Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Refiguring a Dynamax 8" Schmidt Corrector

  • Please log in to reply
886 replies to this topic

#51 DAVIDG

DAVIDG

    Cosmos

  • *****
  • topic starter
  • Posts: 8668
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 31 January 2017 - 01:54 PM

I wonder if you could just replace the Dynamax corrector with a Meade or Celestron one? I mean, they seem like the same optical design.

 It is not the same optical design,  for one the focal length of  a DX-8 is 2110mm vs 2000mm for a C-8.  I could measure across the radius of the plate and determine what the profile of the Schmidt curve is. When back calculate the perimeters needed to make it by the classic vacuum pan method. The reason I'm going the way I am is that this method requires less specialized tools so there is a better chance that someone else might want to give it a try and it will work for other brands of Schmidt correctors as well. I've seen other manufactures that aren't very good as well. 

  As for paying someone to fix one of these, I have 4 hours into this project and if I charged $50/hr that is getting close to what  I bought the scope for. I'm nowhere near done. I'm  guessing I'll put at least 20 hours into this to get it right, maybe more. 

 

                    - Dave 


Edited by DAVIDG, 01 February 2017 - 09:56 AM.

  • tim53, rolo, Live_Steam_Mad and 8 others like this

#52 andycknight

andycknight

    Viking 1

  • -----
  • Posts: 626
  • Joined: 13 Aug 2010
  • Loc: UK

Posted 31 January 2017 - 02:15 PM

Mitrovarr, on 31 Jan 2017 - 6:26 PM, said:Mitrovarr, on 31 Jan 2017 - 6:26 PM, said:

I wonder if you could just replace the Dynamax corrector with a Meade or Celestron one? I mean, they seem like the same optical design.

A good question - one I would like to know the answer to as well...

 

At first glance it seems feasible that if Dynamax and Celestron primary's have the same power (i.e. F2) and the secondary multiplier is 5x (and hence the Schmidt corrector if located in roughly the same position would require a similar correction).

 

Another possible concern is if the curve on the secondary (i.e. is it spherical or aspherical) is different.

 

Finally there is the issue that Celestron have been know to hand correct the secondary for errors on the primary.

 

I'm guessing that very minor changes (like a F2.1 primary instead of F2) could be handled by moving the Schmidt corrector forwards/backwards in the tube, and then re-spacing the secondary to get the correct F10 focal ratio??

 

Does anyone have any more info??

 

Regards

 

Andy.

 

Edit: Just read the Op's answer, Is the difference in Focal length due to a minor difference in the secondary spacing??


Edited by andycknight, 31 January 2017 - 02:18 PM.


#53 starman876

starman876

    Hubble

  • *****
  • Posts: 19076
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 31 January 2017 - 02:40 PM

Is it worth it to take a good SCT and gut it to fix a DX8??   


  • Jon Isaacs and tim53 like this

#54 Mitrovarr

Mitrovarr

    Vanguard

  • -----
  • Posts: 2430
  • Joined: 12 Sep 2004
  • Loc: Boise, Idaho

Posted 31 January 2017 - 02:50 PM

Is it worth it to take a good SCT and gut it to fix a DX8??


No, but Meade and Celestron SCTs have been around a long time, and some have had their bodies fail or be destroyed (rust, flood damage, dents, etc.) There is the occasional one parted out. Although correctors are the most easily broken element and are hard to come by.

#55 Bomber Bob

Bomber Bob

    Voyager 1

  • *****
  • Posts: 14994
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 31 January 2017 - 03:02 PM

If I couldn't fix the DX8 corrector, I'd try taking the guts from a vintage C8 and fitting them to the DX8 tube / hardware -- just like putting a replacement lens in a Classic with a shattered objective (Hi Robert!  Your Tinsley is what I'm thinking of.)



#56 starman876

starman876

    Hubble

  • *****
  • Posts: 19076
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 31 January 2017 - 03:21 PM

If I couldn't fix the DX8 corrector, I'd try taking the guts from a vintage C8 and fitting them to the DX8 tube / hardware -- just like putting a replacement lens in a Classic with a shattered objective (Hi Robert!  Your Tinsley is what I'm thinking of.)

