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1/10 Wave Mirror Advantages

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#1 Altair54

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Posted 16 November 2019 - 07:29 PM

Hello all, what is the advantage of the 1/10th wavelength mirror?  The Questar has a 3.5" aperture so the light gathering power is limited.

Thanks.



#2 coopman

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Posted 16 November 2019 - 07:53 PM

Better quality optics allow a scope to provide a better image.  However, a 3.5" scope will never be more than a 3.5" scope from a light gathering perspective, and this is true for any given aperture that you wish to insert into the sentence. 


Edited by coopman, 16 November 2019 - 07:53 PM.


#3 TOMDEY

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Posted 16 November 2019 - 08:22 PM

Also explains, yet another reason... why good big scopes and optics cost so much more than smaller ones. You have to maintain that absolute wavefront quality over a lot more area. No free lunch.

 

But sigh... I still want a Q-7!    Tom


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#4 EquusSpeculabundus

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Posted 16 November 2019 - 08:57 PM

Bragging rights and a marginal improvement on image quality (visually) over standard "diffraction limited" optics, though the cost to achieve such wavefront number will enhance your perception in a very similar way to what many audiophiles experience when buying $1000 gold plated TOSLINK optical cables, enabling you to see volcanic activity on Io. Just don't tell refractor guys tho because their 90mm obstruction free scopes can out resolve anything not refactor of any aperture and will make you feel bad with your scope.

 

Ok joking aside, 1/10 wavefront optics will give you better image quality specially on extended and high contrast objects, which a 3.5" scope would be mostly be looking at (terrestrial, planets, moon, sun, etc.) not so much on point sources unless you get satisfaction of seeing text book quality airy disks when seeing allows you.


Edited by EquusSpeculabundus, 16 November 2019 - 09:05 PM.


#5 JamesMStephens

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Posted 16 November 2019 - 10:02 PM

 

Ok joking aside, 1/10 wavefront optics will give you better image quality specially on extended and high contrast objects, which a 3.5" scope would be mostly be looking at (terrestrial, planets, moon, sun, etc.) not so much on point sources unless you get satisfaction of seeing text book quality airy disks when seeing allows you.

But I do!  The Questar ad my 6" Intes Mak are the only two telescopes I own that ever show me diffraction rings.


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#6 EquusSpeculabundus

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Posted 16 November 2019 - 10:26 PM

But I do!  The Questar ad my 6" Intes Mak are the only two telescopes I own that ever show me diffraction rings.

I do too, Im not saying that in a negative way it is satisfying indeed. Going from a generic brand Bird-Jones to a 150mm mak was a joy to finally see a perfectly round airy disk with some rings.  


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#7 Kevin Barker

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Posted 17 November 2019 - 01:42 PM

Personally I do not think as an observer  I would with 3.5" see  a discernible difference in a 1/8th wave PV  and 1/10th PV wave scope at the eyepiece. Coatings differences might be the difference if any.

 

Perhaps with a bigger aperture and perfect seeing??  Perhaps with perfect seeing and a star test with a lot of magnification some folks might be able to tell.

 

Having said that 1/8th wave PV optics are pretty **** good. Much improved over 1/4 wave PV. I would guess that a lot of folks could tell the difference between 1/4 and 1/8th wave in good seeing??

 

It has been my observation that once a scope gets past about 1/6th wave PV then the images are pretty darn good at the eyepiece. Any gains after that are very subtle and probably due to other factors.

 

So 1/10th wave PV optics cost a lot more for little if any real gain.



#8 Erik Bakker

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Posted 17 November 2019 - 03:58 PM

Differences are there to see. The total wavefront error of the image reaching the observers's eye is that of the atmosphere + telescope optics (including diagonal and eyepiece). So the better the optics of a telescope, the more detail is visible, or at least a stabler image with similar detail more often than that of a slightly lesser optic. And that does leave the satisfaction of focussing a perfect optic out of the equation. 



#9 TOMDEY

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Posted 17 November 2019 - 08:41 PM

It becomes substantially-academic around quarter-wave single-pass wavefront... green... especially for large scopes. For a Q-3.5, which is a cute little scope... opting for the tenth-wave would be prudent. For a Q-7 it would be substantially overkill.

 

One thing to keep in mind is that most manufacturers overstate, even grossly overstate, the wavefront quality delivered to the focal plane by the entire instrument aka PM + SM + alignments + stability etc. A lot of times it is only the PM tested in the lab that they are hanging their hat on. So a delivered "tenth-wave" scope probably actually is closer to quarter-wave right outa the box.

