November 20, 2025
Description
The tube end ring is combined with the secondary mirror spider. In my opinion, it makes no sense to screw on the tube end ring and also the spider for a 3D-printed part.
The mirror holder is centered by design, it is not possible to adjust the center position of the mirror holder and therefore the mirror.
The standard secondary mirror spider with 4 vanes leads to 4 diffraction spikes at bright stars, with 3 spider vanes you will get 6 spikes which are a little bit smaller.
See https://www.telescope-optics.net/spider.htm
At first I tried to run 3 vanes straight down the center, similar to the original spider. Printed in PETG - and it was far too flexible, especialy with rotational forces.
The design was therefore modified so that the spider vanes run tangentially to the mirror holder in the center. Also, the print material was changed to carbon fiber reinforced PLA - and I was pleased. It is quite stiff, I would say even stiffer than the 4 thin sheets in the original spider.
The holder was recessed to provide more rigidity around the focuser (to reduce focuser tilt) but support is needed.
The vanes are only 1.5 mm thick.
See the picture of the "first light". 6 spikes instead of 4 as expected. I like it.
The spikes do not run directly to each other in the center, I don't see this on the bright star. On less bright star it can be seen better (see pic). Decide for yourself whether this is acceptable for you.
Be sure that the focuser is not intruding the light path. As the spider vanes do not match the direction of the focuser tube, there will not be an overlapping with spikes from the spider vanes but additional spikes instead. This will lead to two additional spikes - see picture of a star test and the corresponding picture of the intruding focuser tube.
In case you decide to move the primary mirror closer to the secondary mirror, you can find a spacer ring here: https://www.printables.com/model/1305532-spacer-ring-to-move-the-primary-mirror-closer-to-s
If it is not possible to get the focuser not intruding the light path, it's maybe better to keep the existing spider vanes or search for another model with straight vanes and orient one of the vanes to the focuser.
Weight ~200 grams.
PLA-CF (carbon fiber reinforced PLA) recommended for printing. Stiffness is very important, don't use PETG or similar “soft” materials. PLA should also be fine, but it is less stiff.
Nozzle 0.6 mm, layer height 0.3 mm, (Bambulab) PLA-CF, 2 wall lines, 5 top and bottom layers, infill 20%, infill pattern honeycomb. Brim recommended if bed adhesion isn't very very strong. I decided to print with brim instead of throwing failed prints to the trash.
Support needed (only on print plate). It's not mandatory to get a nice face (it's no contact surface), so simple support is possible. Or use a different support material for the interface.
Support of the countersink in the center is not needed. Therefore it is recommended to manually paint the surface for support - as shown in the picture.
If you print with a Nozzle 0.4 mm and layer height 0.2 mm inrease wall lines to at least 3 and top and bottom layers to 7-8 (stiffness reason).
Be aware of selecting “Inner/Outer” for “Walls printing order”, otherwise the overhang line is “printed in the air” (see slicer-pictures).
A secondary Mirror spider for 200 mm (8") Skywatcher is available here:
https://www.printables.com/model/1347494-secondary-mirror-spider-for-skywatcher-newton-200
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mechanical0815
