33 replies to this topic

[Jeff B]

Yup, a second sample, again, snagged off of the open range .  I couldn't resist and this does help to address the usual comment(s) concerning sample of one testing. 

 

The testing and evaluation of the first sample are documented here:

 

https://www.cloudyni...c-test-sw150ed/

 

This SW150ED has a nice Moonlite 2" focuser, which is sooo much better than the stock focuser, with an excellent visual back and was nicely collimated with the center of the objective, requiring just a small tweak for perfection.  The focuser does not have the internal glare stops that the stock focuser has (and none are needed as stray light suppression is excellent), resulting in a much larger fully illuminated FOV over stock.  The ML focuser also has a much shorter length than the stock focuser and transition piece assembly, making it bino-friendly with all of my bino-viewers when using my Baader BBHS silver diagonal.  An excellent upgrade. 

 

The stock rings were the only accessories included, which, again, suites me just fine.

 

I wish would have checked the alignment of the objective with my Cheshire eyepiece like I usually do but I was anxious to do the star testing with the excellent seeing.  The star test was horrible, showing at least one wave of astigmatism, easily seen even at low power.  I was quite taken aback especially considering the truly excellent results from my first test sample.  Subsequent checking of the objective with the Cheshire showed the two dots from the reflections off of the rear facing lens elements were way off from the image of the Cheshire aperture dot.  I was major bummed. 

 

Then I started thinking about it.  The image seemed to be pure astigmatism, nothing else.  Astigmatism is the dominant off axis aberration in most refractors.  I also noticed that panning an out of focus star image over to the edge of the FOV of my 7mm UO ortho, resulted in no astigmatism.   The native astigmatism of the ortho at the field edge was cancelling the objective's astigmatism.  So, I was certain the lens cell was cocked off axis on the tube.  The problem is, there is no provision for cell collimation adjustment with  this design.  I was discouraged but then I had a closer look at the end of the cell that threads on to the tube and could see a "reveal" of a couple of tube threads that was uneven around the perimeter of the tube suggesting the cell was cross threaded on to the tube.  Examination of the other 150ED sample showed no such condition.  

 

Sure enough, using my strap wrench to back the cell off of the tube confirmed the cross threading and subsequent cocking of the cell on the tube.  The threads, while accurately machined, have a rather coarse pitch, which allows easy cross threading of the tube and lens cell.  Once the cell was properly assembled to the tube, the astigmatism promptly disappeared, even at 340x.  In fact, the subsequent star tests of Pollux, Castor and Procyon at high power with/without a green filter were excellent, perhaps a bit under corrected with ~1/6 or 1/7 th wave of coma that dissolved over time as the scope settled out thermally.  

 

All of this is backed up with the DPAC testing.

 

First the green LED and white light images.  

 

It's getting late and I'll comment tomorrow, but have a go at it if you wish.

 

Jeff

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Edited by Jeff B, 13 May 2023 - 11:51 PM.

[RichA]

Yup, a second sample, again, snagged off of the open range .  I couldn't resist and this does help to address the usual comment(s) concerning sample of one testing. 

 

The testing and evaluation of the first sample are documented here:

 

https://www.cloudyni...c-test-sw150ed/

 

This SW150ED has a nice Moonlite 2" focuser, which is sooo much better than the stock focuser, with an excellent visual back and was nicely collimated with the center of the objective, requiring just a small tweak for perfection.  The focuser does not have the internal glare stops that the stock focuser has (and none are needed as stray light suppression is excellent), resulting in a much larger fully illuminated FOV over stock.  The ML focuser also has a much shorter length than the stock focuser and transition piece assembly, making it bino-friendly with all of my bino-viewers when using my Baader BBHS silver diagonal.  An excellent upgrade. 

 

The stock rings were the only accessories included, which, again, suites me just fine.

 

I wish would have checked the alignment of the objective with my Cheshire eyepiece like I usually do but I was anxious to do the star testing with the excellent seeing.  The star test was horrible, showing at least one wave of astigmatism, easily seen even at low power.  I was quite taken aback especially considering the truly excellent results from my first test sample.  Subsequent checking of the objective with the Cheshire showed the two dots from the reflections off of the rear facing lens elements were way off from the image of the Cheshire aperture dot.  I was major bummed. 

