Lets play a game! Part 2 Can a doublet be apochromatic?

[Lady Fraktor]

Outside information is good but do try to put it in your own words instead please.

What does it mean to be apochromatic in the first place? Do you actually need 3 or more elements to accomplish this or can you achieve this with 2 elements?

What do the terms achromat, ED telescope, ED apo, semi-apo and apochromatic mean to you?

[Jones]

Some say they are, but only flourites really can be. If you don't have a triple, you got a 2and class scope.

[JayTee]

If you don't have a triple, you got a 2and class scope.

Not necessarily. If your doublet focuses the different parts of the visible spectrum within the narrowly defined criteria for an Apochromat, then you only need those two lenses.

Cheers,
JT

[Bigzmey]

Some say they are, but only flourites really can be. If you don't have a triple, you got a 2and class scope.

No no, quadruplet is 1st class and triplet would be 2d.

[Jones]

Yes there are some exceptions like the Vixen 103 and 115, but they cost so much, you may as well get a larger triplet for the same cost. China makes fine dash 53 triples.

[Lady Fraktor]

Extra Low Dispersion glass does not an apochromatic make of itself, a refractor using regular flints and crowns can deliver apochromatic performance if designed and assembled properly.
I know of four people that have done this and sold them commercially though none are being made currently.

Lets keep the discussion based on the above questions going

[turboscrew]

What I googled about the terms, I think doublet can be apochromatic, but making an apochromatic doublet must be quite hard, like solving a system of three equations with two variables. With some equations it can be done, but not in general.
That is, apochromatic duplets probably can be only made for some specific aperture/focal length combinations using specific kinds of glass.
I wonder if "ED" only means that some extra low dispersion glass type is used as the positive probably suggesting that the negative lens is thin or has lowish refraction. Maybe less overall chromatic aberration is implied.

Thoughts of someone that knows next to nothing about refractors...

[Kerry C.]

Extra Low Dispersion glass does not an apochromatic make of itself, a refractor using regular flints and crowns can deliver apochromatic performance if designed and assembled properly.
I know of four people that have done this and sold them commercially though none are being made currently.

Lets keep the discussion based on the above questions going

I am going to ask these beginner questions because I really want to learn. So, sorry if they are a "well duh" to those already in the know.
Does the "ED" mentioned above refer to the Extra Low Dispersion glass?
If so does ED mean the way light separates in the lens like a prism makes a rainbow of colors?
I am guessing I don't want that, ergo "extra low dispersion" (less separation) of light not lots of it, as in the prism.
However, as I think about this, those that use a monochrome cameras take separate photos capturing only certain light colors, so do I want these light colors to actually be separated?
Yes I am confused...
Kerry

[Bigzmey]

By textbook definition Apochromatic is a type of refractor telescope which is designed to bring blue, red and green wavelengths to the same focal point as oppose to Achromatic which designed to bring just the two wavelengths to the same focal point.

Therefore, semi-APO term should be outlawed. You either bring 3 color wavelengths to the same point or you don't.

I believe confusion about APO, ED, etc comes from the fact that CA-free or CA-reduced and APO terms are used interchangeably. CA-free design can be achieved in many ways, but APO should be applied to a specific refractor design.

Reflector is a CA-free design, but people don't call them APO. Achromatic refractor can be made in a way that it is essentially CA-free. Should we call such achromat APO? This is where confusion arrives. To avoid this confusion the simplest is to go by definition, if a refactor telescope bring three wavelength to the same focal point within specified error this is an APO design.

What specified error? I don't know I am not an optic physicist. But from my experience I know that some poorly executed "APO" triplets show more CA than better executed doublets.

[Lady Fraktor]

What I googled about the terms, I think doublet can be apochromatic, but making an apochromatic doublet must be quite hard, like solving a system of three equations with two variables. With some equations it can be done, but not in general.
That is, apochromatic duplets probably can be only made for some specific aperture/focal length combinations using specific kinds of glass.
I wonder if "ED" only means that some extra low dispersion glass type is used as the positive probably suggesting that the negative lens is thin or has lowish refraction. Maybe less overall chromatic aberration is implied.

