28 replies to this topic

[CreatorsHand]

Early last Summer a friend offered me her late husband's 10' fiberglass dome at a cost I couldn't pass up, so I started looking for a telescope to put in it, since I wanted the Canon 500mm f/4L lens that had been my main OTA for the last several years available for wildlife photography. After looking over reviews and the information on Stellarvue's website, I decided I wanted an SVX102T Raptor. What could be better than an American-made telescope with a tested Strehjl of .99 or better? I even communicated with Vic at Stellarvue about it. Unfortunately, when I stated looking at the total cost of the telescope, FR/FF, FF and assorted accessories and my budget, I realized that I wasn't going to be able to afford one. When I came across an older SVR102T Raptor (Serial No. 107) that I could get for less than half the cost of the SVX102T Raptor, I decided that was close enough, as it was likely the same model that was reviewed very favorably in Sky and Telescope in 2013. My one disappointment was that it predated when SV issued certificates so I didn't have a test certificate; as an engineer, having an objective analysis meant something to me. When members here started posting DPAC (Ronchi Autocollimation) test results, I realized that here was a chance for me to do an objective test of my SVR102T Raptor to see what I have, so I designed and built a DPAC test rig and read all the posts of other tester's results. Thanks to all the information posted and shared by several CN DPAC testers, I figured out most of what I needed to do. Email and CN message chats with a couple of them filled in the holes. I found 3D printer plans by Moshen, a member here, but didn't have a 3D printer, so I hired him to make them for me. He told me about a source for 8K resolution Ronchi screens, and I found a place that will print digital files on slide film at 8K resolution and ordered some. He also recommended some lED lights and I orderd some form Digi-Key. I also picked up a 6" optical flat on ebay. After a number of testing iterations, I finally have some results that I am willing to share, although I still have a few things to improve on (engineers are often perfectionists...).

 

My telescope set up for testing:

 

 

My makeshift testing bench and testing setup (with my C14 in it, the box is quite stable):

 

 

 

[CreatorsHand]

Here are the current results, using  a 130lpi Ronchi screen:

 

 

 

 

 

 

 

[CreatorsHand]

 

 

 

 

Contrast has been significantly enhanced (100%) to bring out details. Overall, I am happy with these results, although I still need to work on the photography aspect. Analysis, well, that is a different matter, and I am not knowledgeable enough about lenses yet to even start to postulate what these Ronchigrams mean, although I have picked up enough to believe that this telescope is decent. Unfortunately we have had too many cloudy nights to be able to truly test it out. My main interest is imaging, but I expect that now that I have this I will put some eyepieces in the SV diagonal that came with the telescope and so some visual observation at some time when it warms up a little. I would welcome any analysis that anyone would care to share, both to understand this scope and to learn about DPAC testing and analysis. The spots on the images are small defects in the surface of the flat; I need to check the reverse side and see if that is any better.

[Maciek_Cz]

Pretty nice results. I have one remark about the photos taken at focus. The problem is that in Ronchi at focus we don't know the location of the lines of the Ronchi screen. It may happen that Airy disk will hit exactly between the lines. The result will be an image with no trace of the shadow as if it were null. And that's what we see in your at focus photos. Unfortunately, the knife does not cut Airy disk.

Edited by Maciek_Cz, 18 April 2023 - 02:58 PM.

[Scott in NC]

Green looks nearly perfect to me.  Maybe a touch of undercorrection, but nothing that I can imagine would show up visually.  In red I suspect that the widening of the Ronchi bands near the periphery is related to your photographic technique rather than due to something going on with the optics of the scope itself, but maybe others who are more knowledgeable can chime in on this.  And Maciek beat me to the comment on the at-focus images.

 

Very nice job overall, and welcome to club DPAC!

[Maciek_Cz]

Green is at the Strehl level of 0.98-0.99. And indeed, there should be no shadow in the at focus position. Maybe red isn't any worse. And maybe we actually see such a nice null.

