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Optical Quality ?

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#1 George Methvin

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Posted 25 January 2013 - 10:06 AM

There is alway a lot of talk about the optical quality of telescopes, are the optic great ,good or just fair. I thank many time a scope that has good optic can get a bad score because of other factors. Many factor decide how a scope will perform and I will not go though all of them here, just a few that I know hurts my scopes ablilty to perform up to its full ability. I will use my setup as a example, I have a Meade 10 LX200 classic and it has very good optics, I do most of my observing from my driveway, there are house on each side of me front and back. My scope really does not have much of a chance to shine in this setting. Now to the problems frist I have my scope setup on concert thats been in 104deg heat all day in the summer so it never really cool down second all the houses around me are throwing up heat waves in the sky in the summer and winter plus my sct never really gets a good chance to cool down with all that going on. On top of all that you have to hope its a good clear steady nite with good seening. Not many of use have to perfect place to setup our scope. So I understand when I setup my scope in this type of setting that it's not going to perfrom any were near its ability, it amazing it does as well as it does. So if you have a scope in this type of setup don't be to harsh on it optical quality you may have a scope that has great optic just in a bad setting. Get that scope out in the country away from the houses and lights and then you will be better able to judge the optical quality of your scope. Hope this helps, clear skys.

#2 csrlice12

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Posted 25 January 2013 - 10:10 AM

I've always said the best piece of astronomy equipment you can buy is the "gas filter". NOTHING enhances a scopes performance like dark skies.......

#3 George Methvin

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Posted 25 January 2013 - 10:34 AM

So true lol. When I do get the chance to take my scope out to the country with dark sky and set up on grass my scope really shines it's like a new scope.

#4 Eddgie

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Posted 25 January 2013 - 10:40 AM

As for optical quality asessment, a properly conducted star test, or tests like the Roddier test can do a very good job of establishing the quality of an individual sample, and nothing should be implied about others scopes of the same model until multiple examples have been tested, or a baseline of performance for that vendor can be established.

As for optial quality, I think most people would struggle to tell the differnce in performance at best focus between a scope with good optics and great optics. Only when the optics get into the "OK but not great" range will even an experienced observer be able to pick the better scope blind without doing a star test (the test is very sensitive).

So, the difference in a good scope and a great scope is not so much that it is easy to see.

Your point about conditions makeing a difference in the view you get though are completely true. Picking a good observing site or observing conditions is a big contributor to the quality of the view you can get.

#5 George Methvin

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Posted 25 January 2013 - 11:19 AM

Yea Eddgie that is so true. Most scope like all thing do there best in the right setting. You should not expect your scope to perform at its best in poor setting then blame the optic for the poor images you see.

#6 oldtimer

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Posted 26 January 2013 - 01:50 PM

Scopes with truely grat optics can only be optimized under truely great skies. Here in the midwest where we are under the jet stream 90% of the time my average optics are all I really require.

#7 Jon Isaacs

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Posted 27 January 2013 - 04:10 AM

Yea Eddgie that is so true. Most scope like all thing do there best in the right setting. You should not expect your scope to perform at its best in poor setting then blame the optic for the poor images you see.


My own attitude is that most telescopes have reasonable, decent, pretty good optics. It's my job to make the best of those optics, make sure the scope is 100% as ready as it can be and then just enjoy what it has to offer. If I do my part of the job, I find most any telescope will reward me with some wonderful moments.

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#8 City Kid

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Posted 27 January 2013 - 10:01 AM

George, my situation in my yard is just like yours. The only direction I can point my scope without looking over a rooftop is straight north. It's a rare night that I'm not looking through heat waves rising from the rooftops when I'm trying to view planets. This limits my planetary viewing to the 150x-200x range most nights. Fortunately planets aren't my thing so I can live with it.

Having said that, even in those conditions the difference between my stock mirror in my XT10 and the Zambuto mirror I replaced it with was quite obvious. I'm certainly no eagle eyed observer so perhaps this says more about my stock mirror than it does about the premium one. I believe Eddgie when he says most observers couldn't tell the difference in focus between a good mirror and a great one on most nights. Maybe my stock mirror was just ok. I just know it didn't take perfect viewing conditions to tell the difference in my case.

