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Refractor Doublet Lens Spacer Material

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#1 Tom Duncan

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Posted 22 January 2019 - 08:56 PM

If you need to replace the three foil spacers common to older refractors I found some .003 thick 1.5" wide adhesive backed metal tape at my local hardware store that works perfectly if that's the thickness you need. Made by 3M, it's called "High Temperature Flue Tape", part number 2113NA, barcode number 0-21200-01699-8, pix attached.

 

Cut off a piece about an inch long, put it on a piece of flat glass, use an X-Acto knife with a fresh blade and a straight edge to cut out the pieces. I made mine 1/16" wide by 5/32" long. Use the tip of the X-Acto knife to pick up the tiny piece and place it in position on the lens element. Use something like the barrel of a fat Sharpie pen to roll it down, flattens it down reducing any creasing and secures it to the glass. 

 

Tom Duncan 

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#2 starman876

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Posted 22 January 2019 - 09:26 PM

I have been using that tape for years.  Even sticky on one side.  That is great tape for a lot of projects.   Holds lens cells on the end of tubes really well if you are testing a lens.


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#3 Stargoat

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Posted 22 January 2019 - 09:29 PM

After trying several types of materials to make spacers that have included HVAC tape and or automobile pin-striping material, IMHO here is the best material that I've come across yet so far.

 

See these links for product that I use. It is the silver foil color that is bright silver which closely resembles the original spacers. I'm a stickler about wanting the spacers to match as close as possible to the original ones used.

 

Silver Foil Adhesive Material for Telescope Objectives Needed 0.003" Spacers.

 

Stainless Silver Foil Adhesive Material for Telescope Objectives Needed 0.003" Spacers.

 

With that said, they also make a silver stainless material that I have bought, but have not tried yet. That may even be a more perfect match to the original spacers as it may be a slightly less bright silver color.

 

This material is very easy to work with. Colors are called "Silver Foil" and Stainless Silver".

 

This material is now what I use and it is sold at different craft/hobby stores here in the USA and even Australia.

 

The material is within a few tenths of 0.003" which includes the extremely thin non messy adhesive on back side.

 

IMHO much better than HVAC type which has too much adhesive on the back side. Also better than the Flue Tape which is too sticky, so less forgiving to place in the exact location.

 

From what I've worked on, the majority of Astro Optical and Swift etc objectives appear to have used 0.003" material best I can measure.

 

When you peel off the white backing paper from this foil, there is a very thin amount of self adhesive on the foil. After peeling off the backing material, the thickness is 0.003" with Calipers and 0.0285" with a Micrometer.

 

Cut longer/wider piece than needed with a very sharp Fiskars brand or like type of fabric scissors and then trim with a new razor blade to exactly match the size of the orig spacers. Use the new single edge razor blade to peel off part of backing on the piece trimmed. Use special tweezers to gently hold material and set the spacer in the correct location. Then use the end of a rounded toothpick to gently finish adhering the foil to objective.

 

It helps to have a hobby magnifier that I wear on my head and flip down over my eyes and or reading glasses similar to what a jeweler uses for close up work. I need stronger magnification than my reading glasses to see real close up to locate the spacers in the correct location just off the beveled edge of the objective for the spacers that do not overlap the edge.

 

Your mileage may vary as they say, but I've had excellent results so far with this material.


Edited by Stargoat, 22 January 2019 - 09:31 PM.

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#4 zakry3323

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Posted 22 January 2019 - 09:59 PM

Thanks for the heads up! I've been having to layer aluminum tape, with mixed results.  


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#5 apfever

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Posted 23 January 2019 - 12:05 AM

0.003" seems to be a common tape thickness.  I've measured the adhesive heat shield aluminum tape we use on our snowmobiles. I have a couple of different roles around here, no idea where they came from originally and both are 0.003" give or take a few tenths as Stephen mentioned.  I also used a 3M two sided regular 'Scotch' size tape to hold my Teflon bearing strips to my Edmund 8" RA shaft, that tape measured 0.003" thick.