 

But Questar is far more likely to deliver stated quality than most others. Part of what you are paying-up for. I measured a Questar-7 in the lab, autocollimation interferometry... and it came out to as shown below... 0.3 wave, scaled to green reference wavelength... which most folks here would gasp as unacceptable. Keep in mind, those fringes are true interference fringes... not that horribly desensitized Ronchi stuff. And being double-pass, the fringes look twice as bad as in-use wavefront.

 

What I'm saying is --- this is a typical superb Questar! True 1/3-wave, delivered to the eyepiece or camera, is operationally-indistinguishable from perfection.    Tom

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#10 JamesMStephens

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Posted 17 November 2019 - 09:40 PM

When I bought my Intes Mk-67 (6" Rumak) in 2000 I decided the extra $200 for a hand-picked Deluxe scope would amount to paying two-hundred dollars more for a distinction that would only be measurable in a bench test.  I've often wished I had spent a little more, but that scope is outstanding.  Side by side with an 8" SCT you just might think it's the better telescope!



#11 Simoes Pedro

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Posted 18 November 2019 - 02:15 PM

Just to add that two 1/10 wave optics can have different Strehl ratios.

 

Link below shows Q3.5 w/ 1/5 wave and 0.96 Strehl @ 532 nm.

 

http://r2.astro-fore...sche-feinheiten

 

In practical terms, for maximum performance at a given size you need to go with refractors, not things with obstructions and mirrors.

 

My rule of thumb is: "if you see the first diffraction ring, then it is good enough". Of course, as size increases, atmospheric limitations kick in and it becomes harder to see the diffraction pattern.


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#12 cbwerner

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Posted 20 November 2019 - 10:25 PM

Do you Spotify? I don't - I Tidal. Because on Tidal I can get high resolution, audiophile quality downloads.

 

And I can hear the difference! Especially late at night when using my excellent Oppo headphone amplifier and my quality B&W PX7 headphones.

 

. . . Or can I?

 

I'm honest enough with myself to know that if I was forced to do a double blind comparison test of the high res and regular res files, that I probably would fail. But I don't care. I *do* believe that I can tell the difference, but the real difference is that I get more enjoyment out of the Tidal high res files.

 

Some care, some don't. I would suggest approaching your question from the same angle.


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#13 Darkskyaz

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Posted 23 November 2019 - 11:25 PM

But I do!  The Questar ad my 6" Intes Mak are the only two telescopes I own that ever show me diffraction rings.

Believe it or not, my 8" Meade SCT shows diffraction rings, though not as distinct as my 3.5" Questar. I do enjoy seeing them as well. 


Edited by Darkskyaz, 23 November 2019 - 11:25 PM.


#14 JamesMStephens

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Posted 24 November 2019 - 04:47 PM

Believe it or not, my 8" Meade SCT shows diffraction rings, though not as distinct as my 3.5" Questar. I do enjoy seeing them as well. 

I believe it.  



#15 bobsorenson

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Posted 26 November 2019 - 04:19 PM

Hi,

cbwerner's comment about audio got me thinking about my schooling in music production at Berkeley school of music. In a critical listening class our hearing would be tested by listening to music while random bands 20 Hz wide would be cut or boosted by varying db and we would try to discern. Impossible at first, but after many weeks of very careful attention, the senses become much more sensitive. This type of structured training is invaluable and even the most die hard "expert" will benefit greatly.

 

I wonder if there is a similar structured exercise to develop seeing skills in the "observational arts"?


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#16 RichA

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Posted 27 November 2019 - 05:03 PM

Hello all, what is the advantage of the 1/10th wavelength mirror?  The Questar has a 3.5" aperture so the light gathering power is limited.

Thanks.

There are differences.  Four scopes were used at Starfest (Canada) with different wavefronts and you could see the difference pretty easily between say 1/4 and 1/10th wave.


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#17 cbwerner

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Posted 27 November 2019 - 07:29 PM

IIRC, I believe the difference in play here is 1/8th vs 1/10th.



#18 luxo II

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Posted 29 November 2019 - 11:32 PM

But sigh... I still want a Q-7! Tom

Tom, the Santel MK91 I had cured me of that, totally. The light grasp was noticeably superior to a 7”, and it was optically superb. And I have since upgraded to an APM 10” mak that is 1/11 wave - for the complete system - not just the OTA.

There really is something to be said for a scope that can stick every photon exactly where it ought to be. In average seeing where lesser scopes struggle both the MK91 and the APM are stunning - stars are still little points doing a dance but these two easily outperform the rest on the observing field.

Admittedly a baby Q might be nice one day when I am too frail to safely lift a big mak.

And yes the difference between ¼ wave and 1/10 wave optics is easily discernable.

Edited by luxo II, 29 November 2019 - 11:42 PM.

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