Phil

#9 dpwoos

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Posted 27 January 2013 - 10:57 AM

I have almost always found a top-notch mirror to produce more satisfying views, regardless the conditions. I think this is because, even when the conditions aren't good, there are nevertheless many moments when the seeing cooperates and the target snaps into focus. A good observer learns to concentrate on these good moments and ignore the rest, and so has a satisfying experience at magnifications that others with less skill and/or lesser optics find unproductive. The other factor is that I think aberrations are additive, and so bad mirror plus bad seeing will always be greater than bad seeing alone. Really, I can't remember the last time I observed with conditions so bad that our club's acknowledged best optics didn't produce superior views.

#10 dan_h

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Posted 27 January 2013 - 11:05 AM

It's a rare night that I'm not looking through heat waves rising from the rooftops when I'm trying to view planets. This limits my planetary viewing to the 150x-200x range most nights. Fortunately planets aren't my thing so I can live with it.

Having said that, even in those conditions the difference between my stock mirror in my XT10 and the Zambuto mirror I replaced it with was quite obvious.


My experience is quite different. For me, it doesn't really matter what bowl you put it in, the soup tastes the same.

When I look at Jupiter through the heat plumes of the neighborhood, the view is a mess. Many nights I can't see any detail at all, just a poorly defined globe with some hazy brownish stripes. I don't care who's name is on the optic. If that's what is presented through the atmosphere, that's all you're going to see.

I believe there are occasions when a better objective will allow one to see more but that is in marginal seeing, not soupy urban winter nights.

dan

#11 CounterWeight

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Posted 27 January 2013 - 11:28 AM

I've always said the best piece of astronomy equipment you can buy is the "gas filter". NOTHING enhances a scopes performance like dark skies.......


I add that also a good guide and or charts (hardcopy, emedia whatever) as to what will be good to look for / at on the evening? ;)

#12 Eddgie

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Posted 29 January 2013 - 09:53 AM

I believe Eddgie when he says most observers couldn't tell the difference in focus between a good mirror and a great one on most nights. Maybe my stock mirror was just ok. I just know it didn't take perfect viewing conditions to tell the difference in my case.


Sadly, the Orion mirrors apparently have in the past varied quite a bit in quality. I read a review here on CN maybe last year where someone had a new Go-To dob and when tested, the evaluation showed up maybe a 3rd of a wave of SA. It sounded to me like the individual did know how to do the test for SA properly, so I have to take that as a valid asessment. Later, I think someone else tested it and concurred that the mirror was well under the "Good" catagory.

The difference of a few Strehl points is to me impossible to see visually. I don't think that on the in-focus image, the average observer would be able to tell a .95 Strehl scope from a .99 Strehl scope (though it will show in the star tests).

It is only when the Strehl deviates to perhaps .1 (99 vs 89 for example) that the difference becomes easier to detect.

I like the rating that Royce puts on his web page, so I tend to follow that classification:

.92 to .93 Strehl = Good
.93 to .94 Strehl = Very good
.95 and above Strehl = Excellent.

My own experience was that having owned Intes Micro scopes made to 1/6th (standard quality) and 1/8th (deluxe), it was just about impossible to see a difference.

Ah, but this is Intes Micro. With their scopes, you now that no serious error exists. There will be no truned edges, no zones, and no astigmatism.

All of these are factored into Strehl, but they way they change the image varies greatly. For example, a bit of astigmatims can indeed damage the contrast for very fine detail. A zone on the other hand, will scatter light across the focal plane and not localize the damage. Both can lower the Strehl, but the way the light is concentrated or not concentrated (based on the kind of defect) determines how it affects viewing extended objects.

Anyway, the 1/6th wave telescopes were superb performers, and I doubt that most people could tell a standerd Intest Micro scope from a Deluxe model.

I recommend Royces page for optical quailty, and I recommend the book "Star Testing Astronomimcal Telescopes" for learning how different fabrication errors will effect the performance at the eyepeice. All defects lower Strehl, but it is where the light goes (close to the Airy Disk vs scattered across the field) that determines how the telescope will perform on contrast transfer.

http://www.rfroyce.com/standards.htm

#13 Eddgie

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Posted 29 January 2013 - 10:12 AM

As a follow to my previous post, Astro-Foren.de has tested a great number of telescopes over the years. I use the site all the time (it has helped me improve my asessment of optics during star testing).

