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8" f/9 apochromatic triplet + 2 element field flattener

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#1 Mike I. Jones

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Posted 13 January 2018 - 11:18 PM

Hello refractor folks,
Normally I'd post this over in the ATM/optics forum, but the more posts I read in the Refractor forum, the more I thought this might be interesting to some readers here. A few years ago I traded for a beautiful, pristine 8.3" diameter blank of Schott KzFS5, complete with 5-wavelength melt data. I did several designs using it, and this one was the best, and the one I'll eventually get around to making. It goes a step farther than normal 3-color crossing apos, and achieves true 4-color crossing performance that also holds up well over the 0º-40ºC temperature range with slight refocusing. I also designed a nice 2-element field flattener for it that completely covers a full-frame DSLR sensor format with essentially diffraction limited imagery.

You are welcome to the design prescription, to play with, build your own, scale down or up, whatever you like. This is the optical design. I'll post up the layouts, spots, Strehl plots and focus shift curves next.
Mike

8f9 apo + FF prescription.png


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#2 Mike I. Jones

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Posted 13 January 2018 - 11:19 PM

These are the layout plots, without and with the field flattener.

 

8f9 apo + FF layouts.jpg



#3 Mike I. Jones

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Posted 13 January 2018 - 11:21 PM

This design performance is athermal with slight refocusing over temperature.  Here are the spot diagrams for +20ºC, 0ºC and +40ºC for the 8" f/9 apo by itself over a narrow field, then with the 2-element field flattener over the full-frame 35mm sensor format.

 

8f9 apo + FF spots.jpg

 



#4 Mike I. Jones

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Posted 13 January 2018 - 11:23 PM

These are the chromatic shift curves, plotted at 82% pupil radius, without and with the field flattener.

 

I have no idea when I'll get around to making this lens, but I hope it's before too long.  I still have to order the other glass blanks, with melt pedigrees, and re-optimize one more time for real glass.  This should be a really nice refractor.

 

8f9 apo + FF focus shift.png


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#5 Mike I. Jones

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Posted 13 January 2018 - 11:26 PM

Here are the Strehl curves without and with the field flattener.  Sorry for the small font - Zemax doesn't offer a way to scale it yet - but you can click on it for a larger view.  Notice that all the vertical Strehl scales start at 0.8.

 

8f9 apo + FF Strehl analysis.jpg



#6 ji4m

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Posted 14 January 2018 - 12:17 AM

That's a fascinating design.  Will the lens cell design be more challenging with the large air space?  I've read enough about your efforts to recognize your experience - which glass type gives you the most concern for the build?  



#7 Astrojensen

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Posted 14 January 2018 - 03:10 AM

This is quite similar to the Zeiss B objective. Fascinating! What are the tilt/wedge and spacing tolerances? Be careful not to design something that is virtually impossible to accurately collimate, like the Zeiss B... The Zeiss B has spacing tolerances measured in one-digit microns. 

 

http://www.uni-tuebi...-B-Objektiv.pdf

 

 

Clear skies!
Thomas, Denmark 


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#8 Mike I. Jones

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Posted 14 January 2018 - 12:04 PM

That's a fascinating design.  Will the lens cell design be more challenging with the large air space?  I've read enough about your efforts to recognize your experience - which glass type gives you the most concern for the build?  

I don't think the large airspace will be an issue.  I'll machine a spacer of the right width to hold the L2 and L3 elements at the right center thickness, after all lenses are finished.

 

I've never worked S-FPL55, and I'll have to be careful due to its high CTE.  Grinding isn't an issue as no significant thermal gradients are generated, but pressing a pitch lap into contact will take longer and involve slowly warming the glass.  I'm using diluted, ball-milled ZOX like Carl Zambuto uses, so sleeking shouldn't be an issue.

 

This refractor, like other one-off projects, will be iteratively optimized.  Tolerancing is done, so I know how precisely I have to hit all the radii and center thicknesses.  Controlling and minimizing wedge is easy with large lenses like these, and will be held to <10µm.  The first redesign will be after I get all the glass blanks and their melt pedigrees.  Then as I "finish" each radius to within tolerance, the remaining radii, center thicknesses and airspacing are tweaked, until all lens elements are done.  The only remaining degrees of freedom are then the airspaces.  That's when I make the spacer rings.



