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Synta's Wonky, Plastic Focusser Fix

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#1 Sky Muse

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Posted 13 May 2018 - 01:08 PM

Synta's base 6" f/5 Newtonian proved not to be such bad gamble.  Its primary's parabola is quite good actually, my having witnessed snap-to focussing on at least one occasion, and with my old-school oculars.  But the focusser itself, per the title of this thread, is what it is, and it has been one bane of my existence since we were first introduced.  You simply cannot expect a tight, smooth and solid racking action, straight and true, throughout the drawtube's length/range, and with these coffee-cake tray-liners...

 

coffee cake.jpg

 

Every other component is somewhat first-rate, but not this travesty.  Now, I had acquired a new, old-stock, 1.25" Parks, all-metal focusser, and made in Japan many years ago.  It's perfect, and light-years ahead of this one.  It will require adding a metal plate to strengthen its mounting.  I may do that, eventually, but I decided recently not to give up on attempting what seems to be, at first, the "repair of a sand castle with cake frosting".  The rack itself on the drawtube even, the teeth, is of chromed plastic.  My first attempt ended in failure, as all I had on hand was self-adhesive PTFE tape, but it was a bit too thick, therefore the drawtube was far too tight a fit, so I ground down those areas on the drawtube where the strips would contact it, but not exactly to the ISO9001 standard.  "Hey, if Synta didn't, why should I?"...

 

old drawtube2.jpg

 

That was not the way to go about it.  Last May, being the original owner of this Orion, I was "blessed" with the ability to get another drawtube for it...

 

Orion receipt - 050817.jpg

 

drawtubes.jpg

 

                 whee.gif                                                       uhhh5.gif

 

 

 

 



#2 Sky Muse

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Posted 13 May 2018 - 05:03 PM

The drawtube, fresh from Orion, is not ready to use as is.  Now, if you're like me, you wouldn't trust someone who manufactured a joke like this to properly blacken the interior of the drawtube.  It was certainly black enough inside, but not a chance, so I took my trusty rattle-can of ultra-flat black and spritzed it again...

 

drawtube interior.jpg

 

Before I did that, however, I prepped the outside of the drawtube for the third and last step.  Note how the lower two-fifths of the tube there is not as shiny...

 

drawtube prep.jpg

 

You simply don't want a chromed barrel jutting into the light-path within the optical-tube; no, not ever...

 

drawtube prep3.jpg

 

My, that looks like one of those fancy anodised-black metal drawtubes, eh?  On the outside, I used bog-standard satin-black, as the surface will be in contact with its new runners.  I figured satin would be best; not semi-gloss, and certainly not gloss-black.  In any event, I went over that area of the tube with a blow-dryer on low, and perhaps to "bake", and accelerate the curing of, the paint.  Still, I will want to let it alone for at least a day before its integration; that is, I should wait. grin.gif   Before painting, I make certain that the surfaces are grease, oil and dust free.  I use 91% rubbing-alcohol, then a tack-cloth, for that.

 

Would that Synta provided its plastic drawtubes just so. fingertap.gif   Not a chance, which is why you must...DIY.



#3 Sky Muse

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Posted 14 May 2018 - 03:10 AM

I guess I've been working on this all day, or for most of the day.  It's about time now to retire, but not until I finish this portion of the thread.  The fix was a resounding success, and my quiet victory over Synta's failure to equip this telescope and others within their base line with at least a good-quality focusser.

 

The materials required for this particular project...

 

focusser materials.jpg   

 

The double-sided Scotch® tape is obvious.  The large roll of tape is self-adhesive aluminum, which is used for air & heating ductwork.  I don't know its exact thickness.  If I had to guess, I'd say 0.005" or less.  The white strip there, and to take the place of the coffee-cake tray-liners, was cut from a sheet of PTFE, or Teflon®.  Here's the packing list of the PTFE I had purchased...

 

PTFE2.jpg

 

The sheets in the box...

 

PTFE.jpg

 

I used the thickest of what I had ordered for this project: 0.020" thick

 

First, the inside of the focusser's housing was cleaned and degreased with 91% rubbing-alcohol.  Three(3) 3/4"-wide strips of the aluminum ducting tape were then laid down first within the drawtube cavity of the housing, vertically, in a triad, and evenly spaced.  The 1/2-wide double-sided Scotch® tape was then laid on top of those, then the 3/4"-wide strip of PTFE on top of those.  Any excess that jutted out from the bottom was trimmed off flush...

 

drawtube integration.jpg

 

The drawtube was then inserted and secured with its hardware...

 

drawtube integration2.jpg

 

new focusser.jpg

 

Even when racked fully up and out, there is no slop, nor any binding as it's racked along its full range.  I can now, at long last, collimate this Newtonian properly.

 

PTFE will pull straight up, perpendicularly from the double-sided tape, and with no effort.  However, the PTFE will not budge in the slightest when pulled parallel to said tape, therefore with the drawtube inserted the PTFE will never slide out; that is, perhaps until the tape becomes so old that it fails utterly, if ever.  This fix is viable for plastic focussers found on practically all Chinese telescopes, whether a refractor or Newtonian, and no matter the brand.