[CreatorsHand]

Pretty nice results. I have one remark about the photos taken at focus. The problem is that in Ronchi at focus we don't know the location of the lines of the Ronchi screen. It may happen that Airy disk will hit exactly between the lines. The result will be an image with no trace of the shadow as if it were null. And that's what we see in your at focus photos. Unfortunately, the knife does not cut Airy disk.

Thank you Maciek. If it makes any difference, the at focus images were captured in between the inside focus images (always first so I can keep track) and the outside focus images, and as I remember, the one line of the Ronchi screen stayed dead center as I adjusted focus. I have made up a couple slides using razor blades (one with the spine at the center and one with the blade edge at center, both with the "cover" or "handle" removed) and the printed slide holders I got with my eyepieces from Moshen, and plan to try using them to see what I get.

[CreatorsHand]

Green looks nearly perfect to me.  Maybe a touch of undercorrection, but nothing that I can imagine would show up visually.  In red I suspect that the widening of the Ronchi bands near the periphery is related to your photographic technique rather than due to something going on with the optics of the scope itself, but maybe others who are more knowledgeable can chime in on this.  And Maciek beat me to the comment on the at-focus images.

 

Very nice job overall, and welcome to club DPAC!

Thank you Scott! With the red images in particular, I was having trouble getting the LED to illuminate the full circle, and I missed that I had erred on the outside focus image. This is a set I put together before I decided to clean the objective, and it is better illuminated, although it is still subject to a little shading on the top (which was the bottom of the opening in the eyepiece)::

 

[CreatorsHand]

I don't know if this means anything, because I really am unsure of what I am doing, especially with a knife test, but this is what I got just now with my knife edge slide and the red LED. I believe it is focused correctly, looking at the scale on the focuser, although I don't see it change too much either side of here. The shadow didn't change. To the best of my knowledge, after careful measurement the knife edge is at the center of the slide.

 

 

[Bob Campbell]

Looks like a very well corrected optic, especially in green. Is a 2013 SV 102 imported optics? Based on the discussions on various threads, and the excellent green result it  is.

 

You made a very wise purchase, methinks.

 

Bob

Edited by Bob Campbell, 18 April 2023 - 05:36 PM.

[JMW]

I bought a used SVR102T about 8 years ago. Its a good scope and a very easy size to deal with it. I have larger refractors but sometimes I want a shorter refractor with a wider field of view. 

 

I am glad you are happy with yours. 

[PhotogTom]

To my eyes, Green looks excellent, and neither red nor blue is less than very good. 

 

Of course, my eyes aren't really trained in this type of testing either. 

[CHASLX200]

Looks good.

[Scott in NC]

To my eyes, Green looks excellent, and neither red nor blue is less than very good. 

 

Of course, my eyes aren't really trained in this type of testing either. 

When a scope is near perfect it doesn’t take much training to see how good it is.  It’s when there are significant defects then it helps to have experience with this sort of thing to determine what the issues actually are.  

[Kitfox]

Paul, a knife edge or Foucault test is more about movement than an image of a single position of the blade.  What you are looking for is what the "shadows" look like as you slice into the converging and diverging light rays inside, at and outside the focal plane.  Finding the exact focal plane is easy, if you move the knife in and the "shadow" moves in the same direction, you are inside focus; if opposite the direction of blade movement, you are outside focus.

 

Once you find focus, you watch the apparent image as you slice into the point of light and watch the way the light is cut off.  With a perfectly spherical figure (mirrors; this is not exactly true with multiple component lenses, but almost), the light will appear to go from full illumination to smooth gray and fully black almost instantly.  Any shadows and gradation as you slice the light cone apex indicates an issue with the lens.  Unless it is an aspherized lens, which makes interpretation more complicated.  Analogous to a parabolized mirror.

 

Note that this is only true with a single wavelenght at a time with a lens; chromatism will totally foul up a white light Foucault test.

 

But my key point is that the Foucault test is a test that follows the movement of the edge, not at any single position.  And that is only true for spheres; once you start figuring away from a sphere, specific locations are key to following the progress of "pushing" glass.

