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Equipment Discussions >> Reflectors

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azure1961p
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Reged: 01/17/09

Loc: USA
Top Opticians in the US for reflectors
      #5683979 - 02/16/13 06:05 PM

Everyone mentions Zambuto, Lockwood, royce and maurizio di sciullo for reflectors but no other names seem to surface. Another is out there - Peter Ceravolo in Canada comes to mind but little else . Anyone have any other names or comments?

Pete


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ausastronomerModerator
Carpal Tunnel
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Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: Top Opticians in the US for reflectors new [Re: azure1961p]
      #5684048 - 02/16/13 06:47 PM

Hi Pete,

Maurizio Di Sciullo used to sell his scopes under the name of Excelsior Optics. These were designed as specialist long focus tubed planetary newtonians, either F6 of F8. To my knowledge Maurizio hasn't produced a scope or mirror in about 10 years, due partly I believe to poor health. If you can find one second hand they are outstanding. Similalrly Peter Ceravolo hasn't built an amateur scope in about the same time period, concentrating on scientific and Government type work over the past 10 years. If you can find one of his second hand they are outstanding.

The other 3 all have excellent reputations. Along with Mark Suchting in Australia these are the only people I would consider buying my next mirror from, if I ever have a need to buy one. I have eliminated OMI on the basis that they only work with 2" thick glass substrate, which doesn't suit me or my scope design parameters. In the future moving forward I will only ever order a mirror made from minimum thickness substrate in the interests of fast cooling. In smaller slower apertures I would also consider people like Gordon Waite, Alan Raycraft, Dick Wessling and a few others.

Cheers,


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Markovich
professor emeritus
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Reged: 05/22/07

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Re: Top Opticians in the US for reflectors new [Re: ausastronomer]
      #5684139 - 02/16/13 07:50 PM

Steve Swayze makes fine mirrors.

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Pinbout
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Reged: 02/22/10

Loc: nj
Re: Top Opticians in the US for reflectors new [Re: Markovich]
      #5684190 - 02/16/13 08:31 PM

Quote:

Steve Swayze makes fine mirrors.




say it three times, swayze swayze swayze....


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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Top Opticians in the US for reflectors new [Re: Pinbout]
      #5684310 - 02/16/13 09:45 PM

Thanks guys. I'm sorry to hear about Maurizo having health issues. Gosh I remember his posts in 96' on the AOL Observers Outpost where he'd post these stunning photos of Jupiter made with a ten inch of his crafting. Outstanding. I recall Ceravolo going commercial/govt but I hoped he might still have done work for the amateur. The others I've heard of - Swayzee to be sure.

Thanks for the input.

Pete


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jgraham
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Re: Top Opticians in the US for reflectors new [Re: azure1961p]
      #5684330 - 02/16/13 10:00 PM

Ed Jones is one of the best that I know. He worked with Dick Wessling for many years.

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aatt
sage


Reged: 07/26/12

Loc: CT
Re: Top Opticians in the US for reflectors new [Re: jgraham]
      #5684374 - 02/16/13 10:47 PM

Terry Ostahowski seems to be very well regarded, but I don't know if he can match Carl Z et al.

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galaxyman
Vendor - Have a Stellar Birthday
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Re: Top Opticians in the US for reflectors new [Re: Pinbout]
      #5684449 - 02/16/13 11:42 PM

Quote:

Quote:

Steve Swayze makes fine mirrors.




say it three times, swayze swayze swayze....




Yep, as in my 22" Telekit.

Karl
E.O.H.


Chesmont Astronomical Society - www.chesmontastro.org
Galaxy Log - http://www.youtube.com/user/GalaxyLog4565?feature=mhee
HASB - http://www.haveastellarbirthday.com
Telekit (Swayze optics) 22" F/4.5 Dob
Homemade (Parks Optics) 12.5" F/4.8 Dob
TMB/APM 8" f/9 Refractor”The Beast”. One great DEEP SKY achro
ES 6" f/6.5 achro. Good one
Celestron Omni XLT 102 refractor.
Celestron 10x60mm Binos


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zawijava
scholastic sledgehammer
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Reged: 10/06/07

Loc: Wells, Maine 04090
Re: Top Opticians in the US for reflectors new [Re: azure1961p]
      #5684459 - 02/16/13 11:51 PM

Waite; Stevens; and I think there's a Kennedy out there as well(?)

Quote:

Everyone mentions Zambuto, Lockwood, royce and maurizio di sciullo for reflectors but no other names seem to surface. Another is out there - Peter Ceravolo in Canada comes to mind but little else . Anyone have any other names or comments?

Pete




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siriusandthepup
sage
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Reged: 02/14/06

Loc: Central Texas, USA
Re: Top Opticians in the US for reflectors new [Re: zawijava]
      #5684475 - 02/17/13 12:11 AM

Don't forget John Hall. Pegasus Optics.
John will still do a nice 12" to 25" size range mirror for ya.


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Calypte
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Reged: 03/20/07

Loc: Anza, California
Re: Top Opticians in the US for reflectors new [Re: aatt]
      #5684493 - 02/17/13 12:26 AM

Quote:

Terry Ostahowski seems to be very well regarded, but I don't know if he can match Carl Z et al.



Why do you think he can't?


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GlennLeDrew
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Re: Top Opticians in the US for reflectors new [Re: Calypte]
      #5684502 - 02/17/13 12:37 AM

Ceravolo hasn't gone completely over to industrial optics. He makes a 12" field-corrected Cassegrain, of the Dall-Kirkham flavor.

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jpb30
scholastic sledgehammer


Reged: 06/18/10

Loc: Uzès ( France)
Re: Top Opticians in the US for reflectors new [Re: GlennLeDrew]
      #5684608 - 02/17/13 03:47 AM

Terry is certainly one of the top in US

Mirrors of Z was tested by a lab pro in France, result: good optics but not very exceptional

Mirro Sphčre in France makes better mirrors, but more expensively...


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Sgt
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Reged: 12/17/05

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Re: Top Opticians in the US for reflectors new [Re: jpb30]
      #5684638 - 02/17/13 04:58 AM

Do you have a link?

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nirvanix
Carpal Tunnel


Reged: 06/07/07

Loc: Saskatoon, SK
Re: Top Opticians in the US for reflectors new [Re: Sgt]
      #5684668 - 02/17/13 05:54 AM

I've heard good things about Normand Fullum but I don't own one of his mirrors. I'm considering him for a refigure.

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dpwoos
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Reged: 10/18/06

Loc: United States
Re: Top Opticians in the US for reflectors new [Re: nirvanix]
      #5684752 - 02/17/13 08:35 AM

I've spent some time with a couple of Norm Fullum's scopes at Stellafane, and the views were outstanding!

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nirvanix
Carpal Tunnel


Reged: 06/07/07

Loc: Saskatoon, SK
Re: Top Opticians in the US for reflectors new [Re: dpwoos]
      #5684977 - 02/17/13 11:09 AM

Quote:

I've spent some time with a couple of Norm Fullum's scopes at Stellafane, and the views were outstanding!




Thanks for letting me know dpwoos, that gives me some confidence to use him. He seems like a very enthusiastic craftsman to say the least!


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steelhead
journeyman


Reged: 01/27/11

Loc: Smithers, BC
Re: Top Opticians in the US for reflectors new [Re: nirvanix]
      #5684995 - 02/17/13 11:21 AM

Norm is highly recommended!

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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Top Opticians in the US for reflectors new [Re: steelhead]
      #5685058 - 02/17/13 11:57 AM

Zambuto test:
http://www.astro-foren.de/showthread.php?9233-Carl-Zambuto-Newton
http://www.astromart.com/forums/viewpost.asp?forum_post_id=170264
And you can find literally hundreds of posts and threads extolling the virtues of the star images using a mirror from Carl Zambuto.

There have been many threads here on CN asking about premium mirror makers and there are probably 20-30 of them, many of whom haven't been mentioned here (e.g.OMI, Galaxy, Nova, Mark Harry, etc.) but whose names will probably be brought up soon.

Just remember, we do not pay for "the best optics money can buy", we buy "the best optics money can buy for a reasonable price".


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HunterofPhotons
Pooh-Bah


Reged: 04/26/08

Loc: Rhode Island, USA
Re: Top Opticians in the US for reflectors new [Re: azure1961p]
      #5685070 - 02/17/13 12:01 PM

Paul Jones at Star Instruments.

dan k.


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Howie Glatter
Vendor


Reged: 07/04/06

Re: Top Opticians in the US for reflectors new [Re: azure1961p]
      #5685215 - 02/17/13 01:19 PM

One of the best hasn't been mentioned yet -
Michael Spooner


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dpwoos
Pooh-Bah
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Reged: 10/18/06

Loc: United States
Re: Top Opticians in the US for reflectors new [Re: Howie Glatter]
      #5685327 - 02/17/13 02:19 PM

And, as I said in another post, many clubs have one or more members that are top-notch mirror makers. There is a guy in our club that does outstanding work, and I would be (and am) as thrilled with one of his mirrors as I would be with anyone else's. I would always check in with these local mirror makers first.

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John Kuhl
sage


Reged: 11/10/05

Loc: SoCal
Re: Top Opticians in the US for reflectors new [Re: dpwoos]
      #5685370 - 02/17/13 02:52 PM


+1 on Mike Spooner, great mirrors.


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careysub
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Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Top Opticians in the US for reflectors new [Re: Starman1]
      #5685377 - 02/17/13 02:53 PM

Quote:

Zambuto test:
http://www.astro-foren.de/showthread.php?9233-Carl-Zambuto-Newton





Quite impressive. For the whole mirror the RMS is 1/54.7 wave (Strehl 0.987). When the central zone is masked out (as in a Newtonian reflector usage) it is 1/66 wave (Strehl 0.991), and it is extremely smooth.

I'd pay Carl's prices for that mirror!

(Amusingly Google Translate thinks that "Glattesten" - smoothest (I think) - means "Smooth Estonians").

He does say that it is below the level of top manufacturers (if the translation is correct) but I think that we are now talking about mirrors that come at come multiple of Carl's prices for slight improvements that are beyond the ability of any amateur to detect.


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Darren Drake
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Re: Top Opticians in the US for reflectors new [Re: careysub]
      #5685396 - 02/17/13 03:03 PM

No one has been making mirrors longer than Dan Joyce of Chicago. He will e ven take on students and let them make mirrors under his instruction.

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Jeff Morgan
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Loc: Prescott, AZ
Re: Top Opticians in the US for reflectors new [Re: Darren Drake]
      #5685413 - 02/17/13 03:36 PM

Quote:

No one has been making mirrors longer than Dan Joyce of Chicago. He will e ven take on students and let them make mirrors under his instruction.






I always hoped DJ would make a go at the commercial business, but he's just not motivated that way.


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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Top Opticians in the US for reflectors new [Re: HunterofPhotons]
      #5685627 - 02/17/13 05:50 PM

I'd be curious to know what happened to George Clements - the PARKS OPTICAL optician at least circa 1990s. Got two essentially perfect mirrors from him. Never hear of him however.

Pete


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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Top Opticians in the US for reflectors new [Re: azure1961p]
      #5685903 - 02/17/13 08:29 PM

Steve Kennedy. He's produces larger aperture mirrors and they are among the best.

Quote:

Everyone mentions Zambuto, Lockwood, royce and maurizio di sciullo for reflectors but no other names seem to surface. Another is out there - Peter Ceravolo in Canada comes to mind but little else . Anyone have any other names or comments?

Pete




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siriusandthepup
sage
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Reged: 02/14/06

Loc: Central Texas, USA
Re: Top Opticians in the US for reflectors new [Re: azure1961p]
      #5686086 - 02/17/13 10:19 PM

Quote:

I'd be curious to know what happened to George Clements - the PARKS OPTICAL optician at least circa 1990s. Got two essentially perfect mirrors from him. Never hear of him however.

Pete




+1

I would love to know about this guy also. I have one of his 8" f/6 Parks mirrors and it is exquisite! I wasn't able to track down much information for him at all.


