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  #21  
Old 15-01-2014, 09:30 AM
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I am very interested in the Hubble 20" Ultra Light as it ticks all the boxes. It's shown here:

http://hubbleoptics.com/UL20.html

[/url]
At those prices and fast focal ratios and using plate glass - with no optical guarantee or optical certification that can be challenged , I would say you would have to take the claim of 0.95 Strehl with a grain of salt . The structure certainly looks light.
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  #22  
Old 15-01-2014, 10:09 AM
glend (Glen)
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Yeah, the more I think about it.....and that secondary hanging out front like that will be a problem for stray light and required shrouding around my area..and as you say performance with a grain of salt. I'll go back to Dennis Steele and see if he can do a 20" kit.
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  #23  
Old 15-01-2014, 10:20 AM
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AG Hybrid (Adrian)
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Hubble Optics mirrors have been tested and had their results posted on Cloudy Nights. The few that have been done are test around the 0.90-0.92 Strehl mark. But, as any mirror maker will tell you Strehl isnt everything. To be honest with ourselves, any mirror above 0.90 strehl will probably be a really good mirror that will be good enough for any of us who aren't chasing the numbers.

I will either be ordering the 18" f4 or the 20" f3.7 Hubble Optics ultralight at the end of 2014. Its exciting that there are other options than Obsession ultralights on the market. That being said. Skywatcher is scheduled to release a 18" ultralight dobsonian at the end of 2014. Interesting times ahead.
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  #24  
Old 15-01-2014, 10:48 AM
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Hubble Optics mirrors have been tested and had their results posted on Cloudy Nights. The few that have been done are test around the 0.90-0.92 Strehl mark.
You would need to qualify that: were they mirrors in the larger ultra fast sizes like F3.7 ? And who did the tests - was it with a foucault test ? Or Null interferometer . I would go further and say that an true 0.8 Strehl mirror free of significant figure distortion problems in a large fast size would be a great performer . Unfortunately I've found a number of epic fails in 20" + sub F4 size amongst a number of USA makers - I would be very surprised if a Chinese company were doing a lot better for half the price !

The fact is that large sub F4 mirrors are much harder to make well - polishing and testing techniques that work easily for smaller slower mirrors can start to fail when applied to larger mirrors - no matter how great your enthusiasm might be .
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  #25  
Old 15-01-2014, 11:19 AM
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I guess Barry and I must be examples of status-seeking 'non-serious' observers. Although both of us can set up in about the same time as smaller dobs ( it's the same number of components to assemble), do so singlehanded, and although I concede having a dedicated trailer for the scope is a bit more of a commitment, it makes heading out observing as simple as hitching it up and driving off. Plus you can store a lot of camping crap in the trailer with the scope - I'm self contained, big battery and inverter as well.
Still - perhaps you should ask someone who lives nearby who observes with a 20+ if you can tag along for a night? Most of us status seekers are only too happy to show them off!
Cheers,
Andrew.
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  #26  
Old 15-01-2014, 11:32 AM
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I love showing off my Phallic symbol!
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  #27  
Old 15-01-2014, 11:56 AM
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Originally Posted by Satchmo View Post
You would need to qualify that: were they mirrors in the larger ultra fast sizes like F3.7 ? And who did the tests - was it with a foucault test ? Or Null interferometer . I would go further and say that an true 0.8 Strehl mirror free of significant figure distortion problems in a large fast size would be a great performer . Unfortunately I've found a number of epic fails in 20" + sub F4 size amongst a number of USA makers - I would be very surprised if a Chinese company were doing a lot better for half the price !

The fact is that large sub F4 mirrors are much harder to make well - polishing and testing techniques that work easily for smaller slower mirrors can start to fail when applied to larger mirrors - no matter how great your enthusiasm might be .
I think the mirror tested was 16" f4.5? I believe it was fully bench tested. I can't remember all the details but the paper work that came with the mirror had a very high strehl. Like in the high nineties. The owner didn't believe it and he was right. Apparently the paperwork that came with the mirror only tested one axis of the mirror - whatever that means. I don't know the full details of how its done - I imagine you do. But, the results were inflated. It still turned out to be a great mirror. The owner didn't want or thought it needed to be re-figured.

