38 replies to this topic

[Jeff B]

Here are the double pass (aka DPAC) images of a recently tested TEC 140FL, S/N076, belonging to a CN's member, Paul, from the Indianapolis Indiana, USA area.

 

The DPAC rig includes a 12", 2.25" thick flat with 133 LPI screens with holders,  green and white LEDS.   Images were taken with my cell phone's excellent camera, motion stability activated, manual focus and white balance towards the warm side to knock down the blue a bit.  ISO is 50 with 1/80 second exposure for green LED shots and 1/3000 for white light.  The only image processing other than color channel isolation/blending is done on the at focus green images, specifically a 20% pop in contrast to help extract & enhance visibility of any structure in the wave front.   

 

The color montage images were constructed from inside/outside of focus white light "master" images.  Specifically, the primary red, green and blue channels are isolated using an old version of Paint Shop Pro.  The yellow and orange images represent blends of the green and red channels.

 

This was one of 5 scopes tested over an 8 hour period.  The scope was very close to room ambient temperature when brought into the basement test area and had ~ 3 hours to further stabilize, dew shield pulled back.

 

First the scope set up in my DPAC testing "facility" along with the white light master images.

 

Jeff

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[Jeff B]

Here are the inside/outside of focus color montages and the contrast enhanced at focus image.

 

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[RichA]

Looks pretty nice; no real spherical, a possible edge issue and perhaps a small zone near the edge and we could always wish the surface was smoother, colour correction looks good!.  Nice work!

[scoale]

Agree with Rich A.  I'm seeing indication of turned edge.  Spherical looks very good. Smoothness seems to be comparable to my TEC 140fl with some rippling, etc.

[121601]

Looks pretty nice; no real spherical, a possible edge issue and perhaps a small zone near the edge and we could always wish the surface was smoother, colour correction looks good!.  Nice work!

FYI, The Dec 2003 Sky and Telescope review (pp 54-58) of the first TEC 140s models (by Dennis di Cicco) displayed similar (actually better) ronchi patterns (Green laser).   Dennis has also reviewed  Stellarvue telescopes quite extensively (102 and recently 127 models).  CS
 

Edited by 121601, 16 March 2023 - 04:30 PM.

[Kitfox]

You’ve gotta love fluorite in the hands of a great optician…

[Noltimier]

Hi Jeff.

 

Please excuse my ignorance.

 

I see in the first photo of post #1 what appears to be the ronchi grating and LED on top of the diagonal.

If this is the case, does testing the optical train with the diagonal make a difference in the outcome of the test.

 

I would make the assumption, that if the diagonal is reasonably flat, there probably isn't much of a difference...

 

Best regards,

Mark

Edited by Noltimier, 16 March 2023 - 05:50 PM.

[RichA]

Agree with Rich A.  I'm seeing indication of turned edge.  Spherical looks very good. Smoothness seems to be comparable to my TEC 140fl with some rippling, etc.

Nothing will ever match the horror of seeing this done with a early-1990s C14 that looked like a vinyl record that's been sitting on a hot-plate.

Edited by RichA, 16 March 2023 - 06:26 PM.

[CHASLX200]

Nothing will ever match the horror of seeing this done with a early-1990s C14 that looked like a vinyl record that's been sitting on a hot-plate.

I wonder what my 1980's C14 would have looked like. Like most SCT's i have owned out of around 60 it was more than lack luster bad.

[Scott in NC]

Hi Jeff.

 

Please excuse my ignorance.

 

I see in the first photo of post #1 what appears to be the ronchi grating and LED on top of the diagonal.

If this is the case, does testing the optical train with the diagonal make a difference in the outcome of the test.

 

I would make the assumption, that if the diagonal is reasonably flat, there probably isn't much of a difference...

