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Do really need to Airspace my Doublet?

ATM Lens Making Refractor
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#1 Taosmath

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Posted 24 July 2021 - 12:17 PM

A question from my ignorance here.

 

I have a very inexpensive 70mm F4.3 refractor that I've been tinkering with to see if I can turn it into a decent finder.

 

With a 5mm EP in it, it was apparent that the Achro doublet was improperly collimated.  Airy pattern was a fan, not rings.

 

On disassembly the the objective was two elements separated by a thin (~ 1mm) plastic ring which was poorly made with rough edges.  As far as I could tell by eye, the two adjacent surfaces the lenses were a match so, just to see, I put the two lenses in contact (they appeared to fit exactly), reassembled and tried it out.

 

The collimation was now near perfect - nice Airy rings at 60x.

 

I was thinking about trying to make a spacer out of a loop of nylon fishing line, but then I wondered if I needed to bother.

 

By putting the spacer ring on top of the second element, the clamping ring held everything in place without rattling.  I could also bond the two elements together with Canadian balsam or similar.

 

So is there a good optical reason to fiddle with making a spacer if direct contact appears to work? (at least as indicated by Airy pattern).

 

Thanks


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#2 ngc7319_20

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Posted 24 July 2021 - 12:34 PM

You will likely see spherical aberration if you fill the airspace.  But you could make some quick test with water or baby oil.

 

These lens designs get big coma if the airspace is not uniform -- that is, if one spacer is thinner or thicker.  Sounds like what you experienced.  It is very sensitive -- sometimes 0.001" difference is all it takes to make coma.

 

I would avoid contact at the center since will damage things long-term.  Stuff tends to move a little with vibration and temperature change, and will cause grinding.  I would put 3 tiny pieces of black tape at the edge to space it.  The fish line thing will work, too, so long there are no "nubs" where you cut it.


Edited by ngc7319_20, 24 July 2021 - 12:35 PM.

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#3 Richard O'Neill

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Posted 24 July 2021 - 01:07 PM

 An alternative is three small shims of kitchen aluminum foil.


Edited by Richard O'Neill, 24 July 2021 - 01:09 PM.

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#4 DAVIDG

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Posted 24 July 2021 - 01:26 PM

 The air space in an objective is an optical element just like the glass elements. Light leaves a high refractive index (glass) and enters a low one (air) then back into a  high  refractive material again. The angle that the light follows is determined by Snell's Law.  Take the air space away and you change the correction of the objective. 

     You want the air space to be uniform. Since three independent points define a plane I would use three separate spacers of the same thickness as the original plastic ring and place them at 120 ° centers.  Your lens is most likely a Fraunhofer design were the two inner radii are close to the same but slightly different. So if you shiny monochrome or semi-monochrome light on the lens, you will see interference rings between  the two element.

  If the air space is uniform they will be round and centered, if not the spacers many not be exactly  the same thickness or one placed slightly inward vs the rest or might have a burr on the edge.

   You should see a pattern similar to this when the air space is uniform. I used a CFL bulb which is uses a number of monochrome wavelengths to make "white" light so you see a multicolored interference pattern. 

 

 

                      - Dave 

lens interference pattern CFL bulb.jpg


Edited by DAVIDG, 25 July 2021 - 08:13 AM.

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#5 Pinbout

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Posted 24 July 2021 - 01:34 PM

Adj’ing the airspace will change SA error. So removing the spacer will change the SA. If you change what’s stock you have to test the lens. Do a ronchi on a star. Ronchi against an oil flat…done vertically. 


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#6 Taosmath

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Posted 24 July 2021 - 02:14 PM

Thanks for the explanations and the suggestions.  I understand a little better now.

 

The kitchen foil shims sound like a great idea.  I can try that out today since I have kitchen foil and I can adjust to the same thickness as the original shim by using my calipers.

 

Thanks !



#7 ccaissie

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Posted 25 July 2021 - 11:13 AM

 An alternative is three small shims of kitchen aluminum foil.

AYE.  

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#8 DAVIDG

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Posted 25 July 2021 - 11:55 AM

 Just remember that the design of the objective called for a certain air spacing.  As I said that  air spacing is optical element just like the glass so the thickness is as  important as the  thickness of both the glass elements.

    I have achromats that range in  spacing from 0.003" to an 80mm Brandon from the '50's that a has spacing of 0.500" ( 1/2 an inch !)  between the elements 

 

           - Dave 



#9 BGRE

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Posted 25 July 2021 - 05:22 PM

Measuring the residual aberrations for several spacer thicknesses should allow the appropriate spacer thickness to be estimated for homing in on the optimum spacer thickness.

#10 oldtimer

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Posted 05 August 2021 - 11:45 AM

Usng 3 little foil spacers is a pain in the arse as they are hard to keep in place when reassembling. I just use 3  1/8" squares of scotch tape which sticks nicely.

 

Gary (oldtimer)



#11 dan_h

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Posted 05 August 2021 - 12:33 PM

Usng 3 little foil spacers is a pain in the arse as they are hard to keep in place when reassembling. I just use 3  1/8" squares of scotch tape which sticks nicely.

