38 replies to this topic

[Polyphemos]

To be more specific, Anatomy of an AT72EDII Focuser.

 

I acquired this focuser from maniack, who generously gifted it to me in the hope I could use some of the parts from it to repair my own AT72EDII focuser which as damaged in a fall. As it turns out his was damaged even more than mine, also from a fall, but I thought it fun and possibly useful to dissect the focuser to see what makes it tick.

 

I believe it’s generally representative of many dual speed focusers and hopefully we can demystify some of the parts and hardware it’s made from so that others faced with the need to perform adjustments, maintenance, and repairs to their own focusers won’t have to do so beginning completely in the dark.

 

 

Here is the focuser maniack sent me ready for disassembly.

 

 

 

[Polyphemos]

Beginning at the top of the focuser body:

 

 

A = Pressure adjustment screws that act upon the top of the draw tube; there are two.

 

B = Screws to affix a finder shoe; there are four, two on each side of the focuser centerline.

 

C = Grub screws which cover threaded holes in the focuser body; there are two with no apparent function other than to seal threaded holes, though the holes could possibly serve as alternative locations for mounting finder shoes.

 

[Polyphemos]

Looking inside the focuser body:

 

 

The two “A” screws act against a thin Teflon strip which acts in turn against the top of the draw tube. Two additional Teflon strips which are fixed in position, one of which can be seen in the center left of the photograph,  further support the draw tube.

 

[Polyphemos]

Looking at the pinion assembly at the bottom of the focuser body:

 

 

D = Screws that attach the pinion assembly to the focuser body; there are two.

 

E = Thumbscrew that pushes on the draw tube brake.

 

F = Threaded holes for grub screws that adjust the height and level of the pinion assembly against the focuser body, and which impact alignment between the rack and pinion. There are four 2mm grub screws within the threaded holes.

 

 

Only the threaded holes for the attachment screws “D” perforate the bottom of the focuser body. Note the helical pinion crossing the slot within the focuser body into which the rack attached to the draw tube fits.

[Polyphemos]

Separating the pinion assembly from the focuser body:

 

 

The attachment locations “D” and the height and level adjustment locations “F” are revealed internally.

 

 

One more look at the bottom of the pinion assembly before disassembly.

 

Things will become a bit more complicated from here as I disassemble the pinion assembly, and therefore more challenging to document clearly, so I’ll take a temporary pause from further posts to determined the best course of action. 

 

[Barlowbill]

I just ate all my fingernails

[scout]

Thanks for showing us these steps so far, Polyphemos.

 

Does it seem possible to switch the fine focus knob to the other side of the focuser? The pinion assembly picture doesn't look like it's symmetrical, so I'm not sure if it would work.  I've been considering putting the fine focus control on the opposite side of my AT72EDII, bit I'm hesitant to open the assembly and possibly mess up the smoothness of the focuser.

[Polyphemos]

I just ate all my fingernails

Courage, my friend!

[Polyphemos]

Thanks for showing us these steps so far, Polyphemos.

 

Does it seem possible to switch the fine focus knob to the other side of the focuser? The pinion assembly picture doesn't look like it's symmetrical, so I'm not sure if it would work.  I've been considering putting the fine focus control on the opposite side of my AT72EDII, bit I'm hesitant to open the assembly and possibly mess up the smoothness of the focuser.

I do believe it’s possible to flip the pinion assembly, and I think others may have done so.  I’ll check more carefully for you when I reassemble the focuser.

[Polyphemos]

Removing the focuser wheels:

 

 

1 - Remove the grub screw that covers a hole in the pinion support and that must be removed to access the grub screw 2 on the stem of the left focuser wheel. * Note that there are two grub screws on the left focuser wheel stem and both may need to be loosened to remove the wheel.

 

2, 3, 4 - Loosen the grub screws to remove the three focuser wheels. All grub screws in this procedure are 2mm.

 

 

The focuser wheels are removed.

 

 

Prepare to remove the pinion shaft from pinion assembly:

 

 

5, 6 - Loosen the brake adjustment grub screw (5) and thumbscrew (6).

 

7 - Loosen grub screw to release the internal bearing from its support housing.

 

8 - Loosen grub screws to partially release the fine focus assembly from its housing; there are three grub screws.

 

 

9 - Loosen grub screw to complete release of the fine focus assembly.

 

The pinion shaft is now ready for removal.

