20 replies to this topic

[Douglas729]

Under the title heading FRINGE TRACING is an ACTIVE FRINGE NUMBER. Is that simply a designation of a particular test or is it more significant?

On the MTF Graph I assume one would want to see the Red and Blue lines as close and parallel to the 45 degree Ideal line as possible.

How can an achromat doublet attain a 0.956 Strehl Ratio with p-v of 1/5? or is that a distinct measurement independent of the amount of light captured within the airy disk circumference?

[Eddgie]

For achromats, I think Rohr can only post the MTF result for one color (though I may be wrong about that).

Usually though, If you look at the report, he specifies a wavelength of light, and this is the wavelength that the Strehl is posted for.

To do Polychromatic Strehl (the damage to contrast that results from all of the defocused light) he would have to know the exact formula for the instrument and then calculate the contrast loss for all of the different unfocused rays. This would not be necessary of course for a pure reflector.

For SCTs and MCTs though, he also cannot include all of the different wavelengths so basically what you get is just a single line that shows the performance based only on the known errors. Note though that he often does measure individual Strehl in SCTs for best correction, and often SCTs are not as corrected quite as well in Green as they are in other colors. Just an interesting side note.

But since chromatic aberration in these instruments is usually smaller than even the best APOs, it matters little.

So, what you get is a Strehl for the green line only, and this is important because this is where the design is supposed to be the best, and knowing the green line Strehl tells you the optical quality for that exact wavelength which is really the only place the Strehl can be high (assuming that the goal was to produce the best green line performance, which of course it is in an Achromat).

In essence, it allows you to compare fabrication quality of the optics from one sample to the next, but it makes it hard to say for example, how a 6" f/8 achromat and a 5" APO would compare.

This then is just the "quality of the curve" and nothing to do with the true total polychromatic Strehl which is always always lower, even in APOs Even the very best APOs will not have the same correction in all spectrums as they do in green, though very good ones can come very close).

That means that the Strehl you see is useful because it tells you how well the curve is cut, but it does not give you the complete picture.

In the MTF, again, you are only seeing the green line Strehl because the polystrehl would have to be manually calculated.

When the MTF chart shows a red and blue line, this indicates that the system has astigmatism, and you are seeing the contrast for the sagittal and tangental planes.

When Astigmatism is present, there is no exact focus of all the rays, so you are seeing how the contrast would be if the Sagittal plane where brought to focus, and if the tangentialplane were brought to best focus. This is indicated in the MTF plot by the words "Tang" and "Sag" in the legand in the upper right corner of the MTF plot.

Of course the instrument will perform mostly somewhere between these lines.

Hope this helps.

[Eddgie]

As a follow-up to my previous post, here is an example of a 6" f/8 achroamt test.

Now if you were to compute the Polychromatic Strehl of such an instrument, it would only be in the low .6 range. Polychromatic Strehl is the total of all energy lost with the total effect on the contrast of the system.

Green line weighted Sthrel is of course only considering the light lost in green. A system could be .98 in green, but the contrast still could still suffer due to the lowering of contrast by the other colors that are unfocused around the Airy Disk and across the face of an extended target.

So the testing you see usually is very specific to green light and ignores the contrast loss of the defocused colors.

In this example that I am going to send you, you will see many of these details.

Again a 6" f/8 achomat will only have the low contrast threshold sensitivity of a 4" APO. (Yes, not making this up. This is what bad CA does and you can find the chart showing this right here: See figure 74 )

Ok, here is the example I am going to use...

This is a 6" f/8 achomat. Again, you can see from the previous link that this instrument will not do much better than a 4" APO on low contrast detail, but with a Strehl tested at .85, one would think that it would easily outperform a 4" APO on low contrast detail.

If the polychromatic Sthrel were to be factored in though, the curve of a perfect 6" f/8 would at best look like the curve shown in Figure 74 above.

And since the Rohr's test shows the Strehl to be much better than this, you can now see that he is only looking at the green line.

Here is the report I am going to use:

Rohr's report on 6" f/8 Achromat

And he says this. In the report, he lists the wavelenght as 532nm.

Notice also that this scope does indeed have coma, astigmatism, and sherical aberration (and in fact also has a slight turned edge kind of defect easily seen in the wavefront graph).

The chart also shows the Tang and Sag lines and again, in the upper right corner of the MTF you can see the legand for these.

What this report says then is that in green light only, this scope has a .85 Strehl, which is as these things go, at the very low end of acceptable optical quality.

But if you factored in the true performance (effects of the defocused energy on encircled energy and contrast), you would find that this particular instrument would struggle to beat a perfect 4" APO. Again, refer to figure 74 above and consider that in the plots shown, the 6" f/8 achomat was given the benefit of being "Perfect" in terms of the actual curve on the lenses.