 

Best regards,

Mark

I’m obviously not Jeff, but I asked him about this once as I was curious too.  His response was that this is a diagonal of known good optical quality (good alignment, no astigmatism, no spherical aberration which you wouldn’t expect anyway with a non-prism diagonal, etc.), and he’s previously performed DPAC testing with and without this diagonal and no differences were seen.

[Jeff B]

Sorry, had to take a break.

 

Always a good question concerning the use of the AP diagonal in the DPAC rig.  

 

Why?  It's much easier to work with for me and the alignment of the diagonal is perfect.

 

Does it add its own aberrations to the images?  Yes, but I can't see any, certainly with the inside/outside of focus images and even the at focus images, and DPAC properly done can see into the mud, 1/50th to 1/100th wave mud.

 

To "validate" using the diagonal, I, a while ago, made some very careful with/without diagonal images and mirror/prism diagonal images on the same scope (Orion 110ED) in green and I believe inside of focus. I nailed the with/without comparison using some witness marks (normally called scuffs in the coating) to line up focuser axial and grating lateral positions.     Attached is the comparison image.  Go, ahead, without looking at the title by placing your cursor over the image, tell me which one uses the diagonal.

 

I use the same diagonal for all of my DPAC testing.

 

Jeff

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Edited by Jeff B, 16 March 2023 - 06:53 PM.

[davidc135]

I've got to say the at focus null looks very rough but I well believe that the objective performs beautifully! I wonder what figure an interferometer would put on all those hills and hollows. Maybe very little? (posts 1&2)

Very little spherochromatism.

 

David

Edited by davidc135, 16 March 2023 - 06:59 PM.

[Scott in NC]

I see very good spherical correction, very nice control of chromatic aberration, and a small turned up edge (very similar to what I’ve seen on my AT80EDL) which likely isn’t a big issue.  But I’d be a bit more concerned about the zones and roughness about 3/4 of the way out to the edges.  What’s your take on this, Jeff?

 

And I’m sure that the owner, Paul, has star tested this scope before.  Did he say anything about that or the visual images provided by this scope?  And if you’d rather that we just discuss the actual DPAC results, just let me know.

[Noltimier]

Sorry, had to take a break.

 

Always a good question concerning the use of the AP diagonal in the DPAC rig.  

 

Why?  It's much easier to work with for me and the alignment of the diagonal is perfect.

 

Does it add its own aberrations to the images?  Yes, but I can't see any, certainly with the inside/outside of focus images and even the at focus images, and DPAC properly done can see into the mud, 1/50th to 1/100th wave mud.

 

To "validate" using the diagonal, I, a while ago, made some very careful with/without diagonal images and mirror/prism diagonal images on the same scope (Orion 110ED) in green and I believe inside of focus. I nailed the with/without comparison using some witness marks (normally called scuffs in the coating) to line up focuser axial and grating lateral positions.     Attached is the comparison image.  Go, ahead, without looking at the title by placing your cursor over the image, tell me which one uses the diagonal.

 

I use the same diagonal for all of my DPAC testing.

 

Jeff

Thanks Jeff !

Much appreciated.

 

Best regards,

Mark

 

[SchrödingersCat]

Jeff, we've seen similar artifacts, in other DPAC images, that looks like a turned edge. Could you speak to that?

[Scott in NC]

Jeff, we've seen similar artifacts, in other DPAC images, that looks like a turned edge. Could you speak to that?

I don’t think it’s an artifact, but rather truly a sign of a mild turned up edge.  But I’d like to hear what Jeff thinks too.

[Jeff B]

Some of my impressions:

 

Chromatic Corrections:

 

The best spherical correction is in green, right where it should be in a current generation objective, and overall it's neutral, very good to excellent, excellent neglecting the edge.  Not much more to say other than from ~80% - 85% span, there is subtle inflection in the line shadows towards over mild correction that extends out to ~95% span.

 

Spherochromatism is very well controlled with minimal over correction in blue and under correct in red.

