Dane.Kouttron

DIY Vari-focal 'Dobsonian' Telescope

Creating a huge imaging telescope in under 20 hours (aka a project 'make Hubble cry')
What?


This documents a simple low cost 'dobsonian' telescope construction. This design differs as it doesn't actually use a giant ultra-expensive concave glass mirror. Instead we demo using a vari-focal mirror constructed from low-cost mirrored acrylic with a point source load. Our goal is to view and image celestial bodies at high magnification, on the cheap.                                                   


Primer
Dobsonian Telescopes are Large and Have only reflective optics, no lens's whatsoever. This project takes the 'low cost' aspect of dobsonian telescopes further and attempts to make a parabolic disk reflector as the main optic collector.

A low distortion parabolic dish for the dobsonian telescope would allow for low budget citicen scientists to have a much higher power device at an extremly low price.
Design
StepDescriptorImage
1. The Maths To determine the focal length, the amount of deflection, and the massive height of this pseudo-portable telescope, some simple geometry was mashed out on the miters whiteboard.
The math was then backed up against a simple CAD mode. FEA (Finite Element Analysis) was run on the deformable acrylic to prove our hypothesis. The Acrylic forms a 'meniscoid' type shape (4th order eq that approaches the desired parabola).
<add some matlab here>
The scope was modeled in solidworks, to check angles.  PDF Layout CAD Design LINK
2.The Base Constructed from simple plywood and 2*4 spars
3. Measure thrice... Using a bicycle rim, we made a uniform circular loading point where
3. Simple Electronics The bi-directional motor drive was a simple 2 switch setup, as basic as it gets. Listed here in case someone else tries this.

use a rocker switch that prevents both buttons from being activated simultaneously.

or just DO NOT press both simultaneously.
4. High Torque Motor Assemb. This is a simple motor scavenged from a printer. it has an internal gearbox of approx 1:3000. The output speed is very low to allow for a safe deformation of the acrylic.
5. The Top
Issues
Topic Notes Images
Cutting Giant Reflective Acrylic We ordered a 24" * 24" piece of reflective acrylic, and used the MIT Architecture laser cutter (massive 24*36" laser cutter) to cut our square into a 22" circle.

note that lack of access to a laser cutter isnt the end of the world. mcmaster has 12" pre-cut discs.
The Bonding Agent The bonding of the cable to the mirror (for actuating deflection, we initially used a 'uber epoxy' which lacked uber-ness.
Clamping Clamping the epoxy. Epoxy's take a while. We used a lamp to accelerate the process. Still took a few hours

After waiting (building other parts of the scope) we mounted the hardware and after a few attempts (20lbf  for deflection) the epoxy failed. LAME. The back of the mirror was rather difficult to bond to.
The Bolt Finally after exhausting other opportunities of bonding agents, we opted for a bonding agent + bolt combo. We found the center of the mirror, drilled precariously, and applied tension. Volia, a meniscuoid!
Laser line was made by passing laser pointer thu a cylindrical glass object. The bolt caused an unbalanced loading on the mirror and some optic issues. Note the mirror still has protective coating.
Development Pictures
Emily measuring the center loading point the mirror, behold the perfection... yet another use for welding rod...

Closeup of mirror (bolt causes deflection)

Pre-Final testing setup

Who made this project? 
Dane Kouttron Emily Krupczak

Conclusions:
There's a reason scope optics & mirrors cost as much as they do.

Special Thanks:
MCMASTER-CARR (for being awesome, and shipping things so fast)
MITERS



Dane.Kouttron
Rensselaer Polytechnic Institute 
Electrical & Electrical Power
631.978.1650