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Achromatic vs semi-apo vs apo


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I appreciate how these bring the focus of the different wavelengths to differing points, but is this effect most marked when imaging, or does it also have a massive impact when using them for visual work also?

Thanks.

James

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On high magnification a low end achro will show too much chromatic aberration on bright objects, like the moon and Jupiter. Medium to high end achros will show CA only on very high magnification, +100x .With my ED I never noticed any CA either through imaging or visual.

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I see a tiny splash of CA around the brightest stars (eg: Sirius, Vega) with my ED doublet refractors (I don't like the term semi-apo - it's either apo or not !) but nothing around the lunar limb or Jupiter and Saturn unless the scopes are a bit out of focus when a little green or violet appears depending which side of focus you are.

Personally, I prefer not to see CA as far as possible but I know others don't find it an issue for them. I'm a visual observer only and don't do imaging.

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For me it seems most obvious when imaging, especially with the little 70-80mm refractors I use. Visually, the objects (moon / planets) are much brighter than the CA round the edge, so my eye/brain filters the CA out. In imaging, the camera compresses the dynamic range of brightness so the objects seems dimmer and the CA relatively brighter. Very generally, the larger the aperture and faster the focal ratio, the worse the CA appears.

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Sorry, i didn't mean to offend with the semi term, it's just something i've seen used a fair amount.

Is there a system / nomenclature for how much chromatic abberation an optical system causes with the trie apochromatics at the end which causes none?

Thanks for the replies.

James

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Sorry, i didn't mean to offend with the semi term, it's just something i've seen used a fair amount.

Is there a system / nomenclature for how much chromatic abberation an optical system causes with the trie apochromatics at the end which causes none?

Thanks for the replies.

James

:D No offense taken, it's just a term preference.

I'm not aware of the system though. Sticking around, somebody might know :)

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Apochromats are simply achromats with improved, near perfect, colour correction! The term semi or half apochromat has a long history. In the early 20th century, Carl Carl Zeiss sold a doublet "half-apochromat" designated "AS" or "Astro-Spezialobjektiv," with about half the CA of a conventional achromat.

Not all doublet ED scopes are equal even though they show less CA than an achromat of the same focal length and apaerture. My IKI 70ED (also sold under many other names) shows enough colour to be annoying and I have a permanently mounted Baader fringe killer in the diagonal. My SW Equinox 120 shows a lot less colour, nothing but a splash of violet around bright objects which I do not find distracting (though it may still be reducing contrast under some circumstances).

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The correct meaning of the term Achromat is to use two different glass types to bring two wavelengths of light to the same focus. The use of two different glasses produces a focus point vs wavelength plot like a shallow U the exact shape and flatness of which determines the quality of the final image, i.e. the CA.

Apochromats use three different glasses to bring three wavelengths of light to the same focus. Doing this produces a plot of focus point vs wavelength that is like a very shallow S on it's side and has much less CA than Achromats.

When the ED glasses were produced they allowed an Achromat to have better performance ( i.e. CA ) than traditional achromats and often were given the marketing hype of Semi-Apo's

to distinguish them from the older designs while still only needing two lenses which are cheaper to manufacture than three lenses.

Of course, using the ED glass in Apochromats will improve their performance as well but it not so noticeable as they were very good to start with.

Nigel

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Doesn't seem to be hard to find "papers" on such things.

http://www.telescope...t/refractor.htm

Understanding them is often a tad more challenging? :p

But the gist of the above is that a 4" F4.9 APO is "7x better" colour corrected than a 4" F15 Achromat? ;)

As they point out though, this is less noticable visually 'cos the eye sensitivity peaks in the green etc.

Random thought: IIRC, Achromat false colour goes as the CUBE of aperture at fixed f-number?

After all, there has to be a reason people shell out for APOs. And I suspect it's not just colour correction.

My much loved ST102 shows significant "surface roughness", spherical aberration etc. etc. :)

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With regards colour correction we use 3 elements because we get a flatter curve of wavelength to focal length then with 2 so the correction is better, assumes it is designed reasonably. New - yet to be developed - glasses may allow 2 elements to be apo with regards to CA, the wavelength to focal plane being flatter and so qualifing as true apo. Basically FPL-53 is better then FPL-51, so a yet to be made FPL-55 may mean true apo CA performance from 2 elements. May be have to wait for FPL-57. :grin:

Additionally focal ratio comes into it, making an f/5.5 lens is still pushing the glass properties even with a triplet, making an f/7.5 doublet with FPL-53 is I suspect better with regards to CA. With respect to this I would very torn in favour of selecting an f/7.5 FPL-53 doublet instead of an f/5.5 triplet, especially if the triplet used FPL-51.

Thing is triplet does not mean it is apochromatic, just that it should, or can, be.

Might be a bit controversal to say but I would guess that a cemented lens will perform a little less better (at least potentially) then a seperated. The seperated allows for the second and third to have different radaii and the seperation also enables another factor to be adjusted. If cemented then the second and third face have to have the same radaii so you lose one factor to adjust for best performance.

So, to me at least, glass type, focal ratio and construction all come in to play. Oh yes the cost also.

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I think the critical factors include the glass used in the mating element, as much as the ED element, plus the standard of figure and polish and the quality of the lens cell holding the objective.

Control of CA is through using the correct combination of glass types / refractive indexes I think.