 

Secondary spectrum is similarly well controlled showing minimal red/blue fringing bordering the ronchi line shadows, even at the tips, in the white light images and it's very white between the shadows and the purple images are only slightly pink.

 

Smoothness/polish

 

Some folks have already called the polish rough.  I call it a bit lumpy.  I don't see any fine scale, grainy or jagged ripple.  I see more smooth small to moderate scale features with smooth transitions that are fairly uniform over the figure.  I'm guessing here, that they are maybe ~1/20 to 1/30 wave in amplitude, if that.  Remember, I try to get the ugliest looking at focus images I can.  And then I make them look worse by hyping the contrast to high light details.  The overall polish is actually a bit smoother than my LZOS 254 objective, which is capable of resolving extraordinary low contrast surface features on Jupiter, Saturn, Uranus and Mars.  But the texture is there and, with just two surfaces to polish, I'd personally like to see a bit smoother figure even though the difference between the polish of this objective and a "perfect" one would be most likely be completely invisible.  But I'm picky too.

 

Edge

 

Yes, the edge has an under corrected hook from the 95% span on out, and of, another guess, 1/10 wave amplitude.  I consider it mild/moderate as there can be diffraction effects at play at the edge too.   At focus, this would be almost impossible to pick out in a star test.  You might catch it out in the inside/outside of focus rings at very high power during a star test, perhaps in combination with that mild over correction starting at the 80-85% span (I have to wonder if that and the under corrected edge might compensate for each other).  I suspect its effect on low level contrast threshold would be very small.  Another guess

 

Manufacturer's Comments

 

I asked Paul to run my DPAC results past Yuri for his take on them, especially in light of the edge.  Yuri replied quickly and said to use the information he provided, including the wave front profile plot,  "anywhere".   His information seems to confirm my take on the edge and smoothness with Yuri's estimate for the edge error being 1/16 wave. I can see the mild over correction from 80% to 95% span as well as the edge hook in the profile plots too.

 

Overall, in double pass, I find this to be a very good objective with excellent spherical and secondary color corrections.  I hope Paul will post his visual impressions having used the scope for about two years now.

 

Jeff

 

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[Mike Spooner]

Jeff,

In another forum topic you and I both noted how the modern digital cameras accentuate contrast features in the auto collimation tests compared to visual inspection. I have a couple of Ronchi photos of a mirror close to focus (null ) that approximate that difference but since this is the refractor forum…

 

Point is we can detect some pretty subtle details that really look worse than the minimal impact they have on the image. The intrinsic quality seems to get short changed by the quantitative analysis- it’s neat to see the auto collimation tests providing a visual reference for qualitative purposes. I was thinking today (while reading a long, similar thread) how of all the optics and scopes I’ve built over the decades, that not one have I ever even wondered what the Strehl ratio was! Must be something wrong with me!

 

Mike Spooner

[fate187]

I like how Yuri chimed in to this test backing the DPAC results .

[Erik Bakker]

Refreshing to see the DPAC results of this scope and the responses, including that of it’s manufacturer.

[nicknacknock]

A big thank you to Yuri for being supportive and providing extra disclosure. 

 

THIS is what a master optician, proud of his work, should do. 

 

Added this test as well to the DPAC repository

[snommisbor]

A big thank you to Yuri for being supportive and providing extra disclosure. 

 

THIS is what a master optician, proud of his work, should do. 

 

Added this test as well to the DPAC repository

Why Im a TEC fanboy, even though at the moment I dont have one. Still on the 110 list should he ever bring that back. Have had a 140 a 160 still miss the 160ED but sold for the AP130GTX. But on the horizon someday I plan on getting a TEC180. Be my ultimate scope.

[Paul Morow]

And I’m sure that the owner, Paul, has star tested this scope before.  Did he say anything about that or the visual images provided by this scope?  And if you’d rather that we just discuss the actual DPAC results, just let me know.