 

 

I don't know if this means anything, because I really am unsure of what I am doing, especially with a knife test, but this is what I got just now with my knife edge slide and the red LED. I believe it is focused correctly, looking at the scale on the focuser, although I don't see it change too much either side of here. The shadow didn't change. To the best of my knowledge, after careful measurement the knife edge is at the center of the slide.

 

 

DPAC SVR102T-6947I-2.jpg

Edited by Kitfox, 18 April 2023 - 06:37 PM.

[CreatorsHand]

Looks like a very well corrected optic, especially in green. Is a 2013 SV 102 imported optics? Based on the discussions on various threads, and the excellent green result it  is.

 

You made a very wise purchase, methinks.

 

Bob

Thanks for the confirmation Bob. It is my understanding that the optics were imported at the time that this telescope was made, although that exact date is a little murky, and SV apparently doesn't have any records to help. According to the Sky and Telescope article I referenced, the SRV102T Raptor that they reviewed was tested in 2013, shortly after NEAF that year, where SV had it on display, but the article didn't hit the magazine until September, 2013. The author, Dennis di Cicco, seemed pretty impressed with it. That review is still referenced on the SVX102T page, so that gave me confidence.

[CreatorsHand]

Paul, a knife edge or Foucault test is more about movement than an image of a single position of the blade.  What you are looking for is what the "shadows" look like as you slice into the converging and diverging light rays inside, at and outside the focal plane.  Finding the exact focal plane is easy, if you move the knife in and the "shadow" moves in the same direction, you are inside focus; if opposite the direction of blade movement, you are outside focus.

 

Once you find focus, you watch the apparent image as you slice into the point of light and watch the way the light is cut off.  With a perfectly spherical figure (mirrors; this is not exactly true with multiple component lenses, but almost), the light will appear to go from full illumination to smooth gray and fully black almost instantly.  Any shadows and gradation as you slice the light cone apex indicates an issue with the lens.  Unless it is an aspherized lens, which makes interpretation more complicated.  Analogous to a parabolized mirror.

 

Note that this is only true with a single wavelenght at a time with a lens; chromatism will totally foul up a white light Foucault test.

 

But my key point is that the Foucault test is a test that follows the movement of the edge, not at any single position.  And that is only true for spheres; once you start figuring away from a sphere, specific locations are key to following the progress of "pushing" glass.

Thank you for the explanation Kitfox. I recently got a copy of Suiter's book on star testing, but haven't had a chance to read much of it; that may help explain more as well. The challenge would be figuring out how to incorporate that into the test setup I have. At least I have a better understanding of what is involved.

[Kitfox]

Just remember, the book for star-testing lenses has not been written yet...

 

 

Thank you for the explanation Kitfox. I recently got a copy of Suiter's book on star testing, but haven't had a chance to read much of it; that may help explain more as well. The challenge would be figuring out how to incorporate that into the test setup I have. At least I have a better understanding of what is involved.

[CreatorsHand]

I remember someone recently posting that in addition to DPAC tests, it would be helpful to hear about observing sessions or see actual images captured with the telescope being tested. I have had very few opportunities to use this telescope since I got it due to too many clouds, but here is a single, 3-minute unguided sub of the North America Nebula captured with my SVR102T with the SFFR102T 0.8X FF/FR. It was captured with my Ha mod 60D, and I was using an NBZ duo NB filter, mounted double (side-by-side) with my 500mmf/4L on my 900HDA. This is uncalibrated; simply brought into Lightroom, with a little noise reduction applied and an adjustment (increase) to the Black Point.

 

 

[nicknacknock]

And added to the ever growing repository of DPAC tests

[Maciek_Cz]

Your telescope is perfect.

 

Let me explain what I mean. For a detailed analysis, it is worth taking several photos in the vicinity of the focus.

The Airy disk for this telescope in red is ~11 microns in size.

Suppose you move the knife a few microns from the center and analysis based on one image becomes very difficult.
I know, I write too much about AOS, but I have no other tool.

Currently, as part of testing, I am analyzing ronchigrams posted here on the forum.

And for example, if the lens is characterized by a small zone in the middle, and we take only one photo at focus,

sometimes blurred, we can choose the parameters for the simulation completely wrong, so that the results will differ significantly from reality.