Who knows what the quality of their sub-4 mirrors are like? For the price I would settle for diffraction limited. I mean, if the one I get isn't to my liking. I can always send it to you for a re-figure, right? Right?
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  #28  
Old 15-01-2014, 02:35 PM
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Well today GSO has thrown a cat amongst the pigeons by indicating they will be introducing a 20" dob later this year. As per my other thread about the announcement, I had emailed the head office this morning asking about future product development in the dob range and had a response this afternoon from Susan Hsu at GSO confirming they were going to 20".

Here is the full content of her email:

Hi Glen,


we plan to make the 20” dobs later this year!



regards,



susan hsu




Then I rang Luke at Andrews Communications to see what he had heard, and after a discussion out of my earshot he said there had been some discussion about the 20". I told him to keep me in mind for #1.

So it's good to see the production big boys getting into the 20" bracket, and this follows on from GSO's recent introduction of the larger 16" truss RCs.
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  #29  
Old 15-01-2014, 07:43 PM
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Unfortunately I've found a number of epic fails in 20" + sub F4 size amongst a number of USA makers - I would be very surprised if a Chinese company were doing a lot better for half the price !
I disagree Mark.
The highly distorted value of the Yuan gives the Chinese a significant advantage in labour costs. To put it another way, a Chinese optical company can get a full months work out of their technicians for the same price as just two days graft from an American. There is just no way that a US company can compete on those terms even at half the price. You would know yourself that you can't even buy the raw materials for making optics here for the same price as you can get the completed item including coating and shipping.

The Chinese might still be a bit behind in the development curve but they are catching up pretty fast. There is nothing (racially) that makes them any less capable of learning how to perform any given mechanical task, so while the imbalance in their currency persists I can only see the merit function leaning further their way.
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  #30  
Old 16-01-2014, 07:15 AM
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Making high quality large fast mirrors is about expertise and knowledge in testing and figuring that is at a much higher level , and no amount of cheap labor can make up for the lack of that . It will happen eventually but it is not there yet .



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I disagree Mark.
The highly distorted value of the Yuan gives the Chinese a significant advantage in labour costs. To put it another way, a Chinese optical company can get a full months work out of their technicians for the same price as just two days graft from an American. There is just no way that a US company can compete on those terms even at half the price. You would know yourself that you can't even buy the raw materials for making optics here for the same price as you can get the completed item including coating and shipping.

The Chinese might still be a bit behind in the development curve but they are catching up pretty fast. There is nothing (racially) that makes them any less capable of learning how to perform any given mechanical task, so while the imbalance in their currency persists I can only see the merit function leaning further their way.
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  #31  
Old 16-01-2014, 08:44 PM
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Mark, if they can successfully and routinely pump out 16" RC optical sets then they are already a lot better than opticians at the Osterhauski/Mike lockwood level (and at half the price)
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  #32  
Old 16-01-2014, 10:24 PM
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Originally Posted by clive milne View Post
Mark, if they can successfully and routinely pump out 16" RC optical sets then they are already a lot better than opticians at the Osterhauski/Mike lockwood level (and at half the price)
I agree that Hubble Optics is not your run of the mill mass production mirror maker. They have got some really specialized products. Not to mention a production variant sandwich mirror for general sale.

After some reflection I might skip on the big scope and pick up a 12" mirror from these guys and retrofit my current scope some time this year. A rapidly cooling 12" mirror sounds good to me.
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  #33  
Old 17-01-2014, 05:21 AM
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Isn't it GSO (Taiwan), and not Hubble, that is the company that has started producing mass market 16" RC in this region?
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  #34  
Old 17-01-2014, 06:23 AM
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Too big for me is is that I need a ladder or another step-up to reach the eyepiece. So with my height (1.96m) a maximum focal length of 1.80m is the limit. I have a GSO 40cm (f/4.5, so f=180cm) which is just not too big. Despite its mass of 64kg, I can easily transport it with a modified wheelcart.
Of course the so-called 'suitcase Dobs' or 'travel dobs' with a very lightwelight construction a 40cm f/4.5 can be even lower because the rotation axis and the counterweight (which is the main mirror) are very low.

With shorter f/ ratios a 50cm dob can also meet these requirements but then a coma corrector is mandatory.
Commercial Dobs over 40cm are rather rare particularly those below f/4.

