Telescope Fine Focus Assembly - Bresser Messier 254 Dobsonian

2024-01-10 21:43:17

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Telescope Fine Focus Assembly - Bresser Messier 254 Dobsonian

Summary

Fine focus arrangement (6:1) for Bresser Messier 10" Dobsonian Telescope. May fit others.

This is a DIY alternative to the fine focus knob commercially available for ~£93. Once fitted, you will have one knob which moves the focuser at the normal rate, and one which moves it at 1/6th the rate. The modification is fully reversible.

The assembly of this arrangement is straight forward, but I've provided (probably) overly detailed instructions. For those who aren't mechanically minded: All the screws are small, none of them need to be very tight, don't force anything.

Other parts required:

2 x M2.5 x 6mm Cap Head Screws
2 x M3 x 12mm Cap Head Screws
1 x Jackson Brothers 4511DA 6mm Ball drive dual anchor (I bought mine from eBay for around £12)

Original file location:
https://cad.onshape.com/documents/1dbb00339e42cf85fb3b3296/w/751956d19cdf779257ad3ff6/e/0e43900906e9a600decb83c7

Post-Printing

Assembly Instructions

Modifications to ball drive:
1.a Optional: Shorten the input shaft of the ball drive to 16mm to allow the knob to sit flush with the 3D printed mount.
1.b Flush grease - the ball drive is supplied packed with thick sticky grease. In order to make it move more freely, wash some or all of the grease out with penetrating oil while spinning the drive.
1.c Optional: Repack with light grease, this can be pushed in through the output side of the drive.
Ball drive assembly to 3D printed mount
2.a Optional: Tap 2x M2.5 & 2 x M3 holes in the the 3D printed mount
2.b Assemble the ball drive to the 3D printed mount using the 2x M2.5 screws.
2.c Insert 2xM3 clamping screws into the 3D printed mount
Fit focuser knob to ball drive
3.a Remove the knob from the short side of the focuser by slackening the grub screw
3.b Fit the knob to the input shaft of the ball drive and tighten the grub screw.
Fit fine focus assembly to focuser
4.a Align one grub screw on the ball drive output with the access hole in the side of the mount. (note the rotation direction to the next screw)
4.b Align the flat on the telescope focuser shaft to the anticipated alignment of the ball drive grub screw
4.c Fit the fine focus assembly to the focuser and tighten the grub screw through the access hole.
4.d Tighten the 2x M3 clamping screws
4.e Rotate the knob 90 degrees to access the other grub screw & tighten