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Sad Sack SCT - Criterion Dynamax 8

catadioptric classic equipment SCT
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916 replies to this topic

#876 starman876

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Posted 26 March 2017 - 09:23 AM

and just how long do you think we can talk about this subject BeatingADeadHorse.gif


Edited by starman876, 26 March 2017 - 09:23 AM.

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#877 davidc135

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Posted 26 March 2017 - 09:33 AM

I'd pay $250 like a shot if everything apart from the corrector was very good. Way cooler than a Celestron or Meade. The trouble is I'd have to pay nearly the same to get it shipped over here.

 

David


Edited by davidc135, 26 March 2017 - 09:36 AM.

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#878 Bomber Bob

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Posted 26 March 2017 - 09:33 AM

Reading a thread and posting to a thread are voluntary acts -- not compulsory.

 

I'll be done once I get imagery made of Jupiter, an Airy Disk, and bench tests of the optics.

 

Y'all can keep the thread going until CN runs out of disk space.  How's that??


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#879 terraclarke

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Posted 26 March 2017 - 12:24 PM

You're hurting my feelings...not cool.

(((((((((((((((((HUGS))))))))))))))))))

 

                 grouphug.gif


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#880 terraclarke

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Posted 26 March 2017 - 12:26 PM

and just how long do you think we can talk about this subject BeatingADeadHorse.gif

Until we make it the longest and longest running thread on CN! So far that title is held by the modern, black, C90 spotter! Our goal is definitely doable! :rofl:


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#881 terraclarke

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Posted 26 March 2017 - 12:33 PM

despite that i'm keeping mine and is still usable with limitations on use

I had a friend who inherited his grandmother's 1963 Rambler and kept it! Kinda the same!


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#882 rolo

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Posted 26 March 2017 - 12:48 PM

i think it because it came close to being great

then failed

 

Exactly.  And we've never forgiven Criterion for it.  I think in part because of the Dynascope - the Dynamax was the opposite of that low cost EQ reflector with decent optics.

Times were changing, big Newtonians were on the way out and Criterion was just trying to stay in business. Every Newtonian company of the day is long gone..


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#883 terraclarke

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Posted 26 March 2017 - 12:52 PM

Well, the other transferable skill would have been making water-heaters! :lol:


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#884 davidmcgo

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Posted 26 March 2017 - 02:19 PM

Well, the other transferable skill would have been making water-heaters! lol.gif

They would probably still have used the cardboard tube!

 

Dave


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#885 clamchip

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Posted 26 March 2017 - 08:15 PM

I started working on my RV-8 today.

I'll bet this is the same tubing as the Dynamax 8 uses.

I'm going back to work where I left it 2-1/2 years ago.

I made the mistake of putting foam rubber pads under the

mirror and the collimation would change by the minute.

The cell is very deep so it needs something thick, but not

foam rubber.

Robert

 

Criterion 002.JPG

Criterion 009.JPG

Criterion 007.JPG


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#886 clamchip

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Posted 26 March 2017 - 09:29 PM

I imagine everyone's seen this history of Criterion but just in case here:

http://www.lpl.arizo...criterion.html/

Towards the end of the article is the Dynamax.

 

Robert


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#887 Bomber Bob

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Posted 27 March 2017 - 11:16 AM

Loose Tube Repair -->  It's been over a month, and the epoxy has held the tube to the base.  The DX8 has been in the shed the whole time, and temps have been 90+ to 30F.  I had the OTA parallel to the ground when not in use.  Chock another one up for Gorilla Glue products!


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#888 Bomber Bob

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Posted 26 April 2017 - 10:25 PM

Bought the stuff today to make corrector masks for my DX8:  1"  1.5"  &  2"  sizes.  NWS forecasts clear skies Monday night, and I'll have these ready by then.  We'll see what effect - if any - blocking the non-circular edge of mine makes.


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#889 terraclarke

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Posted 27 April 2017 - 07:04 AM

How's about an opaque 8" disk? :question:


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#890 TOM KIEHL

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Posted 27 April 2017 - 07:08 AM

How's about an opaque 8" disk? question.gif

Doesn't it have one of those already OEM . poke.gif  rofl2.gif 


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#891 starman876

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Posted 27 April 2017 - 07:43 AM

I think they work best with the dust cover on.


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#892 ftwskies

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Posted 27 April 2017 - 08:55 AM

Bought the stuff today to make corrector masks for my DX8:  1"  1.5"  &  2"  sizes.  NWS forecasts clear skies Monday night, and I'll have these ready by then.  We'll see what effect - if any - blocking the non-circular edge of mine makes.

I'm so confused, JW...  I know your corrector had a ragged edge, but isn't that already hidden when it's installed?  Are you making some sort of (tiny) off-axis aperture masks?  :hmm:



#893 Bomber Bob

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Posted 27 April 2017 - 09:20 AM

Yeah, the retaining ring hides its dirty secret.  The edge masking is to test the idea that some of the performance issues are nearer the rim than the middle.


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#894 davidc135

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Posted 27 April 2017 - 03:53 PM

Some posts back you were setting up an optical bench but you already have everything to hand to assess the DX-8 even if there is no flat (?). Why not use the Tinsley 6'' in a single pass collimated test? Not perfect but it'll give you a very good map of the sct's wavefront.

 

Layout is simple; torch/diffuser, slit at focus of DX-8 then in reverse; cassegrain facing the Dynamax and knife edge at it's focus. Or, instead of knife edge, Ronchi grating in and around focus. The knife edge can be quantitative. The two optical axes should be parallel but can be, should be off centre.

