Grumps

[DaveS]

****Rant Warning****

Well, I've been looking at telescope brochure (No names, no law suit)

and I see a well known line of ED Achromat refracters described as "Fluorite Apochromats". On further reading they turn out to be doublets using FPL 51 or 53 glass.

I think there is, shall we say, a "Terminological Inexactidude" here, since you can't make an Apo lens with two elements, any more (And for the same reason) you can't solve two simultaneous equasions in threee variables.

By definition, an apo lens has to bring three wavelengths to a common focus, and you need at least three different optical materials to do so.

Now good, very, very good though the FPL glasses are they aren't Fluorite, which is, again by definition pure crystalline Calcium Fluoride.

FPL 53, may even be better than Fluorite practically speaking.

Now (Assuming you've got this far) am I being a boring pedant? But consider...some of us know enough to read between the lines and realise what we're getting, but a beginner might not.

Think..what if someone bought an eyepiece thinking it were a Plossl, and it turned out to be a Huygenian, pretty miffed I think would be a good description, likewise if they thought they were getting a true Fluorite Apo at a fraction of teh true cost and found they hadn't.

I suppose you could say "Caveat Emptor" but that would be a bit severe.

Oh well, rant over, exit Victor Meldrew mode/

Dave

[arad85]

I thought I read somewhere that the coatings helped with the convergence. Also, you've more than 3 wavelengths to focus exactly. The spectrum is continuous and doesn't just comprise R, G and B

Could be talking rubbish of course....

[DaveS]

I know there's more....didn't old Newton count all of seven . Seriously, the definition of an "Apochromatic Lens" is that it will bring three wavelengths to a common focus.

Granted these ED Achromats are better than yer bog-standard jobby, they still aint apos. Call them ED achromats or Super Achromats and you're ok, call them apo and the TSOs should come calling

Dave

[arad85]

Having looked at Wiki (oh yes.. see: Apochromat - Wikipedia, the free encyclopedia) I'd say you only need two lenses to get that S-shaped curve... Three wavelengths in focus, from two parabolic (but opposite) curves...

[adamsp123]

OOOOOOOH my.

This could be good - the apo not apo debate, I actually find them quite fascinating. There is usually some good ones on CN that are worth a read.

In my books all the visible colours should focus to exactly the same point to be truely apochromatic, so best buy a Newtonian reflector, but then of course the light goes through the lenses of the EP, are they apochromatic? Ah hell, you can't win!

[A McEwan]

I call mine an Apo. So there.

[adamsp123]

I call mine an Apo. So there.

you mean your "semi apo" 120ED that is just like mine

ooops there's another debate

[barkis]

A manufacturer would have a hell of a job selling them if they were called Nearly Apochromatic. I think most know that whilst they are certainly not without CA, they do a decent job.

Paying the large difference for a true Apo Triplet is not what a lot of people prefer. I own a 150mm f8 achromat. The colour is unmistakable, but I still love the beast.

Ron.

[AndyH]

I hear your rant and concur. It annoys the poo out of me too !!

But...... 2 well mated lenses will make an apo. Think of the now discontinued Takahashi fluorite doublets for example.

I have a Burgess doublet with a Fluorite crystal and a Lananthum mating lens. Stunningly sharp and zero colour in or out of focus, even with insane high powers on bright objects.

I bought that scope because it was a proper fluorite scope and that's what I specifically wanted. I had always dreamed of the Tak doublets but didn't have the finances to get one.

In theory mine is a super achro cause it has 4 crossings or something!! All I know or care about is that's smashingly good.

On some sites/forums cyberspace fist fights occur when these topics are dicussed !!

Cheers,

Andy.

[AndyH]

I must admit I quite fancy a 150mm F5.9 achro. Most certainly not an apo but yum yum all the same.

Andy.

[Whippy]

I thought CN had a makeover when I started reading this thread .... IMO, the term Apochromat has been used/abused (delete where applicable) to now mean if a scope shows none, or virtually no false colour in focus then it's basically an APO.

Does it really matter? It depends how fussy you are I suppose. I've owned a few 'fracs now, 2 triplets (WO ZS110, FLT98), one ED doublet (WO ZS66) and three achros (TAL100R x2 and a Vixen 80S)and I liked all of them visually so I guess it's not a huge issue for me. Imaging though is a different story of course!

And yeah, I keep looking longingly at a 6" achro too!

Tony..

[DaveS]

Hi again, just got back from lighting a show at my am-dram group. Clear night but the people next door are having a "Party" and throwing light everywhere :mad:.

Um back OT my other grump with the description is calling an ED glass doublet "Fluorite" which is quite another thing altogether, there is a big difference between a doublet using fluorite and one using ED glass.

Back in the day, and thinking of microscope objectives (Where the term Apo was first used) fluorite objectives were called "Semi-Apochromats", but they called things what they really were, and didn't suffer from ad-man-itis.

I'm a bit tired and will post more on this sometime tomorrow if it's still active.

