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85mm f/15 doublet

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#1 chantepierre

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Posted 23 November 2021 - 04:01 PM

Hello dear ATMers,

I'll be tackling a new project in the next days, weeks, and it will of course span to months like any project : grinding and polishing of a 85mm f/15 littrow doublet, with blanks that David (davidc135 here) sent me in exchange for a silicone pitch mold. It seems a great winter project, not taking too much space and doable quite quietly compared to a bigger mirror. Thanks again David.

 

Quoting David :

> The flint R3 is used to grind the crown surfaces R1 and R2 whose RoC are all .4 of the f.l of the objective. A glass tool is required for plane R4.

Thicknesses of each lens don't matter. Wedge may be less than .025mm but even that doesn't need to matter precisely if the cell is supplied with centering grub screws. With these the objective can also be adjusted to correct atmospheric dispersion if wished.

 

And Wikipedia :

> Le doublet Littrow consiste en une lentille crown équiconvexe (R1=R2), suivie d'une lentille flint avec R3=-R2 et la dernière face plate. Ce doublet peut produire une image fantôme entre R2 et R3 en raison des rayons identiques. Il peut produire également une image fantôme entre le rayon infini R4 et le bout du télescope.

 

I have three pieces of glass :
- one white
- one with a deeper yellow tint
- and a blue one (a float glass tool).

 

260015214_646368649699262_138503842089274190_n.jpg

(The picture reads crown/flint/ordinary plate glass)
I guess that the white glass is the crown element, the yellower is the flint, and the blue will help plane R4.

 

I have a spherometer and will build a wedge-checker from plans I've seen online.

I also have an 1966 ukrainian flat found on Ebay that is supposedly flat enough if we trust its handwritten certificate.

 

My assumptions :

 

- I will use the flint element to grind both surfaces of the crown element, so I should try to avoid thinning it too much, and alternate working R1 and R2

- R3 can be figured to a sphere of the correct ROC like a telescope mirror with Foucault / Ronchi / Bath tests, then used for interference testing of the R1 & R2 surfaces

- R4 must be made like a flat, so the A/B/C method described in the books should work, with surface A being R4, surface B being the front of the float glass tool, and surface C being the back of the float glass tool.
- I should be able to test R4 against my known good flat

- The grinding experience of the unfinished lens I started with a friend (he has no time to continue at the moment) should allow me to control edge chipping to a minimum.

 

It seems logical to the newbie that I am that I should work all surfaces to the same grit before moving to a finer one, but maybe this isn't the way ?

 

I wish you clear skies,
Lucas


Edited by chantepierre, 23 November 2021 - 04:06 PM.

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#2 davidc135

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Posted 23 November 2021 - 08:26 PM

Good luck with the achromat. Should be a fun project.

 

The flint is much denser than the crown, s.g of 3.6 against 2.6

 

It's better to use one side of the float tool to rough grind r1 of the crown after which the flint r3 (which has already rough ground r2) will do all the smoothing on both r1 and r2. Otherwise the flint will be thinned unduly.

 

I would just use the other side of the tool to work the flat, in conjunction with the spherometer to begin with, and later with the flat. Is the flat transparent on both sides? I'm assuming so. (In the three plate method 3 separate discs of glass are needed.)

 

The first job would be to flatten all four lens surfaces and chamfer the edges. Work on any wedge from the start.

 

It sounds right to work all surfaces at each grit stage before moving to a finer one.

 

Because of astigmatism I always polished work surface down and so r2 and r3 became mutually concave. I think this prevented ghost images but (in theory) caused some over-correction of colour. In practice, in an F/15 achromat, I doubt if it matters but you could always make r4 five waves or so convex to compensate.

 

David


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#3 dan chaffee

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Posted 23 November 2021 - 10:24 PM

If you haven't worked flint glass, there are some things to consider. It chips and

scratches at the slightest provocation. Don't go light on the edge bevel; 1.5-2mm

minimum width. I didn't do this on my last refactor and regretted it.  It takes

longer to polish than the crown.   Should be a fun project. Littrows can be oiled and you can

even test the lens without completely polishing R2 and R3 with oil.  I would plan

on oiling it anyway, you'll get 90-91% transmission without coating.