 

Then I started thinking about it.  The image seemed to be pure astigmatism, nothing else.  Astigmatism is the dominant off axis aberration in most refractors.  I also noticed that panning an out of focus star image over to the edge of the FOV of my 7mm UO ortho, resulted in no astigmatism.   The native astigmatism of the ortho at the field edge was cancelling the objective's astigmatism.  So, I was certain the lens cell was cocked off axis on the tube.  The problem is, there is no provision for cell collimation adjustment with  this design.  I was discouraged but then I had a closer look at the end of the cell that threads on to the tube and could see a "reveal" of a couple of tube threads that was uneven around the perimeter of the tube suggesting the cell was cross threaded on to the tube.  Examination of the other 150ED sample showed no such condition.  

 

Sure enough, using my strap wrench to back the cell off of the tube confirmed the cross threading and subsequent cocking of the cell on the tube.  The threads, while accurately machined, have a rather coarse pitch, which allows easy cross threading of the tube and lens cell.  Once the cell was properly assembled to the tube, the astigmatism promptly disappeared, even at 340x.  In fact, the subsequent star tests of Pollux, Castor and Procyon at high power with/without a green filter were excellent, perhaps a bit under corrected with ~1/6 or 1/7 th wave of coma that dissolved over time as the scope settled out thermally.  

 

All of this is backed up with the DPAC testing.

 

First the green LED and white light images.  

 

It's getting late and I'll comment tomorrow, but have a go at it if you wish.

 

Jeff

Nice job, as usual!  Not as good an example as the last 150mm.  Might be time for shooting before, in-focus and after focus shots of actual diffraction disks at high power to get an idea of the impact on it of what we see in the DPAC. This shows over, under and no spherical aberration of a star.

 

Like this:

 

Edited by RichA, 14 May 2023 - 01:09 AM.

[CHASLX200]

Wished you could test the one i had.

[Wildetelescope]

Yup, a second sample, again, snagged off of the open range .  I couldn't resist and this does help to address the usual comment(s) concerning sample of one testing. 

 

The testing and evaluation of the first sample are documented here:

 

https://www.cloudyni...c-test-sw150ed/

 

This SW150ED has a nice Moonlite 2" focuser, which is sooo much better than the stock focuser, with an excellent visual back and was nicely collimated with the center of the objective, requiring just a small tweak for perfection.  The focuser does not have the internal glare stops that the stock focuser has (and none are needed as stray light suppression is excellent), resulting in a much larger fully illuminated FOV over stock.  The ML focuser also has a much shorter length than the stock focuser and transition piece assembly, making it bino-friendly with all of my bino-viewers when using my Baader BBHS silver diagonal.  An excellent upgrade. 

 

The stock rings were the only accessories included, which, again, suites me just fine.

 

I wish would have checked the alignment of the objective with my Cheshire eyepiece like I usually do but I was anxious to do the star testing with the excellent seeing.  The star test was horrible, showing at least one wave of astigmatism, easily seen even at low power.  I was quite taken aback especially considering the truly excellent results from my first test sample.  Subsequent checking of the objective with the Cheshire showed the two dots from the reflections off of the rear facing lens elements were way off from the image of the Cheshire aperture dot.  I was major bummed. 

 

Then I started thinking about it.  The image seemed to be pure astigmatism, nothing else.  Astigmatism is the dominant off axis aberration in most refractors.  I also noticed that panning an out of focus star image over to the edge of the FOV of my 7mm UO ortho, resulted in no astigmatism.   The native astigmatism of the ortho at the field edge was cancelling the objective's astigmatism.  So, I was certain the lens cell was cocked off axis on the tube.  The problem is, there is no provision for cell collimation adjustment with  this design.  I was discouraged but then I had a closer look at the end of the cell that threads on to the tube and could see a "reveal" of a couple of tube threads that was uneven around the perimeter of the tube suggesting the cell was cross threaded on to the tube.  Examination of the other 150ED sample showed no such condition.  

