91 replies to this topic

[Hummel]

Hallo all,
I am considering purchasing the Orion (England) 250mm f/4,8 reflector. (Actually the Teleskop-Service version.) The scope is speced with a 1/6 Lambda p/v wave mirror but for 378€ more there is a 1/8 wavelength option. So, my question is, what kind of difference is that and is it worth 30% more for the upgrade?
Also, any opinions about the scope would also be appreciated.
Thanks.
- Michael

[hoof]

Not that much. Also be wary of different techniques for measuring the same value. Two different techniques used on the same mirror could yield two different values, e.g. one company might test a mirror and find it 1/5th wave, and another 1/7th wave. This is also why you see suspicious Strehl ratings of >0.99 every so often. Usually one test yielded that, but a more "rigorous" test might give a value more like 0.95.

Assuming the 1/6 vs 1/8th is accurate and using the same measuring procedure and equipment, you're unlikely to tell the difference when using the 'scope. Also, the quality of the secondary is just as important. You could have a 1/20th wave mirror, but have a 1/2 wave secondary, and get poor quality views.

[johnfdean]

There have been more flaming done over what accuracy and importance an objective's wave length is that just about any other topic involving astronmy. The number of experts on the topic are truly amazing.

As the post above (hoof) indicates, in general, the smaller the fraction the better. BUT beware exactly how the test was done.

[sixela]

The scope is speced with a 1/6 Lambda p/v wave mirror but for 378€ more there is a 1/8 wavelength option.

In the case of OO, usually not worth it. Their 1/6th lambda p/v on the wavefront mirrors are usually just as good but with a couple of very localised errors that mean they can't sell them as 1/8th lambda mirrors.

[longfocus]

Hallo all,
I am considering purchasing the Orion (England) 250mm f/4,8 reflector. (Actually the Teleskop-Service version.) The scope is speced with a 1/6 Lambda p/v wave mirror but for 378€ more there is a 1/8 wavelength option. So, my question is, what kind of difference is that and is it worth 30% more for the upgrade?
Also, any opinions about the scope would also be appreciated.
Thanks.
- Michael

Many claims have been made, visually speaking. The truth is you cannot see a difference unless you do lots of imaging and in the context of imaging. Visually - at focus where it counts with the appropriate eyepiece - your eye will not distinguish. If you don't do imaging, save your money.

[tjswood]

You will hear many opinions on whether one can see a difference between 1/6th wave and 1/8th wave at the EP. According to optical theory, when you hit 1/8th wave, you are getting to the point where the spherical aberrations are negligible and hence barely detectable. 1/6th wave on the MTF will have an effect on contrast to a greater degree - but whether it is detectable in most conditions at the EP is a debatable topic. I would conted that experienced observers, given both optics, that over time they and in different settings / conditions would be able to discern a difference, how much though, is again dependent upon the observer and is debatable.

If you stick with 1/8th or better, optically / theoretically speaking you are close enough to perfection on the MTF that you can remove all doubt... and then you can focus on other factors like CO, thermal issues, etc.

If given a choice between the two, I would choose 1/8th wave, but your mileage may vary. Even still, just because its says 1/8th, you have to understand how the testing was done and on how many points on the mirror to reflect the true reality. If you stick with reputable optics with rigid QC, you typically do not have to fret this aspect though... there is a reason that optics like Zambuto, Harry, etc are typically 1/8th or better - they must know something!

[Cosmosphil]

Michael,
How experienced an observer are you? That is a lot of extra cost for the marginal difference. If you think you can see a difference and the extra cost is not a problem then go for it. You will have purchased some peace of mind. However, I find I can't normally tell any difference unless there is larger improvment. Say 1/4 to 1/8th or 1/8th to 1/20th. And, even then, only at very high powers usually on the moon, planets or trying to pop doubles assuming all other influences (collimation, seeing, the observer's state of health, etc., ad nauseum) are not intruding into the experience. Also, as mentioned, I would ask what the spec is on the secondary.
I would pay for peace of mind. Many others would not bother.

