724 replies to this topic

[Jeff B]

Scott, what is your screen's frequency in LPI?

 

Jeff

[peleuba]

Scott, what is your screen's frequency in LPI?

 

 

133LPI

[Kitfox]

Question from an old-fashioned optical guy…those in focus images look remarkably like knife edge impingement at focal plane.  Am I wrong?

[Scott in NC]

Question from an old-fashioned optical guy…those in focus images look remarkably like knife edge impingement at focal plane.  Am I wrong?

Steve, I guess it’s “sort of” like a knife edge, but not really.  One side looks darker than the other because you’re looking at a single Ronchi band (off to one side) at the focal plane.

[DAVIDG]

Steve, I guess it’s “sort of” like a knife edge, but not really.  One side looks darker than the other because you’re looking at a single Ronchi band (off to one side) at the focal plane.

 It is  a knife edge if you adjust the position of grating  just right so one the lines is the knife edge. DPAC testing is the same as doing a Ronchi or Foucault knife test  on a  spherical mirror. With  a spherical mirror your testing at the radius of curvature and  a perfect sphere brings all the light to a perfect focus so it shows a null with a knife edge and dead straight lines with a Ronchi screen.

 A telescope designed for astronomical viewing  has the optics designed to bring parallel light to a perfect focus. In DPAC testing your using the telescope  backwards with the light source at the focal plane so the light exiting the telescope  if the optics are perfect is parallel.  That light is reflected off the flat and back into the telescope were it comes to focus. Any errors in the optics cause errors in the parallel light and those errors are doubled when reflected back off the flat into the telescope.  DPAC is simple yet a very powerful test. As the images in this thread show, you can see if the optics have errors and don't have to measure anything and reduce data to determine the errors.

    The Clarks invented DPAC and it is the reason why at  their time,  their optics were consider better other telescope makers. It wasn't  some secret polishing method or some special glass. It was because they had a test that easily showed them if their lenses were figured correctly or not and showed them what the errors were. Other makers like Brashear also knew about DPAC and used it to figure their lenses. Brashear understood how valuable the test was and how important their optical flat were to the success of making high quality optics. At night their flats were stored in safe ! 

   Here is the DPAC results for a 5" Brashear object I own. It tests very well.

             

   And here is the DPAC results for 3" Mogey objective. Mogey didn't use DPAC but a pinhole in the roof of their shop as an artificial star and tried to figure their lenses using it. The result are nowhere as good.  

 

 

 

  

             - Dave 

[Bomber Bob]

Bob, do you have a wider image that shows your whole rig all set up with a scope?

Sure....
 

 

3 identical platforms:  Flat Holder, Lens Holder, Ronchi Assembly.  Each platform has levelling feet, and I took care to make sure the 3 holes are centered & easy to align.  When I built it, I had all vintage / classic refractors at 4" & smaller apertures, so the lens holder was built for the lens cell -- I take it off the scope -- and it has 4 large threaded knobs to center & hold the lens cell in place.

[Kitfox]

Dave, thank you for that wonderful explanation, it makes sense and clarifies the origins. And brings back some long-lost memories of an optics class back in Univ of North Carolina’s physics department. Not all good memories, alas 

Edited by Kitfox, 18 February 2023 - 11:05 PM.

[Scott in NC]

 It is  a knife edge if you adjust the position of grating  just right so one the lines is the knife edge. DPAC testing is the same as doing a Ronchi or Foucault knife test  on a  spherical mirror. With  a spherical mirror your testing at the radius of curvature and  a perfect sphere brings all the light to a perfect focus so it shows a null with a knife edge and dead straight lines with a Ronchi screen.

 A telescope designed for astronomical viewing  has the optics designed to bring parallel light to a perfect focus. In DPAC testing your using the telescope  backwards with the light source at the focal plane so the light exiting the telescope  if the optics are perfect is parallel.  That light is reflected off the flat and back into the telescope were it comes to focus. Any errors in the optics cause errors in the parallel light and those errors are doubled when reflected back off the flat into the telescope.  DPAC is simple yet a very powerful test. As the images in this thread show, you can see if the optics have errors and don't have to measure anything and reduce data to determine the errors.

