971 replies to this topic

[Hyokin]

no more updates in this thread

[DAVIDG]

no more updates in this thread

 I was at Stellafane at the beginning of the month and  I picked up another corrector plate from the gentleman who bought out what was left of Criterion when they went under.  This one was suppose to be from at the end of the production when Criterion was owned by Bausch and Lomb and used in  the 8001 series which some have reported have good optics. When I get a chance I'll test it on my optical bench and report here the results. 

  So this project is not dead just slowly moving along. As I have said I have many projects, plus a  full time job as research engineer/chemist and I'm the Director of Mt Cuba Observatory and over seeing the installation of 1.3 meter telescope which will be the largest operating telescope on the East Coast when it is completed in the Spring. So I have a bunch of "stuff" on my plate. 

 

https://www.mountcub...g/new-telescope

 

                       - Dave 

Edited by DAVIDG, 30 August 2022 - 10:50 AM.

[tim53]

Awesome!  Maybe by the time I'm ready to tackle one of my DX-8s, I'll know how!  

[Gil V]

David - if that plate needs a future home, please keep me in mind!

Looking forward to your evaluation of the glass once you get the opportunity to examine it.

Edited by Gil V, 31 August 2022 - 08:47 AM.

[DAVIDG]

David - if that plate needs a future home, please keep me in mind!

Looking forward to your evaluation of the glass once you get the opportunity to examine it.

 Gil,

   If  I find or make one that works the way it should, as I said I promised it will come your way for the all the help you provided me with this project.

 

                        - Dave 

Edited by DAVIDG, 31 August 2022 - 12:02 PM.

[mfalls]

 I was at Stellafane at the beginning of the month and  I picked up another corrector plate from the gentleman who bought out what was left of Criterion when they went under.  This one was suppose to be from at the end of the production when Criterion was owned by Bausch and Lomb and used in  the 8001 series which some have reported have good optics. 

I just purchased the B&L 8001 Pro 8 inch SCT  OTA listed on CN ads (Joe Sunseri).  Looking forward to comparing views with my best Dynamax 8 when the seeing is good. I have a 6 inch flat but doubt I will do any DPAC testing anytime soon. Perhaps I will be able to make the trip to Stellafane in the next few years. Joe indicated the 8001 OTA outside diameter is 9 inches, the Dynamax is more. Perhaps the corrector plate diameters are the same.

Edited by mfalls, 06 September 2022 - 02:33 PM.

[highfnum]

corrector plate  I believe is 8.25 inch

 

dx8 and celestron plates same size (of same era)

 

I tried that already 

it does not work!

[DAVIDG]

 A DX-8 and a C-8  are of different optical  design so as HighFnum stated swapping in a  C-8 corrector plate wouldn't work.  If you have made optics you'll understand that even if the design is the same, every  part is unique and if you want a diffraction limited or better system, the complete system needs to be  figured to cancel all the residue errors coming from all the surfaces. 

   As I have said before, a multielement optical system is the analogous  to an analog electrical circuit were each part has a tolerance range and you need to tune the circuit to compensate for the tolerances and bring the system into spec. Same goes with the optical surfaces, that is why Celestron was hand figuring each secondary  to compensate and correct any residue error in the primary and corrector and Meade and Criterion were swapping in different secondaries in and  out to find one that brought  the system into "their spec" not so much diffraction limited but what they stated was "good"

 

                  - Dave 

[highfnum]

Dave Meade?

that's a surprise

[DAVIDG]

Dave Meade?

