1204 replies to this topic

[peleuba]

Preface

 

I kindly ask that no other DPAC images be added to this thread so as not to confuse the reader.  Unless it’s of the same make/model scope that I’ve tested (SVX180).  Hopefully a robust exchange of ideas and opinions will follow especially how it relates to claims and verification as well as thoughts on what is truly important in a high performing Apochromat.

 

===============

 

Look, you know it, I know it - it’s never been more confusing to buy a high-end telescope…  We’ve got test reports, supply chain crunches, and lotteries     to navigate.  And, for those lucky enough to purchase an APO, questions remain.  I mean, how many threads have come/gone with the OP asking how to interpret the test report? or how can I to tell if my scope is good or bad?

 

I’ve been a member here on CloudyNights for nearly 19 years; I’ve been an amateur astronomer more than twice that long.  And, in all this time, I don’t ever recall seeing such a keen interest in testing refractors and the metrics used (Strehl, PtV, RMS) to quantify, describe and communicate those tests to the amateur astronomer.  Why all the interest?  Might it be:  (1)  Fancy looking certs being distributed with the OTA’s;  (2) Manufactures making claims of unverifiable quality;  (3) Manufacturers making no claims of quality other then it “meets our standard”.  The reasons run the gamut, but all have to do with that slippery term “quality” and how its defined.

 

A few weeks back I was asked to check out and evaluate a StellarVue SVX180 F/7 APO. The owner dropped it off in late February and I had possession of it for 6 days/evenings spending about 20 hours in total crunching the data, processing the photos and writing a report.  This was the first time I’ve had a critical look at any StellarVue offering.  Sure, as I’ve written previously, I’ve seen a couple of SVX152’s at Cherry Springs but have never critically viewed through one.  Please understand that at low power, or if a camera is in the focuser hooked up to a laptop, every scope looks like it’s a Strehl of 1.0.

 

I’ve always been a straight-shooter, often with an abrupt but to-the-point delivery.  My tone can be terse, but it’s not personal, rather, like a lot of you, I grow frustrated with all the handwringing and debate that occurs over, what amounts to, hearsay, rumor, and anecdotes labeled as truth.  Nothing beats a good set of facts and that’s what I deliver as quickly, concisely, and directly to as many as possible.  In other words, you may not like the messenger, but the message is usually on-point.

 

I am reminded of an exchange I recently had with another CN member.  In a calmer moment, he asks why I place such an (over)emphasis on bench testing?  A fair question and easy one to answer… Testing, in this manner, is the only neutral arbiter of a telescope’s quality. 

 

At the end of the day, I care deeply about the collective joy this hobby brings to us all and am empathetic to the trials and tribulations we as observers go through when trying to determine if a telescope is good/bad/mediocre/great.  It’s rarely easy and is compounded by an unprecedented level of marketing and misinformation bantered about.  Vendors and manufacturers don’t participate often on CN, so it’s up to us to keep cognitive dissonance, hearsay, and innuendo from becoming the memes of CloudyNights.

 

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First Impressions

 

From a fit and finish standpoint, StellarVue got this right.  For decades I’ve admired the look of the StellarVue OTA’s even when I disagreed with the company’s message.  These SVX optical tubes look like telescopes and that’s important me.  The SVX180 – as all large APO’s are, is nose-heavy.  My sense is the OTA weighs in at about 40lbs (I’ve not verified this).   Even so, StellarVue did a terrific job of removing as much excess weight and bulk from the overall package. 

 

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Test Method

 

Over the course of several days, I performed multiple tests utilizing several different methodologies but will focus on Ronchi Autocollimation (aka “DPAC”) because it’s the current craze here on CN; is visual; is easy to understand; and has few places in which error can creep in to skew the results.  In other words, its highly reliable, dependable, repeatable, and defendable.  It’s perfect for determining if an optic is good/bad/mediocre/great.

 

I tested in three wavelengths R-G-B plus white.   My commentary on the actual test will be brief as the bench photos tell the story.   Thank you for reading this far!

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Edited by peleuba, 13 March 2023 - 09:52 AM.

[peleuba]

Green

 

• In a best case scenario, and I'm being generous, this shows about ¼ wave of overcorrection.

• The second photo shows a software model written by CN member Maciek and performed by my friend Jeff B. computing PtV, RMS and Strehl.  While only an estimate and not to take the place of an interferometer, it does get quite close and is good enough for this exercise where I am trying to determine Good/Bad/Mediocre/Great.

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Edited by peleuba, 13 March 2023 - 09:56 AM.

[peleuba]

Blue

 

• ½ wave of overcorrection.  This is the wavelength that the scope tests worst in.

• Not much else to say…  the lens is way out in blue.

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Edited by peleuba, 13 March 2023 - 01:45 PM.

[peleuba]

Red

 

 

• Nearly perfect; very close to ⅒ wave.  The SV Interferogram highlighted a Strehl of .994.  The model gets close at .989.  Overall correction is nearly neutral.  The lens is clearly nulled in this wavelength, which is rather curious. 

