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Mirrors grades 1/4 1/6 /1/8 wavefront differences

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#1 Hartmann

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Posted 24 August 2012 - 07:01 PM

I have to choose a mirror for a future dobson proyect and in the market there are mirrors with different grades or wavefront numbers, so i´m a bit confused about what mirror choose.

obviously, more expensive means a better mirror, but it´s enough a 1/6 mirror for the amateur observer ? gives enough image quality ?

thanks

#2 GlennLeDrew

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Posted 24 August 2012 - 07:51 PM

How large a mirror? What is your primary observing interest? In a nutshell, a big aperture for DSOs allows to be less stringent on quality. A reasonably smooth, 1/4 wave peak-to-valley error on the *wavefront* (if the error is on the *surface*, the wavefront error doubles upon reflection) is more than adequate. In fact, that would be satisfying for pretty much all scopes in all situations. If cost is an issue, better than 1/4 lambda is gravy.

#3 Hartmann

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Posted 24 August 2012 - 09:50 PM

The mirror would be a 10 " f 4,8 for deep sky, and sometimes planets .

#4 MKV

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Posted 24 August 2012 - 10:17 PM

The mirror would be a 10 " f 4,8 for deep sky, and sometimes planets .

For planets you'd need 1/8 wave PV. But in addition to that your collimation and other factors such as tube currents can degrade the image quality very quickly. You will have to be really spot on for collimation.

F/4.8 will be cheaper and will do fine on on stars at 1/4 wave PV (assuming a smooth surface, of course), but on extended objects *moon, plents), you will not get the bets of your aperture.

I would amke sure they give a clear return policy and have the mirror tested professionally by more than one source. I would be very careful believing wavefront claims, as they can be tricky.

Then there is another catch: you cans epcify the wavefront error at the best focus or at the paraxial focus, which is not the same. The wavefrotn error at the best focus is four times smaller than at the paraxila focus. So, if your mirror is advertized as 1/4 wave mirror - at the best focus - that wold less than acceptable, bcause that's as ood goo as it will get - and even that is not true entirely because oyu have other things that affect the image.

This is particulary true of 10-inch and large mirrors of fast focal ratios such as yours. For example, a 10-inch f/4.8 can not be less than 94% parabola, and that's if the best focus will barely give you 1/4 wave error.

Getting a 1/8 wave mirror is better all around. But, again, I would have ti tested to make sure it is 1/8 wave.

Mladen

#5 pstarr

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Posted 24 August 2012 - 10:25 PM

You can see in the link below, the differences in planetary image quality of optics with various wavefront quality. Adding in various wavefront quality plus various central obstruction % is also shown Here. This may help you decide what image quality would be acceptable. Generally speaking, a mirror with a honest 1/8th wave surface, 1/4 wavefront error or 1/16 wavefront RMS, is a pretty good one.

#6 ausastronomer

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Posted 25 August 2012 - 06:46 AM

That simulation of Damian's is very good. I agree with the others that in 10" aperture a 1/4 wavefront mirror (1/8 wave P to V) is adequate for pleasing lunar / planetary views. This in the old school terms is "diffraction limited". On nights of good seeing a 1/8 wavefront mirror will show subtle features that are not visible in a 1/4 wavefront and lesser mirrors. On nights of marginal and mediocre seeing you will not pick much difference at all, if any, between a diffraction limited mirror and a 1/8 wavefront mirror. Of infinitely more importance will be cooling and collimation issues.

If you're a perfectionist and the extra cost isn't going to change the course of your life, buy the best mirror. If not a "true" diffraction limited mirror will do a fine job.

The $64 question becomes, how is the mirror tested and what are the "true" test results for the mirror. I have seen test results, some from "so called" premium US opticians, where the true quality of the mirror, is not consistent in any way with the test results. For example I have seen a mirror from a "so called" premium optician that had an interofogram test certificate stating a strehl of over .95, when in fact the mirror had over 1/2 a wave of astigmatism. Don't ask me who BTW. My point being there are no guarantees. All you can do is buy from reputable opticians, or dealers. For instance in Europe, Orion Optics UK have an excellent reputation for their mirrors. These are sold in various grades and generally they all test up better than their advertised grading. In the USA in smaller apertures around 10", people like Carl Zambuto, Bob Royce, Mark Harry and Mike Spooner have excellent reputations and I would happily buy from all of these people with confidence.

