Arduino Star-Finder for Telescopes

Space is big. Really big. Owning a small telescope can give a lot of pleasure, but it’s often a struggle trying to locate specific objects in the night sky. In this project I made and integrated an Arduino-powered star-finder with a small reflecting telescope, to allow me to easily find the galaxies, nebulae and clusters that I wanted to observe.

What: This device calculates the current position in the night sky of a list of galaxies, nebulae and star clusters. It then measures the current heading and elevation of the telescope, and displays to the user how to orientate their telescope so that the chosen target object will be visible. The electronics are all contained within a convenient handset, attached to the telescope mount on a 0.5m cable.

Arduino Star-Finder for Telescopes

Why: Traditional methods of finding objects in the night sky (using charts, maps, following constellations) are slow and require experience, and motorised ‘Go-To’ telescopes are very expensive. This project finds a happy medium between the two. Clear night skies are not common, so using this Arduino star-finder allows you to make the most of your precious viewing time.

How: The Arduino receives position and time data from a small GPS module. Using this, and known coordinates for a list of objects (my initial code includes 45 astronomical objects, and it’s easy to expand this list if you choose), their current position in the night sky (relative to the observer) is calculated. The name, position and type of the current chosen object are displayed on a small LCD screen on the handset. By using a rocker switch, the observer can choose which of the database’s objects to observe. The heading and elevation of the telescope are measured using potentiometers. The Arduino then compares the position of the target object to the orientation of the telescope, and using LEDs on the handset indicates to the user whether the telescope must be moved up, down, left or right. When the target object is within the field of view, all four LEDs will illuminate. At this point the observer may view the target object through the telescope. The program constantly updates the position information, to assist the observer in tracking the target object as it moves across the night sky. I used a 3D printer to create fittings to house the potentiometers, and to make the handset and battery box holder.

Edit 2020 – new project video!

Please note: this project assumes some prior experience with Arduino, basic amateur electronics, 3D printing and CAD (possibly). I’ve tried to cover all of the background information relating to the astronomy, and justifications about why I designed it how I did. To avoid creating an overly wordy Instructable I’ve therefore left out some minutiae of the project that would be tedious, for instance wiring of LEDs and switches etc. If there’s anything that confuses you, please ask a question and I’ll try and answer. Also don’t forget that the wonderful thing about Arduino is the huge level of online support, so if you’ve got a question, however basic, hopefully you’ll quickly find an answer online too. This is also my first Instructable, so please go easy on me!

**Caution**: when building and testing your telescope star-finder, please take immense care to not view the Sun through the telescope at any time. Doing so will cause permanent eye damage.

Step 1: The Theory

The Theory Arduino Star-Finder for TelescopesTheory Arduino Star-Finder for Telescopes

(I’m sure you’re interested about the astronomy principles which this Instructables project uses, but if you’re not, then please skip ahead a step or two).

The methodology for calculating the azimuth and altitude values for the desired object is well detailed in a book called ‘Practical Astronomy with your Calculator or Spreadsheet’ by Peter Duffett-Smith. I’d thoroughly recommend this book for any amateur astronomer, as it shows you how to calculate many different things. Recent editions even show you how to implement the calculations in spreadsheets.

Input Data:
The inputs to the calculation are as follows: the ‘right ascension and declination’ coordinates for the target object, geographical latitude and longitude of the observer, and the current time and date.

Date and time are received by the GPS module, and time is given as Universal Time (UT). Significantly, Universal Time as measured by a GPS system has never been adjusted for leap seconds since the system was initialised in 1980, hence GPS time is currently 18 seconds ahead of true Universal Time. The Arduino GPS library program compensates for this by automatically subtracting 18 seconds from the recorded time to find Universal Time as necessary for the calculations.

Time Systems:
The initial part of the mathematics is concerned with converting UTC to Local Sidereal Time (LST), which is done via Greenwich Sidereal Time (GST). Sidereal Time is defined as the hour-angle of vernal equinox. GST is the Sidereal Time as observed on the Greenwich meridian (0° longitude). The conversion to LST takes into account the longitude of the observer, as the sidereal time gets earlier or later depending on position relative to the Greenwich meridian.

All relative positions of all astronomical objects are located according to the angles of right ascension and declination (Ra/Dec). These two angles locate the object on the celestial sphere, according to the equatorial coordinate system. As the coordinate system gradually moves (as the shape and orientation of the Earth’s orbit changes gradually), the Ra/Dec coordinates are adjusted. This adjustment is performed in relation to the current astronomical epoch. Presently, the J2000 epoch is current, and the Ra/Dec figures for all astronomical items will be quoted for the J2000 period. The Ra/Dec data is combined with the LST figure to calculate hour-angle (Equation 1). Equations 2 and 3 are then used to determine the current altitude and azimuth of the target object. Azimuth and Altitude are the coordinates of the horizon coordinate system. This is a coordinate system unique to that position on the Earth’s surface, where 0° of altitude is the observer’s horizon. When considering the alignment of a telescope to these coordinates, altitude is analogous to the inclination of the telescope, and azimuth is analogous to the heading.

Coordinate system transformation:

(1) H=LST- α

Where H = hour-angle, LST = local sidereal time and α = right ascension.

(2) sina=sinδ∙sinϕ+cosδ∙cosϕ∙cosH

Where a = altitude, ϕ = observer’s latitude, δ = declination and H = hour-angle.

(3) cosA= (sinδ-sinϕ∙sina)/(cosϕ∙cosa)

Where A = azimuth, δ = declination, ϕ = observer’s latitude and a = altitude.

You will see these formulae being used in the Arduino program shown in a later step.

For More Details: Arduino Star-Finder for Telescopes

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