TheSkySearchers

The notion for this article hit me this morning as I was perusing my email "Astro-Ads".

I thought this topic would be a relatively easy "define and go" type of post. Not so!

I thought it was going to be a simple doublet vs triplet distinction but it is not. I will now refer you to the "bible" on all matters optical -- Amateur Telescope Optics (ATO): https://www.telescope-optics.net/index.htm -- you should definitely bookmark this site if you haven't already done so.

Since we all know what an Achromatic objective is, I'll just start with what the ATO thinks an Apochromatic objective is:

• "Put simply, the term apochromatic specifically implies the mode of chromatic correction, not the level of optical quality. For that, we have and should use appropriate parameters from the realm of optical aberrations."

So this essentially means that if your doublet meets the correction level for both CA and spherochromatism then it qualifies as an Apochromat. Before this morning, I did not know that!

So now you are probably thinking, JT, I need more than this, I need greater depth to fully appreciate the difference. I understand, but for me to explain all that is involved is both long and complex, so here is the link to the Achromats, Semi-APO, and APO chapter of the ATO https://www.telescope-optics.net/apo_refractor.htm.

I must tell you that before you begin this section, you should go fetch a beverage and put on your thinking cap - you'll need it because MATH is involved!

You can thank me for your headache later.

Cheers,
JT

This is an outstanding post, JT !
Thank you for putting this up.

Thanks for posting this. It's sane. It also shows that the "double versus triplet" distinction is not meaningful. It's an oversimplification of a more complex problem. You need to look at the aberrations to see if your instrument is or is not an apochromat. It's not as simple as counting the number of elements in the objective!

A magnificent post. Thank you.

Quite true nFA, though I really dislike the semi-apo term or the majority of people assuming that since they have an ED lens it is automatically apochromatic.

A good post JT, I have had this site bookmarked for quite some time and reference it quite often.
I was quite upset when it went offline a couple years ago, luckily nFA found a copy of the site online which helped out with the withdrawal
Some people may find this site a bit easier to reference as the writing style is a bit easier to follow: http://www.handprint.com/ASTRO/ae1.html

Lady Fraktor wrote: ↑Sun Aug 18, 2019 6:32 amSome people may find this site a bit easier to reference as the writing style is a bit easier to follow: http://www.handprint.com/ASTRO/ae1.html

I just noticed that at the bottom of the introduction page of the ATO there is a link to the very same site you just linked to.

JT

IMHO, an ED should be fine for visual, that a triplet APO is more for AP?
I don't know a lot about refractors, I am considering an ED at the moment though

I've been doing it with mirrors for 51 years LOL

Ylem wrote: ↑Sun Sep 01, 2019 3:06 pm IMHO, an ED should be fine for visual, that a triplet APO is more for AP?
I don't know a lot about refractors, I am considering an ED at the moment though

I've been doing it with mirrors for 51 years LOL

Maybe on ED doublet for visual and triplet for AP. It's the common wisdom but it oversimplifies. I'd rather have a doublet with better optical surfaces and less scattered light than a triplet any time for anything, visual or AP. If you use narrow band filters for AP the triplet is irrelevant and even an achromat is good.

With better optical surfaces a doublet can outperform a triplet at high magnification on lunar/planetary. The triplet advantages only show up with equal quality optics on one shot color AP.

One other point to consider is maintenance. In the last 20 years or so I've only had to collimate a doublet once but it was a simple though tedious task successfully done. I'd never even try to fix a triplet. It'd have to be sent back.

I'm too dumb to do AP
And yes the simplicity and maintenance would make a doublet more my style

Apo's have most common short focal distances, right?
Achro's, apart from widefielders, have longer focal distances. Mostly done to suppress CA. One of the extra's gifts a long focal refractor gives is the longer focal depth. Vulnerability to seeing decreases with longer focal depths. So an f/10 refractor is less vulnerable to seeing than a f/6 refractor. Let alone a classical f/15 refractor.
Just a free gift from an achro to an observer in the attempt to suppress CA. Quite a difference!

Hello all,

the different levels of correction in the refractors: achromatic - apochromatic - superachromatic are nicely visualized in the graph from WIKI


https://en.wikipedia.org/wiki/Superachromat

On the other hand, even an ED triplet is still a halfAPO. To make a full APO, 2 wide spaced ED lenses are needed, not counting the other lenses correcting the spherical aberration and flattening the field.
That's the reason why some of the imaging people take the big APO telelenses, so far they can manage their high costs.

For visual and with the medium scope speeds, and ED doublet is by far enough.

Best,
JG

I have used ED doublets and triplets. I notice the difference in ap. Doublets are great for mostly visual on f6 or f7 scopes. the one thing everyone forgets is the aberations from your own eyes!

Mandrew wrote: ↑Thu Jan 23, 2020 7:11 am I have used ED doublets and triplets. I notice the difference in ap. Doublets are great for mostly visual on f6 or f7 scopes. the one thing everyone forgets is the aberations from your own eyes!

I just KNEW it was gonna be my fault.
It's always my fault!

When it comes down to the cows coming home, all we really have to go on is putting our trust into the products we buy. Often, after the fact, the sad truth surfaces.
I got my Orion Triple APO ED80T CF because it ticked all the buttons for my purpose, and mostly was reachable in price. My next option at that time was $1700.00 more. For a 100 mm aperture. 20 mm for 1700 bucks? Not me!
Where I to do it over now, there are a lot of more choices I could choose from. And probably much more reliable companies.
Starizona has a tutorial that, sadly, I cannot access at the moment. But it is an excellent comparison between Acro and Apo refractor lenses. And swayed me to my triplet.
Because, to me and my needs, my telescope had to be a camera lens first. Then able to do observation with as well. And a bonus (to me) is being able to do terrestrial photography with it as well.

