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jacksdad

Dual speed r+p focuser

9 posts in this topic

First published project here, so bear with me...

 

This is being done on the stock Skywatcher focuser fitted to the Explorer 150PL, but it'd work the same on anything.

 

IMG-20170420-WA0028.thumb.jpeg.73db2d45031b6abddbe1e8148587561e.jpeg

 

Since my son got his 'scope we've managed two short sessions with it, and that's been plenty to discover how awkward the single speed arrangement is to use.

 

Following the first go I stripped it down and cleaned out all the swarf and glue they seemed to use in lieu of lubricant before applying actual grease and adjusting the preload - which improved things massively but it could be better.

 

So... A dig around in the useful stuff box produced this:

 

IMG_20170421_120925183.thumb.jpg.96f0a5efdfaa9ca61a1de5245e3b8ab9.jpg

 

That's an 8.5:1 ratio slow motion knob.

 

Now just to drop out the shaft from the focuser.

 

IMG_20170421_121013797.thumb.jpg.fc927a254e78ac335da85ce2ec6b5ef6.jpg

 

Will they fit together?

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Posted (edited)

Like your thoughts, if you can get it to fit the spindle or shim the spindle to fit and anchor the base of the slow motion drive you should be on to a winner, looking forward to your results. The knobs may be a screw fit onto the shaft as the SW 130P or could be a tight glue fit

 

Best wishes, now where's my bits box cos I known I have several of these beasties myself

Edited by BXRO

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Alternatively could you just fit a much bigger wheel on one side

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1 minute ago, happy-kat said:

Alternatively could you just fit a much bigger wheel on one side

But to get the same ratio of control it'd have to be 8.5 times the diameter, and that would mean the tube (and the viewer's head) would get in the way ;)

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:-) that would be a tad big, lol, fair point

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To continue:

 

The reduction drive needs modifying as it's limited in rotation to something near 280° (to match the limits of an average potentiometer or variable capacitor) - the method to achieve this varies, but on this one it was simply a lug on the planetary cage which I snipped off - now it carries on turning. I didn't take a pic of this, but it's not very interesting anyway.

 

Inside should look something like:

 

IMG_20170421_121222303_HDR.thumb.jpg.c0196678874186c3a4ece7d988aead6d.jpg

 

Now, turning the sun gives the reduced drive, but sticking something knob like onto the planet cage means it's possible to retain the 1:1 'fast' focus too.

 

Another dig about found this random piece of plastic (maybe it was a lens cap at some time, dunno)

 

IMG_20170421_121126171.thumb.jpg.f1d621cb0d2a04d0603a1a25e5493bbe.jpg

 

Still, that's getting ahead of myself.

 

So, the original knob needs to come off the shaft. It was a bit reluctant and needed heat, hitting and turning to break the threadlock compound, but eventually it unscrews. Careful here if you want to do this, the spindle is chromed brass (knobs are aluminium) so it would be really easy to shear on the thin portion near the middle if you just grab both knobs.

 

IMG_20170421_121107880.thumb.jpg.1936de0d67aa60947ea764abcb806831.jpg

 

This reveals a threaded portion of approximately 5mm, which obviously doesn't fit nicely in the 1/4" hole of the drive. Precision shim stock to the rescue:

 

IMG_20170421_130446694.thumb.jpg.bda08c34b57d7ab33035c213aa435b3d.jpg

 

A few turns worth gives a nice tight interface (actually aimed for needing to screw the spindle home, which I got) and a nip on the lockscrew holds well.

 

IMG_20170421_125852201.thumb.jpg.759a2a9fbb631b4250609c75650decac.jpg

 

To be continued...

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Looking very promising! Eagerly awaiting the next episode :) 

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So that random plastic cap type thing, that needs holes in...

 

IMG_20170421_144959319.thumb.jpg.6731a4326ae4855dcc4cf14156ccb550.jpg

 

Using the original scale dial as a template is accurate enough for a handwheel, and then using that dial as a washer was the route of least resistance.

 

IMG_20170421_145156061.thumb.jpg.4249df36f1345a2d9b1ca28c601c3def.jpg

 

That takes care of the 1:1 part (almost, I need to trim that flange off and put a grip on, and possibly flood it with epoxy or something maybe) and putting the original knob off the drive gives the 8.5:1 reduction.

 

IMG_20170421_145227758.thumb.jpg.786637296c4e41a214e31a19176a477a.jpg

 

While holding the drive housing, turning the large cap (fast/coarse focus) moves the spindle 1:1 - with a little extra resistance due to having to spin the planets and sun, but it's still easier and smoother than the bare focuser was to begin with - while turning the little one a single turn (slow/fine focus) moves the spindle approximately 0.12 revolutions.

 

As it stands, that's as far as I've got. I want a bigger 'fine' knob that's easier to grab, and there's the teensy job of affixing the whole caboodle to the focuser housing.

 

Hopefully I'll get to at least some of that later today...

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