121 replies to this topic

[Scott in NC]

I met a new friend yesterday and he graciously let me test three fine scopes of his, a Stellarvue SVX127D doublet, a TEC 140FL triplet, and a TeleVue NP127is quadruplet.  

 

First let's start with the doublet.

 

I've previously tested another sample of a SVX127D, so I was eager to see how this one compared. My image taking skills have advanced since the time I tested the first one 6 months ago, so please excuse the differences in image quality that don't really reflect true differences in the two samples of this scope. Overall I think the DPAC results looked reasonably similar.  

 

For those who aren't familiar with the SVX127D, 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.

 

Here's the scope set up for testing:

 

[Erik Bakker]

Looking forward to all your reports Scott!

[Scott in NC]

Stellarvue SVX127D f/8 doublet

 

Green inside / outside focus:

 

 

Red inside / outside focus:

 

 

Blue inside / outside focus:

 

 

White inside / at / outside focus:

 

[Scott in NC]

I see pretty nice spherical correction, and although red may be the best, green is nearly identical (overcorrected in both colors). Blue is the farthest off (also overcorrected), but considering how good red and green are, blue is certainly acceptable. It would be extremely difficult to get red and green so close without having blue a little off with a doublet. There is a prominent central zone, and a few concentric shallow polishing marks.  I believe that Vic Maris has stated that they're due to hand figuring, and I've seen this on very nice Astro-Physics and TEC refractors too. The edge appears to be very slightly turned up, but this is minimal and I suspect shouldn't have much impact to the overall image.  The fact that when taken in white light and then separated into R, G, and B, there is a similar amount of banding present, means that all three colors focus at close to the same point.  This is very good for a doublet.  Finally, the minimal amount of false color suggests very good control of chromatic aberration for a doublet.

[Scott in NC]

Next is the TEC 140FL, which is the f/7 oil-spaced fluorite triplet successor to the older f/7 oil-spaced FPL-53 triplet.

 

 

 

[Scott in NC]

TEC 140FL f/7 fluorite oil-spaced triplet

 

Green inside / outside:

 

 

Red inside / outside:

 

 

Blue inside / outside:

 

 

White inside / at / outside:

 

[Scott in NC]

Spherical correction is a little better on this triplet vs. the doublet that we previously tested (and that's what you'd expect to see). Green and red have nearly perfectly straight bars, and blue is not that far out.  If green looked like blue does here, we'd still call this a pretty decent set of optics (not perfect, but still nothing to complain too much about, and I suspect something like that would still give very nice images of the sky when actually observing through it). 

 

Notice the "hooking" at the edges of the Ronchi bands?  While I was in the process of doing the testing, I thought that that this might represent a turned-down edge, but now that I'm actually seeing the images on my laptop, I think it may be more a reflection of the zone (seen as a circle at the 80-90% radius region). But that's only of academic interest, as the exact distinction may not really matter.  Chromatism is very nicely controlled here.  The doublet above is good, but this is even better, as one would expect from a fluorite triplet.  The only way to make this better would be to increase the focal ratio to f/8 or f/9, but then the scope would lose one of its strengths, its excellent portability for a 5.5" triplet. I like this scope, and if it weren't for the zone near the edge, I'd consider it to be pretty close to perfection. 

[Scott in NC]

And last but not least, the quadruplet, a TeleVue Nagler-Petzval (NP) 127is. This is a very fast f/5.2 scope with two sets of doublets. I don't think that TeleVue has ever disclosed the glass type used, but it's been surmised to be FPL-53.

 

[Scott in NC]

TeleVue NP127is f/5.2

 

Green inside / outside:

 

 

Red inside / outside:

 

 

Blue inside / outside:

 

 

White inside / at / outside:

 

[Scott in NC]

Now this is very interesting (in a good way), and something that I don't recall ever seeing before.  Of course I've also never seen DPAC images from a TV NP127is before.  Green and red are slightly under corrected, and to a similar extent.  Blue though (which is typically the least well corrected for SA) actually looks the best corrected with this scope.  The green and red Ronchi bands are still so nearly straight that I'd consider the amount of under correction to be quite minimal.  But what I find most interesting is that this is the most "balanced" (for lack of a better term) set of optics that I've ever seen. 

It's impossible for refractive optics to completely correct for SA in all visible wavelengths, so by necessity one or more wavelengths need to off, if even slightly.  But here, it looks like the optical designers strived to get the best possible correction for SA throughout the entire visible spectrum.  My guess is that it takes quadruplet optics to achieve that, and they executed that design philosophy extremely well here.  Finally, notice the extremely shallow "zone" near the center?  I hesitate to even call it a true zone, as it's so shallow that that area likely simply represents polishing marks.  But at that level the distinction is merely academic, and from a practical sense really doesn't mean anything.

The bottom line is that this is one of the best set of optics that I've ever tested, and I've tested some pretty nice ones.  I told the scope's owner not to sell this one, and if he ever does decide to, to give me a ring first.  Not that I need any more scopes, but if I ever decide I'm in the market for another refractor, this is one that I'd gladly take off his hands.  Maybe all the NP127s are just as good, but I really have no idea since this is the only one that I've ever seen tested  Regardless, IMO this one is a keeper.

