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JustinJ
member
Reged: 08/07/08
Posts: 69
Loc: Central Pennsylvania
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What effects do reflectivity have on the secondary? I am starting to think that I may want to go with a Galaxy secondary with 98% reflective coating. After much reading and seeing the responses to the thread, I think the secondary is good at 2.14 size.
The images are fine in my scope but I am trying to squeeze the best performance out of the 10 inches aperture. I have already flocked the tube, which helped. I would like to draw out more contrast. Would a more reflective secondary help? Would a 3 percent increase of reflectivity help or be noticeable at the scope? I know that a bigger telescope would work, but it is not an option.
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Discovery PDHQ 10 f6
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 10412
Loc: Amargosa Valley, NV, USA
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You may or may not notice it, but it certainly can't hurt. And as part of a whole spectrum of tweaks and adjustments to get as much out of your scope as you can, it's an appropriate component. What Jeff said above is correct -- considering the amount of money you may spend on eyepieces, the price difference between a standard secondary and a premium is pretty small.
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"S.O.E." (Sauron's Other Eye) 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Next Project: The "Eye of Sauron" Observatory!
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10917
Loc: Los Angeles
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Quote:
Some considerations, more from a math wonk point of view:
1) The relevant error of a secondary mirror is not normally across the whole mirror, but the area reflecting light from a star in or near the center of field. In regular Newtonians, it is most but not all the secondary's area.
Not really. Even the on-axis ray utilizes the entire secondary mirror's surface. If it is 100% illuminated, that is. Progressively off-axis rays utilize less until, if the secondary is the correct size, at the edge, the ray utilizes only 70% of the surface of the secondary and 30% of the ray misses the secondary entirely. Think of the on-axis reflection from the primary as a light cone and you will quickly grasp the idea of a star point as a wave rather than a thin line.
Quote:
Consequently, if you move a secondary up closer to the focal plane, then a star's small light cone there would normally have less of the mirror's error. But if the mirror's full error happened to be concentrated in that tiny reflection zone, you might be as bad off as if reflected off the whole mirror (or even worse off).
Perhaps, but this distance of secondary to primary would result in only a point on axis being fully illuminated, an impractical size for a secondary.
Quote:
2) If you have an otherwise perfect mirror except for an area exactly displaced by some constant fraction of a wave, so that all rays trace to the exact point, wavefronts from the displaced section will be out of phase with the others. The effect would be a distortion in the diffraction pattern, with some light lost from the central Airy disk and shifted to the rings. Visually, this would be seen mainly at high magnifications as a subtle deficit of contrast.
Conclusion: Wavefront errors on secondaries matter just as much as on primaries, and a mirror buyer might as well treat those errors as additive, except for an adjustment for the secondary by the ratio of a star's reflection diameter to the mirror's (minor axis) diameter. If surface error is stated instead of wavefront error, primary mirror surface errors are doubled, and secondary surface errors are multiplied by 1.4, times any reflection-size adjustment.
As I do the math, the reflected error at 45 degrees is .717 times the size of the surface error measured at a 90 degree angle to the surface. So the error on a secondary is less important than the same size error on the primary due to that 45 degree reflected angle.
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Like others, I'd be skeptical of a 1/30-wave mirror as being really that accurate. Plus, when you add even a very good primary's error, a 1/30 wave secondary isn't significantly better than a 1/14 wave secondary.
True. Measuring a 1/30th wave error (and how is that measured, RMS, P-V, wavefront? etc.?) requires tools that are more accurate than that--simply unlikely in most lab situations. I'd rather have an enhanced-coated 1/8th wave secondary than a standard aluminum 1/20th wave because I'd see more with the higher reflectivity. [well, maybe--a tiny percent might not be noticeable].
It would be interesting to do an optical comparison between different accuracies of secondary. Such differences would be, I fear, swamped by normal variation in seeing conditions.
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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Chris Z
super member
Reged: 03/24/04
Posts: 109
Loc: Chicago, IL
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Quote:
As I do the math, the reflected error at 45 is .717 times the size of the surface error measured at a 90 degree angle to the surface. So the error on a secondary is less important than the same size error on the primary due to that 45 degree reflected angle.
Yes. The scattering due to surface roughness is less important as you look at steeper angles. This can be verfied by looking at the reflection off a flat, somewhat rough object. Looking at steeper angles, the reflected image becomes brighter. I'm looking at the reflection off a tile in my kitchen and confirming this as I type.
Chris
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Orion SVP 120mm f8.3
Obsession 12.5" f5
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backwoody
scholastic sledgehammer
Reged: 01/08/07
Posts: 890
Loc: Idaho USA
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Quote:
As I do the math, the reflected error at 45 is .717 times the size of the surface error measured at a 90 degree angle to the surface. So the error on a secondary is less important than the same size error on the primary due to that 45 degree reflected angle.
