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FCT-100 is here :)

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#76 n2068dd

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Posted 14 December 2014 - 12:14 AM

Hi,Derek

 

Thank you.

About for FC-65, it was converted from my cloudy TS-65P triplet made around 1972.FC-65 lens unit is compatible for TS-65P barrel.and TS-65P is famous for cloudy lens.Most of old Takahashi refractor is suffer from degradation of the coating.Maybe, baking temperature was not so high to keep lens shape accuracy.Indeed TS-65P veiwing was so sharp at that time.Ironically All of non coat fluorite element had never been cloudy.Nowadays Takahashi developed hard and longtime lasting coat for all lens element.TOA,TSA or FSQ is better choice. One problom is their paintings.Older grey green or black is far more better than current ivory green.

 

It's same of Triplet fluorite 90F made for Takahashi first apo.The picture was my 90F in short time owned.Previous owner had been sent to the famouse repolish shop to clear up cloudy 90F.But visual performance had badly changed.At least worse than my FC-76.so,90F returned to the previus owner and he sent to Takahashi.

Tkahashi checked ill lense shape and changed new element for it.but,Note that spare lens is not perfect.At that time their lens had variation in the quality.They select each lens for better pairings.so it is not certain that changed element could be the best.

gallery_210261_4807_42518.jpg

 

Generally, repolish will go worse in any lens.especialy in a urtra wide lens of 35mm format.If you hope it better,all element should repolish for the correction.After that I never buy a repolished lens.Reason for the degradation of the coating is humidity and some gas of the fluorite element,Tkahashi said.Japan have rainy season,Tokyo or Osaka is not the best places for keeping telescopes.My FC-65 was from Osaka,it is slightly bigining to the degradation on the edge.Reducer of FC-76D is fit for FC-65,Takahashi said.I mainly use FC-65 for astrogragh.I am thinking it's enough,and very pleased of it.

 

Regards

Hiromu


Edited by n2068dd, 14 December 2014 - 02:08 AM.

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#77 n2068dd

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Posted 14 December 2014 - 12:33 AM

New coat ,Takahashi called multi-coat ,have good durability,from 1986 or7,no need for worry about humidity.

I am living in north Japan of Hokkaido,climate is better than Tokyo,except heavy snow.There's no sign of cloudy in about 30years.

Here's my FCT-100 set on a genuine pillar.

I remember of summer in'87 first light of jupitor by FCT-100 ,so crips image.Incredible detailed image for diameter 4 inch,I could not sketch,220x ,with eyepiece Hi-OR 2.8mm.I wish I cloud pushed more high magnificasion. In the next time, shot saturn with NikonF2 very long time exposure of 30 seconds.That photo,selected the magazine 'Tenmon guide' readers photo,was best image of 4inch and more like 8inch reflecter at that time.

gallery_210261_4807_366094.jpg


Edited by n2068dd, 14 December 2014 - 02:00 AM.

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#78 edif300

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Posted 14 December 2014 - 05:13 AM

Hi Hiromu,

 

Takahashi (according the information they said) are using multi coated lenses since october 1986. Since 25th June 1992 also the Fluorite lenses are multi coated. So the coating degradation problem by very high humidity would be on lenses before 1986. But what I see, this problem doesn't occur in all cases on telescopes that were stored in high environmental humidity conditions so maybe there are certain concrete trigger or the coating quality itself also helps. Or am I wrong?



#79 Fomalhaut

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Posted 14 December 2014 - 05:54 AM

Hi Hiromu,

You really are a rich source of information and I am happy you have found this thread and are contributing to it!

In the early 90ies I had a Vixen Fl102/900 that reportedly had been especially selected for a test report. When I got it, its objective had slightly started fogging but nevertheless, it's performance was even better than that of standard Fl102s - more like Zeiss APQ100/1000.

I wonder now if it had been produced with an earlier partner-glass (maybe an earlier model?), thus providing better test-results...

If my informations are correct, the FCTs partner-glasses are of BK7 and K3 (can you confirm this?) that have a very good long-time stability plus are both multicoated. The uncoated fluorite-crystal lens is sandwiched well-sheltered in between.
Anway - my FCT100's objective-lens looks brand-new even in the brutal flashlight-test, a real pleasure to behold, and this after 22 years.
(BTW, the humidity of the air here in Switzerland only seldomly rises above 50%.)