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polaraligned
professor emeritus


Reged: 12/26/08

Re: Top Opticians in the US for reflectors new [Re: HunterofPhotons]
      #5689732 - 02/19/13 09:09 PM

Quote:

Paul Jones at Star Instruments.

dan k.




Unless something has changed, as of a few years ago, Paul flat out was not doing parabolic mirrors anymore, and had not for a while. He had no interest in quoting me for a large mirror.


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ml96737
member
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Reged: 02/10/12

Loc: Big Island, Hawaii
Re: Top Opticians in the US for reflectors new [Re: polaraligned]
      #5690179 - 02/20/13 04:18 AM

There's quite a few people out there, both amateur and professional who can make practically perfect mirrors. Many of the names mentioned so far are very good, but even they have occasionally produced a "lemon" (Steve Kennedy made a big goof by undercorrecting a mirror by 1/2 wave or so, and John Hall of Pegasus produced several bad ones, and even tried to cover it up by false testing and threaten Mr. Rohr! http://www.astro-foren.de/showthread.php?10080-Pegasus-Newton-320-1500 )

Therefore, Carl Zambuto really stands above even this group of experts. AFAIK, he has never produced a "bad" mirror. What makes him so amazing is he has automated the figuring process by machine and can do it repeatedly and perfectly in a relatively short time. I think he produces around 50 or more "perfect" mirrors per year!


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Calypte
Pooh-Bah


Reged: 03/20/07

Loc: Anza, California
Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5690668 - 02/20/13 11:26 AM

Quote:

What makes him so amazing is he has automated the figuring process by machine and can do it repeatedly and perfectly in a relatively short time.



He's not the only one who figures mirrors this way.


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Norm Meyer
sage
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Reged: 02/08/09

Loc: Warren, ME 04864
Re: Top Opticians in the US for reflectors new [Re: Calypte]
      #5690869 - 02/20/13 01:12 PM

What about Mark Harry he makes excellent small to
medium sized mirrors. I have one of his 8" F7.5 mirrors
and it is excellent.

Norm


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Mike B
Starstruck
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5691269 - 02/20/13 04:28 PM

Quote:

...and John Hall of Pegasus produced several bad ones



I'm not a Pegasus "fan", and have never had one of his optics... i am unfamiliar with the backstory of the circumstances you've described. But i *do* note that the date on the forum notes you linked is over five years old.

These discussions of this mirror maker & that can sometimes turn ugly, and frequently sad stories get trotted out and re-aired. My concern here is that an event from five years ago, or ten years ago, etc. seems to NEVER be forgotten- the maker of a poor, or even less-good mirror seems to be forever haunted- even tho perhaps they have made dozens & dozens of perfectly fine optics, before, and since.

I would not wish such a standard be applied to myself or my work, which is no more or less "perfect" than anyone else's... nor would anyone wish to be pummeled unendingly with their past deviations from perfection.

Just sayin'... Be careful what allegations we toss about... be careful what tossed-about allegations we believe... and how they might (or might not) even be applicable to today's goals of acquiring a good scope.


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Jeff Morgan
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Reged: 09/28/03

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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5691302 - 02/20/13 04:42 PM

Quote:


These discussions of this mirror maker & that can sometimes turn ugly, and frequently sad stories get trotted out and re-aired. My concern here is that an event from five years ago, or ten years ago, etc. seems to NEVER be forgotten- the maker of a poor, or even less-good mirror seems to be forever haunted- even tho perhaps they have made dozens & dozens of perfectly fine optics, before, and since.




Yep, reputations take years to earn and minutes to lose.

In the case of the master optician, it's easy to imagine that every so often they could turn out a Dog.

What's unfathomable is that a small operation (often a one-man shop) could let such a Dog get out the door.

A moment of bad judgement and time/money pressures perhaps.


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John Kuhl
sage


Reged: 11/10/05

Loc: SoCal
Re: Top Opticians in the US for reflectors new [Re: Jeff Morgan]
      #5691330 - 02/20/13 04:57 PM


There are great mirrors, but there is no such thing as a "perfect" mirror.

Best, John


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Mike Lockwood
Vendor, Lockwood Custom Optics
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Reged: 10/01/07

Loc: Usually in my optical shop
Re: Top Opticians in the US for reflectors new [Re: Jeff Morgan]
      #5691413 - 02/20/13 05:45 PM

Quote:

Yep, reputations take years to earn and minutes to lose.
In the case of the master optician, it's easy to imagine that every so often they could turn out a Dog.
What's unfathomable is that a small operation (often a one-man shop) could let such a Dog get out the door.
A moment of bad judgement and time/money pressures perhaps.



I don't think it's unfathomable - it's a very real possibility in the of manufacturing items that are difficult to test and manufacture. Probability says that defects happen.

While I try to improve my testing methods every day, I take the attitude that it's only a matter of time before I make a mistake. I hope not to, but it's possible, and I acknowledge that.

Think about it - a typo made when entering a radius of curvature, or when writing down the measured ROC will certainly cause erroneous test results. How often might that happen? Once every 500 mirrors? Every 1000 mirrors? Probability says if you make enough mirrors, it will happen eventually.

It is important to note, though, that a defect once in a great while is much different and far less concerning than gross, frequent, or systematic errors, which, sadly, I have seen quite often in mirrors that come to me for testing.

However, it's how one handles a mistake that is important. If it happens, I won't react with denial or by abusing, belittling, or blaming the customer - I will check out the mirror and fix it if there is an issue. I will also explain what happened to the client and do my best to never make the same mistake again.


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Mike B
Starstruck
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Top Opticians in the US for reflectors new [Re: Mike Lockwood]
      #5691437 - 02/20/13 06:03 PM

Quote:

A moment of bad judgement and time/money pressures perhaps.



We've gotta give them permission to be human. They go thru "stuff" just like everyone, and it can affect their work, and their QC:
1) A simple typo, like Mike said above,
2) Normal & seasonal illness, maybe combined with deadline & business pressures... "not hittin' on all six" as they say,
3) Communication busts with staff... by their very definition, a "bust" can exist primarily because no one realizes it exists, or else they'd correct it,
4) Long-term issues, like a worker (or the boss!) with ongoing personal issues, a family loss, alcoholism, impending divorce, etc.

A lot of this is how they handle the problems that result, and how persistent the "issues" might be. Some folks have their act together, more-so than others. I'd dare say someone with CZ's long-term reputation is not only a great optician, but a highly organized & disciplined individual. Yet it doesn't mean that he's the ONLY one making fine mirrors; Mike L & others have certainly shown what can be accomplished!

Also factoring into this is how *WE* handle the problems that have resulted. Which is why i hate to see one who is likely a decent enough mirror-maker get hammered over past QC issues. Especially LONG-past issues.


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5691538 - 02/20/13 06:54 PM

Mentioned so far:
Zambuto
Lockwood
Royce
di Sciullo
Ceravolo
Suchting
OMI
Waite
Raycraft
Wessling
Swayze
Ostahowski
Kennedy
Pegasus
MirroSphere
Fullum
Harry
Jones
Spooner
Joyce
Clements

Not mentioned yet:
Nova
Orion UK

It seems the list is well-populated, but would all of them be considered in a list of "Top Opticians"?
I had an Ostahowski mirror that was excellent, but my Zambuto is more excellent.
I've seen a nearly-perfect 16" GSO mirror that blew me away it was so good (as good as any mirror I've ever seen), but GSO wouldn't be in a list of "Top" mirrors because of variability from mirror to mirror.
Perhaps that is what differentiates the "top" makers from the "good" makers--an attention to perfection that results in every effort being superlative.


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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5691569 - 02/20/13 07:15 PM

I read recently on the Zambuto Groups that CZ is now producing 200+ mirrors a year.

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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Top Opticians in the US for reflectors new [Re: Starman1]
      #5691584 - 02/20/13 07:29 PM

Each mirror maker offers different options. Some don't figure mirrors faster than F/4.5 or 4.0. Others don't make anything larger than 16" or 20" or less than 22" aperture. Before choosing a mirror maker, it depends on what kind of mirror you are after. For instance, Steve Kennedy and Mike Lockwood are the only premier fast mirror makers producing top quality mirrors below F/4.0 and at apertures far larger than 24" -- and these two really shortens the list. Steve Kennedy broke this important barrier first back in 2005. Steve came out of Celestron and has make over 2,000 mirrors to date. Before Steve K., there were only custom-figured mirrors at F/ 4.X and slower for amateurs. CZ only recently has moved to a faster F/4.0 after many years at F/4.3-5.X.

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davidpitre
Post Laureate
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Reged: 05/10/05

Loc: Central Texas
Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5691758 - 02/20/13 09:00 PM

Quote:

John Hall of Pegasus produced several bad ones, and even tried to cover it up by false testing and threaten Mr. Rohr! http://www.astro-foren.de/showthread.php?10080-Pegasus-Newton-320-1500 )





I don't want to pile on or hurt feelings, but John Hall has produced more than a few stinkers. I owned one, and have talked with a number of folks who felt likewise about their experience. I don't know about his better mirrors (and I'm sure some will chime in about their good mirrors) but because of consistency issues, he should not be listed with the "top".


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ausastronomerModerator
Carpal Tunnel
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Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: Top Opticians in the US for reflectors new [Re: Mike Lockwood]
      #5691771 - 02/20/13 09:19 PM

Quote:

It is important to note, though, that a defect once in a great while is much different and far less concerning than gross, frequent, or systematic errors, which, sadly, I have seen quite often in mirrors that come to me for testing.

However, it's how one handles a mistake that is important. If it happens, I won't react with denial or by abusing, belittling, or blaming the customer - I will check out the mirror and fix it if there is an issue. I will also explain what happened to the client and do my best to never make the same mistake again.




Hi Mike,

You make some extremely good points and it's a pity all opticians don't take a leaf out of your book. Everyone is human and will make a mistake at some time in their life. How they handle it is what matters. If it's properly handled it will just fall through the cracks. The customer will be out a little in observing time and use of the scope, he won't be out of pocket financially. In these situations invariably the opticians reputation will remain in tact, if not enhanced due to the good customer service and fault rectification.

Unfortunately a couple of opticians who continually get glowing reports on internet forums; and already given a big rap by several people in this thread, have a less than favourable success rate IME and unfortunately have not even wanted to hear about the lemon they produced, or wanted to rectify the issues, when confronted about them. The people offering favourable comments on internet forums usually give their positive reports based on their sample of one good mirror that they may have experience with, and not be aware of the number of not so good mirrors these opticians have produced. I know you have had to refigure a couple of these lemons. Mark Suchting had to deal with another. In all cases at no cost to the optician who produced the lemon. I am aware of a couple more that are yet to be refigurred and the owner has continued to tolerate a less than acceptable optic for the money paid.

Unfortunately in the world of constant litigation which we now live in, people keep pretty tight lipped about their bad experiences.

It's a pity they can't all deal with a bad mirror in the fashion you have outlined.

Cheers,


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azure1961p
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Re: Top Opticians in the US for reflectors new [Re: Peter Natscher]
      #5691844 - 02/20/13 10:10 PM

Nice points Don. I'd bet they all can and have produced fine mirrors but consistency would seem to be one of the real TOP GUN attributes of the best.

Pete


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azure1961p
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Re: Top Opticians in the US for reflectors new [Re: siriusandthepup]
      #5691859 - 02/20/13 10:19 PM

Quote:

Quote:

I'd be curious to know what happened to George Clements - the PARKS OPTICAL optician at least circa 1990s. Got two essentially perfect mirrors from him. Never hear of him however.

Pete




+1

I would love to know about this guy also. I have one of his 8" f/6 Parks mirrors and it is exquisite! I wasn't able to track down much information for him at all.




Lol, love your qoute in your signature.

Yeah, I've googled for Clements and he's not to be found. I spoke to him once over the phone, its not like he was some old codger. It'd be interesting to find out what happened.

