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  #21  
Old 15-03-2019, 08:10 PM
bratislav (Bratislav)
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Originally Posted by JA View Post
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
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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
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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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