[F.Meiresonne]

The scope is speced with a 1/6 Lambda p/v wave mirror but for 378€ more there is a 1/8 wavelength option.

In the case of OO, usually not worth it. Their 1/6th lambda p/v on the wavefront mirrors are usually just as good but with a couple of very localised errors that mean they can't sell them as 1/8th lambda mirrors.

yeah , OO is really good. Mirrors of 1/6 PV are more closely to 1/7. My 1/8 is actually a 1/8.5

If i had knew that on beforehand i would have settled for a 1/6 PV...

[Hummel]

Thank you all for your responses. Tjswood, Phil, I am not very far along in this hobby (less than a year with a Vixen R135). And longfocus, I will not be imaging, visual only. Freddy, sixela, good to hear that Orion Optics has a good reputation. (Teleskop-service is also pretty highly regarded here.) And hoof, johnfdean, you seem to imply that the difference, though certainly there and noticable to an experienced observer, is not so significant.
So, the consensus seems to be that the difference is quite small and at my level probably not even detectable. On the other hand, I am fortunate to be in a position in life where I am able to pay (up to a point) for nice toys. However, 378€ would buy a couple of nice LVW eyepieces. All things considered, I will probably pass on the 1/8. There is also the consideration that I will rarely if ever be using the scope in perfect seeing conditions. I do live in a fairly dark rural area but have no car to easily get to really dark sky.
Thanks again to all. This really helps me.
- Michael

[F.Meiresonne]

However, 378€ would buy a couple of nice LVW eyepieces.

Exactly, you could even go for a pan of Nagler. Perhaps the money is benter spent for a hiqh quality eyepiece.

[sixela]

1/6th wave on the MTF will have an effect on contrast to a greater degree

Not necessarily. The point spread function of the aperture degrades the MTF, not peak to valley errors. In the case of OO mirrors, their 1/6th wave mirrors have an encircled energy function and Strehl ratio that are much, much better than those of a 1/6th wave PTV error mirror whose error is a global conicity error (or large zone).

Even still, just because its says 1/8th, you have to understand how the testing was done and on how many points on the mirror to reflect the true reality.

It's a (quite well executed) Zygo interferometer test.

Ignore the PTV error and look at the Strehl and you'll see what I mean.

Let's stick to the question at hand -- we are talking about Orion Optics mirrors, not hypothetical mirrors with a wrong conicity yielding 1/6th wave PTV errors on the wavefront.

http://www.astrosurf.../testOO300.html

shows a typical OO 1/6th wave mirror, with a Strehl well above 0.95 (even in green light - the report does use 632nm light, so you have to rescale the RMS error to yield one in green light).

The thing is, this is what you get typically, but not what you are guaranteed to get...but I've seen enough interferograms for 8" OO mirrors to know that the 1/6th wave mirrors are really mirrors coming off their 1/8th wave mirror process but usually just missing the peak-to-valley specs by a hair.

In the example posted above, the Strehl is much better than what the PTV would imply if the error were global: the peak error is caused by a defect covering an extremely small portion of the mirror's edge.

Make the bevel a bit larger and it becomes a 1/8th wave PTV error mirror, but with essentially the same point spread function.

If I were them, I'd start selling the mirrors by Strehl ratio and make more money .

[Hummel]

Thank you sixela. That helps me feel better that the 1/6 will be just fine for visual observation at my level.
And Freddy, a new Nagler is also a nice idea.
- Michael

[Lucifigus]

Many claims have been made, visually speaking. The truth is you cannot see a difference unless you do lots of imaging and in the context of imaging. Visually - at focus where it counts with the appropriate eyepiece - your eye will not distinguish. If you don't do imaging, save your money.

