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Simple Triplet with only 2 glasses?

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#1 starcanoe

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Posted 25 January 2021 - 09:22 AM

Hi all.

 

Have some smaller optical glass blanks...3 inch give or take.

 

In terms of color correction is there any advantage of a triplet made of a glass with an abbe number of 35 ish and an abbe number of 55 ish over a doublet made of the same glass types?

 

I guess there is one advantage in that you can oil the inner surfaces of the triplet for better light transmission and or no ghosting. Also I suppose the additional degrees of freedom allow you to correct for coma and or other abberations.

 

Plus good practice for a larger refractor project later that uses the good and expensive stuff.

 

Also....in the f15 ish range plus or minus 5.

 

Can give better optical glass numbers later if needed.

 

Oh...or a triplet that uses three different glasses but none of them are abnormal dispersion to any great extent.



#2 jimhoward999

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Posted 25 January 2021 - 10:00 AM

There is probably no advantage to a triplet using normal dispersion glasses at F/15.  Do you know the exact glass types?  If so it could be ray traced quickly to verify for sure.



#3 ccaissie

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Posted 25 January 2021 - 12:40 PM

a 3" f/15 doublet is about as good as it gets re: secondary color.  Larger and shorter designs is where you need to control the secondary spectrum and require a special glass or a third glass.  Rutten and VanVenrooij "Telescope Optics" discuss this in great clarity and have some design examples.


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#4 sg6

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Posted 25 January 2021 - 12:58 PM

Think there is some advantage, you get to control (hopefully and to some extent) 3 wavelengths. Not sure you need to have 3 different glasses. Did design a triplet years ago and cannot recall if it had 2 glasses or 3 glasses.

 

It was a LONG time ago and the design was on paper with calculators - no computers!!!!

As said a long time ago. Those days just before the XT's came out, all 8 bits of them lol.giflol.giflol.gif The 16 bit AT's came out 3 years later grin.gif grin.gif grin.gif 

 

I would use/try 2 crown elements with a sandwiched flint.


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#5 davidc135

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Posted 25 January 2021 - 02:16 PM

Very much doubt if there's any improvement with splitting one of the two glass types into two components as regards secondary spectrum. Whilst keeping the objective an aplanat, substituting the extra degrees of freedom of the air gap by those of the third lens and gaining some extra light transmission by oiling, that could be so.

 

David

 

PS I've just seen a table in which a doublet and triplet apo using just the same two glasses are compared and the triplet is noticeably better. This in a thread 'Fluorite vs FPL53' started by Lucullus in Feb 20. PPS: It's down below posted by Jon Isaacs.


Edited by davidc135, 25 January 2021 - 03:22 PM.

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#6 jimhoward999

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Posted 25 January 2021 - 02:42 PM

I ray traced it assuming the two glasses to be Bk7 and F2.    It turns out these is a small advantage to the triplet.  The secondary spectrum is the same whether you use a doublet or a triplet.  It is about 1/2 wave between d and F-C.   The on-axis performance is the same,  doublet or triplet.    However, if you add and air space of about 1.5" between the Bk7/F2 doublet and a third BK7 element to make an air-spaced triplet you can improve the off axis aberrations.  You can readily correct over a 2" format with a triplet, whereas you have some monochromatic aberrations with a doublet.


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#7 starcanoe

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Posted 25 January 2021 - 02:49 PM

Thanks for the info.

 

Makes sense to me that the air space helps some.

 

Will dig in my glass pile later and see exactly what I have.

 

Thanks again for input folks.



#8 Jon Isaacs

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Posted 25 January 2021 - 02:55 PM

I don't know nothing.. :)

 

But if you look at Vlad's work on Polychromatic Strehl, it does seem like a 6 inch F/8 triplet with ZKN7/FPL-53/ZKN7 is considerably better than a 6 inch F/8 doublet with FPL-53/ZKN7.

 

How this translates to a standard achromat, I have no idea. Examples 61 and 63. 64 is a K7/FPL-51/K7 which is essentially identical to the FPL-53 doublet. 

 

achro_apo.PNG

 

https://www.telescop...romatic_psf.htm

 

Jon


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#9 starcanoe

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Posted 25 January 2021 - 03:00 PM

Thanks Jon...

 

But I think part of the reason those work better is because those glasses have abnormal dispersion..

 

Me...I just have an Abby Normal brain....



#10 starcanoe

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Posted 25 January 2021 - 03:07 PM

Okay...

 

Looks like I have 

 

FN 11  1.621   36.2

 

And I believe what an F2? blank?  Oh wait...it is an SK6 or N-SK6 1.620 60.3 (probably would not use that blank...)

 

SK 51 1.620 60.3

 

Crown...what exact type?  1.523    58.6

 

Another one  1.626    54.5

 

The FN11 and the SK51 and the Crown? 1.523  58.6 would  probably be the ones to work with....maybe the 1.626 54.5 if desperate ( the thing is WAY convex).


