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CFF92 Under Test

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#1 peleuba

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Posted 13 April 2018 - 10:35 AM

I will never make a mirror or grind a lens, nor will I win an award for any photograph that I take through a telescope.  The reason being is that the quality that I require exceeds my ability to deliver.  So, sometimes the best tool to reach for in cases like this is my credit card...

 

One aspect of the hobby I really enjoy is testing.  It boils down to two reasons:

 

(1) to understand how flawed my telescope is and how these imperfections degrade the view.

(2) to verify claims made by manufacturers. 

 

So, I've had my CFF92 since October of 2016.  Its been a fantastic companion.  And, because its so short, with the balance point almost directly in the middle of the OTA, the mount requirements are minimal.  Build quality is very nice, and optics seem to be first class.

 

I have star tested the CFF dozens of times both indoors on a collimator (parallel point source) and outside on a real star.  As many of you know, the star test is easy to do but a PITA to interpret mainly because there are several hard to control environmental issues to account for that really have nothing to do with the figure of the optic like atmospheric turbulence and the temperature delta between the lens/mirror and the night sky.  I much prefer indoors.  

 

As sensitive as it is, the star test is a qualitative measure (except when using Roddier).  Its always bothered me to hear that an optic has been pronounced good/bad based on a quick rack in/rack out of the focuser.  This occurs at star parties ALL of the time. 

 

For the last 5 years or so on the CN ATM forum there has been a lot of chatter on the pros of using Double Pass Autocollimation.  This is another qualitative test, but when combined with the star test, really gives great confidence on the overall condition of the optic.  I've always intuitively known that its better when two test methods agree with each other.    In fact, I have never seen a mirror or lens that does well in DPAC do poorly in use.  Roland Christen figured his lenses using DPAC prior to building his Cerevolo spherical wave interferometer.  And, TEC mentions DPAC use in their test methods for their compound telescopes.

 

So I purchased back my Lawrence Livermore National Laboratory 10" optical flat (~1/20 wave) from a friend - I sold it to him years ago.  And, while expensive, this  was the easy part.  I still had to design/build a stand for the flat so it could be leveled to the telescope as well as an eyepiece containing both a Ronchi grating and an LED light source - still leaving enough space in the eyepiece to view the test and take photos.

 

Don't scoff at the the use of a Ronchi.  In DPAC its a very powerful test - giving you an indication of the spherical correction, edge condition as well as an overall picture of the zoniness/smoothness of an optic.  DPAC/Ronchi is not be confused with a "single pass" Ronchi test when you throw one of the commercially available Ronchi testers into a focuser of your sub F/4 Newt and you see straight lines.  Testing a fast optic in this single pass manner is quite insensitive to errors.  Double pass is very sensitive - its as the name says - the light used in the test traverses the optic twice (through the lens bounced off of a flat and back through the lens to your eye or camera) so the errors noted are doubled.  Aberrations are much easier to see.  Conversely, if no errors are seen - bands are "jail bar" straight - you have an excellent optic.

 

Here are the results for my CFF92.  Ignore the minor thermal anamolies within the star test.

Attached Thumbnails

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  • Startest.jpg
  • DPAC4.jpg

Edited by peleuba, 13 April 2018 - 12:11 PM.

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#2 Erik Bakker

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Posted 13 April 2018 - 10:39 AM

Very nice Paul waytogo.gif



#3 rustynpp

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Posted 13 April 2018 - 10:51 AM

Very cool, thanks for posting! Can you interpret the results for folks like me who don't know what to make of them? Those lines look "jail bar" straight to me, but I don't have the experience necessary to pick out any subtle flaws. 



#4 donadani

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Posted 13 April 2018 - 11:01 AM

perfect optics ! waytogo.gif



#5 elwaine

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Posted 13 April 2018 - 11:19 AM

Thanks, Paul. 

 

How does a double pass Ronchi test relate to the optic’s Strehl ratio?  I.e., if a reputable company includes documentation of a high Strehl ratio, can one conclude that it will have an excellent result in a double pass Ronchi test?



#6 Aleksandr Naumov

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Posted 13 April 2018 - 11:19 AM

Could you please explain how did you make an eyepiece with Ronchi and LED inside?



#7 peleuba

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Posted 13 April 2018 - 12:09 PM

Very cool, thanks for posting! Can you interpret the results for folks like me who don't know what to make of them? Those lines look "jail bar" straight to me, but I don't have the experience necessary to pick out any subtle flaws. 

That's the beauty of DPAC - you don't have to be an expert on any particular aberration.  If the lines are straight and smooth you have a very well corrected optic.  Remember any error that this test uncovers is doubled - so even small erros are easy to see.   


