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

Discuss your refractor type scopes here.
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Achromat Vs Semi-APO Vs APO - What's The Difference?

#1

Post by JayTee » Sat Aug 17, 2019 9:58 pm

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
Main: C-stron CPC1100 #2 Scope: 8" f/7.5 Dob mounted Newt AP Scopes: TPO 6" f/9 RC, ES 80mm f/6 APO G&G Scope: Meade 102mm f/7.8, Bresser 102mm f/4.5 Guide Scopes: 70 & 80mm fracs Mounts: C-stron AVX CGEM & GT Alt-Az, Meade DS2000 Cameras: Canon T3i (x2), ASI120MC Binos: 10X50,10.5X70,15X70 (x2), 25X100 EPs: ES: 21 100°, 30 82° X-Cels: 9, 12, 18, 25 Clubs: RCA & HAS
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#2

Post by Don Quixote » Sat Aug 17, 2019 10:11 pm

This is an outstanding post, JT !
Thank you for putting this up.
SCOPES: ES127 f/7.5, ES80 f/6, SW100 f/9. Meade LXD75 8" Sct f/10, Meade 2120 10" Sct f/10, Bresser 152 f/6.5 Newt.
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#3

Post by notFritzArgelander » 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!
Scopes: Refractors: Orion ST80, SV ED80 A f7; Newtonians: Z12 f5; Catadioptrics: VMC110L, Intes MK66. EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker III, Vixen Porta II, Celestron CG5, Orion Sirius EQG
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#4

Post by Zeroed4x » Sun Aug 18, 2019 2:18 am

A magnificent post. Thank you.
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#5

Post by Lady Fraktor » Sun Aug 18, 2019 5:55 am

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.
Proper Telescopes: Antares 105 f/15, Celestron 150 f/8, Stellarvue NHNGDX 80 f/6.9, TAL 100RS f/10, TS 102 f/11, UR 70 f/10, Vixen ED115s f/7.7
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#6

Post by Lady Fraktor » Sun Aug 18, 2019 6:32 am

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 :lol:
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
Proper Telescopes: Antares 105 f/15, Celestron 150 f/8, Stellarvue NHNGDX 80 f/6.9, TAL 100RS f/10, TS 102 f/11, UR 70 f/10, Vixen ED115s f/7.7
Mounts: Celestron AS-GT, Celestron CG-5 w/ Argo Navis & tracking motor, SLT w/ pier & tripod mods, Manfrotto 028b w/ SV M2C, Mantrotto 055Pro w/ 128RC, TAL MT1C, Vixen SXP w/ HAL-130 & half pier
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#7

Post by JayTee » Sun Aug 18, 2019 6:38 am

Lady Fraktor wrote:
Sun Aug 18, 2019 6:32 am
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
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
Main: C-stron CPC1100 #2 Scope: 8" f/7.5 Dob mounted Newt AP Scopes: TPO 6" f/9 RC, ES 80mm f/6 APO G&G Scope: Meade 102mm f/7.8, Bresser 102mm f/4.5 Guide Scopes: 70 & 80mm fracs Mounts: C-stron AVX CGEM & GT Alt-Az, Meade DS2000 Cameras: Canon T3i (x2), ASI120MC Binos: 10X50,10.5X70,15X70 (x2), 25X100 EPs: ES: 21 100°, 30 82° X-Cels: 9, 12, 18, 25 Clubs: RCA & HAS
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#8

Post by Ylem » 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
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#9

Post by notFritzArgelander » 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.
Scopes: Refractors: Orion ST80, SV ED80 A f7; Newtonians: Z12 f5; Catadioptrics: VMC110L, Intes MK66. EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker III, Vixen Porta II, Celestron CG5, Orion Sirius EQG
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#10

Post by Ylem » Sun Sep 01, 2019 9:20 pm

I'm too dumb to do AP ;)
And yes the simplicity and maintenance would make a doublet more my style ;)
Clear Skies,
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#11

Post by John Baars » Sun Sep 29, 2019 1:26 pm

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!
Telescopes in Schiedam : SW 150mm F/5 Achromat, SW Evostar 120ED F/7.5, Vixen 102ED F/9, OMC140 maksutov F/14.3, SW 102MAK F/13 on Vixen GPDX.
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#12

Post by j.gardavsky » 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
Image

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
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#13

Post by Mandrew » 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!
Devout bino lover and Lunaholic! Scopes - Celestron RASA 8, Orion 180mm Mak-Cass, 6"f8 dob, ST80. binos - Orion 7x50, Oberwerk 8x56 LW, Oberwerk 15x70 Deluxe, Oberwerk 100mm ED Binocular telescope.
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