61 replies to this topic

[Jeff B]

This is the intro

 

https://www.youtube....h?v=pTFbYzaQl2s

 

More detailed stuff later.

 

Enjoy!

 

Jeff

[Enyo]

Great video, saw it first posted on AP's group. Now we know what you look like!

[Dean J.]

This is the intro

 

https://www.youtube....h?v=pTFbYzaQl2s

 

More detailed stuff later.

 

Enjoy!

 

Jeff

If a rocket scientist says Double Pass Ronchi Testing isn't rocket science, then who am I to argue? 

 

The video was fun and informative.

[deSitter]

This is the intro

 

https://www.youtube....h?v=pTFbYzaQl2s

 

More detailed stuff later.

 

Enjoy!

 

Jeff

Wow that's a jail I wouldn't mind being in! What a test!

 

-drl

[PatientObserver]

This is the intro

https://www.youtube....h?v=pTFbYzaQl2s

More detailed stuff later.

Enjoy!

Jeff

Thanks for sharing. I enjoyed seeing how the test is done rather than just reading about it.

[AstroApe]

Awesome video and an even awesomener scope! Thanks for sharing 

[kgb]

Awesome!

[Kitfox]

That 175 is nice, but that little fella in the ROR isn’t going anywhere!  It looks so small in pictures, tho.  10” of amazing glass…  I’ll never forget the impression it gave me.  Overwhelming at the time, for sure. 

 

If those viewing this can’t tell, Jeff is one of the best hosts there ever was.  Just a nice guy.  I’m looking for an excuse to knock on his door again.  Soon!  (Hint, hint  )

[PatientObserver]

Jeff, what is the cheshire eyepiece you use to test collimation?

Edited by PatientObserver, 08 July 2024 - 10:35 AM.

[DBPeel]

Good video, I appreciated seeing the process.  Thanks.

[fate187]

So cool to see you in person after reading your posts for years! That LZOS 254 on the mount ...

[Scott in NC]

Excellent job, Jeff!

 

If you get a chance, would you mind posting some of the Ronchigram images here so that we can also look at them without having to view the video each time? Or if you don’t have time, maybe later tonight I can try to get some screenshots to post.
 

[t.r.]

Great video and content. I see a retirement hustle for Jeff packaging DPAC kits complete with the printed screen holder…talk about getting DPAC into the hands of the masses!! 

[deSitter]

Great video and content. I see a retirement hustle for Jeff packaging DPAC kits complete with the printed screen holder…talk about getting DPAC into the hands of the masses!! 

Oh what an excellent idea! Seconded!

 

-drl

[denis0007dl]

Nice to see you Jeff, finally 

 

Awsome datas, awsome telescope, I simply love that finish and precision AP does!

 

One of my friends says telescope have bad center zone defect, which spreads light into airydisc, and that lines have some roughness.

 

He also says that deep valley in center can not be removed.

 

While Im not expert here, maybie Jeff or others can comment it?

Attached Thumbnails

• 
• 

[Scott in NC]

Great video and content. I see a retirement hustle for Jeff packaging DPAC kits complete with the printed screen holder…talk about getting DPAC into the hands of the masses!! 

Jeff is the master DPAC tester, but I feel the need to do a shout-out to my son, who designed and 3D printed the Ronchi screen holder for Jeff.

 

Edit: So as to not short-change Paul (peleuba), I posted a clarification here: https://www.cloudyni...edf/?p=13554469

Edited by Scott in NC, 08 July 2024 - 10:09 PM.

[deSitter]

Nice to see you Jeff, finally 

 

Awsome datas, awsome telescope, I simply love that finish and precision AP does!

 

One of my friends says telescope have bad center zone defect, which spreads light into airydisc, and that lines have some roughness.

 

He also says that deep valley in center can not be removed.

 

While Im not expert here, maybie Jeff or others can comment it?

There is an obvious central zone and turned edge, yet the DPAC looks superficially OK. This is a case where the star test would show how it does in focus and would certainly show these errors more directly than with DPAC. Interferometry is not a panacea for testing.

 

-drl

[peleuba]

Jeff is the master DPAC tester, but I feel the need to do a shout-out to my son, who designed and 3D printed the Ronchi screen holder for Jeff.

 

 

Scott - Wait - what?  The Ronchi holder that Jeff is using looks like identical to the one you're son printed for me from the plans I sent to you back in May, 2023 via private email.  

 

If there is an improved version, I'd like to hear about it.   As you know, the Ronchi holder is critical to holding the screen and LED in alignment with both the objective lens and autocollimation flat.  Your son is excellent (I don't want to use his name in public) and if he made an "improved version" using the plans I sent to you please don't be bashful about.  Innovation helps us all.  

 

In the same email, I also sent plans for a 3D rendering of a Foucault/Knife Edge holder. 