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#8 peleuba

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Posted 13 April 2018 - 01:15 PM

Here is a thread where one is made out of plumbing pieces and another from empty eyepiece barrels/housings:

 

https://www.cloudyni...llimation-test/

 

https://www.cloudyni...0mm-dpac/page-4

 

I made mine from an old Meade 5000 26mm Plossl.  I removed the optics, mounted a Roncho sceen (133 LPI) inside the eyepiece then cut out the filed lens eyepiece cap for the LED.

 

I also have a stand that I can clamp the ronchi grating and LED to that keep both aligned with the focuser opening of the telescope.  There are many ways to do this test.   



#9 peleuba

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Posted 13 April 2018 - 01:26 PM

Thanks, Paul. 

 

How does a double pass Ronchi test relate to the optic’s Strehl ratio?  I.e., if a reputable company includes documentation of a high Strehl ratio, can one conclude that it will have an excellent result in a double pass Ronchi test?

 

It correlates in this manner:  if a telescope has straight lines in a DPAC test, it will have a high Strehl - mid 90's or better.  If telescope comes with a "test report" with a high strehl noted, it may have a smooth straight DPAC test.  For the second part of your question, please see my first post and why I test optics.  Basically, I test for my own knowledge/intellectual growth AND to verify claims made by manufacturers.  I have seen some mirrors that arrived with very nice Strehls on test reports only to fall apart when tested.



#10 moshen

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Posted 13 April 2018 - 02:13 PM

Hey Paul, looks like a great optic as you already know.

I just got in a 1/20 wave 6" flat so I can DPAC. Have you tried taking photos of foucault? It's much more sensitive and will also show inconsequential zones but you can get a feeling of the smoothness of the figure. I would be curious how the foucault looks.

Roddier is also a very good tool to quantify a lens. Here's one I did of an out of collimation lens and an indoor star. Roddier constructed wavefront and then its simulated star test match the real life version very well.

Using Roddier and an artificial star is probably as close to interferometry you can get without actually doing interferometry.

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  • Out of collimation.jpg


#11 moshen

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Posted 13 April 2018 - 02:22 PM

It correlates in this manner:  if a telescope has straight lines in a DPAC test, it will have a high Strehl - mid 90's or better.  If telescope comes with a "test report" with a high strehl noted, it may have a smooth straight DPAC test.  For the second part of your question, please see my first post and why I test optics.  Basically, I test for my own knowledge/intellectual growth AND to verify claims made by manufacturers.  I have seen some mirrors that arrived with very nice Strehls on test reports only to fall apart when tested.

I've played with ronchi software to simulate different aberrations in fast optics in DPAC it it seems the limit for detecting SA is around 1/6 wave which puts it lower 90's - this is dependent on focal ratio of course. It also seems coma and astigmatism are harder to detect which can lower strehl even more. That's why I really like Roddier which isn't much harder than DPAC.

 

The foucault image is interesting info that only itself can give though.

So best thing is to combine and correlate all three which is accessible to any amateur, DPAC, Roddier & Star Test lol.gif



#12 peleuba

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Posted 13 April 2018 - 03:02 PM

I've played with ronchi software to simulate different aberrations in fast optics in DPAC it it seems the limit for detecting SA is around 1/6 wave which puts it lower 90's - this is dependent on focal ratio of course. It also seems coma and astigmatism are harder to detect which can lower strehl even more. That's why I really like Roddier which isn't much harder than DPAC.

 

The foucault image is interesting info that only itself can give though.

So best thing is to combine and correlate all three which is accessible to any amateur, DPAC, Roddier & Star Test lol.gif

 

I don't agree with this.  Test is more sensitive then 1/6 wave if talking about pure spherical.  Astig is can be seen in Ronchi bands.  Coma is hard to detect, but Coma and Astig are VERY easy to see in a star test.  There in lies my point of having multiple test methods.

 

I know a fair amount about  the Roddier test.  I was the co-moderator on the Roddier Yahoo group.  My good friend an observing partner - Dr. John Biretta HST/M87 fame - was the moderator.  You don't need Roddier to tell you you have a good/bad optic.  One uses Roddier to quantify the "goodness".   DPAC will tell you most of what you need to know unless you want to play the numbers game.

 

You mention an artificial star - hopefully you made sure the light is parallel as seen by the telescope under test or your results will not be valid.  

 

Yes, have used the Foucault test.  I had a 110mm APO some years back that was not very good for what I like to use a telescope for.  I documented results from a variety of tests in my report back to the manufacturer - including Foucault.   But, frankly, the star test told the whole story - Foucault just confirmed it.  



#13 moshen

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Posted 13 April 2018 - 03:23 PM

I agree DPAC and star testing is all anyone really needs to 'pass' an optic.

 

I do like trying to quantify the "goodness" which is why I like Roddier. I've done Roddier on a real star and artificial star and definitely get more consistent numbers with an artificial star because of the stability of the image. The artificial star (as a point source) introduces some undercorrection and it needs to be far enough for it to be small but for small refractors that distance isn't large. Roddier has a section where you put in the artificial star distance and with the rest of the parameters for the optic calculate and subtract the undercorrection introduced (you probably already know this given your Roddier background but just mentioning it for others).