that's a surprise

 You have to understand that one of the main reasons that Celestron started to make SCT was they could make them cheaper then  other types of cassegrains and make a profit  In a classic cassegrains you have two aspheric surfaces. Those surfaces need to be hand figured and that  takes time and skill which equals money. Figuring out how to mass produce the corrector allowed for a spherical primary. Both can be made quicker with less skill and that reduces costs and increases profit. The only aspheric surface is the secondary and I don't care what the "experts" over in the Cats and Cass forum say, the secondaries are aspheric. Celestron hand figured theirs. Meade and Criterion wanted to compete with Celestron so they needed to cut production costs. So instead of hand figuring the secondaries here in the USA were skilled labor cost money,  they did what many companies do, they had them made in  a country that has cheaper labor costs.  Anybody that has made optical surfaces knows  that each one is a little different so what both Meade and Criterion did was swap secondaries in and out  when testing the corrector/primary combination to find a secondary that when paired with the primary/corrector would meet their "spec" 

  Criterion knew they needed to make their SCT different then a C8 and not an exact copy so the optical design is different in focal length which makes the power of the corrector plate different and why you can't swap a C8 corrector in a Dynamax 8 and get a diffraction limited image. 

 

                       - Dave 

[tim53]

Yes.  If you want to swap in a celestron corrector, you also would have to use the celestron secondry.  And if you have a corrector and a secondry from a C-8, there are plenty of people who'd be lining up at your door for the chance at them to replace a broken corrector plate in theirs.

 

The only hope, then, for improving a DX-8 is something like what Dave is doing.  Or, live with the fact that a DX-8 is best kept to lower powers.

[firemachine69]

 I was at Stellafane at the beginning of the month and  I picked up another corrector plate from the gentleman who bought out what was left of Criterion when they went under.  This one was suppose to be from at the end of the production when Criterion was owned by Bausch and Lomb and used in  the 8001 series which some have reported have good optics. When I get a chance I'll test it on my optical bench and report here the results. 

  So this project is not dead just slowly moving along. As I have said I have many projects, plus a  full time job as research engineer/chemist and I'm the Director of Mt Cuba Observatory and over seeing the installation of 1.3 meter telescope which will be the largest operating telescope on the East Coast when it is completed in the Spring. So I have a bunch of "stuff" on my plate. 

 

https://www.mountcub...g/new-telescope

 

                       - Dave 

 

 

 

I have a 40mm Celestron Plossl to donate for your new scope. You know, public outreach and all that. You'll have to confirm if your focuser can handle 1.25" eyepieces, however.   

 

 

Back on topic...  

 

 

Were those Criterion scopes single MgF2 coatings only? How did they make that happen? I was under the impression the wider the surface became, the harder it was to evenly coat the optical surface? 

 

 

Loving this thread! This is what I joined CN for!

[DAVIDG]

I have a 40mm Celestron Plossl to donate for your new scope. You know, public outreach and all that. You'll have to confirm if your focuser can handle 1.25" eyepieces, however.   

 

 

Back on topic...  

 

 

Were those Criterion scopes single MgF2 coatings only? How did they make that happen? I was under the impression the wider the surface became, the harder it was to evenly coat the optical surface? 

 

 

Loving this thread! This is what I joined CN for!

 The process used to aluminize a mirror is the same that you use to apply MgF2.   I have done it in my lab a couple of times. It just a bit more critical that you get the thickness correct which require it to by 1/2 wave of thickness to act as an  anti-reflective coating and the surface is heated.  I'm sure Criterion set them out to a  coater that did a number of the them at a time to reduce cost like other optical companies do today.

   Thanks for the offer for the eyepiece but the new 50"  scope only has a port for $50K CCD camera.

 

                           - Dave 

Edited by DAVIDG, 07 September 2022 - 08:28 PM.

[davidc135]

The primary in a DX-8 is slightly slower than those of a Meade or Celestron by 5.5% (needs checking). As SA varies inversely as the cube of the f ratio, the Celestron corrector would be 17.4% too strong assuming similar optical design ie all spherical mirrors. The secondary would have to be figured to match but you would end up with a system pretty well corrected for coma. I make it 2.8 waves under-correction needed which might be tricky with a 2'' diameter mirror.

 

It shouldn't be assumed that the Criterion primary mirror is fault free as mine has strong astigmatism, although I need to check that the mirror surface is responsible.

