The Synta Wonky, Plastic Focusser Fix

[Alan64]

I've had Synta's base-model 150mm f/5 Newtonian, an Orion-branded "StarBlast 6", since the fall of 2012.  I didn't start using it much until a couple of years later.  Since that time I've enjoyed it quite a bit, and eventually flocked and blackened the interior of the optical-tube.  The particle-board mount that came with it had been long since abandoned, up in the attic now where it will remain until it rots, my preferring instead a tripod-type alt-azimuth for the Newtonian. 

There was only one thing left to do: enable the focusser's drawtube to rack in and out, smoothly, straight and true, and for an exacting collimation and the best images the parabola might produce.  Until yesterday, whilst somewhat satisfactory, I did not have a focusser quite up to par.  The problem all along was with these, and what I call "coffee-cake tray-liners", or for packaged cinnamon rolls, albeit of plastic instead of paper, and for the drawtube to glide against...

I could actually see the wonky path the drawtube took as it was racked in and out.  The rest of the telescope is somewhat first-rate, particularly the primary-mirror cell and the secondary's spider-assembly.  The parabola of the primary isn't bad either, not at all, with my having witnessed snap-to focussing at high-power on at least one occasion, with slightly above-average oculars, and with a 3x barlow integrated to boot.  During my initial attempts to fix the focusser, I ended up spoiling the original plastic drawtube.  Then a year ago I was able to get a replacement, but only by being the original purchaser.  A real stickler for that they are, Orion in California...

I gave the inside of the replacement a fresh spritzing from my rattle-can of ultra-flat black, then I prepped the outside for more...

...but in satin-black.  I didn't want a chromed end jutting into the light-path.  That step is optional, of course, but the next is not, I daresay...

PTFE, or Teflon®, sheets, and of varying thicknesses...

For this step, I chose the 0.020" or 0.508mm thick sheet.  The materials used: said PTFE in three 3/4" or 19.05mm wide strips, double-sided transparent tape 1/2" or 12.7mm wide, and aluminum-foil tape(perhaps 0.0025" or 0.0635mm thick, and the strips cut to the same width as the PTFE)...

First, the interior of the focusser's housing was degreased with 91% rubbing-alcohol.  I then applied the three strips of aluminum tape in a triad around the drawtube's cavity, then the double-sided tape, then the three strips of PTFE...

Both the aluminum tape and PTFE should be degreased as well, after handling with the fingers, and before and after applying the double-sided tape, respectively.

The drawtube now racks in and out smoothly, straight and true, and with no discernible slop...at long last.  Incidentally, the PTFE will lift right up off the double-sided tape, easily, but pulling on it parallel to the tape exhibits the opposite effect: it will not budge in the slightest.

The visual-back of my own is not 1.25", but 1.255" or so.  I lined the inside with a strip of the aluminum tape, and now it's quite close to the ideal, and a snug fit for my eyepieces and other accessories...

 

I stopped just short of extending the tape past the two holes for the locking screws, then pressed and burnished the tape down.

This fix is for all imported plastic focussers of overseas manufacture, whether for a Newtonian or refractor.  Would that, one day, it would not need to be so.

Edited May 15, 2018 by Alan64

[Stu]

Nice piece of work Alan, wobbly focuser/image shift are very annoying so I’m sure this makes a good improvement to your enjoyment of the scope.

[Alan64]

Thank you.  I do hope that this post not only helps, but also encourages others to fix theirs similarly.

[Stu]

5 minutes ago, Alan64 said:

Thank you.  I do hope that this post not only helps, but also encourages others to fix theirs similarly.

Yes, agreed. You’ve laid it all out very clearly so it should be very helpful for others.

I’m interested (for other projects) as to how you get the PTFE to stick to the tape? Was this the degreasing step?

[Alan64]

Thank you again.  Yes, you always want a grease-and-oil free surface where any adhesives are concerned, paints too, and you definitely don't want to omit that in this project.  For other applications, as I understand it, the surface of PTFE must be etched with a chemical compound, involving an acid I think, and then the ability to use a permanent adhesive, or a glue of sorts, presents itself. 