 

Sure enough, using my strap wrench to back the cell off of the tube confirmed the cross threading and subsequent cocking of the cell on the tube.  The threads, while accurately machined, have a rather coarse pitch, which allows easy cross threading of the tube and lens cell.  Once the cell was properly assembled to the tube, the astigmatism promptly disappeared, even at 340x.  In fact, the subsequent star tests of Pollux, Castor and Procyon at high power with/without a green filter were excellent, perhaps a bit under corrected with ~1/6 or 1/7 th wave of coma that dissolved over time as the scope settled out thermally.  

 

All of this is backed up with the DPAC testing.

 

First the green LED and white light images.  

 

It's getting late and I'll comment tomorrow, but have a go at it if you wish.

 

Jeff

Interesting that your Null shows the same central “Nub” as the previous one.  That and the overall performance  definitely shows the consistency of the automated machine polishing.   I remember the thread when these first came out, and there was much debate about the “mystery” glass and potential quality.  It seems to me that your results more or less validate the early reports of some of the more experienced visual observers from that time.  In your discussion for the previous sample, you mentioned that the results reminded you of those you have generated for some of the older Pre -ED glass AP Starfires.   From what I remember of results published by you and Paul, that seems to be accurate.   You guys are generating quite a catalog of data.  It would be an interesting exercise to compare results from one of these SW scopes with one of the older 6 inch AP scopes, and one of the 6 inch AP/TAK/LZOS/CFF… Triplets.  

 

JMD

[scooke]

Wished you could test the one i had.

Here you go.  Chas sold this to me and I subsequently sold when I got a TOA150.  It was an excellent sample with Moonlight focuser and Parallax rings.  Pictures are not great, it was the second scope on tested with DPAC.

[scooke]

This was from February 2020.  

[CHASLX200]

This was from February 2020.  

It did 450+ easy on the planets.

[Jeff B]

Wished you could test the one i had.

Me too!

[Jeff B]

Here you go.  Chas sold this to me and I subsequently sold when I got a TOA150.  It was an excellent sample with Moonlight focuser and Parallax rings.  Pictures are not great, it was the second scope on tested with DPAC.

Ok, that's sample three.  It looks like an outside of focus image.  Is that right?

 

Well done.

[Jeff B]

This sample is not a neutrally corrected in green/yellow as my first sample, but it is still very good and overall simulates out to be around 1/7 wave undercorrected in green.

 

The figure is what I call a "compound figure", specifically, the inner ~70% span tends to be neutrally corrected then smoothly transitions towards undercorrection out to the edge with no abrupt hooks there.  I have seen this before, making it a little difficult to simulate this with Macek's program.  I matched the outer 30% curvature to the lines, which undercorrected the center, but that was a conservative approach.  

 

You can see the secondary spectrum is rather low as the centers of the R/L outer bands show some blue/red fringing.  You see a bit of blue spherochromatism in blue as the blue tips to the R/L bands as well.  The green at focus image shows to me a subtle hexagonal shape to the zone transition.  I also see very mild radial lines.

 

DPAC shows everything! 

 

Here are this sample and the first sample compared in white light, inside of focus.  There is a color balance difference because I used a slightly cooler white balance setting when taking the sample two images.  They are more similar to me than different, with sample one being just a hair width overcorrected with the second sample being overall, slightly undercorrected.  Out under star testing the only differences I saw between them were, maybe, the second sample was indeed subtly undercorrected when examining the inside/outside rings in green.  At focus they were identical to me except for that small bit of thermal related coma that went away in the second sample.  All this at 340X.  At a more use full magnification, I could detect no differences between them with the air overhead, which was really quite steady....but not Chas-in-Florida perfect  .

 

Also attached are the color montages.  This sample seems to have best correction a little left of green, while sample one was best corrected in yellow.  However this is just an interesting observation as spherochromatism in both samples seems very well controlled from  green to red.

 

I will update my 150ED + Chromacor thread to include my Chromacor experiments with sample two.

