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Mid 80's C8 as bad as Dynamax 8 ?

catadioptric classic optics SCT
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#26 CHASLX200

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Posted 09 August 2019 - 05:20 AM

The last C8 i had was just as bad as any D8. Looked ok at 70x but after that total mush. Seems 9 out of 10 SCT's i had were bad to ok at best. Just 6 stood out as good out of around 60.



#27 CHASLX200

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Posted 09 August 2019 - 05:22 AM

Looks like a 90mm Chinese Mak!  

 

I had an observing buddy in the mid-70's who used an orange tube C8.  It never could get even close to the views that my home-made 8" f/7 Newt put up, side-by-side.  And that was years before Halley.  That was my first exposure to the fact that SCT's weren't generally as good as a same-aperture Newt.  Not until I got a modern C6 about ten years ago, that I finally started getting really good images from an SCT.   

But you can't make a few buy that there are plenty of bad SCT's out there as they don't know any better.



#28 CHASLX200

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Posted 09 August 2019 - 05:24 AM

 Be careful when you see  a planetary image that are  post processed and made up of hundreds of selected frames that have been stacked, sharpened and contrast enhanced. That doesn't show the true quality of the optics. What would be better is too show  just one single unprocessed frame. That is a better representative of what you actually see in the eyepiece.  

  I bet if you took images through the C-8 that is being discussed here, post processed them and posted them here that most what say they are very good. 

 

                   - Dave 

None of the images are what you really see in the eyepiece. All faked out and  made to look much better than it really is.


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#29 starman876

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Posted 09 August 2019 - 06:22 AM

 A thread about SCT bashing.  Everyone's favorite lol.gif


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#30 bobhen

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Posted 09 August 2019 - 06:55 AM

Not "bashing" per se; just a “you get what you pay for” reality check.

 

You just can’t get smooth 1/8th wave system optics out of these scopes, let alone consistently at the price these scopes are built.

 

If a SCT has rough optics with a zone and is ¼ wave it does not go back on the line for refiguring, it goes out the door – to be profitable it has to. The high-end makers send the optics back until they reach a quality spec, not a price-point spec.

 

Having said that, I have owned 8, 10 and 11” SCTs and enjoyed them all (for a total of 31-years of SCT use) for what they were: reasonably priced, very compact scopes, with decent aperture, that are at least competent in many areas of observing and imaging.

 

But as the 1989 S&T article stated: "if you want a scope for lunar/planetary observation (which implies high quality optics) you would be better off with a refractor or long-focus Newtonian." Which is, of course, the truth – not bashing.

 

Bob


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#31 tim53

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Posted 09 August 2019 - 09:41 AM

None of the images are what you really see in the eyepiece. All faked out and  made to look much better than it really is.

I've heard more ridiculous statements in my time, but I can't recall them at the moment.


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#32 Bomber Bob

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Posted 09 August 2019 - 09:46 AM

You just can’t get smooth 1/8th wave system optics out of these scopes, let alone consistently at the price these scopes are built.

 

Compact, affordable, versatile... yes.  I like all of that with my C5 (and my other CATs in general).  But my 3" F15 refractor is better-suited to serious lunar / planetary.  And that's OK.  I don't think it's bashing to point those facts out -- newbies need to know what they're getting before they buy.



#33 clamchip

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Posted 09 August 2019 - 10:08 AM

The best C8 I've ever owned is one I own from a ebay Goodwill auction and I think it was stripped of it's protective locker

kicked and neglected. I picked it up and what a sad day that was.

The goodness from the whole traumatic experience was it required a complete overhaul and a careful assembly with no

time clock or lunch bell to distract me from perfection.

Mechanical alignment of the optics, squareness of the tube-cells-baffle tube, etc. and exhaustive colimation I'm sure paid  

off because it is truly fabulous.

 

Robert


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#34 DAVIDG

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Posted 09 August 2019 - 10:26 AM

The best C8 I've ever owned is one I own from a ebay Goodwill auction and I think it was stripped of it's protective locker

kicked and neglected. I picked it up and what a sad day that was.

The goodness from the whole traumatic experience was it required a complete overhaul and a careful assembly with no

time clock or lunch bell to distract me from perfection.

Mechanical alignment of the optics, squareness of the tube-cells-baffle tube, etc. and exhaustive colimation I'm sure paid  

off because it is truly fabulous.

 

Robert

 That is great so please post a DPAC image so others can see what a good one should show when optically  tested. It would great to see some excellent examples  via DPAC  as well,  to get an idea of what the the odds are of getting a good one. 

 

                     - Dave 


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#35 starman876

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Posted 09 August 2019 - 10:52 AM

I've heard more ridiculous statements in my time, but I can't recall them at the moment.

Not everyone understands the science behind imaging


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#36 Boom

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Posted 09 August 2019 - 10:59 AM

Not everyone understands the science behind imaging


..and not everyone understands the science behind optical evaluation.
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#37 starman876

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Posted 09 August 2019 - 11:53 AM

..and not everyone understands the science behind optical evaluation.

and some just pretend they know a lot.   



#38 starman876

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Posted 09 August 2019 - 11:55 AM

 That is great so please post a DPAC image so others can see what a good one should show when optically  tested. It would great to see some excellent examples  via DPAC  as well,  to get an idea of what the the odds are of getting a good one. 