Regards

Chris

#80 n2068dd

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Posted 14 December 2014 - 07:03 AM

Hi Hiromu,

 

Takahashi (according the information they said) are using multi coated lenses since october 1986. Since 25th June 1992 also the Fluorite lenses are multi coated. So the coating degradation problem by very high humidity would be on lenses before 1986. But what I see, this problem doesn't occur in all cases on telescopes that were stored in high environmental humidity conditions so maybe there are certain concrete trigger or the coating quality itself also helps. Or am I wrong?

 

Hi,Inaki

 

Yes,it's certain.That's said some element of the glass will precipitate to the lens surfase,or some gas will volatilize.Takahashi said some gas.so,I thought a trigger may be the temprature.

I had some experiance of lens without coating.Old Zeiss tesser whitch non coated easily went cloudy by humidity and finger print

and alcohol.I was surprised to know a lens will rust and no one can toch the raw lens.Old Takahashi lens looks no coat,but they were all coated.Coat was the protection in some way not only for the reflection loss.At least,cloudy lens is far more better than repolished clear lens.

Mr.Hata said new multi-coat is baked hard than old coat.

 

Regards

Hiromu



#81 n2068dd

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Posted 14 December 2014 - 08:07 AM

Hi Hiromu,

You really are a rich source of information and I am happy you have found this thread and are contributing to it!

In the early 90ies I had a Vixen Fl102/900 that reportedly had been especially selected for a test report. When I got it, its objective had slightly started fogging but nevertheless, it's performance was even better than that of standard Fl102s - more like Zeiss APQ100/1000.

I wonder now if it had been produced with an earlier partner-glass (maybe an earlier model?), thus providing better test-results...

If my informations are correct, the FCTs partner-glasses are of BK7 and K3 (can you confirm this?) that have a very good long-time stability plus are both multicoated. The uncoated fluorite-crystal lens is sandwiched well-sheltered in between.
Anway - my FCT100's objective-lens looks brand-new even in the brutal flashlight-test, a real pleasure to behold, and this after 22 years.
(BTW, the humidity of the air here in Switzerland only seldomly rises above 50%.)

Regards

Chris

Hi,Chris

 

Thank you.

But there's so many heavy refractor mania in Japan,I'm not so much.

FL102 used FKzF and it's said TAK used BK7.Takahash had never opend the element unit.so I don't know what was.

Do you know this site?

http://www.astrophot...kkei/sekkei.htm

The owner is the famous lens designer Mr.Kato who designed FL102 at VIXEN.FL102 has high reputaion than FC100 in Japan.

 

Regards

Hiromu



#82 Steve D.

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Posted 14 December 2014 - 09:50 AM

Here's another link with info on the FCT line of scopes.  http://www.astrosurf...FCT-100/FCT.htm



#83 Derek Wong

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Posted 14 December 2014 - 03:22 PM



FL102 used FKzF and it's said TAK used BK7.Takahash had never opend the element unit.so I don't know what was.

Do you know this site?

http://www.astrophot...kkei/sekkei.htm

The owner is the famous lens designer Mr.Kato who designed FL102 at VIXEN.FL102 has high reputaion than FC100 in Japan.

 

Regards

Hiromu

 

 

Hi Hiromu:

 

Thank you so much for clarifying this issue.  I think that the Tak doublets used KZF2 although I have never seen that directly confirmed.  This chapter http://rohr.aiax.de/chapter%204b.htm has the following quote:

 

"One must understand that both KzFS1 and KzFSN4 are extremely sensitive to water and will quickly be etched by it, if it is allowed to stand on their surfaces for more than a few minutes.  Therefore, these glasses--and indeed, all the most useful short-flints--need careful protection from moisture if the lens they make up is to last a long time."

 

I presume that proper coatings and baking will prevent issues in these scopes, and that is why all the modern Tak and Vixen fluorite doublets are still holding up.  I have a Tak FCT 65 triplet that has uncoated elements, presumably different than the doublets.  It spent most of its life in dry California and I see no evidence of clouding at all.

 

Derek



#84 Larry Geary

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Posted 15 December 2014 - 12:55 AM

I have a Tak FCT 65 triplet that has uncoated elements, presumably different than the doublets.  It spent most of its life in dry California and I see no evidence of clouding at all.

