Tales of the Lazy Stargazer

Volume One, Number Four
page is still under construction

EL EL Siderostat

El-El Siderostat (Elevation-Elevation)
By Alan Rifkin (The Lazy Star Gazer)

Prototype Number 1, October 2000

What is an El-El Siderostat

An Elevation Elevation Siderostat is a moving, flat, front surface mirror, mounted in a computer controlled double gimbal.
This configuration can also be called Tilt-Tip, as the double gimbal allows the light path to change in elevation in two axis.
It is placed in front of the objective lens of a stationary telescope, and just by controlling the mirror, you can track images moving at a siderial rate and motion.

Pros and Cons of the Design


No net light loss because, due to extra mirror no star diagonal is needed
Comfort, siting and no position shifting
Eyepiece is always in the same place
Eyepiece can actually be inside of a building
No tripod, stable mount
Aligned either North-South or East-West or no alignment is needed
Easy to build, no precision bearings or parts
Oblong mirror for Tilt/Tip
No image shortening for Tilt axis because of oblong mirror
Very low cost needle bearing


Limited Sky coverage with only 45 degrees tilt usable, depending on the size of the mirror
Added cost of Mirror
Tracking path calculations are complicated
Finder scope useless

Notes and Comments

Why El-El instead of Alt-Az?
I have no precision machining capability.The only tools I used were a Torch, Saw and Drill Press.
Oblong or rectangular vs. round mirror allows more efficient area mirror usage.
A normal Siderostat configuration is Alt-Az with the Azimuth axis coaxial with that of the telescope. This is a very complex mechanical configuration.
See the works of Oscar Knab for examples of this design.
Caution, clicking on images will bring up large 300k versions of the images.

First Light was at the Christmas 2000 Solar Eclipse.

Basic Components

First you need a telescope. Refractors work best, because of the geometry of the system and the usually smaller aperture of the refractor. I have a TeleVue Genesis, that I love.

Then you need a tripod to hold the telescope. In this case I use a Quad-Pod. There are times when you just need four legs. In this case you want to sit underneath the scope. This design is similar to a carpenter's horse, but with one set of legs turned perpendicular to the other set for stability.
The front legs are fiberglass bar, the back legs are stainless steel pipe, and the cross members are nylon bar.

The next part is the moving mirror assembly. The quality of the mirror is the most important factor, but you also want to have smooth and accurate movement. In this design I used pin bearings, because they are cheap and easy to make. The stepper motors came from two perfectly good 5 1/4 Floppy Drives.What else are you going to do with them? The low cost steppers give you accurate open loop control. The timing belts came from an old Epson dot matrix printer

Two stepper drivers will be needed. There are many on the market. I used a circuit board from Mel Bartels and assembled the rest.

The final part is how to control the steppers. I used an old notebook computer. The software is an open-ended project. I will be rewriting that for the rest of my life as I come up with new ideas.

Here is it all assembled at the 2001 Stellafane Convention. The mirror is sitting on the grass, and the wheelchair is for the person using the system.
All contents © 1996-2001 by Alan Rifkin. All Rights Reserved.