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  #21  
Old 15-03-2019, 08:10 PM
bratislav (Bratislav)
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1. if it has 3 zero crossings on the focus v wavelength curve (3rd order cubic function) then it's a triplet and apochromat and
2. if it has 2 zero crossings on the focus v wavelength curve (2nd order parabolic function) then it's a doublet and achromat
3. if it has 4 zero crossings on the focus v wavelength curve (4th order quartic function) then it's a superachromat



So to answer your question: YES, NO and MAYBE
Well, again you are completely and utterly wrong.
A doublet can easily have 3 zero crossings, and there are couple of glass combinations that will allow a doublet to have FOUR zero crossings (see one of my posts above).
Yes, apochromat or superachromat can be done with just two lenses.
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  #22  
Old 15-03-2019, 08:16 PM
JA
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Originally Posted by bratislav View Post
Well, again you are completely and utterly wrong
A doublet can easily have 3 zero crossings, and there are couple of glass combinations that will allow a doublet to have FOUR zero crossings (see one of my posts above).
Yes, apochromat or superachromat can be done with just two lenses.
Good I like that . Always a chance to learn, but just so we're clear when was the last time I was completely and utterly wrong (since you didn't mention it previously)? In any event read carefully what I said rather than partially quote it to attempt to make a point. One can hardly be "completely and utterly wrong" when one prefaces one's comments with

Quote:
Originally Posted by JA View Post


Well it really depends on how exactly or inexactly one cares to define an apochromat:

Is it defined by
1. the number of lenses in the design or
2. by how well it corrects for aberration (typically spherical and chromatic) ?

Some would say that

1. if it has 3 zero crossings on the focus v wavelength curve (3rd order cubic function) then it's a triplet and apochromat and
2. if it has 2 zero crossings on the focus v wavelength curve (2nd order parabolic function) then it's a doublet and achromat
3. if it has 4 zero crossings on the focus v wavelength curve (4th order quartic function) then it's a superachromat

Others simply say that the apochromat corrects better than the achromat and leave that as some sort of definition.
Just in case you missed it I was demonstrating two opposing points of view. I thought that would have been clear and that it didn't need further explanation.

So please now join in the conversation and answer Jeremy's original question, instead of relying on the crumbs from someone who tries to. EDIT: My apologies, I just noticed you did now answer his question- Well done. Now that wasn't too hard was it?

XXX

Best
JA

Last edited by JA; 15-03-2019 at 08:42 PM.
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  #23  
Old 15-03-2019, 08:22 PM
bratislav (Bratislav)
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This is a KzFSN2 and FPL 53 doublet (one of Skywatcher ED120/900 variants after a redesign).
3 color crossings. Two lenses.
And plenty of those measured by Wolfgang Rohr, confirming the performance in his lab.

BTW, we'd have very similar curve ("S" curve, denoting three crossings) if we paired KzFSN2 with FPL51, FPL52, FCD-1, FCD100 or FPL55. We can also use some Lantanum glass to have same effect. And don't get me started on Fluorite.
Plenty of doublets with 3 crossings.
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Last edited by bratislav; 15-03-2019 at 08:32 PM.
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  #24  
Old 15-03-2019, 08:23 PM
bratislav (Bratislav)
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Do you guys actually read the answers or just go along?
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  #25  
Old 16-03-2019, 12:27 AM
Ukastronomer (Jeremy)
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Even more confusing is, is a Semi Apo Filter what it says, or just a fringe killer ???
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  #26  
Old 16-03-2019, 10:29 AM
brian nordstrom (As avatar)
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The cheaper ones are just basically a light yellow filter , the more expensive ones eg. Baader ' Semi APO ' do quite a good job , I used one for years on my Saxon 150mm f8 Achro ( yard cannon ) and enjoyed the results before getting a Chromocor made by Aries in Russia , these really work but very expensive .

This set up with the Chromocor compared well to a friends 152mm Astro Physic's f8 Starfire APO ( $7000 ) on all objects up until about 350x then the APO took the lead as it stayed sharper than my $1200 Achromat at those high powers .

I did find that the ' Semi APO ' style filters worked well in the 6 inch refractor but dimmed the image quite a bit in the 3 and 4 inch achromat's I had at the time .

Brian.
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Even more confusing is, is a Semi Apo Filter what it says, or just a fringe killer ???
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  #27  
Old 25-03-2019, 07:29 AM
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I've also wondered how good these new range of doublet APOs that are on the market these days.

Tak has several fluorite doublet scopes again, 100mm aperture range, and then there are FPL51 and Lanthanum doublets some up to 150mm aperture.

I have had 3 doublets in the past - an FS102, an FS152 and an Orion ED80.

They were all quite good for visual but lacked in imaging being a bit poor in the blue channel. Blue rings around bright stars was where they were weak.

Whereas the oiled triplets I have had were all much more perfect, TEC110 F5.6 Fluorite, TEC180 F7 fluorite, AP140 F7.5 FPL 53 oiled triplet.

Colour correction is not an issue with AP Riccardi Honders 305 F3.8 and Planewave CDK17 F6.8 and these scopes have lensed correctors. The Honders has a large objective plus a corrector (a triplet I think).

I have not noticed in any of the telescopes I have used that had correctors/flatteners/reducers that colour correction became an issue. So this APO aspect is more about the objective lens not the lenses at the imaging/visual end of the scope so much it would seem.

Anyone used these new Tak doublets for imaging or are they regarded more as a visual instrument? Also these new FPL51 and Lanthanum doublets? They are marketed by Teleskop Services, APM, AstroTech and probably Skywatcher and maybe others (Sharpstar etc).



Greg.
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  #28  
Old 25-03-2019, 07:34 AM
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I might add I have read Roland Christen say an APO doublet with triplet like correction was possible to make. I am not sure if he ever made one but all AP scopes seem to be oiled triplets with the 160 being an airspaced triplet and the 130GTX having one air spaced element in its triplet.

But no APO doublets from AP. Even if you could make a superb doublet it would be pushing it uphill marketing-wise to get it accepted as good and not just hype. Especially at the prices its likely to be sold at.

I have briefly read some reviews about the APM 140mm doublet. APM now sell a 152 doublet same setup, for the same price as the 140!

There is so much choice these days in the telescope market, particularly the APO market. Its hard to get past all the marketing hype and rebranding to see if the scope would be OK or not.

Best again to resort to owners' reviews and inspect images from these scopes. Hard to lie then.

Greg.
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