Kitfox described it very well in his post.

[jimandlaura26]

I possess an SVR-102T of this type from this era. I have obtained consistently outstanding visual results for all manner of targets. I have no bench tests to share. That said, if memory serves Vic Maris stated at the time that measured results were typically on the order of 0.96 Strehl. Mine had a report stating 0.956 Strehl, though don’t remember color used and don’t recall where I put that paper. But, I do put my faith in what I have observed over the years for this and other SV scopes I have owned. Enjoy your SV telescope!

[Eddgie]

This is what I would expect to see in a an excellent 4" f/7 FPL-53 triplet. 

 

Not only does this scope have excellent optics, the polychromatic Strehl is probably in the .985 range. 

 

We see a lot of optically excellent doublets but the spherochomatism is almost always causing spherical aberration in blue and red. At f/7, even with FPL-53, the polychromatic Strehl of an f/7 doublet of this size and speed would likely be only around .92 so even if the FPL-53 doublet was perfectly made, the performance would be that of a .92 Strehl scope.

 

Here, the designer seems to have corrected green and this means that with the FPL-53 in a triplet configuration, the red and blue can also be very highly corrected. For scopes corrected to be best at 620nm (which seems to be common today), it seems like it is hard to keep the spherical aberration in blue under control if the green line is where the design was set for best correction, keeping red and blue well corrected would seem to be easier.  

 

 

 

Great quality in a 4" FPL-53 Triplet makes for an outstanding telescope. You are lucky to have this one. 

Edited by Eddgie, 19 April 2023 - 08:52 PM.

[CreatorsHand]

This is what I would expect to see in a an excellent 4" f/7 FPL-53 triplet. 

 

Not only does this scope have excellent optics, the polychromatic Strehl is probably in the .985 range. 

 

We see a lot of optically excellent doublets but the spherochomatism is almost always causing spherical aberration in blue and red. At f/7, even with FPL-53, the polychromatic Strehl of an f/7 doublet of this size and speed would likely be only around .92 so even if the FPL-53 doublet was perfectly made, the performance would be that of a .92 Strehl scope.

 

Here, the designer seems to have corrected green and this means that with the FPL-53 in a triplet configuration, the red and blue can also be very highly corrected. For scopes corrected to be best at 620nm (which seems to be common today), it seems like it is hard to keep the spherical aberration in blue under control if the green line is where the design was set for best correction, keeping red and blue well corrected would seem to be easier.  

 

 

 

Great quality in a 4" FPL-53 Triplet makes for an outstanding telescope. You are lucky to have this one. 

Thank you for the confirmation, Eddgie! The guy I bought it from said that the telescope was spectacular and that he would put it up against any 150mm or smaller refractor; I was thinking that was optimism, but maybe he wasn't too far off. I especially like that I have less than $2,000 into this, including telescope, rings, dovetail, both a 0.8X FR/FF and FF, and SV's best 2" diagonal. Now I just need some clear skies...

 

I don't know how much difference this would make, but the LED I used had the peak and major wavelengths at 640nm-660nm, so this should be representative of Ha; as an imager that is also encouraging.

[Eddgie]

The guy I bought it from said that the telescope was spectacular and that he would put it up against any 150mm or smaller refractor; I was thinking that was optimism, but maybe he wasn't too far off. 

Many people think there is no difference between ED and Super Low Dispersion glasses (SD) such as FPL-53, but here we have an example that shows us what a perfect SD triplet can do. Nothing else (for given focal ratio and aperture) is as efficient as a high quality SD triplet. Not only is the scope excellent in green, it is excellent in red, and something rarely seen, excellent in blue as well. This is what sets SD Triplets apart from the pack. They correct not just one color, but all colors to a very high level. 

 

I too have seen smaller FPL-53 triplets seem to magically match the performance of larger ED refractors and much larger achromats.  

 

You have a real prize there. Finding FPL-53 triplets is not hard, but finding them with this high of a quality of manufacture is. This is about as perfect as telescopes get.