 

 

Some of my impressions:

 

Manufacturer's Comments

 

I asked Paul to run my DPAC results past Yuri for his take on them, especially in light of the edge.  Yuri replied quickly and said to use the information he provided, including the wave front profile plot,  "anywhere".   His information seems to confirm my take on the edge and smoothness with Yuri's estimate for the edge error being 1/16 wave. I can see the mild over correction from 80% to 95% span as well as the edge hook in the profile plots too.

 

Overall, in double pass, I find this to be a very good objective with excellent spherical and secondary color corrections.  I hope Paul will post his visual impressions having used the scope for about two years now.

 

Jeff

 

The TEC140FL star test is pretty good, but not as textbook perfect as my AP155 f/9 EDT and CFF92 f/6.9; those two instruments are very good references. The in-focus concentric rings are crisper and cleaner than the out of focus star test. Some of the edges of the out of focus rings are soft, not as hard & crisp as the in-focus image. The outside focus concentric rings might appear a little thinner too. I don’t consider myself an expert star tester or anything else. The overall star test appears pretty good, but just more things to talk about, not textbook perfect.

 

After viewing the lunar & planetary images through the TEC140FL one forgets all about the possible less than perfect star test.

 

On nights of good seeing the TEC140FL really delivers awesome lunar & planetary images. I have owned the AP155EDT for nearly 30 years and the TEC140FL is right there trying to hang with it. There are only subtle differences between the two APOs visually. The AP155EDT shows slightly more, but they are very close, the extra 15mm aperture is much closer than one would think. The TEC140 really performs like a 6-inch class instrument. I have been very pleased with its optical performance over the last 2 years; actually, surprised how good it is. I love the scope!

 

I went into this process knowing there are no perfect lens, just nearly perfect lens, and not so perfect lens. The DPAC testing really takes no prisoners and your telescope optics are laid bare naked for the world to see. I really appreciate my buddy Jeff showing us the light. Our community has learned are great deal from these tests and a few more telescope data points are good for this forum.

[Scott in NC]

Thank you so much for sharing that, Paul.  And also thanks so much for letting Jeff test your TEC and share those results here. DPAC results are nice, but as as been mentioned before, they show *everything*, and can make the very nicest lenses look like there might be something wrong with them (at least for those who aren’t experienced at interpreting them, that is).  Combining those results with actual star testing and visual observation is what’s really needed to give the whole picture.

 

You’ve got a very nice scope, and just reading about that makes me pine for my former TEC140ED.  But I know it went to a good home, and I’m not exactly slumming it with what I replaced it with.  

[Jeff B]

Jeff,

In another forum topic you and I both noted how the modern digital cameras accentuate contrast features in the auto collimation tests compared to visual inspection. I have a couple of Ronchi photos of a mirror close to focus (null ) that approximate that difference but since this is the refractor forum…

 

Point is we can detect some pretty subtle details that really look worse than the minimal impact they have on the image. The intrinsic quality seems to get short changed by the quantitative analysis- it’s neat to see the auto collimation tests providing a visual reference for qualitative purposes. I was thinking today (while reading a long, similar thread) how of all the optics and scopes I’ve built over the decades, that not one have I ever even wondered what the Strehl ratio was! Must be something wrong with me!

 

Mike Spooner

Exactly sensi Mike and your input is always helpful and most appreciated!

 

It was interesting in that looking visually first before image capture with a preliminary shot with the green LED, the edge condition appeared as just a bit of a subtle blob at the tips of the lines inside of focus with no thin lines seen between the lines creeping laterally in and the edge.  The small disturbances seen from 80% to 95% span were completely unnoticed by both of us as well.  But the image from the camera clearly shows them.  Ditto for the at focus image, which, live, visually, looked rather boringly smooth and difficult to get a good shot of, requiring very careful positioning of the grating and a longer (1/30th second) exposure.  

 

As this is MY thread, please feel free to post your comparisons Mike.

 

Jeff