Pete


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Pinbout
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Re: Top Opticians in the US for reflectors new [Re: Norm Meyer]
      #5691864 - 02/20/13 10:24 PM

Quote:

What about Mark Harry he makes excellent small to
medium sized mirrors. I have one of his 8" F7.5 mirrors
and it is excellent.

Norm




up to 14in and is quick. I wish I could get one of his... but I'm aflicted with poor artist syndrome...


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jpcannavo
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Re: Top Opticians in the US for reflectors new [Re: ausastronomer]
      #5692286 - 02/21/13 07:30 AM

Quote:


Unfortunately in the world of constant litigation which we now live in, people keep pretty tight lipped about their bad experiences.




And there is another force that might be at wok here as well. If one were to get a lemon from one of the "real big names", there may be fear that publicizing it, on a forum such as this, would rattle "cherished beliefs" and be taken as somehow being politically incorrect, possibly generating hostile responses. I myself fell victim to this sort of hesitance, not being able to bring myself to "out" the very well know optician who gave me a less than up to spec mirror, that moreover remained unimproved after returning it for a touch-up.
What we really need is some organized body of objective consumer reporting. We have some of this - Wolfgang Rohr, etc. - but not nearly enough. I do think I remember a while back Dave Bonandrini talking about an upcoming/ongoning report on the third party testing of a large number of commercial mirrors - including those made by our cherished premium "luthiers" - but I don't think it materialized. Interestingly, the big sources for objective info - S&T and Astronomy mag etc. - will publish monthly objective evals of commercial scope and equipment, but not individual mirrors. Wouldn't it be neat to see S&T have a monthly column called "Mirror Test Reports".
Given our intense focus (!) on optical quality, it is indeed strange that very little of this actually happens. This absence is even evident in our forums. While they do have review sections, look at the categories. There is no specific section dedicated to reports on mirrors. Sure they report on large and small dobs, etc, but this is not quite the same thing, and parallels the similar absence in our our magazines. Strange indeed.

Edited by jpcannavo (02/21/13 09:54 PM)


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Darren Drake
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Re: Top Opticians in the US for reflectors new [Re: jpcannavo]
      #5692394 - 02/21/13 09:26 AM

Yes Wolfgang Rohr's website is an amazing source of info on so many mirrors and lenses along with their detailed reports. It can be a little frustrating reading through the translated German but still I find myself going through the many reports frequently so as to familiarize myself with the general quality and consistency of various lens and mirror makers. I wish there were more sites like this one especially in English. The book Bonandrini mentioned here a few years ago has apparently been mothballed or seriously delayed. In any event I hope another optical testing site makes an appearance sometime in the not to distant future.

Edited by Darren Drake (02/21/13 01:43 PM)


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EJN
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Re: Top Opticians in the US for reflectors new [Re: Darren Drake]
      #5692594 - 02/21/13 11:56 AM

Quote:

The book Bonandrini mentioned here a few years ago has apparently been mothballed or seriously delayed.




The so-called book, as well as a lot of other stuff Bonandrini claimed, is
about as real as Manti Teo's dead girlfriend.


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Doug Culbertson
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Re: Top Opticians in the US for reflectors new [Re: EJN]
      #5692629 - 02/21/13 12:16 PM

Quote:

Quote:

The book Bonandrini mentioned here a few years ago has apparently been mothballed or seriously delayed.




The so-called book, as well as a lot of other stuff Bonandrini claimed, is
about as real as Manti Teo's dead girlfriend.




Careful. I got a similar post removed from the OTO for stating something very similar regarding David B's now mythical book.


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Mike Lockwood
Vendor, Lockwood Custom Optics
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Re: Top Opticians in the US for reflectors new [Re: jpcannavo]
      #5692749 - 02/21/13 01:17 PM

Quote:

And there is another force that might be at wok here as well. If one were to get a lemon from one of the "real big names", there may be fear that publicizing it, on a forum such as this, would rattle "cherished beliefs" and be taken as somehow being politically incorrect, possibly generating hostile responses.



I can't say why people don't usually speak up, but one very real risk is that the user/client/buyer might be wrong. Mirror testing is not easy, and when you put it in a telescope, the number of variables involved goes up dramatically. It takes experience to determine what the real problem is.

Many issues with mirror cells and other telescope problems have been blamed on the optics, and many true optical problems have been blamed on mirror cell and other issues. Many owners of telescopes blame seeing or equilibration for poor image quality simply because they believe it can't possibly be an optical problem. In reality, it can be.

This is why I advocate that those who aren't expert testers, but still care about optical quality, go to star parties, walk around, and use different instruments over the course of a night. This allows time for optics to cool off. Learn what poor collimation looks like, and learn to recognize it and other issues in images so you don't blame the optics for it. Take notes, including eyepieces used, and see what instruments really perform best at high power.

In short, do your own research, and don't believe everything you hear or read. A trip to a far-away star party may be expensive, but it is less expensive than the loss one might take on a telescope that doesn't perform.

Quote:

What we really need is some organized body of objective consumer reporting.



Aside from the risk of being sued for libel even if the test report is honest and correct (a risk that is greatly reduced by the tester being located in another country!), another risk is that the third party tester/reporter might make a mistake. While some do great work, others might not, and the risk of inaccurate test results goes up dramatically as the size of the mirror increases. If the tester doesn't do it full-time, he/she may be more likely to make a mistake than a professional optician.

Quote:

Given our intense focus (!) on optical quality, it is indeed strange that very little of this actually happens. This absence is even evident in our forums. While they do have review sections, look at the categories. There is no specific section dedicated to reports on mirrors.



The explanation is simple - it is difficult to test mirrors, and any bad report will be countered by a glowing report. Then it turns into a *BLEEP* contest, rather than a discussion.


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ml96737
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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5692984 - 02/21/13 03:49 PM

Quote:

Quote:

...and John Hall of Pegasus produced several bad ones



I'm not a Pegasus "fan", and have never had one of his optics... i am unfamiliar with the backstory of the circumstances you've described. But i *do* note that the date on the forum notes you linked is over five years old.

These discussions of this mirror maker & that can sometimes turn ugly, and frequently sad stories get trotted out and re-aired. My concern here is that an event from five years ago, or ten years ago, etc. seems to NEVER be forgotten- the maker of a poor, or even less-good mirror seems to be forever haunted- even tho perhaps they have made dozens & dozens of perfectly fine optics, before, and since.

I would not wish such a standard be applied to myself or my work, which is no more or less "perfect" than anyone else's... nor would anyone wish to be pummeled unendingly with their past deviations from perfection.

Just sayin'... Be careful what allegations we toss about... be careful what tossed-about allegations we believe...




These are not allegations, but facts. 3 Pegasus mirrors were tested by interferometry by Mr. Rohr, and were found to be unsatisfactory.

But, the real issue is not so much the bad mirrors, but the fact John Hall guaranteed these mirrors to be "1/10 wave". When in fact they were below 0.80 Strehl! And, when this was brought to his attention, he attempted to cover it up and threaten the evaluator!

Now, maybe in the past 5 years Mr. Hall has completely changed his methods of mirror making, and his attitudes, but then on the other hand human nature is what it is...

As a followup to this whole issue of "bad mirrors" getting out, it strikes me as very poor QC, that very few mirror makers take the small extra effort to star test their "finished" product before sending it out? It would takes just a few minutes to do a careful star test, and that would reveal any of these problems mentioned. This lack of proper final testing of optics is the root cause of quality problems.

It could be so easily avoided at very little cost of time or equipment!


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Calypte
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Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5693144 - 02/21/13 05:05 PM

Quote:

As a followup to this whole issue of "bad mirrors" getting out, it strikes me as very poor QC, that very few mirror makers take the small extra effort to star test their "finished" product before sending it out? It would takes just a few minutes to do a careful star test, and that would reveal any of these problems mentioned. This lack of proper final testing of optics is the root cause of quality problems.

It could be so easily avoided at very little cost of time or equipment!



This post is oblivious to the realities of commercial mirror-making. I have no opinion about the alleged fault-free excellence of Zambuto or the claimed failures of Pegasus, since, to my knowledge, I've never seen an example of either. But what this post expects is that the mirror-maker already owns a scope of every possible configuration, every mirror-cell size and focal length, so he can star-test every possible permutation of a completed mirror within the size range he chooses to produce. And "just a few minutes" to mount up each mirror for star-testing? How long does it take the person making this demand to remove a previous mirror from a scope, then mount a new mirror and collimate the scope, then wait for a clear night of good seeing? Possibly a mirror-maker could limit their available product to a selection of a very few apertures and focal lengths. A friend of mine makes batches of dozens of 6s, 8s, 10s and 12.5s for a famous supplier of optical products. He also usually has at least one or two 17.5 or larger mirrors in the shop, often of very short focal lengths, often for reworking from someone else's product. I've seen several of these in his shop for eventual mounting in high-end astrographs made by famous manufacturers (they usually don't make their own optics). To mount up each mirror in a scope designed to accommodate each is simply impractical. Refractor manufacturers can star-test every single scope, but they are testing completed scopes (just put it on a mount and see what it does), and some of the old-time reflector makers star-tested each scope. Again, they were testing completed scopes. Even so, apparently a few bad scopes were shipped. The airy expectations of this post are precisely why my friend abandoned most of the amateur market. He tests his mirrors with interferometry, and he supplies a report and interferogram. I've seen testimonials to the excellence of his mirrors in this forum, and I have little doubt that some of the other mirror-makers cited in these posts can provide equivalent quality. But if Zambuto, Swayze, Royce, et al, want the low-end market, they can have it! My friend views that market as a cesspool of unrealistic demands and a path to poverty.


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ml96737
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Re: Top Opticians in the US for reflectors new [Re: Calypte]
      #5693263 - 02/21/13 06:07 PM

Quote:

Quote:

As a followup to this whole issue of "bad mirrors" getting out, it strikes me as very poor QC, that very few mirror makers take the small extra effort to star test their "finished" product before sending it out? It would takes just a few minutes to do a careful star test, and that would reveal any of these problems mentioned. This lack of proper final testing of optics is the root cause of quality problems.

It could be so easily avoided at very little cost of time or equipment!



This post is oblivious to the realities of commercial mirror-making.

But what this post expects is that the mirror-maker already owns a scope of every possible configuration, every mirror-cell size and focal length, so he can star-test every possible permutation of a completed mirror within the size range he chooses to produce.

The airy expectations of this post are precisely why my friend abandoned most of the amateur market....My friend views that market as a cesspool of unrealistic demands and a path to poverty.




With all due respect sir, making an adjustable star testing apparatus for a wide range of mirrors is not difficult at all. Basically, a fixture that can take the size and FL range the maker produces, with a few high quality secondaries and eyepieces. The costs of such simple testing system is a fraction of what it costs to put together a professional optical shop. If you doubt it can be done, then contact Mr. Kennedy. http://www.kennedy-optics.com/process.htm He can explain how he star tests each mirror and makes a profit doing it too!

Reading between the lines here, it appears the real issue is the fear that many professional opticians have of the accuracy and sensitivity of the star test. Better to just finish it up to the level you are familiar with, ship it out with a "certificate", and go do the next one, right?

Also, I get the impression you are implying the standards of the amateur community are too high, above that of the professionals, and are unrealistic? That may be partially true, and my knowledge of several professionally acquired and produced mirrors substantiates that. Eg. Maria Mitchell Obs has one with a bad turned edge, as does the 30" at Lowell Obs. The McDonald 82-inch was 1 wave or worse until Texereau refigured it... The list is quite long.

Zambuto makes mirrors for everyone, and he doesn't ship it until its essentially perfect. He does a large number of mirrors and has no "lemons". If ever a customer wasn't satisfied, he will fix the problem. No if, ands, or buts.

That's quality, service, professionalism, as it should be, IMHO. The rest is just excuses.

Mike


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mark cowan
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Re: Top Opticians in the US for reflectors new [Re: Calypte]
      #5693276 - 02/21/13 06:14 PM

Quote:

My friend views that market as a cesspool of unrealistic demands and a path to poverty.