 

I'd say that the secondaries of all three makes from this period were nominally spherical. If Celestron retouched their 2ndries to improve the overall optics that's stretching the definition of aspheric.

 

I don't have a C8 so I can't say for sure but my Meade LXD55 2ndry certainly came out as spherical in an optical test. This ke test used a 50mm binocular objective, a small Newt diagonal and preferably but not essentially a beam splitter prism. If anyone has a C8 they can settle the question.

 

Details of test layout at post46 of 'Celestron sct 2ndries; spheres or aspheres?' started 10th June 22 in ATM subforum.

 

David

Edited by davidc135, 08 September 2022 - 03:33 AM.

[highfnum]

"If you want to swap in a celestron corrector, you also would have to use the celestron secondry."
 

 

did that first 

NG

 

no good

!

[DAVIDG]

"I'd say that the secondaries of all three makes from this period were nominally spherical." I strongly disagree with this from directly testing the many Celestrons I have worked on . I have the design  for the DX-8 that  only works well with a aspheric secondary. Documentation published in different books with information stating from the founder of Celestron that  they aspherized  the secondaries and the fact that the both Meade and Criterion had their secondairies  made  by other companies  which only make sense if they were aspherics to reduce labor costs. Also Celestron stated that they also increased the power of the corrector to over correct the wavefront from the combination of the corrector and primary to act like a hyberbola in  a Ritchey Chretien and now to correct that wavefront you need an aspheric secondary. If you  know optics  you understand that the reason why you make a Ritchey is because it has no  coma and that comes from an aspheric hyperbolic primary and aspheric hyberbolic secondary.  Meade was/is most likely doing the same with there correctors as well to compete with Celestron in optical performance.  

   The coma in an all spherical  SCT is about what f/5 Newtonian  has which is not  great of astrophotography  and the many images published  by  a commercial SCT show much better coma correction.    It is  an optical fact that to reduce coma in  a SCT to  an acceptable level for astrophotography that there are three ways to do this  and they are to the place the corrector at the radius of curvature of the primary which would double the tube length. aspheric the primary or aspheric the secondary. Since the secondary is the smallest that is the surfaces of choice.  It is also an optical fact that if the secondaries were spherical collimation would not be as critical as  it  is because a spherical surfaces has no optical center but an aspheric one does. So again more proof of aspheric secondaries. 

   Here is  a link to the Celestron White paper on their Edge design, on page 11 there is a picture of the secondary being figured,  if you have actually made convex apsheric surfaces you immediately note the shape of the pitch  lap is peddle shaped.. That is the shape you use to aspherize a surface and not  to make it  spherical. https://s3.amazonaws...aper_final.pdf 

   There is no published statement by Celestron, Meade or Criterion saying their secondaries are spherical but I can provide referenced to published statement by Celestron that they are aspheric.  The evidence is out there but  people that do not understand both optics and business don't want to accept it. 

 

    Here is just one of  a couple I can point to from Celestron own literature 

 

          We are not satisfied

with just good performance, however. We there·
fore set up each of these optical systems in a laser
collimator capable of detecting 1/100lh wave
errors. We then carefully hyperbolize the secondary
mirror to bring the optical system of each Celestron
to a perfect optical null. The Celestron optical
guarantee is as follows: using a point source at
infinity (star test) and with the system properly
collimated, a knife edge shall indicate a clean optical null; when tested with a l00-line Ronchi grot·
ing with three lines intercepting the cone, the
shadow bands shall appear straight. 

 

 

                    - Dave 

Edited by DAVIDG, 08 September 2022 - 10:52 AM.

[davidc135]

When the dividing line between a manufacturer's self promotion and useful information is blurred I wouldn't rely too much on anything from Celestron or the others.

 

They use the word 'hyperbolize' above very likely because it sounds more impressive than figure or just tweak. Similarly, a petal lap is prettier than other shapes. Too cynical? Or the secondaries may have come off the polishing machine over-corrected and need taking back to a sphere.

