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Let the antique speak for itself...

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#101 DAVIDG

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Posted 20 February 2019 - 01:58 PM

 The elements or lens is not reversed. If they were the Ronchi bands would be badly bowing. Unfortunately the lens has problems  from the asymmetrical pattern of the Ronchi bands.  You'll see astigmatism in the images and washed out planetary detail. Maybe it is spacing problem or maybe it is just a poorly figured lens. It happens more then you think. 

   I would double check that the  spacers are at 120° centers, if not your not going to get a uniform air gap and will not be able to center the interference rings. Next is to  place three tabs of tape on the back lip of the cell at 120° centers and place the lens in the cell so the air spacers are over these taps. On the front element place three taps of tape over the air spacers. Now when the retainer  ring is screwed down it will only come in contact at these three defined points and you have even pressure on the air spacers. You should now be able to center the interference rings. Next is to retest the lens in Double Pass. If it still shows the asymmetric pattern, it means the lens is poorly figured and the only way to fix it is refigure it.

 

              - Dave 


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#102 John Higbee

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Posted 20 February 2019 - 02:33 PM

Dave - if it were a spacing problem (spacers are not the proper thickness) how should I go about determining the proper air gap for this lens?

 

John


Edited by John Higbee, 20 February 2019 - 02:45 PM.


#103 starman876

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Posted 20 February 2019 - 04:16 PM

Now that I think about it the spacers on the cell where the lens rests were not lined up with the spacers of the lens.  This is one thing we can fix straight away.   Next we can put some spacers on the crown where  the retaining ring comes in contact and see if that helps.  We can also add some spacers to center the newton rings and test again and see if it improves.  We can also try changing the spacing of the lens and see if the DPAC improves.


Edited by starman876, 20 February 2019 - 04:17 PM.

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#104 DAVIDG

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Posted 20 February 2019 - 10:00 PM

 The pattern is asymetrical so either the air gap is wedged or one of the elements has astigmatism. Note on the right side of the image of the lens being tested in double pass the lines are pretty straight but the bottom of the left side they bow outward. Changing the spacer thickness isn't going to fix that.  

    I would remove the spacers and place the elements in direct contact. If the interference rings are round and centered that showed that both of these surfaces are figures of rotation. If the pattern is off centered and/or asymetrical then on of the surfaces is stigmatic. Put the spacer back in and assembly the cell back in the cell so the spacers are over any raised areas in the cell. If the cell doesn't have them, then add them.The lens should have slight rattle in the cell if not the retainer is too tight. Hopefully you'll now be able to get the interference pattern round and centered.  If so then put the lens back on the test bench and see what the Ronchi bands look like. If you still see the asysmetrical pattern then the  figure on one or more of surfaces is bad. The only way to fix that is to refigure the surface(s). Good luck.

 

                   - Dave 


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#105 John Higbee

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Posted 21 February 2019 - 05:48 AM

Thanks, Dave.  We're getting back together this weekend and will do this...and will report what we find.  John



#106 John Higbee

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Posted 25 February 2019 - 04:50 AM

Johann and I had another session with the 6" objective yesterday, and made some significant progress in objective alignment and DPAC.

 

Last weekend, we had several unresolved problems at the end of the session:

  • Signficantly skewed Ronchi lines on the DPAC
  • Newton rings that were skewed away from the center of the objective
  • An objective that fit so tightly into the cell that there was no slight "rattle" heard after objective reassembly

David G. gave us some good advice on troubleshooting, which we followed yesterday:

  • Removing the cell from the objective, and checking the shims and air gap between the crown and flint
    • Shims were in good shape, and air gap was even around the objective perimeter
    • Using a template, found all three to be symmetrical (120 degrees apart, plus or minus a degree)
  • Checking the three support points at the bottom of the cell for proper placement
    • Using a template, found all three to be symmetrical (got similar spacing results as for the shims)

We also cleaned the inside of the cell: 

  • Removed a lot of dirty film from the inside cell walls 

When we re-assembled the objective, we ensured that the objective shims and the lower support points were aligned.  The objective went in on the first try with no difficulty.  We also ensured that the objective retaining ring was installed "hand-tight, then backed off slightly"...and we did get the "slight rattle" that had been missing previously.

 

Newton rings were symmetric and centered...and we had three straight lines on the DPAC testing!  (...and there was great rejoicing! grin.gif ).  It's possible that one element may have very slightly cocked in the cell during last week's activities, due to the dirty residue on the cell walls...that, and/or the misalignment of the objective shims to the cell support points, may have been the cause of last week's problems.

