19 replies to this topic

[Scott in NC]

Now that the word is getting out that I'm having a blast with refractor testing, other CN members have been asking me to test their scopes for them.  I've now got a backlog of results awaiting posting, and now that I finally have a leisurely Sunday afternoon available, here is the next set of results.  

 

This is a Stellarvue SV70T 70mm f/6 triplet containing one FPL-53 element sandwiched between two mating elements (not sure what type of glass is used for those). The optics (and I assume tube and focuser as well, but I have no confirmation of that) were sourced from China, as this scope was built in the pre-SVX days, before Stellarvue switched to in-house optics production.

 

As you'll see, this is a fantastic example of a small, moderately fast triplet, with excellent spherical correction, nice control of false color, and excellent polish.  This is also the smallest aperture scope that I've tested, and seems to fit in with my general observation that it's much easier to produce well-corrected small optics than larger optics.

 

Here's the scope set up for DPAC testing, although before taking the final DPAC images I propped the scope up on a book so that it was parallel to the table surface, as the first time I tried I got some vignetting in my photographs.

 

[Scott in NC]

Now for the DPAC results.

 

Green inside focus:

 

 

Outside focus:

 

 

Red inside:

 

 

Outside:

 

 

Blue inside:

 

 

Outside:

 

 

White inside:

 

 

At focus:

 

 

Outside:

 

 

[Stellar1]

Looks like a killer little scope!

[Scott in NC]

Spherical correction is excellent in both green and red, and so good in blue that if green looked like blue did here then we'd still consider this a reasonably well corrected optic.  Blue is usually the furthest off, but with this particular scope blue is still remarkably close to the other colors.  

 

The white light images show very nice control of chromatic aberration, and I only see a hint of blue near the periphery. I'm not sure that I could really make it out with my eyes while doing the testing, and it takes blowing the image up on a computer screen to really bring out the mere bit of false color that you see here.  

 

Polish appears to be excellent as well, and I can barely make out a hint of a small zone at about 1/3 the radius. Not that one can assign exact numbers to a qualitative test like this, but if I had to take an educated guess I'd say that that's so shallow that it's likely well under 1/10 wave, and nothing that would be visible visually or photographically under actual use under the stars.

 

There may be a hint of a very small downturn of the very edge of the lens, but this is so close to perfect that when I see something like this I'm always hesitant to say that there's a true deviation of the edge vs. simply a diffraction artifact.  Regardless, this is so close to perfect that I seriously doubt that it would cause any visual or photographic issues.

[Scott in NC]

The bottom line is that this is a very nice scope.  Stellarvue doesn't sell these any longer, and I believe they were originally sold for somewhere in the $800-1000 price range.  I'm all tapped out on scopes of my own for now, but if the owner were willing to sell it and I wanted something small for either AP or visual use, this would be worthy of serious consideration. IMO its only significant limitation for visual use would be its small aperture. Hopefully one of these days I'll get a chance to test it out under the stars before its owner comes to reclaim it!

[gstrumol]

Have you ever written out the full list of scopes you've tested?

[Scott in NC]

Have you ever written out the full list of scopes you've tested?

Not exactly, but we’re keeping a repository of DPAC test results here on CN: https://www.cloudyni...-on-refractors/

 

There are somewhere around 80 scopes listed, and I’ve done around 20 of those.  Plus I still have a few that I’ve tested and just haven’t gotten around to posting yet.  I’m trying not to bore you guys with everything that I’ve done! 

[SandyHouTex]

Very nice.

[Scott in NC]

I had forgotten that the owner of this scope sent me the Zygo interferometry report from this particular SV70T until he reminded me today.  And no, I don’t know for sure whether this was tested in red or green, but if the test was performed by SV then it was in green red.  If the test was done by the manufacturer then I suppose it could be either red or green.  But as you can see from the DPAC results above, in this particular case it may not matter, as this scope was figured excellently for both wavelengths.  

 

Curiously, I don’t see anything to indicate on the report that this was the report for this particular scope, but this was indeed the report that the owner received at the time that he purchased the scope.  There’s a number 161 at the bottom of the report, but that’s not the serial number of the scope.  Maybe it’s the lens number?

 

 

Edit: I meant to say “red” where I said “green” above.

 

Edited by Scott in NC, 05 January 2024 - 11:05 AM.

[RichA]

Now for the DPAC results.

 

Green inside focus:

 

IMG_8887 green inside.JPG

 

Outside focus:

 

IMG_8889 green outside.JPG

 

Red inside:

 

IMG_8887 red inside.JPG

 

Outside:

 

IMG_8889 red outside.JPG

 

Blue inside:

 

IMG_8887 blue inside.JPG

 

Outside:

 

IMG_8889 blue outside.JPG

 

White inside:

 

IMG_8887.JPG

 

At focus:

 

IMG_8888.JPG

 

Outside:

 

IMG_8889.JPG

I wonder if the turning of the edge is actually due to the retaining ring exerting pressure on the edge of the lens?

[Scott in NC]

That's certainly a possibility.  However, as it's not my scope, and the edge looks so close to perfect, I'm not going to mess with trying to adjust it.  I'll leave it up to the scope's owner as to whether he'd like to attempt adjusting the retaining ring once I return the scope to him.  I'll definitely discuss the risks and benefits of that procedure with him, but right now my advice is to just leave it alone.

[jmiller1001]

I have one of these scopes - Scope #95/Lens #21 bought in 2016 from OPT at their physical store in SoCA.  I had it out the other night on my AM5 doing some widefield EAA.  It's a great scope and is comparable in optical quality to my CFF 92 and 105 - yep, it's that good.  And, it's been all over the world as my travel scope.  I'll never sell this one.

