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Goto Optical Design

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#51 Bomber Bob

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Posted 03 June 2017 - 09:47 AM

Remember that when these telescopes were made, high-power was sought-after for double stars, planetary observing, and detailed lunar features.

 

And I think Goto chose the lens design and built the OTA for that purpose; whereas, my 60x700 & 60x900 refractors are more general purpose.  I can use lower powers on the Goto and the views are very good, but that's not where it excels.


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#52 gitane71

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Posted 03 June 2017 - 10:41 AM

I have not seen a 60mm f/20, but would love to. 

I am just guessing here, No expert on these.

In a thread a while back, brass tubes of commercial refractors from around the late 50's, 60's ear was discussed.  I think it was Terra Clarke  ? that mentioned surplus brass tubing from after WW2 might have been used because it was  available, affordable.  Chuck mentioned 'a question of quality of polish and figure'.  And JW thought maybe the tube assembly was built around a choice of a certain lens.  I wonder if the concave was an element of some optical system,  two concave surfaces that can be easily checked and carefully done,  ground and polished to military specs, and a crown was added to make an achromat.  ?  

I so often hear 'Aperture Rules' and I wonder if this is an example of how Quality is a critical factor as well, this being a particular design Very well done, giving excellent results. 
?  


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#53 Stew44

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Posted 03 June 2017 - 10:41 AM

 

The above diffraction images are easily seen when the scope is out of collimation.  So it appears the real question that we need to quantify is when the coma starts to show up and would become objectionable to the observer.  More effort, but could you do more diffraction images at 10%, 20% and 35% of field?    

 

Got the Ronchi eyepiece.  On an artificial star, my copy of this scope is very straight lines on both sides of focus.  But lots of them for a 10L/mm from what I read so need to make sure it's not simply the grating I'm seeing (saw that in another thread).   

You need to focus till 3 lines are visible for maximum sensitivity. Try both sides of focus. Often one side is dramatically clearer than the other.

 

You can also focus till you have one line acting like a knife edge. Any zones, hills, etc will become visible then assuming good seeing.

 

Okay, this is very helpful.  Wasn't aware you could focus to fewer or greater number of lines.  Ordered a Hubble artificial star this morning but for now was back out at the sunlit ball on top of flagpole.  At seven lines I see everything straight lined dropping to six the lines on one side start to go away.  At five only lines in image about 2/3 of circle.  Remaining lines straight.  Does collimation affect this test?  I think I'm pretty close, but probably could tweak a little more.  And this isn't a dot, on the pole ball, but probably a bright spot a 100 yds away.  Thoughts?



#54 Chuck Hards

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Posted 03 June 2017 - 10:46 AM

 

 

The above diffraction images are easily seen when the scope is out of collimation.  So it appears the real question that we need to quantify is when the coma starts to show up and would become objectionable to the observer.  More effort, but could you do more diffraction images at 10%, 20% and 35% of field?    

 

Got the Ronchi eyepiece.  On an artificial star, my copy of this scope is very straight lines on both sides of focus.  But lots of them for a 10L/mm from what I read so need to make sure it's not simply the grating I'm seeing (saw that in another thread).   

You need to focus till 3 lines are visible for maximum sensitivity. Try both sides of focus. Often one side is dramatically clearer than the other.

 

You can also focus till you have one line acting like a knife edge. Any zones, hills, etc will become visible then assuming good seeing.

 

Okay, this is very helpful.  Wasn't aware you could focus to fewer or greater number of lines.  Ordered a Hubble artificial star this morning but for now was back out at the sunlit ball on top of flagpole.  At seven lines I see everything straight lined dropping to six the lines on one side start to go away.  At five only lines in image about 2/3 of circle.  Remaining lines straight.  Does collimation affect this test?  I think I'm pretty close, but probably could tweak a little more.  And this isn't a dot, on the pole ball, but probably a bright spot a 100 yds away.  Thoughts?

 

You need to be on-axis for the Ronchi test to have any meaning.  At least as close as possible.  With an artificial light source, the closer the source is to the viewing position, the better.  The objective need to be square with the test axis, so if testing an OTA, yes, collimation is important.



#55 Stew44

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Posted 03 June 2017 - 10:55 AM

Thanks Chuck!



