19 replies to this topic

[Denis Jack]

  Might be an dumb idea or question.
  But could an Achromatic and a Chromatic (as long it be the same of the convex on the Achromatic set, like taken from another identical Achromatic set), be merged in or to form an Apochromatic set? Or simply wouldn't achieve a better result, nor work?
  Sorry, that got me wondering for quite some days already. 

[KBHornblower]

An apo triplet needs to be designed as a set, with each lens made to a very precise formula.  The odds against getting a good match from any old crown or flint lens are astronomical.

[Sean Cunneen]

There are a couple of prescriptions laying around for a corrector for achromats... Google chromacor for a commercial attempt. They are expensive and have their drawbacks, but they are around...

[SandyHouTex]

  Might be an dumb idea or question.
  But could an Achromatic and a Chromatic (as long it be the same of the convex on the Achromatic set, like taken from another identical Achromatic set), be merged in or to form an Apochromatic set? Or simply wouldn't achieve a better result, nor work?
  Sorry, that got me wondering for quite some days already. 

I would need to know what a "Chromatic" is.

[gstrumol]

I would need to know what a "Chromatic" is.

I think he means just a single lens.

[jimhoward999]

Adding a third lens element to an existing achromatic doublet would in every case degrade its color correction performance.    You would not get an apochromat and you would not have an achromat anymore either. 

 

To make an achromat you need the sum of the powers over the v-numbers to be zero.   An existing achromat already has that property with its two elements.  Adding a third to would upset that balance.

 

To make an apochromat you need three elements where the sum of  powers/v-number is zero,  and also the sim of partial-dispersion x power/v-number is zero.

 

It is possible to improve the secondary spectrum of an existing achromat by adding zero-power doublet comprising two elements with the same v-number and equal and opposite powers but differing partial dispersions.  This is occasionally done, but not to my knowledge in astronomy....although maybe the above mentioned Chromacor does this.

[RichA]

  Might be an dumb idea or question.
  But could an Achromatic and a Chromatic (as long it be the same of the convex on the Achromatic set, like taken from another identical Achromatic set), be merged in or to form an Apochromatic set? Or simply wouldn't achieve a better result, nor work?
  Sorry, that got me wondering for quite some days already. 

Someone designed a 2 element apochromat in 2008 to use a Maksutov corrector up front to create a much higher level of chromatic correction, I wonder how it would act on "just" an achromat?

[lylver]

It is a dumb question. In fact it is hidden in the definition of ordinary glass.

 

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But i need to gives some definitions before...

 

An achromat correct color from F 486nm (limit of sky blue and sea blue) to C 656nm deep red ray (modern version is F' 480nm to C' 644nm)

They correct well or not.

 

To be enough corrected for terrestrial uses, they need to match a criteria : 5/8 lamba maximum defocus from green (e ray) to red (C ray) and from green to blue (F ray) : this is Conrady criteria that defines achromatic.

 

Second level criteria is good enough for astronomy use. This is 1 full lambda for e to F and e to C. (Sidgwick). This is astronomy achromat, not terrestrial achromat.

 

Less makes some "color TV" when looking through eyepieces at standard magnification (D/2 in mm). This is not achromatic.

 

Why that ? what is normal conditions ?

It is because the eye itself is not achromatic but only at very small entry pupil : 0.7mm to 0.4mm depending on the observer.

Instrument needs to be enough corrected for not to deceive the eye.

 

At terrestrial level (meaning Conrady) you need to equalize illumination : so to have a standard magnification resulting in 2mm pupil working.

Ex : 51mm (2inches) achromat needs to work at x25 to have same illumination that eye see normaly.

 

So Conrady rule was edicted to match for the above condition.

I will let you think alone about the level of correction needed to match high magnification and/or astronomy conditions.

They are lower (astronomy) and/or higher (magnification) respectively.

 

What is apochromat ? It is an top achromat that also correct color from blue to violet. (486 to 434nm)

This is a requirement for blue photosite for the photography for which pic sensitivity is around 460nm

The eye doesn't need this so acurate because :

a) Blue cone eye receptor are sparse (1 on 16)

b) Resolution/accuracy of the eye is very low or null on this range

 

Supplement from TMB for modern definition : i won't go into details for it, only retaining the fact that Thomas M Back though for two needs.

a) Thomas wanted to match the requirement for photography that are broadband.

b) He thought about high magnification when edicting the rule of lambda / 4 for F and C ray. (to compare 2/8 with 5/8 lambda for Conrady) and at least 0.95 strehl or l/8 at 555.5nm

So apochromat can be used at smaller entry pupil/higher magnification. 5/2 more or ~0.8mm pupil in terrestrial use and 0.5mm is astro use (4 times more). This is the 2D criteria that is announced in commercial sells.

Note : achromat/high end apochromat can go higher in peak and magnify higher in some conditions.

 

Semi-apochromat : they where made for easy high magnification, without photography requirement. In fact... they were frequently use for astrophotography in a time where apochromat were very expensive. Astrophotography is also less demanding than terrestrial (super-apo level with electronic sensor small photosite now, very small comparing to argentic photography)

 

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Ordinary glass makes achromat, this is why they are called ordinary.

 

Chromatism is calculated as a difference of glass index variation between two different glass to calculate what we call secondary spectrum : what makes or not color to be corrected for the eyes.

Fusionned color of the spectrum appears light yellow or near white. A little lack of deep blue or violet is not important for the eye. Under sunlight, it is too few in intensity as the very far red hues.

 

This graph illustrates this "normal variation" for glasses. Using glasses that have the same slope (this is an FC calculated slope Vd depends on index on F ray, index on C ray and index on D ray ) cannot correct anything else than a FC dispersion...