 

[Polyphemos]

Remove pinion shaft:

 

 

The pinion shaft slides off to the right. If it resists removal it’s either because one or more grub or thumb screws weren’t sufficiently released, or, more likely, there’s a burr on the shaft that’s hanging up on the internal bearing on the left side of the pinion assembly body. A light tap with a small plastic hammer usually gets the shaft past the bearing.

 

* Note the location of the black plastic saddle on the brake bar for reassembly later.

[Polyphemos]

Disassembling the fine focuser mechanism:

 

 

There are three grub screws that secure the pinion shaft to the the fine focus assembly. It’s best to remove these completely to better access the slotted retaining/pressure ring. To prevent unwanted movement after assembly and adjustment the slotted ring is secured in place using what appears to be a locking compound that will require softening.

 

Here I’ll pause again to let some acetone applied to the locking compound to do its work and hopefully allow me to proceed.

[K-night]

You're doing a great job on this tutorial.

 

I hope you're getting to the disassembly steps I'm waiting for; bearing cup/race, ball bearings & cage.

[maniack]

Thanks for showing us these steps so far, Polyphemos.

 

Does it seem possible to switch the fine focus knob to the other side of the focuser? The pinion assembly picture doesn't look like it's symmetrical, so I'm not sure if it would work.  I've been considering putting the fine focus control on the opposite side of my AT72EDII, bit I'm hesitant to open the assembly and possibly mess up the smoothness of the focuser.

See this thread on reversing the focus knob assembly for the AT72EDII.

[scout]

See this thread on reversing the focus knob assembly for the AT72EDII.

Thank you!

[Polyphemos]

Completing the fine focuser disassembly, almost:

 

 

The acetone has done its work, the slotted retaining/adjusting ring has been unscrewed, and the parts securing the eighteen (18) small bearings have been slid back towards the captured shaft bearing.  From right to left are the slotted ring, spring wave washer, and small bearing race half.

 

 

A closer look.

 

Ideally, at this point you’d be able to remove the pinion shaft from the brass fine focuser ball bearing housing. More normally, burrs raised by the three grub screws we removed earlier prevent the shaft sliding off. With determination and fortitude it’s possible to worry the shaft off from the brass large ball bearing cage but it’s strictly speaking not necessary. If you decide it must come off do not use the captured bearing as a fighting position if you don’t have a ready replacement.

 

 

It’s now possible to slide the pinion shaft to the left and expose the brass cage for the three large bearings and remove them. With the large balls removed the fine focuser shaft can now also be removed.

 

 

Balls big and small.

Edited by Polyphemos, 10 April 2025 - 07:05 PM.

[Polyphemos]

Here’s the problem:

 

 

It’s hard to see in person, and even harder to see in a photograph, but within the area circled is a very small spot flattened by one of the large balls when the right side fine and coarse focuser wheels hit the ground. That’s what make’s the periodic sticking spot while turning the focuser wheels.

 

I’ll smooth the spot with a fine Emory board while spinning the shaft in a lathe. It will make things better but as dented as this shaft is only a new replacement would completely fix the problem. Even if fixed the rack teeth took some damage as well, so this focuser is pretty much a write off.

 

Anyway, I’ll massage the fine focuser shaft and reassemble the focuser and document those parts and procedures I think might be helpful. A big thank you to maniack for providing the focuser which without I’d have had to do house chores this afternoon.

[Polyphemos]

Repairing the fine focuser shaft:

 

 

I employed a bit of fine emery paper, probably something like 2000 grit, wrapped around a chopstick to gently smooth the dented surface of the radiused portion of the fine focuser shaft. I pushed against the back of the chopstick while spinning the shaft on a mini-lathe at it’s highest rotation speed. The fine focus shaft chucked into hand drill could almost certainly provide the same result.

 

The damaged area that was very difficult to see was not at all difficult to feel through the chopstick to my pressing finger. As I worked the damaged area the oscillation caused by the dented surface could be felt to diminish. I didn’t remove all of the damage, not wanting to change the dimensions of the radiused surface excessive and possibly create other problems, but I probably removed 85% or 90% of it.

[PKDfan]

A most excellent tutorial Polyphemos !

These aren't the simplest of mechanical marvels to make so i admire your willingness to dissect the problem and to show the rest of us cowards exactly how its really done.

I need to readjust my 100ED's Crayford as its long sleep loosened up the drawtube to a frightening level and this helps to ease the angst of it if it all goes horribly wrong.

Sideways so to speak. Lol.



