I can't quite agree with that comment entirely.
With smaller aperture telescopes (under say 12") the differences are fairly noticeable to a skilled observer under good observing conditions between say a diffraction limited mirror (1/4 wave) and a 1/10th wave mirror. However without star testing under excellent conditions it gets quite difficult to tell the difference between say a 1/7th or 1/8th wave mirror and a mirror which is 1/10th wave or better, for the simple reason a mirror that is a genuine 1/7th or 1/8th wave mirror is an exceedingly good mirror in any case. Any differences between mirrors at this level are very subtle at best and only detectable under the very best conditions.
With larger aperture telescopes (over say 15" aperture) it gets progressively more difficult to separate them, once they are better than diffraction limited, smooth and free of astigmatism. It is certainly still easy to pick a mirror which is worse than diffraction limited (1/4 wave). Larger aperture scopes are more noticeably affected by the subtleties of seeing and thermal equilibrium than smaller scopes. More often than not with a scope of this aperture you are limited by seeing and thermal equilibrium not optical quality, provided the mirror is better than diffraction limited, smooth and free of astigmatism.
What you will invariably find with these larger mirrors, from whichever premium maker you care to choose, is that most of them are no better than 1/6th or 1/7th wave across the full face of the mirror at the worst point. This of course still correlates to a true strehl in the very high 90's and will deliver excellent views.
Quote: the occasional flashes of truly good seeing are more likely to be seen and caught because the better optic has less scattered light.
Happy owner of-- A Mag 1, 12.5 inch Porta Ball A Dual Axis Equatorial Platform A PST Double Stack
Quote:It's more complex than you give it credit. I can't agree entirely with your comments. Your comments really only apply to areas that do not experience dramatic temperature changes from day to night.
Quote:You also need to keep in mind that in regards to the number of 18" plus aperture mirrors running around the world, there are an infinitely greater number of them 2" thick at minimum, than there are < 2" thick. For that matter there are also a lot more mirrors in the 12" to 18" aperture class mirrors which are 2" think running around the world than there are which are less than 2" thick. 2" thick was the norm with 10" to 20" mirrors for decades. At one time it was even a selling point to have a "full thickness mirror".
Quote:You need to remember that I am more than capable of properly star testing a telescope to confirm its optical quality. In addition, a very skilled optician is a friend of mine and I have seen some of these mirrors on the test stand, under bench test conditions.
Quote:Mike, when you say larger "spot", you are not referring to the airy disc?To me that's a gray area I don't fully comprehend. It's easy enough to see how at least one ray could miss the Airy disc or some target "spot", but really the Airy disc forms when the entire wave comes to focus. And if one ray is not cooperating (the wavefront P-V), then it get's sent to the diffraction rings when the diffraction pattern comes to focus. Right?
Quote:The Airy disc is determined by aperture, not figure.
Quote:A poor 6" figure does not make an Airy "spot" larger than 0.92" arc, it redistributes light throughout the pattern. Is not the result of a poor optic a normal Airy pattern with more light spread into the rings? Or does that offending ray actually show as blurring somewhere in the first minimum?
Mike Lockwood - Owner, Lockwood Custom Optics. 20" F/3 MX Starmaster, 14.5" F/2.55 self-built Newt., nine self-built scopes, 4.25" to 30" http://www.Loptics.com/
Too many scopes - my wife wants back her family room...
Quote:For me there is a difference.
I had heavily light polluted skies in Atlanta. Often very turbulent. Had a Meade Starfinder 16.
Scope was OK for viewing, but not great. Re-figured the mirror from 1/4 wave to 1/9. The difference was striking.
Period. Regardless of conditions.
AS of NSW
AS of Hunter Valley
Fact: I like reflectors and big ones. All the refractors I own are either finderscopes, binoculars, or riflescopes.
18"/F4.5 Obsession #1333 fully optioned (OMI Optics)
14"/F4.5 SDM #33 fully optioned (Zambuto Optics)
10"/F5.3 SDM #50 fully optioned (Suchting Optics)
Pentax XW's, T4 & T5 Naglers , UO HD orthos, 6mm Delos, 8mm Radian, 8.5mm Pentax XF, 13mm Ethos. 12mm Celestron Illum Astrometric eyepiece. Rainbow Optics Spectroscopic Eyepiece Set. Orion Optics UK 250 lpi Ronchi grating eyepiece. 2" Astronomiks UHC, OIII & H-Beta filters, 2" DGM Optics NPB filter. 2" Variable Polariser. 2" Hoya colour filters.
Leupold 10x42 & Pentax 16x60 binos.
Quote:You seem to know a lot about a mirror that you have never been near?
Quote:Personally, I have not been dissuaded from my belief that a substantially better (than diffraction limited) optic results in (frequently) substantially better views, though I have found your explanations thought-provoking.
Quote:So, yes, the blurring does move into the minima, effectively enlarging the spot and making it less distinct from the surrounding energy.
150mm MCT f/13, 31% CO
"People say I'm in denial. I disagree."
Quote:As I understand it, the distance to the first ring stays the same even as the star image grows fainter, so the true size of the Airy disc does not diminish with magnitude, only the apparent size of the spurious disc.
Quote: Really, no anecdotal evidence is going to change your mind, and in fact that is the only kind of evidence that we have!
