179 replies to this topic

[DAVIDG]

So preparing  to fix mine and the have worldest best DX-8, I needed to test the primary to see if it had any issues. So I first set the scope back up and tested it via double pass autocollimation.  It's one of the worst scope I have ever seen. The test images show a very rough surfaces , with a turned edge and it is over corrected. I'm pretty sure the overcorrection and turned edge is coming from the secondary and the roughness from the corrector.  To check that I pulled the corrector plate off the scope and tested the primary with a knife edge and Ronchi test at the radius of curvature. It showed an excellent spherical primary. I could not see a flaw. It's really too bad Criterion couldn't have got their process down to make the correctors and why they didn't test them I have no idea.

 So here  are some pictures showing how the complete scope tested. First is inside of the focus 

 

 Next is at focus. Notice how you can see the roughness and asymmetrical pattern.  

 

 

 Here is the outside of the focus. Notice how the lines now bend inward indicating over correction and also how jagged they are. They almost look like lighting bolts ! 

 

 Finally a the primary mirror by itself. Jail bar straight Ronchi band indicating an excellent spherical surface. The goal is for the whole scope to test like that in double pass and I'll have a scope that will give just about any other a run for it's money.

 

 

 

 - Dave 

Edited by DAVIDG, 22 January 2017 - 10:17 PM.

[rolo]

Criterion was so close and yet so far! I kind of suspected the mirrors would be relatively good and the issue has always pointed to the corrector.I hope you can fix the optical flaws with the secondary and corrector so at least there could be one good DX8!.

[deSitter]

Nice work, wow, pic = 1000 words.

 

-drl

[DAVIDG]

 The next  step will be to pull the corrector again and test it against my 8" flat to see what is going on, on the front and back surfaces.  Then I plan to cast a flat pitch lap and condition it against the flat so it's surface is also flat. I'll use that to clean up the "flat" side of the corrector. It's doesn't need to be flat just optically smooth. Then I'll make a 5 lobe peddle shaped lap in the which the peddles are designed to fit the Schmidt curve and so that will hopefully smooth out the curve. If that all works I should then have scope that show smooth and clean Ronchi lines but will most likely show bowing which indicates some level of spherical aberration. I'll  tune that out by figuring the secondary until the Ronchi line are straight. 

   The primary is perfect, if they just tested a couple of these completed scopes  they would have seen the problems and hopefully would have fixed it. I just don't get it ?  It almost seem like they didn't understand how to do optical testing. 

 

                         - Dave 

Edited by DAVIDG, 22 January 2017 - 10:44 PM.

[deSitter]

So you have a nice good fast sphere - can you not now just sort of hand-finish the corrector based on your experience?

 

-drl

[deSitter]

Such a good fast sphere itself is something valuable.

 

-drl

[rolo]

 The next  step will be to pull the corrector again and test it against my 8" flat to see what is going on, on the front and back surfaces.  Then I plan to cast a flat pitch lap and condition it against the flat so it's surface is also flat. I'll use that to clean up the "flat" side of the corrector. It's doesn't need to be flat just optically smooth. Then I'll make a 5 lobe peddle shaped lap in the which the peddles are designed to fit the Schmidt curve and so that will hopefully smooth out the curve. If that all works I should then have scope that show smooth and clean Ronchi lines but will most likely show bowing which indicates some level of spherical aberration. I'll  tune that out by figuring the secondary until the Ronchi line are straight. 

   The primary is perfect, if they just tested a couple of these completed scopes  they would have seen the problems and hopefully would have fixed it. I just don't get it ?  It almost seem like they didn't understand how to do optical testing. 

 

                         - Dave 

If this works out and you want to fix other DX8's I have a couple for you!

[Tarzanrock]

 

   The primary is perfect, if they just tested a couple of these completed scopes  they would have seen the problems and hopefully would have fixed it. I just don't get it ?  It almost seem like they didn't understand how to do optical testing. 

 

Oh, they understood the optics and they understood the necessity of the optical testing of the component parts.  They also understood failure analysis and quality control testing of the component parts and the product as a whole.  They chose to ignore it; and, they likely did so because they well knew that few people within the consuming public purchasing their products would know the difference or even have the ability to test those products.  Since the 1980's it has gotten a lot worse.  Defective products are mass produced and mass marketed and they become "throw away" items when the simplest piece of plastic breaks on the product.

Edited by Tarzanrock, 22 January 2017 - 11:03 PM.

[DAVIDG]

So you have a nice good fast sphere - can you not now just sort of hand-finish the corrector based on your experience?

 

-drl

 Yep, as I explained the first step is  to clean up the "flat" side then tackle the schmidt curve with  an engineered shape peddle lap. This is not new. They made the corrector on the 48" Schmidt at Palomar this way.  The advantage here is the curve has been ground into the plate of  the correct shape, it needs to  be smooth out.

  

                     - Dave 

[highfnum]

Best of luck 

Kinda what was suspected

You can turn this into a business 

If simple enough to do

[Gil V]

I'm not at all surprised the primary tested as well as it did. If there is one thing that shop did well, it was mirror-making.

I'd be interested if you could test just the secondary. How would you go about doing that?

[highfnum]

Daveg on the SA point

Don't all CATs have some level of SA

Except at one point

 

 

What would  you say is bigger  problem  roughness or the SA?

