointing at the celestial pole, you don't need to move the scope at all. While if you're pointing at the celestial equator, that needs the fastest tracking rate. What you end up doing is tracking along a circle that represents the declination.</p>

<p>Which means that in order to work out what the declination is, you're going to need angle encoders on both your alt and az axes.</p>

CC BY-SA 3.0
2017-09-18T21:02:43.883Z
2017-09-18T21:02:43.883Z
JerryTheC
<p>I have a <a href="http://en.wikipedia.org/wiki/Dobsonian_telescope" rel="noreferrer">Dobsonian telescope</a>.</p>
<p>It is using <a href="http://en.wikipedia.org/wiki/Altazimuth_mount" rel="noreferrer">Altazimuth mount</a>.</p>
<p>Basic idea of using it is to target the object by moving telescope vertical axis perpendicular to the ground, and an elevation axis that is parallel to the ground.</p>
<p>I have installed two step motors to automate the movement along both vertical and elevation axis.</p>
<p>I would like to find out how can I cancel out earth rotation speed by simultaneously moving both vertical axis and elevation axis motors.</p>
<p>Idea behind it is to point telescope at the object and press the button.
Then the step motors driver software would follow the object as the earth rotate.</p>
<p>I will quote few lines from <a href="http://www.astronomyhints.com/mounts.html" rel="noreferrer">The Basic Astronomical Telescope Mounting Designs</a> to help me explain what I am trying to achieve:</p>
<p>[Using altazimuth telescope...:]</p>
<blockquote>
<p>If you happen to be observing from the North or South Pole, the vertical axis would be aligned with the Earth's spin axis. The nice thing about that would be that when you found an object to observe, rotation in only the vertical axis would be needed to keep the object in the field of view. Rotating at the Earth's spin rate in the opposite direction as the Earth's rotation would keep and object motionless in the eyepiece.</p>
<p>However, for any other latitude on the planet, the vertical axis is not aligned with the Earth's spin axis. This means that to keep an object in the field of view requires motion in both axes. The motion rates will change over time as the elevation angle changes. Tracking objects near the horizon requires mostly changes in elevation, and tracking objects more straight up requires mostly changes in azimuth.</p>
</blockquote>
<p>I need to find mathematical algorithm that will help me solve the problem described in the second paragraph.</p>
<p>Hope this is clear.</p>

CC BY-SA 3.0
2014-12-29T14:34:46.860Z
Nowadays there's usually an Arduino involved somewhere: https://www.google.com/search?q=Dobsonian+%22star+tracking%22+arduino&ie=utf-8&oe=utf-8  Without a latitude wedge, you'll get frame rotation over time. https://www.google.com/search?q=tracking+azimuth+wedge&ie=utf-8&oe=utf-8#q=tracking+wedge+latitude
Wayfaring Stranger
CC BY-SA 3.0
2014-12-30T00:36:21.220Z
@barrycarter clock drive is used with equatorial mounts. My telescope is altazimuth mount. I need to find the algorithm to drive both motors using altazimuth mount.
Kocur4d
CC BY-SA 3.0
2014-12-30T00:41:54.423Z
@Wayfaring Stranger I plan to use Arduino but at this stage I would like to stay away from net ready solutions and see if it is possible to build it from scratch.
Kocur4d
CC BY-SA 3.0
2014-12-30T01:51:03.643Z
@Kocur4d Understood. When I need to make something a little complicated, I usually have a look at the "Make" sites and the like, figure out what's wrong with those designs for *my* purposes and design new from a position of knowing what others have tried. Someone is likely to have a great bearing design, but poor stepper/servo control or vice versa. If you look at a enough designs you can get a feel for what's best for you.
Wayfaring Stranger
CC BY-SA 3.0
2014-12-30T14:09:50.800Z
@barrycarter yes it sounds like a move in the right direction. I will update the question in a minute to make it more accurate.
Kocur4d
<rotation><dobsonian-telescope>
Cancelling out earth rotation speed, Altazimuth mount
Kocur4d
# Cancelling out earth rotation speed, Altazimuth mount

(Asked by: username_0 on 2014-12-29)

I have a [Dobsonian telescope](http://en.wikipedia.org/wiki/Dobsonian_telescope).

It is using [Altazimuth mount](http://en.wikipedia.org/wiki/Altazimuth_mount).

Basic idea of using it is to target the object by moving telescope vertical axis perpendicular to the ground, and an elevation axis that is parallel to the ground.

I have installed two step motors to automate the movement along both vertical and elevation axis.

I would like to find out how can I cancel out earth rotation speed by simultaneously moving both vertical axis and elevation axis motors.

Idea behind it is to point telescope at the object and press the button.
Then the step motors driver software would follow the object as the earth rotate.

I will quote few lines from [The Basic Astronomical Telescope Mounting Designs](http://www.astronomyhints.com/mounts.html) to help me explain what I am trying to achieve:

[Using altazimuth telescope...:]

> If you happen to be observing from the North or South Pole, the vertical axis would be aligned with the Earth's spin axis. The nice thing about that would be that when you found an object to observe, rotation in only the vertical axis would be needed to keep the object in the field of view. Rotating at the Earth's spin rate in the opposite direction as the Earth's rotation would keep and object motionless in the eyepiece.
>
> However, for any other latitude on the planet, the vertical axis is not aligned with the Earth's spin axis. This means that to keep an object in the field of view requires motion in both axes. The motion rates will change over time as the elevation angle changes. Tracking objects near the horizon requires mostly changes in elevation, and tracking objects more straight up requires mostly changes in azimuth.

I need to find mathematical algorithm that will help me solve the problem described in the second paragraph.

Hope this is clear.

**Tags:** rotation, dobsonian-telescope

> **Comments:**
> **username_1**: Nowadays there's usually an Arduino involved somewhere: https://www.google.com/search?q=Dobsonian+%22star+tracking%22+arduino&ie=utf-8&oe=utf-8  Without a latitude wedge, you'll get frame rotation over time. https://www.google.com/search?q=tracking+azimuth+wedge&ie=utf-8&oe=utf-8#q=tracking+wedge+latitude  
> **username_0**: @barrycarter clock drive is used with equatorial mounts. My telescope is altazimuth mount. I need to find the algorithm to drive both motors using altazimuth mount.  
> **username_0**: @username_1 I plan to use Arduino but at this stage I would like to stay away from net ready solutions and see if it is possible to build it from scratch.  
> **username_1**: @username_0 Understood. When I need to make something a little complicated, I usually have a look at the "Make" sites and the like, figure out what's wrong with those designs for *my* purposes and design new from a position of knowing what others have tried. Someone is likely to have a great bearing design, but poor stepper/servo control or vice versa. If you look at a enough designs you can get a feel for what's best for you.  
> **username_0**: @barrycarter yes it sounds like a move in the right direction. I will update the question in a minute to make it more accurate.  

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## 1 answers

### Answer by username_2 on 2017-09-18. Score: 0

Note that how fast - and which direction - you need to move also depends on where you're pointing; it's not a one rate fits everything solution.

For example, if you're pointing at the celestial pole, you don't need to move the scope at all. While if you're pointing at the celestial equator, that needs the fastest tracking rate. What you end up doing is tracking along a circle that represents the declination.

Which means that in order to work out what the declination is, you're going to need angle encoders on both your alt and az axes.


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