It is interesting to note that Celestron for example produces SCTs that do vary in quality, but a surprizing number of Celestron SCTs he has tested have had optics very clearly in the "Excellent" catagory, and some even with Strehl in the .97 range, but quite a few at .95 or higher.

The last Celestron SCT he tested was an EdgeHD 11".

Strehl was .95. Excellent.

Astro-Foren Test Index by scope types

EdgeHD 11", .95 Strehl in Green. slightly undercorrected, but still exce...

#14 David Knisely

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Posted 29 January 2013 - 03:14 PM

Eddgie wrote:

Sadly, the Orion mirrors apparently have in the past varied quite a bit in quality. I read a review here on CN maybe last year where someone had a new Go-To dob and when tested, the evaluation showed up maybe a 3rd of a wave of SA. It sounded to me like the individual did know how to do the test for SA properly, so I have to take that as a valid asessment. Later, I think someone else tested it and concurred that the mirror was well under the "Good" catagory.



Well, it wasn't a Go-To Dob, but my Orion XX14i 14 inch f/4.6 Dob that I reviewed here on Cloudynights. The primary mirror's wavefront error was measured at 1/3.1 wave p-v and 1/12.8 wave RMS by the same optician who later re-figured the mirror to a premium level of quality (the secondary was between 1/5th to 1/6th wave). I had earlier bench-tested the primary with my own equipment and gotten similar results after the scope showed notable spherical aberration in the star test. The mirror was very smooth in surface appearance in the knife-edge test, but the figure was simply wrong (I termed the mirror "mediocre"). I require a telescope's optics to be better than 1/4 wave p-v and 1/14th wave RMS on the wavefront in order to meet my acceptable minimum for quality (what I consider "diffraction limited"). Unfortunately, Orion's primary didn't meet that requirement. Clear skies to you.

#15 Eddgie

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Posted 29 January 2013 - 05:09 PM

Yes, that is the test I remember. And that is what I was saying.. I think Orion's quality does indeed vary, and someone earlier said that they did not think that their Orion did as well as a similar sized scope. My point there had been that between a good scope and an excellent scope, there is not much difference in performance (to your own point that "Diffraction limited" is the minimum quality level one sould allow), and it is likely that the other poster simply had a telescope with less than "Good" optics.

Do you still have the scope? How is it doing now that the mirror has been re-figured?

That is why if I get one of these (and I do want a Go-To dob), I will budget for a mirror job.

#16 David Knisely

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Posted 30 January 2013 - 12:21 AM

Yes, that is the test I remember. And that is what I was saying.. I think Orion's quality does indeed vary, and someone earlier said that they did not think that their Orion did as well as a similar sized scope. My point there had been that between a good scope and an excellent scope, there is not much difference in performance (to your own point that "Diffraction limited" is the minimum quality level one sould allow), and it is likely that the other poster simply had a telescope with less than "Good" optics.

Do you still have the scope? How is it doing now that the mirror has been re-figured?

That is why if I get one of these (and I do want a Go-To dob), I will budget for a mirror job.


Yes, I still have the scope and with the refigured optics (both primary and secondary), it is an absolute killer on both the planets and the deep-sky. Jupiter at very high power is simply to die for, especially now that I have shortened the trusses a little which allows me to use my binoviewer. On deep-sky, when the seeing allows it, I will push past 800x on a fairly regular basis to eek-out fine detail on planetary nebulae. Mike Lockwood did an outstanding job on the mirrors, so I can heartily recommend him for refiguring of large fast telescope mirrors. Clear skies to you.

#17 Eddgie

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Posted 30 January 2013 - 10:12 AM

Good to hear.

And this has been what I have been recommending to people as the ultimate tlescope... A Go-To dob with refinished mirrors. I want one myself.

They are just so inexpensive.

I am looking at the 12" verions. It is slightly slower (less coma, and I don't think you can use a Paracorr with a binoviewer) and while I give up a couple of inches of apeture, I also get a bit of a wider true field, though coming from a C14, it is all "Wide".

Again, very happy to hear that the scope is satisfying to use. I had expected as much.

#18 Gord

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Posted 30 January 2013 - 12:23 PM

Hey Eddgie,

Question for you regarding Rohr's tests (since I don't understand German...). Are the "artificial sky" test images he shows and analyzes all relative to the theoretical resolving power of the scope in question? In other words, a test to show how the particular sample would show star in the real world under perfect conditions that are at or close to the resolution limit?