Edited by starcanoe, 25 January 2021 - 03:36 PM.


#11 Jon Isaacs

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Posted 25 January 2021 - 03:30 PM

Thanks Jon...

 

But I think part of the reason those work better is because those glasses have abnormal dispersion..

 

Me...I just have an Abby Normal brain....

 

The point is that splitting the flint into two separate lens does improve the correction. 

 

Jon


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#12 starcanoe

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Posted 25 January 2021 - 03:37 PM

Oh....I see....will have to look at that chart more carefully later....about off the computer for the day...



#13 jimhoward999

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Posted 25 January 2021 - 07:09 PM

The point is that splitting the flint into two separate lens does improve the correction. 

 

Jon

That is because in the example the lenses are 6" aperture and F/8 as opposed to 3" aperture and F/15.    The lenses in the F/8 system are stronger so there is more residual residual spherical aberration which is improved by splitting them.  

 

That is less so in an F/15 system.

 

Moreover,  FPL53 and ZkN7 are both really crowns even though ZkN7 is being used as the flint. Zkn7 is flintier than FPL53 but has similar partial dispersion.     Using to glasses with a small v-number split requires that the lenses be stronger yet for achromatization.  That all means splitting the power helps in that situation.  But not so much for an F/15 achromat with glasses far apart on the glass chart.

 

One man's $0.02


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#14 jimhoward999

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Posted 25 January 2021 - 08:11 PM

Okay...

 

Looks like I have 

 

FN 11  1.621   36.2

 

And I believe what an F2? blank?  Oh wait...it is an SK6 or N-SK6 1.620 60.3 (probably would not use that blank...)

 

SK 51 1.620 60.3

 

Crown...what exact type?  1.523    58.6

 

Another one  1.626    54.5

 

The FN11 and the SK51 and the Crown? 1.523  58.6 would  probably be the ones to work with....maybe the 1.626 54.5 if desperate ( the thing is WAY convex).

SK51/FN11...both obsolete glass types...would make a better than average achromatic.  If would only have 0.4 waves of secondary spectrum.    If you add in a 523586 element (not K5 but similar) behind, you could get a nice system corrected over a wide field.


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#15 starcanoe

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Posted 26 January 2021 - 12:09 PM

That makes sense that that larger aperture and stronger curves all are helped with a triplet vs a doublet. And that a triplet helps when you want a corrected field as well. Even with just two glasses.

 

Interesting that SK51 and FN11 are good but obsolete glasses...were they replaced with something better or is this another case of "you can't get that good stuff for reason XYZ anymore".

 

Another pairing I have.

 

A 1.573 / 57.4  close a BAK1 glass? paired with a SF (sk?) 12  1.649  / 33.8

 

And 

 

D56-27 1.755 / 27.7  paired with a Hoya 1.620 / 60.3    1228216

 

 

None of these have to work great...small blanks...slow f ratio...just something to play with to build up to a good 4 incher and 6 incher of which I have some very nice blanks for.

 

PS...funny thing I this really elaborate dream last night...it involved my stash of atm supplies and especially glass blank.....in the dream I had those babies stashed in every nook and cranny one could find.  I wish !

 

Minimal investment/ smaller/ easier project with low tolerance and just happy just works makes for a nice low stress project.

 

Thanks again for all the input folks !


Edited by starcanoe, 26 January 2021 - 12:11 PM.


#16 jimhoward999

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Posted 26 January 2021 - 06:55 PM

That makes sense that that larger aperture and stronger curves all are helped with a triplet vs a doublet. And that a triplet helps when you want a corrected field as well. Even with just two glasses.

 

Interesting that SK51 and FN11 are good but obsolete glasses...were they replaced with something better or is this another case of "you can't get that good stuff for reason XYZ anymore".

 

Another pairing I have.

 

A 1.573 / 57.4  close a BAK1 glass? paired with a SF (sk?) 12  1.649  / 33.8

 

And 

 

D56-27 1.755 / 27.7  paired with a Hoya 1.620 / 60.3    1228216

 

 

None of these have to work great...small blanks...slow f ratio...just something to play with to build up to a good 4 incher and 6 incher of which I have some very nice blanks for.

 

PS...funny thing I this really elaborate dream last night...it involved my stash of atm supplies and especially glass blank.....in the dream I had those babies stashed in every nook and cranny one could find.  I wish !

 

Minimal investment/ smaller/ easier project with low tolerance and just happy just works makes for a nice low stress project.

 

Thanks again for all the input folks !

Schott has dropped all but about 120 of their glass types and tweeked the formulas of those that remain for environmental reasons (lead free).

