Swarovski ATX-115: Star-test and Resolution Measurements (1 Viewer)

[henry link]

I recently borrowed a Swarovski ATX-115 to conduct a few basic tests. The PDF below was assembled from one of the three artificial star-tests I conducted (indoors and outdoors night and day). This one was done outside at about 30 meters using a glitter point of the sun reflecting from a Christmas tree ornament to make an artificial star. The center point shows the star at best focus. The left and right images show approximately the same amount of defocus in opposite directions. As usual the photos are much less clear than the views at the eyepiece (at magnifications between 70x and about 150X), particularly the left image which is affected by air turbulence more than the others.

Several aberrations are evident. Firstly, and probably the most damaging is considerable spherical aberration, which causes the right defocused image to show strong diffraction rings while the left defocused image has no rings at all combined with a bright central spot. Coma is also evident. It causes the off-center placement of the central disc in the center focused image as well the asymmetry and uneven illumination in the unfocused diffraction discs. Longitudinal chromatic aberration is pretty well corrected, but the dominant red/magenta CA in the focused image is just visible to the eye at 70x (the lowest magnification I used) and quite obvious at 120X (using the Zeiss Tripler).

Put all this together and the result is a scope with somewhat worse than diffraction limited resolution, about 1.09 arcseconds (125/D) using a USAF 1951 glass slide. Owing to the large 115mm aperture that’s actually the best raw resolution figure I’ve measured for a spotting scope, approximately the equivalent of a diffraction limited 105mm scope and frankly better than I was expecting considering the scope’s aberrations.

However, as I have observed in the past measured resolution seems to be the last thing to go in aberrated optics. The image quality at higher magnifications is already visibly compromised by the time the resolving power drops below diffraction limited. The scope I use as a reference is virtually aberration free (Astro-Physics 93mm Stowaway). Its resolution measured 1.21 arc-seconds at the same time and using the same set-up as ATX-115. Even though the Stowaway has lower raw resolution its image quality actually looks better than this particular ATX-115 because its aberrations are so much lower. At the same magnification it has a cleaner, higher contrast image quality. In comparison the Swarovski image looks like there is a layer of gauze stretched over it, something I have seen many times in scopes with aberrations high enough to prevent diffraction limited performance, which itself should be considered the floor for high quality telescope performance, not the ceiling.

Even though this particular specimen of the ATX -115 did not turn out to be a scope for which I would willingly shell out $5400 USD, I think if I could find one without the coma and with about half as much spherical aberration it could be a contender to replace the Stowaway as a lighter waterproof birding scope.

 

[DRodrigues]

Thanks Henry.

It would be great if you could evaluate the effect of the 1,7x extender on the aberrations you observed.

 

[henry link]

Sorry to say I didn't have a chance to try the extender.

 

[jring]

Hi Henry,

thank you for doing the star test and posting the results, that will be a nice instruction piece for those wanting to learn star testing in here...

A pity that this one ever left the factory, I hope it goes back to Absam with your image and findings. I don't think the extender will help a bit in this case - if the image is already slightly blurry at 60x, it won't get better from there.

Joachim

 

[henry link]

Thanks Joachim. Bad as the star-test looks I don’t think very many people would complain about this specimen. The aperture is so large compared to the normal magnification range that to some extent the warts don’t show. It’s just like an 80mm scope with a top magnification of only 48x. My first impression at 70x was positive. The problems only became apparent by directly comparing that 70x image to a better scope. I’m confident there are units without coma, but finding out how well spherical aberration is corrected in the best of these would require star testing many more samples.

Henry

 

[jring]

Hi Henry,

yes, I believe that at 70x the image seemed ok at first glance, after all the 95mm version also goes up to 70x and has 30% less front element surface.

I'd say that both the coma and the SA both are worse than diffraction limited on their own and together make for a quite bad instrument... that can be recognized at first glance at an artificial star or a quick measurement with a commercial interferometer.

I am still amazed that cheap chinese ed refractors are guaranteed to be diffraction limited (and it's hard to find worse examples which are exchanged w/o discussion when found) while alpha spotter manufacturers don't make any assurance about the optical quality and sometimes tend to get defensive when called out...

Joachim

 

[etudiant]

Hi Henry,

yes, I believe that at 70x the image seemed ok at first glance, after all the 95mm version also goes up to 70x and has 30% less front element surface.