 

Derek

 

 

I'd love to see a picture of the FCT-65 when you have the chance.


Edited by Larry Geary, 15 December 2014 - 12:56 AM.


#85 Derek Wong

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Posted 15 December 2014 - 03:14 AM

Hi Larry:

 

I don't have a great photo but here is one in the original state, and one with different parts put on including a 2" focuser (these changes are reversible).  The tube paint on this particular scope chips extremely easily, and it looks like moths have eaten it.

 

This particular sample was respaced by Mr. Rohr to have a monochromatic Strehl of 0.99.  I have no idea if that had any effect photographically, but from a visual perspective it has no color.  I have yet to test it vs other long focal ratio scopes.

 

Derek

 

FCT 65.jpg

 

FCT-65 Modified.jpg


Edited by Derek Wong, 15 December 2014 - 03:47 AM.

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#86 n2068dd

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Posted 15 December 2014 - 03:20 PM

 

Hi H.I.

Beautiful images !

 

Did you purchase the FCT in 1987? So in year 1986, nearby sixty units were manufactured.

 

You can use the TOA-35fl in order to improve greatly the peripheral correction at prime focus for full frame format. Gives a corrected imagen circle of 44mm at f6.3.

 

Regards

Inaki

Hi,Inaki

 

Thank you so much.

Yes, exactly purchased in1987. At that time I stopped gazing a star from 1976,that was the re-entering the star world.

Without FCT-100,I would not.but soon after that I had to go busy for living,very little time for watching a star.

 

and, yes I know TOA-35FL is conpatible for FCT-100. I've heard from 'STAR BASE' inTokyo.

They made adapter for it.Maybe I'll buy for it.

 

My recent concern is FCT-76.I think It's very rare  in Japan than FCT-100.

I did not see any images in japnease astro magazine.

so,I bought FCT-76 last year,of course second hand,for how it reproduse.

Now I am take it to a dealer overhaul and repaint to gray color.waiting about one year.

 

Here Mr.Hta FCT-100 designer expain how he made.

 

Regards

H.G.

gallery_210261_4807_26778.jpg

 

Hi,there

 

I translated this copy.It may have many mistakes or wrong translation,please permit.

 

Title-------Recommended telescope as astrophotograph by Takahashi

text-------Kenichiro Hata(optical engineer of Takahashi)

 

Recently in astro photograph, we’ve seen great high growth of technical achievements. That made us to develop FC telescopes from 1980 and next to FCT as photo-visual telescopes. As many knows, it’s alternative if which should be based on a photograph or visual use.  
In case of visual use, we should minimize the spherical and color aberrations for sharp center images. and should set focal ratio F10 to F12 for magnify up to 20times large by objective diameters as effective limit. In this case, we are allowed to narrow field correction because of the eyepiece field.
Another case of photograph, we needs not so severe correction of center images, because of the film grain or air streams which affects the image quality. Small focal ratio and short focus length for high speed exposure time forces the telescope as wide angle lens. As for the border zone of the photograph, we must optimize the another aberrations.
This conflict of alternative demand, discussed (Kiichiro Takahashi)with fumbling hard, we finally came to the conclusion as system telescopes with sub-optics of reducer. FC-100 was the first telescope of this concept.

 

FC-100
We set objective diameter 100mm and focal length 800mm for visual use. and reducer will decrease the focus as 590mm. F ratio will be smaller F8 to F5.9 for utilize as astro-camera. Reduction ratio of reducer,we optimized as 0.7. It’s said in visual use based on the human sensitivity, we should design the ray trace of wavelength 656.3 to 486.1nm for adequate correction. Despite astro-camera,we should design wide 435.8nm g-line or more of 404.7nm h-line. For the balance of the objective and reducer,we should set the objective to minimize the color correction, not for force the exessive power load to reducer. Therefor,we select the doublet objective used fluorite crystal instead of the classical KzFS. Fluorite have wide range of ray transparency 125nm to 10micro-m,nealy to the ultra-violet ray. This will be useful for the astrophotograph.Reducer is mainly used for correction of field curvature and coma.fig-1 shows the spot diagram with reducer caliculated mixing each 250 of d,C,F,G-line. D-line is the standard line and used as the best focus point

 