Thoughts of someone that knows next to nothing about refractors...

Making a high quality doublet is no harder than making a triplet overall. Optical design, proper glass choices and then manufacturing to specifications of the design.

[Lady Fraktor]

I am going to ask these beginner questions because I really want to learn. So, sorry if they are a "well duh" to those already in the know.
Does the "ED" mentioned above refer to the Extra Low Dispersion glass?
If so does ED mean the way light separates in the lens like a prism makes a rainbow of colors?
I am guessing I don't want that, ergo "extra low dispersion" (less separation) of light not lots of it, as in the prism.
However, as I think about this, those that use a monochrome cameras take separate photos capturing only certain light colors, so do I want these light colors to actually be separated?
Yes I am confused...
Kerry

ED glass is actually Extra Low Dispersion not Extra Dispersion... marketting strikes again
The glass still has to be complimented by a appropriate mating element for it to do the job required.

[turboscrew]

What I googled about the terms, I think doublet can be apochromatic, but making an apochromatic doublet must be quite hard, like solving a system of three equations with two variables. With some equations it can be done, but not in general.
That is, apochromatic duplets probably can be only made for some specific aperture/focal length combinations using specific kinds of glass.
I wonder if "ED" only means that some extra low dispersion glass type is used as the positive probably suggesting that the negative lens is thin or has lowish refraction. Maybe less overall chromatic aberration is implied.

Thoughts of someone that knows next to nothing about refractors...

Making a high quality doublet is no harder than making a triplet overall. Optical design, proper glass choices and then manufacturing to specifications of the design.

I was thinking of making apochromatic. I think it would be easier to handle three wavelengths using three "optical components" than using just two. Sure, it doesn't have to be apochromatic to be of high quality. And I suppose it's possible to make poor quality apochromatics too...

[Lady Fraktor]

Just to try and get some conversation going, most people seem to think that to be an apochromat there must be three colour crossing.
What about the Takahashi TOA-130 which only does one colour crossing but is considered one of the best refractors?

[turboscrew]

Just to try and get some conversation going, most people seem to think that to be an apochromat there must be three colour crossing.
What about the Takahashi TOA-130 which only does one colour crossing but is considered one of the best refractors?

I was under the impression that three wavelengths is a definition of apochromatic and two vawelengths is a definition of achromatic regardless, really, of the overall chromatic aberrations. The spectral behaviour between the specific wavelengths could be as bad as ever (I guess, in reality, the wavelengths in-between are made to behave somewhat well).
And as such, achromat or ED could, actually, be of better quality than a low quality apo.

Again, thoughts of someone, who knows next to nothing about these.

[Lady Fraktor]

It is not a definition but a small part of the definition.
Obviously there are exceptions to the rule though.
The Takahashi TSA-102 and FS-102 are examples as well.

[Greenman]

To my way of thinking an ED doublet or even an achromat can be considered APO if the design eliminates CA by bringing blue, green and red to the same focal point.

ED glass I assume just reduces the distance between the focal points (the dispersion being lower), and it would need a second, third or forth element to bring the spread focuses together. Semi-APO is a misnomer, it is simply put not APO, so improved achromat would be a more apt name.

All off the top of my head.

No brains were hurt in this process.

[turboscrew]

I think this definition by Thomas M. Back (TMB Optical) of apochromat brings more sense to even semi-apo.
"With the proliferation of apochromatic refractors that are available to the amateur astronomer, it is time to define the parameters of a true apochromatic objective lens. The modern definition of "apochromat" is the following: An objective in which the wave aberrations do not exceed 1/4 wave optical path difference (OPD) in the spectral range from C (6563A - red) to F (4861A - blue), while the g wavelength (4358A - violet) is 1/2 wave OPD or better, has three widely spaced zero color crossings and is corrected for coma."
http://www.csun.edu/~rprovin/tmb/definition.html

[Greenman]

