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XT8 Focusers

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#1 MikeTelescope

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Posted 09 July 2019 - 04:58 PM

Bought a late-model, used Orion XT8.  It came with a flocked OTA, blackened secondary edges, re-worked azimuth bearings, and some age-related cosmetic spots on both mirrors.  Flashlight through the back of the primary shows a number of small spots without reflectivity.  The focuser is the 2" metal rack and pinion, which was a sticky mess with slop and lots of image shift.  I removed the sticky stuff, replaced with white lithium grease, and re-balanced the tensioner strip and pinion box screws. 

 

The focuser is now pretty good, no image shift and only a little lateral slop with heavier eyepieces.  The images are also pretty good.  Under good seeing I could focus on Jupiter at 200x sharply enough to see festoons and detailed GRS.  Darkish-sky views of M51 showed some detail at 100x. Star testing shows a good pattern with very slight undercorrection.  

 

Question is, how much improvement would there be in a more recent XT8 with a single-speed Crayford focuser, as compared to a re-worked and tuned-up 2" metal R&P?  Mainly wondering about focusing ease/accuracy at high magnification, and robustness to heavy eyepieces. 

 

More recent used XT8's come up now and then, and wondering if it would really be worth it to switch to something more recent, albeit still used.

 

 



#2 N3p

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Posted 09 July 2019 - 05:40 PM

The real improvement would be a dual speed focuser, that would deliver all the required precision.

 

On my friend's telescope we replaced this one. His was total garbage, I would not buy one new.

https://www.telescop...ser/p/13039.uts

 

He found a dual speed like this one second hand and it's very precise and pleasant to use.

https://www.telescop...rion dual speed


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#3 Volvonium

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Posted 09 July 2019 - 05:41 PM

I have a 15 year old XT10 with a 2" r&p focuser and did similar upgrades with the pinion box.  Mine has several equidistant long thin strips of of teflon that hold the focuser tube in place fairly well and still look new.  I think it is decent enough that it would be a while before the strips wear down and introduce any slop.   

 

The GSO Crayford with 8" baseplate is a perfect drop in fit for mine, but the Synta Crayford from an equivalent XT10 will not, as it has a different baseplate pattern.  I feel like the original rack and pinion is good enough for high power views, but I wouldn't want to frequently stress the focuser tube with heavier eyepieces.  I had no major problems getting into focus on one spectacular night of seeing, pushing Jupiter to 373X.

 

I don't think it's necessarily worth it to upgrade to a single speed crayford, unless you think you'll be frequently using heavier eps.  Going to a dual speed crayford may be worth it on the other hand if it is a scope you will be using rather often.  


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#4 MikeTelescope

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Posted 09 July 2019 - 06:50 PM

 Mine has several equidistant long thin strips of of teflon that hold the focuser tube in place fairly well and still look new.  I think it is decent enough that it would be a while before the strips wear down and introduce any slop.  

Are these the strips that came with the stock focuser, the white corrugated ones?  Or did you add some extra teflon strips on the tube to take up the wiggle room in the drawtube? 

 

Mine has the original 2 corrugated strips that are 120 degrees spaced, with the black teflon plastic pad at the third point.  I don't know the age of the scope.  The corrugated strips could have worn down some, thus the slight lateral movement. 

 

I mainly notice the lateral movement when I use a heavier zoom eyepiece or a barlow at higher altitudes.  It is not that bad, but if I really want good high-magnification views, I will re-collimate the primary on a de-focused star near my desired target, so that the altitude and focuser distance are about where they will be on the target.  If switching to a single-speed Crayford would eliminate that need, that would be a good thing.  



#5 aeajr

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Posted 09 July 2019 - 08:06 PM

I upgraded the focuser on my XT8i with the Antares 2 speed. Worked well.
https://agenaastro.c...dual-speed.html

It did require that I drill new mounting holes.

This may also be an option. https://agenaastro.c...dual-speed.html
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#6 SteveG

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Posted 09 July 2019 - 11:32 PM

The images are also pretty good.  Under good seeing I could focus on Jupiter at 200x sharply enough to see festoons and detailed GRS.  Darkish-sky views of M51 showed some detail at 100x. Star testing shows a good pattern with very slight undercorrection.  

 

 

 

That sounds like a pretty good mirror!

 

IMO, having a really nice focuser makes a huge difference with any scope. I actually prefer a premium single-speed over a cheap dual-speed. Get yourself a nice Moonlite, and never look back. YMMV.


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#7 Jon Isaacs

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Posted 10 July 2019 - 12:14 AM

Question is, how much improvement would there be in a more recent XT8 with a single-speed Crayford focuser, as compared to a re-worked and tuned-up 2" metal R&P?  Mainly wondering about focusing ease/accuracy at high magnification, and robustness to heavy eyepieces.

