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80mm f/15 GOTO Apochromat. Yes Apochromat

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#1 rfic1

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Posted 21 September 2019 - 05:04 PM

 A exceedingly rare telescope. 1st light tonight approaching. If it performs up to typical GOTO standards,  I’ll be in for a real treat. Seeing is projected to be excellent, so I am going to try and nail down some close double for a 80mm scope.

 

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#2 petert913

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Posted 21 September 2019 - 05:29 PM

WOW !  I like that a lot.  Where did you find that beauty?

 

Give us a report when ready.



#3 Jim Waters

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Posted 21 September 2019 - 05:43 PM

Great - enjoy the scope.  Report back...



#4 agmoonsolns

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Posted 21 September 2019 - 06:35 PM

OMG, you lucky dog, that's gorgeous!

 

Please tell us more about it, inquiring minds need to know.



#5 Chuck Hards

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Posted 21 September 2019 - 06:40 PM

Ah, it's a triplet.  I was wondering why an APO would be f/15.  I don't know if it would be a noticeable difference with a well-made achromat at f/15, except at extremely high powers, but I await your report.

 

Nice find!


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#6 rolo

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Posted 21 September 2019 - 06:46 PM

If not perfectly aligned and of the highest quality it may actually not be as sharp as an f/15 doublet. Six surfaces to figure, polish and coat is not an easy task. At f/15 I question the need for a triplet... I'd like to DPAC and star test that one. Really nice and unique with a great mount toowaytogo.gif


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#7 Scott in NC

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Posted 21 September 2019 - 07:33 PM

Wow--very nice!  I don't believe I've seen one of those before. :waytogo:



#8 walter a

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Posted 21 September 2019 - 09:39 PM

Bob that sure is a good looking rig.  Are those original legs on your televue [carton] mount? 


Edited by walter a, 21 September 2019 - 09:40 PM.


#9 Stew44

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Posted 21 September 2019 - 09:43 PM

Is that a Goto mount?

#10 rolo

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Posted 21 September 2019 - 10:01 PM

Is that a Goto mount?

Carton. Also, for those who may not know the ED on the lens cell is for effective diameter not ED glass.


Edited by rolo, 21 September 2019 - 10:02 PM.

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#11 rfic1

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Posted 21 September 2019 - 11:11 PM

The mount is a Carton/Televue systems mount on old Edmund wood legs sold with the 4” Edmund refractor.

Very stable mount. A little more robust than a Vixen DX. 

 

I did spend some time with the scope tonight

 

First of all I could not detect any false color on Vega in focus. 

 

I did a brief star test test using Altair. Seeing was very good as diffraction rings and airy disc suggested pickering 7-8 seeing.  In focus the lens appeared well  collimated with symmetrical, circumferential  diffraction rings. Beautiful colorless airy disc at 200X. Very nice. I was expecting a great star test.  Intra and extra focus was brought to reveal 4-5 rings. Inside of focus,  the rings where sharp and color free, Outside of focus, there was some purple and greenish tint located centrally within the innermost ring. Rings not as sharp or as bright compared to inside of focus.  Improved using a green filter.  A little bit surprising.  I did not think of it at the time,  but I wonder how it may have looked using the GOTO prism diagonal that came with the scope. Instead I used a mirror diagonal. .  I also had a dew heater in place and wonder if this contributed 

 

Slightly disappointed with the star test, I decided to cut to the chase and observe two doubles that would be challenging for a 80 mm scope.

Delta Cygni proved to be relatively easy. Easy split at 200x using a 5mm Pentax SMC ortho. I could detect separation using a 9mm SMC, (133x) only knowing where to look, otherwise I am not sure I would have picked it up. White primary and a bluish secondary. 

 

Next was Pi Aquilae. My understanding  the current separation  in 1.4”. This one proved to be much more challenging but was clearly split using 200x.  Slight yellow primary with a pale blue secondary. 

 

GOTO made four Apochromatic telescopes in the 80mm size.  80mm f/12.5 and f/15 standard apochromatic triplet, 80 f/15 fluorite doublet  and a Fluorite f/15 triplet.


Edited by rfic1, 21 September 2019 - 11:19 PM.

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#12 agmoonsolns

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Posted 21 September 2019 - 11:25 PM

Wow, I would love to see the 80mm f/15 fluorite triplet.



#13 Bomber Bob

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Posted 21 September 2019 - 11:48 PM

Did you try star testing straight-through (no diagonal)?  I don't use one for my initial tests, even when I know the mirror / prism is clean & aligned.