That might not be as easy as you think. Having repaired a lot of SCT's it may be rather difficult doing that.  Diameters of the mirrors and corrector may not be exactly the same.  Primary center hole could be a different size.  Maybe a whole bunch of things to adjust for. Besides, you would need a matched set of optics to have a decent scope.  Which would mean buying a good SCT and gutting it to have any kind of chance of having a decent set of optics.  Seems to me like an effort that would be really costly and a waste of time.


Edited by starman876, 31 January 2017 - 03:23 PM.

  • Geo31 likes this

#57 DAVIDG

DAVIDG

    Cosmos

  • *****
  • topic starter
  • Posts: 8668
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 31 January 2017 - 03:27 PM

 

If I couldn't fix the DX8 corrector, I'd try taking the guts from a vintage C8 and fitting them to the DX8 tube / hardware -- just like putting a replacement lens in a Classic with a shattered objective (Hi Robert!  Your Tinsley is what I'm thinking of.)

That might not be as easy as you think. Having repaired a lot of SCT's it may be rather difficult doing that.  Diameters of the mirrors and corrector may not be exactly the same.  Primary center hole could be a different size.  Maybe a whole bunch of things to adjust for. Besides, you would need a matched set of optics to have a decent scope.  Which would mean buying a good SCT and gutting it to have any kind of chance of having a decent set of optics.  Seems to me like an effort that would be really costly and a waste of time.

 

 Yep, So watch TV, eat popcorn and polish away on the corrector ! 

 

                       - Dave 


  • Jon Isaacs, Mr Magoo, starman876 and 6 others like this

#58 Bomber Bob

Bomber Bob

    Voyager 1

  • *****
  • Posts: 14994
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 31 January 2017 - 05:06 PM

 

If I couldn't fix the DX8 corrector, I'd try taking the guts from a vintage C8 and fitting them to the DX8 tube / hardware -- just like putting a replacement lens in a Classic with a shattered objective (Hi Robert!  Your Tinsley is what I'm thinking of.)

That might not be as easy as you think. Having repaired a lot of SCT's it may be rather difficult doing that.  Diameters of the mirrors and corrector may not be exactly the same.  Primary center hole could be a different size.  Maybe a whole bunch of things to adjust for. Besides, you would need a matched set of optics to have a decent scope.  Which would mean buying a good SCT and gutting it to have any kind of chance of having a decent set of optics.  Seems to me like an effort that would be really costly and a waste of time.

 

Johann, you forget... I am Bomber Bob.  I got an unknown (and possibly Space Alien) Cassegrain mirror set to fit in my Tinsley.  I'm not saying it would be easy, but if I got a really abused C8 with intact optics... I bet I could shoe-horn the bits in my DX8 chassis good enough for appearances...

 

Yep, So watch TV, eat popcorn and polish away on the corrector !

 

Does sweat hurt the grinding?  Cause there'd be a lot of mine dripping on that old Criterion corrector!  One slip, and catastrophe.  Of course, Terra may be right, and I could make a new one from an old window pane...


  • terraclarke, kansas skies, Augustus and 1 other like this

#59 Gil V

Gil V

    Apollo

  • -----
  • Posts: 1280
  • Joined: 09 Sep 2012

Posted 31 January 2017 - 06:04 PM

Two requests.

1) Cool it with the hate. Those of us who actually built the scopes are not liking that. As someone who provided direct customer support 35 years ago, I respectfully disagree. We really did not receive that many complaints. That is the God's honest truth.


2) David - when you are done with all your work, even if it's two years from now, I'd like to look through the finished product. To me, it will be like "what could have been". Perhaps that could be arranged.

Even if David was available for hire back in the day, Criterion probably couldn't afford him. We never had the cash to pay for top notch talent.

Edited by Gil V, 31 January 2017 - 06:05 PM.