 

Jeff

 

Jeff 

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[Wildetelescope]

So these scopes all look like they have a pretty good figure.  Not the level of perfection one would expect from a $13-20K+ six inch triplet, but certainly better than diffraction limited.  I would expect them to be excellent for visual use.  I would also expect them to be quite nice for someone doing astrophotography and wanting to take advantage of the larger aperture.  I would expect Narrow band imaging would be just fine, and that even imaging with an OSC and UV-IR cut filter would not look bad at all.  

 

JMD

[scooke]

Ok, that's sample three.  It looks like an outside of focus image.  Is that right?

 

Well done.

Actually inside focus.  I managed not to save an outside of focus image.  It was actually a true green but my cellphone changed it to blueish green.

[Orion68]

Jeff,
Which cheshire did you use?

[Jeff B]

Jeff,
Which cheshire did you use?

My Tectron Cheshire.

[Jeff B]

Actually inside focus.  I managed not to save an outside of focus image.  It was actually a true green but my cellphone changed it to blueish green.

Ahh, so it's very slightly overcorrected in green, just like my first sample.

[Jeff B]

Interesting that your Null shows the same central “Nub” as the previous one.  That and the overall performance  definitely shows the consistency of the automated machine polishing.   I remember the thread when these first came out, and there was much debate about the “mystery” glass and potential quality.  It seems to me that your results more or less validate the early reports of some of the more experienced visual observers from that time.  In your discussion for the previous sample, you mentioned that the results reminded you of those you have generated for some of the older Pre -ED glass AP Starfires.   From what I remember of results published by you and Paul, that seems to be accurate.   You guys are generating quite a catalog of data.  It would be an interesting exercise to compare results from one of these SW scopes with one of the older 6 inch AP scopes, and one of the 6 inch AP/TAK/LZOS/CFF… Triplets.  

 

JMD

Funny you should ask.   White LED images where possible, inside of focus, just place the cursor over the image to see the scopes/title.

 

Jeff

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Edited by Jeff B, 15 May 2023 - 07:14 PM.

[Wildetelescope]

Funny you should ask.   White LED images where possible, inside of focus, just place the cursor over the image to see the scopes/title.

 

Jeff

Now THAT is an interesting set of comparisons.  Especially the white light.  I am not sure what the biggest surprise is, the TEC, the AT or the CFF?   I assume the conditions for testing were kept as close as possible and that the color calibration of the images is consistent for all of them.  Is the AT the 152 EDT?   I am no DPAC expert, but that looks pretty good.

 

The AP 6 inch F8 is one of the really old ones.  Before 1988.  I believe it came after that NASA glass scopes, and was one of the first KZFSN-4 glass with an Abbe number of around ~44 or so.   I have the later F9 version.   Is last result in green an AP f9 starfire? 

 

Need to digest all that.  I own one of the old AP F9s so I am very familiar with its performance visually and photographically.  Assuming that its DPAC performance is roughly similar to the F8, that gives me a sort of Rosetta Stone to the rest of therm:-).   My impression is that visually, all these scopes would perform quite well.  If you go looking for CA with the AP F8 or the SW F8,  on bright objects, you will find it, but you have to look for it.  Photographically you will see CA, but not too bad.   A UV/IR filter will help and I do not doubt Chas’ comments about his SW version on Jupiter at high mag.  They track with what I see with my AP f9, only with more Turbulence:-). 

 

At some point I need to take Paul up on his offer to check out some of mine.  Just not enough time in the day.

 

Thanks Jeff!  

 

Cheers!

 

JMD

[AstroCub]

Hi Jeff

 

Thanks for all the great threads you produce, they are appreciated. 

 

As a relatively new girl on the block, and at the risk of asking maybe the most naive question of all time - what do the DPAC pictures actually mean / represent? I mean, I know the green vertical stripes indicate the lens' performance in the green part of the spectrum, ~ 500 NM. 

But honestly I don't know how to tell apart one set of images for a given part of the sprectrum between each OTA, IE I don't know how to meaningfully compare the results. Is thise where strehl comes in? 