    The Clarks invented DPAC and it is the reason why at  their time,  their optics were consider better other telescope makers. It wasn't  some secret polishing method or some special glass. It was because they had a test that easily showed them if their lenses were figured correctly or not and showed them what the errors were. Other makers like Brashear also knew about DPAC and used it to figure their lenses. Brashear understood how valuable the test was and how important their optical flat were to the success of making high quality optics. At night their flats were stored in safe ! 

   Here is the DPAC results for a 5" Brashear object I own. It tests very well.

             Brashear objective.jpg

   And here is the DPAC results for 3" Mogey objective. Mogey didn't use DPAC but a pinhole in the roof of their shop as an artificial star and tried to figure their lenses using it. The result are nowhere as good.  

 

mogey dpac RONCHI.jpg

 

 

  

             - Dave 

Thanks for the education, Dave.  On the Mogey lens, I see spherical aberration and a turned edge, but what does the thicker area on the band on the left represent?  A zone, or just a photography artifact?

[Scott in NC]

Sure....
 

BBs DPAC Gear v3 S02.jpg

 

3 identical platforms:  Flat Holder, Lens Holder, Ronchi Assembly.  Each platform has levelling feet, and I took care to make sure the 3 holes are centered & easy to align.  When I built it, I had all vintage / classic refractors at 4" & smaller apertures, so the lens holder was built for the lens cell -- I take it off the scope -- and it has 4 large threaded knobs to center & hold the lens cell in place.

Nice, Bob—that’s the first one I’ve seen like that.

[Scott in NC]

Thanks for the education, Dave.  On the Mogey lens, I see spherical aberration and a turned edge, but what does the thicker area on the band on the left represent?  A zone, or just a photography artifact?

Ah…I just read your description of the test on that Mogey here:  https://www.cloudyni...gey/?p=10750922

 

Great explanation, Dave.  I’m learning a ton from you guys, and also just from doing this myself.  Some of what I’ve read didn’t make complete sense to me until I actually started the testing process.

[Bomber Bob]

Thanks Scott.  My idea was to make a rig that's simple & reliable, so I get consistent results.  When not in use, it stacks together as a cube -- easy to store in a closet (keeps away the dust!).  But, it is limited to 4" & smaller refractors.  To test larger scopes, I leave them assembled, and just use the flat stand & Ronchi assembly -- I have to get creative to center & align the scope!

[Scott in NC]

Bob, I really like this part of your sig line:

 

Ol' Bomber Bob says:

- Keep it Simple so you'll Use it.

And that’s really apropos here as well.  I’ve shown pics above illustrating how simple my testing rig is.  And while I’m sure I’ll refine it a bit over time, I really like the fact that it’s simple and can be set up in a matter of minutes.  Before I got started I had this vague idea that it was going to be a lot more complex and take a ton more time to set up and fiddle with each time.  There is definitely a learning curve involved, but the process is coming along much more smoothly than I ever imagined it would be.  

 

I have you guys to thank for your outstanding tutelage, and most importantly Paul, who went the extra mile helping me get this set up, even going so far as to personally construct some of the parts right before shipping the gear off to me.  Left to my own devices, this whole project might never have come to fruition.  I’m having a ton of fun right now, and learning a lot about optics in the process.

[DAVIDG]

Thanks for the education, Dave.  On the Mogey lens, I see spherical aberration and a turned edge, but what does the thicker area on the band on the left represent?  A zone, or just a photography artifact?

 Scott

   Here is  a DPAC picture of the Mogey lens were I set the position of the grating to act as a knife edge.  The lens as a turned edge and large hole in the center. For those that have made a mirror and tested by a Foucault knife edge test that looks like a hill but as I said because light is refracted going through the glass, a hole acts like a concave lens and diverges the light  making it focus longer then the rest. So it looks like a hill on a lens

 

 

   The lenses you  have tested are testing very well but if you keep this up and start testing  larger scope and different designs like SCT you start to see some major errors. The problem is many believe they have great optics when they don't and this test easily shows it. I have tested optics for ATMs that have been in the hobby for decades and used telescopes for decade and swear that they know good optics from bad and their scope is great, yet on the test stand they look like the Mogey lens.  My club owned a SCT that came with certificate stating the optics were tested and were good 1/10 wave, yet when tested by DPAC the bands were waving all over the place. The optics were no more then 1/2 wave at best.  