I knew there had to be something I really liked about this business...

Best,
Mark


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Calypte
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Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5693339 - 02/21/13 07:06 PM

Quote:

Also, I get the impression you are implying the standards of the amateur community are too high, above that of the professionals, and are unrealistic?



"Too high" at a price most are willing to pay. "I want 1/20 wave accuracy, peak-to-valley. Oh, I won't pay more than I would for a GSO." My friend says he gets frequent queries like this. Inevitably, they cite a wave-accuracy from some other optician's website. If they can do better with Zambuto, etc., then more power to them. The comparison to Steve Kennedy is interesting, since my friend -- who knows Kennedy very well and has done much business with him -- has his own stories about Mr. Kennedy's quality. But I don't have the story first-hand, and I've personally met Steve Kennedy only very briefly. WRT to the "star test," I thought you might mean with a real star. Again, I think you are not well-informed about the realities of the business, and I'm betting Zambuto & Kennedy would agree!

Edited by Calypte (02/21/13 07:10 PM)


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Mike B
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Re: Top Opticians in the US for reflectors new [Re: Calypte]
      #5693479 - 02/21/13 08:18 PM

Quote:

...another risk is that the third party tester/reporter might make a mistake. While some do great work, others might not, and the risk of inaccurate test results goes up dramatically as the size of the mirror increases. If the tester doesn't do it full-time, he/she may be more likely to make a mistake than a professional optician....

The explanation is simple - it is difficult to test mirrors




It is interesting that Mike L would make this comment at THIS point in the thread... i doubt we'll hear a more concise AND accurate appraisal on the matter.

Some Dob makers *do* test THE actual optic of each scope in THE actual structure wherein it will reside when shipped to its customer. Such tests can even be done prior to coating. However, some don't test in the scope, anyway... you'll receive the structure from ONE source, and the optic from another. They're *ALL* tested as they're produced- many times thru the process, and presumably when finished- in some manner or another. Yet each method of testing has its value... & its limits.

Makers of BOTH types have good reputations, on the whole. Yet as there is in just about any market, there are stand-outs. I know which i'd prefer myself, if i were plunkin' down the payola. Even so, the Dob i use now was acquired "used"... i have no idea how Starsplitter handled the "testing" when the scopes were shipped to the original owners. But i'm confident this one's (OMI) optics are pretty darned good- based on a LOT of scopes i've owned & used & viewed thru over the years.

Should i have turned down the purchase of this Dob simply because OMI has turned out some poor optics in the past? That's a personal decision each buyer would have to make. I'm glad i did *NOT* turn it down! Should i have insisted on "star-testing" the scope prior to signing over the dough?... just to make sure it was a "good one"? Sure, not a bad idea. But i didn't- went ahead on reputation alone... was a daylight transaction!

I can see from both sides of this... from a buyer's perspective, it's not totally clean-cut. Having mirrors tested & the results made public seems a great idea... one who's execution is swamped with devilish details.

Quote:

But, the real issue is not so much the bad mirrors, but the fact John Hall guaranteed these mirrors to be "1/10 wave". When in fact they were below 0.80 Strehl! And, when this was brought to his attention, he attempted to cover it up and threaten the evaluator!



First, 0.80 Strehl is in NO way a "bad mirror"! It may not be "1/10" wave, if that what was being promised. Assuming that's not a 1/10 wave surface, right?

Secondly, you're assuming the tester is entirely correct, and the manuf. is entirely incorrect. Realities are seldom this clear. Add to this that the story is now how many hands old? I'm presuming these three mirrors were not yours.

Additionally, if Mr. Hall truly felt he was being unfairly charged, and his business rep impugned, does he not have the right to defend himself? Does acting in defense amount to some kinda admission of guilt? Do you see how this can appear? We do not know the *true* backstory on this, and unless we were directly involved, how are we to know whether the actions & statements were noble or deceitful?

I, for one, doubt we can or will know... which is why i cringe at these things being brought up all the time. In fact, (as i understand it) CN's T.O.S. stipulates that posters are constrained to speak only of criticisms regarding their own, personal dealings with any specific vendor. And really, the intent is to work things out WITH such a vendor- definitely *NOT* bash them, and certainly not bash them over dated & second-hand info!

Quote:

Now, maybe in the past 5 years Mr. Hall has completely changed his methods of mirror making, and his attitudes, but then on the other hand human nature is what it is...



Oh, so in the minds of some, he'll be forever suspect. I guess that, too, is an attitude, isn't it?

As i stated before, let's all hope such "standards" & "criticisms" are never leveled at the rest of us & our imperfect work, or even our work imperfectly appraised, lest we, too, never recover... remaining tarnished of reputation the rest of our days.


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ml96737
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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5693579 - 02/21/13 09:12 PM

Quote:

Quote:

...another risk is that the third party tester/reporter might make a mistake. While some do great work, others might not, and the risk of inaccurate test results goes up dramatically as the size of the mirror increases. If the tester doesn't do it full-time, he/she may be more likely to make a mistake than a professional optician....

The explanation is simple - it is difficult to test mirrors




i have no idea how Starsplitter handled the "testing" when the scopes were shipped to the original owners.




Why depend on others to test your mirror? In fact star testing is EASY. Compared to what it takes to do accurate bench testing to 1/10 wave, or the complexities of phase shift interferometry, star testing is Trivial

In fact, everyone who buys a mirror should also get Suiter's book on star testing with it, so there is no question about the actual quality of the optic in anyone's mind.

Quote:

Quote:

But, the real issue is not so much the bad mirrors, but the fact John Hall guaranteed these mirrors to be "1/10 wave". When in fact they were below 0.80 Strehl! And, when this was brought to his attention, he attempted to cover it up and threaten the evaluator!




First, 0.80 Strehl is in NO way a "bad mirror"! It may not be "1/10" wave, if that what was being promised. Assuming that's not a 1/10 wave surface, right?




No, Pegasus own website says their guarantee is 1/10 wavefront P-V, 1/36 wave RMS, Strehl 0.97, but interferometrically measured was Strehl 0.78

Quote:

Secondly, you're assuming the tester is entirely correct, and the manuf. is entirely incorrect. Realities are seldom this clear. Add to this that the story is now how many hands old? I'm presuming these three mirrors were not yours.




No, this is first hand from Rohr's public website.

Quote:

Additionally, if Mr. Hall truly felt he was being unfairly charged, and his business rep impugned, does he not have the right to defend himself? Does acting in defense amount to some kinda admission of guilt? Do you see how this can appear? We do not know the *true* backstory on this, and unless we were directly involved, how are we to know whether the actions & statements were noble or deceitful?




No, he was being fairly charged for falsely advertising his products, and failing to correct the deficiency. Rohr has no motive to falsely evaluate Pegasus products. You can compare to Rohr's evaluation of Zambuto mirrors by the same criteria, he shows they are practically perfect as they star test too.

Quote:

...definitely *NOT* bash them, and certainly not bash them over dated & second-hand info!




Telling the truth is not bashing, heresay, or libel, its the truth.


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auriga
Pooh-Bah


Reged: 03/02/06

Re: Top Opticians in the US for reflectors new [Re: ausastronomer]
      #5693736 - 02/21/13 10:59 PM

Quote:

Hi Pete,

Maurizio Di Sciullo used to sell his scopes under the name of Excelsior Optics. These were designed as specialist long focus tubed planetary newtonians, either F6 of F8. To my knowledge Maurizio hasn't produced a scope or mirror in about 10 years, due partly I believe to poor health. If you can find one second hand they are outstanding. Similalrly Peter Ceravolo hasn't built an amateur scope in about the same time period, concentrating on scientific and Government type work over the past 10 years. If you can find one of his second hand they are outstanding.

The other 3 all have excellent reputations. Along with Mark Suchting in Australia these are the only people I would consider buying my next mirror from, if I ever have a need to buy one. I have eliminated OMI on the basis that they only work with 2" thick glass substrate, which doesn't suit me or my scope design parameters. In the future moving forward I will only ever order a mirror made from minimum thickness substrate in the interests of fast cooling. In smaller slower apertures I would also consider people like Gordon Waite, Alan Raycraft, Dick Wessling and a few others.

Cheers,




Dick Wessling made fine short focus mirrors, I have one of his 16" f/4 mirrors which is superb. But alas, Dick passed away three years ago.
Auriga


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Calypte
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Re: Top Opticians in the US for reflectors new [Re: auriga]
      #5693787 - 02/21/13 11:44 PM

I asked my mirror-making friend about star-testing. He does star test with a point-source. He performs this test for astigmatism.

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Mike B
Starstruck
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Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5693869 - 02/22/13 12:44 AM

Mike L, a professional optician states:

Quote:

The explanation is simple - it is difficult to test mirrors




You're stating:

Quote:

In fact star testing is EASY. Compared to what it takes to do accurate bench testing to 1/10 wave, or the complexities of phase shift interferometry, star testing is Trivial.




I think the actual, factual story is that quick & easy testing isn't precise, but tells a knowledgeable & experienced tester quite a bit... within limits. It might also tell a novice tester that his mirror has issues, leading them to ship it off for a re-figure... needlessly.

But a mirror maker could be pummeled for releasing mirrors that "star-tested ducky" when they were, in fact, 0.8 Strehl (okay, 0.78 Strehl ), just as easily as they could be pummeled for releasing 0.97 Strehl mirrors that were, in fact, 0.8. I'm quite sure this is precisely why CZ no longer issues "numbers" or "certs" on his optics... he got tired of playing that game. Good for him! Even a Zambuto mirror will test out with different numbers from different shops... probably not radically different, but certainly not identical. Some testers might even admit that the second decimal place is a bit fuzzy... and a third decimal place is just foolishness.

Any "test" that is truly meaningful, in a quantifiable manner (such as it is), will *not* be "easy". Any test that is "easy" will *not* be quantifiable. Mirror makers (& "testers") that confuse these two get into hot water. I'm sure this has happened more than once. Perhaps these Pegasus mirrors you mention are representative of this? We'll never know.

I know this data has been published on Rohr's website. I'm merely stating there's a LOT that is unknown as to the reactions, motives, background, communications, miscommunications, assumptions... and the entire story- for everyone involved. To state it as cut-and-dried from our perspective, NOW, five years later, disconnected from the people & incidents involved, is the height of presumption.

We also don't know how many mirrors Pegasus (or any maker, for that matter) puts out every year, or how many may be "out there" from many years of production. This example is, what?, THREE mirrors? Out of how many? Did Rohr test several dozen Pegasus mirrors? No? Then what do we know, scientifically speaking?

Not much.

Quote:

Telling the truth is not bashing, heresay, or libel, its the truth.




I don't know 'bout you, but merely reading something on the internet does not a truth make. Perhaps a data point? But conclusions drawn therefrom?... hmmm, i don't think so. If it's not your mirror, your story, then it qualifies as hearsay.

Mike, i'm not worried about 'convincing' you to change your opinion re: Pegasus Optics or its owner. I don't know the man, myself... never looked thru one of his optics. I'd just like to see a measured & balanced portrayal for others reading thru this.

Yes, to learn to "star-test" one's scope is never a bad idea, but certainly not required for enjoying its views. Obsessing over minutiae can be a slippery slope. Many will & have stated that a 0.80 Strehl Dob in the 16-inch range may perform perfectly well most nights (depending on what it is that's off-par from "perfect")- unless one is blessed with incredible seeing. So a lot, if not ALL, of the issue with these three Pegasus mirrors seems to me to revolve around advertising specs & bragging rights... neither of which cuts mustard under the stars.

Which is really where we all wanna be ANYway!


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Whoapiglet
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Re: Top Opticians in the US for reflectors *DELETED* new [Re: Calypte]
      #5694180 - 02/22/13 08:30 AM

Post deleted by Jarad

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pstarr
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Re: Top Opticians in the US for reflectors new [Re: Whoapiglet]
      #5694206 - 02/22/13 08:48 AM

I feel a lock coming on. To digress is not a good thing.

