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Criterion Dynamax

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#26 DAVIDG

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Posted 12 July 2020 - 10:08 AM

" I am sure there are a few good ones out there...... "  but so far it is like Big Foot, the Loch Ness Monster,  and aliens at Area 51, people believe but no actual proof.  Just the opposite every one that is actually benched tested show the optics to be bad, really  bad.

 

  Yes I'm still working on refiguring the corrector when time allows between many other projects. 

 

                - Dave 


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#27 davidc135

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Posted 12 July 2020 - 02:55 PM

P7121397b.jpg

 

Waiting for the planets. I love it!  David


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#28 Stevegeo

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Posted 12 July 2020 - 03:13 PM

I have a dx8 I use mostly for lunar and planetary stuff .. and watch eagles other side of my lake a few miles ..it's heavy , tracks great, tripod is dead on , but above 100x the views are hit and miss depending on temps outside ..
Stevegeo

#29 davidc135

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Posted 12 July 2020 - 04:05 PM

They deserve their reputation but I'm trying out a new corrector. But looks like the clouds are rolling in...  David


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#30 GreyDay

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Posted 12 July 2020 - 04:17 PM

They deserve their reputation but I'm trying out a new corrector.

Can't wait to see the DPAC's Dave, good luck with the weather and startest, supposed to be better up the west coast this week :)


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#31 highfnum

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Posted 13 July 2020 - 07:53 PM

i got one

its usable 

it not the greatest 

it does have excellent tracking

deep sky ok

fine planetary details at high power-- no

 

this forum  has plenty of data on this scope

its the Edsel of the telescope world 


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#32 ANM

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Posted 22 July 2020 - 02:14 PM

i got one

its usable 

it not the greatest 

it does have excellent tracking

deep sky ok

fine planetary details at high power-- no

 

this forum  has plenty of data on this scope

its the Edsel of the telescope world 

I think that's unfair to Edsel...


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#33 photoracer18

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Posted 22 July 2020 - 02:49 PM

I picked one up local to me off CL with the finder, VB, and one eyepiece for $125. That is about as far as I was willing to go for one of these scopes. I think of it as disposable income just to say I had one. I will get around to sprucing it up but based on initial testing I am not going to expect much. That way I will be surprised if it actually has some merit.


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#34 photoracer18

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Posted 22 July 2020 - 02:52 PM

i got one

its usable 

it not the greatest 

it does have excellent tracking

deep sky ok

fine planetary details at high power-- no

 

this forum  has plenty of data on this scope

its the Edsel of the telescope world 

Not really the Edsel because the Edsel was mechanically sound and worked fine, it just looked funny. On the other hand the DX8 looks great but does not perform.


Edited by photoracer18, 22 July 2020 - 02:52 PM.

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#35 peashooter1982

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Posted 25 July 2020 - 10:16 PM

Here FWIW is a personal anecdote. My first astronomical scope was a Lafayette 2.4” Polaris refractor (1962-72). The next decade was spent in college & graduate school in big cities, so there as an observing hiatus. Finding myself in SW Florida (an area of wonderful skies and a nice astronomy club) for my first job, I resumed the hobby and soon purchased a Criterion Dx-6, mostly based on the near-legendary reputation enjoyed by the Criterion Dynascope 6. I was disappointed when at the club star parties, the Dx-6 produced the worst images of any scope on the field. Thinking that it might be a fluke, I then purchased a Dx-8 (there was no such thing as the Internet in those days!). Well, when I took the Dx-8 to the next star party, it again produced the worst images of any scope on the field! Star images blobs, no detail on Jupiter, etc.

I’d had enough. I sold the 2 Dynamaxes for what they were worth, saved up, and purchased a Questar 3.5” direct from New Hope. When I saw the perfect star images on my first dark night out, I knew I’d made the right decision. This was in 1982, and I have enjoyed the Questar ever since (at an amortized cost of about $5 per month).

I wish you luck in your choice... but don’t say you haven’t been warned! ;)
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#36 Bomber Bob

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Posted 26 July 2020 - 08:30 AM

I think my Sad Sack thread for the DX8 was a pretty good assessment of this scope, but it ran LONG, so let me summarize...

 

Knowing what I know now, I would only consider another DX8 under these conditions:

 

- Short on space, but want to see faint fuzzies in town;

- Don't want to mess with an 8" Newtonian on a DOB or EQ;

- Can get the complete kit (scope + wedge + tripod) in good condition or better for $300 or less.

 

- OR, I have the space, and just want a very pretty Display Scope -- a dud that i can yak about.  (The whole DX Story is interesting to me -- in an Interstate Pile-Up sorta way.)