  I read in the forums were  people spending large amounts of money on eyepieces, a 1/30 wave diagonals, carbon fiber tubes, etc etc in hopes of improving the image in their scope but never critically test their optics. None of that stuff is going to matter if the optics are not figured correctly and unfortunately there a lot that are not made to what people believe they are. 

   So my advice is if you have tested your optics and you really know the quality and your happy uses them, that is great. Enjoy !  But  if  you think they are "1/10" or better you might want to check that,  to be sure,  so your not missing out with what truly good optics can show.

 

                       - Dave 

Edited by DAVIDG, 19 February 2023 - 11:37 AM.

[Scott in NC]

Thanks, Dave.  I really appreciate your taking the time to share your insights!  As I’ve said, I’m learning a lot from this whole process.

[Jeff B]

Scott, welcome to Club DPAC!

 

I have learned soooo much from stepping into the DPAC pool than I thought possible, especially with some of the folks here like Dave and Paul who rescued me when I ventured into the deep end.  

 

I'm still learning and finding new ways to use DPAC as a simple, effective, revealing and powerful tool.  In fact, I'm going to venture into the deep end again with another thread fairly soon showing perhaps an interesting use. 

 

I like measurements (no, really!?).  They help me to understand what I see or don't see and are great ATM tools.  In my audio hobby (when I had hearing worth a ****), I used various tools and measurements to design and set up audio components, especially speakers.  I could go only so far with subjective evaluations and comparisons, I wanted to also see what I was hearing (there is actually a very strong visual component to a well sorted system called imaging & soundstaging.).

 

While on the soapbox, I see DPAC as an easy and useful check on vendors too, empowering us consumers.  Trust, but verify.

 

Keep at it Scott, keep refining your skills!

 

And, especially, be that kid again, having fun!

 

Jeff 

[Scott in NC]

Thanks, Jeff!  I count you as one of the people who’ve encouraged me to take the leap.  On the one hand, now I’m wondering why I didn’t do this sooner.  But on the other hand, it’s probably a good thing that I’ve taken my time developing my observational skills.  Now I can finally begin to understand why I’m seeing what I’m seeing.

[davidc135]

Thanks for the education, Dave.  On the Mogey lens, I see spherical aberration and a turned edge, but what does the thicker area on the band on the left represent?  A zone, or just a photography artifact?

Just a photography artefact, I'd say.  David

[Jeff B]

Just a photography artefact, I'd say.  David

Actually, I believe it's real and a combination of the turned edge and another band creeping into view from the left.  If you look carefully, you see it has a green tint in the center.  I've seen that type of thing before where the edge is turned a bit.

 

Jeff

[Kitfox]

Dave, How small was your light source aperture for this test?  

 

You are right, that brings back memories. We did mostly mirrors back in the day, but we did talk about lens testing, but single pass with an artificial star. I had no idea Ronchi gratings and Double pass had been around a hundred years…

 

 

 Scott

   Here is  a DPAC picture of the Mogey lens were I set the position of the grating to act as a knife edge.  The lens as a turned edge and large hole in the center. For those that have made a mirror and tested by a Foucault knife edge test that looks like a hill but as I said because light is refracted going through the glass, a hole acts like a concave lens and diverges the light  making it focus longer then the rest. So it looks like a hill on a lens

 

DPAC AT FOCUS.jpg

 

   The lenses you  have tested are testing very well but if you keep this up and start testing  larger scope and different designs like SCT you start to see some major errors. The problem is many believe they have great optics when they don't and this test easily shows it. I have tested optics for ATMs that have been in the hobby for decades and used telescopes for decade and swear that they know good optics from bad and their scope is great, yet on the test stand they look like the Mogey lens.  My club owned a SCT that came with certificate stating the optics were tested and were good 1/10 wave, yet when tested by DPAC the bands were waving all over the place. The optics were no more then 1/2 wave at best.  