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#14 Jon Isaacs

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Posted 22 September 2019 - 01:11 AM

Wow, I would love to see the 80mm f/15 fluorite triplet.

 

I doubt that it's Fluorite but I could be wrong.  There would be no need for it to be a triplet at 80 mm F/15. I suspect some sort of "short flint" that waa used prior the the inventuon of ED glass.

 

Maybe Thomas Jensen with his knowledge of the early Zeiss apos can comment.

 

Jon



#15 rolo

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Posted 22 September 2019 - 07:18 AM

The mount is a Carton/Televue systems mount on old Edmund wood legs sold with the 4” Edmund refractor.

Very stable mount. A little more robust than a Vixen DX. 

 

I did spend some time with the scope tonight

 

First of all I could not detect any false color on Vega in focus. 

 

I did a brief star test test using Altair. Seeing was very good as diffraction rings and airy disc suggested pickering 7-8 seeing.  In focus the lens appeared well  collimated with symmetrical, circumferential  diffraction rings. Beautiful colorless airy disc at 200X. Very nice. I was expecting a great star test.  Intra and extra focus was brought to reveal 4-5 rings. Inside of focus,  the rings where sharp and color free, Outside of focus, there was some purple and greenish tint located centrally within the innermost ring. Rings not as sharp or as bright compared to inside of focus.  Improved using a green filter.  A little bit surprising.  I did not think of it at the time,  but I wonder how it may have looked using the GOTO prism diagonal that came with the scope. Instead I used a mirror diagonal. .  I also had a dew heater in place and wonder if this contributed 

 

Slightly disappointed with the star test, I decided to cut to the chase and observe two doubles that would be challenging for a 80 mm scope.

Delta Cygni proved to be relatively easy. Easy split at 200x using a 5mm Pentax SMC ortho. I could detect separation using a 9mm SMC, (133x) only knowing where to look, otherwise I am not sure I would have picked it up. White primary and a bluish secondary. 

 

Next was Pi Aquilae. My understanding  the current separation  in 1.4”. This one proved to be much more challenging but was clearly split using 200x.  Slight yellow primary with a pale blue secondary. 

 

GOTO made four Apochromatic telescopes in the 80mm size.  80mm f/12.5 and f/15 standard apochromatic triplet, 80 f/15 fluorite doublet  and a Fluorite f/15 triplet.

Nice report. Did you make sure the scope was well acclimated to outside temp before star testing? It may take a while with a triplet.

 

I too did a short observing session last night with the old FC100.



#16 Blueox4

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Posted 22 September 2019 - 08:00 AM

Beautiful telescope and mount Bob! Agree with BB and maybe startest viewing strait through initially. 



#17 Stew44

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Posted 22 September 2019 - 01:01 PM

Goto doesn't share the need for documenting the quality of their lenses with interferometry and our fascination here in the US with star testing.  They (through some of our Japanese friends), kinda like Larry Braymer at Questar, are focused more on airy disk and overall image quality.  I think there is an advertisement out there where the new ED and APO designs were discussed.  It shows color correction for the designs.  The f/15 is interesting.  Obviously they aren't focused on wide field imaging with these 80mm scopes.   Will see if I can locate that flier. 


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#18 Stew44

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Posted 22 September 2019 - 09:40 PM

Couldn't find the lens brochure yet, but here is Koji Matsumoto's discussion of the 8cm f/15 APO.  Koji is a vendor and a great guy to deal with.

 

https://koji-matsumo...riplet OTA.html

 

Still looking.


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#19 Tyson M

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Posted 22 September 2019 - 10:15 PM

Goto doesn't share the need for documenting the quality of their lenses with interferometry and our fascination here in the US with star testing.  They (through some of our Japanese friends), kinda like Larry Braymer at Questar, are focused more on airy disk and overall image quality.  I think there is an advertisement out there where the new ED and APO designs were discussed.  It shows color correction for the designs.  The f/15 is interesting.  Obviously they aren't focused on wide field imaging with these 80mm scopes.   Will see if I can locate that flier. 

Al Nagler I think shares the same view, if I am not mistaken.  When asked about star testing Tele Vue refractors...I tried looking for where I saw the quote but am turning up empty handed. 



#20 n2068dd

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Posted 23 September 2019 - 01:05 AM

Wow, extraordinarily rare! even in Japan!