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#4 chantepierre

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Posted 24 November 2021 - 02:20 AM

Thank you David and Dan.
Can I use a large (50cm or so) marble plate to grind the surfaces flat as a first step ?
I will find a suitable grindstone to do larger bevels than what I am used to !

David, both sides of the flat are transparent, I will follow your suggestion to use one side of the float to grind a convex surface.

Lucas

Edited by chantepierre, 24 November 2021 - 02:21 AM.


#5 dan chaffee

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Posted 24 November 2021 - 02:34 AM

Can I use a large (50cm or so) marble plate to grind the surfaces flat as a first step ?
 

Yes. I would definitely monitor the wedge amounts; it makes things simpler and

if you ever do larger refractor projects, it's a good thing to get used to keeping under

control.
 


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#6 chantepierre

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Posted 24 November 2021 - 03:19 AM

So, according to David's post quoting the densities of the glasses, I mislabeled the flint and crown blanks. This starts well !

 

To be sure :

- the biconvex lens must be made in the crown blank, which at the same size is the noticably lighter one

- the plano-concave lens must be made in the flint blank, which is the heavier one

 

Is that right ?

 

Dan, I have seen plans for wedge-meters devices online, using a micrometer dial and a spinning plate, with "collimation screws" allowing to make the spinning plate and the dial coplanar. Would that work ? It seems quite simple to build.

 

Lucas


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#7 dogbiscuit

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Posted 24 November 2021 - 03:33 AM

Will be a fun project.

Curious about the glass types, manufacture and catalog number?


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#8 davidc135

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Posted 24 November 2021 - 03:53 AM

So, according to David's post quoting the densities of the glasses, I mislabeled the flint and crown blanks. This starts well !

 

To be sure :

- the biconvex lens must be made in the crown blank, which at the same size is the noticably lighter one

- the plano-concave lens must be made in the flint blank, which is the heavier one

 

Is that right ?

 

Dan, I have seen plans for wedge-meters devices online, using a micrometer dial and a spinning plate, with "collimation screws" allowing to make the spinning plate and the dial coplanar. Would that work ? It seems quite simple to build.

 

Lucas

Yes to the question.

 

I think there is a simpler jig for monitoring wedge. I'll put up a drawing later today. The lens is placed on three supports eg ball bearings at 120 degrees and is located by two lateral stops at 90*. The micrometer is fixed over the lens near its edge. Edge thickness can quickly and accurately be measured at 4 positions as the lens can be easily popped on and off.

 

David


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#9 dan chaffee

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Posted 24 November 2021 - 03:59 AM

So, according to David's post quoting the densities of the glasses, I mislabeled the flint and crown blanks. This starts well !

 

To be sure :

- the biconvex lens must be made in the crown blank, which at the same size is the noticably lighter one

- the plano-concave lens must be made in the flint blank, which is the heavier one

 

Is that right ?

 

Dan, I have seen plans for wedge-meters devices online, using a micrometer dial and a spinning plate, with "collimation screws" allowing to make the spinning plate and the dial coplanar. Would that work ? It seems quite simple to build.

 

Lucas

You have the lens and their curves correct.

 

If I understand your wedge meter proposal, it sounds reasonable. You should

verify the accuracy on several objects of known thicknesses, such as ball bearings.

As long as it measures the edge thickness at exactly the same contact radius for

each measurement, it should work. Even a good quality micrometer will work if

you set up a jig to hold the lens in place perpendicular to the calipers as you check

several points along the circumference.


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#10 davidc135

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Posted 24 November 2021 - 03:59 AM

Will be a fun project.

Curious about the glass types, manufacture and catalog number?

The glasses from Chance Pilkington date from the early eighties and are their hard crown 519604 and dense flint (F2) 620364. Schott's BaLKN3 is close to the crown but I think slightly different.

 

David


Edited by davidc135, 24 November 2021 - 04:01 AM.

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#11 dogbiscuit

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Posted 24 November 2021 - 05:18 AM

I found this in a file I have with various glass manufacturers from 1951.

 

Chance519604.png

 

The flint is in the Pilkington catalog in OSLO.