FCT-100

 Series of FCT, upgrading FC concept, should take advanced level of star spread function, we intended. The objective of FCT consists of fluorite triplet, eliminated the 2nd order spherical aberration and color aberration, making possible a compact short tube and at the same time upgraded the visual use in sharpness and high-contrast. The center image of the best focus field, mixed 4 different wavelength, will be 30% smaller than FC. In fact, you can easily be aware of. Optional reducer for astrograph is consist of gorgeous three elements, which yields to 458.7mm focal length and F4.6. Wide field of 8degrees angle is 1.6time wider than FC-100.
fig-3 shows spot diagram of FCT-100. Angle 0,4degree and 8degree i.e. height of 0,16mm and 32mm with mixed ray of d,C,F,g .

fig-4 shows spherical aberration with eye.

fig-5 shows bigneting image of border zone of FC-100 and FCT-100 both with reducer.

 FCT-100 is gathering more light of 13% up.


Edited by n2068dd, 18 December 2014 - 11:37 AM.

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#87 photiost

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Posted 16 December 2014 - 07:18 AM

Nice scope.

 

Congrats.



#88 edif300

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Posted 16 December 2014 - 09:14 AM

Many thanks Hiromu !!



#89 Fomalhaut

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Posted 16 December 2014 - 10:19 AM

Hi Hiromu,

 

I am answering to two above statements in two different posts of yours:

 

1) "I remember summer 87 first light of Jupiter by FCT-100, so crisp image. Incredibly detailed image for diameter 4 inch, I could not sketch, 220x, with eyepiece Hi-OR 2.8mm. I wish I could have pushed more high magnificasion. In the next time, shot Saturn with NikonF2 (very long time exposure of 30 seconds). That photo [was] selected by the magazine 'Tenmon Guide' as best reader's image of 4inch and more like 8inch reflector at that time."

 

2) "... Series of FCT, upgrading FC concept, should take advanced level of star spread function, we intended. The objective of FCT consists of fluorite triplet, eliminated the 2nd [order] spherical aberration and color aberration, making possible a compact short tube and [at the same time] upgraded the visual use in sharpness and high-contrast. The center image of the best focus field, mixed 4 different wavelength, will be 30% smaller than FC. In fact, you can easily [be] aware of. ..." (Mr. K.Hata, optical engineer of Takahashi)

 

Thank you very much for your kind translation, Hiromu!   {I have very slightly tried to improve readability, only by replacing certain words, sometimes using [ ]. Please tell me should I have got something wrong. Chris}

 

 

 

My own first sight of Jupiter in good seeing at 229x (Hi-Or 2.8mm) was almost shocking: Before that event or meanwhile I might have seen more detail with distinctly bigger instruments in extraordinary seeing, but never more crisp and aesthetically perfect than with this FCT100! And it has beaten all other 4-inchers (regardless of focal ratio) and not a few 5-inchers (plus some 6-inchers also) I have made personal acquaintance of.

 

At FCT100's first starparty in the Alps, a professor of Astronomy at the University of Geneva (that's the city where the CERN is under ground, by the way) took a very long look at Saturn through it and then sighed: “C’est sensationel! Plus beau que là-bas!” (French –> “This is sensational! Better than over there!”).  -  And “over there” was a 150-Starfire standing on its giant mount, showing a big air-bubble covering about 30% of the area of its oiled triplet next morning at daylight…
 
...And all of this in such a nice, small package. It is an outstanding refractor, indeed! Whoever has got one, take care of it!

 

Regards
Chris


Edited by Fomalhaut, 17 December 2014 - 08:21 AM.

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#90 n2068dd

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Posted 17 December 2014 - 07:33 AM

Hi,Chris

 

Thank you for the kindly advise. I will correct.

 

Do you know this page?

http://www.mmjp.or.j...b/data/data.htm

There you can see the first product year and some specs.STAR BASE is the branch shop of Takahashi.

Maybe you all want to know is the whole quantity of the product,but we can not know.

Only 90F is known made 200.

 

We in japan bring the telescopes there when overhole.

STAR BASE will register all client information for their servise.

 

By the way, I want to travel to Alps or Pic du midi,someday.Star would be the stable image,I think.

There, FCT-100 would bring the best performance.

 

Regards

Hiromu



#91 n2068dd

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Posted 19 December 2014 - 02:35 PM

Hi,there

 

I adapted the copy of FCT-65 test report.