 

Although in fairness to them I don't think they actually claim that their scts were corrected for coma. (prior to the advent of ACF and Edges). Are you not reading too much into the 'evidence'

 

Isn't science taking a proposed explanation and practically putting it to the test rather than relying on inferences read into what other people had stated often decades ago. There are reasons why they might have aspherized the secondaries (beyond tweaking) but that doesn't mean that they did.

 

A repeatable, robust test settles it.

 

David

Edited by davidc135, 08 September 2022 - 12:16 PM.

[bremms]

"I'd say that the secondaries of all three makes from this period were nominally spherical." I strongly disagree with this from directly testing the many Celestrons I have worked on . I have the design  for the DX-8 that  only works well with a aspheric secondary. Documentation published in different books with information stating from the founder of Celestron that  they aspherized  the secondaries and the fact that the both Meade and Criterion had their secondairies  made  by other companies  which only make sense if they were aspherics to reduce labor costs. Also Celestron stated that they also increased the power of the corrector to over correct the wavefront from the combination of the corrector and primary to act like a hyberbola in  a Ritchey Chretien and now to correct that wavefront you need an aspheric secondary. If you  know optics  you understand that the reason why you make a Ritchey is because it has no  coma and that comes from an aspheric hyperbolic primary and aspheric hyberbolic secondary.  Meade was/is most likely doing the same with there correctors as well to compete with Celestron in optical performance.  

   The coma in an all spherical  SCT is about what f/5 Newtonian  has which is not  great of astrophotography  and the many images published  by  a commercial SCT show much better coma correction.    It is  an optical fact that to reduce coma in  a SCT to  an acceptable level for astrophotography that there are three ways to do this  and they are to the place the corrector at the radius of curvature of the primary which would double the tube length. aspheric the primary or aspheric the secondary. Since the secondary is the smallest that is the surfaces of choice.  It is also an optical fact that if the secondaries were spherical collimation would not be as critical as  it  is because a spherical surfaces has no optical center but an aspheric one does. So again more proof of aspheric secondaries. 

   Here is  a link to the Celestron White paper on their Edge design, on page 11 there is a picture of the secondary being figured,  if you have actually made convex apsheric surfaces you immediately note the shape of the pitch  lap is peddle shaped.. That is the shape you use to aspherize a surface and not  to make it  spherical. https://s3.amazonaws...aper_final.pdf 

   There is no published statement by Celestron, Meade or Criterion saying their secondaries are spherical but I can provide referenced to published statement by Celestron that they are aspheric.  The evidence is out there but  people that do not understand both optics and business don't want to accept it. 

 

    Here is just one of  a couple I can point to from Celestron own literature 

 

          We are not satisfied

with just good performance, however. We there·
fore set up each of these optical systems in a laser
collimator capable of detecting 1/100lh wave
errors. We then carefully hyperbolize the secondary
mirror to bring the optical system of each Celestron
to a perfect optical null. The Celestron optical
guarantee is as follows: using a point source at
infinity (star test) and with the system properly
collimated, a knife edge shall indicate a clean optical null; when tested with a l00-line Ronchi grot·
ing with three lines intercepting the cone, the
shadow bands shall appear straight. 

 

 

                    - Dave 

The C8 always seemed to have better edge correction than the early Meade 8" that I used . Neither was all that good optically though. Out of the probably 10 SCT's thorough my hands two were decent and one of them was good.

[DAVIDG]

 David,

  Yes they did say they were better coma corrected and again here is  evidence from their own literature.  If the secondary was spherical it would have MORE coma then  most of the systems in that diagram.  Bob Johnson knew optics well and understood what was required to make a compact SCT with good astrophotography performance.  All I hear from others that say the secondary are spherical is just repeating what other say but show no proof . Again if the secondaries were spherical the coma would show in all the many images taken by them.  Please show me some evidence and not hearsay or guesses that the secondaries are spherical. 

