Mirror Quality

Over the past couple of months I've become interested in how to evaluate mirror test data such as strehl ratio, wavefront RMS, peak to valley wavefront, etc. Just what it all means. I read through a long thread (12.5 Mirror)posted here recently but realized I don't have a basic understanding on the subject. And really, a basic understanding is all I want at this time.

My key question - is there a certain point or level of quality above which really it makes no difference to the average amateur astromoner? Sometimes I read where the mirror is 1/8 or better wavelength. Then another mirror is 1/20 or better. Is the difference between 1/8 and 1/20 seen in actual use.

I own an old coulter 13.1 inch red tube which I am quite happy with. I have no test data on the primary but I believe its reasonable compared to the other scopes I've looked through at star parties. Someday I may want to upgrade to a larger truss mounted dob. I would like to specify a minimum quality or at least underastanding what is being offered for the price. I realize that it boils down to a cost relationship, and the cost would likely go up exponentially beyond a certain quality level.

Maybe I'm obsessing here but I just want to know what I'm paying for and don't what to buy a mirror that the last 10% of quality cost a bundle and I can't tell the difference anyways.

I do believe that seeing conditions, dark skys and collimation are as important to bringing out the most a mirror can offer.

Thanks in advance for your comments.

David

The first problem is trying to figure out which numbers are mentioned and how they were derived. 1/4 wave at the wavefront (EP) means the mirror surface is 1/8 wave off of perfect (the error is doubled because of the bounce). There are many other inconsistencies about RMS, MTF & Strehl.
see http://www.rfroyce.com/standards.htm

In the March 1992 issue of Sky & Tel, there is an article that discusses this very topic. In order to determine what can be seen in the EP, Peter Ceravolo (a very respectable optician) built four 6" f/8 Newtonian Telescopes with wavefront accuracies of 1, 1/2, 1/4, 1/10. These were compared by two veteran observers and they were asked to determine which scope had what figure.

The 1 & 1/2 wave scopes were easy to cull out of the experiment. Only in very good seeing conditions were these people able to determine which was better between the 1/4 and 1/10 wave scopes. Terrence Dickinson, one of the observers for the article, said of the experience "Clearly this excercise suggests that there are huge gains in visual performance as optical quality improves to 1/4-wavefront, and less conspicuous benefits with further improvements"

Three of these scopes were also at 1991 Stellafane, and people were asked to view and try and tell the difference. People who knew how to star test were quicker to note the difference between the 1/4 and 1/10 wave scopes. As the seeing improved during the evening, the confidence of the observations went up. Overall, about two thirds of the 103 people were able to tell the difference betwen the two best telescopes.

When it comes to buying a mirror, to get this kind of quality (1/10-wavefront), I think you will have to buy from premium makers - Swayze, Spooner, Royce etc. to increase the confidence that you will be delivered a mirror that actually meets these specs. Or if you are upgrading to a new scope, consider a Starmaster or Portaball for the Zambuto mirror.

Gary Weber

David,

I suggest you get a copy of the article "Optical Quality in Telescopes," by Peter Ceravolo, Terence Dickinson, and Doug George, from the March, 1992, issue of Sky & Telescope. If you don't have a friend with a copy, many college libraries have a good collection of Sky & Telescope.

Another good article is "Testing Paraboloidal Mirrors (What Every Telescope Maker Should Know about Testing Telescope Optics)," in issue 32 of Telescope Making. This magazine is not generally available in the library, but someone could probably dig up a copy of the article for you.

The bottom line is that even a true 1/4 wave mirror, if it's smoothly figured, works well. A true 1/8-1/10 wave mirror is a superb performer. Claims of mirrors much exceeding 1/10th wave, especially if the results are based on Foucault data, should be taken with a grain of salt. I have an 8" f/9 that the Foucault readings claim is 1/54th wave at the wavefront, but it is utter fiction, and I'd be foolish to claim the mirror is that good.

The proper measure of a mirror's quality is a value that reflects the entire mirror surface - RMS or Strehl. Doing some math to produce these numbers from the Foucault test is not that same as actually testing the entire mirror with interferometry. The Foucault numbers I have seen for RMS and Strehl seem very optimistic, and I would not trust them.

