Re: End of an era?


Roland Christen
 


If so the amount of hand figuring now must have been reduced to its lowest limit? -Best, Robert 
Hand figuring is only used to produce an aspheric surface. For example, a doublet made with ED glass and Lanthanum flint will not have a perfect wavefront if all surfaces are left spherical. There is a 5th order defect that can only be removed via hand figuring. On the other hand, it only drops the Strehl ratio from perfect 100% to about 93-94%. BUT the main part of the defect is at the edge where the lens has the highest light grasp. The result is a nicely color corrected doublet with scattered light that hides subtle detail. Lenses like that are made in production with all-spherical surfaces, but if you want perfection, each one has to be individually hand corrected. That's what hand correction means. There are precious few individuals that can do it correctly.

Rolando


-----Original Message-----
From: ROBERT WYNNE <robert-wynne@...>
To: main@ap-ug.groups.io; Roland Christen via groups.io <chris1011@...>
Sent: Wed, Aug 19, 2020 7:02 pm
Subject: Re: [ap-ug] End of an era?

Interesting. I've had my ears open over the years awaiting an autonomous polishing process that essentially had an infinite randomized polishing routine. I suppose someone has come up with such a process and as usual I'm left in the dark - pun intended. If so the amount of hand figuring now must have been reduced to its lowest limit? -Best, Robert 
On 08/19/2020 2:20 PM Roland Christen via groups.io <chris1011@...> wrote:



The more the machine polished the increased frequency of unrandomized polishing over the same space. Thus surface finish errors would accumulate until a human stepped in to perform the final figuring.
This may have been true at one time, but it depends on the geometry of the lap and whether the lap is oscillated over the surface or held stationary. Usually for rapid polishing the lap is held stationary, but then a randomized oscillation is introduced to smooth out the surface. The end result is a very fine finish which is extremely accurate and smooth.

Rolando


-----Original Message-----
From: ROBERT WYNNE <robert-wynne@...>
To: main@ap-ug.groups.io; Roland Christen via groups.io <chris1011@...>
Sent: Wed, Aug 19, 2020 3:42 pm
Subject: Re: [ap-ug] End of an era?

I find it very fascinating to learn of the tricks of your trade. Some while ago I enquired about a Synchospeed and was informed that due repeated passes over the same areas of the blank a truly randomized polish could not be obtained by automated polishing. The more the machine polished the increased frequency of unrandomized polishing over the same space. Thus surface finish errors would accumulate until a human stepped in to perform the final figuring. This was 20 or so years ago and repetitive errors as this may now be accounted? -Best, Robert
On 08/19/2020 9:38 AM Roland Christen via groups.io <chris1011@...> wrote:


I 'll try to reply to some of the things you said below:

Automated polishing machines have always been around. The first ones were called Synchrospeed and were used almost 40 years ago for polishing spherical surfaces. It is a hands-off process, requires no hand work and is fairly efficient. If applied correctly with good tooling it can reproduce 1/10 wave to 1/20 wave surfaces all day long. Normally requires a long break-in period for the tool, somewhere around 100+ pieces before it reaches this level of accuracy, but then every piece after that comes off like peas in a pod. The basic process is still being used today.

Not all optics can use all-spherical surfaces, especially Apo Triplet lenses that are oil spaced. They pretty much require hand correction on an individual basis. Air-spaced triplets can be all-spherical, so they can be produced in large quantities using the above method. The only drawback is the tremendous investment required to obtain hundreds of ED or Fluorite blanks so you can run them on automated machines. A single 6" blank can cost $2000, so if you were to make 500 lenses, you would need to invest $1 million up front, with no assurance that all would actually work out and have customers for them. Very risky for small telescope makers.

As far a Sony polishing .01 micron surface smoothness - smoothness has no real meaning for a lens' accuracy. Surface accuracy is not the same as surface smoothness. You can have a surface full of zones, turned edge, humps etc., but have extreme smoothness. It won't make a good telescope lens.

For small batch processing of astronomical lenses it still makes sense to do the old tried and true pitch polishing of each surface. So unless we can get everyone in the world to buy a refractor telescope, the cost of making highly corrected optics for telescopes won't come down much. Besides, the cost of the glass alone is the biggest driver of the price, not so much the processing.

Rolando



-----Original Message-----
To: main@ap-ug.groups.io
Sent: Wed, Aug 19, 2020 2:29 am
Subject: Re: [ap-ug] End of an era?

Hi Rolando,

Are automated polishing machines good enough for most of the 130GTX elements to be used straight out of the machine for the most part?  Or does it just decrease the overall time needed for fine tuning and leverage the fact that hand figuring a spherical surface is easier than an aspherical surface?  

In a way, with narrowband imaging being very accessible today, poor color correction can be masked. I think the splendor of a flagship APO telescope (or flagship Maksutov) can really be appreciated with visual astronomy as well as the mechanics to optimize thermal stability and baffling to control stray light.

Recently, Sony has said that they can get automated aspherical polishing to 0.01 micron surface smoothness as long as the elements are under 60mm in size.   If you ever decide to make a n Astro-Physics 60mm f12 scope, count me first on the list.  I think it would be a great way to celebrate the end of Covid and instead of a melancholy post on the last 10” Mak for visual observation, you could have a celebration of visual lunar observation especially in light of planned human return to the Moon. 
Alan


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