 

Now, I'm not out of the woods yet with this focusser, for its visual-back is not a 1.25", but a 1.26"...

 

visual-back.jpg  

 

Its resolution will be forthcoming, and the result posted within this thread in future.

 

Incidentally, before my success with this one, I had already revamped the focusser for my Zhumell Z100, a 100m f/4 Newtonian, and in the same manner...

 

focusser fix.jpg

 

It's a little more of a tighter fit with that one, a little stiffer, but with no binding whatsoever.  Now, as I understand, PTFE cannot be glued in place, hence the use of double-sided tape.  The surface of the PTFE  would have to be etched first, and with a dangerous chemical compound.  I'll pass.  Although, I can't help but think that I could score lines into the surface, then use epoxy.  I'll be experimenting with that someday.  

 

It is my sincere hope that this fix will prove helpful to those affected.


Edited by Sky Muse, 14 May 2018 - 03:24 AM.


#4 Sky Muse

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Posted 14 May 2018 - 05:45 PM

It turns out that the visual-back is only a little off from being 1.25" in diameter.  It's actually 1.255" or 1.256", so I got the idea to place a piece of that aluminum ducting tape, and it's near perfect.  I didn't feel the need to go all the way around the inner surface, which is threaded by the way. Instead, I ran the two ends of the tape just short of the two holes for the locking-screws...

 

visual-back2.jpg

 

An eyepiece, and all other 1.25" accessories, will now fit like a glove.  The tape should be burnished after placement, then cleaned with alcohol.  I used the barrel of an X-acto knife to burnish this one.  Incidentally, that's not a dent in the tape within the top image, but merely a reflection.


Edited by Sky Muse, 14 May 2018 - 05:45 PM.


#5 Sky Muse

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Posted 14 May 2018 - 06:07 PM

The collimation is still "right on the money"...

 

collimation-051418.jpg

 

That's through the included collimation cap.  I haven't taken my Tectron tool kit to it yet, but that will wait.  Tonight, I will test out my new zoom-ocular, and to hopefully celebrate the successes of both.



#6 luxo II

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Posted 15 May 2018 - 06:47 AM

You need to do something about the 3 retaining clips protruding on the primary mirror - those are simply gross and will cause 6 gawd-awful diffraction spikes IN ADDITION to the four from the vanes supporting the secondary.

 

There are ways to stop the mirror falling out without obstructing the mirror... there was a thread here a while back.

 

And while you are at it I'd get rid of the diffraction spikes from the vanes - make a curved-vane spider with just 2 vanes matching the perimeter of the primary mirror optical surface. You really don't need an assembly capable of mooring the Titanic to support a secondary in place.


Edited by luxo II, 15 May 2018 - 06:51 AM.


#7 Sky Muse

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Posted 15 May 2018 - 11:24 PM

You need to do something about the 3 retaining clips protruding on the primary mirror - those are simply gross and will cause 6 gawd-awful diffraction spikes IN ADDITION to the four from the vanes supporting the secondary.

 

There are ways to stop the mirror falling out without obstructing the mirror... there was a thread here a while back.

 

And while you are at it I'd get rid of the diffraction spikes from the vanes - make a curved-vane spider with just 2 vanes matching the perimeter of the primary mirror optical surface. You really don't need an assembly capable of mooring the Titanic to support a secondary in place.

I have thought about those improvements, but not for this Synta.  I have a Parks 8" f/5 where I would prefer curved vanes.  Its primary mirror, on the other hand, uses a hard-plastic retaining-ring to hold it in place.

 

Thanks for the suggestions.   



#8 Sky Muse

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Posted 16 May 2018 - 12:43 AM

Last year, I got a Celestron "AstroMaster 70 EQ" kit, but I didn't get it for the telescope that came with it, but rather for the modern equivalent of the venerable EQ-1 mount.  I didn't want one of Synta's Jones-Bird simulations, so I settled on the kit with the 70mm f/13 achromatic-doublet refractor instead.  The focusser's housing, and in dire need of the same enhancement...

 

focusser2.jpg

 

There's another one of those drawtube glides, and curiously, only one.

 

None on the remaining two of the three drawtube supports...

 

focusser.jpg


Edited by Sky Muse, 16 May 2018 - 12:45 AM.


#9 Sky Muse

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Posted 16 May 2018 - 04:38 AM

With the focusser taken apart, I cleaned Synta's "glue", and what they try to pass off as lubrication, out of the focusser's housing, and off of the drawtube's rack and the pinion-shaft...

 

focusser4.jpg

 

The interior of the focusser's housing appeared a bit too reflective when I took a shot of it earlier using the flash, so I carefully masked off this and that and spray-painted the interior with the ultra-flat black...

 

focusser3.jpg

 

It was all apart, so why not.



#10 Sky Muse

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Posted 16 May 2018 - 03:30 PM

The focusser's drawtube is rather long, and probably due to its f/13 focal-ratio, and for a refractor.  Isn't it pretty and shiny...