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Mike B
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Re: Top Opticians in the US for reflectors new [Re: pstarr]
      #5694610 - 02/22/13 12:27 PM

Counting the moments....


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Mike Lockwood
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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5694648 - 02/22/13 12:45 PM

Quote:

Some testers might even admit that the second decimal place is a bit fuzzy... and a third decimal place is just foolishness.
Any "test" that is truly meaningful, in a quantifiable manner (such as it is), will *not* be "easy". Any test that is "easy" will *not* be quantifiable. Mirror makers (& "testers") that confuse these two get into hot water.



Exactly right!

In some cases, the FIRST decimal place can be quite fuzzy.

Nothing comes for free - especially test precision and repeatability. You have to work for it.


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Re: Top Opticians in the US for reflectors new [Re: Doug Culbertson]
      #5694757 - 02/22/13 01:36 PM

I was given the following advice several years ago with regards to picking a mirror manufacturer. It was originally sent to me as an email. I was astounded at what he said as I certainly had not expected such a huge reply. It was also eventually posted on his Yahoo group.

========================
How to choose a mirror - by Carl Zambuto

1. BELIEVE THE REPORTS.
Yes, the online reports from individuals and reviewers when the do say something. They will not say everything, but they -will- say something, so listen to that something and listen to it carefully. And of course, consider the source, whether they have an agenda such as if they are a troll, or a competitor, and so on. But by and large, if 25 different people say the same thing about a product, it is probably true.

2. Find the product that elicits an EMOTIONAL RESPONSE.
Is that not what we are doing here anyway? We are in this hobby for an experience. So listen to what the individuals and scope reviewers say about their emotions. If you get continual responses such as "when the seeing got good I couldn't -believe- what I was witnessing, it looked just like..." and they babble mindlessly on and on, acting like children. That is the product to buy.

3. Pay attention to what you are NOT HEARING.
For example when a reviewer talks about a big telescope being "big", and "tall", and "shiny" and shows "big, bright views" and on and on without saying anything substantial, watch out. Did they address performance? Did they compare it with others on the planets, did they talk about contrast? Pay attention to what they are -not- saying, that is, read between the lines. Look for the substance. If you don't find substance in the review be wary of using it as a sole indicator. If you are not hearing about performance other than the term "this scope rocks", I would take the review with some salt. Remember, in the end you are looking for performance. And if that doesn't happen to be the case, then go for "ego". There are those who can cater to that and you -will- get stroked.

4. Pay attention to which companies are NOT GETTING PRESS and rave reviews on the online forums and review sites.
In other words, which companies are everyone conspicuously SILENT about? Watch out, because that is a clue. If you have a particular manufacturer in mind, try to find performance reports on them. How many are there? What do those reports say and what do they not say? Example, if someone says "I'm a newbie and I just took delivery and looked through mine and compared it to brand XYZ at 100x at a globular cluster and it looked almost the same to me, so I'm happy." Pay attention to that. Another way to say it is, when the best they can do is compare themselves to another product in a non-substantive manner, it is probably not enough.

5. When you do read a negative report, listen to it carefully as well.
What are they saying, and what are they -not- saying? Are they complaining about THEORY, or are they complaining about PERFORMANCE? Again, pay attention to what they are saying, and make appropriate considerations for the subject matter discussed.

6. ADVERTISING.
Uh-oh, here's an interesting one. Who is advertising? Who is not, and why not? How much are they spending? What are they saying in their ads? Do they talk about numbers? Do they talk about test methods? Do they talk about theory? Do they talk about performance? Do they talk about popularity and use general terms, or do they talk about specifics? Can they say a whole paragraph without saying anything? Do they disparage other people's product? If so, why? And what is their guarantee? Do they guarantee numbers?. Do they guarantee performance? Do they guarantee customer satisfaction no matter what? Do they guarantee not to be outperformed? Which is it and why? I'll leave it at that.

7. Now here's another one to look at: Long lines.
I'm going to make a generality here, it may not include all cases but I think this is a fair generalization. If you want to find value, LOOK FOR A LINE. Yes, I mean a waiting list. This does not necessarily mean "the best" in every case because a product does not need to be the best to have VALUE. At the same time, the list of products with lines will probably include the best, because it is always a value to somebody. I'm not saying that a product isn't a value if it doesn't have a line, what I'm saying is take special notice of those that do. Folks overall aren't stupid. Look for where the lines are and go find out why the line has formed. Talk to the people in the line, if you can. You will learn a lot there. I'll say it another way: Generally speaking, mediocrity is easier to come by.

8. Specifically concerning reflectors - CALL THE OPTICIANS.
Don't write to them, because they may not want to write what they might be willing to say on the phone. Get them cornered and ask them what is going on in the big picture. Do that with enough of them and soon you will know more than any one of them.

9. Refiguring services.
This is a sub-point to #8. Ask the opticians, "Whose mirrors are you refiguring?" You see, if all the companies you have listed are in the "extremely good to exceptional" category, then whose mirrors are they all refiguring? Many have refigure services, and I can tell you, they are providing that service. Make a list. Ask each one who does refigures, whose mirrors they are doing. They may not tell you, but if they do, you have gained some VERY valuable information. Pay attention to who did the refigure and what company's optic it was. Find out if one company or two or three are doing refiguring on one or two or many of the others. And of course if applicable, pay attention to who is NOT on the refigure list. Oh, and one more thing. Put your "antennae" up when you do this, to get a sense if you are being told the truth.

10. AMATEUR TESTERS.
Now here's one for you- Go talk to the individuals (amateurs) who wield the knifedge. Yes, I mean the bench Foucault test. They are the "keepers of the secret". There are commercial companies who will say that ATM's (amateur telescope makers) are not qualified to make assessment of the quality of their optic. I SAY DIFFERENT. These "swordsmen of the knifedge" are around, and they make public postings. There are some on this forum, and generally they do not have an agenda. Those people cannot be lied to. In about 60 seconds they can make a good qualitative assessment of the overall quality of an optic on the bench. In another 1/2 hour they can tell you everything there is to know about it and whether it is a dog, mediocre, good, very good, or stunningly superb. One example, there is an individual who posted on SAA some time ago who has tested some 50 optics in the last 5 years. He named names and cited numbers. For those who read it they gained some very good information. Pay attention to individuals like that. Outside of veteran star testers who have evaluated thousands of scopes in the field (very rare individuals) these folks know the very most.

11. See for yourself at a starparty by TESTING AT THE EYEPIECE.
Do you know what a Ronchi grating is? -Very valuable qualitative tool at the eyepiece for the reasonably seasoned user. It does not lie. Take out the eyepiece, point at a bright star and put in the grating. Look for very straight lines, lined up with the spider as a reference. Any noticeable departure from straight lines could mean trouble because it is not real sensitive when used this way. Then look at the end of the lines as they go over the edge of the mirror. If you see any hook whatsoever you have a turned edge. TDE is a very common problem which kills contrast right from the start, and is easy to spot with this method. For those who become more adept at it, try using it with a 2.5x Barlow. I'm not going to get into more specifics but it is worth studying for those who want to. The Ronchi grating can also be used at the radius of curvature of the mirror. In an instant, with a simple $5.00 grating and a light source you can see smoothness of correction, surface roughness, zones and turned edge. You can see all this at a glance, and you can do this right in the telescope!

Carl Zambuto


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ml96737
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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5694862 - 02/22/13 02:30 PM

Quote:

Mike L, a professional optician states:

Quote:

The explanation is simple - it is difficult to test mirrors




You're stating:

Quote:

In fact star testing is EASY. Compared to what it takes to do accurate bench testing to 1/10 wave, or the complexities of phase shift interferometry, star testing is Trivial.




I think the actual, factual story is that quick & easy testing isn't precise, but tells a knowledgeable & experienced tester quite a bit... within limits. It might also tell a novice tester that his mirror has issues, leading them to ship it off for a re-figure... needlessly.

Any "test" that is truly meaningful, in a quantifiable manner (such as it is), will *not* be "easy". Any test that is "easy" will *not* be quantifiable.




With all due respect, this sounds like a cliche, not any factual statement. Please refer to: "Star Testing Astronomical Telescopes", H.R. Suiter ISBN 0943396905. In the first sentence of his book Mr. Suiter makes clear "Telescopes are easy to test...All that is required is a good high-magnification eyepiece..." In fact, for most users, all that is needed is to become familiar with is the 16 pages of Chapter 2 "An abbreviated Star-test manual".

It will become abundantly clear to the tester that the star test is in fact EASY and "Quantifiable and Meaningful".


Quote:

This example is, what?, THREE mirrors? Out of how many? Did Rohr test several dozen Pegasus mirrors? No? Then what do we know, scientifically speaking?




Statistically speaking, if a random sample of 3 mirrors tested unsatisfactory (far below manufacturer's claims), the odds that was simply by chance can be calculated by the one-sample t-test, which in this case would give (using the 3 values measured 0.774, 0.782, 0.80 [assumed]) p=0.0017 which is definitely statistically significant, ie. the observed difference from the claimed 0.97 Strehl to arise by chance selection of the sample alone is 0.17%, that is 99.83% true the measurements represent an actual deviation from the claimed Strehl 0.97 of Pegasus mirrors.

I hope this thread is not shut down, as it really covers this very important and misunderstood topic of how simple and effective the star test really is. And anyone can easily evaluate their mirrors in a clear and definitive fashion by following Suiter's simple instructions.

Mike


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Mike B
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Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5695014 - 02/22/13 03:42 PM

Quote:

With all due respect, this sounds like a cliche, not any factual statement.



Then read CZ's statement quoted in the post above; look how many times he uses the term "qualitative" in regards to knife-edge & star testing. For someone with some experience at doing so, i'd say *yes*, getting a correct qualitative assessment of a mirror is relatively easy, even "quick" if the half-hour quoted above is withing the tester's abilities.

It's the (meaningfully) "quantitative" values that takes the time & expense, and where much of this trouble stems from. This is all i'm saying.

Quote:

Statistically speaking, if a random sample of 3 mirrors tested unsatisfactory...



Stop right there. People boxing up their Dob's primaries & shipping them ovserseas to Rohr for testing hardly qualifies as "random"! The vast majority of reasonably happy Dobists would never do this. This sort of thing is not a "random" situation at all.

Same for the CZ mirrors he's tested... why were those sent? Someone thot they had a lemmon? Somehow i doubt that. What Rohr is doing, at least on the surface, appears to be a real service to the astro community. But it hardly represents "random".

Additionally, there STILL is no info re: how many Pegasus optics Rohr tested; just these three? Or were there more that were just fine- these three being the ones reported on the website? You can stir numbers all you want... if the premise is unfounded, or the data is too small, or worse yet- not truly representative of the species, we are nowhere.

This is a bit like judging a chicken farm by the number of omelets not ordered at local restaurant.

A far better line of reasoning to support your position is described in CZ's point #4. This is a very illuminating point!

His point #5 also bears on this; these three (now famous? ) Pegasus mirrors were hovering right at or slightly below 1/4 wave performance. But "performance" wasn't the issue, was it? It was regarding the *advertising*. So what was being said, and what -wasn't- ?

I've heard & read that Porsche "dumbs-down" the performance specs on its cars, slightly... this way every weekend road-warrior that goes out & puts his foot thru the firewall turns out a 0-60 a couple of tenths faster than the glossy listed in the showroom. "Yay, i got a really good one!" Sure is better than getting a couple of tenths longer, & returning to the dealer in a lather... only to find your tires were a skosh low on air, your track was upwind or upgrade, or your driving skills were 0.75 Strehl. The folks at Porsche ain't stupid.

I guess that's much of what i've been harpin' on... what wasn't said. We spend our time here over-analyzing the iceberg we see above the water... what's hidden from view, well, that's what would be really helpful. Unfortunately, it's probably not forthcoming.


Quote:

The explanation is simple - it is difficult to test mirrors, and any bad report will be countered by a glowing report. Then it turns into a *BLEEP* contest, rather than a discussion.