I think this definition by Thomas M. Back (TMB Optical) of apochromat brings more sense to even semi-apo.
"With the proliferation of apochromatic refractors that are available to the amateur astronomer, it is time to define the parameters of a true apochromatic objective lens. The modern definition of "apochromat" is the following: An objective in which the wave aberrations do not exceed 1/4 wave optical path difference (OPD) in the spectral range from C (6563A - red) to F (4861A - blue), while the g wavelength (4358A - violet) is 1/2 wave OPD or better, has three widely spaced zero color crossings and is corrected for coma."
http://www.csun.edu/~rprovin/tmb/definition.html

Hi Turboscrew,

Interesting, I wonder how many vendors ensure, or even better, certify that performance before marketing their Scopes as APO’s? Very few I expect.

Also would the amateur have the optical bench to test out these differences, very few I expect. Mostly, they are aware that optical performance is subjective, let’s say in the eye of the beholder (and that opens a whole other can of worms).

[turboscrew]

I think this definition by Thomas M. Back (TMB Optical) of apochromat brings more sense to even semi-apo.
"With the proliferation of apochromatic refractors that are available to the amateur astronomer, it is time to define the parameters of a true apochromatic objective lens. The modern definition of "apochromat" is the following: An objective in which the wave aberrations do not exceed 1/4 wave optical path difference (OPD) in the spectral range from C (6563A - red) to F (4861A - blue), while the g wavelength (4358A - violet) is 1/2 wave OPD or better, has three widely spaced zero color crossings and is corrected for coma."
http://www.csun.edu/~rprovin/tmb/definition.html

Hi Turboscrew,

Interesting, I wonder how many vendors ensure, or even better, certify that performance before marketing their Scopes as APO’s? Very few I expect.

Also would the amateur have the optical bench to test out these differences, very few I expect. Mostly, they are aware that optical performance is subjective, let’s say in the eye of the beholder (and that opens a whole other can of worms).

I have no idea. I just wondered, if that (or something similar) is what @Lady Fraktor meant by saying that 3 wavelength crossing is just a part of the definition of APO.

[Greenman]

I think this definition by Thomas M. Back (TMB Optical) of apochromat brings more sense to even semi-apo.
"With the proliferation of apochromatic refractors that are available to the amateur astronomer, it is time to define the parameters of a true apochromatic objective lens. The modern definition of "apochromat" is the following: An objective in which the wave aberrations do not exceed 1/4 wave optical path difference (OPD) in the spectral range from C (6563A - red) to F (4861A - blue), while the g wavelength (4358A - violet) is 1/2 wave OPD or better, has three widely spaced zero color crossings and is corrected for coma."
http://www.csun.edu/~rprovin/tmb/definition.html

Hi Turboscrew,

Interesting, I wonder how many vendors ensure, or even better, certify that performance before marketing their Scopes as APO’s? Very few I expect.

Also would the amateur have the optical bench to test out these differences, very few I expect. Mostly, they are aware that optical performance is subjective, let’s say in the eye of the beholder (and that opens a whole other can of worms).

I have no idea. I just wondered, if that (or something similar) is what @Lady Fraktor meant by saying that 3 wavelength crossing is just a part of the definition of APO.

Me neither, as a vendor of spectrometers our definitions do not marry up well to classical physics. For example:

Our Vis/NIR spectrometer only uses a silicon detector, so it’s really a Vis/SWNIR (maximum spectral end 1100nm ). The InGaAs versions range extends to the end of NIR at 2.5 um, but only starts at 900nm. The visible range in pure terms visually ends at 780, although most would accept 800nm as the start of NIR. So 780nm to 2500 is the NIR window, but due also to physics no one detector can cover this. As you will see above setting expectations is key to my job.

What I’m saying is that vendors name their products to fit a preconceived market, not the nuances of reality. It does not surprise me that telescope vendors do not differ.

The problem is the confusion such marketing produces, and the expectations set in the customers mind. At the end of the day the consideration is are they being mis-sold?

Hard to answer, if the customer is happy with the product and support then, no.