  By the way I contributed to "Telescopes, Eyepieces and Astrographs" and while the SCT design were thoroughly reviews  the design were guesses at exactly what Celestron and Meade were doing. So I will take what Celestron stated as more proof then other saying  they know better.

   Meade and Criterion were no dummies either and I'm sure  they reverse engineered these designs and also Criterion hired away  Bob Geoff from Celestron to help them make the correctors so I'm sure he knew what the design was and why they were doing it the way they  were.  Criterion knew they needed aspheric asecondaries so they had they made in Japan to save money. Meade did the same as well and I said both companies would swap them in and out the find one that worked while Celestron choose to hand figure each one to the needed aspheric correction. 

 

                 - Dave 

 

         

Edited by DAVIDG, 08 September 2022 - 02:33 PM.

[davidc135]

 David,

  Yes they did say they were better coma corrected and again here is  evidence from their own literature.  If the secondary was spherical it would have MORE coma then  most of the systems in that diagram.  Bob Johnson knew optics well and understood what was required to make a compact SCT with good astrophotography performance.  All I hear from others that say the secondary are spherical is just repeating what other say but show no proof . Again if the secondaries were spherical the coma would show in all the many images taken by them.  Please show me some evidence and not hearsay or guests that the secondaries are spherical. 

  By the way I contributed to "Telescopes, Eyepieces and Astrographs" and while the SCT design were thoroughly reviews  the design were guesses at exactly what Celestron and Meade were doing. So I will take what Celestron stated as more proof then other saying  they know better.

   Meade and Criterion were no dummies either and I'm sure  they reverse engineered these designs and also Criterion hired away  Bob Geoff from Celestron to help them make the correctors so I'm sure he knew what the design was and why they were doing it the way they  were.  Criterion knew they needed aspheric asecondaries so they had they made in Japan to save money. Meade did the same as well and I said both companies would swap them in and out the find one that worked while Celestron choose to hand figure each one to the need aspheric correction. 

 

                 - Dave 

 

         

celestron.JPG

I haven't a C8 so I can't test it and say for sure. But I do have proof that my 8'' Meade is spherical using the test referred to in post 939. 

 

Which is better; a practical test where you can see with your own eyes what the figure is, or interpretation of dusty texts? As long as the test has been well thought out I can't see that there is any discussion.

 

David

Edited by davidc135, 08 September 2022 - 02:28 PM.

[DAVIDG]

I haven't a C8 so I can't test it and say for sure. But I do have proof that my 8'' Meade is spherical using the test referred to in post 939. 

 

Which is better; a practical test where you can see with your own eyes what the figure is, or interpretation of dusty texts? As long as the test has been well thought out I can't see that there is any discussion.

 

David

 When I do another C8  I will pull out my test plate and show the interference pattern that it is not spherical. Tim Parker is making a new secondary for his C-14 and I bet  it will need to  be aspherized to null the system. because  the corrector plate has G factor great then 1 to over correct the wavefront from the primary and to fully correct the system for both spherical and coma the secondary is aspherized. 

    I have presented my evidence  from multiple sources spanning many years in a number of threads  and  including what  Bob Johnson stated  who is the founder of Celestron as proof vs what people keep saying and provide nothing to back it up.

  I will state again that the coma correction  would be equal to  a F/5 Newtonian in  a compact SCT with all spherical system and that is an optical fact that  you can't get around. The  coma would be easily visible in any prime focus image taken, yet it is not. These system were designed and marketed for astrophotography and within a short time after they were marketed in the early 70's  , any experienced astrophotographer would have pointed out the large amount of off axis coma if it was present and any review in Sky and Telescope and Astronomy would have done the same.  The only way you reduce the coma with a spherical primary and the corrector plate located near  the focal length of the primary is to  aspherize the secondary to conic of -0.7 or greater. 

   So please explain why would  a company publish information that they aspherize a secondary when they did not, why they would market a telescope that  they state has better off axis correction  then other designs when it can't if the secondary is spherical and  also pay to have a skilled optician hand figure the secondary when it would not be needed ?  

