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This is one for the optics buffs.

Getting back into astronomy after 20++ years I realise a lot has changed! especialy the rise of the Triplet Apo telescope.

Re-reading J. B. Sedgwick "Amateur Astronomer's Handbook" I was reminded of the venerable Cooke Photovisual Triplet, and how (By definition) only a triplet design can be truly "Apochromatic", I was wondering whether these new Apo 'scopes are derived from the Cooke design, or are they enteirly new.

I realise that massive strides have been made in optical glass formulation with low and anomalous dispertion types. Incidentluy I'm a little confused by the reference to "Fluorite Glass" in some ads, since Fluorite is, of course, crystaline (And horribly expensive).

Hope someone can enlighten me.

Dave

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Hi Dave,

Welcome to SGL.

Most new apo scopes being made these days are from artificial Fluorite type class, usually called Ohara FPL-53 and Ohara FFL-51 (53 being considered 'better' than 51) although there are other manufacturers.

Triplets are still considered to be the only way to get a TRUE apochromat but a lot of these new scopes come very close esp. if you are just intending the use the scope for visual work rather than image. Also one "trick" some of them use is to make two of the colours at the end of the spectrum focus at the same point at the expense of the third colour. So blue and green may be focued together at the expense of red.

Gaz

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I'm still a little confused, since fluorite isn't a glass, it's pure (Or should be!) crystaline Calcium Fluoride, the only "grades" would relate to chemical purity and absence of crystal imperfections (Dislocations etc). I do remember reading about Fluoride glasses some time ago, but they were regarded as a bit "flakey" compared to bog-standard crown / flint types. times may have changes since then of course :) .

(Not directed ant anyone in particular)...there is a tendency among non-chemists to confuse eg "fluorite / fluoride" and eg "silicon / silicone" (The number of times I've heard of "silicon implants" ouch! a bit hard (unless you're a Borg :) ) )

Dave

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My (non-chemical!!) understanding is that the artificial glasses have the near to the optical qualities of Fluorite (FPL-53 moreso than FPL-51).

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Cheers :) We've probably got the marketing types to blame for the confusion!

I understand (Sort-of) the UD ED LD SLD etc glasses from Geofrey Crawley's optical dissections in the Amateur Photographer lens tests, though it's been a long time since I did any even half-way serious reading on the subject of optical design.

Dave

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i believe some manufactures add some fluorite CaF2, rather than the pure cyrstal. Ie they are lying a bit...some arent true fluorite.

thats what I believe.

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Dave,

I've also heard rumours of the fragility of true flourite lenses - when the Vixen Flourite doublets came out in the early 1980's there was a debate about whether the FL element would deteriorate over time - they don't seem to have so it was probably just sour grapes from the non-flourite scope makers !.

One factor that has affected lens production are new environmentally-driven laws on lead and other content of glass. My Vixen ED102SS had a comparatively short production life (approx 1999 to 2001 I think) because the unspecified ED glass element did not comply with the new legislation. After a short gap in production Vixen came out with the ED103 which used different (compliant presumably) ED glass although it's focal length had also grown a bit.

John

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Yes, all glasses are now supposed to be ROHS compliant, which means no more heavy lead flints! Fluorite is very fragile, also it has a much larger coeficiant of expansion than glass which makes the mounting of the elements that much harder, FL will also change with temp. When Canon use Fluorite in theit "L" lenses, the element is always well buried in the optical train.

Fluorite will degrade over time, though I suspect that such factors as chemical purity and crystal defects (Or lack of) will have an effect. Nineteenth century fluorite microscope objectives are now all unusable due to crystal degredation, even an early-mid twentyth century objective that I have (A Cooke x45 na 0.95 oil) is no longer as transparent as it was when made.

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Just as an interesting note, Sir John Herschel tells us about object-glasses

built by Dr.Blair, that were free from chromatic and spherical aberration.

These were made some 25 years before the Cooke triplet lenses.

When the Cooke p-v scopes came out, they were regarded as being excellent,

sadly for Cooke's, the new Schott glass that was used turned out to be reactive

with the elements. This led to much public mud-slinging, with claim and counter-claim being made. A nice article on the whole episode can be seen here:

http://www.europa.com/~telscope/hdtaylor.txt

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it's worth bearing in mind that Takahashi went the flourite route for some time but the new FSQ is a modified Petzval four element design. Truly apochromatic from blue to IR and only uses ED glass. Seems to me that fashion plays a big part in lens marketing as opposed to design with various manufacturers trying to outdo each other in hype. I can remember a so-called expert selling Taks decrying my choice of TMB as a "oh dear! a triplet, all those glass to air surfaces", and now Tak make triplest and quadruplets! How does he sell those I wonder?

Dennis

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No doubt the dealer was peeved because you went for a TMB, for Takahashi have been building triplet refractors for 40 years now. The first one decribed as semi-apochromat was built in 1969, then in 1972 they brought out an 80mm fluorite triplet scope. Five years later a 90mm model was introduced. By 1983, the FCT range was in development with many models hitting the market by 1985.

Over the years I've been fortunate to look through many fine and splendid refractors from different makers. But I have to say that one of the best scopes I have is my 30 year old Towa 80mm f/15 Achromat with its standard magnesium lens coatings.

The planetary and double star views are simply stunning.

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When the Cooke p-v scopes came out, they were regarded as being excellent, sadly for Cooke's, the new Schott glass that was used turned out to be reactive with the elements. A nice article on the whole episode can be seen here:

http://www.europa.com/~telscope/hdtaylor.txt

This is an interesting article, and a fascinating piece of telescope history...I've referred to it before.