In the March 1992 issue of Sky and Telescope there was an article about this. In fact, Peter Ceravolo made four, 6" f/8 newts with 1, 1/2, 1/4, & 1/10 wave mirrors. Initially Terry Dickinson and Doug George (highly experienced observers) were provided the scopes over a period of time and asked to rank them. In general, comparing the 1/4 and 1/10 mirrors was difficult to distinguish between them except in very good seeing conditions.

Diskinson found on nights of good seeing, he did spot sharper and finer detail in Jupiter's belts and subtle differences on Mars with the 1/10-wave mirror. He commented that both gave detail on Mars that approached a 5 1/2" Astro-physics refractor.

At Stellafane, they had 103 people look through the three best (1/2, 1/4 & 1/10 wave) which were focused on Polaris. People who knew how to star test optics did better at determining which scope was which. About 2/3 of the people could correctly identify the 1/10 wave scope.

The conclusion: "when the seeing turns good, many persons can indeed make out a slight difference in sharpness and clarity between 1/4-wavefront and 1/10-wavefront telescopes in focus."

The article is quite illuminating and worth finding for a good read. I cannot dig out the article right now, but there is a good summary on SAA here.

[sixela]

In fact, Peter Ceravolo made four, 6" f/8 newts with 1, 1/2, 1/4, & 1/10 wave mirrors.

With global errors (spherical aberration) - pretty much the worst kind of error for a given PTV error...

[tjswood]

So, has this thread concluded that all these premium mirror makers are wasting their time, that all we need is 1/6th wave and everything is hunky dory?

Lest we forget that most rear their head and are most noticeable at under 1/8th wave. Are we trying to hard to play the middle ground here? Nothing beats a premium optic in my book, and I will still tell anyone who cares different try to get 1/8th or better.

[sixela]

So, has this thread concluded that all these premium mirror makers are wasting their time, that all we need is 1/6th wave and everything is hunky dory?

If the Strehl ratio is really 0.97, yes, independent of what the PTV error is.

Once you reach numbers like that, it's more important to know exactly how the Strehl ratio estimate has been derived than to know its numerical value. And if you have RMS errors or a Strehl ratio, the PTV error is the least of your worries.

Peak to Valley error is a completely meaningless as performance criteria (you might as well posit that to know whether a car is fast you have to measure the size of the wheels). And I'm not alone in saying that; just check R.F. Royce's web site if you don't believe me, or ask Carl Zambuto.

I will still tell anyone who cares different try to get 1/8th or better.

It's meaningless. Two "1/8th wave" peak to valley mirrors may be entirely different.

To witness, the OO "1/6th wave" mirror shown above has a Strehl ratio of roughly 0.96 (in green light), while a "1/6th wave" mirror with pure spherical aberration has one of 0.915, and the energy scattered away from the central portion of the Airy disc is more than twice as large.

[Lucifigus]

So, has this thread concluded that all these premium mirror makers are wasting their time, that all we need is 1/6th wave and everything is hunky dory?

Essentially, if you cannot tell the difference between the scope you are using and a so called "premium" set of optics, what you have is fine. If however, you get the chance under good conditions to see a premium set of optics show it's stuff, and you notice it shows more than your scope, perhaps it's time to upgrade.

What it comes down to is look through a lot of scopes and make your own determination as to where you want to put your money.

[tjswood]

And yes, looking through a lot of scopes is good advice, and sometimes not possible for many people or it would take too much time. The advice of going to star parties is good, but determination / satisfaction on scope type and optics comes from repeated use, and not from one isolated 5 min peek through the EP in random seeing. And not everyone has a local astro store loaded with scopes on the floor - take Phx for instance - big city, but an astro dealer wasteland.

That is why it is always best, IMHO, to buy a mirror backed up by some stated documentation of how that mirror performs. (I know - speaking in such terms is blasphemy in todays market). The industry got away from this practice years ago for many reasons (one being cases where the claims were refuted by testing), and that is where the "diffraction limited" phrase was used to describe mirror quality - which IMO is an overloaded term that is meaningless. And, interestingly, this phrase is equated to 1/4 wave ptv by many. And 1/4 wave is not going to stack up well to 1/8th wave in *most* conditions (good or bad seeing) - arguring this point to vehemently shows alterior motive or lack of education on optical theory.