Cheers,

#7 hottr6

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Posted 25 August 2012 - 08:27 AM

You can see in the link below, the differences in planetary image quality of optics with various wavefront quality. Adding in various wavefront quality plus various central obstruction % is also shown Here. This may help you decide what image quality would be acceptable. Generally speaking, a mirror with a honest 1/8th wave surface, 1/4 wavefront error or 1/16 wavefront RMS, is a pretty good one.

Crikey! Great link, but the profusion of terms is mind-boggling:
"wave surface"
"wavefront error" (same as "wave error"?)
"wavefront RMS"
"lambda"
"wave peak-to-valley (PV)"
"wave"
All these terms to describe the same thing, but they differ by powers-of-2. Why is that?

It sure would help if we as a community settled on the same terminology. Standards are critically important in science, but it seems this profusion of terms used to describe the same thing but differing by powers-of-2 is driven by commerce and the need to make something sound better than it really is.

Strehl seems to be the only one true measurement in all of this. So why not drop the "wave" stuff?

#8 Pinbout

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Posted 25 August 2012 - 08:27 AM

I have to choose a mirror for a future dobson proyect and in the market there are mirrors with different grades or wavefront numbers, so i´m a bit confused about what mirror choose.

obviously, more expensive means a better mirror, but it´s enough a 1/6 mirror for the amateur observer ? gives enough image quality ?

thanks




since it's a future project, how about plan for going to delaware in march next year?

Del Marva Stargazers hold their annual mirror making seminar where Steve Swayze, Dave Groski and Dick Parker help instruct people to make their own mirrors.

over a period of 3 days you'll grind and figure a mirror to 1/8 wave.

you'll learn how to test them also.

it's a lot of work but it will be your mirror that you made.

Delmarva puts out a sign up form in january, and classes do fill up.

#9 dan_h

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Posted 25 August 2012 - 09:02 AM

Strehl seems to be the only one true measurement in all of this. So why not drop the "wave" stuff?


Because strehl is not so easily determined. If it was broadly used without any indication of how it was derived, you would end up with wavefront or wave error being loosly translated to an equivalent Strehl. You see this sometimes now from variuos manufacturers and the number is meaningless.

dan

#10 hottr6

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Posted 25 August 2012 - 01:40 PM

Strehl seems to be the only one true measurement in all of this. So why not drop the "wave" stuff?


Because strehl is not so easily determined. If it was broadly used without any indication of how it was derived, you would end up with wavefront or wave error being loosly translated to an equivalent Strehl. You see this sometimes now from variuos manufacturers and the number is meaningless.

dan

Dan, I don't really know what the best measurement is to use. Clearly the "wave" measurements are extremely difficult too, because there are so many different ways to do it. Manufacturers certainly skirt the issue by never stating how their figure was measured.

In the 1960s, auto-manufacturers "cheated" on horsepower measurements by dyno-testing hand-built engines on a stand with no drivetrain or accessories and using av-gas. That practice was thankfully stopped and manufacturers are forced to to use standardized tests.

I think the audio industry is guilty of embellishing electronic equipment specifications, often even changing the units of measurement to fool the unsuspecting consumer. I don't think we see this level of dishonesty from astro-manufacturers, but depending on the "wave" measurement used, the numbers could be a factor of 2 or even 4 better than reality.

I'd still like to see a set of standards set for optical quality. NASA and other agencies use standards all the time in their request for quotes..... I don't see why it should be any different for consumer-grade products.

#11 dan_h

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Posted 25 August 2012 - 01:52 PM

Here is a link to a good explanation of what the numbers mean and why they can be decieving.

http://www.bbastrode...atemirrors.html

dan

#12 GlennLeDrew

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Posted 25 August 2012 - 03:41 PM

The factor of 2 variance you're finding probably derives from the doubling of wavefront errors upon reflection. A 1/8 P-V (peak to valley) surface error caused a 1/4 P-V wavefront error. This is why I stressed this in my first post, above.

