William Optics GT71 focuser rack loose

[PadrePeace]

I’ve had an increasing backlash develop on my WO GT71 triplet over a number of months and finally identified the source. I racked the focus tube out and found that the brass rack gear plate was loose. Both of the bolts holding it onto the focuser barrel were loose and wabbling in their sockets. The outer most one was easy to tighten but the inner (inside the optical tube) was inaccessible. I removed the focuser (two large chromed bolts in the middle of the focuser casing) hoping to get sight of the second rack bolt. The first image shows this stage of the strip down. 

Unfortunately the bolt was only partially visible through the uppermost focuser bolt hole and not enough to get at it to tighten it. As the focuser tube would not extend out any further I decided that I’d have to unscrew the focuser housing from the main optical tube. First I marked a witness line across the joint line on both tube sections lightly with a pencil. This was to show the factory assembled position for when I came to reassemble the scope. I got hold of a pair of strap wrenches from an online retailer which cost just under £10. You can get them cheaper but you will need two. I also used a single layer of grip mat to give the straps something to get hold of. See here: 

https://www.amazon.co.uk/New-Multipurpose-Non-Slip-Mat-Furniture/dp/B00AVJ9GBY/ref=pd_bxgy_328_img_2/257-4866886-4111635?_encoding=UTF8&pd_rd_i=B00AVJ9GBY&pd_rd_r=92cc8659-4415-11e9-a9f4-838997fed375&pd_rd_w=Wf1ms&pd_rd_wg=f4nr2&pf_rd_p=a0365e62-3353-40ad-91cf-d4ca762b18a7&pf_rd_r=YB4ZXES80P827DWXEYRX&psc=1&refRID=YB4ZXES80P827DWXEYRX

This is excellent stuff and I also use it to help grip and unscrew difficult spacers at the camera end. It definitely helped to stop the straps slipping on the tubes. Once the initial stiffness of the joint was ‘cracked’ I found that unscrewing the two tube sections was quite easy. I did this by holding the scope vertically with the lens cell at the top both to stop the threads brim bindind and to stop loose paint or thread material from falling onto the clean and expensive stuff. Once apart, the loose rack bolt was easily tightened. I used a CR-V2 driver bit for this, and then reassembled the tube sections. Be careful not to cross-thread the sections at this stage. Aluminium threads are easily chewed up and you don’t want swarf inside the tube. I found that hand tight was all that was needed to realign the two pencil witness marks. There’s plenty of thread so no point in over doing it. A quick functional check of the focuser movement once refitted confirmed that the backlash had gone. 

Note that there is no need to remove the focuser, it just happened to be a thing I tried to get access but it is definitely not necessary. 

I hope this helps some of you who might have similar scopes and loose rack issues. Finally,  if you haven’t checked yours recently I’d recommend this as a loose or wayward bolt could jam the focuser at best or drop out onto your optics/ filters which would not be a good thing for your images or the glass ware!!!

Clear Skies

[david_taurus83]

Hi. What adapter is that on the end of your drawtube? It allows rotation?

[PadrePeace]

Hi David

It’s the rotator part from the TS-Optics 360° Rotation & Thread Adapter - M63 to M68, M54 and 2". I added a paper 360deg vernier scale to allow me to set a specific frame rotation angle. The Baader click lock is the M68a x1 Zeiss Norm which screws onto the rotator M68 thread and seats my WO 6a flattener/reducer.

[david_taurus83]

29 minutes ago, PadrePeace said:

Hi Pro Star

It’s the rotator part from the TS-Optics 360° Rotation & Thread Adapter - M63 to M68, M54 and 2". I added a paper 360deg vernier scale to allow me to set a specific frame rotation angle. The Baader click lock is the M68a x1 Zeiss Norm Which screws onto the rotator M68 thread and seats my WO 6a flatterer/reducer.

Thanks. I have the Flat 6A II and it has an M63 male thread for the scope side. The only thing I can find is the purpose made WO M63 rotator. Its £130 or so and I've resisted so far by rotating the OTA in the rings but I cant rotate 360° as the focus knobs eventually catch the mounts saddle. Looks like WO will be grabbing more cash off me in the near future ?

Thanks for the neat tip on uncoupling the OTA from the focuser housing!

[PadrePeace]

I was at the Practical Astro Show over the weekend and had a close look at the new range of WO FR with the added rotator. I’m pretty sure they have used the TS rotator screwed to their various FRs. If you can source an M68 to 63 adapter with the right male/female threads and have the back focus to spare then the TS option I used is only 59euros plus p&p. Just saying.....

[david_taurus83]

That's what I was thinking. So the TS rotator threads into the drawtube and leaves an M68 female thread? So I'd need some sort of threaded ring, M68 outer to M63 inner to thread onto the Flat 6A II and then into the TS rotator?

[PadrePeace]

Exactly. Good luck and note that BREXIT might impact costs and supply from Europe

[PadrePeace]

https://www.teleskop-express.de/shop/product_info.php/info/p6692_TS-Optics-M68-System-Adapter-from-M68-to-M63---Riccardi-connection-adapter.html might do you.

[sloz1664]

Did you just tighten the rack bolts or use some sort of thread lock to ensure they will not come loose again.

Steve

[PadrePeace]

Just tightened. 

[david_taurus83]

4 hours ago, PadrePeace said:

https://www.teleskop-express.de/shop/product_info.php/info/p6692_TS-Optics-M68-System-Adapter-from-M68-to-M63---Riccardi-connection-adapter.html might do you.

Given the cost of that and the TS adapter I might as well get the WO M63 Male and female threaded adapter.

[Tudor_Chibacu]

Great tutorial.

I have the same scope with a different issue. The focuser rack is attached well to the drawtube, but the whole drawtube is loose (unless I tighten the 2 screws adjustment screws extremely tight). Even with the screws tighten when the drawtube moves in or out it tilts to the left or to the right depending on the direction of travel. Resulting in tilt.

I was thinking of disconnecting the focuser and try to fix this issue somehow.

Any ideas?

Thanks,

 

[Demon Barber]

When you have got it stripped down check the bearings on the curved plate, one or more could be cracked, the bearings and plate are available from W/O,but the plate will be okay and the bearings are widely available from the likes of bearing services for not a lot,hope this helps.

Mike