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Mirror Quality Comparison

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#1 Spoonsize

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Posted 17 February 2019 - 04:00 PM

So, is there a link to set of images taken that compare 1/4, 1/6, 1/8, 1/10 wave mirrors? Just trying to wrap my brain around this whole issue. Thanks.

#2 eyepiecedropper

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Posted 17 February 2019 - 04:10 PM

http://www.damianpea.../simulation.htm


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#3 Cotts

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Posted 17 February 2019 - 04:12 PM

Your research could do no better than to start here: http://www.damianpea.../simulation.htm

 

Dave


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#4 Cotts

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Posted 17 February 2019 - 04:13 PM

Beat me by two minutes!!!!!   <shakes tiny fist!>

 

Dave


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#5 Pinbout

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Posted 17 February 2019 - 05:28 PM

So, is there a link to set of images taken that compare 1/4, 1/6, 1/8, 1/10 wave mirrors? Just trying to wrap my brain around this whole issue. Thanks.

I never really felt the above link  meant anything to me.

 

what I can say is for a good mirror better than 1/6

 

on a night that my friend is complaining about the atmosphere cause in his discovery 12" dob the stars are bloaty at moderate magnification

 

my 12.5" hubble optic dob, that bench tests really well, has pinpoint stars.

 

but then one night I looked thru his same 12" discovery dob with denk II binos at Saturn and it was one of the best views I've seen.

 

that told me he was under corrected cause the barlows used with binos add correction.

 

and cause of the two different nights I asked him if I could star test his mirror and as soon as he heard me say, " oh look at that..." he knocked the scope off target. so never got a good assessment. I just know he's undercorrected.


Edited by Pinbout, 17 February 2019 - 05:29 PM.

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#6 TOMDEY

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Posted 17 February 2019 - 06:03 PM

In actual practice, it's often (usually?) a Pandora's Box. Nuff to make me believe a true, performing, smooth 1/4-wave (PV green single-pass wavefront) entire optical system (mounted PM, SM, Star Diagonal, field elements, eyep or camera... eye... in the field, ready to use...) is a lot rarer than we would like to believe.

 

I've done optics my entire working life... and just got jaded regarding claimed specs (was most often on the certifying/delivering to customers end!) Our beloved customer, "Sam"... routinely beat the snot out of us to certify and back up field performance --- Good for Him, and Good for us!

 

So, when I see an Ad blithely claiming 1/20-wave, "diffraction-limited" Rayleigh-res, 98% Strehl... on a bargain-priced consumer telescope... well, it actually turns me off, rather than attractive. Unfortunately, the true quality shops and dealers can get lost in the cacophony or unfounded superlative claims.

 

Best go with those who have built up great reputation over the years... and that usually means "paying-up."    Tom


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#7 LarsMalmgren

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Posted 18 February 2019 - 11:51 AM

Mel Bartels has a good summary of thoughts about mirror quality, i think.

https://www.bbastrod...atemirrors.html

 

I kinda like Bratislav's star test scale.


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#8 CrazyPanda

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Posted 18 February 2019 - 05:49 PM

From that article:

 

The images when viewed from a distance of 50cm from the screen are equivalent to a power of 1000x used on the telescope

 

What size screen? What resolution in PPI? These are kind of important criteria to establish.



#9 Cotts

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Posted 22 February 2019 - 12:02 AM

See this old CN thread, especially the first post.  https://www.cloudyni...quality-optics/  A lively discussion ensues!

 

Three of the  four telescopes that Ceravolo made were shown at Starfest that year as well.  I remember many people stopping by for a look.  Saturn was the target and I was able to rank the scopes correctly as were many other observers...  but the 1/4 and 1/10 scopes were very close...

 

Dave



#10 Pinbout

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Posted 22 February 2019 - 07:54 AM

 

See this old CN thread, especially the first post.  https://www.cloudyni...quality-optics/  A lively discussion ensues!

a trip down memory lane

 

interesting lockwoods - traverse approaching 1 shows significant improvement even between same wavefront errors.


