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Dynamax 8 image compared .

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#1 Stevegeo

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Posted 12 February 2020 - 06:38 PM

I took my newly acquired dynamax 8 and pointed it across my valley to a cell phone tower . Using a Celestron cell phone holder I took a shot of this tower with a 8 to 24mm zoom (Celestron)   at  24 mm setting .. loosing daylight fast I have not the chance to go down further . 

According to Google this tower is 3.35 miles away .  Now I have had others tell me that the Dynamax 8 has horrible optics. 

I will make further comparisons to my C8 using a 20 mm and  12mm eyepieces  with both scopes as I can , taking shots the same time, same day,  and post results and let the CN members chime in.  Stay tuned .. 

 

 IMG_20200212_153233.jpg

Stevegeo Area3751 observatory otisco lake ny 


 

#2 Steve Allison

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Posted 12 February 2020 - 07:11 PM

Looking forward to your comparisons and images!


 

#3 markb

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Posted 12 February 2020 - 08:40 PM

Apparently some were very good. By all reports most were not due, most likely, to rough correctors. From Davidg's thread on the 8 repair it sounds like cheaping out on corrector glass substrate quality played a part, although I also recall reading that the tooling should have been better.

 

I assume the lucky good ones came from batches of better quality glass, but that is a true guesstimate based on what I've read.

 

I you have a good one enjoy it, really. Mechanicals were decent.

 

I have the B&L version made after they bought the tooling. My first 'bought new' scope, I assumed inexperience and local seeing explained why it seemed to be pretty awful. After gaining knowledge and experience it was pretty easy to diagnose it as ... rough corrector.

 

Comparing it years later to a sharp Celestron was eye opening (and heart rending to some extent).

 

I might as well have flushed the money down the sewer. B&L had a good reputation at the time, and I relied upon it. I think I ordered it directly from B&L, too.

 

If I had more knowledge at the time it would have gone back, and I suspect they would have made it right.

 

The issues were fixed eventually in the 8001 series, forgive the fog of time if I got that wrong.

 

The Dynamax primaries and secondaries are said to be very good, but mine has not been out of the closet in 35 years or so for further examination. I don't recall flaws beyond the rough corrector though, not sure if I figured it out after learning how to do extrafocal testing.

 

On the plus side, 20-25 years ago I picked up a Rumak meniscus of about the right diameter. Just a tad too large to install and try out when I bought it, the now-affordable 3D printer will let me generate an adapter (or several if I need to adjust mirror spacing) to fit the slightly different diameters, after I move west.

 

It will be an interesting project with low expenditure risk.


 

#4 G-Tower

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Posted 13 February 2020 - 09:05 AM

Here's a more scientific comparison. The DX8 DPAC is typical, rough corrector with other issues. The C8 is from the Halley era which were supposed to be the worst Celestron produced. Lines should be straight and smooth. Personally, from the dozens of tests posted in this forum, I've never seen a good one. There was a guy here who tested eight and all had the same corrector issue.

 

 https://www.cloudyni...x-8/?hl=dynamax


 

#5 CHASLX200

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Posted 13 February 2020 - 09:21 AM

Just slap some power to it on a planet and that will tell ya the real story.


 

#6 DAVIDG

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Posted 13 February 2020 - 11:39 AM

 When taking images to show optical quality, the subject needs features that are the size of the theoretical resolution of the optics and the magnification needs to be high to resolve these features. Now you can see   if the optical quality will allows these features to be resolved.  If both condition are not meet, high quality optics and poor one will give the same results and a false positive result to the test. If your comparing two telescope and don't know the actual quality of the optics of the "reference"  then  the results only prove that one maybe better then the other. If the one your comparing is equal to the other all that means is those  two examples are the same but not the one your comparing also has  excellent optics. 

   It has been reported that some  DX-8 give good images but at what magnification ?  Benching testing of all the example shown here show very poor quality optics. If the magnification is too low optical errors will not show in both photographic images and also visual ones.  An example of this are typical binoculars. The lens in them have an  F-ratio around F/4 to f/6 so the chromatic aberration is 1/2 wave  or worse when the objective an  is achromat made of type BK7 and F2   but since the magnification is fixed at  a typical 7x to 10x, the errors don't show or show very little.  Take one those same lenses out of those binoculars and take the  magnification up high enough to resolve the features at the theoretical limit and the image is a blurry mess. 

   The best way to test optical quality is a  bench test under controlled conditions. 

 

                        - Dave 


Edited by DAVIDG, 13 February 2020 - 01:07 PM.

 

#7 Bomber Bob

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Posted 13 February 2020 - 01:07 PM

Point your DX8 at Mars, Jupiter, or Saturn.  At just 30x per inch, or 240x, see what you get...