 

As a final (unrelated) step, we cleaned and tested the 3" f/6 objective for the finder on the 6" Spacek.  I had not been able to disassemble the finder previously, but with Johann's help we got it apart, and found that one of the objective shims was out of the air gap entirely (was on the edge of the objective...we reinstalled it after cleaning both elements).  During reassembly, Johann demonstrated for me what DPAC results for a reversed objective and flipped element looked like...as Dave G. says, DPAC will immediately show you those conditions if they exist!  Finder is reassembled, shows fine images, and is reinstalled on the 6" OTA.

 

All in all, a great day!  Many thanks to Johann, for his support and his patience teaching a refractor "newbie" about optical maintenance and testing!

 

John   


Edited by John Higbee, 25 February 2019 - 04:53 AM.

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#107 Steve Allison

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Posted 25 February 2019 - 06:28 AM

Outstanding!

 

As a new Tinsley 4 inch F/15 refractor owner, I am rooting for you!

 

The weather has prevented me from putting my Tinsley through its paces, but a daytime peek at a light pole insulator revealed a mighty promising set of inner and outer Fresnel rings, despite the shimmer in the air!

 

Are these vintage Tinsley refractors cool or what?

 

Steve


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#108 starman876

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Posted 25 February 2019 - 08:16 AM

Johann and I had another session with the 6" objective yesterday, and made some significant progress in objective alignment and DPAC.

 

Last weekend, we had several unresolved problems at the end of the session:

  • Signficantly skewed Ronchi lines on the DPAC
  • Newton rings that were skewed away from the center of the objective
  • An objective that fit so tightly into the cell that there was no slight "rattle" heard after objective reassembly

David G. gave us some good advice on troubleshooting, which we followed yesterday:

  • Removing the cell from the objective, and checking the shims and air gap between the crown and flint
    • Shims were in good shape, and air gap was even around the objective perimeter
    • Using a template, found all three to be symmetrical (120 degrees apart, plus or minus a degree)
  • Checking the three support points at the bottom of the cell for proper placement
    • Using a template, found all three to be symmetrical (got similar spacing results as for the shims)

We also cleaned the inside of the cell: 

  • Removed a lot of dirty film from the inside cell walls 

When we re-assembled the objective, we ensured that the objective shims and the lower support points were aligned.  The objective went in on the first try with no difficulty.  We also ensured that the objective retaining ring was installed "hand-tight, then backed off slightly"...and we did get the "slight rattle" that had been missing previously.

 

Newton rings were symmetric and centered...and we had three straight lines on the DPAC testing!  (...and there was great rejoicing! grin.gif ).  It's possible that one element may have very slightly cocked in the cell during last week's activities, due to the dirty residue on the cell walls...that, and/or the misalignment of the objective shims to the cell support points, may have been the cause of last week's problems.

 

As a final (unrelated) step, we cleaned and tested the 3" f/6 objective for the finder on the 6" Spacek.  I had not been able to disassemble the finder previously, but with Johann's help we got it apart, and found that one of the objective shims was out of the air gap entirely (was on the edge of the objective...we reinstalled it after cleaning both elements).  During reassembly, Johann demonstrated for me what DPAC results for a reversed objective and flipped element looked like...as Dave G. says, DPAC will immediately show you those conditions if they exist!  Finder is reassembled, shows fine images, and is reinstalled on the 6" OTA.

 

All in all, a great day!  Many thanks to Johann, for his support and his patience teaching a refractor "newbie" about optical maintenance and testing!

 

John   

Will be a fantastic day when you get the scope back together and test it under the stars.  Also, I was humbled by the focal length of the Tinsley.  Here I thought an 8 foot long bench would have been long enough to test the objective.  I had to rig up an extension to bring the ronchi screen with the LED out far enough to test the objective. 


Edited by starman876, 25 February 2019 - 08:19 AM.

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#109 DAVIDG

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Posted 25 February 2019 - 12:12 PM

 Glad to hear that when properly assembled the lens tested well. You saw the power of Double Pass Autocollimation testing and it showed you that now things are  correct. Without out, it would have been much more difficult to isolate the problem and fix it or worse it might not never have been corrected !  With the test showing three straight Ronchi bands the lens will give an excellent image.

  As I have said many times, test your optics and don't assume anything about their quality, spacing, and alignment marks. Let the test tell you when things are correct. 

 

                 - Dave 


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#110 Bomber Bob

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Posted 25 February 2019 - 12:27 PM

With all the refractors I've tested with DPAC, only 3 showed major issues.  And in those cases, rotating one of the elements fixed the lens:  the Lafayette 76x910, the Goto 60x1200, and that unknown 5" F5 triplet.  Would I have seen these errors visually -- without DPAC?  Yes.  But correcting lens alignment outdoors is much harder & time-consuming than indoors and mounted on the rig.

 

Thanks again, Dave, for encouraging us to check -- and see -- for ourselves.

 

Oh, and when the spacers are missing, the DPAC rig is the best place to experiment with different materials & thicknesses. 


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