(Zero-shift focus).

•A problem and solution for Black Diamond ED120mm Refractor.

Introduction.

Perfect focus for deep space astrophotography is critical and time-consuming enough, without having to worry about it changing halfway through an exposure or shoot.

That’s a problem with some telescopes. They move. Mine in particular, has a hefty camera hanging off the back of the focus tube. As the telescope rolls over through the night, the weight of the camera pulls on the focus tube as it shifts from horizontal to vertical.

One would think that the lock knob under the focuser would do the trick and it does, to a limited extent. I’ll explain.

Using the numbers for focus in MaxIm (image capture program), you can get your stars very sharp. As focus improves the numbers increase. If they decrease, you are losing focus. Once the focus is as good as it gets, the idea then is to lock the focuser with the little lock knob underneath. Unfortunately, this tightening shifts the focal plane just a fraction, causing a drop in the numbers again.

One way to avoid this is to leave the lock knob undone, or partially tensioned so you have some resistance during your manipulations of the focus knob. Unfortunately, you run into the shifting problems I mentioned as the weight starts to move rearward during tracking.

Investigation and analysis.

The adjustment of the focus tube is not a rack-and-pinion with teeth and a gear as I had imagined. Instead, the focus tube has a flat milled along the outside bottom curve and the driver is a thin stainless steel rod between the focus knobs that rolls along the milled flat. The rod is tensioned to push the focus tube up against two roller bearings set at 60 and 300 degrees above the milled flat. The lock knob through the housing has a plastic bush set in the end of the thread, to stop it from damaging the anodized focus tube inside.

As a consumer grade telescope, not expressly designed for photography, the tolerances of the fittings are not precise enough that zero-shift is possible.

Solution.

It is best to avoid drilling holes that could damage the optical tube assembly, (OTA). To keep it in pristine condition I designed a solution that clamps, rather than screws to the telescope.

To do this, a backstop-brace and plate was constructed, comprising of a 3mm thick aluminium ring, four lengths of 6mm round aluminium bar and a 6mm thick, round sheet of Perspex.

Remove the three screws that fix the focus unit to the outer tube and fit the aluminium ring over it. Make the ring to fit the diameter of the outer tube exactly. Make it wide enough to allow four holes of 7mm and the nuts to suit. The focus unit is slightly larger in diameter and so provides a solid stop for it. Re-attach the focus unit.

Construct a backstop plate out of Perspex, with a M6 x 20mm adjustment knob in the centre. Cut some ports in it for the cables to the back of the camera, and to allow airflow to the cooling fan. Drill 4x 7mm holes in it at equal stations around the plate.

Lastly, make four struts out of 6mm aluminium rod and thread a few centimeters of each end. I used my drill press and an old bearing to get the rod square in the thread die. Once the thread was started nice and square, I removed it from the drill press and finished them off in the vice. Alternatively, you can buy steel threaded rod, but there is a weight sacrifice to consider. They need to be long enough and with enough thread to reach behind the camera and have some adjustment available. I cut mine approximately 350mm long.

Fit and function.

Once the plates are positioned and joined to each other via the rods, some spring washers and nuts, the focusing is still accessible and the camera has room to move. When you have reached that perfect focus, you can set the little adjustment knob in the rear plate against the back of the camera to stop it from sliding any further back.

Soft Jaws.

Steven Leahy from Joinit Plastics very kindly sent me a sample of his injection molded soft jaws for a vice. I must say, there couldn’t be a more perfect tool for this kind of work. With its well designed gripping channels at 45, 90 and 180 degrees, it held the work beautifully without marking my soft aluminium. Because of the brilliant idea of magnets in the soft jaws, I didn’t have to hold them in place every time I opened and closed the vice.

In gratitude to Joinit Plastics I would like to provide the opportunity for anyone else who would like to order a set of these here.

http://www.joinitplastics.com.au/