The real test of a telescope is its performance under the night sky. Does it provide nice, detailed views of the Moon and planets? Under good seeing at high power, are the star images nice, tight Airy disks, with a nice uniform first ring of the diffraction pattern visible, without extra light floating around? (Because the disk goes to a minimum intensity as you reach the edge, and it does not have a hard edge, brighter stars will show a slightly larger Airy disk than fainter stars.)

Numbers can actually be dangerous. I have heard of perfectly happy telescope owners becoming unhappy owners simply because an optical expert star tested their telescope and put a number on it. Then there are folks who like to wave "optical certification" around, even if it has numbers that are far too accurate and have far too much precision for the test methodology.

Clear skies, Alan

David,

If the world were perfect and everyone measured Strehl universally the same way, I would want a Strehl ratio of at least 95%. This is because the Strehl of the total system is impacted by the central obstruction and to a degree, diffraction of the spiders. A central obstruction of 20% (by diameter) would give a perfect mirror a Strehl of .96. Add to that the effects of the spider and you are probably down to .92-.94 (and this you can't do much about). Following this reasoning, a mirror of .95 Strehl in a newt with a 20% obstruction would have a total Strehl of about .87-.89--comfortably above the generally accepted level for the diffraction limit of .80 for Strehl.

I agree with Gary, however, and that is Strehl can be misleading due to the various ways it is measured and reported. A manufacturer could report a high Strehl ratio, for example, where a mirror of lower Strehl, could be smoother and provider better contrast. Like one of the other respondents wrote, the true test is in the observing. I would keep a mirror without a test document or with a any Strehl ratio listed on a test document over a 99+% Strehl, if I were satisfied with the image contrast and overall quality.

As far as effects of atmosphere, my own experience convinces me that excellent mirrors outperform mediocre mirrors in any seeing. Sorry, but that has been my experience consistenly over the years. Others feel differently, but just take note at the different star parties under different conditions and I think most would agree.

Best wishes,

Randy

I'd say that level is 1/4 wave to 1/5 wave of spherical aberration level, but talking about the entire telescope system, not any single element. Some time ago I was playing with low-contrast resolution limits determined by the MTF, the way it is outlined by Rutten and Venrooij. It is, of course, both approximation and generalization, but should give some basic insight in this matter.

If we express drop in limiting low-contrast resolution as aperture reduction, reduction at 1/8 wave of spherical aberration level is only ~2%. At 1/4 wave, it is ~11%, and at 1/2 wave s.a. it is as much as 57%. What these few figures illustrates clearly enough, is that drop in limiting low-contrast resolution is very slow up to ~1/8 wave s.a. level, noticeably faster between 1/8 wave and 1/4 wave, and then nearly plummets as the system error exceeds 1/4 wave level.

Contrast of resolved details is also lower, and more so than the drop in limiting resolution. Approximately, for the 1/8 wave level, contrast of resolved details is up to 7% lower, while for the 1/4 wave level it is up to 20% lower than in a perfect aperture.

For a Newtonian, 1/8 wave s.a. level is out of question, if for nothing else, due to the central obstruction: 20% c.o. alone puts it down to 1/6.5 wave level. A 15% c.o. alone would put it at 1/8.9 wave level; but should we ad as little as 1/10 wave of spherical aberration from the primary mirror, the system goes down to 1/6.8 wave level. Add a bit (or a bit more) of thermal errors, miscollimation, pinching, astigmatism, turned edge, zone, diagonal... - and it becomes clear that it is hard to keep the system error for larger, fast Newtonians above 1/4 wave, regardless of the quality of primary mirror. Of course, less than good primary would certainly make it impossible.

How good is good enough? The bad news is that for ~12" aperture and larger, the combined error of all other sources besides primary is likely to be ~1/4 wave, or larger. In other words, the error budget is already spent, and the primary has to add as little as possible to the final error.
Beyond 1/8 wave to 1/10 wave s.a. (or 1/27 wave to 1/33.5 wave RMS) wavefront level, the difference in error added by the primary becomes entirely negligible for an average amateur.