[Erik Bakker]

Wonderful reports with great images Scott.

Thank you for your efforts to make all those and share your findings here!

[scoale]

First, I am tremendously appreciative of Scott making the drive over to test these three scopes of mine.  He is as nice a guy as you are going to meet.  Second, I encourage our experts to weigh-in for the benefit of our CN members, like me, who are trying to learn as much as they can about evaluating their optics.  Nobody is going to hurt my feelings if they call out a short-coming with any of these scopes.  Lastly, it has been very interesting to compare what I have learned about my scopes while observing and star testing to what DPAC has revealed.  I found the DPAC results to be a great confirmation of some of what I knew, while also providing fresh insights on aberrations like the turned edges which I think DPAC does a better job of revealing.

 

Scott, thanks for making this possible.  Hopefully when my TSA 120 is back from TAK NA we can add another scope to the DPAC library.

[mehdymo]

Very informative, thanks for sharing this.

[Scott in NC]

It was quite an enjoyable process doing this testing and also getting to meet Stephen. The more of these I do, the more I learn, and so please if any of you who have been doing this way longer than I have disagree with my interpretations of the test images, please speak up. And also a big thanks to everyone who’s taken the time to read through all of this!

[areyoukiddingme]

First, I am tremendously appreciative of Scott making the drive over to test these three scopes of mine.  He is as nice a guy as you are going to meet.  Second, I encourage our experts to weigh-in for the benefit of our CN members, like me, who are trying to learn as much as they can about evaluating their optics.  Nobody is going to hurt my feelings if they call out a short-coming with any of these scopes.  Lastly, it has been very interesting to compare what I have learned about my scopes while observing and star testing to what DPAC has revealed.  I found the DPAC results to be a great confirmation of some of what I knew, while also providing fresh insights on aberrations like the turned edges which I think DPAC does a better job of revealing.

 

Scott, thanks for making this possible.  Hopefully when my TSA 120 is back from TAK NA we can add another scope to the DPAC library.

Thanks for making your scopes available. I'd be very interested to read your thoughts on subjective comparisons and how they align (or otherwise) with the DPAC results.

[Scott in NC]

From my discussions with Stephen yesterday, it appears that he’s a much more accomplished star-tester than I am (and I’ll admit that that’s always been a weak point of mine). But what I found fascinating was how well what he had previously noted on star tests corresponded to what we saw on yesterday’s DPAC testing. There were no contradictions between the two tests, and in fact even before we looked at the DPAC images he was able to tell me what we’d most likely find. It’s always nice when results can be validated by using a different method of testing.

[Echolight]

Nice. I like that the TV doesn't weigh so much. Doesn't need a huge mount.

[PhotogTom]

Great testing, much appreciated.

 

for educational purposes, it would be interesting to see or read about the Star tests done on the 3 scopes by the owner. Not necessary, of course, but it is a kind of test that we are all equipped to perform.

[Scott in NC]

Great testing, much appreciated.

 

for educational purposes, it would be interesting to see or read about the Star tests done on the 3 scopes by the owner. Not necessary, of course, but it is a kind of test that we are all equipped to perform.

I’m sure Stephen will be happy to chime in with that when he sees your post.

[PhotogTom]

It’s up to him, of course. And he certainly doesn’t have to.

[scoale]

Great testing, much appreciated.

 

for educational purposes, it would be interesting to see or read about the Star tests done on the 3 scopes by the owner. Not necessary, of course, but it is a kind of test that we are all equipped to perform.

 

Stellarvue star test below.  Single, unstacked, image.  Artificial star @ 100ft, green filter, 2.5x barlow, asi 290 camera.

 

You'll note no obvious astigmatism, coma, or collimation issues.  The spherical error appears very good.  The central zone is easily seen at greater defocus and is also easily seen on the test certificate Stellarvue provided.  The turned edge is much more apparent with DPAC.

 

In focus, the SVX puts up a very clean airy disk at 290 and 338x.  Views of Jupiter are comparable to my other 5" refractors.

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[PhotogTom]

Thanks. I’ve much to learn, but this helps a lot.

[Kitfox]

Hello, Scott and Stephen!  I was awaiting these results with great interest. I was always curious about how the petzvals would test on Scott’s rig, and I am pleasantly surprised. I have quite a few petzvals, two of which are TV, and I love them all!  I’m still not sure whether I want Scott coming to burst my bubbles on some of my favorites, but I’m getting closer!

 

I’m interested in seeing how the TEC star tests compared to the SV.  I would not be surprised if that zone isn’t apparent in the intra- and extra-focal images, and less so in-focus as it compresses into the inner ring (at least).  One question, Stephen, do you have the SV star test images in color or just monochrome?  They are really good, and some of the small “defects” could be pinhole aperture or air turbulence related. Very impressive. 

[Joe G]

Great job Scott!

[Cbaxter]

That 127is test is very interesting. Another great test thread! Thanks!