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.

Comments

[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!