  • deSitter, tim53, Bill Griffith and 5 others like this

#60 rolo

rolo

    Cosmos

  • *****
  • Posts: 8327
  • Joined: 14 Jan 2007
  • Loc: GA

Posted 31 January 2017 - 06:25 PM

I don't see any hate here. Could you point out which post expresses hate? You really shouldn't take things so personal it's not your fault.


Edited by rolo, 31 January 2017 - 06:32 PM.

  • terraclarke and TOM KIEHL like this

#61 rolo

rolo

    Cosmos

  • *****
  • Posts: 8327
  • Joined: 14 Jan 2007
  • Loc: GA

Posted 31 January 2017 - 06:30 PM

If I couldn't fix the DX8 corrector, I'd try taking the guts from a vintage C8 and fitting them to the DX8 tube / hardware -- just like putting a replacement lens in a Classic with a shattered objective (Hi Robert!  Your Tinsley is what I'm thinking of.)

With the $200 you have invested in you DX8 OTA why would you spend a few hundred more on C8,create a lot of work for yourself making the optics work, and in the end you have to disclose it's a good DX8 but with Celestron optics.  Maybe I'm missing something but I just don't get this one.


  • terraclarke, TOM KIEHL and Augustus like this

#62 Augustus

Augustus

    Stardust Telescopes

  • *****
  • Vendors
  • Posts: 7465
  • Joined: 26 Dec 2015
  • Loc: Connecticut

Posted 31 January 2017 - 06:34 PM

If I couldn't fix the DX8 corrector, I'd try taking the guts from a vintage C8 and fitting them to the DX8 tube / hardware -- just like putting a replacement lens in a Classic with a shattered objective (Hi Robert!  Your Tinsley is what I'm thinking of.)

The DX8 has a focal length about 100mm longer than that of the C8, thus it might not even work. If you like the mechanics of the DX8 fork and tripod so much, get a C8 OTA and put it on the fork.



#63 starman876

starman876

    Hubble

  • *****
  • Posts: 19076
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 31 January 2017 - 07:22 PM

 

 

If I couldn't fix the DX8 corrector, I'd try taking the guts from a vintage C8 and fitting them to the DX8 tube / hardware -- just like putting a replacement lens in a Classic with a shattered objective (Hi Robert!  Your Tinsley is what I'm thinking of.)

That might not be as easy as you think. Having repaired a lot of SCT's it may be rather difficult doing that.  Diameters of the mirrors and corrector may not be exactly the same.  Primary center hole could be a different size.  Maybe a whole bunch of things to adjust for. Besides, you would need a matched set of optics to have a decent scope.  Which would mean buying a good SCT and gutting it to have any kind of chance of having a decent set of optics.  Seems to me like an effort that would be really costly and a waste of time.

 

Johann, you forget... I am Bomber Bob.  I got an unknown (and possibly Space Alien) Cassegrain mirror set to fit in my Tinsley.  I'm not saying it would be easy, but if I got a really abused C8 with intact optics... I bet I could shoe-horn the bits in my DX8 chassis good enough for appearances...

 

Yep, So watch TV, eat popcorn and polish away on the corrector !

 

Does sweat hurt the grinding?  Cause there'd be a lot of mine dripping on that old Criterion corrector!  One slip, and catastrophe.  Of course, Terra may be right, and I could make a new one from an old window pane...

 

I do not care if you are the original master of all modifications.  It would be a total waste of time ,money and energy.


  • rolo, TOM KIEHL and Augustus like this

#64 kansas skies

kansas skies

    Apollo

  • *****
  • Posts: 1393
  • Joined: 02 Dec 2012
  • Loc: The middle of nowhere, USA

Posted 31 January 2017 - 07:58 PM

"I do not care if you are the original master of all modifications.  It would be a total waste of time ,money and energy."

 

Isn't that the ultimate objective of retirement?


