27 replies to this topic

[Howard_C]

After viewing innumerable DPAC posts the last few months I’ve finally succumbed and joined the dark side, and obtained a flat with holder, LEDs, Ronchi screens and screen holders from (or with the assistance of) several helpful CN members. Here is my setup:

 

 

 

 

The flat is 6” uncoated 1/10 wave +.  The screen/LED holder in the focuser holds the 3mm RGB 4-lead LED, which is connected to the home brew switch box (on the white board) which has a separate dimmer control for each color.  Next to the box there’s a separate screen/LED holder for the white LED.  The power supply on the left is my bench supply, but a cheap wall wart works just as well.  Finally, my ancient but sturdy World Books and Nikon D40 DSLR complete the setup (I always knew those World Books would come in handy someday!).

 

Here are my results:

 

 

 

Visually, I am quite pleased with this scope (I don’t image) and hope my pics do the scope justice.  I haven’t noticed any false color.   It’s not light for its aperture, but it gives steady views with an XW3.5 (the highest mag EP I’ve used with it) when mounted on an M2C and Gitzo carbon tripod.

 

Your impressions of the results and suggestions are welcome! 

 

Howard

 

 

[Jeff B]

Great work Howard.  And it's not the dark side as you are helping to move the dial of knowledge away from the dark and mysterious and into the light.  Thanks for that.

 

I like how you arranged your colors!

 

Seems the objective is a little undercorrected across the board, more so in the red.  Polish is smooth with a mild, circular center zone but smooth transition outward.  A nice edge too.

 

LCA/secondary spectrum seem very well controlled.

 

A nice scope. 

 

I like that you have a very even exposure from red to blue.

 

Can you please describe your camera a bit more (lens, settings...)?  Also, I assume you used a 133 LPI screen?  Which processing software do you use?

 

Well done sir.

 

Jeff 

[Scott in NC]

Nicely done, Howard!  These are much better than my first images from back when I started DPAC testing.  I don’t have much to add to what Jeff already said, but find it interesting that spherical correction is nearly the same in R, G, and B.  The edge might have a very slight downturn, but when it’s this small I often have a hard time deciding whether it’s real or just a diffraction artifact.  Regardless, even if it’s real then it’s most likely too minimal to make a difference.  This looks like a pretty nice optic.

[Howard_C]

Can you please describe your camera a bit more (lens, settings...)?  Also, I assume you used a 133 LPI screen?  Which processing software do you use?

 

Jeff 

Hi Jeff -
 

I used a 138 LPI Ronchi screen and Nikon 18-55mm lens (set at 55mm). 

 

Shutter speed was 1/10 sec.

 

I used Paint.net for image cropping, resizing, and combining. 

 

Thanks much for your many detailed test reports, which have been instrumental in my decision to learn how to perform my own tests!

 

Howard

[Howard_C]

Nicely done, Howard!  These are much better than my first images from back when I started DPAC testing.

Hi Scott -

 

Only because I had the opportunity to learn from your experience and your work to spread the use of DPAC testing in the community. 

 

Thanks much for your efforts!

 

Howard
 

[Howard_C]

I would also like to thank CN member David Lim, who  was instrumental in setting up my test bench.  Also CN member Moshen for providing his excellent Ronchi screen/LED holders.

 

Howard

[Scott in NC]

Glad you’ve joined the bandwagon!  I really like how you’ve presented your images all lined up for easy comparison.  That’s something that I really need to learn how to do.  I use GIMP for image processing, but am not sure if it has a function that allows one to do that or not.

[sdedalus83]

Nicely done, Howard!  These are much better than my first images from back when I started DPAC testing.  I don’t have much to add to what Jeff already said, but find it interesting that spherical correction is nearly the same in R, G, and B.  The edge might have a very slight downturn, but when it’s this small I often have a hard time deciding whether it’s real or just a diffraction artifact.  Regardless, even if it’s real then it’s most likely too minimal to make a difference.  This looks like a pretty nice optic.

