Piggyback Mount

Improvise, adapt and overcome.

Even though the plan on paper is for a piece of 60 X 10mm Flat aluminium bar as the raw material, I found this nice piece of thick door landing strip trim. The outside dimensions have changed, but all the holes are in the same place. No major machining necessary.

All I had to do was drill and tap holes, a bit of countersinking, and file a couple of slots out to fit over some screws protruding from the outer tube assembly, (OTA).

I found that the trim was nice and stiff, with no flex, which was my concern for mounting the ED 80 nice and solid. The inside profile also was perfect to fit over the curve of the OTA.

Here it is, mounted to the telescope, ready for the rings to bolt on. The extra tapped holes up the centre at 50mm apart, are for adjusting the distance of the ring mounts.

I took my designs to a machinist to have a go the proffessional way. Unless you are owed a favour, this is usually too expensive way to go. For the cost of doing a one-off job like this on a CNC machine, you would be able to buy a very nice telescope with it's own mounts.

I will source raw materials of standard stock soon and show how the real DIY can be followed by the average home handyman.

Stay tuned....

Click image to enlarge

This image represents the Piggyback rail mount for Celestron Nexstar 11 GPS .

Click image to enlarge

Now back to basics.

As promised, here is the way to go for the average home handyman with basic to moderate access to simple hand and power tools.

Step 1.

Marking and cutting.

The challenge of marking and cutting a straight line around a round tube is made even more critical to the home hobbyist, by the fact that he has limited funds for raw materials. Mistakes can be costly.

A simple tool can be made to achieve this. Take a piece of angle steel or aluminium and scribe two lines on the inside. One is the thickness of the ring to be cut and the other is the centre line of the ring for drilling holes. Centre-punch and drill two holes, one on each line, large enough to jamb your marking scribe into for a tight fit. Finally, hold the angle against the side and end of the tube whilst circling the perimeter with the scribe. When you cut, make sure the bench and vice are solid and not shaking everywhere as you cut. Keep the cut line close to the vice jaws to minimize chatter and improve the accuracy of your cut. If you don’t want to mark the soft aluminium tube, pad the vice jaws with some rag or wood. Cut just outside the line if you are not confident you can stay right on it. It is easy to file down to the line later to clean it up. Finer tooth hacksaw blades are a slower cut, but often a neater one.

Step 2.

Drilling and tapping.

Four holes around the perimeter of each ring are needed. The bottom one is to screw the ring down to the rail. If using 6mm bolts, (The tapped holes already in the rail are M6) then a 6.5mm un-tapped hole needs to be drilled in the ring.

The remaining three are drilled at 5mm then tapped at 6mm, for the alignment adjustment screws. These three are spaced equidistant around the ring, with one at the top and two at the bottom sides.

If taps are available, but no tap wrench, clamp the end of the tap in a vice and place the rings holes over the tap. Rotate the ring to produce the thread. Be careful and don’t rush this. Aluminium is very sort and when dealing with a thin wall thickness, it is easy to strip the thread. Ideally, the hole diameter should be equal to the wall thickness of the aluminium.

Once the holes are done, test fit the rings to the rail.

Step 3.

Finishing.

Use a fine half-round file, then medium sandpaper, then fine sandpaper to smooth the rings down to a shiny finish. Scotch-brite pads are excellent for producing a near-mirror finish. Finally, cut and polish compound or even toothpaste and a cloth will bring the aluminium up beautifully. You may wish to consider taking them in to an anodizing shop. Anodising is relatively cheap and surface hardens the aluminium, making it more scratch resistant. You can also pick neat colours.

Step 4.

Mounting.

Screw the rings down tight onto the rail, mount the rail to the OTA of your main telescope. Place your smaller telescope centrally in the rings, using the alignment adjusting screws. To protect the OTA of the smaller telescope from scratching by the adjusting screws, wrap some electrical tape around the OTA where the screws contact.

Lastly, align your two telescopes by pointing and centering your main telescope at a distant, unmoving object, (I find the tips of distant antennas useful.) Then adjust the alignment screws on the rings to point and centre the smaller telescope on the same object.

You’re done!