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Post your Cat/Cass CO measurement

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#51 Riccardo_italy

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Posted 29 August 2017 - 03:07 AM

If we have a perfect unobstructed system, Strehl will be 1.

Also a perfect obstructed system has a Strehl equal to 1, because the ratio is computed taking into account the existence of the obstruction also in the "ideal" scope.


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#52 treadmarks

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Posted 29 August 2017 - 09:22 AM

 

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

 

Jon, good question, I should have been more clear. I did not go that far. As stated, that measurement is for the secondary mirror. I am aware of the issue with the Apex 127 so I did try to visually determine if the baffle is significantly larger than the secondary. It seemed clear to me that it is not.

 

I was satisfied when I inspected it and did not feel the need to go further. But I realize others might not be, so I'll edit my post to make that clear.


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#53 Karl Fabian

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Posted 29 August 2017 - 06:13 PM

Meade 6 inch F5 Schmidt Newtonian: 2.14 inch elliptical secondary giving 35.66% obstruction by diameter 13% by area.


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#54 luxo II

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Posted 06 June 2019 - 07:35 PM

Anyone measured the Bosma Maksutovs ? The CO looks big... >30%



#55 jjack's

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Posted 07 June 2019 - 03:21 AM

It's difficult to mesure exactly the obstructed part on a telescope, more on a mak-cass beecause of the baffle cone. The best way is to take a picture thru the eyepiece holder : https://www.cloudyni...ur-obstruction/



#56 Eddgie

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Posted 07 June 2019 - 07:32 AM

But, that far from the offending first ring, I cannot imagine that 5th ring being much of a problem, anyway. 

 

This is the essence of the MTF graphs that show the effect of the central obstruction.

 

On point sources this does not matter unless you were trying to see the very faintest of secondary stars that happened to be just this far out from the primary in a double star system.   

 

That is for stars though.

 

The easy way to think of contrast transfer, (and what the MTF charts show) is that as contrast transfer is lowered, a white line on a black background will appear wider, and a black line on a white background will appear narrower.  The size of the line in depicted by the MTF Chart with one line pair to the left side, and the numer of line pairs the aperture can resolve on the right, so that as you get closer to the right, the lines are far narrower than on the left (which is always 1 line pair per millimeter at the focal plane).  The right side will depend on the focal ratio (which defines the number of line pairs the system can resolve and is independent of aperture, though you can calculate it out from the aperture as an angular size). 

 

For extended targets one has to remember that every "point" on the surface of an extended surface spreads the light from this point over a very wide area around that point, and right next to that point on the surface of the extended object is another point and it is sending spreading a bunch of light around it, and the point next to it is spreading the light aroundgh  it, and so on and so on.

 

And this is where you see the sag in the MTF chart.  The chart is showing how different frequency (size) features is being affected by this diffused light coming from everywhere across the extended surface.  The MTF chart is showing that contrast on detail smaller than the gap between the first ring and Airy Disk is actually improved (the lower right of the scale,) but the big  curve in the sag you see in MTF charts corresponds pretty much to detail that is between the size of  one Airy Disk diameter and maybe 4 or 5 Airy Disk diameters.

 

So, a faint, dark detail 3 or 4 arc seconds in diameter could be swamped with the light that is being transferred from the points that surround this feature that have bright halos around them over this feature, which lowers the contrast of this feature.

 

This is why the MTF chart is so important.  It is showing how light form these "Infinite" number of point sources that are in the dark and light areas of an extended target are lowering the contrast of neighboring darker or lighter features, and for visual use, we are generally talking about small, low contrast details that would be a five or six Airy Disk diameters in size.

 

And the point is that a 20% loss of contrast on a small feature that stars with low contrast (small festoons on Jupiter) can make the difference between seeing that or not seeing that. Once it falls below the contrast sensitivity threshold of the observer, it can no longer be seen.

 

MTF then is all about how much contrast for different size features in an extended target is preserved from the starting value.    A small feature that starts with only 10% contrast will be lost when the system loses 50% of that starting contrast, resulting from only a 5% contrast on the observer's retina, which will cause it to fall below the contrast sensitivity threshold of the observer.

 

 This last statement is why the area of the sag in the MTF chart is often referred to a "the important visual frequencies."  Since camera systems generally have far better contrast sensitivity than the human eye, they might still catch this feature down to about 2% contrast, but most human eyes will have a contrast sensitivity of between maybe 2% for very large low frequency line pair and 15% for very fine line pairs.   This is why the CO size is allowed to be much larger for systems that  have been designed with imaging in mind.  You trade off some contrast to get a better illuminted field.

 

Long story short?  That very energy in the fifth ring is exactly what MTF charts are showing, and when you add the faint glow coming from the infinite number of points on the surface of an extended target, the effect is to lower contrast all across the surface because it is not just the light from the fifth ring of one point but the light being redirected from an infinite number of points and this is what contrast transfer means.  The diffraction of the system moves energy from where it should be to where it should not be, and this lowers the amount of contrast the system can transfer to the observer or the camera.


Edited by Eddgie, 07 June 2019 - 07:44 AM.

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#57 Asbytec

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Posted 07 June 2019 - 09:36 AM

<snip>

 

And this is where you see the sag in the MTF chart.  The chart is showing how different frequency (size) features is being affected by this diffused light coming from everywhere across the extended surface.  The MTF chart is showing that contrast on detail smaller than the gap between the first ring and Airy Disk is actually improved (the lower right of the scale,) but the big  curve in the sag you see in MTF charts corresponds pretty much to detail that is between the size of  one Airy Disk diameter and maybe 4 or 5 Airy Disk diameters.

 

<snip>

 

Long story short?  That very energy in the fifth ring is exactly what MTF charts are showing, and when you add the faint glow coming from the infinite number of points on the surface of an extended target, the effect is to lower contrast all across the surface because it is not just the light from the fifth ring of one point but the light being redirected from an infinite number of points and this is what contrast transfer means.  The diffraction of the system moves energy from where it should be to where it should not be, and this lowers the amount of contrast the system can transfer to the observer or the camera.

Yea, I get what you're saying and don't disagree generally. I have seen my scope's obstructed PSF out to about 5 diffraction rings on very bright stars and get a feel for the range and extent of contrast limitations imposed by the added diffraction effects.

 

Orion 150 MCT in Focus (Final).jpg

 

If we use the infinite point source model, of course the most offending diffraction ring is the first one. In my case, there is another fairly bright one out there, the fourth ring at about 4x Rayleigh, but it's not terribly bright to the eye. The fifth one is very weak and is only stable enough to be seen at all in very calm moments of good seeing. So, it's been my contention the sag in the MTF occurs primarily at the first ring, but that sag seems to depend on the area of the obstruction as does the rule of  thumb. (They do not draw MTF graphs with lumpy curves corresponding to individual rings, just one general sag in a smooth curve.)  

 

The obstruction on my 150 MCT (post secondary baffle mod) is 44 mm relative to 150 mm aperture or about ~30%. Using the (1-o)D contrast transfer rule of thumb, this puts the theoretical threshold bright low contrast detail on the MTF on par with a ~106 mm aperture with a Rayleigh criteria of about 1.3" arc. The 106 mm refractor has a offending first bright ring, too, just not as severely affecting bright low contrast detection. This is consistent with the first bright diffraction ring of the obstructed 150 mm aperture being 1.63 Lambda/Dmm ~ 185/Dmm ~ 1.2" arc and larger. It's my understanding this vicinity is the bulk of the contrast loss, especially on bright extended objects and close double stars (like 42 Ori which sits on the first ring of a 6" aperture). 

 

Yes, there is some damage a bit further out as evidenced by difficulty with high DeltaMag double stars out to several arc seconds, but the bulk of the damage seems to be near the offending first bright ring of the larger aperture. I presume this is the low point on the MTF for an obstructed scope. 

 

From Telescope Optics.net figure 106, "RIGHT: Contrast transfer with 32% obstruction (ο=0.32) nearly coincides with that of (1-ο)D aperture (dashed red line) approximately in the 0<ν<1/3 frequency range (blue area). This accounts for most extended details, from about 1.4 times the Airy disc diameter up."

 

https://www.telescop...obstruction.htm

 

The "(blue area)" mentioned above is the left hand side of the MTF less than 0.4 (actually about 0.33) spatial frequency where the sag seems to be at maximum and consistent with 1.4x Airy diameter and larger (and I believe consistent with the first diffraction ring and further out). By the time we get out to the fifth dim ring, the damage is (should be) much less because the offending ring is much dimmer...is all I was saying. I cannot imagine that dim 5th ring contributing significantly to contrast loss at over 4x Rayleigh at  <0.2 spatial frequency and lower frequencies where contrast begins to normalize with increasing angular distance. Not relative to the first bright ring, anyway, closer to 0.4 spatial frequency where the bulk of the damage is (appears to be) done. 

 

He goes on to say in the next sentence, "However, due to the contrast recovery at higher frequencies, the resolution limit for bright low-contrast details (BLC) is less than 10% lower than in a perfect aperture."  Here, I believe he is talking about the area inside the first ring and smaller (less than 1.4x Airy diameter) on the right hand side of the "(blue area)". I guess this means Jove's white ovals, something like that, which are probably low contrast but pretty small. Yea? 

 

Unless I am totally whacked... :) 



#58 TG

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Posted 12 July 2019 - 01:13 AM

Mewlon 250 (classic, non CRS): 80mm CO = 32%.



#59 Astrojedi

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Posted 18 July 2019 - 10:23 AM

Seeing the effect of central obstruction on contrast is really difficult if not impossible for the human eye. 

 

While all these diffraction, MTF and contrast theory/equations presented by folks here are accurate there really is no empirical evidence that these differences are perceptible to the human eye to the extent stated.

 

Optical quality is the main factor in loss of contrast. I had the epiphany when I cut out a 30% obstruction and stuck it in the center of my very expensive .98 strehl 5.5” APO. I could barely tell the difference on Jupiter. I was so surprised that I double and triple checked. But no, the difference was barely perceptible.

 

If you have a fine 5 or 6” APO I recommend you try this yourself.