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#12 rflrs

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Posted 24 November 2021 - 10:16 AM

I just checked my old Gee-Wyld tool.  It has:

 

              HC519604          DF620364

 

lambda^0    2.2755233E+0      2.5552054E+0
lambda^2   -8.7399531E-3     -8.6205092E-3
lambda^-2   1.1317492E-2      2.2547765E-2
lambda^-4   2.7778083E-4      8.7146250E-4
lambda^-6  -1.5068080E-5     -2.7370738E-5
lambda^-8   1.0122437E-6      4.9410668E-6

 

as coefficients for the "Schott" formula, i.e.: the
square of the [relative] refractive index is the
sum of coefficients (given) times powers of the
wavelength (lambda, in microns).

 

Richard


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#13 dan chaffee

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Posted 24 November 2021 - 01:44 PM

The glasses from Chance Pilkington date from the early eighties and are their hard crown 519604 and dense flint (F2) 620364. Schott's BaLKN3 is close to the crown but I think slightly different.

 

David

I thought the prescription for glasses for a Littrow was K7 F2.  How close is K7 to BaLKN3?


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#14 davidc135

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Posted 24 November 2021 - 02:22 PM

K7 is 511604. Perhaps it's as good as the hard crown which I don't think is available anymore.  David


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#15 davidc135

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Posted 24 November 2021 - 02:33 PM

A simple jig to measure wedge. It's probably better to have the micrometer and one of the three supports in line.

 

PB241792.JPG

 

David


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#16 rflrs

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Posted 24 November 2021 - 03:55 PM

I thought the prescription for glasses for a Littrow was K7 F2.  How close is K7 to BaLKN3?

The simple 85 mm f/15 solutions are (with t2=0):

 

                HC519604/DF620364             K7/F2

 

R1                 525.088                   507.773

t1                     9.5                       9.5

R2=R3             -535.018                   -536.97

t3                     6.5                       6.5

R4                 37318.9                   29492.6

 

If the solution is "bent" until the crown is isoconvex, that radius is 530.006 mm for the

HC519604/DF620364 pair and 521.964 mm for the K7/F2 pair.  This amount of bending

makes the last radius 109616 mm for the HC519604/DF620364 pair and -50930.9 mm

for the K7/F2 pair.  It is slightly better for the HC519604/DF620364 pair when it is

restricted to a Littrow solution.

 

Richard


Edited by rflrs, 24 November 2021 - 04:11 PM.

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#17 Mike I. Jones

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Posted 24 November 2021 - 04:50 PM

The purely all-spherical Littrow design for these glasses doesn't look good.  Lots of SA and primary color.  If you allow R4 to be very slightly curved and aspheric, you can get a much better performance, while still leaving R2 = R3 = -R1.

 

Attached File  Lucas 85mm f_15 achromat.len   780bytes   15 downloads


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#18 lylver

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Posted 24 November 2021 - 09:56 PM

Hi Lucas, don't bother about coma, I propose a F'C' to Oiii-Halpha design (like Mike to more like planetary)

 

Step 0

make a flat back with the flint r4=0, this will make a good wedge for all.

Protect the flat-back : adhesive or anything that will make some space between the surface and potential abrasive. Alcool or acetone at end will not cause problems if needed to remove glue remains.

 

Step 1

do r2=r1=r3=520mm.

Control the wedge as stated, include the noname glass in the process ; flint is much softer beware !

 

At the end of this first step you would have an equiconvex crown, two mirror like surface flat-back.

This will give you a mirror surface to control radius by focal length or biconvex easy to check by Silbermann 2 fl method

 

Step 2 :

Then lengthen r1 to 535mm minimum to 544mm maximum using the noname glass. The difference of curvature is easy to check on a foucault between the flint and the ordinary glass tool.  The ratio of curvature is the key if you have an other base fl than 520mm.

... make it fit in between for your chromatism (1283mm to 1308mm fl as specs with this)

The noname glass will permit to retune if curvature is to be modified.

 

I recommend 540mm as a basis (FC "on foot"), polish enough for testing fl and chromatism with the spacer.

 

Spacing is 1mm for r1=535mm to 1.5mm for r1=544mm. Easier tolerance so you can adjust at end.

 

At this f/D tin spacer will work but prefer 1mm 3D printed plastic ring and add after 1 to 3 tin 0.2mm small spacers pile on the flint when tuning sharpness. (spherical aberration is easy to tune on 85f15)

 

This will do a high strehl on edge of the Moon coma present but not bothering. High f/D are nice for any fabrication errors

 

Wedge control and patient regular move are the key : let the randomness do the job. Don't push move to much.

