Mounting rail for Telescopes, Prism rail / Dovetail, Vixen style, length 260 mm

Usage

Be aware it is plastik and not as durable as aluminium.  Don't use it with too much load (the question what is “too much” is difficult). Use it only for smaller Telescopes. And the colder the less load!
 

Prism rail

I want to use my Skywatcher 150/750 PDS (f/5) on an iOptron CEM70. But the Skywatcher prism rail has an overall length of 210 mm with hole to hole around 180 mm what is too small for the saddle of the CEM70 which has 200 mm. The next bigger size of a Skywatcher rail is 335 mm which is way too long for an 150 f/5.
Additionally I designed the rail with more height. I had a “near collision” of the 150/750 with a tripod foot of my Skywatcher HEQ5. To get (a little bit) more distance additional height was considered. Also stiffness and durability is much higher with a thicker rail.

Mounting

If you have a Skywatcher too you can use the original screws for mounting the rail on the tube clamps.

Otherwise screws M6 and maybe 1/4" (I don't have one) should fit:
Hexagon head bolts: ISO 4017 / DIN 933 M6x25 mm (min) up to M6x35 mm (max).
A washer is recommended: DIN EN ISO 7089 A (previous DIN 125) 6.4 x 12 x 1.6

Printing

I recommend to use PETG, it is durable and gets not as brittle as other materials when the temperatures are low (~ -10 °C).
Part weight ~ 205 grams.

To obtain a strong rail print with rather more than less wall loops and top/bottom shell layers.

I printed it on a Bambu Lab X1 Carbon with nozzle 0.4 mm, line width 0.42 mm, layer height 0.3 mm, 5 wall lines (= 2.1 mm, more is not a mistake) and both top and bottom 5 layers which means 1.5 mm (exactly 1.45 mm because of first layer with 0.25 mm).

(update 18.05.2025)
I printed one more with a nozzle 0.6 mm, line width 0.72 mm, layer height 0.3 mm, 4 wall lines (= 2.88 mm) and both top and bottom 5 layers. This results in more strength and faster printing, even with a Bambulab nozzle - I achieve 19 mm³/s with a 0.4 mm nozzle and 28 mm³/s with a 0.6 mm nozzle.

Wether you print with nozzle 0.4 mm or with 0.6 mm switch ON “Alternate extra wall” to get a good bonding between infill and the walls (5 wall lines on every second layer and 4 wall lines in the other layers). Further switch OFF “Infill combination”.
If you don't do this, the result can be seen on the picture - I would not use this for my telescope.

Strength

It does not make sense to print with 100% infill, that's only waste of material (and printing time). Stiffness is the most relevant parameter, it increases with the square of height - thats the reason why I increased the height, wider was not possible .
The highest load occurs in the mid of the rail on the side surfaces when the telescope is  oriented on the side (first and foremost bending and torsion).
I used 30% infill which I think should be enough for my needs. If your load is quite high (telescope, Finder scope, camera, …), which is not recommended, consider to take additionally more infill, maybe up to 60%, but more important are the amount of wall lines.

Edit 18.05.2025: printing infos, slicer-file nozzle 0.6 mm and picture added