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Crayford focuser drawtube slides right out

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11 replies to this topic

#1 erin

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Posted 08 January 2020 - 05:31 PM

Hi all,

 

I took delivery of a 90mm refractor today (SVbony 90mm f5). It has a 2 speed crayford focuser and this is my first time using a crayford. When I picked the scope out of the case, just tipping it a little made the drawtube slide right out of the focuser (to the internal stop). No amount of tightening the screws on the bottom helps, except for the thumbscrew which really just locks it. 

 

I am guessing this isn't how it should work? I have heard of these focusers sometimes slipping under heavy loads, but I can't attach a diagonal and eyepiece to it unless that big thumbscrew is almost totally locked. I don't dare point the scope up. Thanks!

 

Erin



#2 mastersniper

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Posted 08 January 2020 - 05:57 PM

not sure which specific focuser is on that scope but every crayford focuser I have used has a seperate friction screw near the lock screw (may be below flush, my EON's grub screw is quite far below flush to engage at all) that controls the tension on the draw tube.  maybe it made it out to you with that screw missing somehow?   is there an empty threaded hole? just forward or back from the lock screw?  


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#3 vtornado

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Posted 08 January 2020 - 06:01 PM

Hello Erin, can you post  a pic of the bottom of the focuser?

 

I think mastersniper is on the right track.

If one of my synta focusers looks like your I will experiment with mine.


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#4 erin

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Posted 08 January 2020 - 06:04 PM

I can try to get a pic later, but there are two small screws, one above and one below the lock screw. I have tightened both screws as much as possible and even then the tube slides right out when tipped the least bit vertical.



#5 msl615

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Posted 08 January 2020 - 06:30 PM

Are these accurate pictures of your scope, top and bottom of the tube?

 

You see the two small screws on the top of the tube? Those SHOULD be the tension adjusters on the sliding plastic/teflon pieces inside the focuser. 

 

1. Try adjusting those to tighten the tube.

2. The two on the bottom are already as tight as they can go?  Those should be the ones that adjust the tension of the spindle gear against the tube....not really the ones you want to adjust. 

 

I suppose there is the possiblity that the teflon spacer strips inside the focuser are not there. Can you pull out the tube entirely and look inside?

 

Hunt down "adjust crayford focuser" on the web and you will find a ton of DIY advice and videos.

 

Keep us posted. 

 

Mike

Attached Thumbnails

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  • bottom.jpg

Edited by msl615, 08 January 2020 - 06:31 PM.

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#6 erin

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Posted 08 January 2020 - 07:09 PM

Thanks everyone! I will do some research and let you all know how it goes.

 

Still, I can't seem to find anywhere that talks about this kind of "slippage" (I might call it "fall-age) being normal. It isn't, right? Just so I know, going forward. Since I will be exchanging this one anyway.



#7 vtornado

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Posted 08 January 2020 - 07:55 PM

Hi Erin, if your focuser looks like Mike's, then mine is not the same.

I have seven small hex cap screws surrounding my lock knob.


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#8 ButterFly

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Posted 09 January 2020 - 12:49 AM

This happens on my ES when I go to center the tube.  The ES tube rides on bearings that can be adjusted with eight screws to get the centering and tilt just right (four on top, four on bottom; four on objective side, four on eyepiece side).

 

Total looseness sounds like a tube bearing issue and not a focuser adjustment issue.  I am looking at those three shiny screws (two visible in the pic on the left in post #5 and the third on the right between the focuser bottom and the optical tube dovetail).  Those look like tube bearing adjustments to level the tube.  Can someone confirm that?  If those are not bearing on the tube, it can slide in and out freely.

 

If you do get the tube tight again, stick a collimated laser in the focuser to check the centering.


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#9 erin

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Posted 09 January 2020 - 07:22 AM

Thanks Butterfly. I think there is more to this than some adjusting. Using the focuser wheels with this scope is basically optional. 



#10 Jon Isaacs

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Posted 10 January 2020 - 05:57 AM

Thanks Butterfly. I think there is more to this than some adjusting. Using the focuser wheels with this scope is basically optional. 

If you posted a photo of the underside of the focuser, we could make more useful suggestions... .

 

I've worked on a lot of focusers..

 

Jon


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#11 erin

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Posted 18 January 2020 - 01:57 PM

Jon, my focuser looks exactly like the one in Post #5. I ended up returning this scope and picking one up in the classifieds. The 2nd one still slides, but not as badly, and I was able to swap the 8mm screws for 10mm length screws and it worked!

 

The screw that adjusts the tension is the one to the left of the locking thumbscrew (see Post#5, picture on the right). Now the drawtube doesn’t slide freely when tilted up and I don’t need to rely on the locking screw for tension control.

 

Thanks everyone for your help!



#12 Jon Isaacs

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Posted 19 January 2020 - 04:51 AM

Jon, my focuser looks exactly like the one in Post #5. I ended up returning this scope and picking one up in the classifieds. The 2nd one still slides, but not as badly, and I was able to swap the 8mm screws for 10mm length screws and it worked!

 

The screw that adjusts the tension is the one to the left of the locking thumbscrew (see Post#5, picture on the right). Now the drawtube doesn’t slide freely when tilted up and I don’t need to rely on the locking screw for tension control.

 

Thanks everyone for your help!

 

The thumbscrew with the "tension" label, that adjusts the force between the pinion and the drawtube, it is not a focuser lock. With a Crayford, the focuser lock normally pushes directly on the the drawtube flat and is offset from the pinion.

 

The facts that it says tension and it's centered over the pinion tells me it's for adjusting the the drive force.

 

Jon




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