  I read in the forums were  people spending large amounts of money on eyepieces, a 1/30 wave diagonals, carbon fiber tubes, etc etc in hopes of improving the image in their scope but never critically test their optics. None of that stuff is going to matter if the optics are not figured correctly and unfortunately there a lot that are not made to what people believe they are. 

   So my advice is if you have tested your optics and you really know the quality and your happy uses them, that is great. Enjoy !  But  if  you think they are "1/10" or better you might want to check that,  to be sure,  so your not missing out with what truly good optics can show.

 

                       - Dave 

Edited by Kitfox, 19 February 2023 - 07:39 PM.

[DAVIDG]

Dave, How small was your light source aperture for this test?  

 

You are right, that brings back memories. We did mostly mirrors back i the day, but we did talk about lens testing, but single pass with an artificial star. I had no idea Romci gratings and Double pass had been around a hundred years…

 In a Ronchi test the ideal size of  the light source is the width of  the lines on the grating and the slit needs to  be   parallel to the lines on the grating. So if you just simply put the light source ( green LED ) behind the grating so it shines through it the grating makes slits that are the perfect width and always parallel to the lines on the grating. So it is a self aligning test !  So the light goes through the grating, through the optics being tested, reflected off the optical flat, back into the optics being tested and through the grating were you see the results

   Yep the Clarks were doing it way back when and used to test and figure their very large lenses. They had a 48" flat. It was displayed at Stellafane maybe 15 years ago but unfortunately it was placed in the hot Sun and that causes it to crack. If you didn't get publication "English Mechanic" were  articles on optical fabrication was published or live near  a major library you didn't know about these tests.  It wasn't until "Amateur Telescope Making" was published in the 1920's that techniques to grind both mirrors and lenses was more easily available. Russell Porter writes about DPAC in ATM book 1 . We are pretty sure the person that made the optics for the famous Porter Garden Telescope which uses 6" f/4 mirror used DPAC to figure the mirrors.

 

                   - Dave 

[Scott in NC]

Well, I should have stopped while I was ahead, but my son came home from college for the night and asked me to test out my prized FS-102 so he could get a feel for how the testing process works.  This scope was created in 2001, and I’ve owned it since 2014.  It was my first introduction to Takahashi, and the optics easily bested any other scope that I had previously owned, in particular with its ability to resolve close double stars.  But as I’ll soon demonstrate, the DPAC testing uncovers some deviations from perfection.  No, it’s not terrible, but I so much wanted to see as perfect a test as I got with my SV80L 80/600 LOMO.  I’m heartbroken.

 

[Scott in NC]

Takahashi FS-102 f/8 fluorite doublet

 

Inside focus:

 

 

At focus:

 

 

Outside focus:

 

[Scott in NC]

Someone please correct me if you think otherwise, but I see a very small turned down edge and mild overcorrection.

 

No this isn't bad, but I'm frankly shocked given the beautiful views that this scope provides of the heavens.  I realize that DPAC is an especially rigorous test, but I can't fully reconcile these test results with the scope's actual star test. Now I'm almost afraid to test my other two prized Taks, the FS-128 and FS-152 (although the 152 may be a little too large for my 6" flat).

 

Edit: Rereading this a month later, I see that I inadvertently wrote the direction of the turned edge backwards, and I’ve corrected it for accuracy.

Edited by Scott in NC, 23 April 2023 - 09:07 AM.

[PawPaw]

Scott,

I make it a point to always retest several times and that usually takes me a couple of days.  I will admit it the lines look very slightly bowed outside and inside focus but still good.  Retest and watch the lines as you rock slowly back and  forth between out and in.   Remember DPAC is 2x errors.

 

I don't know who to credit for the following saying........always good to remember:

 

"Test an optic for someone, and you've disappointed him for a day.  Teach him to test his own optics, and you've disappointed him for a lifetime.   

Optical testing has ruined more telescopes than any other cause.:

 

Cheers

 

Don

[Jeff B]

Complete trash. Send it to me Scott and I'll get rid of it for you......