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Replacing Focuser Lunt 40mm

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#1 Ray777


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Posted 12 February 2024 - 07:46 AM

After testing out my new Lunt 40mm, (very happy by the first light results by the way)..  I decided to see if I could find a replacement/upgrade for the original Helical Focuser

as it really felt not up to the quality of the rest of the scope.  I had some issues with it causing sagging and wiggling of the Diagonal, even more so

when I put my camera in the diagonal, which made it almost to heavy it seemed.. which resulted in a 2nd problem the focus ring getting loose (which I had to fix by tightening the little screws a bit..)

..which just made the focusing more painful, needing to use a bit more force to turn it.



So I went looking for a replacement focuser,  with the Feather Touch focuser obviously being the best candidate.  However, it was nowhere in stock,.. and the price for it here in Europe is ..$$$..

I looked online, (also on these forums) and came around the COMA Focuser for Lunt 50  by Adriano Lolli from Italy. (about € 95 euro's for the standard version,

and € 135 euro for the micro-focus version..(which is the same with only some extra knob's to help with focusing more precisely) 

Originally made for the Lunt 50mm, I looked further into the design of the COMA focuser and the differences between the Lunt 40 and 50mm and found both were capable of 

using the Feather Touch Focuser so both used the same M35x0.5 Threads for the focuser.


Looking a bit further into it, it seemed that the Lunt 40mm however used an adapter between the Telescope Tube and the original Helical Focuser to go to the required M35x0.5 threads.
..and according to some other users (and correspondence from Lunt themselves), that adapter was threadlocked on both the telescope-tube side and the adapter side..

According to Lunt you needed to send it in to Replace the focuser..  But living in Europe and reading about the extremely delayed production at Lunt (probably because of the high demand for this April's Eclipse),

i decided to do it myself. 

Since the focuser could be replaced, it was clear the threadlock adhesive wasn't unmovable object to overcome.  ..Just needed to know how to, and 'thread' the disassembly with care..


So I ordered the COMA focuser for Lunt 50 by Adriano Lolli and once arrived began my Lunt 40mm "modification" 

Here is a quick Manual,  (Follow along with the first picture)



Quick Manual

Most import tip  ---DON’T! do anything with brutal force!..

And remember it’s at your own risk

So even-though it isn’t very difficult to do, …just in case you don’t feel sure enough to do this,
….then you probably shouldn’t do it, to dodge user error.


If you feel sure enough, then follow on. 


Do this in a room where dust won't be to much of an issue as you open the telescoe tube back-side and don't want excess amounts of dust enter in.

(Keep something nearby to cover up the open backside against dust.


Okay here we go.




The Helical Focuser is threadlocked glued in two places I carefully first unscrewed the backend of the telescope.

That back-end on the telescope side, is home to the M35x0.5 thread adapter. (The blue line piece)

If it is locked tightly, use a hair dryer to warm it a bit to soften the adhesive. (just warm it, don’t heat it up RED-HOT!)


--Then use gentle and small (counter clockwise) turns to get it loose. 


Cover up the now backside of the Telescope tube against dust




For me the threadlock between the adapter and the telescope-tube was not super stuck, 

..between the Helical focuser and the adapter however, it took more grip and a bit more force.

Hence why I first removed it from the telescope tube. 


To remove the Helical focuser from the adapter  I then simply held the adapter and focuser at these two “Purple Line” points 

(the adapter with a piece of cloth to create some steady grip) and on the focuser side with my other hand.  


Then slowly I tried to rotate them clock and counter-clock wise until I felt the adapter beginning to move inside the threads.

..I carefully kept on rotating them until the Threadlock adhesive had given way completely.

---AGAIN: DON'T brutally Force it!  (a bit force is okay, as they can take it, and you probably need it)

but if it is locked too tightly, use a hair dryer to warm it a bit to soften the adhesive. 

Once loose enough I could rotate the Focuser out of the adapter.

..and there it was  the same M35x0.5 threaded connection for the Feather-touch  (or in my case for the COMA Adriano Lolli ‘non rotating’ focuser for Lunt 50.) 


The remaining glue residue on the threads was easily wiped of both threads with a fiber cloth (making movements with the threads)

Now simply ‘carefully' screw back the Adapter on the Telescope tube and carefully tighten it.  

(If you want to you could first check no dust has entered the tube, and maybe use a hand rocked blower (one's used for camera sensor cleaning), to blow out any dust..)

Screw the COMA focuser onto the M35x0.5 adapter thread, and again 'carefully tighten it, then put in your diagonal blocking filter..

....and you’re good to go!




I've added some extra pictures to compare, 


The quality difference is quite noticeable.


And really like the extra inward & outward focus travel with the COMA Focuser, and how smooth it feels turning the focus compared to the rather stiff or shocking-loose original Helical focuser.


Also with no movement at all from the diagonal inside it, even when using my rather heavy camera setup or big Zoom eye-pieces in it,

it really gives it a much more robust, professional and saver feeling whilst using it.


I haven't been able to test the new focuser with imaging yet, as he weather has sucked for days... but hopefully today I get a chance.


So not at all a bad modification/upgrade for the little Lunt 40mm!




If you'd like to know what I get with the Lunt 40mm, check out my other post.

Attached Thumbnails

  • Lunt 40mm (Modificatie) 1B.jpg
  • Lunt 40mm (Modificatie) 2.jpg
  • Lunt 40mm (Modificatie) 3.jpg
  • Lunt 40mm (Modificatie) 4.jpg

Edited by Ray777, 12 February 2024 - 07:46 AM.

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