#9 mogur

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Posted 14 January 2018 - 06:03 PM

Looks like that FF really throws a monkey wrench into things, especially at lower temps. tongue2.gif



#10 Mike I. Jones

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Posted 14 January 2018 - 08:43 PM

True, I could nudge the weighting for this design to try to even out the chromatic performance over temperature a little more.  But the field flattener is still doing a very nice job of cleaning up the imagery.  These spot diagrams are for the 8" f/9 lens without and with the field flattener for the same FOV, a 1.4" circular FOV that encloses a full-frame DSLR sensor.  The improvement in image quality with the field flattener is quite dramatic.  The lens by itself has super-sharp imagery near the center of the FOV, but there aren't enough degrees of freedom in a closely spaced triplet to correct for field curvature.  The human eye isn't bothered as much by field curvature as the eye can accomodate a few tenths of a diopter over the eyepiece FOV with no significant fatigue.  All your close-spaced apo triplets have similar field curvature.  My Tak TOA-130 even shows tiny smears for stars in the edges and corners of the FOV, which go away when I use the Tak field flattener.

 

8f9 apo + FF spots over 1_4in FOV.jpg



#11 Mike I. Jones

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Posted 14 January 2018 - 08:46 PM

MTF curves show that the field flattener restores image contrast as well as the lens by itself at the center region of the FOV, and over the whole 35mm format.  Photos taken with this lens and FF should be pinpoints right to the edge.  

 

See, what I'm doing here is trying to get me to start working on it sooner. grin.gif

 

8f9 apo + FF MTF.jpg



#12 starcanoe

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Posted 14 January 2018 - 09:33 PM

That's some sweet stuff right there.

 

Being your own boss.The problem is the employees are lazy layabouts and the boss is a jerk :)


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#13 CounterWeight

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Posted 15 January 2018 - 11:24 AM

Certainly fascinating! thanks for posting in here.  Also curious about how the large air gap / cell will play out in reality. Am curious about if you will have any coatings applied?  I have only dabbled in OSLO and that was some time ago, does OSLO have provision for entering coatings?  Thank you.


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#14 Mike I. Jones

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Posted 15 January 2018 - 02:47 PM

Certainly fascinating! thanks for posting in here.  Also curious about how the large air gap / cell will play out in reality. Am curious about if you will have any coatings applied?  I have only dabbled in OSLO and that was some time ago, does OSLO have provision for entering coatings?  Thank you.

I could be wrong, but I don't think the freeware version of OSLO can do coatings.  I do everything in the "professional" version of Zemax, and translate into OSLO-EDU when someone asks.  I think I'll get the lenses coated, but that's always a sweaty situation.  Part of the price people pay for commercial refractors is risk money to cover the supplier's costs of replacing any lenses fractured during coating.  It's a wonder they don't cost twice what they do.  With my own hand-made lens, if a coater breaks a finished lens, it's pretty much "sorry about that", and I'm out a lens.



#15 ji4m

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Posted 15 January 2018 - 03:36 PM

The risk inherent in coating a lens only makes sense to me if the potential reflection causes trouble (ghost image perhaps).  The loss in the first surface, for example, seems innocuous, just a little less transmission.  Maybe the last surface loss is similar.  Is that reasonable?



#16 Mike I. Jones

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Posted 15 January 2018 - 04:43 PM

If I didn't know some coaters I trust with my hard-earned lenses, I probably wouldn't have them coated.  The increase in transmission would only be a few percent.  However, the KzFS5 is a "stainer", and it's better to get it AR coated to help environmentally seal the surface.  It's in the middle rather than being exposed, but it's still a good idea to me.



#17 bratislav

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Posted 15 January 2018 - 07:11 PM

Mike, why so slow? F/9 is almost painfully slow, and definitely quite long by photographic standards (presenting a long moment arm to any mount - even heavyweights from AP and Paramount are going to balk at that). Remember new breed of CCD/CMOS cameras have very small pixels and are very unforgiving.

I'd let extremes "bleed" out of theoretical Airy disk and get it down to at least f/8, the existing top shelf APOs are in the same boat (most likely worse) as far as residual chromatic aberration is concerned, yet noone complains about color in their big AstroPhysics, TECs and like. CFF/Pal Gyulai is even pushing their 230mm to f/7 !

