The making of a goto telescope

Hi everybody

I recently met Mr. Tushar Purohit from IUCAA Pune on his visit to
Aurangabad and got an opportunity to show him the automated telescope
I made. Impressed, he said that he would publish an article about how I
made the telescope, in an international astronomical magazine that IUCAA has
started.
So, here is that article:

“Making of a goto telescope”

I am Anant Kale, student of class 11, Nath Valley School, Aurangabad.  I am interested in astronomy, physics, mathematics and I wish to pursue higher studies in physics.

I got interested in Astronomy after joining the astronomy-club in our school which involves students in activities such as star-gazing, watching special celestial events and making telescopes. I would specially like to thank my physics teacher Mr. Suryavanshi because of whom I was able to attend a telescope making workshop in IUCAA Pune.

Last summer, I set out to make a low cost, fully automated, ‘goto telescope’ using a 100mm diameter primary mirror with a focal length of 650mm. It has full ‘goto’ capabilities and can be operated using any smart phone, laptop or tablet pc.

The user can enter coordinates in altitude-azimuth form or equatorial form using a smart phone or laptop and the telescope will slew to the desired coordinates. The total cost of this telescope was approximately Rs 7,500 and took 2-3 months to make.

The primary mirror was made at a mirror making workshop in IUCAA, Pune, under the guidance of Mr. Tushar Purohit. After manual grinding and polishing the glass for about 20 hours, the integrity of curvature of the mirror was tested using ronchi test and the mirror was coated with aluminum.

The brightness of images formed in a telescope is proportional to the square of the aperture. Thus it is exponentially more difficult to locate faint objects on small telescopes. Goto capabilities are thus very valuable for amateur astronomers using small aperture-telescopes. They enhance the viewing experience by eliminating the time and effort often spent in locating objects of interest, in the polluted urban skies.

Making the telescope was an enjoyable and informative experience. It helped me learn many things about telescope-making, optics and electronics.

The optical tube is a standard size PVC plumbing pipe attached to the mount using a metal clamp. The clamp is a 2mm thick steel plate bent into a circular shape with two nuts on diametrically opposite ends to bolt the supports. The telescope mount is of Dobsonian design which is simple to make and easy to motorize. The mount is small and compact with a height of 18 inches. It can be used as an alt-az mount as well as an equatorial mount. When it is placed on a surface parallel to the ground it behaves as an Alt-Az mount; when placed on an inclined surface with angle of inclination equal to altitude coordinate of the pole star, it can be used as an equatorial mount.

To motorize the telescope, high precision stepper motors were used for both axes (14PM-M201 MINEBEA CO. stepper motor, 200 steps per revolution) giving an accuracy of 0.075°. All the electronics are controlled using a microcontroller Arduino Due. The Arduino series microcontrollers are versatile and easy to use. They can be programmed using C++ language. They can receive inputs from sensors, serial monitors or computers and give desired outputs. The Arduino Board was programmed to receive the coordinates of the celestial object as input from a laptop or even smart phone.

The microcontroller calculates the number of ‘steps’ that the stepper motor has to rotate depending on the step angle and gear ratio and sends the corresponding output to a motor driver. In this case there are 200*25/360=13.89 steps per degree. The motor driver comprises of a dual H-Bridge integrated circuit(L298). It provides the necessary power gain to the output from the Arduino. As the motor rotates the telescope slews to the correct coordinates.

For the altitude/declination drive, a stepper motor with a worm gear pair of gear ratio 1:25 was used. The slew speed was set to 2 degrees per second. However there were several difficulties while making the azimuth/RA drive as it required a significantly more powerful motor while maintaining precision. The first attempt using the same stepper motor with a worm gear was not satisfactory as it could not provide enough torque. In the modified design, the drive will be provided to the circumference of the telescope instead of the axis thereby reducing the torque-demand.

   

For convenience and ease of use, a hand-held controller was added to operate the telescope manually. This is very useful when the coordinates are not known and to make finer corrections to compensate for drift.

This whole telescope making experience has been very instructive and enjoyable and has further increased my interest in astronomy. I hope to set out on bigger and better projects in the future.

  

 

PICTURES TAKEN FROM MY TELESCOPE