 

Interesting you feel DPAC is more sensitive to 1/6 SA, perhaps I was simulating too fast of an optic - the lines looked pretty straight at 1/6 SA when I was messing with software sims. I'll play with it myself since I just got a flat. I would definitely be happy to find if it was more sensitive.

 

Thanks for sharing your setup! I'm going to work on CADing up and 3D printing a simple LED/Ronchi eyepiece holder.



#14 peleuba

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Posted 13 April 2018 - 03:48 PM

 

 

Thanks for sharing your setup! I'm going to work on CADing up and 3D printing a simple LED/Ronchi eyepiece holder.

 

I would purchase one from you if you perfect the design.  Seriously, I would.



#15 moshen

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Posted 13 April 2018 - 04:06 PM

I would purchase one from you if you perfect the design.  Seriously, I would.

The stuff I design and print for myself is pretty crude, but it's easier for me to do it in software and hit print than finding various parts to cobble together. If whatever design I come up with would work for you then I'm happy to just print and send you one for free.



#16 moshen

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Posted 13 April 2018 - 04:13 PM

Also since you have more experience with DPAC, if you already have an ideal design in your head you can PM me with some hand drawings and I can translate it to CAD and I'll try it out and send you one too.



#17 Mike Spooner

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Posted 13 April 2018 - 05:30 PM

Back in sci.astro days we had lots of discussions on testing. Then, as now, I was using DPAC and folks were exploring the double sensitivity of the setup. It is much easier to see using DPAC in a stable shop vs eyepiece Ronchi single pass on a star with seeing and all. But remember that in DPAC we are working at the native f/ratio whereas using Foucault at C of C we are actually working at increased sensitivity because the mirror is at twice the working f/ratio on a star. The star test is exquisitely sensitive but difficult to assess with mixed aberrations. With f/ratios below 4, even DPAC needs very careful setup with respect to the optical axis - on one f/3.3 mirror I made, minute parallax errors from eye and source placement was enough to impact the correction significantly.

 

BTW, Paul, what was the frequency of the Ronchi grating you used? Nice looking optic, too!

 

Best,

Mike Spooner 



#18 peleuba

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Posted 13 April 2018 - 07:41 PM

Back in sci.astro days we had lots of discussions on testing.

 

>>BIG Snip<<

 

BTW, Paul, what was the frequency of the Ronchi grating you used? Nice looking optic, too!

 

Best,

Mike Spooner 

 

Hi Mike,

 

I remember those days...

 

The Ronchi grating is 133LPI.


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#19 Paul G

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Posted 14 April 2018 - 08:48 AM

I miss the sci.astro days.
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#20 Jeff B

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Posted 14 April 2018 - 09:39 AM

Wow, looks really good to me Paul. 

 

I do both DPAC and star testing too to "pass" an objective lens.  The star testing is mainly for astigmatism, coma and obvious zones while the DPAC lets me see the overall correction all at once (I can also see zones as I approach focus with DPAC).  The thing is I can do DPAC whenever I want and in less time than it takes to set the scope up outside.   I've also found that DPAC testing lenses faster than say F6 is more difficult as small alignment errors can mess with the results.

 

Jeff


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#21 lionel

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Posted 15 April 2018 - 12:47 AM

Could you please explain how did you make an eyepiece with Ronchi and LED inside?

 



#22 peleuba

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Posted 15 April 2018 - 01:12 PM

The thing is I can do DPAC whenever I want and in less time than it takes to set the scope up outside.   I've also found that DPAC testing lenses faster than say F6 is more difficult as small alignment errors can mess with the results.

 

Agree that DPAC is very easy top setup and perform.  And, as you say, it gives you nearly a complete qualitative frame of reference as to the overall correction of the lens.



#23 John O'Hara

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Posted 15 April 2018 - 04:13 PM

So, is this the new Stowaway?  Sure looks promising. 

 

Nice scope, Paul!  



#24 peleuba

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Posted 15 April 2018 - 04:48 PM

Hi Joh

 

So, is this the new Stowaway?  Sure looks promising. 

 

Nice scope, Paul!  

 

 

Hi John - Hope to see you at Cherry Springs sometime this year.

 

I don't know if the CFF92 will ever have the cache the Stowaway does but optically the CFF is one of the best small aperture telescopes I have ever looked through and tested.

 

Initially, there was only a run of 15 produced and I heard that this is now sold out.  I don't if another run will be produced.



#25 John O'Hara

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Posted 15 April 2018 - 05:25 PM

Paul,

 

I'm hoping to be up in may, and at the two star parties at CSSP.  If you have that scope with you, I'd love a look through it!

 

John




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