 

David


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#895 GeneT

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Posted 28 April 2017 - 06:54 PM

I almost bought a Dynamax 6 in 1980 but was warned off it and bought a C5 instead.  Ever since, I've been wondering what Dynamax telescopes were really like.  Earlier, I had owned an RV-6, which was excellent, but I got out of astronomy for a few years and sold all my equipment.

The RV-6 was an excellent telescope. When Dynamax brought out their SCT line, I had high hopes. I bought a Dynamax 8. The optics were horrible. It was the worst telescope I have ever owned. The Coulter 13.1 is a close second. I sold my Dynamax 8 and bought a C8. What a difference. It was a great planetary telescope, yielding nice contrasty, crisp views of the planets. The mechanical aspects of the C8 were also excellent. The telescope moved throughout the viewing arch effortlessly. The tracking was right on. I then jumped up to 20, and 18 inch Dobs before settling on my keeper telescope--a 12.5 inch, F5 Portaball.  


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#896 Gil V

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Posted 29 April 2017 - 03:45 PM

Gene, I love your post.

The scopes that I have now are - a Dynamax 8 and a Coulter 13.1".

Love them both to death.
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#897 GreyDay

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Posted 29 April 2017 - 08:19 PM


I made the mistake of putting foam rubber pads under the

mirror and the collimation would change by the minute.

The cell is very deep so it needs something thick, but not

foam rubber.

cork does the job really well and you should be able to get the thickness you need.


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#898 starman876

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Posted 29 April 2017 - 10:44 PM

Some posts back you were setting up an optical bench but you already have everything to hand to assess the DX-8 even if there is no flat (?). Why not use the Tinsley 6'' in a single pass collimated test? Not perfect but it'll give you a very good map of the sct's wavefront.

 

Layout is simple; torch/diffuser, slit at focus of DX-8 then in reverse; cassegrain facing the Dynamax and knife edge at it's focus. Or, instead of knife edge, Ronchi grating in and around focus. The knife edge can be quantitative. The two optical axes should be parallel but can be, should be off centre.

 

David

you just went above his paygrade with those instructionslol.gif


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#899 GreyDay

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Posted 30 April 2017 - 05:26 AM

 

you just went above his paygrade with those instructionslol.gif

 

and mine confused1.gif  

I've been folowing DAVIDG and his dynamax refiguring enterprise, i don't understand all of whats said in that thread but the picture updates show the progress in a way the layman understands.

 

I was kinda hoping BB would be posting pics of the corrector figure on his scope, sure he'll get there at some point.


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#900 davidc135

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Posted 30 April 2017 - 08:19 AM

 

Some posts back you were setting up an optical bench but you already have everything to hand to assess the DX-8 even if there is no flat (?). Why not use the Tinsley 6'' in a single pass collimated test? Not perfect but it'll give you a very good map of the sct's wavefront.

 

Layout is simple; torch/diffuser, slit at focus of DX-8 then in reverse; cassegrain facing the Dynamax and knife edge at it's focus. Or, instead of knife edge, Ronchi grating in and around focus. The knife edge can be quantitative. The two optical axes should be parallel but can be, should be off centre.

 

David

you just went above his paygrade with those instructionslol.gif

 

Not at all! But I should have taken some more time.

Perhaps you have done a knife edge test at the ROC of a spherical mirror? This is just the same. What does a perfect sphere show? Why the even greying? What if there are zones or a turned edge or it's smooth but not spherical. Why are some parts then lighter and others darker? How can you measure what you see in terms of waves of error?

 

The slit at the focus of the first telescope acts like a point source of light.. A pinhole could be used but the slit is easier. Make it fine. Although either scope can be used as the collimator it's maybe better to use the Tinsley rather than the DX-8 here. Then alternative scopes can be tested.

Light travels in reverse through the scope and exits as a collimated beam of parallel light 6inches across. This beam's wavefront is as perfectly plane as the errors of the Tinsley allows. Now, this 'perfect' collimated 'starlight' enters the front of the telescope to be tested and is brought to a focus where it can be tested by a Ronchi grating, a knife edge or viewed with an eyepiece. As the light passes through the faulty optics of the scope under test, parts or zones of the wavefront, which now should become uniformly spherical, are advanced or retarded by whatever errors they've met along the way and will then focus longer or shorter. This is what's shown by the knife edge or Ronchi. It's good to have the two alternative tests. For example the Ronchi may show a diffraction curling back effect around the edge which might be mistaken for a turned down edge. The knife edge can be used to check on this.

 

A couple of practical tips: make the slit 1/4inch long, tape the blades onto a flat eg glass surface. On the glass side away from the scope and facing the torch tape something eg tissue paper to diffuse the light.

At the other scope's focus ensure the testing knife edge or Ronchi is aligned parallel with the slit which will be obvious in practise.

 

Try it on a Zeiss and see what perfection looks like. Compare that with the DX-8 and maybe think Wow! the errors are set out in such detail and so precisely. Swap for the Ronchi and compare. Which tells me more. Swap for the eyepiece and check/confirm astigmatism and general standard of imagery.

 

Many contributing to and following this thread will be practical, especially BB. I see long hours of careful, skillful work spent on restoring classic scopes. So much for pay grades.

 

Most in this forum will have a collection of scopes including some with excellent optics, better than 1/8 wave and maybe better than many flats.

 

You can set up these tests with a Ronchi or knife edge, with a flat or auxiliary scope. Pros and cons. Versions of the same thing. I find  the knife edge test quick, easy, informative and fun. It takes maybe half an hour to set up. Why not try it and see for yourself?

 

Yes, as Greyday says, pictures would explain things best. I must work on that.

 

David


Edited by davidc135, 30 April 2017 - 10:07 AM.

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