 

My 1980s Meade 826 spoiled me against Big CATs, including that very fine Vixen VMC200L -- which really could deliver on fuzzies & planets.  Even if I include the $$$ spent buying & restoring the short Meade SF EQ, and the upgraded Lumicon helical focuser, I'm still out way less than I spent on my DX8 Experiment, and I have a Newt with first-rate mirrors that delivers the views on everything I turn it to.


Edited by Bomber Bob, 26 July 2020 - 08:31 AM.

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#37 Terra Nova

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Posted 26 July 2020 - 11:22 AM

My 1980s Meade 826 spoiled me against Big CATs, including that very fine Vixen VMC200L -- which really could deliver on fuzzies & planets.  Even if I include the $$$ spent buying & restoring the short Meade SF EQ, and the upgraded Lumicon helical focuser, I'm still out way less than I spent on my DX8 Experiment, and I have a Newt with first-rate mirrors that delivers the views on everything I turn it to.

It’s hard to beat a good Newt! They can be made short and still, like an apo refractor, if the mirror is well figured they will be very versatile and in my view, beat an SCT every time on an inch per inch basis, usually for half the cost! A 6” F6 or an 8” F5 is very affordable and still of manageable size.


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#38 tim53

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Posted 26 July 2020 - 12:34 PM

It’s hard to beat a good Newt! They can be made short and still, like an apo refractor, if the mirror is well figured they will be very versatile and in my view, beat an SCT every time on an inch per inch basis, usually for half the cost! A 6” F6 or an 8” F5 is very affordable and still of manageable size.

In addition, Newtonians are "hyperapochromatic".


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#39 Kasmos

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Posted 26 July 2020 - 01:20 PM

I think my Sad Sack thread for the DX8 was a pretty good assessment of this scope, but it ran LONG, so let me summarize...

 

Knowing what I know now, I would only consider another DX8 under these conditions:

 

- Short on space, but want to see faint fuzzies in town;

- Don't want to mess with an 8" Newtonian on a DOB or EQ;

- Can get the complete kit (scope + wedge + tripod) in good condition or better for $300 or less.

 

- OR, I have the space, and just want a very pretty Display Scope -- a dud that i can yak about.  (The whole DX Story is interesting to me -- in an Interstate Pile-Up sorta way.)

 

My 1980s Meade 826 spoiled me against Big CATs, including that very fine Vixen VMC200L -- which really could deliver on fuzzies & planets.  Even if I include the $$$ spent buying & restoring the short Meade SF EQ, and the upgraded Lumicon helical focuser, I'm still out way less than I spent on my DX8 Experiment, and I have a Newt with first-rate mirrors that delivers the views on everything I turn it to.

If you ever want a classic 8" SCT, get a early die-cast or sand cast Celestron. They can sometimes be had for only about $100 more. IMO, not only do they look better, they are built much better than a Dynamax. 

 

Yes, they might not beat a good Newt, but some put up great images, plus I'll take the space saving, ease of storage, easy set up, and the position of viewing.


Edited by Kasmos, 26 July 2020 - 01:20 PM.

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#40 Chuck Hards

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Posted 27 July 2020 - 09:32 AM

Yes, they might not beat a good Newt, but some put up great images, plus I'll take the space saving, ease of storage, easy set up, and the position of viewing.

 

I would argue that a well-designed DobNewt beats the best SCT of same aperture as far as ease of setup and takedown, as well as ergonomics in use.  The trade-off is lack of single-axis tracking.



#41 Kasmos

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Posted 27 July 2020 - 12:59 PM

I would argue that a well-designed DobNewt beats the best SCT of same aperture as far as ease of setup and takedown, as well as ergonomics in use.  The trade-off is lack of single-axis tracking.

Every design has it's pros and cons, so it's always a bit of 'different strokes for different folks'. I can't say that I haven't seen some nice Dobs and understand many of the reasons for their appeal, but 'many' of them just don't rub me as a nice instrument. When it comes to telescopes I'm old fashioned and the mechanics of them is a big part of what appeals to me. In other words, I prefer them to be on tripods and piers with gears, clamps, levers, knobs, motors and such. Hence, I always cringe when someone wants to make one out of a classic Newt.... or sometimes, heaven forbid, a refractor. shocked.gif


Edited by Kasmos, 27 July 2020 - 12:59 PM.

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#42 clamchip

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Posted 27 July 2020 - 01:16 PM

What surprises me is the images and photos that come from SCT's, absolutely jaw-dropping stunning

for the aperture. 

I guess it goes to show the design is a camera lens first, a visual instrument as a secondary purpose.

 

Robert


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#43 davidc135

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Posted 29 July 2020 - 05:10 PM

P7291413 (2).JPG

 

A DPAC snapshot of the DX-8 corrector. No wonder their reputation is poor. Here, the ke is from the right. Apart from strong over-correction, the surface is covered by inch size hills and hollows. Out by around 1 wave.

