DIY Astromaster EQ114 motorization

[Swifty35]

Hello all! I am in the midst of completing software to finish motorizing a celestron eq114 telescope mount. I am using hour angle and declination from stellarium to slew to my targets, however there is a significant level of error in the pointing of the instrument. It has finally occurred to me that the hor ange and dec affect each other. For example, if i want to point to an object at the local meridian at 120 degrees dec the ra axis must move 90ish degrees and then the dec must move 30 degrees "up". As the object of interest moves away from the meridian the error becomes more pronounced, so I am looking for assistance with the math to compensate for the 2 axes being dependent on one another when pointing the telescope. Thanks. 

[malc-c]

Why try and reinvent the wheel.... there are already  lots of existing 3rd party DIY alternatives to commercial products like synscan.

I'm no expert, but I don't think there is such a correlation between the two axis.  In an ideal world an EQ mount will only need to track in RA.  The DEC will stay the same.  To track a target, the mount needs to move at the same rotational rate of the Earth but in the opposite direction. So 360 degree / 24 = 15 degree per hour or  0.25 degree per minute.  The DEC axis won't need changing as the track of the scope will match the arc the target forms.  Something that is 30 degree above the meridian will always be 30 degree above the meridian as the meridian itself is an arc from East to West.  The only time you would need to control both RA and DEC axis would be if you were using an ALT / AZ mount (like a Dobsonian) as the "height" of an object will change as the AZ axis is level and not tilted to the latitude of the observer 

[Cornelius Varley]

8 hours ago, Swifty35 said:

For example, if i want to point to an object at the local meridian at 120 degrees dec the ra axis must move 90ish degrees and then the dec must move 30 degrees "up"

I think you might need to read up on the definition of declination. Declination is a value between 0 and +/-90 degrees from the celestial equator.

As malc-c stated, if the equatorial mount is tracking correctly, then only the ra axis will be operating. If you are trying to tracking manually and you are finding that the dec axis needs to be constantly corrected then the mount is not correctly polar aligned.

[Swifty35]

So this is a diy equatorial mount and I am writing the software to slew to objects based on hour angle and declination. Sorry if that was not clear in the original post. I realized my math was off when I tried to point at an object near my local meridian and a declination of around 120 degrees. The way I have the program written only the dec axis moved significantly. However, the further from the meridian I was pointing the closer I was... but still off. That is because with a good polar alignment the instrument is pointing to 0h0m0s HA and 90 Degrees Dec.  With the alignment being better further from my local meridian I think that there is an exponential error involved. This means that to point 30 degrees above the polar axis the software must calculate the position of the RA axis so that the DEC is actually pointing above the polar axis, and then begin tracking. Once I have my target centered in the eyepiece the mount tracks wonderfully, but a little goto function and understanding of the mechanics is the real goal.

I am hoping that there is someone around who actually understands the way their mount works and possibly the math behind it. I am ok with geometry, but plotting a position on a sphere is something I could use some help with. 

[Robindonne]

My sister just sold her 130eq after she asked me why tracking by hand was so hard.   I looked at that mount and it surprised me celestron sells that crap.   Gears were so rotten and tight that turning the slomo knob almost made me turn the whole mount. Im very curious if tracking by motors can be accurate on this mount

[malc-c]

7 hours ago, Swifty35 said:

So this is a diy equatorial mount and I am writing the software to slew to objects based on hour angle and declination. Sorry if that was not clear in the original post. I realized my math was off when I tried to point at an object near my local meridian and a declination of around 120 degrees. The way I have the program written only the dec axis moved significantly. However, the further from the meridian I was pointing the closer I was... but still off. That is because with a good polar alignment the instrument is pointing to 0h0m0s HA and 90 Degrees Dec.  With the alignment being better further from my local meridian I think that there is an exponential error involved. This means that to point 30 degrees above the polar axis the software must calculate the position of the RA axis so that the DEC is actually pointing above the polar axis, and then begin tracking. Once I have my target centered in the eyepiece the mount tracks wonderfully, but a little goto function and understanding of the mechanics is the real goal.

I am hoping that there is someone around who actually understands the way their mount works and possibly the math behind it. I am ok with geometry, but plotting a position on a sphere is something I could use some help with. 

 

I still feel you need to follow the above advice and read up on celestial coordinates.  Declination runs from +90 at the NCP to -90  at the SCP, with 0 degrees being the celestial equator.  Right Ascension runs  from 0 degrees North, through 90 degrees East to 180 degrees South, and 270 degrees through West.  I think you are somehow applying Altitude and Azimuth system to an EQ mount, especially with comments of the DEC axis being above the Polar Axis ???  The mount you are using is an EQ mount so it will have its Azimuth axis tilted over to your latitude so that it is parallel to the Earth's Axis, and thus becomes the Right Ascension Axis 

Assuming you have perfect polar alignment, so in order to track an object only the RA to counteract the Earth's rotation.  The Declination of an object is fixed (other than the planets), and will be XX degrees YY minutes and ZZ seconds either on, above or below the celestial equator which is 00:00:00 degrees declination.  The declination is an angular movement, so if a target is 30 degrees above the celestial equator and you want to slew to a target at 20 degrees, then you need to drive the DEC axis NN steps of the stepper motor to move the mount 10 degrees.  However you first need to set the starting point so the software you are using has that datum point (which is why all goto mounts do a one, two or three star alignment).  Now you could  centre a star of known declination and then set the mount to that location and once centred press an alignment set button to zero out the coordinates.  However any star on the celestial equator will not always be visible at certain times of the year.  You could set the mount to +90 degrees Declination and point the mount at the NCP.  But as there is no clear identifying star (Polaris is a few degrees off the NCP so that's not really useful) this can make alignment difficult and can introduce more error than aligning at the celestial equator.

Once you have found a suitable and reliable way to set that initial starting point then it's fairly easy to work out how to drive the mount to a given position.  Stepper motors have a set number of steps per degree of rotation of their shafts, so if the motor has 10 steps per degree, 3600 steps will give a full rotation of the motor.  You then workout (or obtain from the manufacture) the ratio of the gearing for the mount.  Lets say that the ratio is 20:1 just to make the math easy, so for the mount to rotate one complete revolution the motor has to run for 20 x 3600 = 72000 steps, which equates to 200 steps per degree the mount is moved.  So in the example above, to move 10 degrees the controller needs to send 2000 steps to move the DEC axis.  In reality it's a little more complicated as the ratios are not as simple, but hopefully you get the idea.

[Cornelius Varley]

2 hours ago, malc-c said:

Right Ascension runs  from 0 degrees North, through 90 degrees East to 180 degrees South, and 270 degrees through West.

Someone also needs to read up on the definition of right ascension 

 Right ascension - Wikipedia

[malc-c]

Peter, I was just trying to simplify  things... if you start factoring hours seconds and minutes the poor bloke will get confused -   But yes, I probably confused him slightly...

Quote

Right ascension is the celestial equivalent of terrestrial longitude. Both right ascension and longitude measure an angle from a primary direction (a zero point)

This was what I was trying to relate to, that he would need some form of starting point, much the same was as the meridian line is used....  

Edited January 5, 2021 by malc-c