#56 Bomber Bob

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Posted 03 June 2017 - 11:20 AM

I wonder if this is an example of how Quality is a critical factor as well

 

I think it definitely applies with optics (besides other products), and it should correlate with the amount of time & attention given to the components.  With the Japanese Classics, Royal (or Yamamoto, or APL, etc.) are better than TOWA; and, Goto are better than Royal; and, that's reflected in the higher prices.  It should also show in better materials & construction, and it does in most cases.

 

My quip is that Goto just had to be different.  Well, Mr. Goto wanted to be the Oriental Zeiss.  To get there, you do what you have to do with the lens design, materials, and production methods & tolerances.  My 452 is unlike any of my other small EQ refractors -- as my Zeiss is a high-quality oddball.


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#57 Stew44

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Posted 03 June 2017 - 11:27 AM

And we need to remember that Goto placed large aperture telescopes in MANY of the Japanese government and university observatories.  We're still seeing some after sixty years finally being removed for more modern equipment.  Were they Fraunhoffers?  I don't think so.  Largest lens I've seen in price sheets is 200mm.  Even then I think it was only $3500.


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#58 deSitter

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Posted 03 June 2017 - 01:14 PM

And we need to remember that Goto placed large aperture telescopes in MANY of the Japanese government and university observatories.  We're still seeing some after sixty years finally being removed for more modern equipment.  Were they Fraunhoffers?  I don't think so.  Largest lens I've seen in price sheets is 200mm.  Even then I think it was only $3500.

It is possible that they chose a design that minimized the amount of glass needed for those larger scopes. Avoids cool down woes and mechanical sag.

 

-drl



#59 DAVIDG

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Posted 03 June 2017 - 01:40 PM

 

 

The above diffraction images are easily seen when the scope is out of collimation.  So it appears the real question that we need to quantify is when the coma starts to show up and would become objectionable to the observer.  More effort, but could you do more diffraction images at 10%, 20% and 35% of field?    

 

Got the Ronchi eyepiece.  On an artificial star, my copy of this scope is very straight lines on both sides of focus.  But lots of them for a 10L/mm from what I read so need to make sure it's not simply the grating I'm seeing (saw that in another thread).   

You need to focus till 3 lines are visible for maximum sensitivity. Try both sides of focus. Often one side is dramatically clearer than the other.

You can also focus till you have one line acting like a knife edge. Any zones, hills, etc will become visible then assuming good seeing.

 

Okay, this is very helpful.  Wasn't aware you could focus to fewer or greater number of lines.  Ordered a Hubble artificial star this morning but for now was back out at the sunlit ball on top of flagpole.  At seven lines I see everything straight lined dropping to six the lines on one side start to go away.  At five only lines in image about 2/3 of circle.  Remaining lines straight.  Does collimation affect this test?  I think I'm pretty close, but probably could tweak a little more.  And this isn't a dot, on the pole ball, but probably a bright spot a 100 yds away.  Thoughts?

 

 

   What you see is when your far from focus is many closely spaced lines like this |||||||| . As you approach  focus the number of lines will decrease and the spacing between them will increase | | | |. You continue adjusting the focus until you see  only three lines. If you have well corrected optics you'll see |  |  |. If you see ) | ( or ( | ) then you have under or over corrected spherical aberration. Any departure from dead straight lines when you have  focused to show three lines indicates some amount of aberration. 

   This is  a single pass test  so if you use it on a real or artificial star  the sensitivity is about 1/4 wave.

 

                - Dave  


Edited by DAVIDG, 03 June 2017 - 01:42 PM.

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#60 DAVIDG

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Posted 03 June 2017 - 02:00 PM

 If the design is correct you have two concave surfaces , one flat one and one convex one So from a  testing stand point it is easier to test them then   other designs.  The two concave surfaces can be tested directly via the Foucault test. The flat surface tested by interference against a Master flat surface. The convex would require a concave test plate and tested by interference.  The convex surface would have the highest level of error  being tested with a test plate, the other three surfaces could tested with a high degree of confidence of their surface quality. So the elements could quickly be tested and one would have  a higher degree of confidence that when assembled into an objective it would perform well without any farther tests.  Since time is money I believe this may have been the reason why this design was chosen over others.