[highfnum]

To gilv: I took out my dynascope 4 inch my first scope that I got in 67

50 years,ago! That little mirror still puts up a real crisp  image

[Terra Nova]

They had to understand optical testing of mirrors to produce a primary of that quality and to turn out RV-6s and Dynascope 8s of the quality they are known to have. For some reason tho, they must never have understood how to test the correctors and they may not have even had a decent understanding of SCT optics in general. Newtonians on the other hand were their forte.

[Gil V]

Tara, I am of the opinion that we did the best we could making our scopes. There was something about the 8" correctors, though, they just weren't smooth enough.

As I described previously, we only polished one side of the plate. I remember we tested a piece that we polished on both sides, and there wasn't a significant improvement.

The 6" plates were really good, though. I'll profess that until you pry that opinion from my cold, dead hands.

[Terra Nova]

Du (Orion61) also attests to the quality of the 6" Dynamax. I have also heard that from at least one other source. The 6" scopes show up on the market very seldom. They are either really good and no one is letting them go or they were produced in very low numbers. Kind of like the old C5s.

[Chuck Hards]

Dave, are you going to polish the back of the secondary and test it through the rear side?  That was my plan.

[DAVIDG]

Dave, are you going to polish the back of the secondary and test it through the rear side?  That was my plan.

No, once the corrector is finished I'll put the scope all back together and figure the secondary until it the complete system nulls. Your just making more work for yourself figuring the corrector by itself since there will always be some residue errors. By figuring the secondary until the complete system nulls under double pass you have corrected for all the errors in the system. 

[DAVIDG]

Daveg on the SA point

Don't all CATs have some level of SA

Except at one point

 

 

What would  you say is bigger  problem  roughness or the SA?

 No, in  theory when made correctly there should be no on axis spherical aberration when the system is focused for infinity. 

 

                           - Dave 

[DAVIDG]

Tara, I am of the opinion that we did the best we could making our scopes. There was something about the 8" correctors, though, they just weren't smooth enough.

As I described previously, we only polished one side of the plate. I remember we tested a piece that we polished on both sides, and there wasn't a significant improvement.

The 6" plates were really good, though. I'll profess that until you pry that opinion from my cold, dead hands.

 Gil,

   No offense  but I don't believe the 6" were any better. I have tested  a number of 4" ones and you know about the 8" ones posted here and those have all been bad. Unless you tell me that the 6" correctors were made by a different method I can not see how they would be good and the 4" and 8" ones bad. Also your test method of looking at a  laser positioned across the room isn't sensitive enough to detect anything but the grossest errors.  I'm still puzzled why better test methods were not used.  All this stuff is "Optics 101" for any one in the business. 

   As for the Newtonians  I have tested  maybe 50 RV-6's  over years and they have all be undercorrected but when smooth they are at or just below a 1/4 wave. As I have said many times there is a  very good reason why they have a  50" focal length. It is because in Jean Texereau's book 'How to Make a Telescope' there is  a listing of  mirrors for  what focal lengths they can be left spherical and be just at 1/4 wave. For a 6" one that was 50".  My own RV-6 that I purchased new in 1976 had a spherical mirror with turned edge. I have since refigured it and now it gives excellent images.The 4" Dynascopes use mirrors that were almost f/12, again  being spheres. I have tested  at least  ten RV-8 and those mirrors have all been spheres but at that aperture and being f/8 they are above a 1/4 wave and not diffraction limited.  

   So Criterion was smart enough to understand that if you made Newtonians that had long enough focal lengths, the mirrors could be spheres or very close to that and be at or below 1/4 wave.  Testing spherical mirrors is very simple and easy. As I have said many times most people are using telescopes that are  much worse then this so when they  look through  a true 1/4 wave or better systems they are very impressed and they should be since these system deliver very good images. 

 

                       - Dave 

Edited by DAVIDG, 23 January 2017 - 03:00 PM.

[DAVIDG]

Best of luck 

Kinda what was suspected

You can turn this into a business 

If simple enough to do

As a hobby this will be fun but I couldn't make a business from  it.  It's going to take me many hours to fix the corrector and if I paid myself the going rate around here of about 80/hr what   a car mechanic charges, I would have to charge around $2000 to refigure a system.  

 

                       - Dave 

Edited by DAVIDG, 23 January 2017 - 03:00 PM.

[Terra Nova]

Amazing. I have never owned or even looked thru an RV-6 but from all the accolades, I assumed that they were better that 1/4 wave and were figured as parabolas. Glad I stuck with Edmund and Cave. Edmund 6" and 8" mirrors were parabolic as were Caves.

Edited by terraclarke, 23 January 2017 - 10:23 AM.

[Geo31]

As I have said many times most people are using telescopes that are  much worse then this so when they  look through true 1/4 wave or better systems they are very impressed and they should be since these system deliver very good images. 

 

                       - Dave 

 

Well, you've certainly convinced me and I'm sure a lot of others.

Edited by Geo31, 23 January 2017 - 10:23 AM.

[DAVIDG]

Tara, I am of the opinion that we did the best we could making our scopes. There was something about the 8" correctors, though, they just weren't smooth enough.

As I described previously, we only polished one side of the plate. I remember we tested a piece that we polished on both sides, and there wasn't a significant improvement.

The 6" plates were really good, though. I'll profess that until you pry that opinion from my cold, dead hands.

 Gil,

   If you tested them and there wasn't any improvement then  it sounds like you knew there was a problem. Also how did you test the 6" plates and know they were better ?

 

               - Dave 

[Napersky]

I've really admired the Dynamax and may end up buying Darren Drake's perfect Golden Tripod.

I especially liked the advertisement's I believe one may have announced they were tested to 1/10th wave with interferometry.

 

Mark