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Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

Posted: Fri May 29, 2020 12:36 pm
by jbelden
JT,

Thanks for the links, good reference material to read up with not just one beverage but a couple. My only experience in optics at the amateur level is I only made a dozen parabolic mirrors for Dobs and Newts, largest was an 18".

BTW, I see your in HI, I used to do projects for HECO so I made several flights to HI from ATL back in 2006 - 2012.

Joe

Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

Posted: Fri May 29, 2020 5:20 pm
by j.gardavsky
A triplet, or doublet plus singlet, allows to make a well corrected light gathering lens system with a higher speed, than the ED doublets.

The doublet plus singlet became a widespread design for the fast (F/4) objectives in the premium binoculars.
With the increasing aperture, it is the 4 wide spaced lenses design, 2 of them ED, like in the Leica 82mm APO Televid.

Otherwise, even an achromatic doublet can offer an "apochromatic performance" in the eye relief space behind the eyepiece, when the eyepiece is overcorrected to flip the focus deviations on the short wavelengths, and in the way like the yellow curve in the graphic I have posted before.
But even then, outside the range of the three crossings, the deviations will be much larger if compared with a true APO.

Best,
JG

Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

Posted: Tue Oct 06, 2020 3:09 pm
by John Donne
Although coming very late to the table, three fine courses already consumed and the wine bottles empty, I have found this thread a pleasure to consume.
I have nothing to add but my thanks to the chef, Mr JT, who set the table for this conversation.
Clear skies to all !

Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

Posted: Sat Jan 30, 2021 1:40 pm
by TareqPhoto
I asked about this topic or similar before and got so many answers, and still can't understand it, only that triplet is better as long it has quality design of the scope first, and with a better optics it will be even better, so it is like 3 things in 1, you can be good with two minimum, but not only 1, so for that it is like saying a doublet with two requirements is better than a triplet with 1 requirement.

That made me lost, as i rushed buying first APO, and now i keep searching for the second not sure if it should be a triplet or doublet and dunno how quality design it is and even not sure if the optics must be this or that, all within a budget, and within a must focal length i am looking for.

Re: Achromat Vs Semi-APO Vs APO - What's The Difference

Posted: Sat Jan 30, 2021 4:57 pm
by AntennaGuy
notFritzArgelander wrote: Sat Aug 17, 2019 10:15 pm 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!
Surely, you can just look at what it says it is, on the box that it came in, right??
:dance: :eek: :twitch: :whistle: :whistle:

Re: Achromat Vs Semi-APO Vs APO - What's The Difference

Posted: Sat Jan 30, 2021 5:56 pm
by notFritzArgelander
AntennaGuy wrote: Sat Jan 30, 2021 4:57 pm
notFritzArgelander wrote: Sat Aug 17, 2019 10:15 pm 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!
Surely, you can just look at what it says it is, on the box that it came in, right??
:dance: :eek: :twitch: :whistle: :whistle:
Yes! Everyone knows that "triplets are better than doublets" so lets ditch the trips and get sextuplets! :lol: As long as I have fingers and toes to count elements, that's all that matters, right? :popcorn:

Re: Achromat Vs Semi-APO Vs APO - What's The Difference

Posted: Sat Jan 30, 2021 7:50 pm
by TareqPhoto
notFritzArgelander wrote: Sat Jan 30, 2021 5:56 pm
AntennaGuy wrote: Sat Jan 30, 2021 4:57 pm
notFritzArgelander wrote: Sat Aug 17, 2019 10:15 pm 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!
Surely, you can just look at what it says it is, on the box that it came in, right??
:dance: :eek: :twitch: :whistle: :whistle:
Yes! Everyone knows that "triplets are better than doublets" so lets ditch the trips and get sextuplets! :lol: As long as I have fingers and toes to count elements, that's all that matters, right? :popcorn:
Maybe i won't go to that much elements, so eliminate one element, how about a quintuplet then?