Unless of course you intend it as mainly visual instrument, with only occasional use for astrophotography. Which would be shame (almost), large APOs are formidable imaging instruments.

 

That large gap is going to be tough to maintain in the cell that has enough give not to crack the lenses in sub zero temperatures...

Great project, good luck with it.


Edited by bratislav, 15 January 2018 - 08:07 PM.


#18 Mike I. Jones

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Posted 16 January 2018 - 01:46 PM

Mike, why so slow? F/9 is almost painfully slow, and definitely quite long by photographic standards (presenting a long moment arm to any mount - even heavyweights from AP and Paramount are going to balk at that). Remember new breed of CCD/CMOS cameras have very small pixels and are very unforgiving.

I'd let extremes "bleed" out of theoretical Airy disk and get it down to at least f/8, the existing top shelf APOs are in the same boat (most likely worse) as far as residual chromatic aberration is concerned, yet noone complains about color in their big AstroPhysics, TECs and like. CFF/Pal Gyulai is even pushing their 230mm to f/7 !

Unless of course you intend it as mainly visual instrument, with only occasional use for astrophotography. Which would be shame (almost), large APOs are formidable imaging instruments.

 

That large gap is going to be tough to maintain in the cell that has enough give not to crack the lenses in sub zero temperatures...

Great project, good luck with it.

Hi Bratislav, yes, I'm wanting it more for a very nice visual scope than for photography.   I have a Tak TOA-130 and field flattener so the photography scope is covered.  All scopes ride on a Paramount ME.

 

The thermal design model currently uses 6061 aluminum for the cell, but I haven't frozen on that just yet.  Whatever the cell metal, I'll be using Artus shim stock in direct contact with the lenses.  My mantra is to never allow metal to touch optical glass of any kind.  The Artus website is http://www.artuscorp.com/.  I used Artus shims for decades while at Lockheed Martin for optical systems with far worse temperature extremes, and am very comfortable with it.  



#19 calypsob

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Posted 16 January 2018 - 01:52 PM

Mike, why so slow? F/9 is almost painfully slow, and definitely quite long by photographic standards (presenting a long moment arm to any mount - even heavyweights from AP and Paramount are going to balk at that). Remember new breed of CCD/CMOS cameras have very small pixels and are very unforgiving.

I'd let extremes "bleed" out of theoretical Airy disk and get it down to at least f/8, the existing top shelf APOs are in the same boat (most likely worse) as far as residual chromatic aberration is concerned, yet noone complains about color in their big AstroPhysics, TECs and like. CFF/Pal Gyulai is even pushing their 230mm to f/7 !

Unless of course you intend it as mainly visual instrument, with only occasional use for astrophotography. Which would be shame (almost), large APOs are formidable imaging instruments.

 

That large gap is going to be tough to maintain in the cell that has enough give not to crack the lenses in sub zero temperatures...

Great project, good luck with it.

I think you could work around the slow aperture with some of the modern high gain cmos bodies.  Narrow band may be a challenge but at a darksite you could surely pull off sub 120s RGB data


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#20 Riccardo_italy

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Posted 16 January 2018 - 03:08 PM

Mike, why so slow? F/9 is almost painfully slow...

The larger the diameter, the slower the f-ratio required to keep constant color correction.

 

An 8" f9 is slightly more challenging - in terms of color correction - than a 6" f7. No triplets I knows with 6" aperture are faster than f7.



#21 bratislav

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Posted 16 January 2018 - 07:40 PM

 

 

 

No triplets I knows with 6" aperture are faster than f7.

 

Maybe you haven't looked too hard. There's plenty of big, fast triplets on the market, perhaps most extreme being APM/LZOS 304/2280 (12" f/7.5!!!) and 203/1420 (8" f/7), with above mentioned CFF 230 (9") at f/7. APM/LZOS used to sell 150/900 Fluorite triplet (6" f/6), but obviously there is no market demand for it.

For visual, one can push the envelope even harder, as our eyeballs aren't particularly sensitive at <450 and >650 nm (unlike CCD). But if you want (and can afford) the VERY best, Mike's APO is going to be hard to beat. 

BTW, I couldn't get 4 color crossings, but my KzFS5 data aren't obviously as precise, and my ancient copy of Zemax only allows simple glass model (nd, Vd and Pgf). 




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