 

                     - Dave 


Edited by DAVIDG, 03 June 2017 - 06:13 PM.

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#61 TG

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Posted 03 June 2017 - 02:37 PM

 If the design is correct you have two concave surfaces , one flat one and one convex one So from a  testing stand point it is easier to test then the other designs.  The two concave surfaces can be tested directly via the Foucault test. The flat surface tested by interference against a Master flat surface. The convex would require a concave test plate and tested by interference.  The convex surface would have the highest level of error  being tested with a test plate, the other three surfaces could tested with a high degree of confidence of their surface quality. So the elements could quickly be tested and one would have  a higher degree of confidence that when assembled into an objective it would perform well without any farther tests.  Since time is money I believe this may have been the reason why this design was chosen over others.

 

                     - Dave 

I think Dave hit it on the head. Otherwise, there is nothing to recommend this design over the Fraunhofer one which has been around since the early 1800s.

 

Tanveer.



#62 Stew44

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Posted 03 June 2017 - 02:47 PM

So in this thread currently is more information on using this Rhonchi eyepiece than I got in two pages of instructions that accompanied it and anywhere else I've searched on the net.  Thank you all. 

 

Goto wasn't making thousands of lenses.  Yet their shop may have been very small by Nihon Seiko standards.  I believe from the post I made of the American who ordered a telescope from them to take back to the states, that they were very proud of their optics.  I think even with the limitations of this design these optics are quite special.  Certainly they would have been looking for efficiencies, because I think demand, especially for the larger institutional scopes, was quite robust.  Probably a combination of all of the above got them to the design we are enjoying today.


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#63 Nave

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Posted 03 June 2017 - 03:22 PM

 If the design is correct you have two concave surfaces , one flat one and one convex one So from a  testing stand point it is easier to test then the other designs.  The two concave surfaces can be tested directly via the Foucault test. The flat surface tested by interference against a Master flat surface. The convex would require a concave test plate and tested by interference.  The convex surface would have the highest level of error  being tested with a test plate, the other three surfaces could tested with a high degree of confidence of their surface quality. So the elements could quickly be tested and one would have  a higher degree of confidence that when assembled into an objective it would perform well without any farther tests.  Since time is money I believe this may have been the reason why this design was chosen over others.

 

                     - Dave 

I love this explanation. Just love it. 

 

Goto hardware is premium relative to other retail telescopes at the time. 

As a manufacturer, what are your choices?

 

1. Commit to the luxury market and not concede on price - smaller volume/greater margins. 

2. Concede on price, and embrace thinner margins relative to your lower cost competitors. 

3. Find cost savings elsewhere and work a broader upper-to-luxury segment in defense of your margins. 

 

If Dave is right, and the economics of testing afforded them some cost savings with their particular optical choice, then they could surrender some on price (thereby allowing them to compete better with retail RAO despite greater hardware costs), but still lay claim to the luxury retail market. 

 

It would be so interesting to see marketing materials from back in the day - to show how Goto sold its wares relative to other retail manufacturers. 



#64 Stew44

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Posted 03 June 2017 - 03:57 PM

Besides the Lazlo Hy-Score marketing in the US (which I don't think was much different than anyone else except Unitron), Goto primarily marketed large institutional refractors and planetarium projectors in Sky and Telescope, IIRC.  No idea what they did in Japan, but had to be very well respected institutionally.



#65 Chuck Hards

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Posted 03 June 2017 - 11:04 PM

Mine isn't a Hy-Score brand import and appears to have been made for the domestic Japan market.  Dreamweaver (Keith) has a Japanese-speaking relative who is working on a translation of the label on my box.  We have a partial translation now, will post it soon.  Many thanks to Keith!


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#66 Steve_M_M

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Posted 03 June 2017 - 11:38 PM

Back in the 1970s when in high school I worked doing drywall finishing in the summer. One job was at a very unusual 3 story house being built by owner using mahagoney and teak. Had a finished observatory on one end of the 3rd floor with a 6" GOTO refractor. When I went back in 1982 after getting out of the Navy the house was gone, someone said it had a fire and about 50% burned down, the rest was leveled and the wood sold. Always wondered if the scope survived, and if it did where it ended up. Was an awesome looking scope and mount, shame I never had the chance to look through it.