The comment below is from John Lightholder, an optics expert and maker of premium primaries. I think it's instructive in this situation:
"A good and honest 1/10 wave for the secondary would be OK. The issue there is, as we move farther from the optical surface [primary] within the converging light path, the less important is large scale surface quality. Small scale surface quality still matters though, so as not to scatter light. For instance, we routinely place colored filters, even colored gelatin within 1" of the focal plane and never consider its optical quality. Full aperture Mylar solar filters matter less as they are thin, and thus have little chance to distort the incoming wavefront. A full aperture optical window would matter greatly because of its thickness. It would have to hold to the standard and be both smooth and have both faces/planes parallel.
The best quality secondary should be a goal; you just don't have to go overboard about it. True, it's one-half the optical system, but it's close enough to the optical plane that large-scale errors make less difference visually. The most important thing there [with a secondary] is to have a smooth surface, along with a smooth coating. I've always had good mirrors with good coatings from ProtoStar.
A friend of mine bought a 1/20 wave 3.5" secondary from another company, and of course, the paper certificate says.... 1/20. That's too exact. A TRUE and certified 1/20 wave mirror would be financially out of reach of most in the ATM community."
Hope this helps. I'd go with an honest 1/10 wave secondary, preferably a ULS quartz, for its excellent surface quality (all the way to the edge) and thermal stability.
c/s,
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woody
a parsec farther out...
12.5" f/4.8 custom truss dob, EQ platform
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dave brock
member
Reged: 06/06/08
Posts: 36
Loc: New Zealand
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Hi Don.
Here is a crude sketch to illustrate why I believe the error is increased by a factor of 1.4x
What am I missing?
Dave.
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jpcannavo
sage
Reged: 02/21/05
Posts: 303
Loc: Long Island New York
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Have a look...
http://www.telescope-optics.net/newtonian.htm
Although I am not entirely satisfied with the analysis (I am wondering if the intersection between geometric and physical optics is a bit oversimplified)it may be worth some discussion in this thread. For one, I bothered by the arguement (orthogonality etc.) relating change in path length (which is actually 2.8t here) and wavefront error.
Joe
Edited by jpcannavo (08/18/08 08:18 AM)
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jg3
sage
Reged: 05/27/07
Posts: 207
Loc: near Auburn, CA
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Quote:
(jg3) The relevant error of a secondary mirror is not normally across the whole mirror, but the area reflecting light from a star in or near the center of field. In regular Newtonians, it is most but not all the secondary's area.
(Starman1) Not really. Even the on-axis ray utilizes the entire secondary mirror's surface. If it is 100% illuminated, that is.
If the secondary is barely big enough to fully illuminate the center of the field, then the central light cone reflects off the entire secondary. But most Newtonians have a secondary somewhat bigger than this, to fully illuminate a larger field. Consequently the central light cone does not reflect off the entire secondary surface. Each star in the fully illuminated field reflects off different, but overlapping, portions of the secondary surface. The fully illuminated field of view uses the whole secondary mirror, but any star or point in that field uses most but not all of the secondary. (Just look into your focuser; when you eye is in the fully illuminated zone on the focal plane, you should see the entire primary, *and some stuff around it*, reflected in the secondary.)
High wavefront accuracy of a secondary has little if anything to do with distortion (warped mapping of the field); the purpose is to provide an intact wavefront of each star or point within the field, so that the diffraction pattern is as close to Airy's as possible. Eyepieces, even the best ones, distort the image on a par with fun-house mirrors, by geometrical necessity of mapping a small disk to a large sector of a sphere.
Quote:
As I do the math, the reflected error at 45 degrees is .717 times the size of the surface error measured at a 90 degree angle to the surface. So the error on a secondary is less important than the same size error on the primary due to that 45 degree reflected angle.
Correct but misunderstandable. The wavefront error reflected at 45 degrees is 1.414 (i.e. sqrt(2)) times the surface error. The wavefront error reflected at 90 degrees is double the surface error. Consequently, a given error on the secondary surface is 71.7% as deleterious as that same error on the primary surface. But a given *wavefront* error of the secondary is equally deleterious as the same *wavefront* error on the primary (absent further information about the exact shapes of the errors and the remote chance that they cancel).
Filters and other optical devices near focus don't need the same wavefront accuracy across the whole device, because the light cone of any star (or point in the sky) is so small that it doesn't suffer the error of the whole device, only the error within the tiny circle it crosses. That's a reason why roughness is the critical error in optical components near the focal plane. (Many precision optical components, especially those designed for handling small beams, are rated in terms of wavefront error *per unit distance* across.)
