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Mid 80's C8 as bad as Dynamax 8 ?

catadioptric classic optics SCT
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#1 rolo

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Posted 07 August 2019 - 09:09 AM

A few moths ago I borrowed a friend's C8 that he suspected may have optical issues. He complained that the focus was soft and contrast was lacking specially at higher magnifications. It just didn't perform as good other C8's he owns so I agreed to test it for him. I'm no optical expert but it looks pretty bad. Almost as bad as the Dynamax 8's and B&L 8000 that I've tested. I estimate this C8 to be from the mid 80's. Star testing mainly showed rough optics with unevenly illuminated diffraction rings that looked like bad zones. Maybe there be some truth to the Halley's era C8's rumors?

 

Here's a pic of a DX8 vs this C8.

 

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#2 Chuck Hards

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Posted 07 August 2019 - 09:21 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.   



#3 rolo

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Posted 07 August 2019 - 09:36 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.   

I've had very good results with a couple of C6's. Better than most C5's I've owned.


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#4 Chuck Hards

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Posted 07 August 2019 - 09:40 AM

The only C5 I've ever liked is the white-tubed C5+, but even that one isn't as good as the C6.  


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#5 John Rose

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Posted 07 August 2019 - 10:43 AM

I have never thought of the Halley era quality issues as a "rumor". The company president later admiied in an interview that the quality had badly slipped at the time.

 

John


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#6 highfnum

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Posted 07 August 2019 - 10:45 AM

they both  look pretty bad

c8 slightly less so 


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#7 ccwemyss

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Posted 07 August 2019 - 10:47 AM

I had a 1982 C8 that was optically OK, but not great. Later in that decade, during the Halley craze, quality control was less consistent. Celestron made the mistake of thinking that the Halley enthusiasm in the market would last, and then had financial troubles, resulting in selling the company. It wasn't until the Fastar change that they got their act back together. They had also solved the corrector print-through problem by then. 

 

Chip W. 


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#8 rolo

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Posted 07 August 2019 - 10:52 AM

I have never thought of the Halley era quality issues as a "rumor". The company president later admiied in an interview that the quality had badly slipped at the time.

 

John

I think a few other manufacturers of mass produced scope had similar issues during the Halley craze.



#9 ngc7319_20

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Posted 07 August 2019 - 11:00 AM

Dunno.  The first thing I see in your Ronchigram of the C8 is a collimation problem.  I'd want to fix that and test again.  Yes there are some weak (?) circular zones in the C8.  The DYNAMAX looks much worse to me.


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#10 DAVIDG

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Posted 07 August 2019 - 11:14 AM

 Typical 1/2 wave or worse optics I keep talking about when you actually the  test scope.  The corrector is rough and  it has astigmatism, not collimation problems. You can see it is astigmatism because the bands on the left are  bowed and the one on the right is straighter. If it was collimation issue the  band would all bow in one direction.  It also shows that these corrector are variable in their figure and not interchangeable so the odds are low that your not going to get a diffraction limited image by swapping one into a scope that had a  broken one. 

  Try loosening the retainer ring on the corrector and see if the pattern becomes more symmetrical.

 

                   - Dave 


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#11 ngc7319_20

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Posted 07 August 2019 - 11:43 AM

 Typical 1/2 wave or worse optics I keep talking about when you actually the  test scope.  The corrector is rough and  it has astigmatism, not collimation problems. You can see it is astigmatism because the bands on the left are  bowed and the one on the right is straighter. If it was collimation issue the  band would all bow in one direction.  It also shows that these corrector are variable in their figure and not interchangeable so the odds are low that your not going to get a diffraction limited image by swapping one into a scope that had a  broken one. 

  Try loosening the retainer ring on the corrector and see if the pattern becomes more symmetrical.

 

                   - Dave 

Yes, there could be astig as well.  I doubt loosening the retaining ring on the corrector will help since it is zero power and transmissive.   Both the front and back surfaces get bent by any forces, so not much happens.  It is not like a mirror with only the front surface is getting bent.  If it is astig, I would try rotating the corrector + secondary mirror, or just the secondary mirror.  If you get lucky any astig will be spread among the elements and can be made to cancel.



