Jump to content

SkySurveyBanner.jpg.21855908fce40597655603b6c9af720d.jpg

Mirrors 1/12 WAVE vs 1/8 wave..... ?


Recommended Posts

If you are actually talking about 1/12 and 1/8 of a wavelength then you will not really tell the difference.

What MAY occur is that to try and get 1/12 wave there is a greater degree of QA applied and the final result has a better chance of being good.

Another factor is what is actually measured ?

If the surface is flat to 1/12 that sounds good, but if the overall profile is not good then you have a poor mirror that is "flatter".

When I read these specifications I often find that different "units" are used.

Is 1/12 peak to trough, or +- 1/12, meaning 1/6 peak to trough?

Whose mirror are you looking at ?

Link to comment
Share on other sites

My layman's understanding is as follows:

Its a way of measuring how 'perfect' the mirrors surface really is.  When a mirror is ground the finished product can be scrutinised with highly sensitive equipment to reveal the topography of the mirror. This kit can detect errors as small as 0.00003mm ! This seeing of such small errors reveals a 3D surface of the mirrors peaks and troughs and is presented as an image in the form of a Zygo report.  The mirror somewhere on its surface will have a highest peak and a lowest trough (not the edge of the mirror and the middle of the curve but the error in the machined surface overall).  This distance between the mirrors highest error (peak) and lowest error (valley) is the mirrors Pv wavefront error (its worse range of error across the whole surface.  The smaller this difference the better !

These reports are bundled with some mirrors to prove it fits the criteria.  A 1/8 wave mirror is less perfect than a 1/10th wave mirror, as is a 1/10th is less perfect than a 1/12th. 

The more perfect the finished surface the better the control of light & therefore the final image at the eyepiece.   I am unsure how detectable to the human these differences are as eyesight is a huge limiting factor for most.  I have a 1/10th wave mirror and its lovely, I have not seen through a 1/8 of 1/12 of the same aperture to compare. 

Link to comment
Share on other sites

If depends a bit whether the difference is 1/12 vs 1/8th lambda P-V (peak-valley) or RMS (root-mean-square). P-V measures focus on the worst deviation from perfect figure, RMS looks at the average deviation. For a given mirror, RMS values are lower than P-V values, and comparing a mirror with 1/12th lambda RMS to a 1/8th lambda P-V is meaningless. A short explanation is here:

http://www.cyanogen.com/help/quickfringe/QUICK_FRINGEPeaktoValley_vs_RMS.htm

Depending on the type of error, P-V values can be 2.8 to 7.2 times higher than RMS, A factor of 4 is often quoted, so a 1/8th lambda RMS would be 1/2 lambda P-V and 1/12th RMS would be 1/3 lambda P-V. This is a gross simplification, and RMS values are generally a better guideline. More detail here (P.30 in particular)

http://www.optimaxsi.com/PDFs/RulesOfThumbOptomechanics.pdf

Link to comment
Share on other sites

Might need to re-locate to Texas (excellent seeing) to notice any difference. You probably won't notice here in blighty. I wouldn't.

I've heard people comment that a SW 150PL gives the same view as my 1/10 wave OO model. And its all down to the UK seeing.

Link to comment
Share on other sites

If you are actually talking about 1/12 and 1/8 of a wavelength then you will not really tell the difference.

What MAY occur is that to try and get 1/12 wave there is a greater degree of QA applied and the final result has a better chance of being good.

Another factor is what is actually measured ?

If the surface is flat to 1/12 that sounds good, but if the overall profile is not good then you have a poor mirror that is "flatter".

When I read these specifications I often find that different "units" are used.

Is 1/12 peak to trough, or +- 1/12, meaning 1/6 peak to trough?

Whose mirror are you looking at ?

Thx Ronin.... I have a Royce mirror in my Dob.....

Mark

Link to comment
Share on other sites

(Not being an optical engineer, I apologize if I have any of these details wrong, as I'm trying to quote info from the Zambuto site)

The best measurement of the quality of a mirror is thought by many experts to be the Strehl ratio. A perfect mirror would contain 83% of the light from a point source within the first difraction ring. The Strehl ratio is the percentage (usually given as a fraction) of that 83% that a real mirror is measured to contain. An excellent mirror would have a ratio of 0.97-0.99, a very good mirror 0.94-0.97, and so on, down to a so-called "diffraction limited" mirror at 0.80, which actually is poor, and its faults are evident. The Strehl ratio is properly determined by measuring not just a few, but hundreds or thousands of points on the mirror surface. It supposedly will give the least ambiguous indication of the quality of an image a mirror can produce with extremely good seeing conditions.