Dave

[GazOC]

The distinction usually seems important to people who've dropped £4000 on a refractor that has a Chinese ED counterpart selling for a quarter of that...;O)

[RichieP]

Well to me - Acro - some false colour, can live with it or filter it out

- Apo - no visble (to me) false colour

But then again I am no expert.

If it suits some people to look down on doublets using FPL-53 instead of actual flourite then that's their beef. I simply don't care.

[AndyH]

I don't think it's looking down on anything.

It's the fact that an already brilliant glass, fpl53, is being called fluorite in advertisments, which is plainly wrong. fpl53 is fpl53 and fluorite is fluorite.

Anyway most of the best apo's out there are using fpl53 in their designs now.

Cheers,

Andy.

[DaveS]

I actually think FPL 53 may be better overall than fluorite. Not in the absolute constringence, but I would guess it's more robust, and being a glass would have a lower coeficient of expansion, hence less sensitive to temperature changes.

Also I'm not so naive as to think that a triplet must automatically be better than a doublet, it should be, but much will depend on the quality of the glass, the design, and manufacture.

But back to my original grump, I do think the brochures should call things what they really are and not confuse people who may genuinely not know the difference. But then the brochures are probably written by people who wouldn't know FPL 53 from window glass.

I've no problem with terms like "ED Achromat", "ED Doublet", or even "Super Achromat" But "Apo", "Apochromat", or "Fluorite" mean something totally different to what is the case.

I think I've got this off my chest so I'll stop grumping.

Dave

[Izar]

Just out of curiosity, does any company in the world

manufacture a refracting telescope that meets the Abbe definition

of a Apochromat telescope?

[DaveS]

I think so.

IIRC, the Abbe criterion is for bringing three wavelengths to a common focus, for which you need three distinct glasses, ie a triplet.

Therefore, in theory any triplet design should fulfill his criterion (Note emphasis!). BTW, according to the optical diagrams, the Tak 4-element astrographs are NOT apos, since they are a modified Petzval design, using a combination of a doublet objective lens and a widely separated doublet field flattener (In a true Petzval design, the rear component is a cemented (I think) doublet). A true apo version would use a triplet objective.

Another BTW, I now think calling FPL 53 "Fluorite" is only a slight stretch, since it is a Fluoro-Phosphate glass, probably using mainly Calcium Fluoride in the melt, though also containing other things, ie Silica (I don't think you can make glass without it) and possibly Potassium Phosphate.

An interesting point is that the high refractive component to an ED doublet could be an "ordinary" crown glass, compared to the low index, low dispersion FPL glass. If this element was placed on the outside, (Protecting the FPL 53 component) then we would have an inversion of the usual achro design, first ascribed to Gauss.

Of course I could be talking so much hot air, those who know better are welcome to correct me.

Dave

[johninderby]

I think that one of the problems with defining what makes a true APO is that the advances in glasses and coatings over just the last ten years makes it possible to get APO performance out of a scope that doesn't meet the definition of an APO.

While there is the high end true APO, there's a bit of a grey area in the middle where a scope that's not high end, but still meets the APO definition, might be beaten by an excellent example of what is technically a non-APO.

With problems being caused by such a rigid definition maybe we need something more flexible such as an APO classification system. Perhaps they could be called a Class "A" APO, a Class "B" APO and a Class "C" APO.

Can't see manufacturers going for it, but it would be a useful system for someone to give their opinion of how good a scope's optics were when reviewing it.

John

[laser_jock99]

By definition, an apo lens has to bring three wavelengths to a common focus.

Dave

I think you'll find the spectrum is pretty well continuous.

[DaveS]

*** Exasperated Tone*** I know full well that the spectrum is continuous .

I hold to my point that an apo by definition has to bring three wavelengths to a common focus.

I agree that residual colour (AKA secondary spectrum) in a well designed doublet using ED glasses may be lower than that from a poorly designed "apo" using less well chosen poorer spec'd glasses.

I think the term "Super Achromat" would be appropriate for a doublet using ED glass with a greatly reduced secondary spectrum, reserving "Apochromat" for true triplet designs. I don't think "Semi-Apochromat" would take off.

Dave

[Ben Ritchie]

I don't think "Semi-Apochromat" would take off.

I've seen a few things advertisied as semi-Apo, it was very popular a few years ago with far-east achromats (e.g. the William Optics zenithstar ones). It was never entirely obvious what it meant

But there's always been lots of advertising guff around refractors, and the fluorite thing is just the latest. Whatever taxonomy anyone would come up with would just get twisted round by marketing keen to sell a fast doublet.

[DaveS]

Referring back to an earlier post of mine, early fluorite microscope objectives were described as "Semi Apo", but they were being sold by opticians to scientists who appreciate accuracy.

The reason I suggested "Super Achro", rather than "Semi-Apo" is that "Super" appeals more to the ad-men than "Semi", and thus less likely to be twisted. Or not

Dave

[Ben Ritchie]

Yeah, but I think "apochromat" trumps "achromat" in marketing-speak regardless of a "super" prefix. They've got us all terrified of achromats

[DaveS]

Ok, I'll give you that one :D !

Which brings us back the the last comments in my original post "Caveat Emptor"

Dave