Here are some construction tips gained from adapting several telescopes for StarTrak use.

Tools

Preliminaries

Creating_scales from the SetCirc.exe program

Cutting the azimuth circle

Marking the azimuth circle

Drilling the hole for the azimuth pivot

Attaching the altitude scale

Here's what you need:

• a drill (preferably electric) and a drill-bit the same diameter as the telescope's azimuth pivot, a jigsaw (an electric one makes the job much easier), a thumbtack (to help you mark out the azimuth circle), a permanent marker pen (with, say, a 0.3 or 0.5 mm nib), compasses for drawing a circle, and a rule marked in millimetres.

 I guess if you want to be pedantic, you might ask "Where's the glasspaper? And the pencil sharpener?" As you read on, it may occur to you that a paintbrush is also needed, and maybe a chunky screwdriver to open the paint can...

Let's assume that you have a Dobsonian telescope, and that it has large-diameter tubes for its altitude bearings. For measuring the altitude angle, you can create a linear scale (marked 0-90 degrees) to fix to either the inner surface or the outer surface (whichever is more convenient) of your 'scopes's altitude bearing tube. (At another part of this website, you can see photographs illustrating this.) The SetCirc.exe program (which you can download from this website) can print such a linear scale. Please note that the SetCirc.exe program runs under Windows 95 / NT (or later releases of Windows), and does not run under other operating systems such as on a Mac or under Unix.

The length of the altitude scale strip needs to be one quarter of the circumference of the tube. (Here's a further tip: measuring the inside circumference can be difficult, but here's a trick to make it a lot easier. Acquire some thin plastic strip, for example the strip that hardware stores sell for ironing on to the sawn edges of melamine-faced chipboard. This can easily be curled up and fitted to the inside diameter of the altitude tube, then taken out, uncurled and measured to determine the inner circumference. Divide the distance by 4 and you have the required length of the altitude scale.)

Of course some scopes (particularly those made by the major manufacturers) don't have the open-ended type of altitude bearing tube that allows a linear scale to be fitted in this way. Or maybe the altitude bearing tube has a diameter too small for a useful linear scale to be fitted. In such cases, it's better to fit a circular scale (similar to the azimuth scale), though this means a little more work. See the photos of 'scopes for an example.

These are the templates created by the SetCirc.exe program (the program can be downloaded from this website).

When you run the program, the first stage is 'calibration', that is, determine the precise number of dots-per-inch (DPI) resolution of your printer (the calibration print is not shown in this photo). Based on the horizontal and vertical DPI figures you supply, the program prints what it thinks is a 100mm x 100mm square. When this is printed, measure the true width and height, and if not 100mm x 100mm, adjust your DPI settings accordingly.

If you want to see more detail, click on the photo.

When you have determined the printer's DPI resolution accurately, you can accurately print the altitude scale and the template for the azimuth circle, as shown in this photograph.

The SetCirc.exe program creates four altitude scales, annotated in different ways. This gives you a choice (e.g. whether you wish the strip to be on the left-hand side of the 'scope, or on the right-hand side, on the inside or the outside of the altitude bearing tube).

For the azimuth template, the SetCirc.exe program prints four pages which you join together to make a large 90-degree protractor. 

Here we see the limb extremities of a StarTrak devotee cutting an azimuth circle from some 2mm MDF (medium-density fibreboard). If you use an electric jigsaw this is quite an easy operation, and can be done quite accurately freehand - just follow the pencil line. Don't cut the central hole (for the azimuth pivot) at this stage - this comes later. But be sure that you can find the centre point of the azimuth circle - there will probably be a tiny hole left by your compasses.

The azimuth circle will benefit from a few coats of paint. Based on experience, a first coat on each surface of quick-drying (acrylic) primer/undercoat, finished by a second coat on each surface of quick-drying (acrylic) gloss provides a very good finish. The board may warp a little after one surface has been painted, but applying a coat to the opposite surface will restore the board to being flat.

If you want to see more detail, click on the photo.

When the azimuth circle is ready to have degree scales marked you can use the template from the SetCirc.exe program. Before using the template, draw two perpendicular diameters across the azimuth circle to divide it into quadrants. Cut the template to a smaller radius than the azimuth circle, and tape it to the azimuth circle.

Insert the thumb-tack into the centre of the circle. Now take a rule or other straight edge and abut one end against the thumb-tack. Arm yourself with the marker pen and 'extend' the radial lines from the template out to the edge of the azimuth circle. The photo illustrates this well. From experience, this method of 'extending' the template markings is the best way to achieve good results. Not recommended is poking a pin through the template (to mark the azimuth circle underneath) - this technique gives inferior results.

If you want to see more detail, click on the photo.

Using a permanent marker pen will mean that the markings will not smudge if the azimuth circle gets wet, and the markings are unlikely to become faded by light.

When all four quadrants have been marked, the last job on the azimuth circle is cutting the hole for the azimuth pivot.

Fix pieces of waste wood above and below the centre of the azimuth circle, and use the two perpendicular diameter lines to mark the centre of the circle on the piece of waste wood into which you will be drilling.

If you want to see more detail, click on the photo.

..and here's the final azimuth setting circle, fitted to the base of the telescope. (Well actually it's a different azimuth circle off another 'scope, but it illustrates the point).

If you want to see more detail, click on the photo.

Before cutting the altitude scale, apply a strip of quality transparent tape to protect its surface. You make a neater job if you apply the tape before cutting the scale.

You can stick the altitude scale to the 'scope's altitude bearing tube with some PVA-stick adhesive.

The last thing to do is to make some reference markers for the two scales. No hints or tips here, since this is quite straightforward, and details will depend on your scope. You need two markers attached to the rocker box - one near its base to mark the azimuth scale, and one near the altitude bearing tube to mark the altitude angle. That's it!  

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