FPL 53 Fluorite.

[ollypenrice]

Yes, like Dirksteele I don't think the gass type tells you much of itself.

Olly

[John]

1 hour ago, ollypenrice said:

Yes, like Dirksteele I don't think the gass type tells you much of itself.

Olly

I agree. The match with the other lens elements, figuring and polishing, and quality of the lens cell are just as important, if not more so.

I guess anyone using flourite is going to take some care though as it's blooming expensive stuff !

TAL's 6 element Apolar 125 used no "exotic" glass elements at all but still managed excellent control of false colour as I recall from the sample I tried. 6 elements in 3 groups along the tube poses some other issues though which I feel that TAL did not quite pull off with that design.

I believe that some of the older Astro Physics designs did not use particularly special glass but still have a fearsome reputation.

 

 

[t0nedude]

Yes, agreed with all that has been discussed. The original topic was merely to bring to the attention the term Fluorite and FPL 53 being used interchangeably and misleadingly.

Using extra elements in a lens will create wonderful CC without special glass being needed of course (and is not necessarily the be all and end all in visuals obs) and many other factors will ultimately count just as importantly in the optical train, including but not necessarily limited to: Optical finish, Multicoatings, Lens Cell, Focal Length, EP's used, Observer skill, Seeing Conditions, Lens acclimatisation/tube currents Etc. Probably even more, but the gist has been 'got' so to speak. 

Tony.

[DaveS]

Indeed. The legendary Cooke Photovisual Triplet used glasses that wouldn't be considered exotic today, pre FPL 53 or even Lanthanum glasses.

I guess the ED glasses have in some respects made the lens designers' job easier.

[cs1cjc]

my 2p's worth...

All current optical glasses are "eco-glasses" and contain no Lead nor Arsenic.  Glasses which contained these elements were either reformulated or dropped from the catalogues in the early years of this century.  The reformulated glasses are easy to spot, because their code has the prefix "N-", so for example ZKN7 has become N-ZK7.   There is an excellent article describing the impact of the change to eco-glasses here: A_Brief_History_of_Optical_Glasses

The new glasses have refractive indices that are very close (to four significant figures or so) to their namesake in the less environmental past, but often have very different physical properties to the point where many glass types have been dropped from optical glass catalogues completely, because they could not be well manufactured. 

Other glasses have less good physical properties, including one glass that is particularly important to amateur astronomy, ZKN7, which was a very good glass, inexpensive (only 1.34x the cost of BK7 according to my old copy of  TELESCOPE OPTICS) match to Fluorite and all ED glasses.  The reformulated N-ZK7 is a problem glass for high quality telescope objectives, with too many bubbles and/or striae (variations in refractive index due to inhomogeneity in the glass) to the point where both Roland Christen of Astro-Physics and Yuri Petrunin of TEC, have reported that they will never use this glass again, "once bitten, twice shy".  The Lanthanum crown (sometimes machine mis-translated to coated) glasses now generally used with S-FPL53 in doublets are not such a good match for suppressing secondary spectrum and are also significantly more expensive (~10x the cost of BK7).

"FPL 51 (Glass) < FPL 53 (Glass) < Caf2 - Fluorite (Crystal)" is a very commonly believed truism to the point that Sky-Watcher USA has written that it is not viable for them to sell S-FPL51 based telescopes.  If you add the words "for fast refractors. FPL 51 (Glass)..." then I would agree completely.  Most amateur astronomers want fast short reactors rather than long  ones, even though long focal lengths allow better chromatic correction and give a flatter field which should be important to older observers like myself with decreasing accommodation (Presbyopia).

S-FPL53 is not without its problems.  It is allegedly more difficult to work than S-FPL51 and Fluorite has its own issues because it is a crystal rather than a glass.  The difficulty of polishing S-FPL53 is such that Ohara has introduced a new S-FPL55 even more expensive glass for critical (military?) customers with the advantage of improved polishing characteristics.  Putting S-FPL53 in the middle of a oil spaced triplet not only protects the delicate S-FPL53 from the environment, but would also help minimise the impact of any imperfections in polish.