Attached Files


Edited by lylver, 24 November 2021 - 10:41 PM.

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#19 davidc135

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Posted 25 November 2021 - 11:22 AM

The advantage of using these glasses and the any three equal curves and a plane design is to keep everything very simple whilst ending up with an fine objective.

 

In the simple approach:

 

It's an easy thing to remove wedge.

 

Accept coma which is modest in an F/15 achromat- on the scale of an F/10 Newt.

 

Avoid astigmatism by polishing face down. Some of the fine grinding must be face up so care here.

 

Other than for R4 you don't need to measure any curve very carefully, so long as they are all the same. My figure of .4 f.l for R1,2 and 3 is a bit low, 0.415 looks better. These equal radii can be 520,530 or 540.It matters no more than with a Newt.

With flint R3 fine grinding both R1 and R2 there's no freedom to independantly adjust these curves.

 

No need to worry too much about residual design S.A. Each surface may need to be checked for zones, tde or large aspericity and remedied to the point where the accumulated S.A errors of figure and design can be removed in one go from one surface with the objective being Foucault tested in double pass. In this way the air gap is fixed at (nearly) zero or the objective can be oiled.

 

Which leaves CA. In post 16 rflrs gives figures of 530mm for R1,2 and 3 with R4 being 109616mm ie only 8microns or 14.5 waves (550nm) deep and I see from the sign that it is concave, if I've read it right.

I found that by fine grinding the crown with the flint and by always polishing face down each surface became slightly more concave than planned and so, by accident,the colour correction ended up about right.

Seems my earlier idea to make R4 a touch convex was wrong.

At F/15 and small aperture I'd have thought there was plenty of latitude regarding CA. IIRC, Ellison thought a 1 thou or 25micron error sag on one surface wouldn't matter in practice- 45 waves! but maybe my memory is faulty.

 

But in post 17, Mike seemed more emphatic- lots of CA? I can't access the file, unfortunately.

 

David


Edited by davidc135, 25 November 2021 - 11:26 AM.

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#20 DAVIDG

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Posted 25 November 2021 - 12:15 PM

  You don't need to use the three disk method to grind  R4 flat. Since you have spherometer you can do it with just the flint blank and the tool glass if you don't have an optical flat to zero it on.

  You start by grinding R4 against the tool to get them to mate and be fully ground to the edge. Keep flipping them over during this process to stay as close to flat as possible . 

   With your spherometer,  zero it on R4 and measure the curve on the tool. You will  be measuring 2x the difference from mechanically flat. If  R4 is concave that means the curve on the tool is convex. So when you measure it you'll see a positive deflection of the reading the spherometer. If the R4 is convex, then the curve on the tool is concave and you see a negative deflection on reading of the spherometer.  So knowing if the R4 is either concave or convex, you can then grind with R4 on top of the tool, or the tool on top of the R4 to make the curve go closer to flat. The goal is to reduce the error by  50%. So when the absolute value that you measure with the spherometer has been reduced by  50% you then switch to the next finer grit.   When you get down to 5um grit  the   spherometer should read close to 0  when you  measure the tool. Now R4 is mechanically flat enough to polish it optically flat.

 

                 Happy Holidays !

                   - Dave 


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#21 rflrs

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Posted 25 November 2021 - 01:32 PM

If you are already filled with designs, do not read this message!

 

The design I previously gave was straight out of the Gee-Wyld
tool -- something from the previous millennium, but sometimes
still useful.

 

I generated two additional designs (using ZEMAX).  The first
design corrects across the Oiii-H_alpha band:

 

Attached File  Lucas 85mm f_wcoma_Oiii_halpha.len   878bytes   2 downloads

 

The middle wavelength here is about 567 nm; spherical aberration
is pretty well corrected at that wavelength.  For those who are
unable to read the OSLO file, the prescription is:

 

SRF      RADIUS       THICKNESS   APERTURE RADIUS       GLASS  SPE  NOTE
 

AST        --         -1.740279 S   42.500000 AS          AIR    

  2    519.824168     10.000000     45.000000        HC519604    
  3   -519.824168 P    0.600000     45.000000             AIR    

  4   -519.824168 P    7.000000     45.000000       DF620364A C  
  5    2.7345e+04    1.2607e+03     45.000000             AIR    

