I have used some information found on internet on how to calculate position of
stellar objects. Most of the information I collected from the following links:
Describe formulas and approach to calculate Hour Angle (HA), knowing location,
current time, and stellar object RA and DEC.
Once I created the code to calculate HA, I used this page to
check if it works OK: http://www.jgiesen.de/astro/astroJS/siderealClock/
, and on a later stage when I added the DB with Messier Objects I used Stellarium (http://www.stellarium.org/)
The main issue I found was not the formulas themselves, but
the way Arduino deals with large numbers and in particular the precision of the
What I found when compared my calculations with the Stellarium ones is about 30 arc. sec
- 55 arc. Sec. Don’t ask me why the error vary J
Anyhow, I thought that 1 arc. min is pretty negligible
value when it comes to visual observations and decided to go ahead accepting
UPDATE: They actually don't vary at all. The differences were because Stellarium uses JULIAN TO DATE, not J2000 Epoch and thus the difference. However this is easily corrected using Star Alignment and the result is that the objects are always in the eyepiece!
Now once we have the HA and DEC for a given object, we want
to make the mount point the telescope at it and to do so, we need to somehow
know how much exactly motors needs to turn. To succeed in this I need to know
what is the worm to gear ratio – i.e. how many turns should the worm do in
order that the gear makes 360°. I have Vixen Super Polaris and quick search on
internet revealed that it uses 144 tooth gear for both RA and DEC shafts.
I have created a simple Excel file (*.XLS) to calculate how much the motors
should turn once I calculated HA and DEC, and also how often to fire the motors
in order to compensate for the Earth rotation.
The same mathematical model I used in the XLS is also
implemented in the code, so what you need to do is simply substitute the values
for your mount in the beginning of the code (rows 7 to 10), and add parameters
of the stepper motor and how you decided to use it (micro stepping).
Now once we calculated ratios, the code will know how much
to turn the motors in order to point the telescope to the corresponding
coordinates and also what is the clock speed so that the mount can track the
You know, this is not a consumer ready product. It allows you to customize it
for your needs and thus is very flexible, but with the flexibility comes some
drawbacks. I have included this “considerations” section in order to make sure
those are not dropped from your To Do list.
motors is a simple task, which however impact the way your calculations
should work. In my scenario Vixen Super Polaris have their original gears put
on the right side of the worm shaft. I however found those shafts to be short
in order to replace the gears with pulleys and thus moved the pulleys on the
opposite side of the shaft (left). This caused that the motor actually needs to
turn backwards in order to increase telescope position and thus I had to revise
the original code I have created. You should definitely revise the directions
you motors turn other ways you will end up with wrong counting of the steps,
thus wrong position. Let me show you what I mean: See row 1924 where I command
motor to STP_BACK, but on row 1927 I increase the counter +=
ACTION: Revise the following parts of the code: void consider_Manual_Moveint
xP, int yP),
and void Sidereal_rate()
VS. Southern Hemispheres: The
code also inverses the FORWARD move depending on the hemisphere you are
located. This is done in a part of the code, where I take care of the Touch
inputs. You might need to revise the code section.
on DEC and RA axis: My code works assuming that both DEC and RA axis use
144 tooth gear. This is the case for Vixen Super Polaris. In case you have a
different setup you might need to revise some portions of the code. If you have
issues to identify those parts of the code, drop me a message and I’ll try to
help you out!
Plan your activities: Now we reached a point where we need to plan our activities
to actually build the rDUINO Scope GoTo system. Rough estimation is that you should be able to
complete the system in 2-3 days considering you have everything needed (HW,
Tools and Materials). It took me something like 4-5 months to complete it,
considering I was only able to work during the night (not every night), and
also I needed to do a lot of research. Putting it all together took me like:
- 1 day to build he electronics plate and live up the system;
- 1 day to pack everything in a box;
- 1 day to fix motors and pulleys to the mount.
Considering that I don’t have a formal training in
electronics, nor experience in soldering (I’m civil engineer, managing people)
I assume it will took you even less time than it took me.
So, what you need to do in order to complete rDUINO Scope GoTo: