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Post your Cat/Cass CO measurement

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#26 TG

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Posted 08 July 2017 - 06:14 PM

M14" f8 = 14.1%


This seems to be by area.

#27 csauer52

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Posted 08 July 2017 - 09:49 PM

 

M14" f8 = 14.1%

By area, yes.  By diameter, which is what we are all using for comparison here and what is generally quoted around the hobby the 14" Meade f/8 ACF is probably in the 35%-45% range.    

 

Dave

 

37% in that case.


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#28 Max Power

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Posted 08 July 2017 - 09:56 PM

If CO mattered, why is my Takahashi CN212 with a 34% obstruction ratio so darn good?
It is just a touch less good than my 10" Zambuto dob with a 20% ratio.
Kinda like the difference between a Radian vs. a Delos eyepiece.
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#29 Cotts

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Posted 09 July 2017 - 09:21 AM

If CO mattered, why is my Takahashi CN212 with a 34% obstruction ratio so darn good?
It is just a touch less good than my 10" Zambuto dob with a 20% ratio.
Kinda like the difference between a Radian vs. a Delos eyepiece.

CO does matter in terms of contrast transfer.  Most folks will just be able to discern the better contrast of a 20% - obstructed telescope when compared to 35% in scopes of mostly equal aperture and quality.  As you say - "just a touch better".   Not everyone will be able to tell a 10% difference, though.... , say, between 25% and 35%.   Differences in optical quality will trump the differences in CO in most cases.  (How often do we get two scopes of equal quality but different CO side-by-side for a test anyway?)

 

It is generally agreed that a CO of 20% cannot be distinguished from none at all - Your 10" Zambuto's high-resolution images will be virtually indistinguishable from those in a 10" APO refractor... at about 1/15 the price.

 

But I don't recommend making that claim over on the Refractors Forum.... grin.gif

 

Dave


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#30 GlennLeDrew

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Posted 09 July 2017 - 06:40 PM

And note that this business of contrast transfer as impacted by the CO's additional diffraction is applicable at the fairly small scale of a few Airy disk diameters. It does not have relevance at larger scales, where other factors such as baffling, blackening, coatings, optical reflections, etc., introduce veiling glare.

 

Comparing two obstructed systems where the principal difference is a not greatly disparate CO relative diameter, one will not detect much to separate them. For instance, 40% vs 30% is not that significant a difference. 


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#31 Jon Isaacs

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Posted 09 July 2017 - 06:52 PM

 

M14" f8 = 14.1%

By area, yes.  By diameter, which is what we are all using for comparison here and what is generally quoted around the hobby the 14" Meade f/8 ACF is probably in the 35%-45% range.    

 

Dave

 

 

14.0% by area = 37.4% by diameter.. 

 

Jon



#32 Jon Isaacs

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Posted 09 July 2017 - 06:57 PM

And note that this business of contrast transfer as impacted by the CO's additional diffraction is applicable at the fairly small scale of a few Airy disk diameters. It does not have relevance at larger scales, where other factors such as baffling, blackening, coatings, optical reflections, etc., introduce veiling glare.

 

Comparing two obstructed systems where the principal difference is a not greatly disparate CO relative diameter, one will not detect much to separate them. For instance, 40% vs 30% is not that significant a difference. 

 

If one wants to experience the difference between a 30% CO and a 40% CO, a scope with a 30% or smaller plus some paper and scissors is a good way..

 

From what I can see, the CO size does affect the fine scale planetary contrast.. 

 

Jon


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#33 Asbytec

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Posted 09 July 2017 - 07:42 PM

"Not everyone will be able to tell a 10% difference, though...."

 

I believe this is true. I reduced my obstruction by removing the baffle from about 38% to 31%. I must say, observing Jove there was no immediate difference I noticed. The immediate differences seemed to be, in very calm seeing, a loss of the 5th faint diffraction ring from barely visible to almost invisible. It was noticeably 'improved.' But, that far from the offending first ring, I cannot imagine that 5th ring being much of a problem, anyway. 

 

The second anecdotal improvement /may/ have been a slight reduction in the diffraction artifact along the lunar rim. No way to really say for sure if it's somewhat diminished, it just looked like it might be a little dimmer. Maybe. (It's no more offending than a violet halo from chromatic aberration.) 

 

However, over time, I did notice my Jupiter observations were producing a bit more detail. That may or may not have been due to the smaller obstruction because, at the same time, I was using a bit more magnification. So, I did not keep the conditions the same after the baffle mod, so it's impossible to tell if the obstruction allowed a slightly greater contrast on small scale. I am sure it did, just do not know how much. So, IME, I think an improvement of 10% +/- is difficult to observe. 

 

"It is generally agreed that a CO of 20% cannot be distinguished from none at all..."

 

If we have a perfect unobstructed system, Strehl will be 1. If we obstruct it by 20%, the light diffracted to the rings will be approximately (1-co^2)^2, including the shading effect, giving a peak intensity of the central disc a value of 0.92. What this tells me is, if it's generally agreed 20% CO is indistinguishable, then so is the difference between Strehl of 1 and a Strehl of about 0.9. (Note: peak intensity is a Strehl equivelent, IMO it is not a Strehl ratio.)

 

For a perfect system, one can withstand an obstruction upwards of 0.33D before things become degraded below a peak intensity of about 0.8. Now, I am sure one can begin to notice a difference between perfect unobstructed and perfect 0.33D obstructed system. But, I am wondering how much of a degradation is visible between a peak intensity of 1 and 0.8? So, between 1 and 0.9 is indistinguishable, but surely the difference between 1 and 0.8 can be observed on small scale planetary detail, lunar contrast, and close unequal doubles. 

 

"From what I can see, the CO size does affect the fine scale planetary contrast.."

 

I often wonder what a 6" APO (peak of 1) can observe that I cannot (through a 6" CAT at a peak of about 0.8) even though my image is apparently a bit softer (but often in better seeing.) I still do not have the answer, except the APO image is a bit "sharper" (on lunar and planetary.) Unequal doubles are easier, too. A 6" APO made short work of 42 Ori, but I did manage it after a valiant effort and some guesswork. (LOL) 

 

Apparently things do not start getting softer until below 0.8 peak intensity (equivelent to about 1/4 LSA unobstructed) when we consider aberration (Strehl) and obstruction effects. Strehl and peak intensity combine to provide the intensity distribution on the focal plane we observe as contrast transfer on very small scales - as Glenn says, the scale of the first diffraction ring, primarily. 

 

I wouldn't worry too much about the obstruction. It has an effect, but it's minimal until you push your scope's peak intensity much below 0.8 with a lesser Strehl and a large CO. Certainly you'll observe a difference between a nice APO and a same obstructed aperture with a similar Strehl. But, it's probably nothing to lose sleep over. 


Edited by Asbytec, 09 July 2017 - 09:12 PM.

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#34 Paul G

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Posted 09 July 2017 - 07:50 PM

AP 254 f14.6 Mak-Cass -- 23%.


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#35 luxo II

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Posted 11 July 2017 - 08:28 AM

I often wonder what a 6" APO (peak of 1) can observe that I cannot (through a 6" CAT 

Effectively nothing.

 

A while ago I sold all my gear and suffered a severe bout of refractoritis, buying a nice 5" f/7 triplet ED APO, which is around 0.95 strehl. One night this was set up with a white C5 on one side and a 6" Skywatcher Mak on the other side.

 

With each working at the same magnification with similar eyepiece series on the same objects (Jupiter, Saturn, clusters and stars), it was practically impossible to tell them apart optically - despite all the hype about refractors being better.

 

I left feeling the refractor was frankly no better than a well made 6" f/8 newtonian that was optimised for planetary - with a clean mirror, well baffled OTA, curved single vane spider, a tiny secondary - and was lighter than the refractor.

 

The only thing noticeably better in the refractor was contrast,and this is especially noticeable on the moon, Jupiter and Saturn. Less-than-optimal baffles, the scattering of light produced by several air-glass surfaces less than scrupulously clean, mirrors that are less than perfect (tiny pinholes in an aged mirror produce a haze), plus spider vanes or central obstructions all contribute to this. In this respect the refractor clearly does have an advantage. 

 

And FWIW I'm now back to using maks somewhat larger than the APO. 

 

If you want to see the effects of CO in combination with other optical defects you can typically encounter, it is nicely shown at http://www.telescope...aberrations.htm


Edited by luxo II, 11 July 2017 - 08:46 AM.

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#36 TG

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Posted 11 July 2017 - 10:27 AM

Gentle reminder: the more we have discussions of effects of COs and refractors vs reflectors etc., the less useful this thread is as a repository of actual data about CO sizes.
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#37 Phil Barker

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Posted 11 July 2017 - 02:22 PM

 

If CO mattered, why is my Takahashi CN212 with a 34% obstruction ratio so darn good?
It is just a touch less good than my 10" Zambuto dob with a 20% ratio.
Kinda like the difference between a Radian vs. a Delos eyepiece.

CO does matter in terms of contrast transfer.  Most folks will just be able to discern the better contrast of a 20% - obstructed telescope when compared to 35% in scopes of mostly equal aperture and quality.  As you say - "just a touch better".   Not everyone will be able to tell a 10% difference, though.... , say, between 25% and 35%.   Differences in optical quality will trump the differences in CO in most cases.  (How often do we get two scopes of equal quality but different CO side-by-side for a test anyway?)

 

It is generally agreed that a CO of 20% cannot be distinguished from none at all - Your 10" Zambuto's high-resolution images will be virtually indistinguishable from those in a 10" APO refractor... at about 1/15 the price.

 

But I don't recommend making that claim over on the Refractors Forum.... grin.gif

 

Dave

 

I've got a boot in both camps with a very nice vixen 115mm fpl53 doublet and a sw180 mak often getting compared.  The obstruction effectively in the mak is 30% although its 32% if you measure the secondary baffle cone.

 

The other day Saturn was stunning in the refractor at around 260 x very sharp lovely disc detail nice colour etc

moved to the 180mm mak and while it wasn't quite as steady ie seeing wasn't quite there the image was more detailed more power could be employed.  I think a 5 inch mak or even a 6 would struggle to show more than the refractor but the 7 clearly does.

 

Back to the obstruction, I'm sure the effective obstruction is indeed around 30% with the diverging rays through the corrector.  Fantastic weapon for planets and double stars in a portable package exceptional for public viewing also.

 

Interestingly I did own a fine intes micro f15 7 inch that had a lower obstruction 25-27% (effective 45mm measured 48mm) and it couldn't handle 2 inch eyepieces whereas  the sw can and does it well.  Pity I can't compare them directly that would be interesting the sw is that good.



#38 chuckscap

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Posted 11 July 2017 - 07:31 PM

AP 254 f14.6 Mak-Cass -- 23%.

oooh nice!



#39 Joe1950

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Posted 11 July 2017 - 08:37 PM

Celestron C-5, very recent vintage, 40%.

 

 

Good, sharp planetary images, easy to set-up. A scope that will be used often.

 

I've been testing scopes so much I haven't looked through them. So I decided to sign the Optical Test Ban Treaty and use, not test. And the size of the CO is probably less of a factor in seeing planetary detail than seeing is. At least where I live. shrug.gif


Edited by Joe1950, 11 July 2017 - 08:38 PM.

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#40 dweller25

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Posted 12 July 2017 - 02:41 AM

Intes Micro Mak, 180mm, F/15, secondary obstruction  = 27% (same contrast as a 5" APO)


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#41 Gil V

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Posted 12 July 2017 - 10:10 PM

Dynamax 8 - 2.781"

 

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#42 Cepheus Elf

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Posted 13 July 2017 - 02:10 AM

As best as I can measure (eyeballing with a ruler), my new Celestron Omni 127 C5 has a CO of 48mm which I believe equates to 38% (diameter) 

 

Cheers,

Mick 


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#43 KarlL

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Posted 13 July 2017 - 02:56 PM

ETX-125???

 

I'm having to sell everything to pay bills (including the "Orange Scope" in the "Pimp My XT6!" thread In Reflectors). I may try the ETX once again after everything has settled and it's time to rebuild.

 

Thanks to everyone who's measuring. It's a big contribution.


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#44 Joe1950

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Posted 13 July 2017 - 04:36 PM

Sorry to hear that Karl. I'm in the same boat. May have to liquidate some things. My sewer line went bad and replacing it was a major, major expense. And the homeowners insurance wouldn't pay 1 cent!

 

Good luck.



#45 checcocpb

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Posted 28 August 2017 - 03:23 AM

My Intes MN 78 obstruction is 13,4%

 

Cheers


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#46 Kunama

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Posted 28 August 2017 - 04:57 AM

TEC MC200/15.5  26.6%


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#47 Edwin

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Posted 28 August 2017 - 06:25 AM

Tal 200K F10 35%


Edited by Edwin, 28 August 2017 - 06:26 AM.

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#48 treadmarks

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Posted 28 August 2017 - 12:51 PM

Orion Apex 102mm Maksutov (F/13): 31mm secondary mirror / 30%

 

I measured this a while back... The manufacturer's specification is confirmed. However the CO% on their spec sheet is inaccurate; says 34% but should be 30%.

 

edit/disclaimer: Measurement is for secondary mirror only, not baffle tube. My visual estimate is that the baffle tube is not larger than the secondary mirror to any appreciable degree.


Edited by treadmarks, 29 August 2017 - 09:26 AM.

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#49 Jon Isaacs

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Posted 28 August 2017 - 09:10 PM

Orion Apex 102mm Maksutov (F/13): 31mm secondary mirror / 30%

 

I measured this a while back... The manufacturer's specification is confirmed. However the CO% on their spec sheet is inaccurate; says 34% but should be 30%.

 

Did you remove the corrector and measure the diameter of the secondary baffle?  The 127 mm version of this scope has a 48 mm Baffle, which combined with the effective aperture of ~120 mm means the CO is ~40%.  

 

They state that the secondary mirror obstruction is 39 mm, 31%.. But the baffle has to be included.. 

 

Jon


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#50 JohnH

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Posted 28 August 2017 - 10:21 PM

M809 - 31%

 

MN76 - 20%


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