#12 Chuck Hards

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Posted 07 August 2019 - 12:13 PM

popcorn.gif


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#13 bobhen

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Posted 07 August 2019 - 12:59 PM

A few moths ago I borrowed a friend's C8 that he suspected may have optical issues. He complained that the focus was soft and contrast was lacking specially at higher magnifications. It just didn't perform as good other C8's he owns so I agreed to test it for him. I'm no optical expert but it looks pretty bad. Almost as bad as the Dynamax 8's and B&L 8000 that I've tested. I estimate this C8 to be from the mid 80's. Star testing mainly showed rough optics with unevenly illuminated diffraction rings that looked like bad zones. Maybe there be some truth to the Halley's era C8's rumors?

 

Here's a pic of a DX8 vs this C8.

Get a copy of the December, 1989 Sky & Telescope. They reviewed the mechanical and optical quality of 3 new Meade and 3 new Celestron 8” SCTs.

 

One of the Meades was “unusable”, as stated in the review, because the secondary could not be collimated or remain collimated. The Ronchi bands on all the Meades were all bowed. All the Celetsrons showed straighter bands, not perfect just better than the Meades, but all the Celestrons showed a rougher figure. 

 

Maybe this line at the end of the review says everything you need to know about the optics in these SCTs: "The truth is, if your primary goal is to observe the planets, you should consider a refractor or a long-focus Newtonian."

 

My old DX8 was no worse. At least the collimation held.

 

Bob


Edited by bobhen, 07 August 2019 - 01:00 PM.

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#14 highfnum

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Posted 08 August 2019 - 09:40 AM

this is a ronchi (NOT DPAC) of my orange tube late 70's c8 

images are good IMHO

c8ron1.jpg

mrs0918.jpg

 


Edited by highfnum, 08 August 2019 - 09:43 AM.


#15 Chuck Hards

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Posted 08 August 2019 - 09:56 AM

Is that second pic Mars, or the worst case of "dog-biscuit" I've ever seen a Foucault test produce?   ;)  


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#16 DAVIDG

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Posted 08 August 2019 - 10:34 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 


Edited by DAVIDG, 08 August 2019 - 10:35 AM.

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#17 highfnum

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Posted 08 August 2019 - 11:08 AM

mars silly

views and star image

are pretty good

but david i know

your standards are high

i can guarantee its better than

my dx8!


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#18 Bomber Bob

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Posted 08 August 2019 - 11:19 AM

I don't have a problem with folks posting a "stacked" image, so long as they state that, because that's the way I sketch planets.  I've stayed at the eyepiece for an hour or more (with breaks to stretch my neck!), and I sketch each detail as it appears / when the seeing allows.  I've had nights with 2 or more dozen individual sketches that I amalgamate into a final single version that I then scan & post.

 

I don't see a big difference.  I have seen a difference using Snap Shot Mode on my ASI120MC:  By the time I click, that perfect view is gone due to the short lag time, and I wind up trash-canning the image.

 

As a minimum, post the scope, camera, and stacking program.  IF you run the resulting image through PhotoShop or other enhancement software, then state that.  If the image you post is average, better, or the worst compared to what you see with the scope, state that, too.  I think I did that when I posted images from the Orion StarShoot 2 that I started out with, because there were lots of my early attempts that were way below what I actually saw at the eyepiece.  (I know, cheap gear, cheap results.)

 

Honestly, if my sketches didn't stink on ice, I wouldn't bother with imaging at all.  I like being able to share a bit of what I saw with others, and that's about the best option for me.


Edited by Bomber Bob, 08 August 2019 - 11:24 AM.

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#19 bremms

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Posted 08 August 2019 - 11:33 AM

The C8 loos a good bit better than the DX8. Currently have two C8's a 74 and one from about 1978 or 79. both are the best out about 10 other that I've had. Ronichgrams show the 74 is quite  smooth ( for an SCT) with a touch of undercorrection. The 78/79 is very well corrected and still smoother than the other C8's I've tested. I would say they are each at least 1/4 wave. Since most of the other C8's were dreadful, I'm pretty happy with these.


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#20 Augustus

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Posted 08 August 2019 - 12:07 PM

 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 

Yep, this 100%. Some of the Dynamax planetary images I've seen here look pretty good but the scopes still suck optically.

 

You regularly see people using C14s that are barely diffraction-limited and getting awesome planetary images......



#21 DAVIDG

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Posted 08 August 2019 - 01:09 PM

mars silly

views and star image

are pretty good

but david i know

your standards are high

i can guarantee its better than

my dx8!