Glasses on the normal line are ordinary glasses

CQFD

 

This is a very simplyfied response but at least one glass need to be abnormal from this group to make an apochromat because glasses near the normal line cannot help to correct, at the same time, the behaviour to be achromat and the behaviour between the F 486 to G' 434nm range.

Edited by lylver, 17 September 2023 - 06:35 PM.

[Argonautt]

Someone designed a 2 element apochromat in 2008 to use a Maksutov corrector up front to create a much higher level of chromatic correction, I wonder how it would act on "just" an achromat?

Indeed, coincidentally looked at what appears to be the relevant paper today: "Adding a Maksutov-like corrector to a refractor apochromat improves the color correction of the apochromat"

 

However... (p.1, first column)

 

 

Whomp whomp.

Edited by Argonautt, 17 September 2023 - 05:23 PM.

[lylver]

Some false idea

Can a single lens objective be achromatic ?

Yes it can.

See a Galilean refractor or the very very long f/D Hershel refractor.

https://www.telescop...lescopes.htm#to

 

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Quiz : what minimal magnification a 60mm diameter apochromat (modern definition) is supposed to achieve in terrestrial condition ?

 

Quiz 2 (hard) : what minimal magnification is a 80mm diameter satisfying Conrady criteria achromat is supposed to achive in astronomical condition ?

3) do you believe this is the regular use of a good achromat ?

Edited by lylver, 17 September 2023 - 06:52 PM.

[Mike I. Jones]

McCarthy corrector for conventional achromats.  Full aperture, not like a Chromacor.

 

https://www.cloudyni...6-f10-achromat/

[lylver]

McCarthy corrector for conventional achromats.  Full aperture, not like a Chromacor.

 

https://www.cloudyni...6-f10-achromat/

I din't understand the goal of you post : you use an abnormal (and costly) glass S-NBH51 and a non-often produced strong flint tha cost more than the initial objective !

[jimhoward999]

I din't understand the goal of you post : you use an abnormal (and costly) glass S-NBH51 and a non-often produced strong flint tha cost more than the initial objective !

The subject of the original topic is adding lenses to an existing Achromat to make an Apochromat.      Mike's post is perfectly responsive to the topic. 

 

The added lenses will of course cost more than the original objective as you need abnormal dispersion glasses to make an Apochromat regardless of whether you design from scratch  or add components to an Achromat.

 

Sorry to butt in.

[lylver]

The subject of the original topic is adding lenses to an existing Achromat to make an Apochromat.      Mike's post is perfectly responsive to the topic. 

 

The added lenses will of course cost more than the original objective as you need abnormal dispersion glasses to make an Apochromat regardless of whether you design from scratch  or add components to an Achromat.

 

Sorry to butt in.

No the subject is : could-an-achromatic-lenses-and-a-chromatic-be-merged-into-an-apochromatic

So no : they can't, it is a question of definition.
chromatic is not a precise word, in optical language, it is difficult to say anything about that.

I would prefer in this case : "can an achromatic lens be corrected ?"

 

I prefer that Mike itself explain this in more than so few words.

His doublets is not a lens, it is a negative full aperture corrector.

Edited by lylver, 18 September 2023 - 12:46 PM.

[duck]

Geez...Mike I Jones takes a lot of heat lately.

 

Here's a way to produce an apochromatic objective for only slightly more that achromat.  Put in a spinning filter wheel which has a different optical thickness for each filter.  Spin at 24 times the number of filters (to get one filter every 1/24 seconds.)  View.  See..you just got better than apochromatic performance on the cheap.

[Mike I. Jones]

I din't understand the goal of you post : you use an abnormal (and costly) glass S-NBH51 and a non-often produced strong flint tha cost more than the initial objective !

To show one of a few methods available to accomplish apo correction of an existing achromat. Not sure how that appeared obscure.

[Mike I. Jones]

No the subject is : could-an-achromatic-lenses-and-a-chromatic-be-merged-into-an-apochromatic
So no : they can't, it is a question of definition.
chromatic is not a precise word, in optical language, it is difficult to say anything about that.
I would prefer in this case : "can an achromatic lens be corrected ?"

I prefer that Mike itself explain this in more than so few words.
His doublets is not a lens, it is a negative full aperture corrector.

Already did, in the referenced link.

Edited by Mike I. Jones, 19 September 2023 - 01:01 PM.

[BGRE]

No the subject is : could-an-achromatic-lenses-and-a-chromatic-be-merged-into-an-apochromatic

So no : they can't, it is a question of definition.
chromatic is not a precise word, in optical language, it is difficult to say anything about that.

I would prefer in this case : "can an achromatic lens be corrected ?"

 

I prefer that Mike itself explain this in more than so few words.

His doublets is not a lens, it is a negative full aperture corrector.

A doublet corrected for colour or not is a lens just as a triplet or a singlet are lenses.

Multielement lenses are often used in camera, telescope and microscope objectives as well as elsewhere.

The thread title does not ask if a single element lens can be used. 

[MKV]

I din't understand the goal of you post : you use an abnormal (and costly) glass S-NBH51 and a non-often produced strong flint tha cost more than the initial objective !

Where in the thread title does it say the glasses must be cheap?!? 

[lylver]

  Might be an dumb idea or question.
  But could an Achromatic and a Chromatic (as long it be the same of the convex on the Achromatic set, like taken from another identical Achromatic set), be merged in or to form an Apochromatic set? Or simply wouldn't achieve a better result, nor work?
  Sorry, that got me wondering for quite some days already. 

 

 

Where in the thread title does it say the glasses must be cheap?!? 

Here guy. => "as long it be the same of the convex on the Achromatic set, like taken from another identical Achromatic set"

 

So please read the start post, this is not a design challenge but a explaination trial

Edited by lylver, 20 September 2023 - 02:21 PM.