I'm guessing this to be the case as what I see in the images displays a lot of what I see in real life (ex. a secondary obstruction making the diffraction ring more prominent), but the scale and resolving of the star images is the same no matter the size of the scope (ex. 55mm or 356mm). I assume he just scales the test setup to match the resolving power of the scope and then shows how that optic renders the image. Does that sound about right?

Thanks,

#19 Eddgie

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Posted 30 January 2013 - 04:31 PM

Well, the star images I think are just shown to show the effects of the optical errors, especially if there is tilt or astigmatism. These are not theoretical. They are actual images taken though his double pass autocollimator.

Sometimes, he shows the off axis stars as well, so this just shows how the errors affect the image.


Remember, his images are going to show deviations that our eye is not sensitive to see. For example, a very small amount of astigamtism may only change the brightness in the rings only a tiny amount.

Just like with images of planets, our eyes are not as good at seeing detail as CCD chips, and his pictures show defects on in-focus stars that might not stand out in the eyhepeice.

The defocus images are likewise actual images, and this is where I have studied a lot of star tests to help me identify different errors when I do my own star testing.

More important is the Strehl and the MTF plot. The MTF plot shows how much contrast the scope will loose vs a perfect one of the same design. If there is astigmatism, he shows the perfect plot again, for a scope of that design) and a red and blue plot for the tangenitial and sagittal focus.

Another site with a lot of star tests is the Abberator site. They show defocus tests of maybe 30 telescopes, and they have Abberator simulations using the actual star test data to calculate optical quality:

http://aberrator.ast....net/scopetest/

Rohr uses a double pass system that is very accurate. Abberator is a useful tool for trying to asess spherical abberatoin as well. The web page above kind of shows some exmaples where this has been done.

Anyway, Astro-foren uses real images from artificial stars for the in focus and defocused images from the actual scope under test.

You can also use the "Translator" function on Google. Text in images will not be translated, but the rest will be, though it is often not clear what they are saying because the translations are not always good for technical content.

#20 Gord

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Posted 30 January 2013 - 09:28 PM

Hey Eddgie,

Yeah I was using the translated links you provided above (thank you!), but as you say, it doesn't all come through clearly. I've also been running certain text from the annotations to the images through the translator on their own to understand the meanings better. I can get a lot from it, but was just wanting to be sure I was clear on the star images, mainly on them all being the same regardless of the optics being tested (ie. they are scaled in the test to match the resolution of the optic and see how it performs).

One aspect of it though that I'm a bit puzzled about is around differences in those images, and even in some of the defocus images. Why? Some from what I thought (or appear...) to be good optics don't show the star images very nicely. In another case, there were noticeable differences in the defocus images, but the optic looked to be very good (Astro-Physics 155). Another example are the star images shown for a lot of achromat lenses. Frankly, they're pretty bad!

Here's a few that caught my eye:

Very nice C8, look at the airy disks
TEC 200 MCT, very ugly star images compared to the C8
A-P 155, not so pretty stars and the defocus looks different
Synta 120ED, compare the images for this one to the A-P

Anyway, very interesting stuff, and to me it would seem like the absolute way to characterize a sample. And yeah, it really is the _sample_ that's being shown, and some clearly show a lot of variation.

I must confess though, I still really don't understand the Ronchi diagram images and what they are saying. It seems when I think things should be good, the summary result isn't what I expected. Interesting stuff I hope to learn.

Clear skies!

#21 Eddgie

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Posted 31 January 2013 - 02:11 PM

The C8 was slightly out of collimation.

In the first image, you see a little box at the bottom that has a line that says "Tilt" and it states that for the quality analysis the tilt was "Removed". This is why you see it in the star test, but in the "Energy Distribution" plot (third image) you see the first ring as even.

This is a really excellent C8 by the way.

For the TEC image, you can see in the star test that there is a bit of astigmatism. As I mentioned in my earlier post, this would be impossible to see on the in-focus image and it is such a small amount that it has on effect on the telescopes performance. The camera shows that there are areas that are slightly briger and slightly dimmer in the rings, an the intensity plot shows it, but for the eye to easily see it, the difference in brightness needs to be about 15%. In other words, for the eye to easily see it, the brightest section of the ring would have to be 15% brighther than the darkest section of the ring. Our eye's contrast sensitivity is not that great.