This is a scope that would not succeed in producing a better, more detailed view than a 5" ED scope. Heck, a 4" APO would probably beat this particular sample at just about anything you choose to point them at.

I hope this helps, and forgive me if I over-answered the question, but most people do not seem to have any interest in MTF and topics associated with the true performance of achomats, that when someone askes, I feel compelled to give as complete a picture as possible.

One note. There is some disagreement about the importance of Polychromatic STrehl for visul use.

For visual use, people say that only the green line correction reallly should be considered. It is just my own opinion, but I think that you need to consider the contrast performance to be somewhere below the best correction in green and the worst MTF plot of Polysthrel (for imaging though the lower line is the correct one).

I personally feel that for visual it is still much closer to the polystrehl MTF curve than to the E line curve. I have seen unfocused blue light obscure delicate detail on Jupiter, Mars, and the moon that was easily visble in a similar sized APO.

I hope this helps..

I would prefer to let the reference materials speak for me and don't see much point in debating it a lot.

I have learned that most people do not want to accept the physics and that is fine enough with me. Not sure where you will fall, but at least you have a lot of material to consider in shaping your opinion.

If you have ever compared a 6" f/8 APO to a 6" f/8 Achromat though, you will quickly realize that the physics are indeed accurate.

[Fomalhaut]

One note. There is some disagreement about the importance of Polychromatic STrehl for visul use.

For visual use, people say that only the green line correction reallly should be considered. It is just my own opinion, but I think that you need to consider the contrast performance to be somewhere below the best correction in green and the worst MTF plot of Polysthrel (for imaging though the lower line is the correct one).

I personally feel that for visual it is still much closer to the polystrehl MTF curve than to the E line curve. I have seen unfocused blue light obscure delicate detail on Jupiter, Mars, and the moon that was easily visble in a similar sized APO.

I agree. Therefore the Strehl of each measured wavelength should be multiplied with its specific human eyes' visual photopic sensitivity, then all the products be summed up and the result finally be divided by the sum of all the weighting factors used.
For example, this weighted polystrehl over 11 wavelengths from 422 to 678 nm was measured by the German amateur astronomer Kurt Schreckling and published at astrotreff.de.
Here are four out of eight results: TSA-102 => 97%, FCT76/487 => 98%, AP Traveler 105/630 => 90%, TEC 140 f/7 => 87% and so on...

Some years ago on astronomie.de, H.Rutten calculated the on-axis weighted polychromatic Strehl (over 12 wavelengths and photopic sensitivity) of a standard 200 f/10 achromat, optimised at best focus to have 50% weighted visual polystrehl only!

Chris

[Mark9473]

I came across the following a few days ago:
http://www.astrotref...&TOPIC_ID=95328
I found the first graph very interesting; it compares the Strehl at different wavelengths between the LZOS 130/1200, the LZOS 115/805 and an unnamed 130/950 ED doublet.

It is immediately clear that looking at the Strehl at a single wavelength does not tell the whole story.

[Eddgie]

It is immediately clear that looking at the Strehl at a single wavelength does not tell the whole story.

Yes, and that was why I spend the time with the OP to explain what he was seeing when he looked at Rohr's test.

I have seen this before. People think that because they saw an achromat with .95 Strehl (in green light) that it is somehow going to be close to an APO in performance.

Now on this particular forum, people are often very quick to denounce the fact that Strehl reports for SCTs do not include the effects of central obstruction.

They will say "How can an SCT have a Strehl of .98 if the central obstruction lowers the contrast by 1/4th wave (which it does not, but that is a different story).

And yet when the look a the test for an achromat that shows .95 Strehl, the talk about how perfect refractors are.

The completely an totally ignore the effects of the unfocused light in all of the other wavelengths of light that are not being brought to best focus and are doing very real damage to the image, and more damage than a filter can ever correct.

Everyone loves to call the SCT ugly, but fast achromts are the ugliest of the ugly. Not only is the contrast much worse than a good APO, everything is purple to boot.

At least in the SCT, you don't have the ugly purple.

Anyway, this was indeed my intention with the OP. Published green light Strehl can only be really useful for comparing the fabrication quality of similar instruments. You could say "this 6" f/8 acromat is better than that 6" f/8 achromat" but without knowing the Polychromatic Strehl, you really can't say much else.

The link I sent above (Figure 74) really does have a great explination of these characteristics and I would hope that one day more forum members take the time to study this material.

And every time I see someone say "Strehl for SCTs is no good because it doesn't factor in the central obstruction" I will respond with "Strehl for refractors is no good because it does not factor in the damage caused by unfocused light.