Edited by Pinbout, 22 February 2019 - 07:55 AM.


#11 Asbytec

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Posted 22 February 2019 - 09:26 AM

My takeaway from Damian's article is:

 

"Other issues such as collimation accuracy, focus accuracy, thermal equilibrium and above all astronomical seeing are really of much greater concern than the small performance differences that actually exist between telescopes under "lab test" conditions. Also it becomes clear that central obstruction is a greatly overblown issue when its affects are placed against those caused by so many other factors (such as those outlined.)

 

Perhaps the biggest concern above them all, is user the friendliness of the instrument. After all, we aren't likely to observe very frequently with something that is difficult or frustrating to use. In the end, the best telescope to own is ultimately a trade off of many different issues and its important not to become fixated on a single issue that in the wider perspective of things isn't of great importance."

 

I currently own a mass produced Orion 150 MCT operating under good to excellent seeing, much better than 2" arc average I guess, in a modest climate allowing easy thermal stability and, along with those conditions, very good collimation. Seeing so good often enough I can see a tiny hole form in the dead center of the defocused diffraction artifact and watch the birth of the next diffraction ring. (You can see this daisy chain behavior in Suiter's book...kind of neat to watch it happen.) 

 

I have no idea what the PV wavefront deviation is, but it does have some indications of spherical aberration (probably both high and low order). I can say it appears very smooth without any visible astigmatism or coma (on or nearly on axis in the smaller FOV) and can push to 0.3mm exit pupil on bright equal doubles and Mars without any "image breakdown" (whatever that means: aberration, surface brightness?). The in focus diffraction artifact is clean and high contrast (even once at 900x), though outside focus is a little lower contrast. It definitely does not show the classic 1/4 PV waves of LSA, or worse, everyone says it must have (and it may well have more than 1/4 PV waves of something). It actually star tests a little better for pure LSA. Regardless, it does appear to be very smooth and free of any observable zones in a star test.  

 

So much so, that scope got more use under tropical observing conditions and I derived more joy using it than any other scope in my observing life time. Surface albedo on Ganymede, fissures in (northern) Martian polar cap, and crater forms 0.9 miles in diameter potentially available in less than 10 minutes set up from the front door. What's not to like? I have often wondered what a quality unobstructed same aperture could do that this one cannot do in likely better than average seeing conditions. Never really got an answer other than just being "sharper." I think I get it. Or maybe I just enjoy observing too much. 

 

I am sure it can be bested by a premium instrument, but I have no complaints. Not one. 

 

My current mass produced 8" F/6 turns out to be pretty good, too. Again, I have no idea what the actual PV value is, but it does not star test to classic 1/4 PV waves LSA. It looks to be better than that (but not perfect or premium quality) based on nearly identical shadow break out diameters on either side. It is slightly under corrected over all based on my perception of inner and outer diffraction ring brightness differences consistent with the shadow break out distance and diameters either side. Again, I can see no visible astigmatism from either the primary or diagonal. I see coma, but I usually collimate that out so that the sweet spot is fairly well centered in the high power FOV, and it looks pretty good on and close to on axis. The moon is pretty "sharp" (whatever that means) with a pretty snappy focus best I can tell. It looks to be smooth enough, at least to the level of seeing (lately) at around Pickering 7/10, with no indication of a severely turned edge. So, I am happy. 

 

Once I got the hang of collimation, fixed the cooling problem, and got a few nights of good (not as excellent) seeing it is beginning to show it's stuff on double stars down toward the Dawes limit (no planetary observations yet.) However, at f/6, I am a little stared for magnification to 0.5mm exit pupil or less, though, for the time being. It has a larger than normal obstruction, so there are some rings around the Airy disc depending on the brightness of the star. But, the star images look good, clean, and no "junk floating around." Well, last night seeing turned ugly and there was nothing but junk floating around. The Airy disc was hardly visible, rings/arcs all over the place, all amid a fuzzy blur. Almost bloated stars. But, that is neither my nor the scope's fault.