I had experimented with the PTFE and the double-sided tape a year ago or so.  I could lift the PTFE straight up off the tape after applying it, with no effort, but when I pulled the PTFE parallel to the tape, it simply wouldn't budge, and I pulled with considerable force; an amazing and delightful discovery that was.  For that reason, with the drawtube in place pressing tightly against the PTFE, the PTFE should never slip out, and for ever how long the tape's adhesive lasts.  No matter then, just remove the old tape, clean said surfaces again, and replace the tape.  Incidentally, the double-sided tape's adhesive is rather tenacious, the Scotch-brand in any event, as is that of the aluminum tape that I used.  I only used the aluminum tape for its slight thickness in addition to those of the other two components.  I'm thinking that there would be some cases where you would not need that extra slight thickness; this and other projects involving trial-and-error, you know. 

  

Edited May 15, 2018 by Alan64

[Alan64]

I am now conducting an experiment with the PTFE and 2-part epoxy, the resin and hardener.  I've found that the PTFE can be very easily scored on one side, cross-hatching it, and with an X-acto knife.  I also scored a piece of plywood as the substrate.  Epoxy readily grabs a hold of common plastic and wood, so I chose wood for this experiment; plastic, wood, no matter...

I applied a thin layer of epoxy to both surfaces, then pressed them together.  I have now set the experiment aside to cure, for at least a day.  I placed a short metal rule on top of the PTFE, and a small screwdriver as the only weight to hold it down...

You want to save the remainder of the epoxy, and to better discern when that between the PTFE and its substrate is cured...

 We shall see...

Edited May 17, 2018 by Alan64

[Alan64]

Said experiment resulted in no difference between using the epoxy or double-sided tape for the adhesive.  Again, when the PTFE was pulled parallel to the substrate, it wouldn't budge.  But when pulled straight upwards. it peeled right off.

I am now looking into the possibility of increasing the surface tension of the PTFE, by passing a flame over it, and from what I had read online.  In that case, scoring the surface of the PTFE might not be done, unless I scored the PTFE before the flame treatment.

[Alan64]

On the flip-side of a five-pence coin...<flip>...we have Synta's wonky, plastic focussers equipped on the refractors, too; gads.

The Celestron "AstroMaster" 70 EQ, a 70mm f/13 achromat, and seen here perched upon my AT Voyager I alt-azimuth...

  

...and brimming full of promise and devil-may-care; more of the latter I expect.

It all started when I had wanted an EQ-1 mount, and for my vintage 50mm f/12 achromat.  I could have gotten the bog-standard EQ-1, in black and silver, but the mount's interfacing with a telescope is not up to current standards, but doable if one is so inclined.  Standing out from that was Synta's zooted-up version, the Celestron "DeLuxe" EQ-1, which comes equipped with a Vixen-type mounting interface; and about time, too.  But the deluxe version is not sold separately.  If you want one, as I did apparently, you must include one of Synta's Celestron telescopes along with it.  I didn't want one of Synta's Jones-Bird simulations, as I have enough work to do, so I chose said refractor instead.  Much simpler, no?  No, not actually...

The refractor's focusser...

This is not your typical plastic focusser for an imported refractor; the focusser's housing...

It looks like a spaceship, and one of those 3D printings to boot.  What will it take, I wonder, and for a smooth, slop-free, straight-and-true racking motion.  The interior...

  

There's another one of those pitiful, plastic, self-adhesive glides, and the only one within the entire focusser.  Note the three drawtube supports.  On the outside, we have these three supports, in addition...

The top outside support is unique, in that it has molded runners...

Next, the fix...

Edited May 19, 2018 by Alan64

[Alan64]

On the side, I thought that the interior of the focusser's housing was a bit too reflective, so I blackened and dulled it with a rattle-can of ultra-flat black paint, whilst carefully masking off the surfaces of the supports and the channel for the drawtube's rack.  I used the blue painter's tape for that.  You also want to ensure that no paint lands on the outside areas of the focusser, the supports, and in general...

I also went ahead and spritzed the inside of the drawtube, as well as blackening the bevelled end of the drawtube which points towards the doublet-lens at the front of the telescope...

The tip of the rack was blackened, too, there at the bottom of the image.

*NOTE: In addition, you want to clean out and away all of Synta's original "glue, and what they try to pass off as lubrication, on the rack of the drawtube, between the teeth even, the gear of the pinion-shaft, and everywhere else, throughout.  During this project, I replaced the "glue" with Super Lube , for example, a Teflon-based lubricant.

[Alan64]

For this fix, six strips of PTFE were used.  Before I cut the strips to size, I treated one side of the long strip with a flame from a mini butane torch, passing the flame over it, back and forth, for a few seconds, and in the hopes of increasing the surface tension for the adhesive, in this case the double-sided tape.  I tried the 0.020"-thick PTFE out first, and with scraps of that inserted loosely between the drawtube and its supports...

But I could only insert those for two of them, not all three.  So then I knew the PTFE would need to be thinner.  In went the 0.015"-thick next.  Success...

The three supports on the interior required just three strips of 0.015"-thick PTFE, and the double-sided tape, as did the two of the outside supports and marked with green arrows...

The outside support at the top, the oddball, required a strip of the 0.020"-thick PTFE, the double-sided tape, and two strips of the aluminum tape, stacked.  It required that in order to bring its level up to the level of one of the interior strips just behind it...

I placed the strip in between the molded runners, and above them in the process, thereby eliminating same.

Now, the drawtube racks in and out, smoothly, straight-and-true, and with no slop whatsoever(as there was before the fix)...

Afterwards, I checked the collimation; not bad, I think...

But yes, it could be better, of course.

[Alan64]

I decided that what I had done to the objective-cell, I would also perform upon the focusser, and in making it collimatable as well...

 

[Stub Mandrel]

On 15/05/2018 at 05:21, Alan64 said:

I stopped just short of extending the tape past the two holes for the locking screws, then pressed and burnished the tape down.

I bought a similar focuser from Astrobin Astroboot, I'm a making a 60 x 228mm guidescope with it, using a lens from Astromedia.

It has the same plastic strips, but benefits from one of them having two adjusting screws plus a locking screw (ideal for a guidescope).

What I have taken away from your post is using some aluminium tape to make the drawtube a closer fit. I also put a short bit between the two screws although i don't suppose they will do anything when the screws are tightened.

 

Edited June 4, 2018 by Stub Mandrel

[Alan64]

Thank you.  There is that, at least.

Yes, the aluminum-foil tape turned out to be the ideal thickness to make the focusser tight enough to allow the battening down of the screws without it slipping out of alignment.  I had tried two strips of it: far too tight; and then one of the double-sided tape with the one foil: still too tight.  I had it right the first time, I found.  I then ripped the last layers off; and applied a single foil, again.  The decorative, scored, bronze tabs, and flat-washers, help in retaining the alignment afterwards.  This was the result of my first attempt at collimating the focusser...

...much improved, but I've removed everything since, and in preparation to flock the tube.  How gracious it is indeed that the current manufacturers have allowed an f/13 achromat to slip out of their factories.  However would that it were an f/15 or f/18 instead.  Incidentally, my brother has a C90.  I've used it more than he has, at night instead, but very little at that.  The Maksutov simulates this 70mm f/13 achromat; but the achromat is lighter, albeit longer, with minimal false-colour, and most importantly, unobstructed; hence my drive in the correction of this one, and with no holds barred.

[DanyalAG]

Hey Alan, I also have a Starblast 6, but I'm really curious about the level of optical perfection in this scope. I'm an absolute newbie, but from what I've learned, F/5 is a fast scope, and so will suffer from many optical abberations. I've no idea how to star-test, so any feedback from you regarding the optical quality of the scope would be highly appreciated.I would prefer a comparison with that Takahashi Apo you have. Thanks. 

(P.S. I also hate the focuser)

Edited March 10, 2020 by DanyalAG

[Alan64]

On 07/03/2020 at 10:23, DanyalAG said:

Hey Alan, I also have a Starblast 6, but I'm really curious about the level of optical perfection in this scope. I'm an absolute newbie, but from what I've learned, F/5 is a fast scope, and so will suffer from many optical abberations. I've no idea how to star-test, so any feedback from you regarding the optical quality of the scope would be highly appreciated.I would prefer a comparison with that Takahashi Apo you have. Thanks. 

(P.S. I also hate the focuser)

Hello Danya,

I had no idea of your having replied until now.   The 6" f/5 Newtonian's only advantage over the 4" f/8 fluorite refractor is that it's a bit brighter, is all.   The refractor is considerably superior in all other aspects.  Star-testing is most simple...

https://popastro.com/documents/PA_jan-mar2009_p12-13_telescopetopics_startesting.pdf

You can use hard, possibly woolen, thin felt strips for the draw-tube's bearings if PTFE is not available.  There are other materials, more common plastics, that may be used as well.  Some have even used plastic inserts within men's shirt-collars.  The goal is to get the draw-tube to rack inward and outward straight and true, with no binding or slop.  The condition of a focusser is just as important as the other parts whilst collimating.