 

These are small numbers but bear in mind the even smaller thickness that can affect the alignment, and the need to check your centering with Newtons rings. I played with my 4" Unitron lens (now sold) using regular kitchen foil adjustments. The foil measures in at a scant 5 tenths (0.0005") which is 1/2 of one thousandth of an inch. This amount on one spacer could consistently move the center of Newtons rings about 1/2 the lens diameter.  I have decent calipers and Vernier mics. I measured things a few times and went around all three spacers on my 4" lens more than once. That 0.0005" shim consistently gave a 1" to 2" shift in Newton ring centering.


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#6 Tom Duncan

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Posted 23 January 2019 - 11:37 AM

A bit off topic, but can you expand on the procedure for producing the Newton rings with a doublet? 

 

Tom



#7 Chuck Hards

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Posted 23 January 2019 - 11:38 AM

Just hold the elements together as designed, and view under monochromatic light.  They pop right out.  You can see them to a lesser extent under flourescent lighting.  


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#8 Tom Duncan

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Posted 23 January 2019 - 11:40 AM

Hmm, what constitutes 'monochromatic' light and is this with the spacers in place? 

 

Tom 



#9 Chuck Hards

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Posted 23 January 2019 - 11:44 AM

Yes, with the lens in the cell, spacers in place, and set up as if ready to install on the OTA.  The rings are interference fringes, can only be seen if the elements are VERY close together.

 

Flourescent light works, but monochromatic light makes them even more visible.  Optical fabricators use monochromatic light sources, typically green, for testing flats and other optical surfaces via their interference patterns.



#10 Chuck Hards

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Posted 23 January 2019 - 11:46 AM

Tom, here's an example of what a pro or advanced amateur glass pusher might use as a monochromatic light source.  LINK



#11 DAVIDG

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Posted 23 January 2019 - 12:09 PM

 A green LED will not work , you need light produced from a gas discharge to make monochrome or semi-monochrome light. A typical CFL bulb will work well.

 

                - Dave

lens interference pattern CFL bulb.jpg


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#12 Terra Nova

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Posted 23 January 2019 - 12:12 PM

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#13 Tom Duncan

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Posted 23 January 2019 - 12:27 PM

So this would be to address wedge, correcting for spherical aberration, correct? 

 

Tom



#14 bremms

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Posted 23 January 2019 - 12:37 PM

Aluminum flue tape has an adhesive that creeps. I stopped using it a long time ago. Fine line tape for automotive painting is my preferred spacer. Thin, non creeping, non reactive adhesive and Polypropylene backing does not stick or react to lenses. it is nearly ideal.


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#15 Chuck Hards

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Posted 23 January 2019 - 01:21 PM

So this would be to address wedge, correcting for spherical aberration, correct? 

 

Tom

  Mostly making sure all three spacers are the same thickness.  I avoid spacers with adhesive, it introduces a variable that sometimes makes even spacing difficult.


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#16 apfever

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Posted 23 January 2019 - 03:10 PM

here's how I used a green pointer laser by diffusing it through a white paper towel or napkin.

 

move the laser around and move your head around the rings should pop out.  

 

You can lay the flint on an optical safe surface with the spacers in place and put the crown on top. No need to put it in the cell to start. Rotating the crown or having a little side shift won't matter a lot. You can move the crown around on the flint and watch the rings. For the most part, the center of the rings won't move a bunch. Put one piece of 0.0005" foil on one spacer and compare how that moves the center of the rings a LOT. 

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#17 DAVIDG

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Posted 23 January 2019 - 03:27 PM

  You should check the interference pattern once the objective is installed back in the cell to be sure that the pattern has stayed centered and is round. 

  A non-uniform air gap will cause lateral color ie the stars turn into short rainbows and planetary detail is washed out and it will  also introduce astigmatism.

 

                - Dave 



#18 clamchip

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Posted 23 January 2019 - 03:47 PM

For Edmund Scientific lenses I've always adhered to .005" thick as mentioned in the Edmund book 

"All About Telescopes"  by Sam Brown.