This article was issued 1987 january at 'Gekkan Tenmon'.this magagine has stopped from some years ago by reason of the sickness of the editor.

Testing wrighter ,Kenichi Abe is amature astronomer. He describes FCT-65 as the best compact telescopes ever made.

gallery_210261_4822_79340.jpg

First page he find a mistake of ring writing fl300 it should be written fl320.

Second page he compared how border image is with RD.He had measure line spread distribution.

FCT-65 border zone is not equal to Nikkor 300mmED.

For visual use,he praised FCT-65,as arey disc is smaller than FC-76 and resolution is better, color rendition is better,totally perfect.

one downside is it's cannot magnify than another.

gallery_210261_4822_208545.jpg

3rd page He measured MTF and color contribution index.FCT-65 has still telescope-like MTF.

For about transmission efficiency, 76% is not so good as Nikkor 300ED 96%. CCI is biased to blue.

Maybe because FCT-65 did not cut any spectrum, he thought.

gallery_210261_4822_121201.jpg

Last page He measured light distribution and image circle as 58fai.

He conclude FCT-76 suited for visual use dispite need FF. Side grey colum is the Takahashi reply.

There wrote They plan to multi-coat. Finally Takahashi will presented this telescope for a reader.

gallery_210261_4822_64310.jpg

FCT-65 was ultra high price at that time, neary equal to FCT-76.and not so much sold.Recently,FCT-65 apeared in auction.and finshed very high price.Maybe FCT-100 have same caracteristics. I hope this article is usefull for FCT user.

 

Regards

H.G.


Edited by n2068dd, 20 December 2014 - 12:09 AM.

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#92 Fomalhaut

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Posted 20 December 2014 - 05:05 AM

Hi Hiromu,

Wow, that's now a comparative test report as comprehensive as should be done - including MTF plus diagrams showing color distribution as well as field illumination and everything!!!
I regret that I can't read anything apart from the diagrams and your friendly translation into English. Anyway - thank's for sharing!



I have just taken another picture of my FCT100 out of a thread where it didn't belong in - feeling this is the better place for it to be...

So, here is the AAA-sample which I'm happy to call mine:


iw9s-1h-9a11.jpg


By the way: Whoever regrets not having got of one of these, nowadays has the chance to go the TSA-road instead. While these contain an FPL-53 lens in their triplet instead of the FCT's fluorite one, their quality-consistency from sample to sample (variations from A to triple-A) may be somewhat more balanced.
Anyway, if I had to replace the FCT (which I don't), then due to the TSA102's different focal ratio I would have to buy some other costly eyepieces with different focal lengths - but optical quality- (contrast-)wise IMO would neither gain nor loose much in the end.
Therefore, my strongest recommendations for "the new FCT100" (withour "F") => the TSA102!

Chris

Edited by Fomalhaut, 20 December 2014 - 06:22 PM.

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#93 n2068dd

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Posted 20 December 2014 - 02:04 PM

Hi,Chris

 

I saw your picture of observing space. It looks so fine among the woods.and the case for FCT-100 looks fine ,too. My surroundings is filled with light polution. Recently, sky is suffer from Chinese dust storm not clear as 30 years ago.At first I did not used case, put  telescope in the original paper cardbox and carryed by may car. Two years later I bought wood case made at Takahashi.

 

Your case is made by yourself?

I looks so fine work.fit as original.Exactly AAA rank is around later version, I know.Takahashi had improved in thoses days.

 

gallery_210261_4807_207044.jpg

Recentry,FCT-100 is used as camera. The focuser of FCT-100 is far more better than cryford. it did not shift or move with heavy ST-8 CCD camera in any angles. For visual watching, I use MK-65 or FC-65 so far. Flatness of MK-65 is so good, I need not FF.

In the dark sky, FC-50 or 65 is good enough. They can show Vail nebula in Syg. F ratio 5.9 with RD is effective and have wide fields. This photo shows how my town is.

gallery_210261_4807_159519.jpg

 

Regards

H.G.


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#94 edif300

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Posted 20 December 2014 - 02:59 PM

Sapporo?

In few months an ice festival :coldday:  :lol:


Edited by edif300, 20 December 2014 - 03:00 PM.