This might be it...

 

https://www.cloudyni...oon-star-party/

https://www.cloudyni...x/#entry2247332


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#67 Chuck Hards

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Posted 04 June 2017 - 12:28 AM

Thanks for those links, Steve.  Amazing telescope!



#68 DreamWeaver

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Posted 04 June 2017 - 01:02 AM

The history of Goto...  http://www.goto.co.j.../about/history/


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#69 Stew44

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Posted 04 June 2017 - 11:55 AM

Thanks DreamWeaver, Goto had 90% of the educational market according to their history in the 50's.  Perhaps another reason for a simpler to test set of optics.



#70 Stew44

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Posted 07 August 2017 - 06:56 PM

I wanted to return here for a more or less final comment.  The lenses we have in the Goto Kogaku Models 103 (Comet), 104, 105 (Hy-Score 452), 106 (Hy-Score 454, and 107 (Hy-Score 453) are the design we have been discussing in this thread.  Essentially a very interesting variation of a Cooke doublet that at the longer focal lengths (f/18.75 for the 103 Comet, f/20 for the 104 and 105, and f/17.33 for the 106 and 107) are superb planetary refractors. They are identified by the fact that they do not use spacers.  They are considered 'Kou' or school telescopes.  Those of us with these telescopes consider them some of the finest we've used at their respective apertures. 

 

On the other side, the 201 (Uranus and Hy-Score 451), 202 (Eros), 203, 204 (Hy-Score 455) and 206 (Hy-Score 456) are Fraunhaufer doublets and all at f/15. They are easily identified by the three spacers separating the two lens elements.


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#71 Bomber Bob

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Posted 07 August 2017 - 08:04 PM

Those of us with these telescopes consider them some of the finest we've used at their respective apertures.

 

If there's a 60mm-class vintage refractor that's better than my GHS 452, I haven't seen it.  The accessories are very high quality, too.



#72 bremms

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Posted 07 August 2017 - 08:35 PM

For a doublet with standard glasses the Fraunhofer doublet is optimal. It's coma free, has minimal aberrations, is not sensitive to collimation.

The Goto lens design has no advantage other than it is easier to make the PCX crown element perhaps. Somebody ran it in OSLO and it was not good at all off axis and below F20 it was hard to control the SA.


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#73 Stew44

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Posted 07 August 2017 - 08:59 PM

For a doublet with standard glasses the Fraunhofer doublet is optimal. It's coma free, has minimal aberrations, is not sensitive to collimation.

The Goto lens design has no advantage other than it is easier to make the PCX crown element perhaps. Somebody ran it in OSLO and it was not good at all off axis and below F20 it was hard to control the SA.

Tanveer did the OSLO work above.  Thanks again, Tanveer.  I think this thread has made your comment abundantly clear. grin.gif  Yet it takes nothing away from the performance at the eyepiece we've experienced.   These are school telescopes that had to meet a minimum performance requirement for government subsidized school purchase (which I am certain they did).  The economies of the design, as pointed out by DAVIDG were critical to make sure that even one telescope per school could be purchased.  The optics being what they are, the build quality is exceptional and they fit a wonderful niche.  The Fraunhaufers were marketed along side these Cookes, providing great cost/performance tradeoff.  Even early on, the Uranus (201) was a Fraunhaufer and the Diana at f/20 (precursor to the Comet (103)) was likely a Cooke.  Goto opened the door for the common man to be able to afford a telescope and their kids to learn practical astronomy at school.  I for one consider these Cooke refractors an excellent value even today and agree with JW as to the 105 I have.


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#74 Chuck Hards

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Posted 07 August 2017 - 09:05 PM

I've looked through a lot of 60mm refractors, and the Goto 105 is near the top of the heap for sharpness and color correction- at least with it's contemporary, narrow-field eyepieces.  If I have time this fall, I want to do a side-by-side with the SPI f/20 and Stellar (Towa) f/20.  


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#75 Bomber Bob

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Posted 07 August 2017 - 09:15 PM

These are school telescopes...

 

So were the Telementors -- also fantastic 60mm-class refractors.


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