104 replies to this topic

[Scott in NC]

I recently acquired a Stellarvue SVX127D for testing.  This is an f/8 doublet apochromatic refractor which is manufactured in Stellarvue's facility in Auburn, CA, with hand-figured optics using a Hoya FCD100 ED element and a lanthanum mating element, as documented on the Stellarvue.com website.

 

As many of you know, I've DPAC tested a number of scopes recently, but I've had a particular interest in obtaining a scope from Stellarvue's SVX series for my own independent testing ever since the report came out on the now infamous SVX180 in March.  While I'd love to test out another sample of a SVX180, I don't currently have the equipment to test a scope that large with any precision, and frankly I don't know if we'll ever see another SVX180 independently tested, as I don't believe that that many have been produced to date.  But what I was really interested in seeing was whether some of the findings from that particular SVX180 could be attributed to the SVX line as a whole, or whether they were only reflective of the SVX180 model (or possibly just reflective of that one particular SVX180).  Prior to testing this particular SVX127 my theory was that the entire SVX line likely had similar attributes, but given the relative ease of fabricating a smaller doublet vs. a larger triplet, we'd likely see that a smaller aperture SVX doublet would likely test better than a much larger SVX triplet.  I'm hoping to eventually get my hands on a 102mm scope from the SVX line, but that will have to await another day.

 

I'm going to present my findings from the SVX127D testing in detail here, but what I do not want to happen is for this thread to degenerate into a 27-page heated argument, where we spend the first page discussing the facts, and then the next 26 pages heatedly arguing with each other.  I'm happy to discuss the merits of DPAC testing, and greatly welcome the opinions of others who are way more knowledgeable than I about testing optics.  But personal attacks will not be tolerated here, whether they be attacks upon me, Vic Maris, other optical testers, etc.  And one more thing I'd like to add.  One perceived "flaw" of the prior SVX180 report was that the author may have had an anti-Stellarvue bias.  Well, this present report is coming from someone (me) who has owned Stellarvue products ever since I bought my very first refractor brand new from SV in 2004, an SV80/9D doublet achromat.  I still have it by the way, and have accumulated quite a few more SV scopes, cases, a mount, a tripod, eyepieces, RDFs, dovetail rails, and other assorted gear over the years.  Regardless, I'm presenting this information "warts and all."  I guess some of you could accuse me of having a "pro-SV" bias, but as you'll see, I'm trying to be as objective as possible, and don't plan on holding anything back.  And for full disclosure, I've let Vic Maris know that I tested this scope and would be publishing a report on CN.  I'd welcome any comments from him too, but please everyone keep in mind that anything that Vic says here has to be confined to the restrictions that our Terms of Service place on vendors.

 

Here’s the scope set up on my optical testing bench (a.k.a., dining room table).

 

[Scott in NC]

First I'm going to present the Zygo interferometer report as supplied by the manufacturer, and as has been previously reported, the SVX models made up until March 2023 were all tested in red.

 

 

Notice that this looks fantastic in red, with Strehl 0.991.

[Scott in NC]

Now for comparison, I present the Ronchigrams from double-pass autocollimation (DPAC, or more accurately, Ronchi autocollimation) testing.  First are the images in red:

 

Inside focus:

 

 

 

Outside focus:

 

 

When DPAC tested in red light, the results correspond reasonably well to what we see with Zygo interferometry in red.  Notice how straight the Ronchi bands are, indicating very good control of spherical aberration.  There may be a very slight upward turn of the edge, which I really only see on the inside focus image, so I'm not going to say with any authority that it's really there, and may be more of a camera artifact.  But regardless of whether it's real or not, I don't think that this would be visible at the eyepiece.

 

Edit: The more I look at this, the more I'm convinced that I see a very slight hint of overcorrection in red, and mainly in the outside-focus Ronchigram.  I'll admit that my images are quite a bit larger than some of the other DPAC-testing CN members have previously posted, which really accentuates any imperfections that smaller images may inadvertently hide.

 

Edit #2: Thanks to a correction made by another member, I’ve corrected the direction of the slight edge aberration from “downward” to “upward.” And now that I’ve gone back and rechecked the Zygo IF report, I can see that it was noted there as well.

Edited by Scott in NC, 23 April 2023 - 09:39 AM.

[Scott in NC]

Now here are the DPAC images in green.  As expected for optics figured in red, green shows a little less tight control of spherical aberration (with some mild overcorrection), but still respectable IMO.

 

Inside focus:

 

 

 

At focus:

 

 

 

Outside focus:

 

 

The finding that concerned me most when I started testing this scope was the concentric rings (or zones) which are seen most easily on the in-focus image, but also on the intra- and extra-focal images.  But after discussion with others, my impression is now that these are indicative of the hand-figuring that was performed when optimizing these lenses.  I've learned quite a bit about testing optics since I started this journey, but I'll admit that I'm still not very knowledgeable about the actual process of making optics, so I welcome input from others who know way more than I do about this subject.