Tilt has again been removed for the AP during the processiong, so you see a very slighty brighter ring on one side in the star test, but he has removed the tilt to calculate the Strehl.

And yes, the Synta has world class optical quality. Not that it is an APO, but for what it is (an ED doublet) it would be meaningless to improve the quality past this point. Again, .95 and better is from a practical standpoint, optically perfect. I Don't think there is an observer alive that could tell a .95 Strehl scope from a perfect one of the same type on the in focus image. Sure you might see it on the star test, but that is what makes the star test so powerful. It shows every deviation from perfect.

The Ronchi test.. It is almost useless for amateurs becasue it is not very sensitive, though I have beena able to see turned edges with it, and you can see them in many of the tests on Rohrs site.

Rohr however, is using a double pass autocollimator. This vastly improves the sensitivity of the test.

And what the Ronchi gratings are doing is shoing the surface figure, and when there are colors, they are for different wavelenght of ligth.

In a perfect reflector, they would be perfectly straight.

In systems that contain lenses (like the SCT corrector) there is often some difference in focus between the different colors of light.

So, the Ronchi shows how much deviation from perfect the surface is. A rough or astigmatic surface will show distorted lines. A turned edge will show a slight bend at the ends of the lines... And so on.

This is a site that one can learn a hugh amount about optics.

For exmaple, someone took offense a while back when I said Mewlons did not seem to enjoy the very high quality that Tak Refractors enjoyed.

Someone basically said I was being silly (not those words, but implied as much), and yet the Mewlons Rohr tested varied in quality be quite a bit, with (as I recall) two out of three falling just out of the "Excellent" catagory.

He has also shown many examples where there was some kind of mechanical or alignment problem that once resolved, greatly improved the performance.

And there are dogs... Not just telescopes that are OK or passabe, but rally bad. There are a couple of Meades, and a Vixen that are total barkers. But I would never say all Meades or all Vixens were not as good as this or that. Clearly though, lemons get through from time to time.

And that is why it is so important for amateurs to be able to star test properly. It is not hard at all, but it is considerably more ivolved than simply racking the focuser in and out.. That tells very little. It is a series of tests and evalluations, each telling you about one specific kind of optical defect and if it is present, and if so, how bad it is.

By doing all of the tests and adding together the results, you can see how good (or bad) a particular sample is.

Most Celestron SCTs are actually quite good. It is very rare to see a lemon, and in fact, many fall into the excellent catagory.

Synta builds excellent scopes.

Zumbuto figures amazing mirrors.

Intes Micro builds consistently excellent scopes.

A few envings reading Astro-Foren and learing to intrepret the results will give you a far better insight into what is likely and what is unlikely when it comes to telescope quality and brands.

#22 oldtimer

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Posted 31 January 2013 - 02:46 PM

Orion doesn't even advertise 'diffraction limited' optics anymore!

#23 Eddgie

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Posted 31 January 2013 - 05:21 PM

Well, to be fair, the term "Diffraction Limited" is somewhat vague.

But clearly, the case sited eariler where the Go-To dob had a mirror that was not reallyup to snuf should be a clear statement that at the low price point these scopes are often sold at, one should not expect to always get a decent mirror.

I'd still like a Go-To dob one day, but it will have to be when my budget also allows immediatly removing the mirror and sending it in to be re-finished.

Even then, you can get an Orion Go-To dob and have the mirror refinished, and still spend less than for a premium dob. And the premium dob won't have Go-To.

So, you get what you pay for, and at the price point, one is not paying for a .95 Strehl mirror. But it is easy to turn a so-so mirror into an excellent one as long as it doesn't have a turned edge.

Or, just chuck the mirror and put in a Royce conical. Now you're talking....

#24 orion61

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Posted 01 February 2013 - 07:34 AM

Dr Clay has been quoted saying the 10" tubes that came from the origonal Classic LX200's are some of the finest he has ever tested with some coming close to null optically.
I've had one. I moved to a 12" Classic that blowes me away every time I get it out on a night of good seeing,
I use my 8" as a test due to the 90 lbs of weight before it gets set up.
Nothing like 12" of mush on a night of bad seeing, when my 127 Mak outperforms it on Planetary.






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