  • BigC, terraclarke, Bomber Bob and 5 others like this

#65 Bomber Bob

Bomber Bob

    Voyager 1

  • *****
  • Posts: 14994
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 31 January 2017 - 08:07 PM

Isn't that the ultimate objective of retirement?

 

Indeed!  It was a CN version of a Monty Python skit -- a farcical idea more complex & convoluted (& expensive!) than what Dave is demonstrating.  You working stiffs -- lighten up!

 

On Topic:  It sounds like both sides have to be corrected, which is beyond my capabilities.


  • rolo, terraclarke, kansas skies and 1 other like this

#66 deSitter

deSitter

    Still in Old School

  • *****
  • Posts: 12129
  • Joined: 09 Dec 2004

Posted 31 January 2017 - 08:19 PM

The truth is, every SCT which aspired to performance commensurate with its aperture would have to undergo this sort of hand tuning. It's just a bad design.

 

-drl


  • tim53 and Augustus like this

#67 deSitter

deSitter

    Still in Old School

  • *****
  • Posts: 12129
  • Joined: 09 Dec 2004

Posted 31 January 2017 - 08:22 PM

..and if you are going to all this trouble, why not make life easy and spend the effort on a classical Cassegrain? I have never been able to understand SCTs.

 

-drl


  • Bomber Bob likes this

#68 Bomber Bob

Bomber Bob

    Voyager 1

  • *****
  • Posts: 14994
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 31 January 2017 - 08:26 PM

Or, if you really want an F8 scope, my Kenko 125C Corrected Dall-Kirkham.  Larger FOV than the C5, better performance on traditional refractor targets (planetary, double stars) than an SCT.  No big honkin' corrector plate -- just a much smaller lens assembly.

 

Because of the Kenko, I'm fine using my DX8 as a compact low power reflector if the corrector can't be corrected.


Edited by Bomber Bob, 31 January 2017 - 08:29 PM.


#69 rolo

rolo

    Cosmos

  • *****
  • Posts: 8327
  • Joined: 14 Jan 2007
  • Loc: GA

Posted 31 January 2017 - 08:38 PM

The truth is, every SCT which aspired to performance commensurate with its aperture would have to undergo this sort of hand tuning. It's just a bad design.

 

-drl

Not so... I'll let you look through the 72 C8 and the C14 Then, you can decide if it's a bad design.. On the contrary, the SCT is an excellent design combining aperture and portability like few other instrument in its size can. Every telescope design that's  excecuted properly will be a good performer, even an SCT. I've seen albedo features on Ganymede with  my C14 and my C8 can show me everything the AP130GT can. Maybe not as contrasty but it's there. 


  • deSitter, highfnum, Live_Steam_Mad and 4 others like this

#70 deSitter

deSitter

    Still in Old School

  • *****
  • Posts: 12129
  • Joined: 09 Dec 2004

Posted 31 January 2017 - 08:52 PM

 

The truth is, every SCT which aspired to performance commensurate with its aperture would have to undergo this sort of hand tuning. It's just a bad design.

 

-drl

Not so... I'll let you look through the 72 C8 and the C14 Then, you can decide if it's a bad design.. On the contrary, the SCT is an excellent design combining aperture and portability like few other instrument in its size can. Every telescope design that's  excecuted properly will be a good performer, even an SCT. I've seen albedo features on Ganymede with  my C14 and my C8 can show me everything the AP130GT can. Maybe not as contrasty but it's there. 

 

I"LL be the judge of that! :) I hope it's true. I just don't trust magic optics.

 

-drl



#71 Mitrovarr

Mitrovarr

    Vanguard

  • -----
  • Posts: 2430
  • Joined: 12 Sep 2004
  • Loc: Boise, Idaho

Posted 31 January 2017 - 09:35 PM

The truth is, every SCT which aspired to performance commensurate with its aperture would have to undergo this sort of hand tuning. It's just a bad design.

-drl


I think there are way too many experienced people happy with their mass-market SCTs for that to be true.
  • rolo and TeiscoMan like this

#72 kansas skies

kansas skies

    Apollo

  • *****
  • Posts: 1393
  • Joined: 02 Dec 2012
  • Loc: The middle of nowhere, USA

Posted 31 January 2017 - 09:57 PM

Now, for my two cents worth...