 

Gary (oldtimer)

That works. I have also used the trim from a sheet of self adhesive postage stamps. 

 

dan



#12 oldtimer

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Posted 08 August 2021 - 11:21 AM

I have sort of a 'scrap book' which over the years I have filled with various bits of telesecope facts and figures. Unfortunately I don't always keep a record of where I read the information. So for what its worth my notes say that for the Fraunhofer design the spacing between the crown and flint lens should be no more than .1 millimeter or .004 inch


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#13 dan chaffee

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Posted 08 August 2021 - 12:07 PM

I have sort of a 'scrap book' which over the years I have filled with various bits of telesecope facts and figures. Unfortunately I don't always keep a record of where I read the information. So for what its worth my notes say that for the Fraunhofer design the spacing between the crown and flint lens should be no more than .1 millimeter or .004 inch

It would be good to have a source for that info.  "Telescope Optics" gives a prescription for a classic 200mm f/15 Fraunhofer doublet with 3.3mm.   A Baker design also gives generous air gap. This reduces the ghost reflections to being undetectable

even with an uncoated lens, in addition to being aplanatic. 
 



#14 oldtimer

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Posted 09 August 2021 - 01:35 PM

More from my 'mystrey notebook' although there are no fast and hard rules for the spacing of Fraunhofer lenses, as a general rule of thumb If the 2nd and 3rd lens surfaces match then the thinist possible spacers that keep the lenses from toughing are suggested.



#15 DAVIDG

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Posted 14 August 2021 - 06:08 PM

More from my 'mystrey notebook' although there are no fast and hard rules for the spacing of Fraunhofer lenses, as a general rule of thumb If the 2nd and 3rd lens surfaces match then the thinist possible spacers that keep the lenses from toughing are suggested.

 As you stated there is no hard fast rules and I would say there is more exception then the norm to using the thinnest spacer.  First a true Fraunhofer design does not have the R2 and R3 equal. They make them different to remove ghost images so spacer thickness varies with the exact design.   In the Baker design which many classic Jaegers lens have,  R2 = R3 and the air space is very wide in 1/8" or larger vs a few thousands of an inch. I have  3" Brandon objective with a huge air spacer as in 0.5" and many Clark objectives has had very large air spacing. 

  The bottom line is that you need to know the design to correctly replace the spacer with the correct thickness or bench test the lens to determine the best spacing. 

 

              - Dave 


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#16 Jon Isaacs

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Posted 16 August 2021 - 05:33 AM

A question from my ignorance here.

 

I have a very inexpensive 70mm F4.3 refractor that I've been tinkering with to see if I can turn it into a decent finder.

 

With a 5mm EP in it, it was apparent that the Achro doublet was improperly collimated.  Airy pattern was a fan, not rings.

 

On disassembly the the objective was two elements separated by a thin (~ 1mm) plastic ring which was poorly made with rough edges.  As far as I could tell by eye, the two adjacent surfaces the lenses were a match so, just to see, I put the two lenses in contact (they appeared to fit exactly), reassembled and tried it out.

 

The collimation was now near perfect - nice Airy rings at 60x.

 

I was thinking about trying to make a spacer out of a loop of nylon fishing line, but then I wondered if I needed to bother.

 

By putting the spacer ring on top of the second element, the clamping ring held everything in place without rattling.  I could also bond the two elements together with Canadian balsam or similar.

 

So is there a good optical reason to fiddle with making a spacer if direct contact appears to work? (at least as indicated by Airy pattern).

 

Thanks

 

I also have one of those 70mm F/4.3 refractors, I bought it just to see what was like.  It has the same symptom.  I did not realize it was actually an air spaced doublet.  So thanks, I will experiment with it.  I also bought a 60mm F/7 doublet with similar problems and I bought a used Celestron 102mm F/9.8 that also has very funky images with a bad flaring.  

 

They all need work. 

 

They say refractors don't need alignment/collimation.  I've got three that say they certainly can.  And collimating a Newtonian is pretty easy by comparison.

 

Jon 



#17 dan chaffee

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Posted 16 August 2021 - 11:49 AM


They say refractors don't need alignment/collimation.  I've got three that say they certainly can.  And collimating a Newtonian is pretty easy by comparison.

 

 

Unless a lens is anastigmatic, it does need alignment to minimize off-axis astigmatism and extreme misalignment

would likely produce noticeable lateral color.



#18 sunrag

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Posted 16 August 2021 - 03:18 PM

I have a D74/3000mm FL achromat that is cemented. It produces horrible star images (looks triangular). From what I have read, pinched optics could cause this result. Could i just free it by removing the cement and air-spacing it?



#19 DAVIDG

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Posted 16 August 2021 - 04:13 PM

I have a D74/3000mm FL achromat that is cemented. It produces horrible star images (looks triangular). From what I have read, pinched optics could cause this result. Could i just free it by removing the cement and air-spacing it?

 The air space is an optical element just like the glass so lenses that are designed to be  air spaced should not be cemented and ones that are cemented should be air spaced.  If you bench test lens you would be able to see if it strained from the cement. If so you can remove the cement and either redo it or oil it. 

 

                  - Dave 


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