I wonder if that’s a characteristic of triplets with two ed elements. The Esprit looks like it behaves the same way.

http://interferometr...esprit.html?m=1

[Scott in NC]

I wonder if that’s a characteristic of triplets with two ed elements. The Esprit looks like it behaves the same way.

http://interferometr...esprit.html?m=1

I’m pretty sure that unlike the scope tested here, the Skywatcher 120ED f/7 Esprit has a single FPL-53 element and two mating elements. But regardless, I’ve seen this in a number of refractors, both doublets and triplets.  I don’t think that it’s particular to any one optical design.

[Lagrange]

Nicely done, Howard!  These are much better than my first images from back when I started DPAC testing.  I don’t have much to add to what Jeff already said, but find it interesting that spherical correction is nearly the same in R, G, and B.  The edge might have a very slight downturn, but when it’s this small I often have a hard time deciding whether it’s real or just a diffraction artifact.  Regardless, even if it’s real then it’s most likely too minimal to make a difference.  This looks like a pretty nice optic.

The use of wide air spaces (inspired by the Tak TOA I believe) is maybe why the spherical correction is almost the same for each colour. Apparently it greatly reduces spherochromatism.

[Ephemeral]

Thanks for sharing!

I believe these scopes ship with test certificates; how did your tests compare with the supplied results?

[Maciek_Cz]

I tried to analyze the ronchigrams for the green color with AOS software.

After fighting with the definition of the middle zone, it turns out that for the green Strehl ~0.94 (definitely higher than 0.90).

Please note that AOS does not analyze the wavefront, but only checks the criterion of hitting the Airy disk for best focus.

If the author of the thread has a certificate, I'd love to see it :-)
The lens is characterized by a center zone and a delicate turned edge.

[Howard_C]

Here you go:

 

 

Howard

[Scott in NC]

Thanks for that information, Howard.  It’s interesting that the manufacturer-supplied Ronchi test looks so different from the one that you provided, almost as if they were from different scopes.  Of course I place more faith in the accuracy of the one that you performed, as it looks more like a real Ronchigram, accurately detailing the edges and central zone. The manufacturer’s Ronchigram shows perfectly straight bands without deviation from normal, which is a rarity in even the best of scopes, and doesn’t state in which color the test was performed (although I’m reasonably sure that it was done in white light based upon its appearance).

 

You still have a very nice scope, but this really helps to illustrate why more of us need to get involved in testing these scopes for ourselves, rather than merely accepting what we’re told.

[Maciek_Cz]

Scott, the answer is simple. The manufacturer performed the Ronchi test in a single-pass system.

Admittedly, rather with a dense Ronchi screen ~10 or 13 lines per mm, but at a large distance from the focus.

This is what it looks like in the simulation as well.

[Scott in NC]

Thanks!  I should have been able to figure that out, but I’m still learning.

 

What do you see that helps you determine the density of the Ronchi screen?

[John Carlini]

Thanks for the test results! I like Long Perng products and it's nice to know how good they are from a scientific standpoint. You must also be a ham radio operator given the cat on the radio avatar and using the word "home brew". -73

[Jeff B]

Scott, the answer is simple. The manufacturer performed the Ronchi test in a single-pass system.

Admittedly, rather with a dense Ronchi screen ~10 or 13 lines per mm, but at a large distance from the focus.

This is what it looks like in the simulation as well.

Yup, single pass.  Also the star test is way out of focus with no mention of the magnification used, however, it does clearly show the center zone.  Getting close to focus at high power, I imagine you might see that center zone as  a "hot spot" in the middle close to one side of focus and a dark spot on the other side, close to focus.

 

To me this is another example of what I call a mild "compound figure".  Ignoring the center zone, the objective seems more neutrally corrected in the middle than out towards the edge with a smooth transition between both areas intercepting the edge nicely.   Delicate is a good word for the transition.

