Meade ETX 125 Telescope Manual Focuser Knob Modification

A replacement for the factory installed focusing knob, which allows precise setup and indication of the focus position for a range of eyepieces, adapters, and photographic equipment. No more refocusing guesswork!

The original scope's construction modification is minimal and easily reversible. But most likely will void your warranty. Be especially careful while handling the mirrors during full disassembing of the OTA.

Inspect the attached Google SketchUp 1:1 model file for all of the details and measurements.

Your comments are welcome!

Step 1: Major Modification

You need access to a milling machine to do this major modification for the factory focusing shaft. It must be completely removed from the telescope beforehand. Pay as much attention as possible to the optical mirror during the removal procedure.

The width of the groove to be milled depends on the diameter of the set screw you have. The grove is about 1.5 - 2mm deep in order to include the top narrow part of the shaft. The set screw should easily slide in the groove, but without much of freedom to prevent back lag.

Step 2: Scrap Some Parts

- I've took off this nice (12 revolutions 100 positions each) knob from a salvaged resistors bank. It's actually inspired the design. I've drilled it's plastic top for the fixed shaft, because the height of the knob is not enough to contain the moving focusing shaft all the way it goes. Plus, it allows me to connect the flexible hand focuser in the future.

- The brass rod, which will serve as the new focusing shaft, was made of the variable resistor's shaft taken from the same resistors bank box. It's diameter is just perfect for this job. The especially nice fact is that the standard drill bit has the exact fit diameter (8/32in) too.
They are all of the same diameter for this size of variable resistors, though, just make sure it's long enough (about 40mm).

- I've cut it from the base flat. The size could be anything a bit longer than your scope's focusing shaft. But the longer, the better. It's easy to cut it later as necessary.

- then I've made a short thread on the factory end with hand tools to match some wide nut, I already had. The nut will be cut off of the hex side, polished on the other side and epoxy-glued to the shaft later, so the precision of the thread doesn't matter much. It's better to make it loose a bit, so you could precisely adjust the nut to the perpendicular position, while gluing or soldering it in place.

- next step is to drill the brass shaft along the main axis to make a good fit for the scope's focusing shaft. I had a smaller hole already in it, so it was easy to make it bigger.

- the last thing with the new shaft is to drill and tap the set-screw hole on the side of the brass shaft. It is better to assemble the focusing knob on the scope at this point and mark the lowest possible point for the set-screw beforehand.

- In order to do that, I had to enlarge the focusing shaft hole in the scope's back to fit the new shaft in it with some tension.

Step 3: Another Scope Drilling

To fix the knob in place I had to drill and tap another little hole in the plastic back of the scope for a small screw to match the groove on the back of the knob. After the initial assembling, I've also marked the knob's own set-screw position on the brass shaft and made a small groove for it on the shaft, to make sure it holds well and wouldn't slipper.

Step 4: Final Thoughts

I've put some white lithium grease inside the fixed shaft, on its outside bottom end, and around the bottom nut. The set-screws are treated with a thread stopper glue.

Everything works flawlessly!
The knob diameter is very comfortable. The dial wobbling is only 2 points. The dial lock (on the side of the knob) allows secure locking of the focus, so nothing would ruin it by an accident.

I've adjusted the scale so the value 1100 is near the end of the thread of the focusing shaft. So I'd never unscrew it completely.

Make sure that all of the parts moves without much of the force. Do not over tight the parts!

The dial on the picture shows 486.5

Comments

[Cowboy1Brian]

I ran across your page as i was searching for the power circuit board (not the motor circuit board). I realize this post is a few years old but, do you happen to know where I can locate a replacement part?

[kouker]

I have no idea, really. I would rather check the CloudyNights forum and ask that question there - very large and helpful community. Also, the Weasner website has a huge archive and plenty of very knowledgeable members.

[chrisletts]

I'm intruiged that you find only 8 points difference between focus positions - on my scope, there's a lot of free play and about 90 degrees between 'up' and 'down' focus. Is there any way to eliminate the free play ?

[kouker]

That's a common problem. I'd recommend to search the weasner.com site for possible solutions and instructions. In short, it involves regreasing the shaft's thread with thicker grease, same for the mirror plate slot for the shaft (sometimes a washer might be useful there). Also check the shaft's spring tension (either replace it, or stretch by hands a little bit). And make certain the set screw on the knob is tight. Good luck, and Happy New Year!

[theRIAA]

are you saying the new one is more precise than the old one? how easy is it to turn? ever think about adding a big weight to the tip to make it spin longer?

[kouker]

Not much for the precision actually. Though, a bigger handle diameter always improve the manual manipulations in precision and in the amount of force required to turn it. And yes, it's easier to turn than the original knob.

Anyhow, the goal was to have some visual feedback while focusing. For example, you can check the values for the two opposite focusing positions which are definitely start getting out of focus, and then choose the average value to set (it works, I have about 8 points between these positions with the default Plossil 26mm). The average will be definitely closer to the perfect focus than any guess.

Another argument for the dial is the professional astronomers trick in the astrophotography. They make shots with all of the dial values between these no doubt out of focus positions and determining the best focus positions comparing the result images. Thus eliminating any guess work.

Also I plan to measure the perfect focus value for all of my eyepieces with the barlow and without, with 45 deg erector prism, and with the visual back adapter, e.t.c. So, I could just attach an accessory, set the dial to the specific value and use the scope.

Nope, the tip is for the flexible focuser :-) The best weight placing idea I plan to implement is here: http://www.weasner.com/etx//techtips/counterweight3.html

[kouker]

>ever think about adding a big weight to the tip to make it spin longer? I believe, it's not even necessary with this mod at all. The protruding shaft's diameter is small enough for the fast rotation of the knob with two fingers as it is. Also the inner dial disk of this particular knob is quite heavy, providing good inertia. Maybe some tape around the brass shaft would add a better grip for that purpose. Thank you for the idea!