 

                     - Dave 

Now Dave, you need to play fair lol.gif



#39 DAVIDG

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Posted 09 August 2019 - 12:18 PM

Now Dave, you need to play fair lol.gif

 I am playing fair. People keep saying that  they have  a good ones but  so far we have only seen bad ones when actually tested going back to  the 1989 Sky and Tel article,  to the present with  examples posted in this forum and other forums on Cloudy Nights.  So instead of  having this be a  bashing of SCT's thread  lets get some actual test results. That will be more informative to see what is really going on with the  quality. This thread asked the question if  it  is true that the ones made during the Halley'Comet years were poor in quality. Getting more examples will help answer that question  So if you have one and can optically test it , post the results. 

 

                     - Dave 


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#40 ccwemyss

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Posted 09 August 2019 - 02:07 PM

I know that you and Johann have larger flats, but I think a lot of people here would be challenged to come up with one big enough to evaluate 8, 11, and/or 14 inch SCTs. I'm working on setting one up with a 6", which I hope to use to do a full survey of my loaner set of (<= 80mm) refractors. I don't even want to think about looking for a bigger flat, given what they've been selling for (and I don't have the time or skill to make one). 

 

It would be cool if we could arrange to set up a DPAC bench at the NEAF booth, and people could bring in their classics to get checked. (Imagine how many people we could disappoint!) That might even give us enough data points to be interesting. 

 

Chip W. 



#41 DAVIDG

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Posted 09 August 2019 - 02:41 PM

 One could make up an oil flat as large as needed and test away. An oil surface is flat to about 1/300 wave so no error from it.  See this thread for how it is done. https://www.cloudyni...limation-test/ 

 

 

                  - Dave 



#42 Bonco2

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Posted 09 August 2019 - 03:00 PM

Was not pleased with my early 70's C8. Sent it back, they re collimated it. Upon return I saw no change in performance. My RV6 provided better planetary images. The C8 was soft with poor contrast. I loved the mechanics tho. I'd love to find one with really good optics.

Bill



#43 terraclarke

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Posted 09 August 2019 - 03:11 PM

I've heard more ridiculous statements in my time, but I can't recall them at the moment.


No? Just turn on the nightly news. Most any night will do! rofl2.gif bawling.gif
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#44 clamchip

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Posted 09 August 2019 - 03:39 PM

 That is great so please post a DPAC image so others can see what a good one should show when optically  tested. It would great to see some excellent examples  via DPAC  as well,  to get an idea of what the the odds are of getting a good one. 

 

                     - Dave 

My largest flat is 6 inch but I do have this plate that may be hopefully around 1/4 wave.

It measures 7-1/4" x 7-3/4" I could give it a try.

Robert

 

IMG_9359.jpg

IMG_9360.jpg



#45 terraclarke

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Posted 09 August 2019 - 03:42 PM

My 1977 isn’t optical perfection but it’s adequate enough, especially in that it’s by far the most classical aperture per inch that I can easily manage to cart out on my deck and then cart back inside when done. Fat chance that would ever happen with an 8” F6 or F8 Newt on a GEM. Like Bill, I really like the mechanics of the thing. Mine has virtually no noticeable image shift, and I love the big setting circles that are easy to read and use. In short, I’m very satisfied. It’s the best $75 I ever spent on astronomy.

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#46 DAVIDG

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Posted 09 August 2019 - 04:02 PM

My largest flat is 6 inch but I do have this plate that may be hopefully around 1/4 wave.

It measures 7-1/4" x 7-3/4" I could give it a try.

Robert

 

attachicon.gif IMG_9359.jpg

attachicon.gif IMG_9360.jpg

 Your 6"  flat with show what is going on. First center it up so you are  testing the 6" center  of the 8" optics of your C-8. That will tell a majority of the story, then offset so the flat is it  over the edge of the 8" optics and see what the edge looks like. 

 

                 - Dave 



#47 CHASLX200

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Posted 09 August 2019 - 05:47 PM

..and not everyone understands the science behind optical evaluation.

That is for sure. 



#48 CHASLX200

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Posted 09 August 2019 - 05:52 PM

 I am playing fair. People keep saying that  they have  a good ones but  so far we have only seen bad ones when actually tested going back to  the 1989 Sky and Tel article,  to the present with  examples posted in this forum and other forums on Cloudy Nights.  So instead of  having this be a  bashing of SCT's thread  lets get some actual test results. That will be more informative to see what is really going on with the  quality. This thread asked the question if  it  is true that the ones made during the Halley'Comet years were poor in quality. Getting more examples will help answer that question  So if you have one and can optically test it , post the results. 

 

                     - Dave 

They were bad all the years they were made that i have owned.  But one 1984 C8 stood out as insane good that was almost close to the many 8" Newts i have owned. It could do 500x and keep ticking. Only C8 out of many that was super. Most were just lack luster to flat out no good.  Here are my best SCT's  The 1984 C8 was the best of all, then two Meade 10" LX200's and one 12" LX200, all 90's scopes. One C5 made around 2000 was super sharp and one Meade 2045 spotter version.  So there is for a fact some super SCT's that were great.  But i have yet to get a bad 8" Newt out of many that i have owned.  I would guess at least 20 or more. Most old school slower Newts.



#49 Don W

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Posted 09 August 2019 - 06:10 PM

Now where have I heard that before?



#50 CHASLX200

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Posted 09 August 2019 - 07:06 PM

Then what should i type?  Say they are all great or all bad and lie?




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