Quote:Don, thank you. It's interesting topic. IME, a difference can be noted, but why? Is it the inherent smoothness or is it the transverse aberration? Yes, I suspect the Airy disc remains the same diameter (pending any article by Sirchz), but does it change appearance other than being dimmer? Does it get mushy and surrounded by faint light at lessor correction and more tight at better correction. If so, is this responsible for the improvement we can see? No reason to doubt that, really, but is a mushy appearance the reason?The diffracted wavefront coming to focus is full of interference, some points on the disc are canceled while others are augmented (CO aside for the moment.) If the optic is perfect, then it adds no more disruption to the phase nor the pattern of interference. However, if the optic does not produce a spherical wavefront, then the greater optical path from different rays changes the pattern of interference.But, the radii of the Airy disc, first minimum and second maximum, etc., should remain constant and based solely on the wavelength and aperture. So, as I understand it, the first minimum is a point of maximum interference cancelling all energy from that point (or at that radii) regardless of the figure. So, the Airy disc will still fall to zero at a set first minimum distance even as the peak intensity falls off. The PSF curve at the very edge of the Airy disc remains very steep, hence the Airy disc well defined. (I was wrong above thinking the slope toward minimum would be more shallow.)So, I am not sure transverse error has a role in giving the Airy disc a mushy appearance. This diffracted and aberrated wavefront still cancels energy at the first minimum, just the pattern (at set radii determined by wavelength and aperture change in relative brightness.) However, it does redistribute the light across the pattern as the total energy must remain the same.Similarly, micro ripple across the entire surface is an aberration, it sends rays here and there and induces some very fine changes in the wavefront. But they tend to cancel over the entire wavefront. Some points are, say, +1/10th and other points are -1/10th from the perfect reference sphere. Without doing the math, I think total RMS is not affected. In other words, what's left to affect the average is the overall wavefront deviation (back to transverse ray's again ) But, I think at focus ray's do not define the pattern seen, they are simply geometrical representations of self interfering wavefront and not a portion of the wave providing energy into the first minimum. As I can understand it, anyway.So, what would give the in focus Airy pattern that washed out look and is this what makes the difference between a better optic and a lessor one? Or is it the redistribution of light across the pattern through interference induced by diffraction, surface deviation (phase), and the CO? I dunno, but I suspect it's the redistribution of light into the rings that makes the difference. CO induced diffraction would simply add to the wavefront's diffraction making the redistribution more pronounced (and a tiny bit more so if the secondary is not perfectly flat?)
Quote:Similarly, micro ripple across the entire surface is an aberration, it sends rays here and there and induces some very fine changes in the wavefront. But they tend to cancel over the entire wavefront. Some points are, say, +1/10th and other points are -1/10th from the perfect reference sphere. Without doing the math, I think total RMS is not affected.
Quote:I guess what I am driving at is, image quality depends on how that complex series of interference collapses to focus. That complex diffraction and aberrant interference both augments and destroys energy at specific locations (determined by wavelength and aperture) leaving the radii unchanged and energy simply distributed differently. The result is more dependent on total RMS and less so on "tightness" (or lack of) in the Airy pattern itself.
Quote:Both Dickinson and George knew the four scopes included mirrors figured to different levels of accuracy. Neither knew which was which. They had to rely on their own evaluations of images at the eyepiece to distinguish one from another. In the end, both were in agreement that the 1/10-wave primary could be distinguished from the 1/4-wave primary, but only when conditions were excellent and careful attention was paid to subtle differences in the view.
Friends call me Duane. Compustar C14, Leo Henzl's Custom C8, 6" Refractor Adv. GT mount, 6" F5 Omni XLT Newt., LXD-75 F4 Imaging SN8, Meade 8" F6 Newtonian, EX Dynamax DX6, RV-6 ETX-90 Astro, Meade 2045 4" SCT, B&L 4000 Vixen/Celestron 80mm F11 JC Penny 60mm AZ/ALT Refractor Binos 25x100
12" Dob, 4" Mak, 3" APO, PST, bunch of green eyepieces & an understanding wife.
Quote: Some minor misunderstandings, though. By radii, I didn't mean radius of curvature or that of a reference sphere - transverse and longitudinal error. I meant the radius of the diffraction rings and Airy disc, trying to describe the diffracted and aberrant pattern comes to focus, phase, etc. I just don't see how the first minimum can grow in brightness (and the Airy disc get larger) with interference cancelling at that point in the image space, if that is indeed what happens. Both of those are set by aperture and wave length. I'm almost positive there is no other variable that affects them.
Quote: 1) the surface is rough on a fine scale.
Quote: For example, when you say the Airy disc is least affected by aberration, I immediately think of the energy that is robbed from it by aberration.
Quote: If memory serves, the rings stayed the same radius but the relative brightness changed.
Quote:I rest my case.
Quote:The mirror has a true strehl of .987 and an RMS error of 1/54.7 waves. Wolfgang claims it is one of the best mirrors he has ever tested.
PLEASE NOTE THE MIRROR HAS A PEAK TO VALLEY ERROR OF 1/7.3 WAVES, so it is "only" a 1/7th wave mirror
Quote:So which one of you can pick this lowly "genuine" 1/7th wave mirror from a "genuine" true 1/10th wave mirror ?
I rest my case.
Quote:PLEASE NOTE THE MIRROR HAS A PEAK TO VALLEY ERROR OF 1/7.3 WAVES, so it is "only" a 1/7th wave mirror
-DannyMy warehouseMy Channel
Quote:Gentlemen - I had the scope, I had the mirror refigured, and I can swear to the results. There was a visible difference. Clusters resolved more clearly. In the Trapezium it was easier to see detail and e and f stars. Planets had different levels of detail.
Quote:Jupiter through aberator with 1/10th wave SA, 1/10th wave stig and 1/10th wave air turbulence.