The problem is that Strehl ratios tend to be quoted that are calculated from P-V or other numbers by makers playing it rather fast and loose with their advertised specs. These involve wishful thinking, and are questionable. Other shops use modified interferomter measurements of relatively few points. This has invited criticism of companies; one comes to mind immediately that offers cheap Dobsonians. A precision interferometer is the only way to make many measurements in a reasonable time, and they are too expensive for many shops. If a shop has an interferomter, it should be able to offer the customer a computer-generated picture showing the shape of the entire surface, making obvious any zones, turned-down edges, etc. Temperature equilibrium and effects of orientation of the mirror can be plainly seen.

The importance of the Strehl ratio is not primarily about how bright the focused image will be, but is more about how much light is diffracted outside the first zone that can degrade the contrast and sharpness. Other specs such as 1/10th wave are today considered outmoded because they can overlook problems. Disclaimer: I cannot say from personal experience that I could tell the difference between a 0.90 and a 0.97 mirror, especially under usual seeing conditions, Since proper Strehl testing requires costly equipment and experience, the prices "reflect " this. I personally would opt for an honest 0.93 or better. I paid extra for a Galaxy 15" with 0.971.

Hal

Link to comment
Share on other sites

I had a 8" 1/10th wave OOUK mirror and in visual terms it was completely stuffed by a Skywatcher uncertified 12" mirror, that I had no reason to suspect was anything other than distinctly average.

Okay, 8" mirrors that I have owned pre and post that example have shown me that 8" is, on average, more capable of punching through UK seeing than a larger mirror and therefore generally more capable of supporting higher magnifications on most nights. But I'd also generalize that on nights that didn't support higher magnifications with a bigger mirror, it was more important to have a bigger mirror than one of a higher certification at any magnification sub 200x.

I'll guess that in such circumstances, which is most of the time over here, angular resolution of a larger mirror rules over figure.

Russell

Link to comment
Share on other sites

I had a 8" 1/10th wave OOUK mirror and in visual terms it was completely stuffed by a Skywatcher uncertified 12" mirror, that I had no reason to suspect was anything other than distinctly average.

Okay, 8" mirrors that I have owned pre and post that example have shown me that 8" is, on average, more capable of punching through UK seeing than a larger mirror and therefore generally more capable of supporting higher magnifications on most nights. But I'd also generalize that on nights that didn't support higher magnifications with a bigger mirror, it was more important to have a bigger mirror than one of a higher certification at any magnification sub 200x.

I'll guess that in such circumstances, which is most of the time over here, angular resolution of a larger mirror rules over figure.

Russell

Hey Russ, just to clarify as I'm not familiar with UK terminology- what exactly does "stuffed" mean? :grin:

Link to comment
Share on other sites

If you are imaging then it seems to me that the higher optical quality is important.  Simply going to larger scopes is great if you can afford the thousands of dollars required for the mount, never mind lugging it around.  That said, and it's been brought up several times, there are many factors that will affect imaging but optics are something I don't want to have to replace down the road and I would rather take a chance with a camera than the mirror.

However, I don't think the issue is really 1/12 vs. 1/4 for visual.  A younger person may be able to notice the difference but an older person's eye will not be as precise nevermind different people have differing visual capabilities.  It's a bit of a nonsequiter form that perspective.  Simply viewing through scopes side by side is not enough unless you characterize the optics of both scopes beforehand and then perform controlled identical scope tests.  It absolutely cannot be done reliably through visual comparison due to the subjectivity of the viewer, non-static nature of the eye, and the human brain's tendency to "fill in the gaps".

At the end of the day I think it comes down to how much do you trust your supplier and it has less to do whether you are ordering 1/8 vs 1/12 optics.  If your manufacturer does not supply measurement data then you are basically playing the lottery.  Some lots may perform quite well while others may not and you may not know it until you start to learn how to push your scope, at which point you'll be outside the warranty.  When you order a scope that includes a report with the optics you are paying for consistency and a guarantee.  Everything else is up to you.

Link to comment
Share on other sites

I agree with that thehand. For me it's about certainty as much as anything. The premium for 1/10th wave optics is not stupidly high, so I guess I would always chose that certainty if affordable.

I think for a planetary scope up to 10 or 12", there's something to be said for having the extra accuracy for very high mags, but it's not something I have visual proof of in a side by side comparison.

My understanding is that SW mirrors are generally around 1/6th wave, but I guess the odd duffer does get through

Link to comment
Share on other sites

Hey Russ, just to clarify as I'm not familiar with UK terminology- what exactly does "stuffed" mean? :grin:

  

You stuffed the 8" scope into the 12" scope?

He means that the 12" uncertified scope beat the hell out of the 8" premium model. Over here, "stuffed" is slang for winning or being better, for example my team stuffed your team (my team won easily).

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.