Any info you could point me to would be appreciated, thanks! 

 

Best,

 

Siouxsie  

Edited by AstroCub, 23 June 2023 - 02:46 PM.

[SteveG]

Interesting outcome! I hate to admit this, but I cross-threaded a lens cell onto a Synta ED100, and it went all the way on except for a thread, maybe two. I later thought something looked off about the tube, but wasn’t sure. I took some carful measurements, and determined it was cross-threaded. I quickly removed it and re-threaded it properly. I never looked through it when it was cross-threaded, which would have revealed my error.

[Jeff B]

Hi Jeff

 

Thanks for all the great threads you produce, they are appreciated. 

 

As a relatively new girl on the block, and at the risk of asking maybe the most naive question of all time - what do the DPAC pictures actually mean / represent? I mean, I know the green vertical stripes indicate the lens' performance in the green part of the spectrum, ~ 500 NM. 

But honestly I don't know how to tell apart one set of images for a given part of the sprectrum between each OTA, IE I don't know how to meaningfully compare the results. Is thise where strehl comes in? 

Any info you could point me to would be appreciated, thanks! 

 

Best,

 

Siouxsie  

Hi Siouxie and please don't feel naive as we all start out a bit short of experience.  I learn by doing and asking and learn best by failing or stumbling during the process (though I only admit that later....).

 

Yes, the straighter the lines the better at any color.  With refractors, the lines will typically bend in different ways and amounts depending on the color.  This is called spherochromatism, the variation of spherical aberration (what causes the lines to curve) with color.  The curvature of the lines with "flip" from one side of focus to the other.  Specifically, when inside of focus, lines that look like )l( show undercorrection where the light from the outer portions of the lens are falling slightly short (closer to the lens) of those from the more central portion of the lens.  Overcorrection is the opposite of that.  By looking at the individual red, green and blue colors derived from the white light images using software, you a can also sometimes get an idea how close, or how far apart when it comes to achromats, those colors focus to each other.   

 

The at-focus images show how smooth and even the wave front is and you can readily see if there are "zones", for example circular zones, and if the polish of the lens is rough.  

 

There is also a thread above where the CN's staff has been doing a great job of archiving DPAC testing and information:

 

https://www.cloudyni...-on-refractors/

 

But DPAC is only semi-qualitative.  You can see stuff at a mere glance but you cannot assign a Strehl number to it.  Maciek here on CNs is developing a bit of software where, under a specific set of boundary conditions, you can generate a Strehl by simulating the DPAC images.  However, in interferometer is the best way to  generate a Strehl......but they scare me off with the set up, quality, precision and repeatability requirements.

 

Keep asking questions Siouxie!

 

Jeff

Edited by Jeff B, 24 June 2023 - 11:08 AM.

[David Lim]

Here is a good reference for ronchi images of various optical abberations.  I use this manual often to remind myself of what I am looking at.  This manual came with a Gerd Neumann Ronchi Eyepiece that I bought several years ago.  Later I realized that this eyepiece is for a single pass ronchi which is not very sensitive compared to double pass method.  The manual came in more handy than the eyepiece itself 

 

 

 Ronchi Eyepiece manual.pdf   127.27KB   73 downloads

Edited by David Lim, 24 June 2023 - 11:21 AM.

[Scott in NC]

Here is a good reference for ronchi images of various optical abberations.  I use this manual often to remind myself of what I am looking at.  This manual came with a Gerd Neumann Ronchi Eyepiece that I bought several years ago.  Later I realized that this eyepiece is for a single pass ronchi which is not very sensitive compared to double pass method.  The manual came in more handy than the eyepiece itself 

 

 

Ronchi Eyepiece manual.pdf

Very nice reference—thanks for sharing that link!  One thing that needs to be pointed out though, is that this reference is for mirrors and not for lenses, but can still be used with some minor modifications.  Spherical aberration (over- vs. under-correction) will still look the same for mirrors vs. lenses.  Edge errors (turned up vs. down) and central hills vs. depressions will look opposite from each other for mirrors vs. lenses.