I've used a variety of materials and if aluminum is used the cutting tool will leave a bur and change

the thickness so be sure and remove this bur by very lightly squeezing the 3 together in a vise with

smooth jaws or a rolling pin, my wife has a really nice one. I prefer the vise, or between the anvil

and spindle of a micrometer.

 

 

Robert


Edited by clamchip, 23 January 2019 - 03:49 PM.


#19 Stargoat

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Posted 23 January 2019 - 03:57 PM

Aluminum flue tape has an adhesive that creeps. I stopped using it a long time ago. Fine line tape for automotive painting is my preferred spacer. Thin, non creeping, non reactive adhesive and Polypropylene backing does not stick or react to lenses. it is nearly ideal.

Exactly why I stopped using HVAC or Flue tape...too much creep and a real pain to work with.

 

I did use a brite silver 3M auto pinstriping tape for spacers and it worked very well but aesthetically was not as good of a match IMHO to the original spacers which I believe are aluminum held in place by some type of optical adhesive.

 

Thus I now use the "Cricut" brand adhesive foil for my spacers that I mentioned above for excellent results. I get my adhesive foil at my local Michaels craft store. A CN member I helped in Australia was able to find the same product at his craft store. So low cost and easily available.


Edited by Stargoat, 23 January 2019 - 04:00 PM.

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#20 Stargoat

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Posted 23 January 2019 - 04:48 PM

Here is another advantage of using the adhesive foil material that I use.

 

My circa early 1958 Lafayette Polaris F-342 objective had the Newton Rings off center. Upon disassembly, I discovered the alum spacers were much thicker than I would have figured. Note that the crown on this Polaris is very thin in comparison to the flint. Normally the crown is not as thick as the flint, but this is the thinnest crown I can recall of all the ones I've taken apart of any brand or focal length best I can remember off top of my head. Thus possibly the reason for the thicker spacer is based on the optical design of this achromat.

 

With the original adhesive removed best as I could, the spacers seemed to measure 0.0095" on two and 0.0090" on one as I compressed tightly when in my calipers to flatten out.

 

With the adhesive foil material I was able to carefully stack three layers of it and cut to size. The three layers sans the white paper backing measured 0.009". Thus my replacement 0.009" foil spacers are close enough to be correct.

 

The final result was now the Newton Rings were centered and a very good star test.

 

Here are pics of before and after the spacers were replaced and the unusual thickness of the elements.

 

 

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#21 Longislandsky

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Posted 07 February 2020 - 05:25 PM

When you put the lenses back did you put larger lense on outside or inside on the install. I had To do this whole procedure in your feed on my lxd55 6” refractor and when I out Then back I’m getting halo around all the stars I’m viewing. Could you Elaborate on what I might be doing wrong thank you

Edited by Longislandsky, 07 February 2020 - 05:26 PM.


#22 Chuck Hards

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Posted 07 February 2020 - 06:33 PM

Thin edge element toward the stars.  The faces that mate have the same curve, just opposite.  Generally the shallower curve faces the sky, the steeper curve is the mating curve.



#23 ccwemyss

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Posted 07 February 2020 - 10:06 PM

I had a Unitron 114 objective that was missing one spacer, and tried the HVAC tape, but it was too thick. I also got some of the Crickut adhesive foil, and it was close but still too thick. Before giving up and replacing all three, I just started trying stuff that was laying around, and discovered that the backing paper for the Cricket foil was exactly the right thickness. That objective was backwards in the cell.

 

I got another one recently that was also backwards, and also had the rings way off center, even though they were all there. One of them was rough, so I burnished it as best I could, then carefully started scraping it with a blade until a little chunk that was folded over came off. Then the rings came to the center. 

 

Had another from a Shrine-Manon that was missing all the spacers (and backwards), and I put that back together with the Cricket foil, and it seemed to work well. 

 

Chip W. 


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#24 Longislandsky

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Posted 08 February 2020 - 12:35 AM

What is cricket foil and where can I find This please very helpful

#25 Stargoat

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Posted 08 February 2020 - 01:23 AM

What is cricket foil and where can I find This please very helpfuls

See my posting in the third message above for links for the Cricut® Adhesive Foil.


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