Perfect apochromatic doublet refractors using S-FPL51, whatever your definition, are available, but they are long and costly, for example see: ASTRO-THEKE.  It is also interesting to optics nerds like me that they use (Schott?) N-ZK7... 

 

[cs1cjc]

On 30/05/2016 at 17:31, t0nedude said:

Indeed.  No surprise to hear that Canon made the optics in the Vixen Fluorites and the Takahashi F model refractors..

Tony.

And recent fast Borg 55FL, 67FL, 71FL and 90FL and the forthcoming 107FL(!) objectives!

[cs1cjc]

On 31/05/2016 at 10:20, John said:

I agree. The match with the other lens elements, figuring and polishing, and quality of the lens cell are just as important, if not more so.

I guess anyone using flourite is going to take some care though as it's blooming expensive stuff !

TAL's 6 element Apolar 125 used no "exotic" glass elements at all but still managed excellent control of false colour as I recall from the sample I tried. 6 elements in 3 groups along the tube poses some other issues though which I feel that TAL did not quite pull off with that design.

I believe that some of the older Astro Physics designs did not use particularly special glass but still have a fearsome reputation.

 

 

The colour correction of a doublet is entirely defined by the choice of glass.  Unlike in a triplet there are no degrees of freedom for the designer to tweak the colour correction.  This is why the Abbe diagram is such a useful tool for the design of a doublet optic. For CeF colour correction, the relative placement of the two glasses on the Abbe diagram tells you all you need to know.  The chart was so very useful before the use of computers for optics: just like a slide rule.

Some of the glasses used in early Astro-Physics refractors were very very special!  Roland Christen bought up unused special glass made to NASA specification.  It is now all used up, it was not an eco-glass, the US manufacturer no longer makes optical glasses, so it is unobtainable and will surely remain so.

Article by Thomas Back: A_Brief_History_of_Astro-Physics_Lenses

[DaveS]

Thanks for those posts Chris. I'll have a look at those links when I'm on the desktop not my 'phone

[John]

Interesting stuff Chris

I can see the point about the constraints on the doublet designer. Did Markus (or whoever designed it ?) do a good job on the APM 140 F/7 that you have ?

It's a scope that I certainly have my eye on as a possible future purchase and I do recall your kind invitation to have a look at yours sometime

I like the concept of "unobtainium" !

[cs1cjc]

6 hours ago, John said:

Interesting stuff Chris

I can see the point about the constraints on the doublet designer. Did Markus (or whoever designed it ?) do a good job on the APM 140 F/7 that you have ?

It's a scope that I certainly have my eye on as a possible future purchase and I do recall your kind invitation to have a look at yours sometime

I like the concept of "unobtainium" !

 

The optical design of the APM 140/7 S-FLP53 ED is by Gerd Düring, a German amateur(!) astronomer and it looks very good to my inexpert eye.  The optical design was checked by 2 different designers (one from Germany, one from USA) and the factory designer and all 3 are agreement that it is a good design.  The secondary spectrum is corrected to the level of 1/13200 of the focal length (for C/e/F) and the polystrehl is 91.7 %.  Düring also designed the 152/8 and the very highly regarded APM coma correcting Barlow.  He appears to ruthlessly maximise the polystrehl, at the expense of strehl in green (e-line).  Markus of APM says that the design secondary spectrum is corrected to the level of 1/13200 of the focal length (for C/e/F) and the polystrehl is 91.7%. 

Mine was measured as follows:

On Green (546 nm) focused (including spherical aberration, coma and Asti.) -> Polystrehl:
F 486nm  0.66
e 546nm  0.9
d 589nm  0.75
C 656nm  0.34

which by my calculation, with weights 0.18, 0.98, 0.78, 0.075 respectively, taken from Eye_Spectral_Response, gives a visual polystrehl of which is less than 0.80, but which rounds to 0.80.  From the same source: Apochromatic_Refractors, this misses an EXCELLENT by a whisker and falls into the GOOD class.  For myself as an owner I am happy that mine rounds to EXCELLENT!  I am also cheered by by the peak measured strehl in C, 656nm, of 0.99 which means the figure must be excellent. 