• Red is not useful to visual observers as the retina is not overly sensitive to red light.  This is why we use red lamps preserve our night vision.

• CCD lenses are extremely sensitive to red and the deep reds (into the IR).  Perhaps the red null keeps stars from bloating during long exposure photography.

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Edited by peleuba, 13 March 2023 - 01:44 PM.

[peleuba]

White

 

• Overall ~ ⅕ wave overcorrection with a rough surface filled with zones and micro-ripple.  

• White is not really a wavelength, rather, it’s the combination of all visible wavelengths.  White gives a reasonable indication of general performance including any chromatic effects of the lens.

• As in green, this lens is barely diffraction limited in white and the surface is rough.  There are several zones which will tend to reduce contrast.  Carl Zambuto would also remind us that the amount of micro-ripple left over from polishing would hinder the contrast, too.

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Edited by peleuba, 13 March 2023 - 11:15 AM.

[peleuba]

Conclusion

 

Typically, refractors are nulled (best corrected) in green while the red and blue wavelengths are balanced but of opposite correction.  As an example, blue may be overcorrected by ¼ wave and red will be undercorrected by the same ¼ wave.  This is the hallmark of a high performing visual instrument – especially when the optic is smooth and green is the nulled wavelength.

 

The lens was described to its owner as being a “world class optic”.  This is not what I would consider world class - not even close.  Its barely diffraction limited in green, way out in blue and perfect in red.   For the visual astronomer, it’s not a good choice.   However, not all is lost – it makes a terrific Hydrogen Alpha solar scope and will be quite good as an astrograph.  But you don’t need a 180mm lens for solar work nor astrophotography.

 

Generally speaking, nothing pushes my buttons harder than marketing-speak from vendors followed by  some uniformed comments in the forums.  We’ve all seen it and have all participated in it.    The language of optics is mathematics - NOT marketing!  There are few shades of grey.  Reality can be harsh but is a much more useful place in which to operate.  This is when bench testing is as its most powerful.

 

In any event, I had high hopes for this telescope for three reasons:

 

(1) Just looking at it resting on the bench makes me want one.
(2) It’s the current flagship of the StellarVue lineup. 
(3) The SVX lenses are (reportedly) made from scratch - start to finish - onshore in Auburn, CA.

 

There will be dissenting opinions on my test results and that is welcomed.  We are all different and harbor differing points of view.  However, that does not entitle critics of me or of the testing to impart their version of reality into the discussion.  I assure you these results are 100% accurate, repeatable, and defendable.

 

You may begin to hear that red light is the “industry standard” for testing so it makes sense that optics  are nulled (best corrected) in red.  This is certainly true for reflective optics (mirrors) whereas the results gleaned in the red wavelength are easily scaled to green (or any other color) using simple Algebra.  Not so in a lens-based system; at least not until the focal ratio approaches ~F/15 because of spherochromatism.

 

My thoughts regarding this “industry standard” of red…  Well, LZOS tests in green.  And Roland Christen has often said that performance in green is most important.  Finally, when star testing, it’s quite common, for best results, to use a green filter.

 

 

To summarize, Its impossible to test in red and extrapolate the result – with ANY confidence - to green in a 180mm F/7 refractor.  This is precisely what my testing showed.    For the visual observer wanting an excellent, general-purpose APO, green/green-yellow is the best wavelength in which to have the scope nulled.

 

Red is a longer wavelength then green, so any surface error (peak or valley) will appear less in amplitude in red then it does in green.  In other words, it’s conceivable (probable?) an optic will test great in red and less so in green as the amplitude of error is larger in green then it is in red even though the test report describes the same error.   In aerospace and industry applications requiring reflective optics, the standard may be red.  But for amateur astronomy where one puts an eye to the eyepiece, green is the wavelength that’s most important.

 

In closing, I want to be clear that this is a sample size of one.  But the scope did pass through the quality assurance at StellarVue and (presumably) met the design criteria and goals of the company well enough so that it could be shipped to a paying customer.

 

After testing a lot of telescopes at the premium level I’ve found that sample-to-sample variability does not exist nearly like it does in the mass-produced segment.  The SVX180 is supposed to be a “flagship” type of telescope for StellarVue.  Sure, there will be minor deltas between scopes within the same run, but they’re typically not material to overall performance.  In other words, considering StellarVue’s excellent customer service and QA program, my sense is this sample is representative of other SVX180’s.  The smaller apertures in the SVX lineup may well be better corrected.  But testing would be needed to confirm as these all have a stratospherically high Strehl in red like this SVX180 does.

 

Thank You, and Very Best Regards.

 

Paul Leuba

Edited by peleuba, 13 March 2023 - 09:45 AM.

[tboss70]

[tboss70]

As someone who knows really nothing about this, doesn't DPAC make the results look twice as bad?

"Yes, DPAC makes the lens look twice as bad. The error you see are multiplied by a factor of 2"

Edited by tboss70, 13 March 2023 - 10:08 AM.

[Erik Bakker]

Paul,

 

Thank you for your report and substance in testing that sample of the StellarVue  SVX180 f/7.

 

Fortunately, scope optics follow the laws of physics and mathematics and will always do.

