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

[turboscrew]

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.

In my work, accurate specs are often crucial. Components are often selected carefully to cover a certain range. But then, the products are seldom for average consumers, but components for controlling machinery.

I suppose, it's fully OK if the actual range is found in the device's specifications.

[Greenman]

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.

In my work, accurate specs are often crucial. Components are often selected carefully to cover a certain range. But then, the products are seldom for average consumers, but components for controlling machinery.

I suppose, it's fully OK if the actual range is found in the device's specifications.

Understood, we do this when we integrate our products into OEM builds and for NPD. For general use spectrometers the tolerances are wider.

I think some telescope descriptions are deliberately ‘woolly’ To trap the unwary, that I think is the gist of @Lady Fractor game. To uncover the marketing from the true specifications.

[turboscrew]

One of the problems is that too often companies are selling "solutions" instead of products. They have decided to know better what people need. An then, if you need something to use in a bit unorthodox way, you are in trouble - you don't get the specs, or at least many things that are important to you are not specified.
It's not unorthodox use, but the FOV is quite seldom specified for finder scopes, even if, I think, it's even more important for star hopping than the magnification or aperture.

[Lady Fraktor]

ED glass lets the optician make a shorter focal length telescope.
Colour correction is almost the same in my 105mm f/15 (1575mm f/l) as a 140mm f/7 (980mm f/l)
Where the 140mm beats it is in the far violet which I cannot see visually anyways.

Thomas Back was an incredibly accomplished optician and telescope designer but his definition is a lessening of the Abbe criteria of apochromatism.
Even though he lowered the definition, even he stated that if you follow the definition there is no actual apochromatic refractors.

[turboscrew]

So to the question: No doublet - or any lens system - can really be apochromat in Abbe-sense?

[Lady Fraktor]

The definition by Abbe was first put in place for Zeiss microscope eyepieces and transferred over to other optics afterwards.
As Mr. Back stated, Abbe never set a end limit on SC so the designer is left to guess at what is good enough.
He also felt that the 'perfect' telescope was the Zeiss APQ 100/1000 refractor and his designs where made to equal or best that level.

Technically you can have a doublet apochromatic using the Thomas Back definition and you will likely never be lacking from its level of performance.
A lot of the current marketing terms used today are offshoots of the Zeiss designations but are not used in the same way.
Zeiss semi-apo meant a achromat that had 2.5x the colour correction of a equivalent achromat, today manufacturers try to convince people that a semi-apo is actually a apochromat...

[Greenman]

A lot of the current marketing terms used today are offshoots of the Zeiss designations but are not used in the same way.
Zeiss semi-apo meant a achromat that had 2.5x the colour correction of a equivalent achromat, today manufacturers try to convince people that a semi-apo is actually a apochromat...

Herein lies the issue, ‘smoke and mirrors’ are used to sell ‘solutions‘ to people who are looking for the perfect low cost telescope that ‘does everything’.

That market wish will comes from the inexperienced, but manufacturers will try to satisfy that cohort of the market. This is why so many low cost telescopes solutions are in the secondhand market after one use. You would not buy a cheap violin and expect to play the flight of the bumblebee on it, but you can be easily sold a telescope on the premise of seeing a galaxy in full colour - as often printed on the box.

This plays over into the higher end telescope market in a more refined, but similar way. Caveat Emptor has been around for millennia and it still goes strong.

[turboscrew]

A lot of the current marketing terms used today are offshoots of the Zeiss designations but are not used in the same way.
Zeiss semi-apo meant a achromat that had 2.5x the colour correction of a equivalent achromat, today manufacturers try to convince people that a semi-apo is actually a apochromat...

Herein lies the issue, ‘smoke and mirrors’ are used to sell ‘solutions‘ to people who are looking for the perfect low cost telescope that ‘does everything’.

That market wish will comes from the inexperienced, but manufacturers will try to satisfy that cohort of the market. This is why so many low cost telescopes solutions are in the secondhand market after one use. You would not buy a cheap violin and expect to play the flight of the bumblebee on it, but you can be easily sold a telescope on the premise of seeing a galaxy in full colour - as often printed on the box.

This plays over into the higher end telescope market in a more refined, but similar way. Caveat Emptor has been around for millennia and it still goes strong.

I've heard that a beginners only gets "screeches" out of Stradivarius. It plays really well, but only in the hands of masters.

[Baurice]

To throw in my 2p worth, I have read an opinion that suggests that a good quality achromat with a focal ration of F/9 and above gives just as good colour correction as any doublet or triplet.

I'd like to try it out one day (or night).

[Lady Fraktor]

What telescope was it?
Depending on the glass used, quality, level of polishing a well made achromat can do quite nicely against an apochromat.

For visual observing an achromat is a fine choice as you normally do not need the extra light spectrum available with a triplet as we cannot see into those ranges anyways.

[Baurice]

Come to think of it, some reflector designs eliminate most chromatic aberration. I have a 5" Maksutov. I find that Earth's atmosphere and eyepieces also introduce false colour but my "Mak" works well with cameras.

[Lady Fraktor]

The Maksutov design is capable of giving excellent views and they make very good planetary/ lunar telescopes.
My favorite Maksutov is the discontinued Meade LX-200 7", a fantastic telescope.

[Lady Fraktor]

Life is a bit hectic at the moment but I will get back to this and clean things up as well as answer questions.

A Ronchi eyepiece, artifical star and a good star test can give you a lot of information about how well your optics are performing.
A DPAC can be set up for a reasonable price as well if you really want to get even more technical.

[Don Alvarez]

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

Yes.

[Lady Fraktor]

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

Yes.

Hello stranger, I hope all is going well for you.

[Don Alvarez]

Hello stranger, I hope all is going well for you.

I been away a while. Don't wanna derail the thread too much, but, yes. Thanks for asking. Things are finally looking up. I may even get to do some astronomy soon.

[Graeme1858]

Ok, so, other than my guide scope, I've only ever owned Newtonians and my current catadioptric SCT. My knowledge of optics and the physics of light is ok but only academic, my experience is zero! I just read through this thread again and I have learnt a lot of terminology from it. Can I pick up on the unanswered @Lady Fraktor statement that 3 wavelength crossing is just a part of the definition of APO. Is the ability to focus all wave lengths of the visible part of the spectrum required, not just the 3 primary colours? Or is there more?

Regards

Graeme

[Lady Fraktor]

There is more, I get regular days off in four days and I will return to getting this all tidied up with answers as well.

[messier 111]

one word , Takahashi .

[s2mikey]

So to the question: No doublet - or any lens system - can really be apochromat in Abbe-sense?

Im not sure but it seems really, really tough since you have just two lenses to work with. Im not an optical engineer or anything close to it but in my travels Ive owned some pretty good doublets from Wiliam, Stellarvue, etc all of which were medium FL ratios and while all were great scopes, none of them were totally color free. They were great scopes nonetheless. I suppose the loger the FL ratio, the better. Of course, it is probably possible with ulta-premium glass and the best mfg methods but then you're talking big bucks. Again, not 100% sure here.

That being said, I know that ES gets some flak in the industry but I must have got a "good one" because my ES 102mm triplet is color free even on bright objects. That extra lens does wonders for false color. Is my ES perfect? Not at all - but I am sensitive to false color so thats why I went with a triplet. Of coruse I still want a TAK at some point and that will likely be a doublet.