 

 Most of the amateurs here can't to see that real GOTO 80mm f/15 apochromat. We were thinking it must be a illusion of many decades ago. I've heard very few ( 10 or less? ) was made including GOTO fluorite doublet. It was made by order, or one lot running. Several years ago, a GOTO super apo 80mm fluorite was appeared in the auction, GOTO telescope staff was so confused what should they do for the restore. Very feared about it. Though happily, the new owner did not ask for them yet. It seems the drawing( blue print at that time) of the lens were somewhere still in the factory. and auxiliary ( or polishing disk) is hard to find, they thought any restoration could not apply.  When I asked them about GOTO 12.5cm ED APO, it was the same. they happily find the blue print which some staff had keeping, though that's it.

 In general, GOTO optics were superior than Nikon and Takahashi. They seem to made hand figured one by one. less curve error, less polishing error in their product. One decade ago, the Japanese telescope magazine tested most of the telescopes and reviewed them including AP,TeleVue in the USA or ordered mirrors in the UK. Among them, GOTO optics were different. The polishing was clean than any of the USA made telescopes and Takahashi or Vixen. Especially, for the professional use,which twice the price though same the spec. Nikon and Pentax had the same well polishing, though some curve errors were seen. We concluded there's some issues on the amateur telescope lens. We easily could see it depends on exactly the price. GOTO was expensive.

 In my town public observatory, there equipped GOTO 10" ED telescope. Two of our club member are the staff in daytime and night time.

 

Regards

Hiromu


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#21 n2068dd

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Posted 23 September 2019 - 02:44 AM

Searching from the some information of the 'standard apochromat' as GOTO said, it seems made of non ED lens. Before fluorite appear, there was  APO triplet used with some flint and crown.Though, small lens radius made them long FL for less spherical correction. F15 means the limitation of the correction. Mr. Takahashi Kichiro said same thing, ' A triplet APO made with ordinary material will force us long tube and made harder in very small curve radius shape. We made some test models, though canceled,'

Considering this background, that GOTO standard triplet is rare and most historical telescope.I think. If not as good correction as ED doublet, it has the worth as made of normal lens material.


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#22 Terra Nova

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Posted 23 September 2019 - 08:47 AM

Zeiss made long focus triplet ‘apos’ a hundred years ago without using special dispersion or low dispersion glass. Those lenses were also notoriously hard to collimate. 


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#23 miniqtone

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Posted 23 September 2019 - 08:52 AM

Zeiss made long focus triplet ‘apos’ a hundred years ago without using special dispersion or low dispersion glass. Those lenses were also notoriously hard to collimate. 

This forum is an incredible place to learn. I am trying to drink it all in!!!

-Terry


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#24 DAVIDG

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Posted 23 September 2019 - 09:09 AM

Goto doesn't share the need for documenting the quality of their lenses with interferometry and our fascination here in the US with star testing.  They (through some of our Japanese friends), kinda like Larry Braymer at Questar, are focused more on airy disk and overall image quality.  I think there is an advertisement out there where the new ED and APO designs were discussed.  It shows color correction for the designs.  The f/15 is interesting.  Obviously they aren't focused on wide field imaging with these 80mm scopes.   Will see if I can locate that flier. 

 One has to understand that viewing an Airy Disk doesn't mean you have good optics.  What determines good optics when viewing an Airy disk is the distribution of  light in the  center disk  vs the rings Judging the ratio of intensity is very hard to do.  The problem is the more spherical aberration you have the more light is removed from the central disk and into the rings. Observers see the diffraction rings and believe they have good optics when in fact they don't. That is why other optical tests are much more informative of the true quality of the optics. 

 

                  - Dave 


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#25 Stew44

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Posted 23 September 2019 - 12:38 PM

 One has to understand that viewing an Airy Disk doesn't mean you have good optics.  What determines good optics when viewing an Airy disk is the distribution of  light in the  center disk  vs the rings Judging the ratio of intensity is very hard to do.  The problem is the more spherical aberration you have the more light is removed from the central disk and into the rings. Observers see the diffraction rings and believe they have good optics when in fact they don't. That is why other optical tests are much more informative of the true quality of the optics. 

 

                  - Dave 

I certainly agree completely.  Strehl is a measure of light in the central peak IIRC.  Hard Airy disk is likely more an indication of good seeing.

 

I do not believe however that Goto opticians in any way skimped on testing/evaluation of their optics.  As n208dd said above, they worked on these optics likely one at a time and did a superb job of both figuring and polishing.   He talks about an 'auxillary' polishing disk that was apparently some standard for each lens design.   Sounds kind of ATM-ish and for the few lenses of each of these designs that were likely made, that seems appropriate.


Edited by Stew44, 23 September 2019 - 01:10 PM.



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