But there still are a percentage of mirrors that will stand behind their quality. Without data or documentation backing up the optics on a particular instrument, we are at the mercy of bad QC stories you hear on some of the overseas optics - that its a mixed bag. Or at the mercy of some of the bigger commercial optics with the same issue. Not everyone is an optics expert, and many have felt "taken" because they found out the scope they bought in their zeal to get a scope turned out to be the infamous "bad" one. (or more than just one in many cases). Or they learn more, and after many hours behind the EP, discover their socpe is not "up to the task" in varying seeing conditions due to poor optics. It happens more than it should. I personally feel that *all* scopes should come with accurate test data backing up their optics - that way all the new folks have to do is understand that they need to look for certain borderline criteria to know what they are buying. And then, just like fine gems vs. flawed ones - the higher rated optics can command a better premium than the lower rated ones. (Some companies do this now - e.g. - IM will let you pay less for a 1/6th wave vs. a 1/8th wave - good biz model IMO). This way, the user can decide how much they want to pay for a particular level of quality, and we have a *real* barometer on how to compare scopes of the same brand, or moreover different brands. -To me, the fact that one person can buy a "lemon" 100ED off the same shelf as an optically sound one of 1/6th or better wave is a travesty, and not good for the hobby. Then we have to sit here and see some tell the stories about "hand picked optics" and the "good ones vs. the bad ones". That makes folks hesitant to buy in the end, and is not good for business or the hobby. Step back and think about it.

And its always ones final determination where to put their money - there is no doubt there, I was stating my preference as you are stating yours.

The bottom line for me - buy the best optic you can get in the chosen type of scope you get and be patient. Every scope is different. Just realize that if a scope is 1/8th wave ptv *and* in the 95% strehl range, there is a lot of optical theory that will back up that you have made a very good choice. And it will work well in all seeing conditions - who wants to buy a scope that caters to the least common denominator anyway - ie - works just as well as a good one in bad seeing. That to me is a very peculiar argument indeed.

Sixela - of course the assumption was that you would not pick a scope with a lower strehl ratio for a given ptv. 1/4 wave = 80% strehl in theory - but it could be worse. And I fully expect you to pick apart my post sentence by sentence and refute every point - a quick look through your post history quickly shows this - do you ever actually start a thread, or is your MO to just find posts where you can prove that everyone else is wrong? (and you are the expert). I guess starting a post and asking a question would actually allude to the fact that you don't know everything!!

[Mark Harry]

All I know, is that on a star, you won't likely see much difference, but for planetary, with what I've seen, if it's below 1/10th, or .97, I'll notice the difference. Perhaps it's because I've practiced a lot. For general purpose, I'm usually satisfied with anything in the 1/6-1/10thPVW. (apertures in the 6-8" size)
If OO does those tests at 632nm, I'd go for the 1/8th. If corrected for 550nm, 1/6th. (general purpose) Mark

[Hummel]

So, has this thread concluded that all these premium mirror makers are wasting their time, that all we need is 1/6th wave and everything is hunky dory?

Lest we forget that most rear their head and are most noticeable at under 1/8th wave. Are we trying to hard to play the middle ground here? Nothing beats a premium optic in my book, and I will still tell anyone who cares different try to get 1/8th or better.

