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Better Secondary for Visual

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17 replies to this topic

#1 epee

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Posted 09 July 2020 - 07:54 AM

What is the finest Wave PV that the human eye can detect?



#2 Richard O'Neill

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Posted 09 July 2020 - 09:09 AM

 If you're asking about wavefront error I think the answer you're looking for is about 1/8 wave but I've read that some distortion effects can be detected beyond 1/20 wave.

 

 However, several years ago an optical resolution test was set up for Stellafane convention attendees. The majority of those who participated couldn't discern the difference between 1/4 wave and an array of otherwise identical telescopes, each with decreasingly lesser wavefront errors. The conclusion I draw from those tests is that error detectability depends on the individual's eyesight and experience. For me, 1/8 wave deviation in the wavefront seems pretty good.


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#3 Allan Wade

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Posted 09 July 2020 - 09:37 AM

That's going to depend a lot on the eye doing the testing. There's a fair range of differing visual acuity out there. Over specing your secondary is insurance money I consider.

 

Last secondary I bought was a 1/30 wave Antares. I could have gone a lower spec, but I spent $5000 on the Zambuto primary, so it didn't make sense to me to save $50 buying a lower spec secondary.


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#4 TOMDEY

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Posted 09 July 2020 - 09:44 AM

Rayleigh would say quarter-wave in the final wavefront at the focal plane. So, if you have a tenth-wave Primary Mirror and a tenth-wave Folding Flat... that would suffice. Many manufacturers quote surface, rather than wavefront --- so the requirement would then become twentieth-wave for each.    Tom


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#5 a__l

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Posted 09 July 2020 - 07:17 PM

That's going to depend a lot on the eye doing the testing. There's a fair range of differing visual acuity out there. Over specing your secondary is insurance money I consider.

 

Last secondary I bought was a 1/30 wave Antares. I could have gone a lower spec, but I spent $5000 on the Zambuto primary, so it didn't make sense to me to save $50 buying a lower spec secondary.

For such a price, the mirror should be accompanied with a great interferogram. My primary mirror (is Zambuto) standard PV 1/4 wf (without interferogram), secondary Terry Ostahowski quartz ~ 1/12 waves (interferogram is available).

I can clearly see the difference in OUT and IN StarTest. Nevertheless, the stars are good (in focus).

 

My 18" is almost a complete analogue in terms of parameters. Only secondary from sitall. 4" secondary price ~ 500 euros.

 

On my Intes StarTest is good. Mirrors sitall.


Edited by a__l, 09 July 2020 - 09:08 PM.


#6 hakann

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Posted 11 July 2020 - 06:41 PM

There is no way CZ can include a aero space tested modern IF ( read real IF ) for 5 grand.
This is ATM and market has set the price on this plano mirrors.
I was about check my CZ mirror in Rochester but test alone was twice the price of a Quartz 18” at f/4.
My idea was raise the discussion level ex here at CN when we talk tolerances as here we can swing out any numbers with absolutly nothing behind it.
But I thought that was not my ’call’ pay for that, but I hope someday someone do that for the premium ATM optic shops as they will not do that.
One can ask, why but my guess its a answer on that to.

But this shops make us happy for a low price, and they do a good work - that’s the way I see it.
If we feel better to think we has a 1 nm RMS mirror that just great.

#7 a__l

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Posted 11 July 2020 - 11:28 PM

There is no way CZ can include a aero space tested modern IF ( read real IF ) for 5 grand.
This is ATM and market has set the price on this plano mirrors.

Let's start with the secondary ones. Talk to Antares optics why their mirrors sell for much less than 5K.
Let's continue about primary and secondary. Talk to Terry Ostahowski. As a final option, talk with TEC Yuri.

 

For ATM interferometry, speak with OpenFringe developers and others (for example MKV). You can vary the parameters of the laser wavelength, ROC, etc. to evaluate your mistake. You can also get synthetic interferograms for mirrors with different parameters. Seek help from professional opticians who have such programs. You can verify your result with StarTest and good Ronchi. OpenFringe allows it. In sum, this is a long work (year or more), but it is interesting and worth the results.


Edited by a__l, 12 July 2020 - 02:06 AM.


#8 hakann

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Posted 12 July 2020 - 02:44 PM

a_l, we might cheer the same way to look at this.

 

Very few in ATM are into this numbers, and I don’t know why really but I has some answers over the years, and most means - if it’s good its good, and for me that’s Ok.
If it is not Ok, the only way is to talk to the shop that did the optic’s.
As I said, market set the price.
I talked to a ‘premium’ maker on high grade coating company and he said yes they are good but no one in ATM will ever pay for it – so here we go price must be what market like to pay.
This product’s they sell is at a fragment what pro mirrors cost, so I’m full aware of that and most that get this optic’s is all Ok in performance ( normally )
It's a win-win situation.
You get allot for 20 grand in big size on even Quartz and it can be at focal way under 3 as well.
That’s miles from pro pricing.
To me look ar this if a company do this for a living, they need to earn money each hours and in the end of year make a profit to.
So the math is easy, as certain hours and minus material and rest of how much the business cost each day will talk here, and in the end of the day it’s no way around it.

I shore wish the premium ATM makers would test just one of the mirrors to be serious tested, but the answer is a big NO.
I’m humble a following the line and think that is all Ok and good and ‘maybe’ the best way to deal with this.