Edited by peleuba, 08 July 2024 - 09:55 AM.

[peleuba]

There is an obvious central zone and turned edge, yet the DPAC looks superficially OK. This is a case where the star test would show how it does in focus and would certainly show these errors more directly than with DPAC. Interferometry is not a panacea for testing.

 

Superficially OK - I don't mean to pick nits but what does that even mean?

 

You are correct that a central zone as well as an edge zone "are obvious".   And that is the beauty of using a Ronchi screen in Autocollimation (DPAC) - it demonstrates the qualitative nature of the spherical correction as well as surface condition of a lens quite quickly and efficiently.  Its not a superficial test.  Also, its not likely that the star test would show these errors "more directly" then DPAC.   Star testing is the sum of a wobbly stack of variables that often overlap each other.  DPAC presents an almost sterile view of the lens (or mirror) at the wavelength tested.  There are extremely few variables to manage which would impart any error into the system.  You cannot say the same for the star test performed outside under the night sky with typical thermal and seeing conditions.  Nevertheless, I would definitely star test the lens on my bench and I am certain the tests methods would agree.

 

To further dissect the DPAC images, its important to know that if something looks like a hill in double pass, its really a valley.  The inverse is also true - if it looks like a depression, its really a high spot.    That central zone zone is likely only ~⅒ wave high/deep.  Its unlikely that it would bleed any light into the diffraction rings surrounding the Airy disk for this reason and its directly in the center which nearly benign.  I am not even sure the edge zone would do much damage as its so narrow and not very deep.   I would also want to make sure this is not made worse in appearance by the effects of diffraction.  

 

I know a little about interferometry and maybe not a panacea, but its the gold standard if needing to supply certification.  DPAC is more useful and practical for an amateur.

Edited by peleuba, 08 July 2024 - 10:58 AM.

[Jeff B]

Paul, you nailed it.

 

I too put the center zone somewhere between 1/10 to maybe 1/12 wave high/deep and I'll publish some other images but I, in part, base this on how big of a "dent" or "bulge" the zone makes in the line shadow, pulling in learning from you, sensei Paul, and others such as CZ and other skilled opticians.  Also, doing a quick measure shows me it covers maybe 12-13 MM of the center.  Putting a perspective on that, if I say, masked it, that's an ~ 7.5% central obstruction, "worst case", which covers maybe .5% to .6% of the surface, in the center.  But, in reality, the feature is smooth and passes light.  In short, yes (!), entirely benign and trivial. But, yup, "DPAC", immediately shows even the trivial stuff....double reality.

 

So, a bad center defect?  Really!?

 

Seen during a star test?   Really?  At least not outdoors and then certainly not at focus.

 

I'll be publishing more images later, some of which may really call into question the hooked edge comments.

 

Jeff 

[rainycityastro]

Is there a DPAC test of Jeff’s LZOS scope anywhere?

[deSitter]

Superficially OK - I don't mean to pick nits but what does that even mean?

 

You are correct that a central zone as well as an edge zone "are obvious".   And that is the beauty of using a Ronchi screen in Autocollimation (DPAC) - it demonstrates the qualitative nature of the spherical correction as well as surface condition of a lens quite quickly and efficiently.  Its not a superficial test.  Also, its not likely that the star test would show these errors "more directly" then DPAC.   Star testing is the sum of a wobbly stack of variables that often overlap each other.  DPAC presents an almost sterile view of the lens (or mirror) at the wavelength tested.  There are extremely few variables to manage which would impart any error into the system.  You cannot say the same for the star test performed outside under the night sky with typical thermal and seeing conditions.  Nevertheless, I would definitely star test the lens on my bench and I am certain the tests methods would agree.

 

To further dissect the DPAC images, its important to know that if something looks like a hill in double pass, its really a valley.  The inverse is also true - if it looks like a depression, its really a high spot.    That central zone zone is likely only ~⅒ wave high/deep.  Its unlikely that it would bleed any light into the diffraction rings surrounding the Airy disk for this reason and its directly in the center which nearly benign.  I am not even sure the edge zone would do much damage as its so narrow and not very deep.   I would also want to make sure this is not made worse in appearance by the effects of diffraction.  

 

I know a little about interferometry and maybe not a panacea, but its the gold standard if needing to supply certification.  DPAC is more useful and practical for an amateur.

"Those bars are nice and parallel and straight, so it's good." That is a superficial assessment. Any test will have a mode where a superficial assessment can lead you to think it's OK. In the star test, it can be very hard to see minor astigmatism because you are looking for lack of circularity in something that is small and very nearly circular. Likewise if you just judge DPAC by "jail bars", and it's hard not to, you can miss things.

 

-drl

[deSitter]

Paul, you nailed it.