#60 Eddgie

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Posted 18 July 2019 - 11:00 AM

Seeing the effect of central obstruction on contrast is really difficult if not impossible for the human eye. 

 

While all these diffraction, MTF and contrast theory/equations presented by folks here are accurate there really is no empirical evidence that these differences are perceptible to the human eye to the extent stated.

 

Optical quality is the main factor in loss of contrast. I had the epiphany when I cut out a 30% obstruction and stuck it in the center of my very expensive .98 strehl 5.5” APO. I could barely tell the difference on Jupiter. I was so surprised that I double and triple checked. But no, the difference was barely perceptible.

 

If you have a fine 5 or 6” APO I recommend you try this yourself.

Well, I suppose that we would disagree on that.    

 

I can see the difference, but to see the difference, you have to be looking at detail that is most affected by the difference and at a power where the eye is most sensitive to the difference.

 

A good example is faint festoons on Jupiter.  These often have very fine dark detail against a lighter background, and this is the kind of feature that really does show where contrast matters.  If the feature is darker against a brighter background, the light from the background surrounding the dark feather can bleed over enough to make it loss just enough contrast to fall below the observer's visual threshold.

 

As Jon says and others say, for larger detail the contrast difference is hard to see because the larger the detail the less contrast is lost, and the easier the eye can pick out a larger low contrast feature than a smaller low contrast feature.  So, for larger scale detail, the contrast does not matter much.   Exactly where it matters is in eeking out the finer, lowest contrast detail.   

 

Once you know exactly where to make the comparison, it is pretty easy to find a feature where one can see the difference.   

 

Now the difference between a 33% obstruction and a 30% obstruction is impossible to see, but the difference between a 36% obstruction and a 17% obstruction is pretty easy to spot if you are comparing the right kind of detail.

 

For highest contrast observing, a 20% or smaller obstruction will be difficult to tell from an unobstructed instrument, but a 33% obstruction difference can be rather easily seen if the right kind of detail is present.   If it is not, it does not matter, but Jupiter is awash with this kind of detail from north to south, east to west.

 

I only recommend Newtonians for planetary observing because after 50 or 60 telescopes, the larger ones have given the best views (12" Newt vs C14 as and example).  As the Newtonain gets larger or slower, the secondary starts to get smaller and once it gets down into the 17% range the contrast loss is quite small.


Edited by Eddgie, 18 July 2019 - 11:02 AM.


#61 Astrojedi

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Posted 18 July 2019 - 12:03 PM

We agree on one thing.... I think larger newts / dobs 12"+ are the best planetary scope due to the larger aperture and resolving power (even better with tracking). For me tracking is important which is why I usually prefer SCTs.

 

But regarding being able to observe differences between scopes with CO and no CO have you really compared two scopes of the same optical and build quality side by side?

 

This is a critical question as we need to isolate this one variable from about a 100 other factors that could impact that subtle contrast you are talking about.

 

When you spend $4000+ on a high end 5" or 6" APO you are not only paying for the highest quality lens but the best ota design e.g. baffling, focusers, lens coatings, even small things like the best available matt black finish on the extenders etc.

 

Next time try your 6" APO with a 30% CO cut out. The image may dim very slightly but I doubt you will see significantly less detail. I did not. And before you imply observer error I can tell you seeing was excellent and I was at 300x and I tried it on multiple evenings with the same results.

 

In my view why refractors always seem to outperform is usually due to the higher optical quality (try buying a premium SCT - although I have been pleasantly surprised by the EdgeHDs) and because they are just easier to use than other designs (e.g. precise collimation, thermal issues etc.). CO is NOT the main factor as it is made out to be on these forums.


Edited by Astrojedi, 18 July 2019 - 12:10 PM.


#62 Asbytec

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Posted 18 July 2019 - 02:41 PM

I'd agree it's not as big a problem, buy itself, as it's often made out to be and other things matter, too. Whether one can notice it or not is one thing. A 33% obstructed aperture should behave as if it has a Strehl like figure of 0.88 for bright low contrast detail over the range of spatial frequencies from 0 to (D - co)/D with improved performance out to the maximum spatial frequency and /maybe/ a bit beyond. Not too shabby. That's right at 0.67 spatial frequency on a scale near 169/Dmm and lower (about 1.7" arc or more for a 102mm aperture). It kind of makes close unequal doubles a bit more difficult, too, and one should notice it there if nowhere else. The obstruction effects are greatest at higher end of this range of spatial frequencies and contrast normalizes at lower spatial frequencies.


Edited by Asbytec, 18 July 2019 - 03:00 PM.


#63 luxo II

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Posted 21 July 2019 - 04:45 AM

try buying a premium SCT...

You can... buy a couple of dozen, keep the best one, and sell the rest.

 

Or buy an Intes mak secondhand - if you can find one, though it will cost more.


Edited by luxo II, 21 July 2019 - 04:46 AM.


#64 Astrojedi

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Posted 22 July 2019 - 04:11 PM

You can... buy a couple of dozen, keep the best one, and sell the rest.

 

Or buy an Intes mak secondhand - if you can find one, though it will cost more.

Premium is not just about optics. Actually if the system is diffraction limited that is more than sufficient.


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