Interestingly, some Cooke p/v's have survived through to present day without tarnish. It's curious that some of these lenses seem to have escaped the fate that most of their siblings suffered. I wonder circumstance rescued these select few?

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I'm still a little confused, since fluorite isn't a glass, it's pure (Or should be!) crystaline Calcium Fluoride, the only "grades" would relate to chemical purity and absence of crystal imperfections (Dislocations etc)...

i believe some manufactures add some fluorite CaF2, rather than the pure cyrstal. Ie they are lying a bit...some arent true fluorite.

Gaz covered this when he mentioned that most of today's apo refractors use Japanese Ohara FPL-53 or FPL-51 glass, with FPL-53 being considered the better.

The first two letters refer to the two most important elements in its composition. The third letter refers to the refractive index: High, Medium or Low. The two digit number refers to the glass type within its family.

F - Fluorite

P - Lead

L - Low refractive index

53 - The 53rd glass in its family

FPL-53 is essentially a man-made alternative to fluorite glass that is more affordable, easier to polish and easier to coat.

HTH

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Worth remembering that wherever there is an APO element there will always be a matching Crown glass element. The quality and manufacture of the Crown is equally as important, as is the quality and design of the cell that holds the elements.

it's worth bearing in mind that Takahashi went the flourite route for some time...

Interestingly (to me at least) it was Canon (the camera/lens manufacturer) who developed the technology necessary to produce fluorite lenses up to 6" and Takahashi (friends of Canon) who first put one in an astro scope.

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Looks like a few people have been reading the article in AN :(

Not yet, but I did cut and paste my responses posted last October in a similar thread :)

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Debates about the pros and cons of lens designs and glass types are interesting but only part of the story IHMO.

Personally I feel that the quality of the figuring, polishing, coatings and mounting of the lens elements are not always given enough weight. Poor attention to any of these factors can produce a poor objective regardless of the glass type used.

John

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I remember talking to an optics designer a little while ago and he said that the lens cell was as important as the lens itself in overall performance. He said I wouldn't believe just how difficult it was to design and make a really top quality one that would get the best out of a lens due to the often conflicting design parameters.

Apparently a temperature compensated lens cell built to the highest tolerances can cost as much as a good lens. Helps explain part of the reason why high end scopes cost more.

John

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Very true John.

I'll be shot if I mention brands but suffice to say some of the very best have struggled with lens cell design. This is also why triplets are so much more difficult to manufacture.

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Interestingly, some Cooke p/v's have survived through to present day without tarnish. It's curious that some of these lenses seem to have escaped the fate that most of their siblings suffered. I wonder circumstance rescued these select few?

I am a new member and so coming to this thread a bit late in the day. I've had a Cooke PV 3.25inch refractor, dating from 1895, since the 1960s. The lens still looks good to me, but maybe I don't know what to look for!

I'm not sure if the tube is Cooke's own as there is no maker inscription. Interestingly, the eyepieces are labelled in mm rather than inches so maybe it's a German tube. The 'scope does need some restorative work, particularly on the rack and pinion focussing. And a respray as you can see ... The head of the tripod was replaced by a solid oak piece to exactly the same size. This needs varnishing but where do you get shellac these days!?

Steve

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Hi Steve, As for your shellac, this should be obtainable from John Boddy`s fine wood and tool store in Boroughbridge in North Yorks :D

John.

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I am a new member and so coming to this thread a bit late in the day. I've had a Cooke PV 3.25inch refractor, dating from 1895, since the 1960s. The lens still looks good to me, but maybe I don't know what to look for!

I'm not sure if the tube is Cooke's own as there is no maker inscription. Interestingly, the eyepieces are labelled in mm rather than inches so maybe it's a German tube. The 'scope does need some restorative work, particularly on the rack and pinion focussing. And a respray as you can see ... The head of the tripod was replaced by a solid oak piece to exactly the same size. This needs varnishing but where do you get shellac these days!?

Steve

Wow, old thread...but what a scope! I've seen pictures of a surviving 5" Cooke triplet too. So selected 5" pv's and under seem to have been able to survive :D. The Europa.com article referred to above mentions that the smaller objectives had slightly modified standard cells, while the larger glass had the ingenious temperature compensating cell. I wonder of one of the metals in this large cell was a factor in the lens deterioration? :clouds1:

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I have a Tak Sky90, a Vixen 102FL and a very old Ross 5" F15 triplet, all appear to be in good order. The first and second element of the Ross have been oiled to give the second element extra support, being biconcave it is very thin in the centre. I have heard rumours in the past of flourite objectives being susceptible to thermal shock and/or soluable if got wet but can recall no instance of this having happened. :D

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Wow, old thread...but what a scope! I've seen pictures of a surviving 5" Cooke triplet too. So selected 5" pv's and under seem to have been able to survive :D. The Europa.com article referred to above mentions that the smaller objectives had slightly modified standard cells, while the larger glass had the ingenious temperature compensating cell. I wonder of one of the metals in this large cell was a factor in the lens deterioration? :clouds1:

Hi Richard,

Thanks for the compliments on the 'scope. I was going to refer you to a discussion I found on the Cloudy Nights forum but I see YOU were the author of the comments I found were of interest! I found your website and the gallery of pics really interesting, especially the 'scope you found for the famous rock guitarist!

Actually, I was a research chemist before I retired and worked on metal corrosion so you may have something there. If there was galvanic corrosion caused by the junction of two different metals in the lens mount then maybe the metal corrosion product might attack the Schott glass. This would be very slow but over the years humidity might transport the corrosion product through the lens assembly if it was soluble.

Or maybe the sprung lens just kept the glass junction tighter?

Steve

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