The largest Dobson ever built was long before John Dobson was even born: Lord Rosse's with 1.8m main mirror but obviously it was not called a Dobson, but it is the same design.
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  #35  
Old 17-01-2014, 06:43 AM
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Originally Posted by clive milne View Post
Mark, if they can successfully and routinely pump out 16" RC optical sets then they are already a lot better than opticians at the Osterhauski/Mike lockwood level (and at half the price)
Clive - Have you seen any tests a 16" GSO RC OPtical tube ? Is there even a complete successful 16" GSO RC in existance ? As I understand it sales of 12" RC's are only a tiny fraction of even the 10" , so I would expect the 16" RC's are more about building brand perception that $. AS I understand it the GSO RC's are automatically polished - not touched by the human hand. I'm not aware that the USA opticians offering any RC sets at least routinely.

Last edited by Satchmo; 17-01-2014 at 05:43 PM.
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  #36  
Old 17-01-2014, 06:52 AM
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I agree that Hubble Optics is not your run of the mill mass production mirror maker. They have got some really specialized products. Not to mention a production variant sandwich mirror for general sale.

After some reflection I might skip on the big scope and pick up a 12" mirror from these guys and retrofit my current scope some time this year. A rapidly cooling 12" mirror sounds good to me.
The optical tests I have seen on HO mirrors by Wolfgang Rohr show a tendency to astigmatism and some turned edge . Good value if you get a good one. The 14" I star tested had turned edge. I have never seen any HO mirrors bigger than 14" in this country .

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Who knows what the quality of their sub-4 mirrors are like? For the price I would settle for diffraction limited. I mean, if the one I get isn't to my liking. I can always send it to you for a re-figure, right? Right?
I wouldn't try and refigure a sandwhich mirror - they distort too much while cooling as they are not homogeneous.

Last edited by Satchmo; 17-01-2014 at 07:10 AM.
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  #37  
Old 17-01-2014, 07:13 AM
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I wouldn't try and refigure a sandwhich mirror - they distort too much while cooling as they are not homogeneous.
HO do a plate and pyrex variants. Would either make any difference to its strength or suitability for refiguring?
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  #38  
Old 17-01-2014, 07:21 AM
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I agree that there are probably no GSO RC 16"s delivered as yet, based on info on the Andrews Communications website. I sighted that development as an example of their (GSOs) movement into the larger scope mass market, which is supported by their advice to me that they will be introducing a 20" Dob later this year, and their apparent competitors 18"dob comiing as well.

From what I have read I expect that most GSO optics are produced on machines, and that does provide for a certain quality level in mass produced products. They don't produce individual mirror test results but 'claim' a certain level of qualty for their products based on the processing system design and sample testing. I would never expect a mass market producer to be able to duplicate the manhours put into a hand made mirror by an experienced optical specialist dedciated to just the high end of the market. But GSO are bringing astronomy to the masses and at a quality level which is very good compared to past industry offerings and they should be commended for that. Quality wil certainly continue to improve as optical robotics are further developed. As a base for further development and upgrades a GSO scope ticks alot of boxes for many people.

Mark, et al, maybe there needs to be a separate topic thread on mirror quality issues and the cost benefit (or not) of custom hand made verse robotic.
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  #39  
Old 17-01-2014, 07:36 AM
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Mark, et al, maybe there needs to be a separate topic thread on mirror quality issues and the cost benefit (or not) of custom hand made verse robotic.
Glen - All commercial mirrors in the world are mostly machine made - the `boutique ' low production mirrors ( ie Galaxy , Lockwood, Kennedy etc ) may get some localized retouching by the human hand with the mirror rotating face up on a machine and a human being rubbing a polisher on top ( maybe 10% ) of the total production time . This last touch up can produce a very smooth accurate surface, better than one made quickly totally by machine and an operato . Some can do this by machine only . Zambuto mirrors are 100% machine polished with no 'hand' work but a skilled machine operator .

What I was referring to by computer machine polishing is a system where the test results are automatically fed into a computer which designs the next run of machine strokes until completion - no human interpretation involved in the loop. This is the system claimed by GSO for the RC optics.

This info should obviate the need for another thread .
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  #40  
Old 21-01-2014, 02:31 PM
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Mark, if they can successfully and routinely pump out 16" RC optical sets then they are already a lot better than opticians at the Osterhauski/Mike lockwood level (and at half the price)
Totally agreed and they have landed a rover on the moon too.
So it is not outside their capability.
Just bear in mind, there are good manufacturers and not so good ones.
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