 

Trust me, I am not trying to start a Stellarvue debate - I fully concur that Stellarvue sourced this scope from Asia (China? Taiwan?) - but, I don't buy that Stellarvue was flipping these scopes without any "tweaking" or "inspecting in quality/cherrypicking" to achieve these type of results.  Perhaps only Vic knows...but, my SV130T is also just as good as my 70.

Edited by jmiller1001, 12 June 2023 - 09:01 PM.

[Reid W]

When these 70s came out 7 or 8? years ago,  a member here bought one.  When he was setting it up inside his dome,  we spent some time comparing the visuals to some larger 90 and 100 apos.  It was a teriffic optic.   

 

Size wise, it seemed  much closer to a 60 class than the 80.

[Maciek_Cz]

I'm coming back to this thread because I've done an analysis of this telescope.

You must admit that if we put good quality photos into the AOS, the selection of parameters is really easy, and therefore the results are close to the truth.

I have always said that you should be patient and honest when using AOS. The method is very sensitive.

A small change in parameters can result in a large change in the results. But this is also the case with interferometric measurements,

a small change in the conditions in the measurement system can result in a completely incorrect analysis.
The telescope under study is great. The best correction is for the red color, it results not only from the wavelength (size of the Airy disc),

but from the properties of the lens itself, which can be seen on the ronchigrams.
I am still working on the next version of the program, in which the program will construct the wavefront and on its basis the p-v, rms and Strehl values will be determined.

In addition, I am preparing a website with a user manual and examples. But all this takes time.

 

Red

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[Maciek_Cz]

Green

 

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[Maciek_Cz]

Blue and white at focus

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[Scott in NC]

7 months later I finally had an opportunity to view through this fine scope tonight, and had a lot of fun with it. It was a chilly 34°F and I really needed a small, light scope since I didn't want to spend a lot of time out in the cold setting up gear.

 

The Pleiades and Double Cluster were spectacular through a 20mm Pentax XW. Jupiter looked okay through a 3-6mm Nagler zoom, but I’ll admit that I’ve become spoiled by viewing it through a 5” apo lately, and really think it needs a lot more aperture than 70mm to show off its most interesting features. But it still looked nice, with well defined equatorial bands and not a hint of CA. And to be fair, I wasn’t really expecting much for Jupiter, as seeing conditions were below average tonight.

 

I didn’t stay outside long enough to get a good feel for how long this scope takes to reach thermal equilibrium, but half an hour was quite enough for me before I needed to head indoors to get warm. Overall I’m pleased with this little scope, and plan to enjoy it for a while.

[Lagrange]

I had forgotten that the owner of this scope sent me the Zygo interferometry report from this particular SV70T until he reminded me today.  And no, I don’t know for sure whether this was tested in red or green, but if the test was performed by SV then it was in green.  If the test was done by the manufacturer then I suppose it could be either red or green.  But as you can see from the DPAC results above, in this particular case it may not matter, as this scope was figured excellently for both wavelengths.  

 

Curiously, I don’t see anything to indicate on the report that this was the report for this particular scope, but this was indeed the report that the owner received at the time that he purchased the scope.  There’s a number 161 at the bottom of the report, but that’s not the serial number of the scope.  Maybe it’s the lens number?

 

IMG_2862.jpeg

I might be wrong but I think a Zygo report can be assumed to have been performed at 632.8nm unless otherwise stated as this is output of the Helium-Neon laser that's used as a standard light source.

 

It would also fit with Maciej's analysis showing that best correction is in red.

 

Looks to be a very good objective though, and shows the standard that can be achieved using good Chinese optics along with stringent quality control.

[Daniel Mounsey]

Spherical correction is excellent in both green and red, and so good in blue that if green looked like blue did here then we'd still consider this a reasonably well corrected optic.  Blue is usually the furthest off, but with this particular scope blue is still remarkably close to the other colors.  

 

The white light images show very nice control of chromatic aberration, and I only see a hint of blue near the periphery. I'm not sure that I could really make it out with my eyes while doing the testing, and it takes blowing the image up on a computer screen to really bring out the mere bit of false color that you see here.  

 

Polish appears to be excellent as well, and I can barely make out a hint of a small zone at about 1/3 the radius. Not that one can assign exact numbers to a qualitative test like this, but if I had to take an educated guess I'd say that that's so shallow that it's likely well under 1/10 wave, and nothing that would be visible visually or photographically under actual use under the stars.

 

There may be a hint of a very small downturn of the very edge of the lens, but this is so close to perfect that when I see something like this I'm always hesitant to say that there's a true deviation of the edge vs. simply a diffraction artifact.  Regardless, this is so close to perfect that I seriously doubt that it would cause any visual or photographic issues.

 

Scott,

Based on my experiences with Stellarvue QC with optics developed in China, this is not too surprising to me. I don't want to sound negative, but based on my experiences, I have often found the in-house SVX's to be inferior by comparison with regard to optical quality and they cost even more. 

[Scott in NC]

I might be wrong but I think a Zygo report can be assumed to have been performed at 632.8nm unless otherwise stated as this is output of the Helium-Neon laser that's used as a standard light source.

 

It would also fit with Maciej's analysis showing that best correction is in red.

 

Looks to be a very good objective though, and shows the standard that can be achieved using good Chinese optics along with stringent quality control.

Yeah, that was clearly a typo on my part, as I meant to say “red” where I said “green.”  I’ll go back and edit my post from last June.