Home made manual microfocuser

[ollypenrice]

I started a wee doodle today but since it's Sunday I don't have all the bits I need.

My Tak FSQ106 has a non functional 10:1 knob and they cost hundreds. Added to that it will be near impossible to identify the right one, etc. (I have an earlier serial number than Tom's but a later design... etc etc.) And if I go Robofocus later I won't want the costly 10:1 anyway!

So I simply bolted a long lever onto the normal knob and it works fine. However, I've started making a threaded push-pull device to push-pull the end of this lever. It seems to be going well. Its only use would be for imaging because the range of movement is small and you need a camera rotator behind the focuser for this system. However, the limited movement doesnt matter because the main lever can be 'declutched ' from the Tak focus knob in a second, using the screw which holds knob to shaft and lever to knob. Get focus about right, tighten screw and use push pull thingy thereafter.

If it works I'll post a pic...

Olly

[JamesF]

Sounds interesting. I look forward to seeing the pictures

James

[Gina]

Me too

[nightvision]

I'm using something similar on a Moonlite CR1 focuser on the 200PDS, it has an RC servo attached to the lever and is controlled remotely from my study. The lever is just a straightened out part of a large jubilee clip. Will eventually do away with the RC Rx/Tx and drive the servo from a USB connection. By using a long lever on the focuser knob and very short one on the servo I can make fine adjustments with no slop or backlash.

[ollypenrice]

OK, here's the final multi million dollar setup!

Total cost of maybe a quid! The key points are that the angle ally lever can be declutched from the Tak focus knob by the turn of a screw. Get coarse focus and then retighten the lever onto the Tak knob. The home brewed fine focus is ultra fine (I'd guess at about 50:1 but haven't measured it) and it gives about 2mm total focus travel. Plenty for imaging. The black knob for the home brewed thing was purloined from one side of the guidescope...

Cheers,

Olly

[nightvision]

Real engineering. Great stuff! Is there a nut on the other side of the angle bracket?

I only had a single speed focuser but now get very fine adjustment using a similar arrangement, the clutch involves releasing the jubilee screw a little, crude but effective.

[JamesF]

Is my interpretation correct that you just push the black knob in or pull it out and the entire rod slides through the black bracket at the other end?

James

[nightvision]

Is my interpretation correct that you just push the black knob in or pull it out and the entire rod slides through the black bracket at the other end?

James

Looks to me like you just turn it and the nut behind (and attached to) the angle bracket is moved up and down the threaded rod.

[JamesF]

Looks to me like you just turn it and the nut behind (and attached to) the angle bracket is moved up and down the threaded rod.

Ah, yes, could be. It's not totally clear on my monitor that the rod is threaded and, if it is, whether that is the means of adjustment.

James

[ollypenrice]

Real engineering. Great stuff! Is there a nut on the other side of the angle bracket?

I only had a single speed focuser but now get very fine adjustment using a similar arrangement, the clutch involves releasing the jubilee screw a little, crude but effective.

Yes, the nut on the hidden side of the alloy angle lever is profiled, crudely, with an angle grinder to make it conical so as to accept changes in angle with respect to the lever. It is locked to the threaded bar with a locknut. There's a washer there, too. The part of the threaded bar passing through the lever has been ground smooth so as not to bind during focus changes.

Interesting that you had the same idea. I used to have a Jubilee holding a lever onto a TeleVue Genesis focus knob, just as you do. In fact TV used to do what they called a 'wand' which was simply an extended lever like ours.

Anyone plagued by a slipping Crayford might benefit from this system since it also holds the focuser where it is.

James, Tony's explanation is correct. Turning the black knob turns the threaded rod which therefore moves through the nut welded to the angle iron fitted to the blue Altair dovetail holding the guider. (What a lousy sentence!)

Olly

[JamesF]

Ah, right. And the spring is just to remove any backlash between the threaded rod and nut?

James

[ollypenrice]

Ah, right. And the spring is just to remove any backlash between the threaded rod and nut?

James

The spring is there to make the adjuster work in both directions. If you turn the black knob clockwise in the pictures - ie to push the lever away from you as you look at the screen - the spring is what pushes the lever. I did try having conical nuts both sides of the lever, locknutted onto the threaded bar, so you'd get a hard mechanical push in either focus direction. However it tended to bind so I let the spring do the job. Using a positive, springless system would be best but would need some precision tools I just don't have.

To my mind a system like this is actually the best way to control fine focus. Done properly it would guarantee a slipless focuser and if you fancied getting very clever you could try to make it self compensating for temeperature. Contrive it that the threaded bar, in contracting during cool down, drew the focus tube out slightly... Heh heh. Probably would't give enough movement!

Olly

[JamesF]

Ah, right, I see now. I was just being dim. Too many people wanting last-minute jobs doing before I go away to be able to concentrate properly

James

[opticalpath]

Very neat solution, Olly. I used a wand on the R&P focuser on my wee refractor and thought of making some kind of tangent arm like yours but wasn't sure how to defeat lost motion in the linkages. Your spring-loading is great!

Next, the stepper drive .... Go on, you know you really want one Adrian

[ollypenrice]

Very neat solution, Olly. I used a wand on the R&P focuser on my wee refractor and thought of making some kind of tangent arm like yours but wasn't sure how to defeat lost motion in the linkages. Your spring-loading is great!

Next, the stepper drive .... Go on, you know you really want one Adrian

If I go for a stepper I'll just bung it on the original Tak knob. Per is determined to cajole me into robotizing the focus and he may have a point. I won't touch electric, though. Steam or nothing!

Olly

[ian_bird]

And if I go Robofocus later I won't want the costly 10:1 anyway!

Olly

Do my eyes deceive me? The words Robofocus and Olly in the same post? Minus the word "Luddite"?

I am amazed.

Cheers

Ian

[NickK]

You know that's a simpler fine focuser than my idea of shoving a borg on the back of the pentax! Does the Tak 106 only have a 1:1 R&P focuser like the pentax?

edit: re-read the opening post.. I'm seriously loosing it nowadays!

[ollypenrice]

Do my eyes deceive me? The words Robofocus and Olly in the same post? Minus the word "Luddite"?

I am amazed.

Cheers

Ian

Cheeky blighter.

So... first light last night. It worked perfectly. I preferred using it to using the Feathertouch on my TEC. A quite significant turn on the black knob produced a small shift in FWHM and the readings were consitent and coherent. A nice clean action on the knob - say a quarter of a turn, moved the FWHM from say I.8 to 1.7, so it was child's play to keep turning till I settled for a rather acceptable 0.77 to 0.98 range. (This is at a tolerant 3.5 arcsecs per pixel, of course, on this widefield sky hoover.)

I had a similar idea years ago and never followed it up. Pity, because the tangent arm idea seems to bring home the bacon. A really well made one with springless push-pull would be even better. Somebody needs to make them.

Olly

[opticalpath]

...... I had a similar idea years ago and never followed it up. Pity, because the tangent arm idea seems to bring home the bacon. A really well made one with springless push-pull would be even better. Somebody needs to make them.

Olly

I agree - a tangent arm is ideal for this sort of application where the range of fine-adjustment required is very small. I must say though that I do like the stepper drive, either direct drive or via belt reduction. Not just for the remote capability, but as much for the positive, consistent (and hands-off) positioning that makes it so much easier to zero in on the sweet spot just as you described with the tangent arm.

Adrian