 

Both of your glass pairings would make fine achromats, with the BAK1/SF2 being slightly superior.  The CTEs of BAK1 and SF2 are close enough you could cement that doublet if you wanted.  And you dont really need a third element

 

Here is an example cemented prescription that works for an EFL of 1125mm

 

L1 R1 = 731.246mm cx

CT1 = 9mm

L1 R2 = -420.390mm cx

BaK1

 

L2 R2 = -420.390mm cc

CT2 = 6.35mm

L2 R2 = -2264.806mm cx

SF2


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#17 davidc135

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Posted 26 January 2021 - 11:10 PM

What would reasonable tolerances be for the above BaK1 and SF2 objective? In particular CT1.  David



#18 jimhoward999

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Posted 26 January 2021 - 11:34 PM

What would reasonable tolerances be for the above BaK1 and SF2 objective? In particular CT1.  David

Other than figure pretty loose.  ±0.25mm would be plenty good enough for the CTs.  Even looser would be okay.


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#19 dan chaffee

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Posted 27 January 2021 - 03:02 AM

I don't recall what your experience level is, but making an achromat is a lot

of work and most ATM'ers would consider it a long term project.  Personally, I

wouldn't make an achromatic objective that was smaller than what I could be excited about

using. My first attempt at making a doublet was a failure because I had an undocumented

flint that I thought was something other than what it was. It was 105mm in diameter,

which if it had been successful, should have done rather handsomely on the solar

system objects, at f/15. As for designs, I second the barium crown prescription, since the

secondary spectrum is usually about 20% less than with traditional crown/flint designs.

I believe that is what D&G uses in their lenses.


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#20 starcanoe

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Posted 27 January 2021 - 08:07 AM

Thanks a lot for that info Jim.

 

The SF12 one sounds like my best short term bet.

 

Yes Dan I'd agree that for most people most of the time that would be very sound advice.

 

But I have a pair of 6 inch F15 Jaegers lenses (which might could use some figuring...maybe..).  I have 6 inch and 5 inch F5 Jaegers len pairs (monster binoc time one day!) I have a 10 inch F8 scope with a mirror that has an off the charts figure. A planet killer 6 inch dob (well for a 6 inch). An 90 mm f8.8 scope that is pretty respecable. A Televue NP101. And a 12.5 inch F5 mirror that I might get up and running a truss/deep sky scope.

 

So any refractor I do make really isn't filling observing need niche.

 

Then I have blanks with good optical glass for a 6 inch refractor and a 4 inch refractor. 

 

Given those facts...for me I think it makes sense to just play with some cheap on hand glass I've got and build a small refactor or two. No pressure to get it up and running. No worries about messing up some expensive glass blanks ( I almost have a phobia about fracking up expensive stuff...particularly expensive stuff I got cheap that I'd probably never buy had I had to pay normal prices for ). As long as these cave man refractor(s) I build (if I get around to it) are fair or even just work okay at low powers I'll be very happy. And I think it will be good practice for me before I tackle the big ones with the good glass...



#21 starcanoe

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Posted 27 January 2021 - 09:29 AM

Jim...was that calculation for an SF2 or was it for an SF12 and SF2 was a typo.

 

Because it is an SF12 I have.

 

Thanks again !



#22 lylver

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Posted 27 January 2021 - 03:57 PM

A known old school two glass triplet : SYW Japan used a cemented semi-apo BAK1-KzFSN4-BAK1

SYW.JPG

In the native doublet form, the two glass combination has much spherical aberration, rendering it quite unusable under f/15 (if over 3" diameter) despite the exceptional dispersion matching.

A flat back triplet : https://www.cloudyni...plet/?p=7893434



#23 jimhoward999

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Posted 29 January 2021 - 03:31 PM

Jim...was that calculation for an SF2 or was it for an SF12 and SF2 was a typo.

 

Because it is an SF12 I have.

 

Thanks again !

I used SF2, but the prescription is almost the same for SF12

 

L1 R1 731.436mm cx

CT1 9.0mm

L1 R2 -420.526mm cc

BAK1

 

L2 R1 -420.526 cc

CT2 6.35mm

L2 R2 -2254.168mm cx

SF12



#24 jimhoward999

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Posted 29 January 2021 - 03:32 PM

I used SF2, but the prescription is almost the same for SF12

 

L1 R1 731.436mm cx

CT1 9.0mm

L1 R2 -420.526mm cc

BAK1

 

L2 R1 -420.526 cc

CT2 6.35mm

L2 R2 -2254.168mm cx

SF12

L1 R2 -420.526mm cx



#25 SandyHouTex

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Posted 30 January 2021 - 01:01 PM

I think Tak already did this.  I think one of their scopes uses two FPL-53 elements with something else in-between (TOA?), and another one of their scopes uses FPL-53 in the middle and two of the same glass as the two outer elements (TSA?).


Edited by SandyHouTex, 30 January 2021 - 01:03 PM.



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