IMS   -465.387988        --         21.650000                   *

The second design corrects across the F-C band:

 

Attached File  Lucas 85mm f_wcoma_F_C.len   879bytes   3 downloads

 

The middle wavelength is aboust 10 nm away -- 557 nm -- from the
value for the first design.  The text prescription is:

 

SRF      RADIUS      THICKNESS   APERTURE RADIUS       GLASS  SPE  NOTE
 

AST        --         -1.722827 S   42.500000 AS          AIR    

  2    525.072638     10.000000     45.000000        HC519604    
  3   -525.072638 P    0.500000     45.000000             AIR    

  4   -525.072638 P    7.000000     45.000000       DF620364A C  
  5    4.3348e+04    1.2611e+03     45.000000             AIR    

IMS   -466.067252        --         21.650000                   *

 

In both OSLO files, I have included the five wavelengths for both designs.  The

refractive indices for HC519604, given directly in OSLO, are:

 

      0.50068400      0.56757352      0.65627250      0.48613270      0.55797890
    1.5238762367    1.5199210856    1.5163689293    1.5249603201    1.5204036311

  

The focal ratio here is f/15.  This makes spherical aberration fairly mild, I think.

In both of these designs, the correction on the last lens surface is less than one

nanometer to bring both the 70% zone and 100% zone coincident with the central zone

at the middle wavelength.  In neither design was an attempt made to correct coma.

 

Richard

 

 


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#22 davidc135

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Posted 25 November 2021 - 03:56 PM

If R4 is made plane:

Removing the 0.5mm air gap in the 2nd prescription in post 21 is some of the way equivalent to altering R4 from plane to 4.3348e+4 concave (20.8microns sag), as regards CA.

 

From post 18, 0.5mm air gap is equivalent to 14microns flint concave sag leaving 6.8microns total to be taken off the centres of each surface during fabrication. That's if the R4 = plane/gap =0 option is chosen. Advantages both ways.

 

David


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#23 chantepierre

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Posted 26 November 2021 - 12:13 AM

Hello David, David, Mike and Richard, and thank you ! Merci beaucoup Myriam !

 

As much as I start to understand mirrors with the two I've done until now, lenses in regard to their chromatic properties still are way over my head, and your diverging designs will enable me to study them. You provided me with a lot of high quality information on wedge-meters, flat making, and achromat designs and I am highly thankful for that.

 

I think I'll go with Myriam's plan because it has been outlined in simple steps while addressing C.A. in a quantitative way ; while I will try to get a grasp of OSLO Edu and read the designs that you all provided, I'm not qualified to assess them yet.

 

I'm still gathering materials and organizing my workspace, and will likely start to grind this monday, as a lens total newbie.

 

On the side, we are starting to grind our first mirror with my club tonight as I will introduce members to this process ; this will only be possible because people like you (and some of you) provided me with high quality guidance for mirrors here in the recent past !

 

This is how knowledge lives on ; this place rocks.

 

Edit : note that I "like" each specific post as my system to be sure I've read it. I'll periodically check if I missed something.

 

Clear skies,

Lucas


Edited by chantepierre, 26 November 2021 - 12:19 AM.

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#24 Mike I. Jones

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Posted 26 November 2021 - 01:57 AM

Oh man, if you could allow R1 to be different while keeping R2=R3, and allow R4 to be very slightly convex!

 

Attached File  Lucas 85mm f15 no coma.len   750bytes   10 downloads

 

Lucas 85mm f15 no coma - prescription.jpg

 

Lucas 85mm f15 no coma - spots 1000x667.jpg

 

 

 

 

 

 



#25 lylver

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Posted 27 November 2021 - 03:52 AM

For fun : a "Clark" like version and the aspheric cemented version (5um center grind on the flint, NOA65 would match)

Attached File  Lucas-Clark 85mm f_15.len   758bytes   3 downloads

Aplanetic & best Petzval sum, flat-back (r4=0), lateral color is contained.

Clark.JPG

Clark work well on CaK band (Venus imaging)

CaK.JPG

Attached File  Lucas 85mm f_147asph.len   820bytes   2 downloads

A bit tricky to do but feasable for the flint : check as a telescope mirroir for parabolization, conic is -0.565.

Cemented-aspheric.JPG


Edited by lylver, 27 November 2021 - 04:24 AM.

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