And everyone will feel better!!!!

[Douglas729]

Ed

I was reading about the importance of Strehl ratio at the Royce Optics website and somehow must have had a brain cramp because I totally overlooked the importance of the Poly Strehl and how it factors contrast within an airy disk.

Nevertheless, your technical explanation taught me more than I previously knew and turning 64 next Tuesday is not helping one bit..in fact I may now be several bits short of a good 64 bit operating system.

[Eddgie]

I hope I did not get overboard in my answer.

It was a good question though, and the topic itself (Strehl vs polychromatic Strehl) is really a great topic.

If you have not had a chance to study the topic on the Telescope Optics website (the one that Figure 74 above is from), I think you might enjoy it very much.

I love that web site. It has so many excellent charts that explain so many things about how telescopes work!

Anyway, I hope that my answer helped and did not do more harm than good..

[Mark9473]

weighted polystrehl over 11 wavelengths from 422 to 678 nm was measured by the German amateur astronomer Kurt Schreckling and published at astrotreff.de.
Here are four out of eight results: TSA-102 => 97%, FCT76/487 => 98%, AP Traveler 105/630 => 90%, TEC 140 f/7 => 87% and so on...

It took a bit of a search but I have found that thread here.

I think some people are not going to like his numbers.

[Fomalhaut]

@Eddgie: I second each statement of yours in this thread.

@Mark: Thanks for finding the thread which I mentioned.
One additional remark: In the greenish painted Strehl chart provided in that thread, one sees (three columns most to the right!) that the only (plus explicitely stated!) reason for the FCT100 falling short of the TSA102 and the FCT76 was the specific sample's suffering from axial coma at the time of testing. Upon my inquiry, the German owner replied that after having fixed the problem, his FCT100 was on the same level as his (also tested) FCT76.

Here it becomes clear that the optical top of polystrehl-performance had already virtually been reached a quarter of a century ago, and this at least by Tak.
What's being done in today's visual market sector is mainly optimization of performance versus price...

Chris

[Eddgie]

I think it is healthy to have this kind of examination of the facts.

Often on this forum (and it spills over to other forums quite often) the attitude is that refractors are "Perfect" when the reality is that it is far more complicated than that.

I welcome these kinds of dialogs that explore how they differ and why someone might feel compelled to spend three times more on a super-APO than on an ED scope.

There is no free ride on the great wave of light that washes over us. Every kind of instrument has some compromise.

To the OP though, as hopefully has become evident by now, the e-line strehl is only really useful for showing the fabrication quality of a given sample, and by itself cannot tell the entire story.

For that, you need to really look at how the other lines are focused and for that you need the design itself.

By comparison, figuring the contrast loss caused by CO in a reflector is childs play.

[Daniel Mounsey]

Ed,

How would you rate or compare a 6" F-8 achromat against an 8" SCT for visual use such as star clusters for example.

[pbsastro]

weighted polystrehl over 11 wavelengths from 422 to 678 nm was measured by the German amateur astronomer Kurt Schreckling and published at astrotreff.de.
Here are four out of eight results: TSA-102 => 97%, FCT76/487 => 98%, AP Traveler 105/630 => 90%, TEC 140 f/7 => 87% and so on...

It took a bit of a search but I have found that thread here.

I think some people are not going to like his numbers.

It is clear the huge color correction superiority of air-spaced over oil-spaced. Given several scopes on each side, there is not margin for the cause being a bad sample. Of course oil-spaced have cool-down time and transmission advantages.
The tables presented are excellent. We have individual strehl for each wavelength, both focused at that wavelength and focused at green. And we have not only one poly-strehl, but one focused at each wavelength. Excellent.
Does anyone know of a similar measurement on a LZOS triplet, to compare LZOS design (small air-space) and Tak design (big air-space)?

[Mark9473]

Does anyone know of a similar measurement on a LZOS triplet

See my first post in this thread.

[Fomalhaut]

Does anyone know of a similar measurement on a LZOS triplet

See my first post in this thread.

=> which is two different LZOS and one standeard ED, chromatic strehl-curves of which (as the other eight mentioned above) also measured
by the German amateur astronomer Kurt Schreckling!!

Chris

[Eddgie]

Well, I would not own a standard C8 anymore either, so keep that in mind. I also no longer recommend these scopes either becaues the EdgeHD is the APO of the SCT world and just like I no longer recommend achromats, I no longer recommend standard SCTs.

As for the 8" SCT vs the 6" f/8, the bigger and brighter the cluster, the bigger advantage the coma free 6" f/8 had over the C8, and this was the factor that made me abandon the SCT design. When using the lowest power, widest field views, the SCT could not hold a candle to the 6" f/8 achomat.