All we need now is that "glowing report", and Mike L will have nailed it.


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Mike Lockwood
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Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5695028 - 02/22/13 03:49 PM

Quote:

With all due respect, this sounds like a cliche, not any factual statement. Please refer to: "Star Testing Astronomical Telescopes", H.R. Suiter ISBN 0943396905. In the first sentence of his book Mr. Suiter makes clear "Telescopes are easy to test...All that is required is a good high-magnification eyepiece..." In fact, for most users, all that is needed is to become familiar with is the 16 pages of Chapter 2 "An abbreviated Star-test manual".
It will become abundantly clear to the tester that the star test is in fact EASY and "Quantifiable and Meaningful".



I will agree that the star test is meaningful, if done properly.

It is useful as a final check on an optic provided the mirror can be tested in equilibrium, and with a flat of known quality used as a secondary mirror.

Star testing a secondary is almost impossible, though. It is best to first (before doing star testing) measure interferometrically the secondary in its holder to verify that the flat is good and it is not being warped by its holder. A mounted flat of known good quality could then be eliminated as a variable or source of significant error in the star test. The secondary's cooling effects cannot be eliminated, though.

Of course good seeing is necessary for both tests on a real star and an artificial one at finite distance. (I note that you are located in Hawaii, a location with better seeing than most, and this will make star testing much easier than in say, my location.) Also proper mirror support and collimation are necessary.

However, the star test is a subjective one because there is no measurement taken. Error amplitudes are estimated based on simulated images, and that is an educated guess. Tests based on the actual image, such as the Hartman, etc., are quantifiable.

Mirrors usually have a combination of defects, and untangling these overlapping issues is extremely difficult. In the case of a mirror having only one defect (say pure over- or under-correction), then it may be possible to get an estimate of the magnitude of the problem if thermal issues do not throw off the results. Additionally, seeing and thermal issues usually obscure defects like roughness on the optical surface and mild edge rolls.

Quote:

I hope this thread is not shut down, as it really covers this very important and misunderstood topic of how simple and effective the star test really is. And anyone can easily evaluate their mirrors in a clear and definitive fashion by following Suiter's simple instructions.



I agree on the thread and star testing - learning to star test is very good and I encourage people to do it. However, thermal effects and other issues make it more complicated than you make it out to be, and it takes time to learn.

I can't count the number of mirrors I have received that "star tested just fine", only to find that they had serious problems. The clients always notice the improvement that refiguring brings, though.


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Jarad
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Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5695050 - 02/22/13 04:00 PM

Quote:

Statistically speaking, if a random sample of 3 mirrors tested unsatisfactory (far below manufacturer's claims), the odds that was simply by chance can be calculated by the one-sample t-test, which in this case would give (using the 3 values measured 0.774, 0.782, 0.80 [assumed]) p=0.0017 which is definitely statistically significant, ie. the observed difference from the claimed 0.97 Strehl to arise by chance selection of the sample alone is 0.17%, that is 99.83% true the measurements represent an actual deviation from the claimed Strehl 0.97 of Pegasus mirrors.





Okay, if you are going to get into the stats, you have made a major incorrect assumption: that the 3 mirrors represent a random sample. The mirrors were almost certainly sent in for testing precisely because they were under-performing. They are not a random sample - a random sample would have to be selected from the production line before any indication of a problem with the mirror.

Let's not turn this thread into bashing a specific vendor. The topic is "Top Opticians". If there are others you think need to be added to the list, then bring them up. You have a right to feel that Pegasus doesn't belong on it, and have made that clear. Let's move on.

Thanks,

Jarad


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ml96737
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Re: Top Opticians in the US for reflectors new [Re: Mike Lockwood]
      #5695145 - 02/22/13 04:53 PM

Mike, I appreciate your input here. I feel star testing is an important, misunderstood and under-utilized test that all telescope owners should become informed about. Hence, my specific responses below

Quote:

Quote:

With all due respect, this sounds like a cliche, not any factual statement. Please refer to: "Star Testing Astronomical Telescopes", H.R. Suiter ISBN 0943396905. In the first sentence of his book Mr. Suiter makes clear "Telescopes are easy to test...All that is required is a good high-magnification eyepiece..." In fact, for most users, all that is needed is to become familiar with is the 16 pages of Chapter 2 "An abbreviated Star-test manual".
It will become abundantly clear to the tester that the star test is in fact EASY and "Quantifiable and Meaningful".



I will agree that the star test is meaningful, if done properly.

It is useful as a final check on an optic provided the mirror can be tested in equilibrium, and with a flat of known quality used as a secondary mirror.

Star testing a secondary is almost impossible, though. It is best to first (before doing star testing) measure interferometrically the secondary in its holder to verify that the flat is good and it is not being warped by its holder. A mounted flat of known good quality could then be eliminated as a variable or source of significant error in the star test. The secondary's cooling effects cannot be eliminated, though.




This should be the normal conditions under which any high quality mirror should be used anyways, so no special issue here.

Quote:

Of course good seeing is necessary for both tests on a real star and an artificial one at finite distance. (I note that you are located in Hawaii, a location with better seeing than most, and this will make star testing much easier than in say, my location.) Also proper mirror support and collimation are necessary.




Again, these are necessary conditions for any premium mirror to perform well.

Quote:

However, the star test is a subjective one because there is no measurement taken. Error amplitudes are estimated based on simulated images, and that is an educated guess. Tests based on the actual image, such as the Hartman, etc., are quantifiable.




This is an interesting point. One could make the argument, why do you need a precise quantitative numerical value, if the star test shows it is nearly perfect? Carl Zambuto doesn't provide any quantitative specs on his mirrors for this reason, basically guaranteeing it will perform at 50x/inch without image degradation. This would be similar to a "good" star test with nearly equal intra/extra-focal appearances. Why worry if it is Strehl 0.95, 0.97, or 0.992? In fact it may be extremely difficult to measure this accuracy so precisey and repeatably by bench testing anyway. (ie. different competent interferometrists could easily disagree on such precision) So there is little point to such numbers. A good star test is as fine as Zambuto's 50x/inch criteria.

Quote:

Mirrors usually have a combination of defects, and untangling these overlapping issues is extremely difficult. In the case of a mirror having only one defect (say pure over- or under-correction), then it may be possible to get an estimate of the magnitude of the problem if thermal issues do not throw off the results. Additionally, seeing and thermal issues usually obscure defects like roughness on the optical surface and mild edge rolls.




Star testing alone is not so useful for mirrors which have a multitude of problems, far off from proper correction, etc. Suiter himself admits that its primarily as a final test for a nearly or finished mirror, where the defects are small. If a mirror is so bad that it confounds a star tester, who would want such a mirror anyway?

Quote:

I can't count the number of mirrors I have received that "star tested just fine", only to find that they had serious problems. The clients always notice the improvement that refiguring brings, though.




Then, they were not star tested properly. Given how sensitive the star test is, any mirror with "serious problems" will show up in a glance as unequal intra/extra-focal patterns. I would direct anyone who falls into this category to Suiter's book, Chapter 2.

Mike


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Jeff Morgan
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Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5696301 - 02/23/13 10:30 AM

Quote:

With all due respect, this sounds like a cliche, not any factual statement. Please refer to: "Star Testing Astronomical Telescopes", H.R. Suiter ISBN 0943396905. In the first sentence of his book Mr. Suiter makes clear "Telescopes are easy to test...All that is required is a good high-magnification eyepiece..." In fact, for most users, all that is needed is to become familiar with is the 16 pages of Chapter 2 "An abbreviated Star-test manual".

It will become abundantly clear to the tester that the star test is in fact EASY and "Quantifiable and Meaningful".




This puts me in mind of the test Sky & Telescope published back in the 90's. Optician Peter Ceravolo made three 6" f/8 Newtonians with differing quality on the mirror. IIRC, it was 1/2 wave, 1/4 wave, and 1/10 wave. The editors felt the two better mirrors would be difficult to differentiate in use. To test this they took these scopes to Stellafane and a large number (150 or so) amateurs volunteered to grade the scopes via star test.

The results were overwhelming, the "random amateurs" had no problems identifying the better mirror by star testing on Polaris.


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Mike B
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Re: Top Opticians in the US for reflectors new [Re: Jeff Morgan]
      #5696388 - 02/23/13 11:15 AM

Quote:

The results were overwhelming




I recall reading up on that just recently... 'tis an incident that keeps resurfacing. What is truly fascinating is they said the test was intended to compare VIEWS, and those between the 1/4 wave & 1/8 were nearly (save for the better moments of seeing, and then subtly iirc ) indistinguishable! But the testers were flummoxed in their efforts because the more experienced observers were deliberately racking thru focus, star-testing, to accomplish the distinction.

This wasn't the experimenters' plan! Some folks cheated.

It's also worth noting that the optician here, Peter Ceravolo, was probably that day's Zambuto... the "wave" differences in these mirrors was specifically & intentionally designed into them. I forget of what type it was, but recall that it was *not* random aberration.

Edited by Mike B (02/23/13 12:22 PM)


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Calypte
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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5696498 - 02/23/13 12:17 PM

Quote:

It's also worth noting that the optician here, Peter Ceravolo, was probably that day's Zambuto... the "wave" differences in these mirrors was specifically & intentionally designed into them. I forget of what type it was, but recall that it was *not* random aberration.



Peter Ceravolo is still very much with us, though probably not making very many 6-inch f/8 paraboloids. What amazed me about this endeavor (and maybe the experienced opticians here don't find it so amazing) was that Peter was able to intentionally arrive at controlled states of uncorrectedness. Everybody here has looked through a lot of bad mirrors (I remember some real laughers in years gone by), but they were accidents, and many "good" mirrors from amateur makers were products of luck rather than control of the process.


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Mike B
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Re: Top Opticians in the US for reflectors new [Re: Calypte]
      #5696519 - 02/23/13 12:33 PM

Quote:

Peter Ceravolo is still very much with us, though probably not making very many 6-inch f/8 paraboloids.



Which brings up a very good point; these were all 6" F8 mirrors. As aperture goes up, the difficulty in achieving a good figure goes up with it... and probably not arithmetically, either. Prob'ly even more difficult is attaining good correction as the "speed" increases!

Today's 18" F4.5 mirrors are not only FAR and away more difficult to achieve high Strehls on than 6" F8's, even in controlled lab conditions... they're also much more difficult to maintain such values outdoors, in a structure, and in the wild!

Quote:

...and many "good" mirrors from amateur makers were products of luck rather than control of the process.



Which maybe goes some distance in explaining the gradual evolution of "offshore" optics, from generally lackluster to now generally pretty good, even excellent.


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Mike Lockwood
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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5696711 - 02/23/13 02:19 PM

Quote:

the "wave" differences in these mirrors was specifically & intentionally designed into them. I forget of what type it was, but recall that it was *not* random aberration.



Pure undercorrection is very smooth, and doesn't degrade images as much as other aberrations with larger transverse error. However, PURE undercorrection is rare - usually the outer zones are more undercorrected than the inner ones, resulting in larger transverse error in the outer zones.

Quote:

What amazed me about this endeavor.... was able to intentionally arrive at controlled states of uncorrectedness.



Generally parabolizing takes a near-sphere and makes it a near-parabola. In the process, correction is added, so if you stop before reaching a parabola, undercorrection is the result. Getting a purely undercorrected mirror takes about as much work as getting a nicely corrected (parabolic) one.

Making an undercorrected mirror with less correction (and more transverse error) near the edge takes less work because it is correcting the outer areas of a mirror that takes the most time. Thus, this is a common defect in many mirrors produced under time constraints.

During cooling early in an observing session, an undercorrected mirror with less correction near the edge can look like it is fully corrected. For decades mirrors have been left undercorrected on purpose so that they may perform a little better under cooling. Obviously this can give a "false positive" star test. During cooling, though, heat and air currents cause their own significant degradation, reducing the "benefit" of the undercorrection.

