Mounting aid for gluing the secondary mirror

Objective / intention

If you change the secondary mirror, the so-called offset must be taken into account when gluing it onto the holder. That's a bit tricky.
With this assembly aid, it can be done very easily.
The design was done in FreeCAD (see below).

Sizes

• Newton 150 mm / 750 mm (6" f/5), secondary mirror d=63 mm (secondary axis), mirror height 12 mm.
• Newton 200 mm / 1200 mm (8" f/6), secondary mirror d=63 mm (secondary axis), mirror height 12 mm.

The model is easily customisable, see below.
If you are not able to do this yourself, you are welcome to contact me - tell me the dimensions of the minor and major axis as well as thickness of the mirror and the offset you have to maintain.

Diameter of the holder it was designed: 32 mm like used in Skywatcher Newtons 6" (150 mm) and 8" (200 mm).

Offset

There are a few sites on the net that deal with the replacement of the secondary mirror.
I used this page (only in German) to calculate the correct offset - but I am sure you will get your result or find another page in your language.

Printing

~ 24-28 grams, PETG recommended, no Supports needed, no Brim.

Nozzle 0.4 mm, 2 Wall loop, infill 5% or more, infill pattern rectilinear, 2 Bottom shell layers, 3 Top shell layers.

Nozzle 0.6 mm, 1 Wall loop, infill 5% or more, 2 Bottom shell layers, 3 Top shell layers.

Infill is not necessary for stability reasons, but for support for overhanging (top shell layer) walls. Take a look at the sliced layers as the slicer software does.

I have also changed the following options in OrcaSlicer:

• “Filter out small internal bridges” to no
• “Internal bridge infill direction” to 90 degree (also depends how you orientate the model)
• “Detect narrow internal solid infill” to no
   

Attention! (1)

You know it, but it's a reminder: be very, very careful with the mirror surface so as not to scratch it (as I did when I painted the spider and mount :-( , that's the main reason for the replacement on my telescope).

Attention! (2)

I recommendend wearing work gloves when dismantling the mirror to reduce the risk of cuts - saved my fingers.

Assembly

Dismantle the secondary mirror to replace it. Examine the area where the mirror will be placed in your printed part and clean it of any artefacts or threads. Insert the mirror into the printed part and check that it is parallel to the outer surfaces. Prepare the mirror holder with your preferred type of adhesives. If you are not sure, search the internet for different adhesives that are recommended.
Turn the mirror and the mounting aid and place it on a (very) soft tissue. Insert the prepared mirror-holder and press it onto the backside of the mirror. Allow the adhesive to dry, lift the mounting aid and push the mirror holder towards the mirror to remove it.

As I had doubts as to whether the adhesive pads I used would really hold in long term, I applied some super glue to both surfaces as a precaution. Superglue is not a good idea itself, but as long as the adhesive pad is flexible enough to absorb thermal expansion, it's okay.

FreeCAD-model:

You are free to change the model if you need to glue in a different dimension of the secondary mirror, but be kind enough to publish it for other people. Thanks! 

Updates

• 08. June 2025: Model for Newton 200/1200 mm (8" f/6) and secondary mirror d=63 mm height 12 mm added, Files renamed for better differentiation.

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