Focus Boss II Mounting Bracket

Overview

Designed for Starlight Instruments Focus Boss II by Optec. May also work with Optec FocusLynx but I don't have one to test with. The bracket is designed to attach the controller to a "wide" D-Series dovetail plate (NOT the narrower Vixen style).

This is a very strong, very rigid, and possibly over engineered mounting bracket. It has a low profile and shouldn't introduce any new things for cables to snag on.

Hardware Needed / BOM

2x M3x100mm socket head cap screws
2x M3 flat washers, DIN 125A or similar, 7mm outer diameter
2x M3 heat set inserts
4x #4-40 x 5/16" socket head cap screws, or really any style EXCEPT flat head
4x #4 flat washers

Stainless steel parts are recommended to help with rust prevention. Washers are specified to help keep the bolt heads from sinking into the plastic part, thus helping with the overall rigidness of the assembly.

Print Settings

Print one part A and one part B. Both parts are needed.

I printed mine in ABS using the structural profile in Prusa Slicer. Vertical shells are 3, horizontal shells are 4 or 5, infill at 15%. Other filaments should work too, provided that they will stand up to your telescope's operating environment.

Assembly Instructions

The M3 bolt hole tolerances are fairly tight to keep the assembly rigid. The bolts will tend to thread into the holes rather than slide easily through them. My recommendation is to use the bolt threads to "auger" or "mill" out the holes by threading them all the way in and then continuing to spin/strip them. I used a power drill and hex bit to do that. Afterwards, the M3 bolts should move back and forth by hand with zero slop, but not so freely that they can fall out. Using a drill bit is less likely to tailor the holes as perfectly, and the resulting assembly may not be quite as rigid.

Given the tight tolerances, I used a bolt to guide in the heat set inserts as well. Simply thread the bolt into the back of the brass insert, but ONLY by one or two threads. You don't want to heat the entire bolt. When setting the insert, it will push the bolt out the other side if you've "augered" the holes well enough. I was able to perform this operation on a heat set press by using a shorter M3 bolt that I had on hand, and stabilizing the process on a riser block to allow for the bolt sticking out the bottom.

The #4 holes for mounting the controller should be spot on, but you can enlarge them if necessary with a drill bit to allow for variations in hole spacing. The #4 washer should easily cover slightly larger holes and allow the #4 bolt to firmly clamp down.

Final Thoughts

My main goal was to ensure a solid clamping mechanism that wouldn't be at risk of falling off, and wouldn't rattle or vibrate during slewing or wind events. As such, the part performed extremely well for me. It's surprisingly strong and rigid. So much so that I designed several others for all of my devices. The low profile allowed placement under a focuser where other brackets had clearance issues.

The M3x100mm bolts are a unique choice. Probably not a stock item for most makers, but readily available online at reasonable prices. I didn't want the clamping mechanism to try and pull apart the layer lines and I think this design worked very well in that regard. It places the majority of the part in compression. The bolts also act as interior structural elements for the assembly.