 

I could be wrong but I can't help wondering if polishing wasn't done at breakneck speed and whether things would have turned out differently if they'd taken a bit more time.  David


Edited by davidc135, 29 July 2020 - 05:13 PM.

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#44 DAVIDG

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Posted 30 July 2020 - 08:18 AM

  We have been through the problems many times

 

  1) The process Criterion developed to get around the Celestron patent was fundamentally flawed. It could never make a good corrector. 

 2) They used poor quality glass. It was just cheap wavy window glass. To have the Master Block method work, the glass surface pulled up against the the Master Block needs to be optically smooth.  Even when you make a corrector with the classic vacuum pan method  you need to start with strain free and optical quality glass or  you'll find out you are  just wasted your time trying to make the corrector.

  3) They never figured the secondary to remove any of the residue errors in the system like Celestron did so again you can never get to well corrected system.

 

   It all  came down to making a profit. For Criterion to stay in business they needed away to make the scope as cheaply as possible since they were trying to under cut Celestron  and sell the DX-8 at a lower cost. I believe they sold it for $100 less than a C8 at the time. That meant they needed to keep their labor cost down, which meant no time to critically test, refigure, test and refigure until they had 1/4 wave or better optics.  So they rolled the dice and if a majority of scopes they sold didn't come back and the seller accept  the quality, that is the quality they were going to continue to make. 

 

             - Dave 


Edited by DAVIDG, 30 July 2020 - 12:49 PM.

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#45 davidc135

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Posted 30 July 2020 - 11:30 AM

I'd have thought $100 could have been spent on a little more care at half a dozen stages in the plate production and resulted in a more compeitive scope.

But there was always the court battle which they were doomed to lose. When they chose not to go the vacpan route their fate was sealed.

 

David



#46 DAVIDG

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Posted 30 July 2020 - 12:48 PM

 The $100 difference in 1975 is about $500 today. I keep coming to the conclusion that they really didn't understand how  to make and test optics. It is Optics 101 to take an optical flat to check the flatness of the glass they were going to using. to make the  correctors  They could have scanned the  sheets coming in and found  the areas  that were optically  smooth enough to make the plates from. They could have  easily tested the glass for  stain in crossed polarized light  and/or place it in front of one the DX-8's  8" spherical primaries and  did a Foucault test. When they finish making the plates they could easily have tested it by again placing it in front of 8" or larger spherical mirror and doing a Foucault or Ronchi test.  Testing a few of the plates they made by their method would have easily shown that their method had major flaws. The finished scope could have also been tested via Double Pass Autocollimation. Again all this stuff is Optics 101 and common in any optical shop that understand what they are doing. It was all available in books and articles at the time so there is no excuse. 

 

                   - Dave 


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#47 Kasmos

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Posted 30 July 2020 - 02:15 PM

One of my biggest pet peeves is when materials, labor, and time are put into a product that is useless form the moment you purchase it. I guess it's all about making money. 


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#48 Stevegeo

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Posted 30 July 2020 - 03:20 PM

IMG_20200716_121416.jpg I have a rather dumb question.. what other differences am I missing that the dx cant be improved to better quality image ?  I know the corrector plate issue , what else am I missing?  Mind you I'm doing just planetary with either.

Looking through  my C8 and my DX8 using the same eyepiece I see very little difference in image , both during the day and night . But maybe that's ME.

I recently had both out for comet viewing and  people I shared both scopes , about 12 in all,  told me they preferred the dx8  over the C8  for the comet, HOWEVER  turning both scopes 180 and checking out Jupiter , all said that the view was about equal, with very little differences .  I used two eyepieces, switching back and forth ..  20mm plossels and  24mm  ...I did not go any higher in magnification. 

 Just a little survey of my own ... I did notice that the dx plugged into 110 outlet on a wedge tracked much better then the C8 on GEM.. but that could be setup error .

Stevegeo  


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#49 davidc135

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Posted 30 July 2020 - 03:51 PM

The 20mm ep would only give a mag of around x100 which isn't a severe enough test. If the seeing only supported that mag then there wouldn't be much difference seen between the scopes. Pushing the mag to 200 to 300 when the air is steady enough would separate out the so so from the good. You could have a good one!

David


Edited by davidc135, 30 July 2020 - 03:54 PM.


#50 Bomber Bob

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Posted 30 July 2020 - 05:09 PM

Yeah... try Jupiter / Saturn at just 30x per inch in your C8 vs. DX8... 240x should definitely show the difference!

 

(My Keeper minimum for a long achro refractor is 50x per inch of aperture.  My Meade 8" F6 Newtonian has no problem staying sharp at 400x when seeing permits.)


Edited by Bomber Bob, 30 July 2020 - 05:12 PM.

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