But as the targets got smaller and dimmer, the SCT had the upper hand. Skipping ahead to Globulars, the C8 showed more stars in brighter Globulars and would better resolve the fainter globulars often showing at least granularity where the achroamt showed a compleatly unrsolved blur.

As I mentioned though, these days, just like with the achromat, I no longer recommend the standard SCT, and I no longer own any of them (except tha C5 which I use only for range work).

I would now recommend the EdgeHD designe over the standard design for pretty much the same reason... It is a much better telscope. Here the difference is at the edge of the field, where the Edeg HD can show every star that even my 6" APO can show at the edge of the field, with the same superb coma free, in focus excellence.

Here though, very interstingly, the EdgeHD8" does not enjoy the same level of advangage over the 6" APO that the 8" SCT did over the achromat. The 6" APO is far closer to the EdgeHD 8" on Globulars. It is hard to see a star in the EdgeHD 8" that can't be seen in the 6" APO. The 6" APO also will allow me to get close to the same granularity on fainter clusters than the 6" acrhomat would ever allow, because in that scope, faint Globulars were almost always not much more than a nebulous glow.

Roland Christen had written this in a magazine review maybe 20 years ago. He said that the APOs better energy contentration allowed it to almost equal an 8" SCT in terms of limiting magnitude. I had doubted that until I did the comparison myself and found it to be true.

But the Achromat could not come close to it. For GLobulars, I found the achromat to be less competent than even the standard SCT. The 6" APO is amazingly close though, and I really ahve to work at it to see more resolution in the EdgeHD than in the 6" APO.

So for larger brighter clusters, I would prefer the coma free achromat over the C8.

For Gloubulars and fainter clusters, I preferred the C8 over the achromat.

For wide field (up to the limits of the EdgeHD 8" of course), the 6" APO has no advantage over the EdgeHD. Both show the same amount of stars with the same fidelity at the edge of the field. The EdgeHD 8" has only the tinest of advantages over the 6" APO at the center of the field on clusters and globulars. I have to work to see it. Now this is in some ways a testament to both scopes becasue for the 6" APO to come so very close to the 8" scope shows the remarkable ability of the APO to concentrate energy. But the fact that the Edge HD 8" that costs 1/5th as much can hold its own on almost all targets but planets speaks volumes about the capabilities of the EdgeHD design.

I no longer recommend standard SCTs or achromats though. The designs have both been eclipesed by the modern ED for refractors (better performacne in every catagory) and the EdgeHD design (far better off axis performance than the older design, which could not even match the achromat here).

Not sure why you are asking though. It seems to me that you may have owned these scopes and have your own opinions.

But you asked and I am happy to answer with my own impressions.

[Daniel Mounsey]

No worries Ed, it probably seemed like a loaded question. Yes, I've conducted tests, but found your thoughts interesting nevertheless. Thank you

[Eddgie]

Well, the observations were not really my thoughts so much as they were the differences I observed.

Of course your own observations may have been different than mine.

When I have a question from someone that I think has a lot of experience like this, I do in fact feel like I am being baited in some way.

But I choose to believe that it is a friendly forum, so I answer what I can when I can, and to the best of my ability.

I skipped to Globulars because they are the extreme test for clusters and the easiest place to see the difference in these scopes.

I would hope people would extrapolate that to faint NCG clusters of which there are many many hundred in reach of the C8.

And I feel compelled to say it if anyone was not clear on on this. The C8 and the EdgeHD 8 are not at all different at the center of the field. The fact that the 6" APO almost matches the EdgeHD 8" on limiting magnitude is not because it is worse than the C8, but because the APO is simply that much better than the achromats were.

The globular performance is just the easiest and quickest way to compare the limits of the four instruments I mentioned.

[pbsastro]

Thanks Mark and Chris, I had missed that, my apologies.
The 130 f/9 has excellent results. On the contrary, the 115 f/7 has very poor strehl on red and blue. I expected it to be similar to the 130 f/9 but it seems similar to the AP and TEC oil triplets. They all (115 and oil triplets) look like semi-apos compared to the 130 f/9 and Taks, looking at the strehl curves alone…
Markus has this design (ideal) strehl curve of the 152 f/8 triplet on the APM website. This is close to the expected based on the 130 f/9 values and given the differences of aperture and f/ratio.

Pedro

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[pbsastro]

I wonder where in EU we can send telescopes for testing. Do Rohr or Kurt sell that service? Any other sugestion?
I would like to get for several wavelengths, strehl focused at green, strehl focused at the measured wavelength, and poly-strehl, as Kurt presents. I would also want transmission values at several wavelengths.

Pedro

[Fomalhaut]

Hi Pedro,
Just sent you a PM.
Chris