 

After that disappointing session, use your DX8 for what it can do:  Deliver good low to medium power views of deep sky objects -- the faint fuzzies.


Edited by Bomber Bob, 13 February 2020 - 01:08 PM.

 

#8 Terra Nova

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Posted 13 February 2020 - 02:23 PM

Apparently some were very good. By all reports most were not due, most likely, to rough correctors. From Davidg's thread on the 8 repair it sounds like cheaping out on corrector glass substrate quality played a part, although I also recall reading that the tooling should have been better.

 

I assume the lucky good ones came from batches of better quality glass, but that is a true guesstimate based on what I've read.

 

I you have a good one enjoy it, really. Mechanicals were decent.

 

I have the B&L version made after they bought the tooling. My first 'bought new' scope, I assumed inexperience and local seeing explained why it seemed to be pretty awful. After gaining knowledge and experience it was pretty easy to diagnose it as ... rough corrector.

 

Comparing it years later to a sharp Celestron was eye opening (and heart rending to some extent).

 

I might as well have flushed the money down the sewer. B&L had a good reputation at the time, and I relied upon it. I think I ordered it directly from B&L, too.

 

If I had more knowledge at the time it would have gone back, and I suspect they would have made it right.

 

The issues were fixed eventually in the 8001 series, forgive the fog of time if I got that wrong.

 

The Dynamax primaries and secondaries are said to be very good, but mine has not been out of the closet in 35 years or so for further examination. I don't recall flaws beyond the rough corrector though, not sure if I figured it out after learning how to do extrafocal testing.

 

On the plus side, 20-25 years ago I picked up a Rumak meniscus of about the right diameter. Just a tad too large to install and try out when I bought it, the now-affordable 3D printer will let me generate an adapter (or several if I need to adjust mirror spacing) to fit the slightly different diameters, after I move west.

 

It will be an interesting project with low expenditure risk.

Based on how they were ALL made (all the same way), what the correctors were made of (all of the same glass), the history (and legal history) of their production (well known), and pretty much every DPAC test that was ever done and presented here, there were NO good ones! (By good I mean anything even approaching defraction limited/ one-quarter wave. Heck, I think it would be going some to assert that they ever even approached half-wave. That said, they do have their adherents and if you are willing to settle for a nice looking scope, with good mechanics (for the most part good, however there were problems with the tube and the way the cells were glued into it), and acknowledge that you have something that you can use for fair-to-middlin’ low-power star fields, and planetary and lunar views up to 25X to 30X per inch of aperture for your best sample, then you’re good to go.


Edited by Terra Nova, 13 February 2020 - 02:25 PM.

 

#9 highfnum

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Posted 13 February 2020 - 04:13 PM

everytime one of these dx8 treads start

i have to take out dx8

i must be trained like a dog


 

#10 Augustus

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Posted 13 February 2020 - 04:20 PM

Based on how they were ALL made (all the same way), what the correctors were made of (all of the same glass), the history (and legal history) of their production (well known), and pretty much every DPAC test that was ever done and presented here, there were NO good ones! (By good I mean anything even approaching defraction limited/ one-quarter wave. Heck, I think it would be going some to assert that they ever even approached half-wave. That said, they do have their adherents and if you are willing to settle for a nice looking scope, with good mechanics (for the most part good, however there were problems with the tube and the way the cells were glued into it), and acknowledge that you have something that you can use for fair-to-middlin’ low-power star fields, and planetary and lunar views up to 25X to 30X per inch of aperture for your best sample, then you’re good to go.

I think the 6es might actually be better due to the low quantity of production and the greater care taken in their manufacture.

 

everytime one of these dx8 treads start

i have to take out dx8

i must be trained like a dog

Bring it tomorrow, I wanna see


 

#11 starman876

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Posted 13 February 2020 - 04:31 PM

Put the scope on a bright star and see what the image looks like. Is the star nice and round with a difeaction ring. If it looks like a comet with some tails you have an SCT.
 

#12 Terra Nova

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Posted 13 February 2020 - 04:38 PM

The I think the 6es might actually be better due to the low quantity of production and the greater care taken in their manufacture.

 

Bring it tomorrow, I wanna see

I doubt it. They were made same way so why would they any better? It doesn’t matter if you take three times longer to make something if you are using the same flawed process and the same poor materials you’re gonna wind up with the same substandard product. It seems like everyone one of these DX threads has someone looking for the golden fleece amongst the flock of black sheep. I’ve just read so much that frankly, I don’t think it exists. I think Chuck H., David G. And Rolando C. would all agree,with me. They’ve sure turned over enough rocks searching for a good one. It’s nice to be optimistic but sometimes you just have to be realistic.

 

To be clear, I’m not ragging on the DXs just to be ragging on them. Like I implied in earlier remarks, as long as a new or prospective owner knows what they’re getting into and realizes the limitations of these scopes, it’s all good, but just remember, good here is relative.


Edited by Terra Nova, 13 February 2020 - 04:58 PM.

 

#13 G-Tower

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Posted 13 February 2020 - 04:40 PM

This is very simple...A "good" DX8 or any other scope is one that is at least diffraction limited. I've seen tests of both DX8 and DX6 and all had the same corrector issues. Until there's a good documented bench test they're all bad in my book.


 

#14 CHASLX200

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Posted 13 February 2020 - 04:58 PM

I doubt it. They were made same way so why would they any better? It doesn’t matter if you take three times longer to make something if you are using the same flawed process and the same poor materials you’re gonna wind up with the same substandard product. It seems like everyone one of these DX threads has someone looking for the golden fleece amongst the flock of black sheep. I’ve just read so much that frankly, I don’t think it exists. I think Chuck H., David G. And Rolando C. would all agree,with me. They’ve sure turned over enough rocks searching for a good one. It’s nice to be optimistic but sometimes you just have to be realistic.

I had a 6" that was ok for low power. Once ya slap up the power the mush comes alive.  Had other brands of SCT's that were even worse. Looking at a land object tells ya much of nothing at low powers. Get it to 30x or more a inch and then see what it does.


 

#15 davidc135

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Posted 13 February 2020 - 05:06 PM

I doubt it. They were made same way so why would they any better? It doesn’t matter if you take three times longer to make something if you are using the same flawed process and the same poor materials you’re gonna wind up with the same substandard product. It seems like everyone one of these DX threads has someone looking for the golden fleece amongst the flock of black sheep. I’ve just read so much that frankly, I don’t think it exists. I think Chuck H., David G. And Rolando C. would all agree,with me. They’ve sure turned over enough rocks searching for a good one. It’s nice to be optimistic but sometimes you just have to be realistic.

 

Yep, my DX-6 is much the same.

 

Believing that Criterion's worst error was rushing the polishing I tried to recreate their vacuum and profiled plate process. Turned out hopeless and although there could be details that might be improved, after a while I lost faith.  David


 

#16 markb

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Posted 13 February 2020 - 05:24 PM

No, on the 6 inchers being better. No, no, no. That's what my miserable B&L is. Same crap different size.

 

Terra, I only passed along the reported claims that a few might have come out okay. Of course that could have been noncritical users, low power use as Dave suggested (and the OP has tried so far), or who knows what. And nobody buys an SCT for low power in my opinion, nor is a low power low resolution image ever going to remotely challenge a high resolution, contrasty and sharp low power image.

 

Too low power use and noncritical users jump out as likely reasons for the reports. I have read about people returning their Dynamax and get just a different stinker from the production line, so I am not inclined to think they hand selected or hand figured any returns (as rumored of C14s at least).

 

I won't argue whether there were or weren't any 'good' ones. My sample size of one says no. In bold face capitals.

 

Several reports have said the mirrors were fine, I'll go along with whatever Dave says, he has hands on experience and, I assume, tested the mirrors. Or go along with anyone that has tested individual components.

 

I would actually like to see reports on the primaries, on their own, for curiosity's sake.

 

G-Tower's link seems to imply an awful mirror in addition to the rough corrector.

 

I have not found any primary-only tests on the Dx, and CN searches only turned up a couple of DPAC images, but I often find CN searches are spotty.

 

Links would be appreciated, as would links on the legal history. I have read several threads on the production . . . 'issues'. I doubt that term is strong enough. I'd hate to see the $$$ figured on useless scopes sold through the Criterion and B&L periods. I will apologize for this, but I think their biggest mistake was not rushing the polishing, it was in not shutting down the line to fix production of consistently bad scopes.

 

Funny that I never saw a negative review while the manufacturers were buying full page ads. A different issue for a different thread!


Edited by markb, 13 February 2020 - 05:31 PM.

 

#17 DAVIDG

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Posted 13 February 2020 - 05:50 PM

" I would actually like to see reports on the primaries, on their own, for curiosity's sake." I can  tell you I tested the primary in my DX-8 and it is a perfect sphere. Just beautiful.  It is one of the reason why I decide to see if I could fix the corrector since the primary was good there was hope on refiguring the corrector then the secondary to make an excellent telescope.  

 

                    - Dave 


 

#18 highfnum

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Posted 13 February 2020 - 07:30 PM

DAVIDG got a question 

why do you have to mess with secondary 

after corrector plate is fixed 

since its a very weak lens?