#13 Wes James

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Posted 25 August 2012 - 05:06 PM

I would wholeheartedly second Danny's above recommendation to attend the Delmarva mirror grinding weekend and make your own mirror. With the guidance of Steve Swaze, Dick Parker, Dave Groski and others- you will come away with a mirror that you KNOW will be as good as anything you can reasonably expect to buy. There will be no doubt. Dick's autocollimator and Steve's Ross Null test setup will let you see exactly how good your mirror is.
This weekend has been going on for quite a few years now, and has a great track record. I attended several years ago and came away with a superb little 6" f/8 mirror. Nothing like building a telescope with a mirror you made yourself.

#14 Pinbout

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Posted 25 August 2012 - 05:35 PM

here's another thing about the Mirror Making Seminar, they don't bother with numbers, both the ross-null and autocollimator are null tests, either its good or not.

you don't have to worry about," is it this number or that number," and get caught up in the strehl or wether the foucault computer programs are giving you accurate numbers.

when you leave there on sunday you will have a very nice mirror that you'll have to send away to get aluminized.

#15 MKV

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Posted 25 August 2012 - 11:07 PM

I'd still like to see a set of standards set for optical quality. NASA and other agencies use standards all the time in their request for quotes..... I don't see why it should be any different for consumer-grade products.

As Danny ("Pinbout") said - null tests is the best approach. Nothing to measure and no numbers to interpolate; it's qualitative. Either you have a null or you don't. What do you need for an optical null test? An autocollimation optical flat and or a Ross lens. A decent Ross lens can be bought for $20. For parameters needed, the RosssNullXP program is free.

Autocollimation flats will cost more and are difficult to make, although the autocollimation test is ideal for parabolic mirrors, and is a double precision test. You will also need a razor! :)

For maximum accuracy, quantitative assessment, an interferometer will work. A Bath interferometer can be built for less than $50. A fringe-reduction programs (such as FringeXP or OpenFringe) are free. An interferometer can be used with or without autocollimation.

If you don't know any of this, it's best to get a second opinion by at least two sources that can test optics for you. The more the merrier.

If you want quality optics for stars and planets, 1/8 wave correction on the wavefront is what you want. That's a tall order for a 10-inch f/4.8 system. So you're not confused, the wave error shold always be specified relative to the wavefront - or the shell of light converging to the focus.

Ideally such a shell would be perfectly spherical. In reality it never is. What 1/8 wave on the wavefront means is that no part of that shell of converging light is behind or in fornt of the rest of it by more than 1/8 of the wavelenght of light, which is usually taken to be 0.00055 mm, 0.55 milli mucrons (mu), or 550 nanometers (nm). In inches it is 0.0000216 or simply 0.000022 inches.

Wavefront error ratings must be specified as either peak-to-valley (P-V) or root-mean-square (RMS). Without going into too much detail, PV is approximately 3.5 times larger error than RMS but this is only a rough estimate (there is no direct correlation between them).

In addition to that, optical surface must be smooth and free from zonal irregularities (or "zones") which will become apparent on the wavefront as well. In mirrors, the zonal errors are doubled upon reflection for light encountering the mirror surface orthogonally (or squarely) for all practical purposes.

A 1/4 wave P-V wavefront error is considered by some to be borderline acceptable (it works fine on stars, buit not the best on planets). The mirror itself is not the only item that can potentially ruin the image. Any telescope has a myrad of other factors which contribute to wavefront error, including astigmatism and coma, stress and strain, collimation accuracy, If your mirror just meets 1/4 wave P-V chances are other contribtors will make it worse and you will have a telescope whose final wavefront error reaching your eye will be much worse than 1/4 wave.

In short, 1/8 P-V is what you want to aim for.

Mladen

#16 Chuck Jennings

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Posted 05 September 2012 - 11:25 AM

I agree with the previous posts. Our mid atlantic mirror making event is one of the best, producing many excellent mirrors. We haven't published the dates on our website yet, but the 2013 Delmarva mirror making event will be March 20th through March 24th.

- Chuck

www.delmarvastargazers.org






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