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New Goto 60mm F20

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#1 bremms

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Posted 31 August 2013 - 08:29 PM

Some of you may recognize this scope from another member.
I couldn't resist this one. Goto #104 60mm F20.
Very interesting scope. First off it's lightweight. The end cells and a few of the fixtures are made of Bakelite (phenolic). the whole scope is quite lightweight, about 12 lbs with tripod. The lens is an odd one, I decided to clean it up a bit and check it out. First thing I noticed is the front surface was more curved the the second.. the first surface of the flint seemed to have the same curve as the less curved crown surface. fourth surface was slightly CONCAVE! The only achromat like that is a Cooke doublet.
The scope is hard to collimate due to the height of the screws and is very sensitive to collimation due to being a Cooke doublet. Really cool scope. Thanks to Steve for letting it out of his collection.

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  • 6056746-goto3.jpg


#2 bremms

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Posted 31 August 2013 - 08:30 PM

another pic

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#3 albert1

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Posted 31 August 2013 - 08:37 PM

Beautiful piece. Congrats.

Interesting notes on the objective.

#4 mustgobigger

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Posted 31 August 2013 - 08:43 PM

I had one on eq mount....best
60mm ever ...congrats.

#5 bremms

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Posted 31 August 2013 - 09:16 PM

I'm guessing those phenolic parts cost more than aluminum. They are not that easy to make. It is a great scope. Good collimation is key with this scope. A Fraunhofer doublet would be pretty insensitive to collimation at F20. There were no spacers in the lens, wondering if they were removed at some point. I can't find any trace of them on the glass.

#6 greju

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Posted 31 August 2013 - 09:57 PM

Looks nice! Cannot wait for a first light report.

#7 DAVIDG

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Posted 31 August 2013 - 10:27 PM

I'm wondering if your lens was assembled wrong in the past and your incorrectly assuming that it is a Cooke type. The Cooke design has a large amount of coma while a Fraunhofer is completely coma free. That is why it is a popular design, especially in "classic" refractor from the 1940's to the 1970's and which are the ones typically discussed here. I don't see why GOTO would have selected a design that was not coma corrected ?
As I said in the past I have come across quite a few objectives that were assembled incorrectly and they would still give a decent image, just not as good as they should.

- Dave

#8 bremms

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Posted 31 August 2013 - 10:34 PM

Took the scope out this evening for a bit. Got the collimation spot on now. The scope is a pleasure to use. The mount has been adjusted and tweaked a bit and it is steady and easy to track objects. The optics are very good, on par with my Unitron lens frankenscope. The Zeiss is a little better, just a little. Not much CA as expected. Tiny bit of SA. I did take the lens back out to install some 0.003 Kapton between the cell and lens as there was more play than I liked. Once collimated epsilon Lyra was very nice. Ring nebula was easy from my red/ orange zone. Giant mosquitoes drove me inside after M13. No stars were really visible, but there was a hint of mottling. 60mm is just kind of small compared to 80mm. The moon should be great in this scope. Although it's tall, it's really easy to carry outside due to the weight. The Zeiss is much heavier, but the mount is a good bit more substantial.

#9 bremms

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Posted 31 August 2013 - 10:39 PM

I'm a bit stumped on the lens design. It's not really a Cooke either. The crown is nearly Flat on S2 and has a weaker curve than R3. Really a strange design.

#10 Terra Nova

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Posted 01 September 2013 - 10:29 AM

Interesting scope Marc. Don't see many Gotos outside of academic environments here in the states. I would imagine that they went with plastic to keep the weight down given the long tube length.

I am amazed that there is any CA at all given the small aperture of 60 mm coupled with the high F-number of 20. There shouldn't be, I see none with my 60x1000, of course it could just be my old eyes, but that lens should be essentially apochromatic. I wonder if Dave might be right and the crown element is flipped. At such a low curvature it would be negligible, but still might account for the CA. I had such a case with my 60x910, looked quite good but still not quite right. I took the objective apart and flipped the crown and low and behold, that was it. This could definitely be the case with yours, especially given that there are no spacers, I would be wondering?

#11 Nave

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Posted 01 September 2013 - 05:01 PM

Having located that Goto on CL and originally alerted Steve to it, I consider myself its midwife. :)

Marc, review this thread for a discussion of unique Goto plano lens specs:
http://www.cloudynig...5959552/page...

#12 bremms

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Posted 01 September 2013 - 08:16 PM

Thanks for the link Nave, I remember that one. My lens has the same design. The crown is plano convex or nearly so, with a double concave flint. a very unique deign.

#13 bremms

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Posted 01 September 2013 - 08:20 PM

I think you are correct Terra, Bakelite isn't cheap to make and it has threaded inserts for all the screws and the cell. The paper work indicates its a " Educational Telescope" . It is so different from all my other scopes. One of the reasons I like it.

#14 Glassthrower

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Posted 01 September 2013 - 09:44 PM

Agreed - a 60mm F20 should appear nearly apochromatic on most targets. Something might be a touch out of whack. I'm guessing there should have been spacers in the lens elements at some point. If it was a design that didn't require spacers, then why not glue the elements in place and be done with it? Something might be a little cattywampus.

Awesome scope and mount though. :)

#15 Steve_M_M

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Posted 01 September 2013 - 10:53 PM

Marc,

Your eyes must be better than mine. I could not see even a hint of CA. But, I also have astigmatism, surgically treated glaucoma, and keratitis at times.