 

I am also sure it can be bested by a premium instrument, but I have no complaints. Not yet. Well, one fault I can find. The anneal could be better, but so far no issues with it. Just gotta be careful not to run it under cold tap water, I guess. lol.gif

 

No complaints mostly because, even though not premium, I am under no illusion how important preparing our scope for observing (collimation and cooling) and better than average seeing are to the obstructed and aberrant image we see. It's not that bad, we'll see what we are supposed to see in most cases. No doubt premium can tighten up the PSF a that much more. Quite likely with (noticeably?) better planetary contrast and unequal double star resolution. However, the minute seeing drops to Pickering 6/10 or worse, we'll all pretty much in the same boat below the diffraction limit, anyway. Not good for high res observing. At least our scopes tend to be better than that. I hope. Sometimes much better. 

 

For the original question and a theoretical comparison on imaging quality, run the numbers using Strehl * (1 - co^2)^2 to get an approximation of the peak intensity and distribution on the focal plane for Strehl ratios as you like from 1 to 0.8. See what you can make of it. Then, using Aberrator, toss in some seeing conditions, maybe some thermals, and some slight miscollimation. If not, make the best with what you have or can afford and enjoy the heck out of it. I get a kick out of both of mine and use them because of it. smile.gif


Edited by Asbytec, 22 February 2019 - 09:40 AM.

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#12 Pinbout

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Posted 22 February 2019 - 09:57 AM

 

and can push to 0.3mm exit pupil on bright equal doubles and Mars without any "image breakdown"

.3 mm exit pupil is all you need per scope

 

when you get below 1mm low contrast stuff starts to dim...


Edited by Pinbout, 22 February 2019 - 02:37 PM.

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#13 gregj888

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Posted 23 February 2019 - 01:43 PM

A few unreferenced data points.

 

When observing test have been done, in good seeing, experienced observers often see improvements to about 1/10th wave (referenced above as well.

 

If 1/4 wave is truly diffraction limited, how can you see improvements beyond that, but wait... That 1/4 wave criteria is almost certainly a mathematical one.   That is it's expecting an otherwise perfect mirror, that we don't have.  We have micro-ripple, flex, surface scatter and any number of slope errors which can be hidden in most of our criteria.

 

Large telescopes were often specified with seeing criteria in mind.  I assume (ya) some fraction of the average or best seeing at the site.  I believe the MMT was originally specked that way.  This was fine until Adaptive Optics showed up that could nullify seeing issues.

 

In most things we use a 3db (1/2 the signal level) point for bandwidth (resolution) measurements.  Even with out imaging we use FWHM.  In AO, bandwidth is at the zero-db point which is where the roll-off starts and well before the 3 db point.  It would seem optics and image formation are more sensitive to aberrations than many other areas.

 

What's all this mean and the point. I don't know how to even approach the math on this, but I'm feeling like a real analysis of those 1/4 wave mirrors probably net something slightly less in reality and we need something better.  Dale in his fringe analysis has a guide at 1/8 wave wave-front error, maybe there?  Maybe if very smooth 1/6 or maybe it takes 1/10th wave.  

 

Just food for thought...   Back to figuring (0.39 rms/-0.95 cc) 



#14 mark cowan

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Posted 23 February 2019 - 05:23 PM

It's the difference between putative measurement and the actual mirrors themselves.  Under ideal conditions there's no particular point at which a mirror with less actual error doesn't distinguish itself, IME.  And some of those errors escape certain measurements.

 

The Ceravolo optics in that trial were unusual in that they ONLY had differing amounts of spherical aberration.  And the conditions IIRC were not, for the most part, ideal. Yet the experienced observers could sort them out.



#15 MKV

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Posted 23 February 2019 - 05:25 PM

A few unreferenced data points.