[Scott in NC]

Now for testing in blue.  Once again, figuring in red (at the long end of the visible light spectrum) may come at a detriment to accuracy in green (in the middle of the spectrum), with even further fall-off in blue (at the short end of the spectrum).  To illustrate that, remember the old mnemonic ROYGBIV?  Figuring at a wavelength in the middle (ideally green) will still result in some fall-off at the ends, but less so than if optics are figured at one of the ends (typically red, or else blue/violet, but no one would actually do that).

 

Inside focus:

 

 

 

Outside focus:

 

 

[Scott in NC]

Finally here are the white light images.  These are useful for judging control of chromatic aberration.  As expected with an FCD100 and lanthanum doublet at a focal ratio of f/8, this scope does quite well.

 

Inside focus:

 

 

 

At focus:

 

 

 

Outside focus:

 

[Scott in NC]

The bottom line is that this scope appears to have fairly decent optics.  Not bad by any means, and certainly much better than the SVX180 that was previously reported here on CN.  I have not yet had the opportunity to test this scope under the stars, but plan to do so as time allows.  And of course it's thunderstorming this weekend, so any star testing may have to wait until at least the following weekend.  I don't have the ability to test the suitability of its optics for astrophotography, but I suspect that it will provide admirable views for visual observation.  I don't consider it to have "world-class" optics along the lines of any of the scopes recently produced by Astro-Physics, but then this is a scope that's not in the same price class, and is also a scope that you can actually have now, rather than entering a lottery for the chance to buy or joining a decade-long waiting list.

 

And with that, I'll be happy to entertain any questions that anyone may have.  Thanks for reading!

[Scott in NC]

One thing that I forgot to mention.  This review is only applicable to the SVX127D samples that were produced prior to March 2023 (i.e., those that were tested and figured with the assistance of a Zygo interferometer utilizing a red laser).  Per the information on their website as well as information that the company owner Vic Maris has posted on CN, Stellarvue has since changed their manufacturing process to start hand-figuring their optics to optimize them in green light, and is now using a combination of Zygo and DPAC testing rather than relying upon red-light Zygo testing alone.  One day I'd love to get my hands on a newer iteration of the SVX127D, but that will have to await another day.

[PhotogTom]

My first instinct would to be to compare it to the AT125EDL. Side by side comparison under good skies.

[Scott in NC]

My first instinct would to be to compare it to the AT125EDL. Side by side comparison under good skies.

That's an excellent suggestion, Tom, especially since I just happen to own one of those.  Another reason that that would be a good scope for comparison is that despite one being a hand-figured scope by a U.S.-based manufacturer and the other a scope imported by a U.S. vendor from a China-based manufacturer, the optical specs are pretty similar (127mm vs. 125mm, f/8 vs. f/7.8, and both utilize Hoya FCD100 and a lanthanum-based mating element).

 

I've previously posted DPAC results from my AT125EDL (although need to redo the images one day), but have yet to compare the scopes side-by-side under the stars.  Regardless, we'll still have to take care not to make any generalizations from that testing, as all we'll be able to definitively say is how one specific SVX127D compares to one specific AT125EDL. Still, it should be interesting and fun to do when time and weather allow.

[Joe G]

Looks similar to the TS Photoline 115 Ronchis I just posted.  Really nice in Green and red.  Blue lagging.

 

What is nice though is the comparison to the reported strehl of 0.991.  While this is likely a nice scope.  It isn't really a polychromatic strehl anywhere close to that.

[Scott in NC]

 

What is nice though is the comparison to the reported strehl of 0.991.  While this is likely a nice scope.  It isn't really a polychromatic strehl anywhere close to that.

Thanks for your input, Joe.  I do think that in red it's probably reasonably close (unless, and I have no way of determining this, the hand-figuring marks, or zones, or whatever they are, actually degrade the Strehl value more than the minimal amount of SA would indicate).  But I do agree that the polychromatic Strehl isn't likely anywhere near 0.991.  And I base that upon what we see with the blue images, as you did with your scope.

[John Huntley]

I wonder if it was sold (new) as a scope that had a Strehl of 0.991 ?

[Scott in NC]

I wonder if it was sold (new) as a scope that had a Strehl of 0.991 ?

It’s possible that I’ve misunderstood your question, but yes, the scope was sold new accompanied by the Zygo interferogram report that I’ve shown in post #2 above.  I’m sorry for the poor reproduction of that report, but that’s all that I have in my possession.  And yes, I’m sure that that’s the report that belongs to that particular scope, as the serial numbers match.

[Lagrange]

Looks similar to the TS Photoline 115 Ronchis I just posted.  Really nice in Green and red.  Blue lagging.

 

What is nice though is the comparison to the reported strehl of 0.991.  While this is likely a nice scope.  It isn't really a polychromatic strehl anywhere close to that.

The report is quoting monochromatic Strehl at 633nm so there isn't necessarily any mismatch with the DPAC results. I don't think any manufacturer provides a measured polychromatic Strehl for their scopes - anything like that is given as a calculated value.