 

focusser5.jpg

 

However, we don't want "pretty and shiny" inside the optical-tube.  The inner end of the drawtube, aimed at the doublet, is bevelled, so I roughed up all of that and blackened it as well...

 

focusser7.jpg

 

The blackening of these components, whilst effecting the fix, is always optional.  It all depends on your personal preference.


Edited by Sky Muse, 16 May 2018 - 03:31 PM.


#11 Sky Muse

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Posted 16 May 2018 - 05:06 PM

This focusser housing(a 3D printing?) reminds me of something, and from a movie...

 

focusser9.jpg

 

This, I think...

 

https://i.pinimg.com...cca55af6b9c.jpg

 

...the ancient spaceship from the movie..."Alien".  Good stuff, that is.  Incidentally, before I took the focusser apart, I racked the drawtube in and out a few times.  The "grease" inside, as the rack and pinion meshed, I could hear it, sounding just like those alien eggs opening up, there inside the spaceship.  Then, that thing jumps out, and you've bought it.  My having thought of all that begs the question, "What's up with Synta's designers?".

 

Well, I don't regret having bought this kit, as it has great potential, both the mount and the refractor.

 

At the top of the housing, you've got this support for the drawtube as it extends outward...

 

focusser8.jpg

 

Those molded runners...really?  I could shave those off, and before applying a strip of PTFE, but I could also place the strip in between them, there in the center.  I'll be deciding on that in a short while.  For this refractor, the fix is proving to be a bit more complicated.



#12 Sky Muse

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Posted 16 May 2018 - 08:14 PM

I took scrap strips of the 0.020"-thick PTFE, but I could only insert two of the three between the drawtube and its supports when inserted into the housing...

 

focusser10.jpg

 

So, now I know I'll need to resort to the thinner stock, perhaps the 0.015"-thick.

 

On the side, I'm conducting an experiment.  The double-sided tape works as the adhesive, in a pinch, but what if the use of epoxy proved successful?  I took a piece of the 0.020" and scored it with an X-acto blade, and very easily, in a cross-hatch pattern, and also scored a piece of plywood as the substrate(common plastic, or wood, no matter)...

 

PTFE-epoxy test.jpg

 

I applied a thin layer of the epoxy to both surfaces, and pressed them together.  I laid a short ruler on top of the PTFE, and a small screwdriver on top of that; not a lot of weight...

 

PTFE-epoxy test2.jpg

 

You want to save the remaining epoxy, and to better discern when the epoxy finally cures and becomes rock-like in hardness...

 

PTFE-epoxy test3.jpg

 

I would rather epoxy the PTFE down onto the drawtube supports on the inside of the focusser, as there may be times when the drawtube will not be compressing same, as when it's racked fully outward.  It may not be a problem in the case of using the double-sided tape either, but an ounce of prevention...

 

We'll see.  


Edited by Sky Muse, 16 May 2018 - 08:17 PM.


#13 Sky Muse

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Posted 16 May 2018 - 09:48 PM

It appears that the thickness of PTFE required for the three interior drawtube supports is 0.015", and with only the double-sided tape as the additional thickness for each...

 

focusser11.jpg

 

                                         whee.gif



#14 Sky Muse

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Posted 17 May 2018 - 08:43 PM

Scoring the PTFE and using epoxy resulted in the same characteristics as when using the double-sided tape: pulling the PTFE parallel to the substrate, it will not budge; but when pulled straight up, perpendicular to the the substrate, it lifts off with no effort.  I have read that taking a flame, from a mini-torch, and passing it over the PTFE for a few seconds, may increase the surface-tension to where one might make use of conventional adhesives.  For this fix, I did just that, but I didn't use epoxy, just the double-sided tape...

 

On the interior of the focusser's housing, I installed three PTFE strips, 0.015" thick, and with the double-sided tape as the only additional thickness.  Note the two strips in the background, indicated by green arrows.  The same for those two...

 

focusser12.jpg

 

For the oddball drawtube-support...

 

focusser8b.jpg

 

...I used the 0.020"-thick PTFE, two strips of aluminum tape, stacked, and the double-sided tape.  This brought that area up to the level of the one seen just behind it...

 

focusser15.jpg

 

Now, the focusser racks in and out with no slop whatsoever, even when extended fully outward...

 

focusser17.jpg

 

                              whee.gif whee.gif whee.gif

 

Mind you, it's a little stiff, racking in and out, but it doesn't bind up in the slightest; smooth as silk it is.  I'd rather it be that than a little too loose, and like it was before the fix.

 

This particular focusser, and for a Celestron "AstroMaster" 70 EQ, has got to be the most convoluted to ever leave a factory...

 

alt-az mounting.jpg

 

For the standard, and sane, plastic focussers of the rest of the imported refractors, the fix should be much simpler, and more akin to those of most of the imported Newtonians.


Edited by Sky Muse, 17 May 2018 - 11:33 PM.



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