As the mirror approaches equilibrium, air currents decrease and the stage is set for good performance.... but the mirror is then undercorrected with large transverse error in the outer zones (much of the area of the mirror), and it can't perform up to its potential.

Quote:

As aperture goes up, the difficulty in achieving a good figure goes up with it... and probably not arithmetically, either. Prob'ly even more difficult is attaining good correction as the "speed" increases!



As aperture goes up, so does the difficulty of star testing, and so does the error in estimating the quality of a mirror with the star test.


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Jeff Morgan
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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5696759 - 02/23/13 02:55 PM

There was another interesting take-away from that article. In the first part, three experienced observers were trying to grade out the mirrors under general observing conditions (not star testing). Two of them were using large APO refractors as a "reference source" and commented on how the best of the 6" Newtonians was keeping up nicely with the APO on Jupiter.

Overall that article was a great effort, I wish Sky & Tel did more articles like it.


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mark cowan
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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5696826 - 02/23/13 03:28 PM

Quote:

It's also worth noting that the optician here, Peter Ceravolo, was probably that day's Zambuto... the "wave" differences in these mirrors was specifically & intentionally designed into them. I forget of what type it was, but recall that it was *not* random aberration.




Still is. The mirrors only differed in spherical correction, otherwise being smooth and error-free. And this is definitely not the case for ordinary mirrors of poor performance. Such an experiment done multiple times under a range of seeing would be considerably more interesting - it's really more of a test of the observers than the optician.

Best,
Mark


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Mike B
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Re: Top Opticians in the US for reflectors new [Re: Jeff Morgan]
      #5696873 - 02/23/13 03:52 PM

Did a li'l digging... for your reading enjoyment:
Royce
Lake County
Neil English
Peter Ceravolo

In that last one, Mr. Ceravolo's, he lands pretty hard on precisely what Mike (ml96737) has been alluding to... i think it's not that most of us would disagree with this, but more about how to go about dealing with it.

Royce discusses the "star-test", and really addresses the matter frankly. Yes, it's a fairly "easy" test to do... and also one fairly easy to misinterpret if one is not experienced enough.

In The Lake County article linked, Jack Kramer mentions a few shops by name, and the results of their optics being tested... perhaps a start on Dave B's "book"? As was stated previously, look who's being talked about, and how... and who's *NOT* being discussed.


And really, just because a shop or an individual is not "being discussed a lot" re: mirror-making proficiency does *not* necessarily mean they cannot or HAVE not produced fine optics, nor does it mean that most of their mirrors can't provide "good enough" views for most folks' use & enjoyment. The Asian mirrors coming out lately are possibly a good example of this?

At some point, tho, the less-gooder mirrors will fail to impress the more discriminating observers, when conditions permit their limitations to be seen... and in a somewhat different realm, the less-gooders will infuriate those who paid for the bragging rights, and who's mirrors have been exposed by "experts" as being not of the Strehl value that was advertised, and paid for.

All are valid concerns, yet not every manuf nor every buyer will fit the same model. Nor should they be expected to.

As always, these discussions are an education, and i for one appreciate them!


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Mike B
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Re: Top Opticians in the US for reflectors new [Re: mark cowan]
      #5696878 - 02/23/13 03:56 PM

Quote:

Still is.




Thank-you, Mark. Did not intend to render Mr. C a "was"... my apologies to him! I did not know whether he was still practicing the craft.


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Starman1
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Re: Top Opticians in the US for reflectors new [Re: Mike Lockwood]
      #5696945 - 02/23/13 04:36 PM

Quote:


During cooling early in an observing session, an undercorrected mirror with less correction near the edge can look like it is fully corrected. For decades mirrors have been left undercorrected on purpose so that they may perform a little better under cooling. Obviously this can give a "false positive" star test. During cooling, though, heat and air currents cause their own significant degradation, reducing the "benefit" of the undercorrection.

As the mirror approaches equilibrium, air currents decrease and the stage is set for good performance.... but the mirror is then undercorrected with large transverse error in the outer zones (much of the area of the mirror), and it can't perform up to its potential.





Eureka! This explains (and when I think about it I slap my head for not figuring it out before ) what I have seen in many mirrors.

I used to wonder why the "2-hour crowd" at Mt. Pinos, who largely had less-expensive scopes (typically 8-12" Chinese dobs), had seemingly such good optics early in the evening and such poor optics later on.

Some people would argue the "seeing had deteriorated", but I noted my Ostahowski mirror (2-3/16" thick, 1 fan) always performed better after several hours.

My Zambuto mirror (1-1/4" thick, 3 large fans) starts out the evening at the ambient temperature, so the first few hours, which were always "iffy" in the Ostahowski mirror, are always very sharp. Thin mirrors rule, where thermal issues are concerned. But only, I guess, if they are good mirrors to begin with.


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Calypte
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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5696985 - 02/23/13 05:03 PM

This is all very interesting. When I got a factory tour of Meade in 1986, years after Cave's collapse, I met people who had worked for TC, and one of them told me that Cave often intentionally undercorrected his mirrors. It was my understanding -- perhaps wrong, based upon what I'm reading here -- is that the mirror was supposed to fall into shape when fully cooled. With my own Cave scopes (1973 8-inch f/6 and 1978 12.5-inch f/5) I always let them sit for an hour or so before doing any serious viewing, and I've never seen anything to suggest that this is the wrong approach. Where I live now, the temp can drop 30 deg F quickly after sunset. WRT Peter Ceravolo, I suspect he's doing mainly commercial work (just a guess).

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ml96737
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Re: Top Opticians in the US for reflectors new [Re: Mike Lockwood]
      #5697741 - 02/24/13 03:15 AM

Quote:

Quote:

As aperture goes up, the difficulty in achieving a good figure goes up with it... and probably not arithmetically, either. Prob'ly even more difficult is attaining good correction as the "speed" increases!



As aperture goes up, so does the difficulty of star testing, and so does the error in estimating the quality of a mirror with the star test.




Mike Lockwood, can you elaborate on that? In general, aperture should not have any theoretical effect on the difficulty or accuracy of the star test. The diagrams in Suiter's book can be scaled to any size instrument and gives the same relative results.

Maybe you are referring to the larger apertures are more adversely affected by seeing? To get the same image scale you need to increase the magnification proportional to the aperture, and that could end up pushing beyond the typical seeing conditions available. For example a 20" f/4 star tested with a 3mm ep would be running close to 700x, which requires quite good seeing for a steady image.

This issue would really apply for the most detailed star testing. Under average or even relatively poor seeing, you can still judge pretty well if the intra/extra-focal patterns look identical or not. Your eyes are quite sensitive to such differences, if you shift quickly back and forth between the 2 focus points, even if blurred somewhat by seeing.

Mike


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VĂ­ctor MartĂ­nez
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Re: Top Opticians in the US for reflectors new [Re: Starman1]
      #5697770 - 02/24/13 04:12 AM

Quote:

Quote:


During cooling early in an observing session, an undercorrected mirror with less correction near the edge can look like it is fully corrected. For decades mirrors have been left undercorrected on purpose so that they may perform a little better under cooling. Obviously this can give a "false positive" star test. During cooling, though, heat and air currents cause their own significant degradation, reducing the "benefit" of the undercorrection.

As the mirror approaches equilibrium, air currents decrease and the stage is set for good performance.... but the mirror is then undercorrected with large transverse error in the outer zones (much of the area of the mirror), and it can't perform up to its potential.





Eureka! This explains (and when I think about it I slap my head for not figuring it out before ) what I have seen in many mirrors.

I used to wonder why the "2-hour crowd" at Mt. Pinos, who largely had less-expensive scopes (typically 8-12" Chinese dobs), had seemingly such good optics early in the evening and such poor optics later on.

Some people would argue the "seeing had deteriorated", but I noted my Ostahowski mirror (2-3/16" thick, 1 fan) always performed better after several hours.

My Zambuto mirror (1-1/4" thick, 3 large fans) starts out the evening at the ambient temperature, so the first few hours, which were always "iffy" in the Ostahowski mirror, are always very sharp. Thin mirrors rule, where thermal issues are concerned. But only, I guess, if they are good mirrors to begin with.




Don, that Ostahowski's primary mirror was too thick to be a 12", now understand that the acclimation times were eternal and mediocre images up at that point in relation to your current Zambuto, there is no comparison.


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Starman1
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Re: Top Opticians in the US for reflectors new [Re: VĂ­ctor MartĂ­nez]
      #5698600 - 02/24/13 05:03 PM

Quote:


Don, that Ostahowski's primary mirror was too thick to be a 12", now understand that the acclimation times were eternal and mediocre images up at that point in relation to your current Zambuto, there is no comparison.



Carl Zambuto told me he would have melted it into a 15" mold.


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Calypte
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Re: Top Opticians in the US for reflectors new [Re: Starman1]
      #5698812 - 02/24/13 07:26 PM

Without knowing any details of Don's Ostahowski mirror other than those presented in his post, it should be noted that Terry's mirror substrates are often supplied by end users or scope manufacturers and may not necessarily represent his own opinion of what would be ideal.

Edited by Calypte (02/24/13 07:27 PM)


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cpr1
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Re: Top Opticians in the US for reflectors new [Re: Calypte]
      #5698818 - 02/24/13 07:32 PM

The Terry mirror I am getting is 1.6 inches thick. New. 16.25 inches from Newport.

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Starman1
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Re: Top Opticians in the US for reflectors new [Re: Calypte]
      #5698860 - 02/24/13 08:06 PM

Quote:

Without knowing any details of Don's Ostahowski mirror other than those presented in his post, it should be noted that Terry's mirror substrates are often supplied by end users or scope manufacturers and may not necessarily represent his own opinion of what would be ideal.



Mine was pyrex from Terry. It was billed as "full-thickness, 6:1" when I got the scope from Discovery.
Full thickness would have been 52.9mm thick, and it was 55mm, so *approximately* the 6:1 thickness for a 317.5mm mirror.
Terry has gone to thinner substrates, reflecting a more contemporary thinking on the subject of mirror thickness. But by Zambuto standards, his mirrors would still be considered thick if above 10:1.
Christopher's 10:1 thickness 16" defines the more current thinking on the thickness of 10" to 20" mirrors.


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Mike B
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Re: Top Opticians in the US for reflectors new [Re: Starman1]
      #5698920 - 02/24/13 09:05 PM

Quote:

...reflecting a more contemporary thinking on the subject of mirror thickness.



Sounds just like the "contemporary thinking" in regards to human obesity as well, as defined by the ever-changing "BMI" ratio.


The primary difference being that Newtonian mirrors keep getting thinner & faster, whereas we, their users, keep getting thicker & slower.


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Calypte
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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5699219 - 02/25/13 01:28 AM

Long before my imaging days, when I was viewing with Cave reflectors (I still have the reflectors, but I don't use them so much these days), the thin mirrors currently in vogue were considered a cost-cutting heresy. People accepted them to save weight, but not in hopes of faster cooldown. Some of us haven't quite caught up to "contemporary thinking." Soon you'll be telling me that galaxies are obsolete

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careysub
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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5699227 - 02/25/13 01:42 AM

Quote:

Quote:

...reflecting a more contemporary thinking on the subject of mirror thickness.



Sounds just like the "contemporary thinking" in regards to human obesity as well, as defined by the ever-changing "BMI" ratio.


The primary difference being that Newtonian mirrors keep getting thinner & faster, whereas we, their users, keep getting thicker & slower.







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jpb30
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Re: Top Opticians in the US for reflectors new [Re: Calypte]
      #5699230 - 02/25/13 01:47 AM

There's nothing like a very thick mirroir in zerodur
It solves the pb of the putting in temperature of the mirror and also the astigmatism of fold of the thin mirrors

Since I use a mirror to zerodur, I do not see returning any more towards the pyrex or other supremax, the zerodur changes the life of the astronomer


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VĂ­ctor MartĂ­nez
sage


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Re: Top Opticians in the US for reflectors new [Re: cpr1]
      #5699298 - 02/25/13 04:32 AM

Quote:

The Terry mirror I am getting is 1.6 inches thick. New. 16.25 inches from Newport.