 

When observing test have been done, in good seeing, experienced observers often see improvements to about 1/10th wave (referenced above as well.

 

If 1/4 wave is truly diffraction limited, how can you see improvements beyond that, but wait... That 1/4 wave criteria is almost certainly a mathematical one.   That is it's expecting an otherwise perfect mirror, that we don't have.  We have micro-ripple, flex, surface scatter and any number of slope errors which can be hidden in most of our criteria...

Greg,

 

Lord Rayleigh, in his own handwriting, states the following: “aberration begins to become decidedly prejudicial when the wave-surface deviates from its proper place by about a quarter of a wavelength” (The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science”, the “fifth series” November 1879 , Number XLVI, Sect 4” page 409)

 

Everyone nowadays assumes this refers to wave-surface deviation as the light reflectors off a mirror or passes through an objective lens. Hardly. It refers to the exit wavefront reaching the eye, after the light traversed all the obstacles in its path. All that Lord Rayleigh stated was that the wavefront reaching the eye should not exceed 1/4 wave ptv, lest the image quality begins to perceptibly suffer.

 

It certainly doesn't mean that a 6-inch f/8 spherical mirror (which at the best focus in theory delivers  1/4 wave OPD) will automatically deliver 1/4 wave deviation at the exit pupil, as is often erroneously assumed or even claimed.  In the real world, a 6-inch f/8 spherical mirror can never deliver 1/4 wave at the exist pupil, no matter how smooth its figure and its surface may be, and has a perfect edge. The fact that there's a secondary mirror and air currents is sufficient to degrade the wavefront beyond 1/4 wave at the exit pupil. 

 

As for how experienced observers claim they can tell if what they're seeing is as good as 1/10 wave pv on the wavefront, they will usually mention "snap focus" as the criterion. In other words, the image just "snaps" into sharp focus. Of course, without an a interferometric measurement of the exist wavefront, this is nothing more than hearsay, and can vary from one observer to another and include a whole range of wavefront errors.  For example, how would they tell if the exit wavefront deformation amounts to 1/9 or 1/11 of a wave? Ridiculous!

 

Dale's criterion of  mirror corrected to 1/8 wave on the wavefront (1/16 on the surface!) in the shop is more likely to deliver something closer to 1/4 at the exit pupil wave under actual nighttime observing conditions  -- provided the conditions are favorable, the mirror is properly aligned, the secondary is a high-quality mirror,  and heat and air currents are effectively suppressed.  Needless to say, such high-precision mirror is not easy to make, or cheap to buy.

 

Mladen


Edited by MKV, 23 February 2019 - 10:51 PM.

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#16 sopticals

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Posted 23 February 2019 - 05:51 PM

Greg,

 

Dale's criterion of  mirror corrected to 1/18 wave on the wavefront (1/16 on the surface!) in the shop is more likely to deliver something closer to 1/4 at the exit pupil wave under actual nighttime observing conditions  -- provided the conditions are favorable, the mirror is properly aligned, the secondary is a high-quality mirror,  and heat and air currents are effectively suppressed.  Needless to say, such high-precision mirror is not easy to make, or cheap to buy.

 

Mladen

Mladen, you mean 1/8 wave on the wave front (1/16 on the surface).

 

Stephen.(45deg.S.)



#17 Pierre Lemay

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Posted 23 February 2019 - 05:52 PM

As for how experienced observers claim they can tell if what they're seeing is as good as 1/10 wave pv on the wavefront, they will usually mention "snap focus" as the criterion. In other words, the image just "snaps" into sharp focus.

I consider snap focus a meaningful test for fast optics (faster than f/5) when conditions are optimal (steady atmosphere; optics well collimated and at equilibrium with the surrounding temperature). I don't see as much meaning for f/8 and slower optics due to the wider field of acceptable focus.


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#18 Pinbout

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Posted 23 February 2019 - 06:53 PM

Mladen, you mean 1/8 wave on the wave front (1/16 on the surface).

