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

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

#21

Post 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
AP1600 GTO AE, AP1600 GTO, Paramount MX+, TOA130, 20" Obsession with ZOC mirror, Meade 14" ACF F8, STL6303E, ST8XME, STXL6303E, FLI Kepler KL400

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

#22

Post 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
6" F/5 Sky-Watcher achro, 2" BBHS Star Diagonal, 2" zenith prism, 1.25" Takahashi prism
Leica 82mm APO Televid
Eyepieces: Docter UWA; Leica B WW and WW Asph. Zoom; Leica HC Plan S and L, monocentric; Pentax SMC XW, O-, XO; Tak MC O, Carl Zeiss B WW, and Pl, E-Pl, S-Pl, W-Pl;
Swarovski SW; Baader Symmetric Diascope Edition; Nikon NAV SW, ; TMB supermonocentric; Rodenstock; Vixen HR; TV Delos
Filters: Astrodon, Astronomik, Baader, Balzers, Zeiss West and East, Lumicon
Binoculars (7x42 up to 15x85): Docter Nobilem, Leica Ultravid, Nikon Astroluxe, Swarovski EL Swarovision; BA8 (Kunming Optical)
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Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

#23

Post 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 !
SCOPES :ES127 f7.5, SW100 f9 Evostar, ES80 F6, LXD75 8" f10 SCT, 2120 10" f10 SCT, ES152 f6.5.
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BINOCULARS: CL 10X30, Pentax 8X43, 25X100 Oberwerks.
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Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

#24

Post 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.
    Telescope: SkyWatcher: Skymax 180mm F15 Mak, StarTravel 80mm f/5 ---- Meade LX70 8" F5 Newt
      Mount: SkyWatcher AZ-EQ6 GoTo
        Cameras: QHY163M, QHY5L-II-M, ZWO [ASI174MM, ASI290MM, ASI385MC, ASI120MC(damaged)], Sony A7r + Canon DSLRs + lenses [Hasselbald, Canon, Sony, Sigma, Samyang], Hasselbald H4D-60
          Filters: Astrodon Ha 5nm, Cyclops Optics LUX-Series RGB, Optolong NB, Baader filters [M&S Neodymium, Contrast Booster], Skywatcher UHC & OIII
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            Re: Achromat Vs Semi-APO Vs APO - What's The Difference

            #25

            Post 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:
            * Meade 323 refractor on a manual equatorial mount.
            * Celestron C6 SCT on a Twilight 1 Alt-Az mount
            Prof. Barnhardt to Klaatu in The Day the Earth Stood Still: "There are several thousand questions I'd like to ask you.”
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            Re: Achromat Vs Semi-APO Vs APO - What's The Difference

            #26

            Post 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:
            Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 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 IV, Vixen Porta II, Celestron CG5
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            Re: Achromat Vs Semi-APO Vs APO - What's The Difference

            #27

            Post 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?
              Telescope: SkyWatcher: Skymax 180mm F15 Mak, StarTravel 80mm f/5 ---- Meade LX70 8" F5 Newt
                Mount: SkyWatcher AZ-EQ6 GoTo
                  Cameras: QHY163M, QHY5L-II-M, ZWO [ASI174MM, ASI290MM, ASI385MC, ASI120MC(damaged)], Sony A7r + Canon DSLRs + lenses [Hasselbald, Canon, Sony, Sigma, Samyang], Hasselbald H4D-60
                    Filters: Astrodon Ha 5nm, Cyclops Optics LUX-Series RGB, Optolong NB, Baader filters [M&S Neodymium, Contrast Booster], Skywatcher UHC & OIII
                      Software: SGPro, PixInsight, APP, APT, DeepSkyStacker, Nebulosity, SharpCap, FireCapture, PHD2, CDC, Photoshop CC
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                      Re: Achromat Vs Semi-APO Vs APO - What's The Difference

                      #28

                      Post 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:
                      * Meade 323 refractor on a manual equatorial mount.
                      * Celestron C6 SCT on a Twilight 1 Alt-Az mount
                      Prof. Barnhardt to Klaatu in The Day the Earth Stood Still: "There are several thousand questions I'd like to ask you.”
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                      Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

                      #29

                      Post 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...)
                      * Meade 323 refractor on a manual equatorial mount.
                      * Celestron C6 SCT on a Twilight 1 Alt-Az mount
                      Prof. Barnhardt to Klaatu in The Day the Earth Stood Still: "There are several thousand questions I'd like to ask you.”
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                      Re: Achromat Vs Semi-APO Vs APO - What's The Difference

                      #30

                      Post 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.)
                      Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 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 IV, Vixen Porta II, Celestron CG5
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                      Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

                      #31

                      Post 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.
                      Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 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 IV, Vixen Porta II, Celestron CG5
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                      Re: Achromat Vs Semi-APO Vs APO - What's The Difference

                      #32

                      Post 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.
                      6" F/5 Sky-Watcher achro, 2" BBHS Star Diagonal, 2" zenith prism, 1.25" Takahashi prism
                      Leica 82mm APO Televid
                      Eyepieces: Docter UWA; Leica B WW and WW Asph. Zoom; Leica HC Plan S and L, monocentric; Pentax SMC XW, O-, XO; Tak MC O, Carl Zeiss B WW, and Pl, E-Pl, S-Pl, W-Pl;
                      Swarovski SW; Baader Symmetric Diascope Edition; Nikon NAV SW, ; TMB supermonocentric; Rodenstock; Vixen HR; TV Delos
                      Filters: Astrodon, Astronomik, Baader, Balzers, Zeiss West and East, Lumicon
                      Binoculars (7x42 up to 15x85): Docter Nobilem, Leica Ultravid, Nikon Astroluxe, Swarovski EL Swarovision; BA8 (Kunming Optical)
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                      Re: Achromat Vs Semi-APO Vs APO - What's The Difference

                      #33

                      Post 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.
                      6" F/5 Sky-Watcher achro, 2" BBHS Star Diagonal, 2" zenith prism, 1.25" Takahashi prism
                      Leica 82mm APO Televid
                      Eyepieces: Docter UWA; Leica B WW and WW Asph. Zoom; Leica HC Plan S and L, monocentric; Pentax SMC XW, O-, XO; Tak MC O, Carl Zeiss B WW, and Pl, E-Pl, S-Pl, W-Pl;
                      Swarovski SW; Baader Symmetric Diascope Edition; Nikon NAV SW, ; TMB supermonocentric; Rodenstock; Vixen HR; TV Delos
                      Filters: Astrodon, Astronomik, Baader, Balzers, Zeiss West and East, Lumicon
                      Binoculars (7x42 up to 15x85): Docter Nobilem, Leica Ultravid, Nikon Astroluxe, Swarovski EL Swarovision; BA8 (Kunming Optical)
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                      Re: Achromat Vs Semi-APO Vs APO - What's The Difference

                      #34

                      Post 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
                      Refractors in frequency of use : *SW Evostar 120ED F/7.5 (all round ), * Vixen 102ED F/9 (vintage), both on Vixen GPDX.
                      GrabnGo on Alt/AZ : *SW Startravel 102 F/5 refractor( widefield, Sun, push-to), *OMC140 Maksutov F/14.3 ( planets).
                      Most used Eyepieces: *Panoptic 24, *Morpheus 14, *Leica ASPH zoom, *Zeiss barlow, *Pentax XO5.
                      Commonly used bino's : *Jena 10X50 , * Canon 10X30 IS, *Swarovski Habicht 7X42, * Celestron 15X70, *Kasai 2.3X40
                      Rijswijk Public Observatory: * Astro-Physics Starfire 130 f/8, * 6 inch Newton, * C9.25, * Meade 14 inch LX600 ACF, *Lunt.
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                      Re: Achromat Vs Semi-APO Vs APO - What's The Difference

                      #35

                      Post 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?
                        Telescope: SkyWatcher: Skymax 180mm F15 Mak, StarTravel 80mm f/5 ---- Meade LX70 8" F5 Newt
                          Mount: SkyWatcher AZ-EQ6 GoTo
                            Cameras: QHY163M, QHY5L-II-M, ZWO [ASI174MM, ASI290MM, ASI385MC, ASI120MC(damaged)], Sony A7r + Canon DSLRs + lenses [Hasselbald, Canon, Sony, Sigma, Samyang], Hasselbald H4D-60
                              Filters: Astrodon Ha 5nm, Cyclops Optics LUX-Series RGB, Optolong NB, Baader filters [M&S Neodymium, Contrast Booster], Skywatcher UHC & OIII
                                Software: SGPro, PixInsight, APP, APT, DeepSkyStacker, Nebulosity, SharpCap, FireCapture, PHD2, CDC, Photoshop CC
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                                Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

                                #36

                                Post 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.)
                                * Meade 323 refractor on a manual equatorial mount.
                                * Celestron C6 SCT on a Twilight 1 Alt-Az mount
                                Prof. Barnhardt to Klaatu in The Day the Earth Stood Still: "There are several thousand questions I'd like to ask you.”
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                                Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

                                #37

                                Post 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! :)
                                Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 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 IV, Vixen Porta II, Celestron CG5
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                                dagadget United States of America
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                                Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

                                #38

                                Post 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
                                Astro-Tech AT 152EDT Ioptron GEM 45 Mount AKA FracZilla
                                Celestron C11 Carbon Fiber CGEM II Mount AKA Cloudzilla
                                Sky Watcher Mak Cas 180 Ioptron IEQ 30 AKA MoonZilla
                                AT 92 on IEQ 30 Pro AKA ClusterZilla
                                Home Made 8 inch Newtonian Reflector on Rocker Box AKA Scopezilla
                                Celestron 4 1/2 114 mm Newtonian Telescope 910 F/L GT Mount AKA Frankenscope.


                                David
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                                dagadget United States of America
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                                Re: Achromat Vs Semi-APO Vs APO - What's The Difference?

                                #39

                                Post 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.
                                Astro-Tech AT 152EDT Ioptron GEM 45 Mount AKA FracZilla
                                Celestron C11 Carbon Fiber CGEM II Mount AKA Cloudzilla
                                Sky Watcher Mak Cas 180 Ioptron IEQ 30 AKA MoonZilla
                                AT 92 on IEQ 30 Pro AKA ClusterZilla
                                Home Made 8 inch Newtonian Reflector on Rocker Box AKA Scopezilla
                                Celestron 4 1/2 114 mm Newtonian Telescope 910 F/L GT Mount AKA Frankenscope.


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

                                #40

                                Post 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."
                                Cats: Lomo mini mak, Acuter Maksy 60, Celestron C5 XLT
                                Refractors: Tak FC60NZ, Vixen custom 60/910, Vixen FL102s, Vixen Ozuma, Zeiss Telementor II, , Goto Kogaku 65mm, Nippon Kogaku 65mm, etc
                                Gallery: https://www.cloudynights.com/gallery/al ... ic-scopes/
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