 

#19 Terra Nova

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Posted 13 February 2020 - 07:50 PM

Markb, with regard to the legal issues:

 

https://casetext.com...rion-mfg-co-inc

 

and:

 

https://law.justia.c...52/612/1526173/

 

As you can see, at the outset, Criterion set out to copy Celestron’s proven process of making the corrector plates. When Celestron got wind of it, they sued as they had already patented their method. The courts stopped Criterion in their track and the were left to scramble and improvise, and beside that, they proceeded to use shoddy glass.


 

#20 DAVIDG

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Posted 13 February 2020 - 08:58 PM

DAVIDG got a question 

why do you have to mess with secondary 

after corrector plate is fixed 

since its a very weak lens?

 Because there will be residue errors in the system both from the primary corrector and the secondary. For example,  the primary can be a perfect sphere but the radius might be off from the exact design so that would make the corrector need to be slightly different then the design  which would require a slightly different figure on the secondary to fully correct the complete system. As I have said before an optical system with multiple surfaces is the same as an electronic circuit with multiple components. In both systems there are tolerance in all the parts. In an  electronic system their are components that  are tunable to bring the system into spec. In optics you pick a surface and figure it to remove the residue errors of the rest. The secondary in a Schmidt Cass is the smallest surface so it is the  logical one to adjust the figure on to null the system.

   This is was one of the reasons why Celestron was hand figuring secondaries on their scope and one of reasons why their optics give better images. 

 

                  - Dave 


 

#21 Stevegeo

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Posted 13 February 2020 - 09:25 PM

Wow folks I am getting an education here , like I said I bought this initially for the tripod , but the scope being such good shape decided I may play with it a bit, find its limits, and keep it for outreach when the visitor to scope ratio is high . Give everyone a chance at least to look . Maybe inspire a few young minds about the night sky ..

So far for me it's just another scope, that's has size and acceptable views... better weather will determine it's worth. 

 

Thanks to everyone  who chimed in .. 

 

Stevegeo Area3751 observatory otisco lake ny 


 

#22 Augustus

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Posted 13 February 2020 - 09:47 PM

 Because there will be residue errors in the system both from the primary corrector and the secondary. For example,  the primary can be a perfect sphere but the radius might be off from the exact design so that would make the corrector need to be slightly different then the design  which would require a slightly different figure on the secondary to fully correct the complete system. As I have said before an optical system with multiple surfaces is the same as an electronic circuit with multiple components. In both systems there are tolerance in all the parts. In an  electronic system their are components that  are tunable to bring the system into spec. In optics you pick a surface and figure it to remove the residue errors of the rest. The secondary in a Schmidt Cass is the smallest surface so it is the  logical one to adjust the figure on to null the system.

   This is was one of the reasons why Celestron was hand figuring secondaries on their scope and one of reasons why their optics give better images. 

 

                  - Dave 

If I recall from Gil's article, they bought the secondaries coated from Japan. No effort was made to hand-figure any surface to null the rest.


 

#23 davidc135

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Posted 14 February 2020 - 03:30 AM

Dynamax failings were mostly down to the roughness of their correctors rather than being greatly under or over corrected. Tho I'm not saying they were perfect. Figuring the secondaries wouldn't have made much difference.

 

Dave G is working to achieve small scale smoothness which may well leave regular, larger scale errors. So it makes sense to correct those by working on the secondary.   David


Edited by davidc135, 14 February 2020 - 03:34 AM.

 

#24 Senex Bibax

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Posted 14 February 2020 - 07:45 AM

Did Criterion manage to manufacture and sell any 8s before being forced to cease and desist? If they did, that might explain claims of the occasional "good" examples...

 

Markb, with regard to the legal issues:

 

https://casetext.com...rion-mfg-co-inc

 

and:

 

https://law.justia.c...52/612/1526173/

 

As you can see, at the outset, Criterion set out to copy Celestron’s proven process of making the corrector plates. When Celestron got wind of it, they sued as they had already patented their method. The courts stopped Criterion in their track and the were left to scramble and improvise, and beside that, they proceeded to use shoddy glass.


 

#25 DAVIDG

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Posted 14 February 2020 - 08:36 AM

Dynamax failings were mostly down to the roughness of their correctors rather than being greatly under or over corrected. Tho I'm not saying they were perfect. Figuring the secondaries wouldn't have made much difference.

 

Dave G is working to achieve small scale smoothness which may well leave regular, larger scale errors. So it makes sense to correct those by working on the secondary.   David

 My own DX-8 tests as very rough but also fair amount of over correction as well. Figuring the corrector is going to change it's power so if you want a diffraction limited system, the complete set of optics needs to tested as unit and corrected as unit.

    This is why Celestron was hand figuring the secondaries in their scope and why Cumberland who makes the optics for Questar tests and corrects each set of optics that goes in them as well. 

 

             - Dave 


 


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