TAL managed to produce a virtually CA free scope using no exotic glass in the Apolar 125. It had 6 elements in 3 groups though - a bit of a handful if the collimation got out of whack !

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With all the hype about FPL53, it's interesting most manufacturers don't mention the mating crown element at all.

I think LZOS and LOMO uses K8 with their OK4 ED glass. Skywatcher said they use Schott glass, which really isn't saying much because Schott makes a lot of glass. Other than that, I don't think anyone else says anything about the crown element.

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The differences are, in practice, something like this;

-You need a full apo, triplet or quadruplet, to have top notch colour correction in imaging. (You don't need it to be anything like as good for narrowband imaging.) Anything less than a good triplet/quadruplet will bloat at least somewhat in the blue and so will some triplets and quadruplets as well. Doublets and triplets overlap in colour correction, a good doublet beating a bad triplet. A top triplet does beat a top doublet.

- Good doublet semi apos (often sold as apos) are, in truth, effectively apochromatic in visual observing.

- Traditional old long FL slow achromats can be effectively apochromatic as well but they do have to have very slow F ratios.

- In visual use what you really gain from a top apochromat over a good slow achromat is the ability to frame a much wider field of view should you wish to do so.

-The bigger the scope the harder it is to obtain good colour correction at a given F ratio. There are no F5 6 inch apos on the market so far as I know. I'm sure they could be made but the price would be lethal.

- Colour fringes don't bother some people but do bother others. I'm afraid I hate them.

Olly

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My 4" F9 Vixen flourite doublet shows less CA than my 5" F15 triplet, however the 5" is a Ross objective over 100 years old so did not benefit from modern glasses. The 5" was refigured for me by the legendary Horace Dall so there's nothing wrong with the resolution. :smiley:

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-You need a full apo, triplet or quadruplet, to have top notch colour correction in imaging.

Olly

.....or a Newtonian which is naturally apochromatic! No issues with different wavelengths focusing in different planes.

Also frequently overlooked is what happens outside the visible spectrum in UV and IR. Apochromats are well corrected for visible light but CCD's can 'see' well beyond. You could get a UV or IR halo with these scopes and full spectrum cameras if not filtered correctly. Again- not an issue with mirror systems.

I might also add that mirror based scopes are not without their own problems!

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.....or a Newtonian which is naturally apochromatic! No issues with different wavelengths focusing in different planes.

Also frequently overlooked is what happens outside the visible spectrum in UV and IR. Apochromats are well corrected for visible light but CCD's can 'see' well beyond. You could get a UV or IR halo with these scopes and full spectrum cameras if not filtered correctly. Again- not an issue with mirror systems.

I might also add that mirror based scopes are not without their own problems!

:grin: A Newtonian is apochromatic until you put a coma corrector in it... Perhaps the only true apochromat is the all-reflecting system such as the RC, which has a little field curvature often small enough to live with.

Good point about wavelengths outside the visible. UV/IR filters come in here.

I'm very impressed that Peter has a lens figured by Horace Dall!

Olly

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Perhaps the only true apochromat is the all-reflecting system such as the RC, which has a little field curvature often small enough to live with.

Isn't it customary to use a glass filled reducer with a RC?

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:grin:A Newtonian is apochromatic until you put a coma corrector in it... Olly

Fair point about the Coma correctors- cheap ones will disperse the light. A quality corrector will have been designed with some colour correction in mind (that's why they cost as much as small ED refractor).

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Fair point about the Coma correctors- cheap ones will disperse the light. A quality corrector will have been designed with some colour correction in mind (that's why they cost as much as small ED refractor).

It is more accurate to say that coma correctors are as expensive as eyepieces. Once set up, the Altair Astro/Astro-Tech/GSO works just fine at less than £100. If you buy a Tele Vue one new, then you will of course pay the price...

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It does occur to me that even the worst doublet, ED semi APOs are better than the Zeiss definition which was for half the CA of an achromat of the same size. Today we would expect a ED scope to have half the CA of an achromat of the same aperture but twice the focal length.

I thought I would also spell out the visual difference that find in practice between what I consider to be good modern ED scope and one that is not so good (nor so expensive!). The Sky-Watcher Equinox 120 ED does have a violet or perhaps more accurately an indigo splash at high magnification around bright stars. However the image of the star itself stays the same colour even on nights when the seeing is unsteady. Star images in the IKI 70 ED not only have a bright halo, but sparkle like multicoloured jewels in unsteady seeing without the fringe killer filter.

The nearest Olly is likely to get to his desired 6" F5 is probably the TEC 110mm F/5.6, which I can only dream of.

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...

-The bigger the scope the harder it is to obtain good colour correction at a given F ratio. There are no F5 6 inch apos on the market so far as I know. I'm sure they could be made but the price would be lethal.

...

Are you thinking about a 6" FSQ?

Pentax once made a 6" F6.4 150 SDP which can be reduced down to F4.9 with its 0.77x reducer.

The only price I managed to find for that scope was one dated to year 2000. 78600 Deutsch Mark for the scope + 2100 DEM for the reducer which translates to €41.3k!!! before adjusting for inflation. :eek:

http://www.astrooptik.com/Pentax/pentax_preise.htm

It makes Takahashi's TOA150's £10k price tag looks cheap.

No wonder I have never seen a photo of one in the wild. When you Google 150SDP, all you get is trade show photos.

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