I'd say that both the coma and the SA both are worse than diffraction limited on their own and together make for a quite bad instrument... that can be recognized at first glance at an artificial star or a quick measurement with a commercial interferometer.

I am still amazed that cheap chinese ed refractors are guaranteed to be diffraction limited (and it's hard to find worse examples which are exchanged w/o discussion when found) while alpha spotter manufacturers don't make any assurance about the optical quality and sometimes tend to get defensive when called out...

Joachim

Sounds like a real market opportunity.
If the Chinese can put documented optics into a robust and waterproof body at a price in the hundreds of dollars, rather than thousands, they will deservedly crush the current market leaders.
I'm actually very surprised that Nikon has not gone that route, they know both the optics as well as the market and they have the needed relationships in China.

 

[Hermann]

Hi Heny,

Thank you for your report and the star test. It shows once again that even a manufacturer like Swarovski doesn't put enough effort into the manufacturing process. Like Joachim I find the test results unacceptable for a product in that price range. This scope can be used with the extender, and at the resulting magnification the image will surely be well below par for what's possible. In fact, even if used without the extender the image quality will be well below what it should be even though many people won't realize scopes like these are in effect lemons. Nobody expects a spotting scope to be as well-corrected as your Stowaway but no scope from a premium manufacturer should have such a bad star test.

I agree with Etudiant, unless Swarovski (and the other major European manufacturers) put their house in order and make sure such scopes don't get to customers they may well get crushed if some Chinese manufacturer starts making scopes that undergo a vigorous quality control - probably at a cheaper price, too.

Hermann

 

[Binastro]

Is the ATX-115 scope too fast and too ambitious to produce very good image performance?

The ED astro refractors are maybe considerably slower and simpler.

Regards,
B.

 

[jring]

Is the ATX-115 scope too fast and too ambitious to produce very good image performance?

The ED astro refractors are maybe considerably slower and simpler.

Regards,
B.

Hi,

the ATX 115 is extremely fast at f4.8... the usual suspects of chinese ED refractors are in the f6-f7.5 range - so quite a bit slower... but if chinese manufacturers manage to test every objective and remove the bad ones, it should be possible for the teutonic trinity too.

The fact that an overly ambitious focal ratio makes the instrument difficult to manufacture should not be the reason for customers getting sub-standard optics.

Joachim

 

[Binastro]

I agree.

Thinking about it, Televue made Petzval refractors capable of 200x magnification.

I also had a Tewe photographic Petzval lens, 600mm f/5, that gave beautiful high power views of Saturn and Jupiter visually with an eyepiece. Perhaps made in the 1950s.
It is rare for photographic lenses to perform this well visually, but it does happen.

I think that both the Harpia and ATX-115 are getting into photographic lens territory, but even so they could be made well enough to take 150x magnification.

Regards,
B.

 

[henry link]

Before I borrowed the scope I scoured the internet for any relevant optical tests of the ATX-115. The closest I came was this test of a 95mm module at Cloudynights.

Swarovski ATX 95 optical test - Refractors - Cloudy Nights

Swarovski ATX 95 optical test - posted in Refractors : I received optical bench test report of Swarovski ATX 95 spotting scope objective module. Here is some pictures. Setup: Summary: Star test: Ronchi and Foucault: A few comments from the tester. Probably some people...

www.cloudynights.com

A Strehl of 0.867 doesn't give it any bragging rights in the world of astronomical APO's, but at least that unit is diffraction limited and would probably have done better if the prism could have been included.

Before I tested it my doubts about the 115mm mainly centered on longitudinal Chromatic Aberration, which actually turned out to be pretty well corrected, suggesting a possible change to an ED glass with a higher Abbe value. I wasn't exactly surprised to see so much spherical aberration in an f/4.8 scope, but I have a copy of an interferometer test of an Astro-Physics 93mm f/5 triplet with a Strehl of 0.99 and a wavefront error of 0.014 RMS, so if the design goal is a no compromise scope it can be done at around f/5.

I don't think spotting scopes are doomed to have second or third rate optics. All five of the 82mm Nikon MonarchEDs I tested over the last year or two were optically better than this particular 115mm Swaro. Three of them were much better, and probably have Strehls well above 0.90. True, the Monarch's focal ratio is a little above f/6, but the optical design of the objective/focuser groups is quite similar to most current spotters including the Swaros.