#95 Derek Wong

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Posted 20 December 2014 - 11:33 PM

Thanks Hiromu for all of the rest of the information.  Can you tell us why the FCT-76 was not more popular in Japan?  It seems like an ideal small telescope.  Also, do you know why only a very few FCT 125s were produced?

 

I am posting the Rohr report of my FCT-65 scope, in the hope that it may be of interest to others.  I have faulty recall - the Strehl without astigmatism (which is likely inherent in the setup) is 0.98.  This file is not on the Rohr site to my knowledge, and it was given to me with the scope so I should be OK in reproducing it here.

 

Mr. Rohr noted that the lens had too much space between the cell and the scope, so it rattled around and was not collimated.  Here are some images (not sure where in the field) before and after correction:

 

FCT 65 Coma.jpg

 

Here is the lens assembly:

 

FCT 65 Lens.jpg

 

A Ronchi test:

 

FCT 65 Ronchi.jpg

 

Correction in different colors (no Strehls for other colors):

 

FCT 65 Colors.jpg

 

Derek

 


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#96 Derek Wong

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Posted 20 December 2014 - 11:38 PM

More FCT-65 files: the Strehl and a map showing a thin edge area that explains why the Strehl is better than one would expect from the p to v values.  In order to optimize the lens at 510nm, different spacers were put in (30 micron, 20 micron - I don't know what the spacing was previously)

 

FCT 65 Strehl.jpg

 

FCT 65 Map.jpg

 

I cannot see color in focus, and the scope takes high power quite well.  One thing I did notice is that there is vignetting that is present just off axis, most likely due to the fast f-ratio and the drawtube diameter.  That was another reason I switched to a 2" focuser.

 

Derek


Edited by Derek Wong, 20 December 2014 - 11:43 PM.


#97 Fomalhaut

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Posted 21 December 2014 - 05:53 AM

At first I did not use case, put  telescope in the original paper cardbox and carryed by my car. Two years later I bought wood case made at Takahashi.

 

Your case is made by yourself?

It looks so fine work.

@ Hiromu:

 

In 1992 when I bought the scope in the shop of Takahashi-Europe, they had a wooden box made by Takahashi for the FC100. When I asked about a box for my FCT100, the importer and shop-owner told me there were none available for FCT-scopes, however he obviously just did not want to go on importing (relatively cheap but heavy) cases all the way from Japan to Europe.

So, I had the one on my above picture made in Takahashi-style by a local carpenter (according to my measurements).

 

@ Derek:

 

After not even Mr.Rohr had fully succeeded in aligning or merely collimating the objective of your FCT65, it does not quite seem to be an AAA-sample, but still is better than most other scopes.

Due to the lens-cell's not exactly fitting, some astigmatism is still visible, indeed.

 

What Mr.Rohr never does is measuring polychromatic Strehl (and weigh it according to human eyes' spectral sensitivity). Another dedicated German Amateur optician (K.Schreckling) did exactly this,  e.g.  with an FCT-76 for which he got a real-world poly(!)-Strehl of 0.98, which is even better than your FCT65's monochromatic Strehl, and which is virtually impossible to be bettered by any refractor in the past or in the future. (That's what I would call an AAA-scope, by the way...)  

 

Since many scopes go to Mr.Rohr not only for being tested but because there is something wrong with, first of all, Mr. Rohr himself mentioned there were scopes he would hardly ever get in his hands ... And indeed, though Mr. Rohr's place is only 300 miles from where I live, he in fact has never got my FCT in his hands, up to now.

 

Regards

Chris  


Edited by Fomalhaut, 21 December 2014 - 07:05 AM.


#98 Derek Wong

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Posted 21 December 2014 - 12:15 PM

Hi Chris:

 

While I have no problem listing Strehls for each wavelength, there are problems assigning one number as a polychromatic Strehl, because it moves from a physical quantity to a physical-biological interaction.  One day I hope to write something about it.  Here are some technical arguments:

 

http://astro-foren.d...tischen-Strehl/

 

However, you don't need all of the arguments to prove that polychromatic Strehl has its issues.  I observed the sun at the calcium K line at an astronomy expo - a bright purple disk, slightly hazy because my eye is not optimized for that wavelength.  The person showing the image was 67 years old and could see NOTHING at all.  If you had two scopes, say one optimized for blue like the current FC series and one optimized for red like the old FC series, two observers of different ages may see completely different things.  In addition, if you have an achromat, having most green in focus and blue and red somewhat defocused in a much different situation than having the "equivalent polystrehl apo" with mostly spherical aberration.  The focused green in the achromat will register on your medium wave visual receptors, and you will see edges better than if everything is a somewhat mushy.