Re: Achromat Vs Semi-APO Vs APO - What's The Difference

Posted: Sat Jan 30, 2021 8:23 pm
by AntennaGuy
notFritzArgelander wrote: Sat Jan 30, 2021 5:56 pm
AntennaGuy wrote: Sat Jan 30, 2021 4:57 pm
notFritzArgelander wrote: Sat Aug 17, 2019 10:15 pm 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!
Surely, you can just look at what it says it is, on the box that it came in, right??
:dance: :eek: :twitch: :whistle: :whistle:
Yes! Everyone knows that "triplets are better than doublets" so lets ditch the trips and get sextuplets! :lol: As long as I have fingers and toes to count elements, that's all that matters, right? :popcorn:
Well, maybe not, in every case, for telescope objectives. But for eyepieces, more lenses is always better! On that, we all surely agree....
:popcorn:

Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

Posted: Sat Jan 30, 2021 8:24 pm
by AntennaGuy
Oh my goodness. Quintuplet refractors exist.
https://agenaastro.com/sharpstar-121mm- ... 21sdq.html
And... sextuplets? Seriously?
https://telescopes.net/store/stellarvue ... graph.html
(Ok, so that isn't intended for visual astronomy. But still...)

Re: Achromat Vs Semi-APO Vs APO - What's The Difference

Posted: Sat Jan 30, 2021 8:58 pm
by notFritzArgelander
AntennaGuy wrote: Sat Jan 30, 2021 8:23 pm
notFritzArgelander wrote: Sat Jan 30, 2021 5:56 pm
AntennaGuy wrote: Sat Jan 30, 2021 4:57 pm
Surely, you can just look at what it says it is, on the box that it came in, right??
:dance: :eek: :twitch: :whistle: :whistle:
Yes! Everyone knows that "triplets are better than doublets" so lets ditch the trips and get sextuplets! :lol: As long as I have fingers and toes to count elements, that's all that matters, right? :popcorn:
Well, maybe not, in every case, for telescope objectives. But for eyepieces, more lenses is always better! On that, we all surely agree....
:popcorn:
Yes, I know that I must be hallucinating when I am sure that my 1+3 element Abbe Orthoscopic eyepieces dive better contrast and definition than a gazillion element wide field. :lol:

(I know you're cranking me up so I might as well play along and do my straight man schtick.)

Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

Posted: Sat Jan 30, 2021 10:39 pm
by notFritzArgelander
PS Doublet, triplet, ....., dodecuplet, glass types simply don't matter. If you are doing AP you simply want the best polychromatic Strehl ratio. That's the only number that matters. Counting the numbers of elements and glass types doesn't count for diddly against having a good Dtrehl test and interferometrically measured wave front errors. For visual only use it's the same deal except you need to make sure that the Strehl is good in green where the eye is most sensitive.

Re: Achromat Vs Semi-APO Vs APO - What's The Difference

Posted: Sun Jan 31, 2021 7:52 pm
by j.gardavsky
TareqPhoto wrote: Sat Jan 30, 2021 7:50 pm
notFritzArgelander wrote: Sat Jan 30, 2021 5:56 pm
AntennaGuy wrote: Sat Jan 30, 2021 4:57 pm
Surely, you can just look at what it says it is, on the box that it came in, right??
:dance: :eek: :twitch: :whistle: :whistle:
Yes! Everyone knows that "triplets are better than doublets" so lets ditch the trips and get sextuplets! :lol: As long as I have fingers and toes to count elements, that's all that matters, right? :popcorn:
Maybe i won't go to that much elements, so eliminate one element, how about a quintuplet then?
A quintuplet is a good solution.
A sort of is my leica APO Televid 82mm: 4 lenses in the light gathering groups, and the fifth lens is the field flattener for the ultrawide 80° aspheric zoom.

So any fast good APO refractor should have 4 lenses plus a field flattener, when the intended use is with the ultrawide eyepieces.

In plain langueage: Any good refractor should be *tuplet, and not just *let

Best,
JG

PS: I forgive the 6th lens in the Leica, as it is just the internal focuser.

Re: Achromat Vs Semi-APO Vs APO - What's The Difference

Posted: Sun Jan 31, 2021 8:23 pm
by j.gardavsky
TareqPhoto wrote: Sat Jan 30, 2021 7:50 pm
notFritzArgelander wrote: Sat Jan 30, 2021 5:56 pm
AntennaGuy wrote: Sat Jan 30, 2021 4:57 pm
Surely, you can just look at what it says it is, on the box that it came in, right??
:dance: :eek: :twitch: :whistle: :whistle:
Yes! Everyone knows that "triplets are better than doublets" so lets ditch the trips and get sextuplets! :lol: As long as I have fingers and toes to count elements, that's all that matters, right? :popcorn:
Maybe i won't go to that much elements, so eliminate one element, how about a quintuplet then?
A quintuplet is a good solution.
A sort of is my Leica APO Televid 82mm: 4 lenses in the light gathering groups, and the fifth lens is the field flattener for the ultrawide 80° aspheric zoom.