 

I too put the center zone somewhere between 1/10 to maybe 1/12 wave high/deep and I'll publish some other images but I, in part, base this on how big of a "dent" or "bulge" the zone makes in the line shadow, pulling in learning from you, sensei Paul, and others such as CZ and other skilled opticians.  Also, doing a quick measure shows me it covers maybe 12-13 MM of the center.  Putting a perspective on that, if I say, masked it, that's an ~ 7.5% central obstruction, "worst case", which covers maybe .5% to .6% of the surface, in the center.  But, in reality, the feature is smooth and passes light.  In short, yes (!), entirely benign and trivial. But, yup, "DPAC", immediately shows even the trivial stuff....double reality.

 

So, a bad center defect?  Really!?

 

Seen during a star test?   Really?  At least not outdoors and then certainly not at focus.

 

I'll be publishing more images later, some of which may really call into question the hooked edge comments.

 

Jeff 

That tiny thing in the center - sure that's nothing - but there is another zone at about 25% and another at about 50%. And what are the specs D and F of this lens? How do you know it's 1/10th wave? Anyway I'm sure the effect of these three zones can be easily seen in a star test. I am not knocking DPAC, for optics makers it's obviously the gold standard, I'm just saying there are complementary ways of testing, and each has advantages. The tiniest astigmatism can be seen in DPAC because we're very good at detecting convergence and rotation of lines.

 

-drl

[peleuba]

"Those bars are nice and parallel and straight, so it's good." That is a superficial assessment. Any test will have a mode where a superficial assessment can lead you to think it's OK. In the star test, it can be very hard to see minor astigmatism because you are looking for lack of circularity in something that is small and very nearly circular. Likewise if you just judge DPAC by "jail bars", and it's hard not to, you can miss things.

 

 

Don't mean to be argumentative and I appreciate the discussion.  I do understand your point, but you are confusing quick with superficial.  These two descriptions (quick/superficial) in this example are only tangentially related.  When one evaluates data from DPAC and views jail bar straight lines like this optic demonstrates, one can reasonably and quickly conclude that at the tested wavelength spherical correction is about ~⅒ wave or better and there are no prominent image degrading zones.  While this is a quick eval, its not superficial.  I don't throw around ~⅒ wave ratings superficially .    I know that you understand optics but also know that when testing lenses and mirrors, words really do matter.  At this boutique level, its all about nuances.

 

Regarding your example on astigmatism...    I guess it depends upon your definition of minor.  I find the star test to be THE quintessential test for astigmatism down to about ~⅒ wave.   To test this, I just used Aberrator to model a mythical 100mm F/8 two element APO.  I found anything less then about a tenth wave and I cannot see/detect the astig without magnifying the Airey disk - and that would be cheating  .

 

DPAC easily shows astigmatism as you move the focuser from intra to extra focus positions.  The bands have an odd way of "chocking" as the focuser travels between the two focus points.   But, even so, I prefer the star test to evaluate astig. 

 

The term "superficial" has a negative connotation.  DPAC is in no way a superficial test methodology.  The test shows the operator nearly all (including astigmatism) aberrations at the wavelength being tested.  Part of its power is that these aberrations are all doubled, that is to say they are worse then then appear in real life.  This is what makes them so easy and quickly detected.  In fact, I would argue its more useful to the amateur then most other tests.  StellarVue now seems to think so.  

Edited by peleuba, 08 July 2024 - 01:03 PM.

[Scott in NC]

Scott - Wait - what?  The Ronchi holder that Jeff is using looks like identical to the one you're son printed for me from the plans I sent to you back in May, 2023 via private email.  

 

If there is an improved version, I'd like to hear about it.  

Well now I'm confused, and perhaps my memory has become faulty.  I thought that this was a different design that my son came up with which would not only hold a Ronchi screen but could also hold a razor blade for knife-edge testing. But maybe the plans did come from you after all, and he simply tweaked them slightly.  Regardless, I don't want to hijack Jeff's excellent thread, so please feel free to PM or text me about this. I would of course like to set the record straight and give credit where credit is due.

 

Edit: Yeah, the more I think about it, the more the actual facts are coming back to me.  I think my son just tweaked it to make it easier to 3D print, and Paul is actually the brains behind the whole thing. My apologies, and thanks for the correction, Paul!

 

Edit #2: After looking at the original 3D print files that Paul sent me last year, and discussing with my son, I now recall what happened. Paul did indeed send me some files and my son used those to print the Ronchi screen and knife-edge holders last year. But the output from the 3D printer required quite a bit of manual labor afterwards to strip out the extra plastic in order to make the final product usable. My son tweaked the design to make the production process a bit more user friendly, plus he made the final product a bit more ergonomic. So I stand corrected in that the original plans did indeed come from Paul and another astronomer friend of his, and then my son tweaked it a bit.  Thanks again Paul for reminding me of the facts!
 

Edited by Scott in NC, 08 July 2024 - 10:07 PM.