 

But it's smooth with what seems to be well controlled and mild color errors.

 

Howard, what does the high power star test tell you?

 

Again, well done.

 

Jeff

[Howard_C]

Thanks for the test results! I like Long Perng products and it's nice to know how good they are from a scientific standpoint. You must also be a ham radio operator given the cat on the radio avatar and using the word "home brew". -73

Hi John –

 

I also have a smaller Long Perng that I intend to test.   Both are fine scopes.

 

Yup, been a ham since the mid-1960’s.  Our cats loved to bask on my tube rigs (like the Heathkit in my avatar) in winter.  I still have my first rig (another Heathkit).

 

73 OM!

Howard

Edited by Howard_C, 19 June 2023 - 03:35 PM.

[Howard_C]

Howard, what does the high power star test tell you?

 

Again, well done.

 

Jeff

Hi Jeff –

 

 

I appreciate your comments!  I’ll try my own star test when the opportunity presents itself and report back.  It will give me a chance to try out the TV 2.5mm T6 that I recently purchased.

 

 

Thanks again,
Howard

[Kitfox]

Been a while since I heard "Heathkit"  Used to fly RC with an old Heathkit.  

 

 

Hi John –

 

I also have a smaller Long Perng that I intend to test.   Both are fine scopes.

 

Yup, been a ham since the mid-1960’s.  Our cats loved to bask on my tube rigs (like the Heathkit in my avatar) in winter.  I still have my first rig (another Heathkit).

 

73 OM!

Howard

[peleuba]

Great work Howard.  And it's not the dark side as you are helping to move the dial of knowledge away from the dark and mysterious and into the light.  Thanks for that.f 

 

Agree.   A terrific job.   The veil of uncertainty for you has been lifted.  One no longer has to wonder if the images the telescope presents are poor because of seeing, cooling or optics.  

 

The only advice I can give to you or others who starting out is this:   When testing an lens at F6.5ish or faster in DPAC, the test will show even a perfect optic to be ever-so-slightly undercorrected.  The reason for this is that the outer rays pass thru the screen at a slightly longer distance than the inner rays.  See the diagram I've attached below.  The distance D2 is longer than D1, so the outer rays bend inward in the test, making it look like the optic is undercorrected.  The only way for the Ronchi to be absolutely accurate is if it had an inward curvature equal to the distance from the focus point, so that D2 = D1.

 

This was pointed out to me by a master optician who used to figure lenses using DPAC decades ago.  In my testing, I have found the effect slight, but noticeable once I look for it.

 

 

 

I appreciate your comments!  I’ll try my own star test when the opportunity presents itself and report back.  It will give me a chance to try out the TV 2.5mm T6 that I recently purchased.

 

I really like the NT6's as well as the 3-6 Zoom for star testing.   There is a persistent myth that says you should use a simple eyepiece like a Plossl or Ortho for the star test.  This is wrong.  You want an eyepiece that is well corrected and will not add any aberrations of its own to the test.

Attached Thumbnails

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Edited by peleuba, 20 June 2023 - 02:28 PM.

[RichA]

Here you go:

 

Founder Certificate.jpg

 

Howard

 

The central zone error shows I believe in the star test, but not in the report's ronchi test.  Though I can see it in yours. 

[Scott in NC]

The central zone error shows I believe in the star test, but not in the report's ronchi test.  Though I can see it in yours. 

That’s what made me initially wonder if the Ronchi test in the report actually came from that particular scope. But I suppose it probably did. I’ve never done a single-pass Ronchi test, and it looks like the single-pass test is even more insensitive than I thought to all but the grossest of optical errors.

[Howard_C]

Been a while since I heard "Heathkit"  Used to fly RC with an old Heathkit.  

Assembling kits was a great learning experience when I was getting started in ham radio.  I remember the suspense when I turned them on for the first time, and the amazement that they actually worked!  Well, most of the time anyway…  (-:  

 

Howard