Thorough testing illustrates that quite elegantly, though the results can surprise those deafened by marketing and hear say, with internet being their playground.

 

Under the stars, an experienced observer can quickly tell what the substance of a particular optic is.

As can an experienced tester after performing a thorough DPAC test.

 

We should remember that the sample size is 1. But it is telling in the case of such an expensive flagship telescope.

 

What I do know is that large triplets like this 180mm take a looong time to reach thermal equilibrium. But I guess you had that tackled over the course of the days you spend testing it.

[peleuba]

As someone who knows really nothing about this, doesn't DPAC make the results look twice as bad?

"Yes, DPAC makes the lens look twice as bad. The error you see are multiplied by a factor of 2"

 

Yes.   DPAC stands for Double-Pass Autocolliomtion.   Which is redundant in and of itself.   Autocollimation is a method of testing that uses double-pass via an optical flat.  Ronchi is the actual test. On the bench, the errors are twice as bad as what is in reality.  This is what makes it such a powerful tool.

 

In other words, the images are twice as bad as you would see if using a Ronchi screen on a real star in single pass mode.  However, the wavefront estimates reflect reality.   The lens is no better then ¼ wave in green and likely worse because of the zones and rough surface. 

Edited by peleuba, 13 March 2023 - 10:22 AM.

[zjc26138]

Thanks for doing the testing and writing the report! 

[Erik Bakker]

The results of your DPAC testing many scopes are similar to my experience with startesting many scopes over the decades. Not all are created equal and on top of that, sample variation exists. Thats why these days I only buy a scope after I have tested it under the stars. And when it’s good, it generally stays with me for a long time, bringing a lot of joy in use.

[Erik Bakker]

The simple version: perfect optics never need downplaying of test results.

[bobhen]

Thank you for all of your hard work on this article and for your exacting attention to detail. The refractor community owes you a big "thank you".

 

You can add TEC to your list of manufacturers that also tests in green.

 

As far as the results go, I could say a heck of a lot right now but I will just let the results speak for themselves.

 

Maybe I missed it, but I did not see a price in your article, SV lists this scope at $18,395. Not a small amount of change. For those interested in purchasing a scope with the same specs, TS and APM also offer a 180mm F7 triplet for around $4,000 "less" than the SV 180 F7. APM lists delivery in 4 to 8 weeks.

 

Thanks again.

 

Bob

Edited by bobhen, 13 March 2023 - 11:00 AM.

[peleuba]

Thanks for doing the testing and writing the report! 

 

Very glad to do it.  I stand on shoulders of giants (Dave G, John B, Roland C,) who've all helped over the last two decades hone my skill set. 

 

Regarding the scope, I was extremely surprised at the result, like seriously surprised.   The scope was sent to me to investigate a possible de-centering of the center element.   After I ruled that out, I put it through a suite of tests...   I was expecting an excellent optic based on all of the test certs getting posted and the absolute rave reviews here on CN of the SVX series.  

Edited by peleuba, 13 March 2023 - 10:58 AM.

[peleuba]

Maybe I missed it, but I did not see a price in your article, SV lists this scope at $18,395. Not a small amount of change. For those interested in purchasing a scope with the same specs, TS and APM also offer a 180mm F7 triplet for around $4,000 "less" than the SV 180 F7. APM lists deliver in 4 to 8 weeks.

 

 

TS seems to carry a lot of the same apertures and focal lengths as the SVX line...  Probably just a coincidence.  

 

I do want to mention something...  The OTA is really, really nice.  It really is a terrific package.   A lot of time went into pairing it down to be as light as possible.  The fit/finish are excellent. 

[Nippon]

I was hoping the SVX 180 would get a glowing report, and I'm sad to read that it did not. I had a Stellarvue 102 mm triplet for a while that was from just before the SVX line. The optics were good but not super excellent. I read so many heated discussions about Stellarvue's marketing claims. After the SVX line came along all that controversy seemed to disappear.

[Erik Bakker]

That is the good thing, fit and finish of the OTA.

[Bill Barlow]

Paul, have you or the owner of this SV 180 had it out under the night sky for any visual,observing?  I guess your message is beware of scopes that only test in red wavelengths of light.  

 

Bill

[nicknacknock]

I would rather have pristine optics in a decent-ish tube, rather than mediocre optics in an excellent tube for 18K optimized in red…

[turtle86]

Excellent work, Paul!

[peleuba]

That is the good thing, fit and finish of the OTA.

 

It was beautiful...  Someone who knew what they were doing designed it.  I've always liked the StellarVue look and feel.

[ckwastro]

Great write-up Paul! Thanks for sharing. 

[Polyphemos]

A wonderfully balanced and objective report.  We need more of these; many more.  Thanks Paul!

[tboss70]

This image with an SVX180 seems to show violet hue around stars. Interesting, though I am not expert enough to determine if this is somewhat 'processing' related, or other.

Sure makes you wonder considering the blue test above.

 

https://www.astrobin...4/0/?mod=&real=

Edited by tboss70, 13 March 2023 - 11:45 AM.