 

Derek


Edited by Derek Wong, 21 December 2014 - 12:29 PM.


#99 Derek Wong

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Posted 21 December 2014 - 12:25 PM

Yes, all of the FCT 65s have spherochromatism and will not match the overall correction of the slower scopes.  However, the purpose of this scope was not high resolution viewing - it was photographs!  Unfortunately, I cannot get my hands on a reducer.  Rohr's measurements state that the astigmatism may be inherent in the system at fast focal ratios, and he does remove it.  The monochromatic Strehl is 0.978, which is fine with me.  In addition, I failed to post that he did add a liner to the lens cell to prevent miscollimation, and to my eyes it is in excellent collimation.

 

The other issue, as Chris mentioned previously, is that there is some variation in the scopes.  I had an older FCT-76 that had a central zone although it gave sharp images.  The current one has an outstanding star test.

 

It is too bad that these scopes are too expensive to make any more.

 

Derek

 

FCT Triplet.jpg


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#100 Fomalhaut

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Posted 22 December 2014 - 07:24 AM

 

1) While I have no problem listing Strehls for each wavelength, there are problems assigning one number as a polychromatic Strehl, because it moves from a physical quantity to a physical-biological interaction.

 

2) The monochromatic Strehl [of your, Derek's FCT65] is 0.978

 

attachicon.gifFCT 65 Strehl.jpg

 

 

1) I know Mr. Rohr doesn't do polychromatic Strehl because he has arguments against or maybe reverse  :question:  has arguments against because he doesn't do it.

And I have been studying his argumentation for years. I'd like to emphasize I esteem Mr. Rohr and what he has done for some of those dedicated amateur astronomers willing to study things in depth.

But on the other hand, I'm also observing the growing importance of the polychromatic Strehl: Mr.Harrie Rutten (co-autor of the renowned book Telescope Optics) is using it, Vla (author of telescopeѲptics.net) is using it, Takahashi is using it and last, but not least Kurt Schreckling measured it for about a dozen real-world refractors some years ago.

The usual procedure is

 

I) The refractor is focused on green (because about there is everybody's highest perceptual sensitivity). 

 

II) Strehl is measured using a green filter and then also in about ten other wavelengths more or less regularly covering the visible spectrum

 

III) In a diagram, the eleven dots can be interpolated by smoothly connecting them, thus obtaining the Strehl curve. (The more measurements the better the curve's continuity.) 

 

IV) Each of the measured colors is weighted according its average human eyes' mesopic or photopic sensitivity.

Here, instead of eleven just five colors (photopic) given for example: 

g - (violet) - 435nm -  0.02 

F - (blue) - 486nm - 0.18

e - (green) - 546nm - 0.98  (555nm => 1.00!!)

d - (yellow) - 588nm - 0.82

C - (red) - 656nm - 0.08   

[Bold numbers => weighting factors!)

 

V) The sum of all color's weighted Strehls is divided by the sum of all weighting factors. 

 

VI) Thus, the resulting weighted polychromatic Strehl (PS) includes full consideration of the average human eyes' spectral response or color sensitivity!

 

VII) Producers and sellers of refractors the PS of which would not be paramount prefer measuring the monochromatic Strehl in one color around 500 - 555 where (even every achromat's) Strehl usually is best and can reach 0.95 and more.   (Of course, this latter procedure is also much cheaper).  

 

Since it would be off-topic, I am not willing to go on discussing PS further on in this thread. 

 

 

2) Sorry, but no, it is not:  As you can see in the table above, Mr. Rohr measured the monochromatic Strehl of your FCT65 => 0.963. Which, by the way, certainly is good enough :waytogo:. 

(The number which you mention is only that part which was produced by astigmatism alone.)

 

 

3)Your three Taks look great! How good is your FCT100?

I'm drooling over your FCT76, but would not be ready to pay what OTOH I myself would also expect for mine...  ;)   

 

Regards

Chris   


Edited by Fomalhaut, 22 December 2014 - 07:34 AM.

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