TRACKING MOUNTS

The stars appear to move because of the motion of the earth. If you are using a telescope at high power or are doing photography, you will need to have a telescope that "follows" them. Over the years, I have built several mounts for that purpose:

This is the double-arm "barn door" drive which I invented many years ago. To get the complete story on this innovation read my original article from Sky and Telescope magazine Feb.1988 and the follow-up article in April 1989 .

There are MANY web pages about this invention: Steve Gagnon's Page , Brian Rachford's page , Peter Barvoets' page . Evan Williams' Page  has the most precise version of this mount ever constructed to my knowlege. Check them out!

Read more about  the Double-Arm Drive and get plans in Stephen Tonkin's wonderful book Amateur Telescope Making , ISBN 1-85233-000-7. Also see a brief discussion in the facinating book Unusual Telescopes by Peter Manly, ISBN 0-521-38200-9.

This device is designed to make it possible to photograph the stars with an ordinary camera, a couple of boards, some hinges and an inexpensive clock motor. The one in the photograph is my original type-3 barn door.

A simple device like this can produce stunning photographs like this one of the milky way.

Or this one of comet Hale-Bopp.

The barn door mount whether single-arm or double-arm is a very simple, inexpensive device. The original, single-arm barn door has been around for many years and was, I believe, first popularized by a fellow naimed Haig back in the 70's. It is sometimes called the Haig or Scotch mount. My improvement in the double-arm version was designed to make the tracking VERY precise, which is important for astrophotography. But even imprecise tracking, like that provided by the single-arm barn door, can be very useful when doing visual observing.

In a static telescope, working at high power (above say 300x), the target moves out of view very quickly. Many people buy expensive mounts for their telescope for just this purpose, often (rightly) spending as much or more for the mount as for the original telescope. But I am poor and cannot afford such luxuries. So here's what I designed:

This is a VERY large barn door drive, with two "wedges" attached to the primary boards. In effect this becomes a sort-of equatorial platform. It can be placed under an ordinary alt-az telescope and will provide a limited amount of tracking. The tracking is not perfect, of course. But for purely visual use, this device works fine. You can center a planet in the eyepiece at 300x or 400x and study it for up to 20 minutes before it leaves the eyepiece. This is much more convenient than having to adjust the telescope every 30 or 40 seconds, then wait for the vibrations to settle down.
   

Here's the telescope in a before and after shot. This is a 6 inch f/10 refractor. The altaz mount is after Richard Berry's design in Build Your Own Telescope.

I have used the same concept  to mount a 4" f/15 "folded" refractor (which uses two internal flat mirrors to compress the length of the telescope):

The tripod is constructed of two by four lumber and some sturdy hinges. Simply cut the two by fours by removing a wedge of material near the placement location of the hinges and use triangular pieces of plywood at the top and as separators. The "spreader" between the legs is pulled by a wing nut on the "tensioner" rod to hold the legs in position. A length of threaded rod doubles as the top bolt and the tensioner. Note the bolts securing the three tops of the two by four sections. The additional strength  provided by these bolts is necessary because the natural forces on the tripod legs tend to separate the legs and pull the top sections of two by four apart.This is a sturdy, low-cost tripod design that is easy to build AND WORKS WELL!

  DEPARTMENT STORE TELESCOPE

The idea can even be applied to very small telescopes. In my article in Astronomy magazine in January 1997 (see my page on  the Inexpensive Telescope) I have designed some improvements for these cheap 60mm refractors. In my article I show how to transform one of these into something more sturdy. As a further improvement, you can add a small equatorial barn door for easy slow motion. It's a bit of overkill for this small scope, but it's a fun little project. And it's a good way to try your hand at the idea of the hybrid mount.

                                                                       IMPROVED MOUNT WITH

         IMPROVED MOUNT                         EQUATORIAL  BARN DOOR

Here's a closeup of the slow motion control: This mount uses a spring to provide tension and keep the mount in place. The drive bolt is a turnbuckle from the harware store imbeded in a wooden knob. This is necessary because, unlike the larger versions where gravity and the weight of the telescopes is enough to hold the arms closed, this mount is for a very lightweight telescope.

The tripod for this telescope is simple and much like the larger ones. In this case, however, the legs are attached directly to the tripod top by hinges. This is also an inexpensive, simple and quick way to make a tripod.

EQUATORIAL PLATFORMS

This equatorial platform won me a merit award at The Riverside Telescope Maker's Conference in 1987. An equatorial platform, sometimes called a Poncet platform after the man who first thought of the idea, is a table of sorts. You place anything on the table and it will follow the stars (at least for awhile). It's similer to the barn door mounts above. Of course, if you are an astronomer, you want to place your telescope or camera on the platform so it will follow the stars. There are a few complications. For high power astrophotography through a telescope the shutter of a camera has to stay open for quite awhile (minutes to hours). The stars move perfectly but most things made by human beings do not. So you need a way to slightly alter the motion of a telescope so that it matches the motions of the stars perfectly. It's pretty easy to make the telescope track a little faster or a little slower through some simple electronics controlling the drive motor. This handles tracking errors in right ascension.

But errors in declination are at right angles to that. The solution is something called a pseudo-declination axis. In the second photo you can see that the telescope is sitting on a board which is slightly tilted. It is hinged at the right side. On the left side is another board arranged so that the big board tilts a little when the little board tilts a lot. At the far left end of the little board is a small motor. (This is very similer to a double-arm barn door mount and, in fact, was the inspiration for the idea.) The motor is reversable so that the big telescope riding on the big board can be made to move very slightly in declination by the touch of a botton on the hand paddle. I could guide the telescope for long exposures and produce photos like this one of The Orion Nebula(M42). It's amazing to think that I was able to get these shots on a mount made of plywood, hinges, wagon wheels and surplus clock motors.

How about this one of the Lagoon Nebula:    

Or these of Centaurus A and The Whirlpool Galaxy:
     

This the current version of that 13 inch telescope and it's new equatorial platform. This iteration is for purely visual use. The equatorial platform is much lighter, which is ok since it is strictly for visual use. Note that telescope is in two sections. This makes it lighter and it fits into my Honda Civic without being entirely dismantled.

Here are some good pages about equatorial platforms:

David Shouldice's Equatorial Platform , Chuck Shaw's Page on the ATM page