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10917
Loc: Los Angeles
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Quote:
Hi Don.
Here is a crude sketch to illustrate why I believe the error is increased by a factor of 1.4x
What am I missing?
Dave.
See the jg3 response below for a better explanation than I gave. It is the ratio of 1.414/2 that is the reason I came up with (it should have been) .707.
But it is correct to say that, barring evidence of the kind of error in the secondary, we should consider the error additive. It is only in the simple, on-axis, divot that a primary mirror's error is 1.414X as damaging as the secondary.
The good news for amateur astonomers is that secondaries are usually made with smoother more accurate surfaces than primaries and typically contribute less to image problems caused by optical errors.
And, while jg3 is correct about an on-axis ray not occupying the entirety of the secondary, if the secondary is chosen to provide 70% illumination at the edge of the largest field used, then the on-axis ray still utilizes most of the secondary, and not just the center of the mirror.
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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HandyAndy
sage
Reged: 01/11/08
Posts: 396
Loc: West Midlands and around
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Hi,
Behind Physical Optics is Quantum Mechanics. Physical Optics is the statistical results of the quantum wave function collapsing.
The light from the star impinges on the primary mirrors surface. The photon emitted at the star travels as a wave function. Hanbury-Brown proposed photons travelling from the star would interact via this wave function and observed the result. When the wave function interacts with the suface of the mirror it is absorbed, the wave function collapses. It either is captured in the surface or is re-emmitted as as another different wave function with the same energy.
The surface of the mirror is a source of these wave functions. It is the aperture and shape of the mirror which determines how the wave function will collapse when it next interacts with a surface(A collection of wave functions).
In the classic two slit experiment we still get an interference pattern on the detector even if only one photon is in the apparatus at once. That is because the wave function travels through both slits and against the surface the slits are in. Each photons wave function can collapse anywhere but the statistical result is the classic Physical Optics inteference pattern and illumination of the surface the slits are in. The wave function has been through the slits even for those which illuminate the surface the slits are in.
We can entangle two photons with different quantum properties and send them off in different directions. When we observe one and determine its unique quantum property then instantly the other photon takes on the other property. This is the 'spooky' nature of quantum theory Einstein did not like.
Again the wave functions emitted by the mirror interact as before with the flat. The shape of the primary affects the intensity of the light across the flat. After that the shape of the flat affects the resultant intensity of the light across the focal surface.
The phase changes produced at the flat are just as deleterious as those produced at the mirror as they are additive.
If the flat is undersized the off axis aperture becomes one circle masked by another and we have a reduced resolution.
Hold a pair of binoculars out on a well lit scene and observe the focal surface floating in space. This is the wave function of the binoculars determining how the photons wave functions collapse.
Cheers. Andrew.
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Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
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TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10917
Loc: Los Angeles
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Quote:
The phase changes produced at the flat are just as deleterious as those produced at the mirror as they are additive.
Additive? Yes.
Just as deleterious? Yes, if of equal amplitude on the wavefront.
But, due to the 45 degree impact angle, as we have calculated, only 70.7% as bad (for an equally deep divot) as on the primary when both surfaces are compared perpendicular to the plane of the surface.
Another thought experiment: The planar light wave reflects from the mirror as a curved, focusing, wave. When it reaches the focal surface of the scope, it has a curve, the Petzval surface. As I see it, the center of the focal surface is farther from the eyepiece than the edges, which is usually described as negative field curvature.
Yet newtonians are usually described as having positive field curvature.
What am I missing here?
What I am missing is that the field curvature actually goes the other way, with the edges farther away from the eyepiece, which is described as positive field curvature. Field curvature is also directly related to the focal length of the scope, with longer having less.
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
Edited by Starman1 (08/19/08 07:53 PM)
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4stargazers
member
Reged: 07/13/08
Posts: 32
Loc: colorado
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I think it boils down to the fact that you use your secondary to view your primary. Now if we say that our primary is 98% reflective and our secondary is 98% reflective then we are not seeing 98% of the light reflected we're seeing 98% of 98%(96.04%) the same holds true to mirror quality if you have a great primary and a poor secondary your view is only as good as your secondary. Lets say that your primary is 98% perfect and so is your secondary then your secondary reduces the quality of your primary view.The only way to view your primary perfectly is to have a perfect secondary. So to say that your secondary doesn't effect your view if its just as good as your primary isn't correct, but I also agree that the difference is probably not noticeable to most.
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meade 4.5'' reflector
etx 60
orion XT8'' dobs
Sirius 40 mm plossl
Sirius 26 mm plossl
Sirius 10 mm plossl
Orion Ultrascopic 2x FMC
3-Element Barlow Lens
Telrad with dewshield plus
and the pulsing light
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