 

More recent used XT8's come up now and then, and wondering if it would really be worth it to switch to something more recent, albeit still used.

 

 

I have reworked quite of number of the Orion/Celestron/Synta rack and pinion focusers.  I often replace the corrugated strips with Teflon.  It's definitely an improvement.  However, the single speed Crayford is a definite step up, smooth and free from rock.  These often come up used in Classifieds or on Astromart.  I see them for $40-$50.  

 

A two speed is nice but the single speed is a definite upgrade and makes a "big" difference.

 

I believe Volvonium's scope was manufactured by GSO since the GSO focuser is a drop in fit. The GSO rack and pinion focuser is definitely better than the Synta/Orion. 

 

Jon


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#8 MikeTelescope

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Posted 10 July 2019 - 12:07 PM

I have reworked quite of number of the Orion/Celestron/Synta rack and pinion focusers.  I often replace the corrugated strips with Teflon.  It's definitely an improvement.  However, the single speed Crayford is a definite step up, smooth and free from rock.  These often come up used in Classifieds or on Astromart.  I see them for $40-$50.  

 

A two speed is nice but the single speed is a definite upgrade and makes a "big" difference.

 

I believe Volvonium's scope was manufactured by GSO since the GSO focuser is a drop in fit. The GSO rack and pinion focuser is definitely better than the Synta/Orion. 

 

Jon

Thanks for sharing your experience.  Where do you get your teflon replacement strips?  Instead of replacing the corrugated strips, would it work to apply thin teflon tape on the moving focuser tube itself to provide a bit more diameter where it contacts the corrugated strips?  I'm a little skiddish about completely removing the corrugated ones.  I'm pretty good at re-working something from 85% to 75%.

 

Are there any drop-in Crayfords for a R&P XT8?  From the documentation on Orion's site, all of their Crayfords require drilling holes in the older XT8's that had R&P's.  See 85%->75%.   



#9 Jon Isaacs

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Posted 10 July 2019 - 12:35 PM

Mike:

 

I get the Teflon from McMaster-carr.  It's 0.015" and etched on one side so it can be bonded. If you had a thin sheet of Teflon you might make it work but it would be more effort than just cutting and gluing the strips in place.

 

I think the Crayford is the way to go.

 

Can you post a photo of your current focuser along with the distances between the mounting screws?

 

I have upgraded an XT-10 rack and pinion to a single speed Orion Crayford and it was a drop in replacement.

 

It was this focuser:

 

https://www.telescop...-hoCSjUQAvD_BwE

 

Jon



#10 MikeTelescope

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Posted 10 July 2019 - 01:56 PM

 

I think the Crayford is the way to go.

 

Can you post a photo of your current focuser along with the distances between the mounting screws?

I plan on measuring tonight.  I did see that Agena has several choices for baseplates for GSO focusers with documented hole spacing.  Perhaps one of these baseplates would be drop-in for the XT8.  I called and they said they don't know, I need to measure. 

 

https://agenaastro.c...d-focuser.html 

 

The corrugated strips look fairly good.  No obvious matting down from age.  My guess as to where the slop is coming from is excessive tension on the pinion box screws.  With the pinion box removed, I tighten the allen screws behind the plastic strip to get minimal slop and smooth movement when pulling the drawtube by hand.  Then I put the pinion box back together, and screw down on the screws little by little to get a good mesh. I think this amount of pressure is pushing the drawtube towards the plastic bar, and a little bit away from the two corrugated pads, enough to allow a little lateral movement.  I think to compensate, I may need to go tighter on the allen screws and back off on the pinion screws.  I spent a few hours on this when I first did the job, but it may have shifted a bit since then.  



#11 Jon Isaacs

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Posted 11 July 2019 - 03:34 AM

I plan on measuring tonight.  I did see that Agena has several choices for baseplates for GSO focusers with documented hole spacing.  Perhaps one of these baseplates would be drop-in for the XT8.  I called and they said they don't know, I need to measure. 

 

https://agenaastro.c...d-focuser.html 

 

The corrugated strips look fairly good.  No obvious matting down from age.  My guess as to where the slop is coming from is excessive tension on the pinion box screws.  With the pinion box removed, I tighten the allen screws behind the plastic strip to get minimal slop and smooth movement when pulling the drawtube by hand.  Then I put the pinion box back together, and screw down on the screws little by little to get a good mesh. I think this amount of pressure is pushing the drawtube towards the plastic bar, and a little bit away from the two corrugated pads, enough to allow a little lateral movement.  I think to compensate, I may need to go tighter on the allen screws and back off on the pinion screws.  I spent a few hours on this when I first did the job, but it may have shifted a bit since then.  

 

This is how I adjust the rack and pinion focusers:

 

I install the drawtube without installing the pinion and adjust the two allen screws so that there is no play in the drawtube while it still moves quite easily.  