Henry

 

[steph.661]

Excuse my ignorance on the subject, but the quality of the eyepiece alone can influence the results?

 

[DRodrigues]

...

Swarovski ATX 95 optical test - Refractors - Cloudy Nights

Swarovski ATX 95 optical test - posted in Refractors : I received optical bench test report of Swarovski ATX 95 spotting scope objective module. Here is some pictures. Setup: Summary: Star test: Ronchi and Foucault: A few comments from the tester. Probably some people...

www.cloudynights.com

A Strehl of 0.867 doesn't give it any bragging rights in the world of astronomical APO's, but at least that unit is diffraction limited and would probably have done better if the prism could have been included.

...

As I mentioned at the CN link and also at Swarovski_X_derivations, the X Objective modules are made to be corrected by the X ocular modules so the test done isn't valid for all scope.
I wasn't expecting that you detect such amount of aberrations since the Swaro quality control is usually considered as one of the best and lower quality samples shouldn't exit the factory to the market...

 

[Binastro]

It may be that comparing the 115mm ATX with the 93mm perfect triplet is not comparing like with like.

I have not seen either scope, but the Swarovski may be more like a telephoto design with a shorter physical length than the 550mm focal length would suggest.

It may be that the 93mm triplet is 3 elements closely spaced at the front.

Telephoto lenses do not test well compared with traditional scopes.

However, the Swarovski should be much better than it is.
It may be that respacing the elements would help, but it seems there is more wrong than that.

Regards,
B

 

[jring]

Excuse my ignorance on the subject, but the quality of the eyepiece alone can influence the results?

Hi,

while it is possible for a bad eyepiece to create artifacts during a star test, it is normally not an issue if it's not really bad and you keep the star in the center.

What Henry probably was referring to when saying "...but at least that unit is diffraction limited and would probably have done better if the prism could have been included." that the ATX series (as most spotting scopes) are designed in a way that the glass path of the prisms (which introduces SA and spherochromatism) is corrected.

With the ATX the prisms are in the "eyepiece" module, which would be more appropriately named "everything beside the objective lens" module.

PS: As for the AP Stowaway with a 0.99 Strehl... yes, it's possible if Roland has a good day, but Astro Physics tends to go the extra mile for their optics like taking delivery of a batch of optical glass, grinding a prism and measure it and then optimize the design for that batch while others take the data from the spec sheet... or having Roland aspherize some elements by hand.
They also tend to have a rather long waiting list... a few years for some models...

Joachim

 

[DRodrigues]

...
I'm actually very surprised that Nikon has not gone that route, they know both the optics as well as the market and they have the needed relationships in China.

The Nikon Monarch series are made in China, so they already went...

 

[etudiant]

The Nikon Monarch series are made in China, so they already went...

That is smart on Nikon's part. Good on them! Now they just need to go the extra mile and document their optics.
Judging by Henry's comment that all five he tested were good to excellent optically, that should not cost them extra.
Factual data differentiating Nikon from everybody else who just uses advertising blather should be powerful.

 

[kabsetz]

Thanks Henry both for your report and for sharing the X95 optical test bench report. Both very interesting.
Let's hope Tammy gets an X115 tested and shares the results - and let's hope he lucks into getting a good sample.

I have done some artificial star assessments on the X115's this year, but not at great length and not with a particularly excellent sample yet. However, like Henry, I have been seeing less chromatic aberration than I would have expected - on average about same as I have come to expect for an X95. Most of the 115's have not had any astigmatism to mention, but thus far all have exhibited more SA than I would like to see in a scope of my own. Coma has varied, but the best ones certainly had much less than what Henry's sample had. As for the descriptions Henry gave for how the image looks, my experience is very similar when comparing my cherry X95 to these X115's. With a ridiculous magnification of 360x (max zoom, XTS 1,7 + Zeiss 3x12 mono), with my ATX95 I see a best-focus Airy disk having a very thin first diffraction ring around it. When slowly de-focusing, the central spot nicely curls into a doughnut, and this happens almost identically in either direction away from focus. With the ATX115, de-focusing toward outside focus the central dot remained until at least three diffraction rings of de-focus.

- Kimmo

 

[henry link]

Thanks for responding, Kimmo. I was hoping you would add your experiences.

I doubt that I will have a chance to test very many more of these, but maybe you or someone else with better access will eventually turn up a specimen with spherical aberration as low as a good 95mm module, so we can know if such a thing exists.

Henry