Hallo tjswood,
I hope that the consensus is not as you suggest. I asked (not knowing that this is something of a touchy subject :o ) so I could make a better informed decision. The questions that others asked and the answers I would give led me to think that I would not see any meaningful difference (i.e., not imaging, new to the hobby, not perfect skies).
I suppose I could afford to spend 30% more but I am also pragmatic. And anyway, a 10" scope would be twice the size of my current one and that alone should make for "better" views.
However, I am sure that the better numbers do make for a better scope. It is a real balancing act, I guess, to put all this in perspective. This thread has generated much for me to think about, though, and that is good. Your comments will definitely be considered (your post of 11/12/07 08:39 PM as well).
Thank you.
- Michael

[Hummel]

All I know, is that on a star, you won't likely see much difference, but for planetary, with what I've seen, if it's below 1/10th, or .97, I'll notice the difference. Perhaps it's because I've practiced a lot. For general purpose, I'm usually satisfied with anything in the 1/6-1/10thPVW. (apertures in the 6-8" size)
If OO does those tests at 632nm, I'd go for the 1/8th. If corrected for 550nm, 1/6th. (general purpose) Mark

Mark, my main interest is DSOs. I was out just last night checking out the clusters around delta Cas and thought "Wow, these are great but it would be nice to see a bit more" (NGC 654, e.g., looked like it would have a lot more to offer with a bit more light gathered).
Anyway, it seems to me that detail is detail, whether DSO or planet. Why would differences be not so apparent for a cluster, for example, as compared to a planet?

Also, here is a link to the OO test description
It appears they use 632nm. What would corrected for 550nm imply?
Thank you,
- Michael

[tjswood]

Hallo tjswood,
I hope that the consensus is not as you suggest. I asked (not knowing that this is something of a touchy subject ) so I could make a better informed decision. The questions that others asked and the answers I would give led me to think that I would not see any meaningful difference (i.e., not imaging, new to the hobby, not perfect skies).
I suppose I could afford to spend 30% more but I am also pragmatic. And anyway, a 10" scope would be twice the size of my current one and that alone should make for "better" views.
However, I am sure that the better numbers do make for a better scope. It is a real balancing act, I guess, to put all this in perspective. This thread has generated much for me to think about, though, and that is good. Your comments will definitely be considered (your post of 11/12/07 08:39 PM as well).
Thank you.
- Michael

Michael -

I am not saying that a 1/6th wave mirror is bad by any means. Its not. I am hoping that is not what you took away from my reply.

The fact that you are newer to the hobby and asking these sorts of questions is a tribute to you! And exactly the point in my post- you have a choice in what you buy optically - and you are exercising it. The fact that you know the wavefront of the optic is the exception rather than the norm today in the hobby, and the basis of my rant - I don't think that is a good practice IMHO (to not state the quality of the optic). And another point I was making - at least you are aware there is an difference between 1/8 and 1/6th, and the optics you are buying offer a choice instead of coming off the shelf with no optical quality statement.

And I agree with Mark on his assertion for planets. (BTW, just so you are aware, Mark is one of the premium mirror makers that *does* document the quality of his mirrors and stands behind them). I love planetary viewing, and I do see a difference between a better mirror as he does. And its one of the main reasons I strive to eliminate all aberrations from the optic by making 1/8th wave or better. This is backed up by optical theory. And sure your interest may be deep sky today, but I have found that one month its globulars, the next is double stars, the next is planets, etc. And no matter what the flavor of the day is, there is always one type of object (for me anyway) that stays in my viewing regimen constantly - planets. And the day you start to study them more is the day you will have wished that you maximized the optical quality. When you buy the best, you only cry once!!

And since you seem to understand optical theory, have you read "Star Testing Astronomical Telescopes?" (by Suiter). It is not the bible by any means, and there are better books on the topic no doubt, but for beginners like you and I it is a great resource. You will see it refuted by some that typically are the ones that don't like to see an educated populous on optics (maybe because of the alterior motive aspect I referred to earlier).

But for deepsky, on most objects - Mark is correct, 1/6th is fine and will work admirably. In fact, if it is truly a 1/6th optic with the commensurate Strehl ratio to back this up, then you are already ahead of half the optics (or maybe more) floating around today.