I'm not one for arguments that have no ends. If it works for you, and your desires you wish to get from whatever end you approach from, then I'm happy for you. Spare me the semantics that may or may not be delivered to your door.
Show me the pictures.
I think here is yet another case where YMMV applies. :twocents:

The science on whether the optics qualify for APO status is pretty straight forward and clear cut. It is how the manufacturers (the marketing guys) choose to misrepresent the data and how much they are willing to bend the English language to sell their products.

Cheers,
JT

OK, we're tending to simplify as of yet.

There are different definitions of apochromat. So far as I'm concerned it means that in AP you'll have no significant chromatic aberration.

The difficulty is that there are different ways of achieving the standard and the standard itself is a bit variable.

We should remember. . . Achromat effectively means there is no CA. But we all know that our nice achromats have a whole lot of CA. So why are our achromats chromatic? It's because we like short telescope tubes. If you make a very long focal length doublet using the typical Crown and Flint glass (and you do it well) then you can effectively get rid of the chromatic aberration and have what we generally consider to be apochromatic performance.

But those very long focal lengths typically require very long and heavy telescope tubes which may require what today we might consider to be extraordinary bracing to prevent sagging along with a very big, heavy, and expensive mount. Your FOV may be rather miniscule. Increasing aperture increases the length, weight, and gangliness very, very rapidly.

Those neat curves showing the comparison between an achromat, semi-apo, and apochromat are interesting but usually deficient. They are typically done at short focal lengths and if applied generally they tend to ignore the different optimizations which the optical designer can make to tune to certain wavelengths.

Oh, well. Another way to look at it is to suggest we usually get what we pay for.


All that said? I actually have a singlet which has no visually discernible chromatic aberration when used. It's an Ed Jones solar projector and it uses an extremely long focal length to achieve its performance. Since it does not rely on a telescope tube it can be a practical device and perform well.

j.gardavsky wrote: ↑Mon Sep 30, 2019 8:35 pm Hello all,

the different levels of correction in the refractors: achromatic - apochromatic - superachromatic are nicely visualized in the graph from WIKI


https://en.wikipedia.org/wiki/Superachromat

On the other hand, even an ED triplet is still a halfAPO. To make a full APO, 2 wide spaced ED lenses are needed, not counting the other lenses correcting the spherical aberration and flattening the field.
That's the reason why some of the imaging people take the big APO telelenses, so far they can manage their high costs.

For visual and with the medium scope speeds, and ED doublet is by far enough.

Best,
JG

Yes. That's the objective criterion. The number of differing wavelengths that share the focal plane is a fact. No argument.

notFritzArgelander wrote: ↑Sun Sep 01, 2019 7:21 pm

Ylem wrote: ↑Sun Sep 01, 2019 3:06 pm IMHO, an ED should be fine for visual, that a triplet APO is more for AP?
I don't know a lot about refractors, I am considering an ED at the moment though

I've been doing it with mirrors for 51 years LOL

Maybe on ED doublet for visual and triplet for AP. It's the common wisdom but it oversimplifies. I'd rather have a doublet with better optical surfaces and less scattered light than a triplet any time for anything, visual or AP. If you use narrow band filters for AP the triplet is irrelevant and even an achromat is good.

With better optical surfaces a doublet can outperform a triplet at high magnification on lunar/planetary. The triplet advantages only show up with equal quality optics on one shot color AP.

One other point to consider is maintenance. In the last 20 years or so I've only had to collimate a doublet once but it was a simple though tedious task successfully done. I'd never even try to fix a triplet. It'd have to be sent back.

What doublets can outperform a triplet at high magnification on lunar / planetary?

Thanks nFA

Refractordude wrote: ↑Sun Mar 15, 2020 2:09 am

notFritzArgelander wrote: ↑Sun Sep 01, 2019 7:21 pm

Ylem wrote: ↑Sun Sep 01, 2019 3:06 pm IMHO, an ED should be fine for visual, that a triplet APO is more for AP?
I don't know a lot about refractors, I am considering an ED at the moment though

I've been doing it with mirrors for 51 years LOL

Maybe on ED doublet for visual and triplet for AP. It's the common wisdom but it oversimplifies. I'd rather have a doublet with better optical surfaces and less scattered light than a triplet any time for anything, visual or AP. If you use narrow band filters for AP the triplet is irrelevant and even an achromat is good.

With better optical surfaces a doublet can outperform a triplet at high magnification on lunar/planetary. The triplet advantages only show up with equal quality optics on one shot color AP.

One other point to consider is maintenance. In the last 20 years or so I've only had to collimate a doublet once but it was a simple though tedious task successfully done. I'd never even try to fix a triplet. It'd have to be sent back.

What doublets can outperform a triplet at high magnification on lunar / planetary?

Thanks nFA

My SV ED80 Access at f7 takes stupidly high magnification with ease as seeing permits visually. If you're doing astrophotography a triplet is still a better bet.

I bought my TOA130NS last year figuring it would be a good imaging and visual scope and it has been my favorite refractor so far that I have owned. I sold my AP155 about 6 years ago but I ended up selling it to a friend since I was always worried about dropping it or messing it up somehow. Probably one of the only purchased scopes that actually gave me a nice return for my money but always regretted selling it. But, the TOA130 has met my needs, simply amazing views of the planets, moon and well the trapezium, simply beautiful. Using high power on this little 5" is not a problem provided the seeing is supportive.
Though, I will state that I bought the TOA130 based on using it for imaging and visual based on some advice from some serious refractor guys out in CA Derek W and Chip L and some others.

Joe