 

There's nothing magic about an SCT. Like all scopes (of any design), there are good examples and poor examples. I have seen very nice SCT's (I'm perfectly happy with both of mine), and I've seen poor examples. I've seen very nice Newtonians that will give my SCT's a run for their money, and I've seen others that make me wonder why the owners even bother. Refractors are not immune, either. Even some of the ED's and (semi?)APO's that I've seen are not that great. As I've said before, most of the time I'm left wondering if the owners of many of these bad examples truly understand the process of collimation or if those scopes really are that bad? I suspect a little of both. Then there is the question of eyepiece quality, along with a plethora of other variables that add in to the equation. The list goes on and on.

 

I think what Dave is trying to accomplish here is to show that given enough attention to detail, even some of the worst offenders can be made into silk purses. By his own admission, it's not something that the original manufacturer could afford to do given the tools that they had to work with. They probably would have been wise to stick with what they were very good at producing, but in a competitive marketplace, stagnation usually amounts to failure in the end. Then again, even with the success of the major American product lines through the years, the glut of cheaper imports eventually took its toll.

 

Dave is a teacher, and a good one at that. I'm following this thread with great interest, and believe me, I am learning from it. My only fear is that people who really don't understand the process of collimation will begin to perform AC tests on their scopes and misinterpret their results. I have to say that I understand the process and the reasoning behind this testing method, but I really don't understand the math behind the choice of lighting, Ronchi screen and the process of actually interpreting the results. Then again, if you do enough internet searching, there's a tremendous amount of disagreement among the experts on this subject as well (there always is). Since Dave is leading this thread, I intend to listen to him and learn.

 

I still have a fear that instead of making a few minor tweaks to improve performance on a scope to an acceptable level (whatever that is), the owners of these older scopes might start polishing away merrily on their mirrors, lenses and corrector plates and make a real mess of things. Then one day, we will be buying used scopes with all sorts of problems. Then again, I also realize that there are a lot of very competent people out there that will be able to follow Dave's lead and end up with the scope of their dreams. I guess it's a bit of a double-edged sword.

 

Oh well, maybe I'm just paranoid.

 

Bill


  • terraclarke, Bomber Bob, paulymo and 1 other like this

#73 deSitter

deSitter

    Still in Old School

  • *****
  • Posts: 12129
  • Joined: 09 Dec 2004

Posted 31 January 2017 - 09:58 PM

My question is - how to refigure that relatively tiny secondary to make it match the other components? Think about it - there are three optical components and four surfaces that all must agree - a Cassegrain has two. What does the SCT design buy you? Speed? Well yes. But I can just deal with an 8" f/5 or 6 Newtonian if I want speed. So yeah, I am max fascinated by this attempt to turn a mass-produced dog into a real telescope.

 

-drl


  • Bomber Bob likes this

#74 Relativist

Relativist

    Cosmos

  • *****
  • Posts: 8151
  • Joined: 11 Oct 2003
  • Loc: OC, CA, USA

Posted 31 January 2017 - 10:08 PM

Please stop the OT posts and personal attacks.


  • Mr Magoo and Augustus like this

#75 Bomber Bob

Bomber Bob

    Voyager 1

  • *****
  • Posts: 14994
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 31 January 2017 - 10:29 PM

I still have a fear that instead of making a few minor tweaks to improve performance on a scope to an acceptable level (whatever that is), the owners of these older scopes might start polishing away merrily on their mirrors, lenses and corrector plates and make a real mess of things.

 

That's a valid concern.  Even with Dave's guidance, I'm reluctant to attempt this corrector correction -- especially tackling the curved face.  And, it doesn't make much economic sense to pay someone to do the work.  In my case, my $250 bargain DX8 would become a $750 to $1000 scope (at least).  I doubt the performance would match that cost!


  • highfnum and terraclarke like this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics