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Questar Repair Bench – Q&A Thread

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#1 Optics Patent

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Posted 28 March 2017 - 10:12 AM

To avoid starting a bunch of different threads with different questions, I'm starting this thread where anyone may post those little technical questions that we have when working on our scopes, or considering buying scopes.  I encourage everyone to participate.


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#2 Optics Patent

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Posted 28 March 2017 - 10:18 AM

Is this Questar finder lens failed?  Can it be repaired?  I have cleaned both surfaces, and no marks are visible at the surface.  I presume the cement between the lenses has failed. This is from my 1990-era distressed Duplex named "Ralph" after the prior owner who abused it and mis-described it.  I have another older Field model with the same effect.  One old Cloudy Nights Thread covered a failed saga of trying to dissolve failed lens cement, and is not encouraging.  The performance isn't that bad, and the cost of a new replacement from Questar is $170 plus shipping.  I have a question pending with Q about the cement used then.  Unless they warn me it is impossible, I plan to try the solvent and reglue project just for fun and fumes.

 

IMG 1573
IMG 1571


A friend has emailed this article on How to Dissolve Lens Cement with Weld-On 3 and I'm checking with Jim at Q on whether they were using soluble cements back in 1990.

Edited by Optics Patent, 28 March 2017 - 03:37 PM.


#3 DAVIDG

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Posted 29 March 2017 - 08:47 AM

 I have repaired many lens. You need to first try heating the lens by placing it in a pan of water.  Using  plastic or wooden sticks separate the elements. If heating of water doesn't work I use my toaster oven set at about 230 F. since it takes higher temperatures to soften the more modern UV curable cements. These UV curable types will not easily react with solvents so soaking them will usually not work in separating the elements.

    Once the elements are separated turn off the heat and let everything  cool back down to room temperature. Clean off the old cement with organic solvent like acetone or toluene. Then use a small drop of mineral oil between them to oil them back together.  The result will be a perfectly clear assembly and very sharp image. 

  Here are a couple of pictures of a vintage Cave 50mm finder scope objected I repaired.

 

                          - Dave 

cavefinderobjective.jpg

caveelementsapart.jpg

caveoiledlens.jpg


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#4 Optics Patent

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Posted 29 March 2017 - 11:39 AM

How should I decide whether to first attempt a solvent soak or a heat soak for a ~1990 lens. Heat seems riskier by probably isn't. Plus it requires no supplies to be ordered. I'll see if I get a hint from Questar.

#5 DAVIDG

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Posted 29 March 2017 - 12:46 PM

 A lens made in the 90's is must likely using a UV curable cement. That material is  a cross linked polymer so soaking it in  a solvent will not dissolve it. What happens is  over time the solvent will  swell the polymer enough to get the element  apart but it takes a while. as in days or longer. Also you usually have to use a nasty solvent like methylene chloride. 

   So too me it much safer and quicker to heat the lens in  a pan of water as a  first try. If you can't get it hot enough in the water to soften the cement that is were the toaster oven comes in. Just be sure not to take the glass out until it  has cooled way down to avoid having it crack.  I keep watching the lens as it warms up and you'll see the cement change when it has soften, then push to the two elements apart with wooden sticks. Once they are apart turn the heat off and let them cool back down to room temp.

   For oil I use the mineral oil you can buy in the  drug store that is medical grade and sold as a laxative.  It is perfectly clear, non toxic,  it won't react with the glass and it won't yellow with age.  You  just need a tiny drop in the center of the concave  element, lower the convex element of top and place a small weight on them. After about an hour take the weight off wipe the excess oil from the edges. If you have enough room you can apply a thin layer of tape to edge to seal it put I haven't had any issue with the oil leaking out of  lens that I have done this too that are over 10 years old.

 

                  - Dave 


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#6 Optics Patent

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Posted 22 April 2017 - 10:12 AM

Boiling water had no effect, and a long bake at 230F also had no effect.  (The there was the inadvertent broil because I forgot I'd left the lens in the oven to cool when the next mealtime came around).

 

So, should I try for hotter temp, or a solvent soak?



#7 DAVIDG

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Posted 22 April 2017 - 11:12 AM

As a chemist, I don't think any type of solvent it going to do the job quickly  Modern day optical cements are UV curable polymers. What these means is the polymer is cross linked by the reaction with UV light. So the material doesn't dissolve in a solvent, sometimes it will soften but that can take many days to weeks for it to wick in between the elements You also need to use nasty stuff like methyl chloride. 

   Did you try pushing the elements a part when they were hot ? I would also try to heat them up to around 250F and again try pushing them apart. I haven't had a lens yet that did not come part for me when heated and using a two sticks to push them apart.

 

                     - Dave



#8 Optics Patent

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Posted 22 April 2017 - 03:26 PM

Thanks, Dave.  I did apply force with two wood sticks on the 230F lens, and nothing budged.  I suppose I could try 250F - what temp limit would you consider safe (I'm also worried about thermal shock because the hot lens will be in room temp air when I'm applying force).  I did order the solvent and can be patient and let it soak.



#9 Joe Eiers

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Posted 23 April 2017 - 09:42 AM

My center black spot on the outside of the meniscus has started to lift along the edges.  I didn't even know it could do that!   How to I glue these edges down?

 

  Thanks for the great thread!

 

  Joe 



#10 DAVIDG

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Posted 23 April 2017 - 11:02 AM

Thanks, Dave.  I did apply force with two wood sticks on the 230F lens, and nothing budged.  I suppose I could try 250F - what temp limit would you consider safe (I'm also worried about thermal shock because the hot lens will be in room temp air when I'm applying force).  I did order the solvent and can be patient and let it soak.

I have positioned the lens in the middle of my toaster oven when pushing the elements apart and so far no problem. Once they apart I shut  the door and turn off the heat and let it cool to room temperature. When it comes to temperature that works what I've done is turn on the toaster oven and watch the lens as it is heating up. The cement will change appearance when it starts to soften and that is when I try pushing them apart. So keep creeping the temperature up until you see something happen.
   I know an optician who passes the flame from a propane torch over the lenses a couple of times and thermal expansion of the glass pops them apart. To risky for me thou.

                 - Dave



#11 DAVIDG

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Posted 23 April 2017 - 11:11 AM

My center black spot on the outside of the meniscus has started to lift along the edges.  I didn't even know it could do that!   How to I glue these edges down?

 

  Thanks for the great thread!

 

  Joe 

 The disk is  a special 3M paint that is applied with  a brush as the meniscus was spun around.  The spot is larger then the metallized spot that is the secondary and  acts has the baffle for the secondary. Questar stopped using the paint because it would out gas and didn't meet military specs so they now use a metal disk that is curved on back to match the front curve of the corrector. It is glued in place with a special RTV that meets military specs for out gassing.

    As for gluing down the edge of the paint spot I would suggest a water based glue since a solvent based one would most likely start to dissolve the paint and make it worse.  I would use a tooth pick to careful apply the glue under the edge of the paint. 

 

                  - Dave 



#12 Optics Patent

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Posted 23 April 2017 - 04:19 PM

After an hour at 300F, the two elements did not separate when pushed opposite directions at their edges.

 

Granted, a better fixture to shear them apart might help.

 

I think a solvent soak is the next test.



#13 DAVIDG

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Posted 23 April 2017 - 06:53 PM

 I hope it works. The problem is that materials that react with solvents also melts or soften at temperatures that are below 300F.  The other issue is that the there is only a thin line  a material that is exposed to the solvent so if it the type that doesn't dissolve to expose new material but swells and softens it may take a long time for the solvent to work itself all the way between the elements.  Most of the modern lenses use Norland 60 or 61 adhesives and I have been able to get them apart by heating them. I think that the cement has soften in the oven but your not able to get enough force on them to pull them part. The cement softens but doesn't melt so it still is very viscous and gummy.

 Does the lens look any different ?

 

                 - Dave 



#14 Optics Patent

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Posted 24 April 2017 - 08:13 AM

Here's the really weird thing.  I think all this heating improved or fixed the lens.  Under harshest lighting it looks a little iffy, but these are all "after" pictures and even the harsh one doesn't look as bad as the first one which showed an internal "fingerprint"-like pattern that is gone.  Under normal inspection it looked pretty bad before, but now I wouldn't even dream of doing more.

 

IMG 1882
IMG 1880
IMG 1879
IMG 1878
IMG 1885
IMG 1891
 
Compare "before":
IMG 1573

Edited by Optics Patent, 24 April 2017 - 08:29 AM.

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#15 DAVIDG

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Posted 24 April 2017 - 09:11 AM

 That makes sense since if the adhesive soften and flowed it would heal itself. What is the exact focal length and diameter ?  Questar is buying  these lenses from  a 3rd party and an achromat of that  size should be in the $30 range. It is most likely Edmund or Thorlabs that is the supplier. 

 

               - Dave 



#16 Optics Patent

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Posted 24 April 2017 - 11:21 AM

That crossed my mind. I can't fault Questar but their price to replace this is nearly $200. I presume focal length is about 1/10 of the scope focal length and I'll measure the diameter later. Now I'm happy with the "cure" considering it is my worst distressed scope and a parts donor for now

Thank you for the suggestion to bake the lens. I'm happy with the unexpected result.

PS. I'm not sure if the cure occurred after the boil, the 230F bake, the unintentional broil, or the 300F bake

#17 DAVIDG

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Posted 24 April 2017 - 12:54 PM

 The don't fault Questar for their pricing but some of the replacement  parts of equivalent quality can be found without the mark up. The 95mm  metal lens cap I found on Ebay is an example. 

   I'm sure what happen it at one of those temperatures the adhesive soften enough to flow but it stays very viscous. Being a small lens it is difficult to get enough force on them  it easily to separate the elements. without  thermally shocking the glass and cracking it or burning your fingers in the process. 

   The spec's list the finder has having  a 100mm fl and if the diameter is 25mm then Thorlabs has this lens https://www.thorlabs...ber=AC254-100-A

Edmund has this lens as another possible replacement. https://www.edmundop...c-doublet-lens/

  Surplus Shed has this one that has a 100mm fl but is bit larger in diameter at 26.5mm https://www.edmundop...c-doublet-lens/

 

             - Dave  



#18 Optics Patent

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Posted 05 May 2017 - 01:22 PM

For the record, the finder lens measured at 0.653" or 16.6mm diameter.

 

Also, I recently asked a roomful of engineers at my optics patent client Leupold (the rifle scope maker) what might have happened.  They all thought I was insane to think that a failed doublet could be improved this way!


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#19 Optics Patent

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Posted 31 May 2018 - 12:39 PM

Has anyone encountered this type of mirror spring system on a 3.5?

 

This is a 1978, and has an excessive focus shift that is more of a loose rattle you can hear and feel, not just see at magnification.    The three springs are all on the same side of the spindle.

 

IMG 5301

 

Update: To answer my own question, I tried taking out that right spring.  It ended the rattle/wobble, and left only slightly-out-of-spec image shift.


Edited by Optics Patent, 31 May 2018 - 09:53 PM.


#20 davidmcgo

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Posted 03 June 2018 - 10:04 AM

Hi,  

 

I've seen posts where the mirror on its focusing plate and hub is removed, but how does one remove the mirror itself to recoat and re assemble afterwards?  Does the plate the focusing arm attaches to thread off the hub or is it pressed and glued?

 

If it is simple, I'm thinking of getting mine freshened up, possibly with enhanced aluminum.  

 

Dave



#21 Optics Patent

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Posted 04 June 2018 - 10:29 AM

I just learned from the factory about what I had wrongly thought were "shims" or "pads" for the image shift issue.  in fact (as I just began to suspect after some inspection) the 4 contact points forward and four points aft are actually pinch points where the thin walled tube is dimpled inward.  They say it's a tricky process, kind of a pain, to get just right.  Too tight and you get excess wear or a mirror that the spring isn't strong enough to move forward.

 

If I ever try playing with this, my first effort will be to commit the machine shop horror of using a 1" micrometer as a C-clamp or vise, and use it a thousandth at a time to create slight permanent deformation. 



#22 davidmcgo

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Posted 24 June 2018 - 01:27 PM

The finder lens on my 1965 is partially separated and did not release after either a long simmer in water or at 250 degrees in the oven.  I was expecting Canada balsam but may be an epoxy since the UV cured didn’t come out until later as far as I know.

 

So I worked some mineral oil around the separated area with a fingertip a few times and it seems to have wicked in and got rid of the rainbow pattern and flare when looking through it.

 

Dave


Edited by davidmcgo, 24 June 2018 - 01:29 PM.

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#23 davidmcgo

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Posted 24 June 2018 - 03:32 PM

I also found removing the primary is doable without removing the control box, just need to remove the focus knob and unscrew front barrel.  Then grasp the threaded focus rod between the mirror and rear plate of the OTA to unscrew and re screw after cleaning.

 

Dave


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#24 davidmcgo

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Posted 25 June 2018 - 12:08 AM

Just a follow up on wicking mineral oil into the decemented area of the finder lens, worked like a champ!  Had a clear evening tonight and all of the wierd flaring is gone, everything is sharp and crisp, and even Jupiter’s moons were visible in the finder mode with the 80-160 eyepiece.

 

Even if I have to repeat the procedure, the bottle of mineral oil was less than $3.00 and finder alignment was not affected by removing and re installing the mirror bracket (I left the mirror alone).

 

Dave


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#25 Optics Patent

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Posted 27 June 2018 - 07:32 AM

A new member messaged a question that others might help with:

 

...I recently acquired my first Q. ...how to take the knob off.
I loosened the set screw but it doesn't slip off.

 

I have commonly encountered this.  It's tricky. 

 

You can't just unscrew until it stops and crank on it because that will stress the internals and lead to image shift.

 

You can't bang on it because there's a delicate mirror on the other end - and you may lead to image shift.

 

You can't grip the rod with pliers because you'll damage the surface and ruin smooth focusing.

 

I have tried light tapping on the end of the knob to loosen it, but you should protect the surface to avoid marring. 

 

Wiggling/rocking side to side can help.  Often the end of the rod is flared at the end and is too tight.  I've had to file down these edges even to get the knob back on.

 

A drop of "loose juice" could help unseize the connection.  Careful about it getting where it doesn't belong or damaging paint.

 

Heat may help too.

 

As a final resort, the knob is inexpensively replaced, so measures that damage it are tolerable, although this has never been needed in my cases.

 

Never mind.  I just thought of this safe and simple solution:  Screw it all the way in so it's against the control box.  Then screw it more.  The wedging effect will push it off.


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