PS - Like you, I am also looking at a 10" scope - one that is good for both deepsky and planets. (like an F6). My 11" I have now is an SCT with the commercial, middle grade optics we have been referring to (probably 1/4 wave at best), and is a great scope for clusters, etc... but when it comes to planets (contrast) and double stars (resolution) it does not do so well. Right along the lines of this discussion.

In the end, my preference is to buy 1/8th wave or better optics. And my preference also is to only buy optics that will document their quality instead of leaving my purchase to chance. (I wish the industry would go that direction for the sake of the hobby). What you decide is best for you is the right thing in the end - I hope seeing both sides of the coin here has helped. But don't buy a scope for bad seeing, sure your seeing won't always be the best, but a better optic will perform better in both good and bad seeing, period. And when the seeing is good, you want a tool that is up to the task... especially on planets.

Also, in the end, astronomy boils down to one thing - *OPTICS*. The saying "Buy the best you can afford" has never had more meaning when it comes to the optics (and the eyepiece, diagonal, etc - the entire optical train - makes no sense to buy a 1/10th mirror and a cheapy 1/4 wave diagonal, right?). You will never regret getting a better optic, IMHO, and just like fine gems, the better ones hold their value more.

Good luck to you in your choice.

[Lucifigus]

I suppose I could afford to spend 30% more but I am also pragmatic. And anyway, a 10" scope would be twice the size of my current one and that alone should make for "better" views.
However, I am sure that the better numbers do make for a better scope. It is a real balancing act, I guess, to put all this in perspective. This thread has generated much for me to think about, though, and that is good. Your comments will definitely be considered (your post of 11/12/07 08:39 PM as well).
Thank you.
- Michael

You may have made the correct decision, only you will know and, after all, it's your money. Before you get too concerned about premium optics, there are things you may wish to consider in trying to ensure the scope you have is offering up the best views possible.

Learn to collimate well and do it often.

Flock the tube, or at least across from the focuser and above the main mirror - this will improve contrast.

If you observe near ambient light sources, consider a light shield beyond the top end of your tube.

Fan installation to get the mirror to ambient temperature is necessary in some climates. Often what we think is poor seeing is really thermal issues. Since I installed fans, I have been amazed at how my seeing conditions improved. You may wish to give this some thought.

Consider using an observing chair as being comfortable is important.

Observe often, and as you gain more experience, you may find you will form opinions about other things, including those discussed in this thread.

Have fun and be happy with what you have.

[Hummel]

I am not saying that a 1/6th wave mirror is bad by any means. Its not. I am hoping that is not what you took away from my reply.

Not at all. And don't think I don't appreciate this discussion.

And since you seem to understand optical theory, have you read "Star Testing Astronomical Telescopes?" (by Suiter).

My understanding is marginal at best. (I do have a math background, though.) I will seek this out.

Also, in the end, astronomy boils down to one thing - *OPTICS*. The saying "Buy the best you can afford" has never had more meaning when it comes to the optics (and the eyepiece, diagonal, etc - the entire optical train - makes no sense to buy a 1/10th mirror and a cheapy 1/4 wave diagonal, right?). You will never regret getting a better optic, IMHO, and just like fine gems, the better ones hold their value more.

That is a very good point. I don't like regrets!
This is not a quick purchase. In fact the seller says there is a six-month lead time for the better optics. That is OK. I am not in a hurry. That is why I am asking and listening.
Thanks again!
- Michael

[Hummel]

Observe often, and as you gain more experience, you may find you will form opinions about other things, including those discussed in this thread.

Have fun and be happy with what you have.

Very good advice. But you are preaching to the choir. (I really like that expression.) Since purchasing my telescope in June I have been out nearly every clear night. I am still learning, of course, and that is what is great about finally putting my abstract knowledge to practical use. There is a big difference between theory and practice. And a big, big difference between seeing a picture of, e.g., NGC663 and finding it and seeing it "live".
- Michael