[AstroCub]

Hi Siouxie and please don't feel naive as we all start out a bit short of experience.  I learn by doing and asking and learn best by failing or stumbling during the process (though I only admit that later....).

 

Yes, the straighter the lines the better at any color.  With refractors, the lines will typically bend in different ways and amounts depending on the color.  This is called spherochromatism, the variation of spherical aberration (what causes the lines to curve) with color.  The curvature of the lines with "flip" from one side of focus to the other.  Specifically, when inside of focus, lines that look like )l( show undercorrection where the light from the outer portions of the lens are falling slightly short (closer to the lens) of those from the more central portion of the lens.  Overcorrection is the opposite of that.  By looking at the individual red, green and blue colors derived from the white light images using software, you a can also sometimes get an idea how close, or how far apart when it comes to achromats, those colors focus to each other.   

 

The at-focus images show how smooth and even the wave front is and you can readily see if there are "zones", for example circular zones, and if the polish of the lens is rough.  

 

There is also a thread above where the CN's staff has been doing a great job of archiving DPAC testing and information:

 

https://www.cloudyni...-on-refractors/

 

But DPAC is only semi-qualitative.  You can see stuff at a mere glance but you cannot assign a Strehl number to it.  Maciek here on CNs is developing a bit of software where, under a specific set of boundary conditions, you can generate a Strehl by simulating the DPAC images.  However, in interferometer is the best way to  generate a Strehl......but they scare me off with the set up, quality, precision and repeatability requirements.

 

Keep asking questions Siouxie!

 

Jeff

Hey Jeff

 

Thanks for getting back and for your explanation! 

 

So, if someone, were to show me 2 DPAC reports for 2 OTAs, and that person said how much better one OTA was compared to another, the better one would have straighter more definded vertical lines?? Any other factors that would distinguish one as being better than another? 

 

I'm asking because when I see DPAC reports in threads here I honestly can't see any difference between one and another, but that's likely due to the fact that I don't know what to look for! 

 

Thanks, best, 

 

Siouxsie  

[peleuba]

 

But DPAC is only semi-qualitative.  You can see stuff at a mere glance but you cannot assign a Strehl number to it. 

 

Hi Jeff - I think you meant that DPAC is fully qualitative but not really quantitative in any exacting manner.

 

Maciek's program is excellent - especially if the optic is smooth and around F7 or longer.  Its shows - directly - how various levels of aberrations affect the Ronchi bands.

 

 

 

So, if someone, were to show me 2 DPAC reports for 2 OTAs, and that person said how much better one OTA was compared to another, the better one would have straighter more definded vertical lines?? Any other factors that would distinguish one as being better than another? 

 

You are in good hands with Jeff, but your question is a good one so I wanted to add some commentary...    Yes.  On a fundamental level, straight lines mean less spherical aberration for a given wavelength.  Zones also affect the straightness as can localized surface roughness.   But you'll be on firm ground saying the straighter the bands, the better the lens - especially if the straightness coincides with the visual peak (Green).

Edited by peleuba, 25 June 2023 - 10:30 AM.

[Scott in NC]

 

So, if someone, were to show me 2 DPAC reports for 2 OTAs, and that person said how much better one OTA was compared to another, the better one would have straighter more definded vertical lines?? Any other factors that would distinguish one as being better than another? 

 

To answer question #1, yes.  That may be an oversimplification though, as slight curvature of the Ronchi bands (representing very small amounts of spherical aberration) may not have any meaningful effect on the views through that scope.  A perfect scope would have no Ronchi band curvature, but you’ll never see that, as there’s no such thing as a scope with Strehl 1.00.  And an excellent scope may have near perfect looking bands in green and red, with a little more curvature in blue.  That shows the effects of spherochromatism (changes in degree of spherical aberration which vary according to wavelength).  Any lensed scope (as opposed to a mirrored scope) will exhibit this to one degree or another.  Also, look at the white light images to see how much color you see.  Even some very nice doublet apochromats will still exhibit some color, and an excellent triplet will exhibit even less or none.  An achromat, no matter how well designed, will show quite a bit more color.