Build a Telescope

For as long as I can remember I have been fascinated by the cosmos and have long devoured books, lectures, and programs on the subject. Though I opted for engineering rather than a Physics degree (I took as many astrophysics courses as I could for fun though) I still desire to see deeper in to space and this year decided why not build my own telescope to do so?

At first I became attracted by the simplicity and effectiveness of the Dobsonian design.


Dog not included. This book is great for anyone interested in the subject:

After reading the book it became evident that the details of the frame construction are somewhat trivial compared to the cost and complexity of machining the mirror. Though I have access to a lathe on campus, it is clear that the size and perfection of the mirror are the factors determining how much light is gathered by the telescope and thus how far and how sharply one can see in to the cosmos. What if there was another way to build a mirror rather than machining it from glass?

I thought why not a liquid mirror? There are plenty of liquids that are reflective. If one was to spin a container with a reflective liquid at the right speed a nearly perfect parabola should form and create a mirror at a fraction of the cost! It turns out I am certainly not the first person to have this idea. See http://en.wikipedia.org/wiki/Liquid_mirror_telescopes.

The problem, of course is the mirror uses gravity to create the parabola and the mirror has to stay stationary and flat. What if there was another way to create the parabola, such as with a precisely controlled electromagnetic field?

I came across a few promising resources:

Researchers in Quebec have found a way to make a ferrofluid reflective, solving one problem.


The hard part is the systems and controls mechanism to keep the fluid parabolic despite rotation and translation. Previous attempts included passing currents through mercury and actuating the surface with a magnetic coil (Ragazzoni and Marchetti). However, the high density of mercury means that strong magnetic fields and thus high electrical currents are required. Liquid adaptive mirrors shaped by electro-capillary effects have also been discussed by Vuelban et al. in 2006.

Researchers from the same university in Quebec outlined their approach of using 37 custom solenoids and a 1mm thick layer of ferrofluid from Ferrotec. Ultimately they achieved a Strehl ratio of .84 using a new controls method consisting of a constant magnetic field surrounding the mirror that allows the response of the actuators to become linear. Brilliant!


Each actuator cost the team $100, which is well outside of my budget but most of the cost came from the commercial 16 channel analog output cards. Perhaps I could just use an Arduino with PWM outputs, or a few Digital to Analog converters from Maxim. The expensive analog output cards seem a bit wasteful.

First though I will need to run a test with some Ferrofluid. Being thrifty I decided to first try mixing tiny iron particles with vegetable oil. I ordered a MICR refill cartridge off of eBay and mixed it with some veggie oil. I then poured a thin layer in to a petri dish and introduced a neodymium magnet. It worked! Kind of...I wasn't getting anywhere near the response required so I will definitely be needing a higher quality ferrofluid.

Next steps: check the Georgia Tech chem lab for some better ferrofluid or contact Ferrotec since they don't have up front pricing information on their web site.