CSS
Lance

[eyespy]

Hi,

 

There is a similar focuser on the AT EDL and Starfield SDL 102mm refractors and you can undo those two main support screws ‘D’ in Post#4 and reverse the focus knob assembly. I was also able to reduce the contact pressure between the rack and pinion slightly by carefully adjusting screws ‘D’ in order to make the focuser extremely smooth to operate.

 

Doug…..

[Yerman]

Good write up nice pictures. I had a similar focuser problem that I posted a few years back. Mine had many issues. The small fine focus shaft on mine was machined off centre. To fix it I machined a new groove on the other end of the shaft. I have made thousands of dollars worth of telescope accessories on my mini lathe not the least of which every adapter known to man.

[Polyphemos]

Hi,

 

There is a similar focuser on the AT EDL and Starfield SDL 102mm refractors and you can undo those two main support screws ‘D’ in Post#4 and reverse the focus knob assembly. I was also able to reduce the contact pressure between the rack and pinion slightly by carefully adjusting screws ‘D’ in order to make the focuser extremely smooth to operate.

 

Doug…..

Good info, Doug. I can also confirm statements and information from maniack and others that the AT72EDII pinion assembly is completely flippable and works equally well whichever side the fine focus knob is on. 

[Polyphemos]

A most excellent tutorial Polyphemos !

These aren't the simplest of mechanical marvels to make so i admire your willingness to dissect the problem and to show the rest of us cowards exactly how its really done.

I need to readjust my 100ED's Crayford as its long sleep loosened up the drawtube to a frightening level and this helps to ease the angst of it if it all goes horribly wrong.

Sideways so to speak. Lol.



CSS
Lance

Hi Lance, call me Jim

 

The present family of two speed focusers from KUO and some others are very well designed and machined, and when in good shape work extremely well. They’re also relatively simple to work on and adjust.

 

Where people shoot themselves in the foot when considering whether to work on their own gear is that they lack the correct tools. They’ll use an Imperial size Allen wrench in a metric grub screw, or a Phillips screw driver on a JIS (Japanese Industrial Standard) machine screw, bugger up the hardware, and conclude they lack the skills necessary. Well, I also lack the skills necessary to achieve a desired outcome using the wrong tools.

 

It’s amazing what a decent set of metric Allen wrenches, metric sockets and wrenches, and a couple of JIS screwdrivers can do to help maintain, repair, and restore Astro gear, and none of those tools need be expensive. I guess what I’m saying is to get some of the right tools and start fiddling. There’s little you can do to harm your focuser and a lot of potential to make it function much better. Plus it can be fun! Good luck!

[Polyphemos]

I had a couple of minutes so I reassembled the focuser. It went together so quickly I didn’t have time to photograph the process, but as they say, “assembly is the reverse of disassembly”, and I do have some thoughts I can share.

 

1.  The large balls are the first items to reinstall, followed by the fine focuser shaft, and before you slide the fine focuser housing over the large balls and their cage. Make sure the groove in the fine focuser shaft is captured by the large balls.

 

2.  Slide the fine focuser housing over the large balls and their cage. Now you’ll have room to install the small balls on the other side of the housing.  My small balls were magnetized and would have been difficult to place, but fortunately I have a set of non-magnetic tweezers which made placing the small balls in their races fast and easy. Most stainless steel tweezers should be non-magnetic.

 

3.  With the large and small balls in place it’s time to thread the slotted adjustment/retaining ring into the fine focuser housing. You want this as tight as reasonable without making turning the fine focus knob overly resistant. Too loose and your fine focuser knob might slip; too tight and you might find the fine focuser action too stiff. You can check progress as you go along if you install the fine focuser knob on the end of the fine focuser shaft.

 

Tasks 1 through 3 took less than two minutes, and there was nothing difficult or challenging about it. The rest of the assembly was even easier.

 

The end result is that the focus action, both fine and coarse, as as good or better than new; as in nearly Feather Touch focuser smooth. That’s the good news. The bad news is that the rack teeth were damaged right where most focusing is done on an AT72EDII, which makes for a rough spot right where you want things to be perfect. Here’s what the rack looks like:

 

 

This is my third time working on smoothing a planetary fine focuser shaft and the results have thus far always been the same: focuser action is restored to like new or better. If you have a two speed focuser with gritty or notchy action it’s quite possible that you can improve its function dramatically with an electric drill, a bit of scrap fine emery paper, and a chopstick.

[eyespy]

Hi,

 

Can you reverse the rack so that the pinion engages with the undamaged end ? If not, a very fine jewellers triangular file should improve the damage to the teeth.

 

Doug…..

Edited by eyespy, 13 April 2025 - 05:26 PM.