So any fast good APO refractor should have 4 lenses plus a field flattener, when the intended use is with the ultrawide eyepieces.

In plain langueage: Any good refractor should be *tuplet, and not just *let.

Best,
JG

PS: I forgive the 6th lens in the Leica, as it is just the internal focuser.

Re: Achromat Vs Semi-APO Vs APO - What's The Difference

Posted: Mon Feb 01, 2021 1:10 pm
by John Baars
j.gardavsky wrote: Sun Jan 31, 2021 8:23 pm In plain langueage: Any good refractor should be *tuplet, and not just *let.
Best,
JG
I just talked to my wallet. He grinned :naughty: and told me to go to the wife. :hand:
:D

Re: Achromat Vs Semi-APO Vs APO - What's The Difference

Posted: Mon Feb 01, 2021 2:48 pm
by TareqPhoto
j.gardavsky wrote: Sun Jan 31, 2021 8:23 pm
TareqPhoto wrote: Sat Jan 30, 2021 7:50 pm
notFritzArgelander wrote: Sat Jan 30, 2021 5:56 pm

Yes! Everyone knows that "triplets are better than doublets" so lets ditch the trips and get sextuplets! :lol: As long as I have fingers and toes to count elements, that's all that matters, right? :popcorn:
Maybe i won't go to that much elements, so eliminate one element, how about a quintuplet then?
A quintuplet is a good solution.
A sort of is my Leica APO Televid 82mm: 4 lenses in the light gathering groups, and the fifth lens is the field flattener for the ultrawide 80° aspheric zoom.

So any fast good APO refractor should have 4 lenses plus a field flattener, when the intended use is with the ultrawide eyepieces.

In plain langueage: Any good refractor should be *tuplet, and not just *let.

Best,
JG

PS: I forgive the 6th lens in the Leica, as it is just the internal focuser.
Don't know which post of this double post i should quote, but it doesn't matter.

Well, there is that scope of quintuplet, i don't know if those two elements or 1 is the flattener actually, because this scope also has a reducer to be used, so now how many glasses it will be there? won't that affect the quality with more glasses and then filters?

Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

Posted: Thu Feb 25, 2021 12:35 am
by AntennaGuy
O', I bought a refractor quintuplet,
And a Barlow in order to dub' lit.
But it didn't do well,
in regard to the Strehl,
so I traded it for a sextuplet!
:Clap: :dance: :lol:
(Yes, I plan to keep my day job. For now.)

Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

Posted: Thu Feb 25, 2021 1:54 am
by notFritzArgelander
AntennaGuy wrote: Thu Feb 25, 2021 12:35 am O', I bought a refractor quintuplet,
And a Barlow in order to dub' lit.
But it didn't do well,
in regard to the Strehl,
so I traded it for a sextuplet!
:Clap: :dance: :lol:
(Yes, I plan to keep my day job. For now.)
Not bad though! :)

Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

Posted: Tue Apr 06, 2021 5:43 pm
by dagadget
well lets see I have an Astro Tech AT152EDT and I have an Astro Tech AT 72EDII on the way. This is going to be a nice fun little experiment. Will see the differences between the two rather quickly I have to believe.

CS

David

Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

Posted: Fri Apr 09, 2021 7:57 pm
by dagadget
The AT 72EDII will be arriving late next week so I will be setting up a test to see how well the scopes compare.

Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

Posted: Mon May 31, 2021 2:26 am
by Bowlerhat
Other than scientific "APO", in practicality there are other definitions. Like "Marketing APO" - which brands just abused the term "Apo" for selling scopes which isn't..and there's also "Practically APO" which an experience of CA-free in non-APO scopes. I mean I use my classic scopes a lot and tbh I don't see CA (because of long focal length). There's also "Lens APO" where people start to define "apo" based on lens numbers..that doublets can't be APO, or the opposite. This is muddled up further by types of glass. FPL 51,53, 55, BK71, FCD 100, etc..

As a person who has used mirrors, achromats, EDs, fluorites..and eventually long achros, I'd say the term is rather flexible in the field. Personally, I'd just stick to Roland Christen's definition: "bringing 3 wavelengths to a common focus and be corrected for spherical aberration at two wavelengths."