 

Then I install the pinion assembly and adjust it to my liking. Once I get it right, I normally put a small drop of crazy glue next to the head of each screw to keep it from turning.  

 

The corrugated strips normally look good and they work as well as they can.  The Teflon reduces the friction so the focuser can be smoother and lighter.

 

I recently help my friend Jack replace the rack and pinion focuser on his Celestron 8 inch F/5,  I believe it uses the same focuser as your XT-8.  Based on the measurements, the NON-GSO focuser plate would not fit. However by using number 6 screws and whatever plate he had, we were able to make a GSO focuser fit. 

 

Jon


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#12 MikeTelescope

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Posted 11 July 2019 - 11:46 AM

This is how I adjust the rack and pinion focusers:

 

I install the drawtube without installing the pinion and adjust the two allen screws so that there is no play in the drawtube while it still moves quite easily.  

 

Then I install the pinion assembly and adjust it to my liking. Once I get it right, I normally put a small drop of crazy glue next to the head of each screw to keep it from turning.  

 

The corrugated strips normally look good and they work as well as they can.  The Teflon reduces the friction so the focuser can be smoother and lighter.

 

I recently help my friend Jack replace the rack and pinion focuser on his Celestron 8 inch F/5,  I believe it uses the same focuser as your XT-8.  Based on the measurements, the NON-GSO focuser plate would not fit. However by using number 6 screws and whatever plate he had, we were able to make a GSO focuser fit. 

 

Jon

I think I got the ideas for re-working the focuser from reading older posts of yours.  I did the adjustments without the pinion in based on a post of yours I read.  I think my problem is that the tension I come up with when the pinion is removed isn't the optimal tension when the pinion is back in and some moderate tension is given to the pinion box screws, which pushes the tube back towards the black plastic strip.  

 

I was going to play with it more last night, but then the seeing was the best it's been in weeks, and it happened to fall on Saturn opposition.  So I spent the allocated astro time viewing instead.  The focuser can get there at least up to the 266x I tried.  Main issue is that I need to get there by going completely out of focus, slowly bring it in, and hope I stop at the sweet spot.  It is nearly impossible to make accurate small adjustments once it has stopped.  Too much sticktion, and the scope vibration takes long to dampen.  This is where a 2-speed might be nice to have.



#13 Sky Muse

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Posted 11 July 2019 - 01:37 PM

I get the plain PTFE(Teflon) from here, non-etched...

 

https://www.eplastics.com/sheets/ptfe

 

It's economical when getting the minimum 12" x 12" sheets in several thicknesses...

 

PTFE2b.jpg

 

I got all four from .005" to .020".

 

I use double-sided Scotch tape to install, along with HVAC-type aluminum-foil tape to build up the substrate and fine-tune the thickness...

 

focusser materials.jpg

 

That's all that's needed, along with scissors or a craft-knife and a ruler.  The foil-tape is laid down first, then the double-sided tape, then the PTFE last...

 

focusser fix3.jpg  

 

The surfaces of the foil-tape and the PTFE must be cleaned and de-greased before assembling.  I use 91% rubbing-alcohol, or 100% acetone, for that

 

If a miscalculation is made, it's much easier to remove this layer or that, over permanently gluing the PTFE in place, and for a more exacting installation; with no slop whatsoever, nor too tight, a true and square racking in and out throughout the range of motion, butter-smooth.  The foil-tape that I use is about .003" in thickness, and works quite well with the various thicknesses of PTFE.  


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#14 Jon Isaacs

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Posted 12 July 2019 - 02:08 AM

I think I got the ideas for re-working the focuser from reading older posts of yours.  I did the adjustments without the pinion in based on a post of yours I read.  I think my problem is that the tension I come up with when the pinion is removed isn't the optimal tension when the pinion is back in and some moderate tension is given to the pinion box screws, which pushes the tube back towards the black plastic strip.

 

What is the purpose of the increasing the load on the pinion gear?  I keep it light.

 

Jon


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#15 MikeTelescope

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Posted 13 July 2019 - 12:12 PM

What is the purpose of the increasing the load on the pinion gear?  I keep it light.

 

Jon

The screws on the pinion box are screwed in enough to remove play in the pinion/rack mesh, but still keep a relatively easy motion.  The focus turn is still pretty easy, but there could be just enough pressure pushing the drawtube back from the 2 corrugated strips to give a bit of lateral wiggle room.  I did take it out to a star party last night and compared it to other R&P's and single- and dual-speed Crayfords.  My R&P by comparison was not all that bad.  The only thing I don't like is having to change collimation when switching between a low-alt target and one near the zenith.  It's a small change, but it's there.  Probably unnoticeable at lower magnifications, but switching between M57 near the zenith, and Saturn at 30deg requires a tweak of the primary mirror to get the sharp image that good collimation gives.  




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