I am very happy with the colour free high quality views in my testing, which to date has sadly been limited.  You can read more about user experience with the APM 140/7 in this thread: APM_140mm_F/7_is_on_its_way!

[cs1cjc]

A lot has been written on Apochromatism by optical design experts.  There are useful essays by:

* ASTRO-THEKE: What_is_a_visual_apochromatic?

* the late Thomas Back: Defining_Apochromatism and other essays from this page: Essays_on_Optics;

* Roger Ceragioli: Survey_of_Apochromats_(General_Considerations_&_Doublets), and Survey of Apochromats (Triplet Systems) and other chapters accessed from this page: ED-Optiken,_Halb-APOs_und_Frauenhofer-Systeme;

* Roland Christen: Color_Correction_in_Refractors and other essays from this page: CCD_Images_&_Essaybs_on_Optics;

* Pal Gyulai: About_the_focal_ratio_of_apochromatic_lenses;

* Chris Lord: Apo_vs_Achro;

* Harrie Rutten and Martin van Venrooij write in: Telescope_Optics, 1988 edition, that in refractors:
"The difference between the green and red/blue focus is called secondary spectrum.  For normal Fraunhofer-type astronomical doublets, secondary spectrum amounts to 0.0005f or 1/2000 the focal length of the lens. The very best visually corrected doublets possible with slightly abnormal glasses are semi-apochromats or half-apochromats. In these designs, secondary spectrum is reduce to approximately 0.00025f, or 1/4000 the focal length.
Still better color correction can be obtained using new but expensive glasses. With them, secondary spectrum is suppressed to 1/8000 of the focal length... However the best color correction comes only with the use of an extremely expensive material called fluorite.  Fluorite is monocrystalline calcium fluoride.  It has a low refractive index (n = 1.43) and low dispersion (V = 95.6). With this material, the  secondary spectrum can be reduced to 1/16000 of the focal length."

* Vladimir Sacek in his comprehensive notes on amateur telescope optics at: SEMI-APO_AND_APO and on other pages.

It is perhaps worth noting that even if semi-apochromatic is just a marketing term, it has a long history.  Zeiss made doublets designated Halb-Apochromat down to F/11 in the early 20th Century.  The three-lens Zeiss Schwerflint (hard flint) apochromats had a secondary spectrum of only 1/3500, not as good as current ED objectives.  Modern lenses are typically faster due to demand so lose some of their advantage over lenses made before ED glasses and Fluorite became available.  Astrophographers typically prefer triplets because the useful spectrum can be made wider with only a small loss of quality at optical wavelengths.

Everywhere compromises have to made!

[DaveS]

I found these articals by manufacturers, so they should be read with a pinch of salt, perhaps as special pleading, first by Altair-Astro relating to the use of FPL 51 / 53 in their "Wave" series of 'scopes

http://www.altairastro.com/ed-glass-types-used-in-altair-wave-series-refractors-and-how-this-affects-optical-performance..html

And by CFF

http://www.cfftelescopes.eu/optical-design.html

Not too technical for the non-optics nerd, and should be approached with your BS filter turned on.

[cs1cjc]

For what it is worth I now have three refractors which show little no colour even under unsteady UK skies,  a Borg 71FL F/5.6, Equinox 120 F/7.5, APM 140/7, which I am happy to call apochromats.  The one that does show colour which I find unacceptable under UK skies without a fringe killer is an IKI (Kunming, S_FPL31 equivalent) 70ED F/6.  I am happy for the to be called a semi-apochromat or just ED refractor.

[Peter Drew]

My elderly 5" F15 triplet, originally by Ross shows more CA (very little) than my 4" F9 Vixen Flourite.(None?)