 

Stephen.(45deg.S.)

ah...and I was thinking he's just loosing it. lol.gif



#19 Dale Eason

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Posted 23 February 2019 - 07:00 PM

Just to clear up what a couple of you have incorrectly stated or assumed about the limit lines I have DFTFringe put on the wave front profile graph.  They correspond to what I believe is the Rayleigh 1/4 PV wave limit on the wave front.

 

Since the profile graph is centered over 0  there will be a line at 1/8 wave on the upper part of the graph and a line at -1/8 wave on the lower part of graph..  Thus indicating 1/4 wave PV of the wave front or 1/8 PV wave on the surface.  

 

Dale



#20 gregj888

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Posted 23 February 2019 - 07:02 PM

 I understand and don't disagree with any of the comments.  Mark's is the closest to the point.  We  use the math as a model, but it's unlikely to relate well to our actual mirrors.  

 

I don't agree that the image continues to improve forever, the wave nature of light dictates a limit (remember I play with AFMs, SEM and such...  want to image the shell of an atom...).  I was musing more about where the limit is from a measurement standpoint and what we should shoot for.

 

The Ceravolo optics in that trial were unusual in that they ONLY had differing amounts of spherical aberration.  And the conditions IIRC were not, for the most part, ideal. Yet the experienced observers could sort them out.

Dale, thank you, I would have figured that out eventually :-)



#21 Mark Harry

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Posted 23 February 2019 - 08:07 PM

Thinking I remember seeing a 6" F/8 being 1/4 wave, notably inferior-
Here's a plot of a spherical F/12. Would think it'd be easy meeting requirements.
Looks a bit dicey to me. (OSLO spots)

Attached Files


Edited by Mark Harry, 23 February 2019 - 08:08 PM.


#22 dan chaffee

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Posted 23 February 2019 - 09:04 PM

I consider snap focus a meaningful test for fast optics (faster than f/5) when conditions are optimal (steady atmosphere; optics well collimated and at equilibrium with the surrounding temperature). I don't see as much meaning for f/8 and slower optics due to the wider field of acceptable focus.

Yes!   I have made and used telescopes ranging in focal ratios from f7.3 to f/80.   The caustic becomes so stretched

above f/10 that there is no reason to expect it to snap in and out of focus.  You can try this with a  fast scope--just put an  aperture mask much smaller than the full aperture and ask yourself if it snaps as decisively as without the mask. And you can

be pretty sure with a sub-aperture mask it is darn good on the wavefront.



#23 mark cowan

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Posted 23 February 2019 - 09:17 PM

 I understand and don't disagree with any of the comments.  Mark's is the closest to the point.  We  use the math as a model, but it's unlikely to relate well to our actual mirrors.  

 

I don't agree that the image continues to improve forever, the wave nature of light dictates a limit (remember I play with AFMs, SEM and such...  want to image the shell of an atom...).  I was musing more about where the limit is from a measurement standpoint and what we should shoot for.

 

Dale, thank you, I would have figured that out eventually :-)

Yeah I shouldn't have said "no particular point", as the difference between, say, 1/50th wave PVW total error and 1/100th wave PVW total error (if such is even possible) is not really detectable. ;) 

 

1/4 and 1/10 sure.  1/10 and 1/20 under ideal condition, detectable by at least some.  Much beyond that the conditions required so as not to swamp the optical train's errors with those of the atmosphere's are unlikely in the extreme, at least on the surface of the earth.  And of course in space no one can hear you go "OMG!". :lol:



#24 MKV

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Posted 23 February 2019 - 10:50 PM

Mladen, you mean 1/8 wave on the wave front (1/16 on the surface).

 

Stephen.(45deg.S.)

Yes, of course, Stephen, thank you.  Just an innocent typo -- rushing to go to dinner. It's fixed.



#25 MKV

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Posted 23 February 2019 - 11:30 PM

ah...and I was thinking he's just loosing it. lol.gif

Really? roflmao.gif




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