Glad it all arrived well. I would be interested to know if spacers make any difference as none of my Goto's have ever had any.

Enjoy the scope,

Steve

#16 bremms

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Posted 01 September 2013 - 11:23 PM

The only time I could really detect CA was on defocused images. Hard to see CA in a 60mm Achro even at F15 let alone F20. Vega looked white with no real CA. The Gotos had no spacers in the lenses from what I can tell. The odd lens configuration seems to be the design. I can find no achromat design with such a lens configuration. It seems to work well, but it does have off axis coma.

#17 Glassthrower

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Posted 01 September 2013 - 11:40 PM

Yes, the whole point of a scope like the 60mm F20 is that you get APO-like performance without the need for exotic glass or extra elements.

It only works on very small scopes, like 50mm and 60mm. The focal length needed for APO-like performance on a 3" or 4" scope becomes prohibitive.

Scratch what I said about the lens being out of whack. I didn't realize that you bought this scope from Steve. He wouldn't have something that was not working right, so it's probably tip-top. :)

Awesome score. I bet that is going to be a great lunar scope.

#18 Steve_M_M

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Posted 02 September 2013 - 12:13 AM


Scratch what I said about the lens being out of whack. I didn't realize that you bought this scope from Steve. He wouldn't have something that was not working right, so it's probably tip-top. :)


I would like to think that is true, but unfortunately scope inventory exceeds time available some times. As Marc knows, I had not even tested this one prior to Marc expressing interest. The Ronchi test looked good and collimation was testy as he noted. What I really noticed when putting this scope through the ringer over the course of a week was how good Meade Series II ortho eyepieces are.

#19 Terra Nova

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Posted 02 September 2013 - 07:15 AM

There are nothing like good orthos with our old long focus refractors in my book. They are the best!

You are right about Bakelite being cool stuff Marc. It's strong, resilient, can be drilled and threaded like metal, is very hard to scratch, won't bend or loose shape, even chips like ceramic. We just don't see it anymore. It was around a lot when I was a kid. Old phones were made out of it, tuning knobs, even the old 78 records. Vinyl put an end to it for some reason.

#20 DAVIDG

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Posted 02 September 2013 - 01:04 PM

Here is a spot diagram of a Baker design 60mm achromat made of BK7 and F4 which are the common crown and flints used in 90% of these classic lens. The Baker design is very similar to a Fraunhofer in that both are total coma free. The spot digram is for a field of view that is 1 degree in diameter or twice that of the Full Moon. The black circle is the size of the Airy disk. Since all the light falls inside the Airy Disk even for the off axis images, the image is totally false color free.
So I'm stubbled as to why GOTO would have used a design that has off axis coma. The coma or lack off it is not glass depended so you can use many different glass types and still have a coma free system.
So even thou others have reported the elements in these GOTO being in this configuration, I'm wondering if what has happened is that we have two or three examples being reported that all were assembled incorrectly some time in the past ?
As for Bakelite, my 1961 Questar has the retainer ring made from it and the very early Quesatrs had the main telescope tube made from it.

- Dave

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  • 6059355-60mmf20.JPG


#21 bremms

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Posted 02 September 2013 - 02:24 PM

The lens has standard index marks and is installed correctly.
the lens elements don't form any standard design in any configuration. Why they didn't use a Fraunhofer or Baker design is beyond me.

#22 greju

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Posted 02 September 2013 - 02:29 PM

I looks like the best "spot" would be at coordinates 0,0. I am kinda slow, what should that tell me. In laymans terms please. ;)

#23 DAVIDG

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Posted 02 September 2013 - 05:37 PM

I looks like the best "spot" would be at coordinates 0,0. I am kinda slow, what should that tell me. In laymans terms please. ;)


The 0,0 position is the center of the field and on the optical axis of the lens. The spot diagram models a field of view that is 1.0 degrees in diameter for a Baker lens design. Typically optical aberration grow as one goes away from the center of the field of view like coma and astigmatism. The black circles is the size of Airy Disk made by this lens. If all the light falls inside the Airy disk then the image at that position is "diffraction limited". The different colors represent the different wavelengths of light. There are many lens designs that are aplanatic which means they are corrected to have no coma or astigmastism off axis. The standard Fraunhofer is one of them and this is why is it so widely used. Almost all classic refractors discussed here use that design. Coma or astigmatism would show up as a pattern that wasn't round and some of the light would fall outside the Airy disk. This Baker lens design shows that all the colors fall inside the Airy except for just the very edge. Even at the edge that tiny bit of light outside the Airy would be invisible and aberrations in the eyepiece would most likely be many time larger. On axis i.e. in the middle of the field of view it is very well corrected with the light from the different wavelengths falling well within the Airy Disk so you won't see any false color.
The GOTO lens is not of this design and from what is reported does show off axis coma. So it is puzzling as to why a company like GOTO used a designed that seem not to be corrected for off axis coma when it is common knowledge that these other fully corrected design have been around for as long as a 100 years. It takes no more effort to make one of these other designs especially for these small diameter objectives. So what are we missing ?

- Dave

#24 Ain Soph Aur

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Posted 03 September 2013 - 12:32 PM

Very nice! I have a SPI 60/1200 that is simply awesome. What I like about the SPI version I have is that It is a long 37" tube so doesn't require a focuser with an extended drawtube like some versions. It is currently a work in progress as I am moving the objective to a push/pull cell, adding a crayford focuser, etc. Unfortunately not a restore with this one, more of a frankenscope when done.


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