 

And yes I know ex Terry O use IF and show numbers and also Antares, and I has a 1/30 secondary from them but it’s masked off to get there and I don’t believe it is that good either.
Still it shore is a good one.
My TO secondary hold it better to edge and was the best in a batch at around 1/12.
On IF there are older systems out there and I did notice a UK company in optic’s sold very high grade IF tested optics but the references was lousy.
So it’s not a easy thing all this.
As I learned the market is even more than happy on this product’s so why should they pay for test or get the most modern test machinery’s for the ATM market.
I’m amazed they can live on this 2020.

 

I also learned if one ever questionise this, it strike back and a pointless debate arises.
Again, it’s a win-win here and better to not discuss the issue..
That’s my side to look at this.

 

 

-You might has a idea ?


Edited by hakann, 13 July 2020 - 09:38 AM.


#9 a__l

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Posted 13 July 2020 - 09:07 PM

hakann, the idea is one. First of all, see StarTest. You can use a smartphone and a processing program for cleaning noise. This is a very good and independent test. You should get something similar (link from Zambuto website):

http://r2.astro-fore...bei-atmosfringe

If you see the difference in IN and OUT it makes sense to think, try to eliminate the mechanics, does not help, then look for who can digitize it (IF). Further make a decision.


Edited by a__l, 13 July 2020 - 10:22 PM.


#10 hakann

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Posted 14 July 2020 - 12:53 AM

I was not talking on my mirrors or my eyes in startest as OP had a question.
I will wait for a ATM’er had something tested serios, but it will newer be is my guess.

#11 a__l

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Posted 14 July 2020 - 02:51 AM

I was about check my CZ mirror in Rochester but test alone was twice the price of a Quartz 18” at f/4.

What were you talking about, that's what I answered.



#12 a__l

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Posted 14 July 2020 - 03:03 AM

So, if you have a tenth-wave Primary Mirror and a tenth-wave Folding Flat... that would suffice. Many manufacturers quote surface, rather than wavefront --- so the requirement would then become twentieth-wave for each.    Tom

Perhaps when it comes to small optics. I doubt that this is really possible in ATM commercial optics on sizes, say > 10-12" (the size is chosen arbitrarily). At a minimum, it should be expensive ceramics.

Then all the related problems are eliminated (according to the list).

 

What is the finest Wave PV that the human eye can detect?

For the telescope (primary and secondary), 1/8 wf is enough.
But if you see StarTest, in some cases, you can determine whether your telescope is better or not.


Edited by a__l, 14 July 2020 - 03:27 AM.


#13 TOMDEY

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Posted 14 July 2020 - 03:24 AM

Perhaps when it comes to small optics. I doubt that this is really possible in ATM commercial optics on sizes, say > 12" (the size is chosen arbitrarily).

 

For the telescope (primary and secondary), 1/8 wf is enough.
But if you watch StarTest, in some cases, you can determine whether your telescope is better or not.

Yeah, that is indeed the reality of it. And, frankly, in most applications... optics really don't need to be ultra-premium grade. Another potential (and depressing) reality is that components may have already been ~picked through~ at the source or associate, and the best ones skimmed off / spirited away. At work (large govt labs) we would deal with vendors to permit us incoming inspection and returns, no questions asked. We'd order a dozen of two, do the interferometry, and return a couple, for immediate replacement. Good for them and good for us. I assume they restocked our rejects... and preferentially sent us the better ones, to begin with! Explains why, when you buy exactly one certified quarter-wave flat... it arrives... and just comfortably meets the spec... is not much better than that.  Tom



#14 hakann

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Posted 14 July 2020 - 07:00 AM

I has read here on CN many reports on 20 w ( or better ) and many do like a 10 or 20 w, and maybe they can do that in a garage but when it comes to commercial sold mirrors in bigger/faster planos. I doubt they are at 4 w in the reality.
I read a Aerospace article before on a 500 mm lightweight in Zerodur and 10 nm RMS ( ca 8 w ) was almost impossible ( to test ) and that was in expensive fixture/cell and in special test room with absolutely no heat source.

-As I said, I would welcome to see a ATM sold mirror tested that way om modern Zygo's.
But at the price we will never see that, so star-test and be happy ( as most are )

 

 

Idea for OP is we might like 8 w, but few will be there in reality.
Then is smoothness, sag/cell, chassie, heat sources etc, and sky conditions.


Edited by hakann, 14 July 2020 - 07:42 AM.


#15 25585

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Posted 14 July 2020 - 08:28 AM

If great mirrors can be made for star diagonals, why can't the same quality be

produced for Newtonian secondaries?



#16 hakann

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Posted 14 July 2020 - 10:15 AM

It shore can, and ex Antares make great 1/30 W at a very low price secondarys, so just has a lip around and all will be fine.



#17 MitchAlsup

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Posted 14 July 2020 - 10:57 AM

Observers with great eyes can tell the difference between 1/4 and 1/8 wave.

But by the time you get better than 1/8th wave you are generally seeing more of the surface profile than the correction profile.



#18 a__l

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Posted 16 July 2020 - 07:43 PM

Observers with great eyes can tell the difference between 1/4 and 1/8 wave.

 

Not entirely correct.
If your telescope is well made, you have a great atmosphere, you will notice the difference with normal eyes. For example on planets.

 

hakann, "old" Zygo or new interferometer doesn't matter. They will determine the difference 1/4 and 1/8.

And even ATM bath too smile.gif

Including astigmatism. This is the main thing.


Edited by a__l, 16 July 2020 - 08:05 PM.

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