 John,

   I'm sure your C-8 is better then your DX8. My point is that post processed  images are  not  a 1:1 match to what you see in the eyepiece and the true optical quality of the scope. I  have seen images of Mars in the eyepiece like your picture with scopes that do have truly good optics while another scope 5 feet away shows a much less detail because of poor optics. 

  What clearly shows the quality of the optics is when the optical bench test images are posted. Then there is no bias.  Up in the ATM forum there is recent thread https://www.cloudyni...t-scope-6in-f8/  were the author refigured his mirror just a few weeks ago with DPAC, posted the images of the final figure which was excellent and on top of that took that scope to Stellafane where nine optical experts star tested it and it  won a first in the optical judging. Double pass autocollimation images like what was posted at the beginning of this thread show more of what is really going on then processed image of the Moon or the planets. 

   I'll bet that if Rolo didn't post the DPAC of his friend's C8 but just stated it  gave poor images and was made during the late 80's, the replies  would be, the  collimation is off, it's the seeing, it didn't have enough time to cool down, the eyepiece might have issues, the diagonal no good etc etc. But with that one test image from DPAC we know the scope has major optical issues. No guessing.  

 

            - Dave 


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#22 Bomber Bob

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Posted 08 August 2019 - 02:09 PM

Some of the Dynamax planetary images I've seen here look pretty good but the scopes still suck optically.

 

I know those weren't any of my DX8 Jupiter photos!  My BB-made Edmund 4 F15 Cassegrain makes MUCH better planetary images, and I built that thing using recycled Chinese parts, bubble gum, and Duck Tape.

 

I would hope that after a year or two on this Forum, most folks would know:  1. The Criterion Dynamax 8 is great to look at... is fine for faint fuzzies.  And that's about it.  2. Like the DX8, the C8 is a mass-produced SCT, and there are quality variations between each C8.

 

If Rolo hadn't posted the DPAC, I still would've taken him at his word, "Star testing mainly showed rough optics with unevenly illuminated diffraction rings that looked like bad zones."  Not everyone has a DPAC rig, and star testing is better than no testing.

 

Here we go, Jupiter in the DX8 versus the Edmund 4" F15 Cassegrain:

 

DX8JUP20170510V03ASR31.jpg Edmund DKC4 - Jupiter 20170611V01AR04.jpg

 

Doesn't prove anything...  I hear those all-gray Dynamax 8s, and orange Celestron 8s are The Best -- almost as good as my orange mid-70s orange C5...


Edited by Bomber Bob, 08 August 2019 - 05:49 PM.

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#23 bobhen

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Posted 08 August 2019 - 03:34 PM

HERE is a link to a short essay by Roland Christen. The essay is titled: Interferograms for Four High Quality Telescopes and Two Commercial Telescopes.

 

Have a look at and compare the images of the high quality optics and those of the commercial optics. Compare the roughness and wavefront.

 

The C8 is Roland’s but I have no idea what year it is. I'm guessing the 7" Mak is a Meade.

 

The test results in the 1989 S & T article (made with less accurate testing methods) look close to the one in Roland’s essay.

 

Bob


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#24 rolo

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Posted 08 August 2019 - 10:12 PM

You regularly see people using C14s that are barely diffraction-limited and getting awesome planetary images......

 A diffraction limited C14 is a beast of planetary scope...



#25 rolo

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Posted 08 August 2019 - 10:29 PM

The C8 loos a good bit better than the DX8. Currently have two C8's a 74 and one from about 1978 or 79. both are the best out about 10 other that I've had. Ronichgrams show the 74 is quite  smooth ( for an SCT) with a touch of undercorrection. The 78/79 is very well corrected and still smoother than the other C8's I've tested. I would say they are each at least 1/4 wave. Since most of the other C8's were dreadful, I'm pretty happy with these.

I've owned dozens of SCT's through the years and some have been as good and better than some mass produced Newtonians. Some scopes that stood out optically that I recall...Ultima 2000, C8 Deluxe, Ultima C8, 1973, 1974 and 1975 Orange tubes and my current 72 tricolor C8 (possibly the best). All four C9.25's were good as were two Carbon fiber C11's, and two CPC 1100's. My old C14 that gave the best planetary images I've seen in any SCT.


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