Dobsonian Equitorial Platform

I own a type of telescope that consists of a Newtonian reflector on a simple, Dobsonian base.  Typically they’re just called Dobs for short.   Dobs offer a great value in that you get a lot of aperture, or size of the scope, for a relatively small amount of money.  The base is a very simple two bearing design that requires no alignment.  You go out, set the scope down, and start pushing it where you want it to go.  For bearings, you have an azimuth bearing which lets you move the scope side to side, and an altitude bearing that lets you point the scope up or down.  Here’s mine.

A downside to the azimuth-elevation style base is that you can’t follow an object in the sky by rotating about a single axis.    Imagine the sky is a big sphere (better known as the celestial sphere) rotating about an axis (known as the celestial axis).  This axis runs from North to South and is angled at whatever your latitude is.  See the crude sketch below to illustrate this.  The celestial sphere rotates around the red axis and the scope rotates about its green axes.

So, as an object rises in the sky at any latitude other than the equator or the poles, it will move diagonally up and over if you watch it.  To follow it with my scope I have to point it up and then over in a stair step fashion.  For the most part this isn’t an issue.  But, if you are at high magnification you’ll be nudging the scope a fair amount because the area you can see becomes smaller as you increase magnification.  If you want to take pictures or video through the scope this becomes big problem because you don’t want things moving in the frame.  There are several approaches to solving this issue.  First, you could put motors on both bearings and have a computer control the motion.  Second, you could buy a $1000+ equatorial mount to put the scope on and ditch the Dob base all together.  Third, you can build or buy a equatorial platform.

Alright, there is a fourth option to build a new mount of the fork, yoke, or split ring type.  I think these mounts would be very large compared to the scope and wouldn’t be a good choice due to the tube’s center of gravity location.

As you might have guessed I’ll be going for option #3 which is the cheapest.  What’s a equatorial platform you ask?  It does two things.  It allows the entire scope to rotate about the axis of the sky and rotates the scope about that axis with a motor to follow the sky.  The basic principle behind it is when two axes are parallel you can follow an object rotating about the first axis by a rotation about only the second axis once you’re pointing at it.  To do this we need to either orient the azimuth bearing’s axis parallel to the celestial axis or create a new axis parallel to the celestial axis and rotate about it.  Due to the nature of the Dob’s base it cannot be tilted to be parallel to the celestial axis because the optical tube will fall out.  If this problem were corrected then you’d still have the issue of keeping the weight of the tube from causing the base to tip over.  These issues have been overcome of course but there is a simpler way.  The equatorial platform works by creating a third axis to rotate about that is parallel to the celestial axis, keeps the Dob base supporting the scope without modification, and keeps the scope from tipping over.

Imagine that there is a cone that has its center axis aligned with the celestial axis and its edge parallel to the ground as seen in the picture below.  The cone could rotate without issue.   We could set the scope anywhere on or within the cone and it would result in the scope rotating about the cone’s central axis.  Let’s do that.  We want a level surface to put the scope on.  So, cut the cone parallel to the ground and trim the ends off anywhere that isn’t below the base of the scope.  If we do this we’re left with the blue object in the sketch below.

Because we don’t want our cone piece rotating on the ground we’ll also use a board with some rollers on it to support the cone piece.  On one of these rollers will be a motor that turns it at the correct rate to result in the cone making one complete revolution per sidereal day.  Since we only have a part of the cone, it will only be able to rotate for a portion of the day.  An hour is a good number people use.  The platform can be built to allow it to rotate for an hour and then be reset.  It is reset by physically moving the section of the cone back to the starting position.

If you’d like to see what others have built check out these sites:  Jan van Gastel’s  Chuck Shaw’s  Equatorial Platforms  Kurt Maurer’s

I’m still in the design phase with mine but I hope to start ordering parts and building soon!

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3 Responses to Dobsonian Equitorial Platform

  1. That’s a good one on the mechanics behind a dob :)) I’ve gotta Dob myself, a Skywatcher 10″ collapsible….ahahah! You’re right about the nudging bit. But, an Equatorial Mount for a 50 kilo dob is gonna cost me a bomb! Hey it’ll be great if you could check out my blog, its on astronomy as well!

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