[Scott in NC]

I’ve had questions about whether the interferogram report showing a Strehl of 0.991 suggests that the manufacturer advertised this scope as having a poly-Strehl of 0.991.  The answer is no—this was measured in one color, red.  And even though I don’t believe that it was initially made clear as to what color laser the Zygo interferometer used, it should be clear that an IF test is only capable of measuring the Strehl in whatever particular wavelength of light that interferometer is calibrated for. I don’t believe any manufacturer would claim that their scope had a poly-Strehl of 0.991. To the best of my knowledge polychromatic Strehl is a value that can be estimated via calculation, but really can’t be directly measured.

[Joe G]

The report is quoting monochromatic Strehl at 633nm so there isn't necessarily any mismatch with the DPAC results. I don't think any manufacturer provides a measured polychromatic Strehl for their scopes - anything like that is given as a calculated value.

I understand that it was tested in red as has been discussed.  The red DPAC ronchis look good. 

 

I wasn't suggesting that they were quoting a polychromatic strehl.  Just pointing out that the blue was below the red and the green.

 

As I said it looks a lot like the DPAC test I just did on the TS Photoline 115 F7.  Really good in red and green.  A little lacking in blue.

 

I just finished a side by side for about two hours comparing that scope (a friend's 115) to my CF Esprit 150.

 

The seeing was pretty good.  For the most part it is what you would expect.  The 150mm gathers 70% more light, so objects were brighter.  The 115 had a bit of color on very bright stars (Capella for instance). But overall I could not imagine not being happy with that scope for its aperture and price point.

 

Nice scope.

 

I imagine Scott's SVX looks nice under the stars as well.

[ris242]

I wonder if it was sold (new) as a scope that had a Strehl of 0.991 ?

The original scopes were sold at 0.99 to 0.996.

[davidc135]

The scope is over-corrected throughout the spectrum with some zones and a turned up edge (focusing long), all of which are small manufacturing errors that I'm sure the maker well understood from the zygo report and that he thought acceptable.

 

Whether it is or not I'd judge by a side by side with another 127 apo that's perfect in green.

 

How does this objective compare with the 180SV as regards wave error in blue? IIRC the SV180 was out by 1/2 wave at that colour. Checking the actual impact of these test results out under the stars seems a good idea.

 

David

Edited by davidc135, 23 April 2023 - 03:06 AM.

[Scott in NC]

The scope is over-corrected throughout the spectrum with some zones and a turned up edge (focusing long)…

David, thanks for your input.  It’s hard to see here, but if there are any edge issues then I’ve been interpreting a small inward hook on the inside-focus images to signify a slightly turned down edge.  Wouldn’t a turned up edge be manifested as an outward hook inside focus?  Or have I gotten this backwards all along?  I do see that the Zygo report shows that the edge is indeed higher than the center.

 

Here’s a chart that I’ve been using as my guide, from this source: https://stellafane.o...onchigrams.html

 

 

Edit: As you'll see later on, David is completely correct here, and the chart above is for mirrors rather than lenses.  So this is indeed a turned up edge, and I've tried my best to correct it wherever I've previously made that error.  But please let me know if I missed one!

Edited by Scott in NC, 23 April 2023 - 12:24 PM.

[RichA]

Finally here are the white light images.  These are useful for judging control of chromatic aberration.  As expected with an FCD100 and lanthanum doublet at a focal ratio of f/8, this scope does quite well.

 

Inside focus:

 

IMG_0651.jpeg

 

 

At focus:

 

IMG_0652.jpeg

 

 

Outside focus:

 

IMG_0653.jpeg

Nice work, sorry if this was mentioned, what size sensor is being used to image these rochi patterns?  

[Scott in NC]

 

Whether it is or not I'd judge by a side by side with another 127 apo that's perfect in green.

 

Neither one of these scopes are perfect in green, but to my eyes the Ronchi bands look a bit straighter on my AT125EDL and FS-128, so these would be good comparison scopes for actual star testing.

[Scott in NC]

How does this objective compare with the 180SV as regards wave error in blue? IIRC the SV180 was out by 1/2 wave at that colour. Checking the actual impact of these test results out under the stars seems a good idea.

The smaller (and easier to fabricate) 127mm doublet looks a bit better corrected in blue than the larger 180mm triplet that peleuba tested last month.  That’s what I suspected I was going to find before starting to test this scope.

[Scott in NC]

Nice work, sorry if this was mentioned, what size sensor is being used to image these rochi patterns?  

My Canon T2i DSLR has an APS-C sensor, and I used a Canon 100mm f/2.8 macro lens for capturing the images.

[RichA]

The original scopes were sold at 0.99 to 0.996.

A awesomely bold claim.  One minor zone or turned edge section would disallow that. It's too bad they can't do spot source intensity measurements of the diffraction disk and the first ring, which would determine as percentages how much is actually going into the two.