The mine is 1,6" thick too.


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kfrederick
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Re: Top Opticians in the US for reflectors new [Re: VĂ­ctor MartĂ­nez]
      #5699365 - 02/25/13 07:19 AM

Seeing the images in a 32 inch f3.6 Lockwood primary I will say that mirror worked unreal good .I think Als coma corrector helped also. I have a Zambuto 17 inch f8 hyperbolic primary in my Chief it is a gem .

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Jarad
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Re: Top Opticians in the US for reflectors new [Re: jpb30]
      #5699369 - 02/25/13 07:25 AM

Quote:

There's nothing like a very thick mirroir in zerodur
It solves the pb of the putting in temperature of the mirror and also the astigmatism of fold of the thin mirrors

Since I use a mirror to zerodur, I do not see returning any more towards the pyrex or other supremax, the zerodur changes the life of the astronomer




Actually, substrates like zerodur or quartz only fix the issue of the figure changing during cooling. They do nothing about the boundary layer caused by the temperature difference between the air and the mirror, which is the more damaging one in my experience.

Jarad


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kfrederick
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Re: Top Opticians in the US for reflectors new [Re: Jarad]
      #5699493 - 02/25/13 09:37 AM

http://www.cloudynights.com/classifieds/showproduct.php?product=74748&sor... Here is some thick Zerodur

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Starman1
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Re: Top Opticians in the US for reflectors new [Re: kfrederick]
      #5699713 - 02/25/13 11:37 AM

Advantages of a thick mirror:
--simpler cell and edge support design
--better bottom-end ballast. Trunnions can be mounted lower, reducing flexure in rocker boxes.
--less flexure-induced astigmatism at low altitude

Advantages of a thin mirror:
--much more rapid cooling and elimination of the boundary layer
--greater portability to the scope
--larger mirrors can weigh the same, yet be larger with all the advantages of a larger size.
--easier to handle mirror when cleaning
--faster cooling in the shop for the optician, so faster to test
--entire scope structure can be lighter due to lower inertia and scope weight
--less glass means lower materials cost and a greater variety of materials available for use as a substrate.

And since the advent of well-designed mirror cells (9-36 points) and edge support systems (cable slings, whiffle trees, etc), it's no longer true that thin = astigmatism in the star images.


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Mike Lockwood
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Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5699783 - 02/25/13 12:36 PM

Quote:

Mike Lockwood, can you elaborate on that? In general, aperture should not have any theoretical effect on the difficulty or accuracy of the star test. The diagrams in Suiter's book can be scaled to any size instrument and gives the same relative results.
Maybe you are referring to the larger apertures are more adversely affected by seeing? ....
This issue would really apply for the most detailed star testing. Under average or even relatively poor seeing, you can still judge pretty well if the intra/extra-focal patterns look identical or not.



I'm saying that people do a poorer job of estimating the quality of larger mirrors. It's an observation that I've made based on the observing reports that I hear, the observing I have done, and the initial test results obtained when they come into my shop for testing/work.

I think part of the reason is that people in general don't ask more of a larger mirror because they don't know what it should do. If it forms fairly decent images and stars are mostly round, they think it's OK. It takes a good night using a properyly set-up telescope with a good large mirror to push up their standards, and most people haven't had that experience.

Also, it's generally true that the larger the mirror, the poorer it tests. My test data demonstrates this regularly. This is not surprising, given that a poor large mirror still usually outperforms a superb smaller one, some error can be gotten away with...... at least until the owner sees what a good large mirror can do and demands better.

Quote:

Actually, substrates like zerodur or quartz only fix the issue of the figure changing during cooling. They do nothing about the boundary layer caused by the temperature difference between the air and the mirror, which is the more damaging one in my experience.



Exactly right. Quartz changes about 1/6 as much as Supremax/Pyrex, and Zerodur really doesn't change at all. Of course one pays a lot of money for the properties of these materials.

Quote:

Advantages of a thin mirror:
--faster cooling in the shop for the optician, so faster to test



Yes, a bit faster to test, but more difficult to test, too.


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Jeff Morgan
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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5700064 - 02/25/13 03:32 PM

Quote:

Did a li'l digging... for your reading enjoyment:
Royce
Lake County
Neil English
Peter Ceravolo




Thanks for the links, nice reads. I also like Carl Zambuto's philosophy on mirror smoothness.


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Jeff Morgan
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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5700085 - 02/25/13 03:40 PM

Quote:

Quote:

...reflecting a more contemporary thinking on the subject of mirror thickness.



Sounds just like the "contemporary thinking" in regards to human obesity as well, as defined by the ever-changing "BMI" ratio.


The primary difference being that Newtonian mirrors keep getting thinner & faster, whereas we, their users, keep getting thicker & slower.






I almost choked on my ice cream!


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ckwastro
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Re: Top Opticians in the US for reflectors new [Re: Mike Lockwood]
      #5701630 - 02/26/13 12:59 PM

Quote:

Actually, substrates like zerodur or quartz only fix the issue of the figure changing during cooling. They do nothing about the boundary layer caused by the temperature difference between the air and the mirror, which is the more damaging one in my experience.




Quote:

Exactly right. Quartz changes about 1/6 as much as Supremax/Pyrex, and Zerodur really doesn't change at all. Of course one pays a lot of money for the properties of these materials.





These materials may even make the boundary layer much more of an issue than it is with Pyrex.

A few years I had a conversation with a friend of mine who is an optics guy. We were discussing the the properties of these two materials and whether or not they are worth the additional cost. He had done some research on the heat transfer coefficient of Quartz and Zerodur and concluded that they retain heat much longer than Pyrex, so while the figure won't change, the effect is that those substances might result in a boundary layer that takes much longer to clear. I suppose one could always use fans to assist the equilibration, but it seems to be trading one problem for another that is probably more damaging to the final image.

Has anyone with a Quartz or Zerodur mirror ever noticed this to be the case in actual application?


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Mark Harry
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Re: Top Opticians in the US for reflectors new [Re: Jeff Morgan]
      #5701842 - 02/26/13 03:10 PM

CZ's smoothness criteria is spot on the money. I even like to see it a bit more stringent than this to describe an optic's capability in reaching what might be possible to see with it. Bravo.
M.


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DHurst
super member


Reged: 03/10/06

Re: Top Opticians in the US for reflectors new [Re: ckwastro]
      #5701848 - 02/26/13 03:13 PM

Sorry to digress, from all the off topic banter, but what happened to
Ed Stevens?
I have a wonderful 14.5" mirror he made and a great 10" re-annealed and refigured by him.


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Peter Natscher
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Re: Top Opticians in the US for reflectors new [Re: ckwastro]
      #5701918 - 02/26/13 03:53 PM

Per given aperture, the quartz mirror can be made thinner and thus lighter-weight than the Pyrex mirror. This creates a mirror of less mass. Less mass will cool faster. Mike Lockwood is figuring 20" quartz mirror with only 1.25" edge thickness. Typical Pyrex at 20" would be 1.5" edge thickness - a lot heavier than the quartz mirror. I think the 20" quartz mirror would cool faster than the 20" Pyrex mirror given the same f/ratio and sagitta curves.

Quote:

Quote:

Actually, substrates like zerodur or quartz only fix the issue of the figure changing during cooling. They do nothing about the boundary layer caused by the temperature difference between the air and the mirror, which is the more damaging one in my experience.




Quote:

Exactly right. Quartz changes about 1/6 as much as Supremax/Pyrex, and Zerodur really doesn't change at all. Of course one pays a lot of money for the properties of these materials.





These materials may even make the boundary layer much more of an issue than it is with Pyrex.

A few years I had a conversation with a friend of mine who is an optics guy. We were discussing the the properties of these two materials and whether or not they are worth the additional cost. He had done some research on the heat transfer coefficient of Quartz and Zerodur and concluded that they retain heat much longer than Pyrex, so while the figure won't change, the effect is that those substances might result in a boundary layer that takes much longer to clear. I suppose one could always use fans to assist the equilibration, but it seems to be trading one problem for another that is probably more damaging to the final image.

Has anyone with a Quartz or Zerodur mirror ever noticed this to be the case in actual application?




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Sgt
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Re: Top Opticians in the US for reflectors new [Re: ckwastro]
      #5702001 - 02/26/13 04:24 PM

Quote:

Quote:

Actually, substrates like zerodur or quartz only fix the issue of the figure changing during cooling. They do nothing about the boundary layer caused by the temperature difference between the air and the mirror, which is the more damaging one in my experience.




Quote:

Exactly right. Quartz changes about 1/6 as much as Supremax/Pyrex, and Zerodur really doesn't change at all. Of course one pays a lot of money for the properties of these materials.





These materials may even make the boundary layer much more of an issue than it is with Pyrex.

A few years I had a conversation with a friend of mine who is an optics guy. We were discussing the the properties of these two materials and whether or not they are worth the additional cost. He had done some research on the heat transfer coefficient of Quartz and Zerodur and concluded that they retain heat much longer than Pyrex, so while the figure won't change, the effect is that those substances might result in a boundary layer that takes much longer to clear. I suppose one could always use fans to assist the equilibration, but it seems to be trading one problem for another that is probably more damaging to the final image.

Has anyone with a Quartz or Zerodur mirror ever noticed this to be the case in actual application?




Are you sure you heard it right from your friend? A Google search indicates quartz has about 1.3 times the thermal conductivity of Pyrex and slightly lower specific heat capacity. I'd also be interested to hear real life experiences.

edit: that should be 1.3 times the thermal conductivity

Edited by Sgt (02/26/13 06:56 PM)


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Tom Polakis
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Re: Top Opticians in the US for reflectors new [Re: ckwastro]
      #5702018 - 02/26/13 04:33 PM

Quote:

These materials may even make the boundary layer much more of an issue than it is with Pyrex.

A few years I had a conversation with a friend of mine who is an optics guy. We were discussing the the properties of these two materials and whether or not they are worth the additional cost. He had done some research on the heat transfer coefficient of Quartz and Zerodur and concluded that they retain heat much longer than Pyrex, so while the figure won't change, the effect is that those substances might result in a boundary layer that takes much longer to clear.





Heat transfer coefficient is not a intrinsic material property, rather, it's a function of the surface condition, air velocity, and other factors. The thermal properties of interest for mirror cooling would be density, specific heat, and conductivity. I looked them up, and they are not significantly different between the three materials. As was mentioned in reply to your post, the quartz mirror can be made thinner, reducing the mass, which is always good news thermally.

Anyway, here are the properties, which I will admit are not taken from one source. Note the striking improvements over Pyrex in thermal expansion coefficient.


Density Conductivity Specific Heat Thermal Expansion Coeff.
(g/cm^3) (W/m-K) (cal/g-C) (C ^-1)


Pyrex 2.23 1.06 0.180 3.25E-06


Quartz 2.20 1.30 0.176 5.50E-07


Zerodur 2.53 1.46 0.196 2.00E-08


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ckwastro
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Re: Top Opticians in the US for reflectors new [Re: Tom Polakis]
      #5702107 - 02/26/13 05:15 PM

Hi Tom,

It's been too long, good to hear from you. Thanks for the material properties.

Honestly (to the previous posters as well), this conversation was several years ago when we were just starting to see quartz coming into play in the amateur market, and we may have been talking about specific heat or some other property, rather than heat transfer. I do apologize for my fuzzy memory on this (which is unusual for me) , but whatever it was he came away thinking that the quartz would hold heat much longer and cause a completely different problem. I'm not sure exactly what prompted that conclusion at this point, but all I can say is that he's a meticulous researcher, and didn't just jump to that conclusion, even it turned out to be incorrect.

It's certainly possible we had not considered the ability to use thinner blanks at that time, which prompted my question about actual application rather than just theoretical. I was curious whether or not that was a noticeable problem with quartz & Zerodur.

Thanks for clearing up my questions about it.

Edited by ckwastro (02/26/13 05:18 PM)


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