 

                      - Dave 

Edited by DAVIDG, 09 September 2022 - 08:53 AM.

[tim53]

I still have the original secondary that came with. my C-14 prototype, though I chipped it.  It would be interesting to test whether it was spherical or aspheric, but I don't have the means to do that.  I had to find a corrector for it, so even if the secondary was aspherized, it wasn't optimized for the corrector I have, and yet it performed "okay".  My new secondary is now polished out (on pads).  Hopefully I'll have time next week to work on it on a. pitch lap.  This week has been unbearably hot and humid, though, so I haven't made much progress.

[davidc135]

   So please explain why would  a company publish information that they aspherize a secondary when they did not, why they would market a telescope that  they state has better off axis correction  then other designs when it can't if the secondary is spherical and  also pay to have a skilled optician hand figure the secondary when it would not be needed ?  

 

                      - Dave 

 

It's words and tests trump words, either to contradict or confirm.

 

Aspherize can mean anything as all optical surfaces are aspherical in some degree. ie a pure sphere is an unachievable thing.

 

The skilled optician might only be reducing what slight errors were left by machine polishing.

 

And you can't always trust manufacturers to be completely truthful.

 

As the C8 secondary would need a conic of -0.7 at least, to fully eliminate coma which corresponds to a wavefront error of around 2 waves compared to a spherical surface, or 2 fringes on the surface. Even more for the C14 so it would stand out in an interference test, or the test that I used.

But if they purposefully aspherised by half that amount to reduce but not eliminate coma, that would count too.

 

David

Edited by davidc135, 10 September 2022 - 03:08 AM.

[Gil V]

Wow, busy thread.

All I can add is a modest correction to the method used for assembling Dynamax telescopes.

We almost never swapped out secondaries. When we assembled the telescope, we collimated and rotated the corrector and secondary together to achieve best image. Occasionally, we’d loosen and rotate a secondary alone, but not that often.

We swapped correctors frequently. That would be the first optical component we’d change out if needed. It was a bit like trying to find the best match. If a primary was flawed, we could usually spot that right away, and we’d reject that outright and then continue.

A rejected secondary was very uncommon.

Not defending the practice, or criticizing others, but to the limit of my 40 year old memory, that’s how we did it.

[DAVIDG]

 So  getting  back to  the original purpose of this thread. Here are pictures of the interference pattern of latest DX-8 corrector I purchased at Stellafane this past July from the gentleman who worked for Bausch and Lomb when they bought out Criterion He  bought out the remaining inventory when they went under and  he told me all the parts sold as scrap and by weight.  He has been coming to Stellafane for many years and selling the remains of Criterion. Over the years I have purchased clock drive parts, focuser, finder rings, spiders, and mirror mount to name a few and recently a number of DX-8 and 4" corrector plates. They have anti-reflective coated and were wrapped in paper like they were meant for production. Since they were AR coated I don't believe they were "rejects" since why would the company spend the money to get them coated if they did some type of optical test and showed them to be bad. 

   I was told that these were made at the end of the Criterion when B&L owned them so the days when they were making the 8001 series which is rumored to have good optics.   

 

     Here is a picture of the back surface.  In a  Schmidt corrector it doesn't have to be optically flat just optically smooth. So it can be a number of waves convex or concave and it doesn't matter but again is needs to be  optically smooth with no astigmatism. This one still has issues.

 

     Here is  a picture of the pattern on the front that shows the Schmidt profile. The pattern is suppose to look like the upper left  This one does look better then the other ones I have.  Next step is to place it in front of my 8" f/2 spherical mirror and see what the Ronchi pattern looks like.

 

  On  a  side note if have read my post over the years I do my best to provide pictures and other forms of actually data to backup my findings and conclusion. As a  scientist this what you do back up your conclusion and to also educate others.  As the saying goes " In God we trust, All others bring data " 

             

                                                - Dave 

Edited by DAVIDG, 10 September 2022 - 12:55 PM.