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Anyone have the CO measurements for Meade SCTs?

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#51 freestar8n


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Posted 15 August 2014 - 05:20 AM

Here's an interesting clue about Rohr's test:




That link shows a Newtonian mirror being tested in a stand by itself - and all the results including Foucault - show a nice secondary obstruction.  Which normally doesn't happen in a Foucault test of a bare paraboloid mirror.  Perhaps that Newtonian also has a baffle inside that is for some sinister reason made oversized.  Note that there are no spider veins visible - so it isn't due to the actual secondary of the assembled telescope.


My conclusion is that the central obstruction that appears in Rohr's tests has nothing to do with the telescope and instead is something in his own test setup.  There may be a large flat involved that itself has a central hole, or a big autocollimator.


He also sometimes does tests where he will use a different, larger sct as an autocollimator facing in.  In that case, the CO would be due to the larger sct.


Anyway - case closed as far as I'm concerned.  Best way to measure CO is by physically measuring the size of the secondary baffle.



#52 Asbytec


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Posted 15 August 2014 - 09:09 AM


As I said on one side the rings are clean and sharp and the other side there are no rings to be found. Brightness of the defocused pattern was the same as was the size, its just one side you could see the rings and the other you could not. The instrument had the sharpest view of mars I have seen. He was told by AP when he purchased the unit it was .99 and then again when he sent it back to AP after many years because he noticed oil around the edge of the lens.  The images in the scope are very, very sharp but this has been the results when star testing the unit on many occasions.


You keep skipping the vital piece of data : is this with a narrowband green filter? If yes, then someone is twisting the truth big time. There is no way an APO with Strehl of 0.99 will form Fressnel patterns like you described ("one side you could see the rings and the other you could not".). 


It's an interesting discussion, a bit off topic, but interesting. 


As I understand what Roland is saying we use a narrow band filter to isolate a specific wavelength (or range), but not to hide the effects of higher order SA.


Here's Roland on filtering...:


"If the outer rings are identical on either side of focus, there is no significant spherical aberration. What you see in contrast difference on the inner rings is due to chromatic effects. Use a green or yellow filter (in fact, use them both at the same time) to isolate the middle of the visible spectrum."


On the differences inside and out due to residual HSA as I suspect Powell argues...


"In this pure form, the Mak-Cass has left over 5th order aberrations and, depending on design, these can be less than 1/10 wave on the wavefront...The RMS value will be better than 1/50 RMS and the Strehl ratio will be exceedingly high. In other words, the optic will deliver a very high contrast image, consistent with the high wavefront rating. When tested on the night sky, the inside and outside diffraction patterns will be quite different."


I suspect this is what he is seeing, along with some color further reducing contrast in the de-focused image. My ~30% obstructed MCT (struggling to return to topic) with lesser correction exhibits similar behavior, except outside focus the rings are still seen with reduced contrast. Scared the bejeezuz outta me. Inside focus is tack sharp, though.  


But, if you look closely, you can tell the outside ring patterns are nicely presented in relative brightness to one another and fairly similar both sides (to the extent one can tell such things.) The patterns either side are different in that contrast has fallen off outside focus, but in-focus is clean as a whistle when seeing allows the pattern to calm down nicely...exactly as Roland and Powell describe.


"I have recently finished exhaustive tests of different 10" F14 Mak-Cass systems, some with these inherent aberrations left in, some with them meticulously removed. All the optics tested between 1/10 and 1/12 wave with Strehl ratios of 98% or better."


Unless Roland is wrong, a premium scope with higher order left in, including a fast APO, can have a very high Strehl (maybe at a single wavelength or all of them in focus) and markedly different patterns either side. Some of what Powell is seeing is residual higher order SA and the rest of it may be color.


After some discussion on the central zone and the CO...


"The other half of the defect occurs at the outer zone. If this is left uncorrected, you will see fuzz in the in-focus image which is highly destructive of contrast. The inside and outside patterns will look the same, causing you to conclude that the optic is textbook perfect."


And from http://geogdata.csun.../startest1.html


"I have an 8" SCT that shows perfectly identical inside vs. outside diffraction patterns, but tests only 1/4 wave..."


Which doesn't make a lot of sense to me, but back in his comments on complex optical system he states:


"...should we do our best to produce smooth high contrast optics, or should we please the star test crowd and do some hand aspherizing to get a more pleasing out-of-focus star image? I can tell you that it is easy to do some rough compensation with quick local polishing at several zones to get more equal inside and outside star patterns, but the result will almost certainly be a loss of contrast."


"The real test of an optic is not so much how the diffraction pattern looks outside of focus, rather, how much extraneous junk is floating around a star when it is in focus."


I have modelled just about every possible combination of defects in Zemax and it simply does not exist. Not if aberrations are kept to level of 0.99 Strehl.


I really do not know what to make of the argument, just saying I think it's possible what Powell is seeing is not only chromatic effect. And that if his patterns are different (lower contrast outside), it might well have a very nice Strehl if we believe what Roland is telling us about his star test experience along with his IF work.

Edited by Asbytec, 15 August 2014 - 09:15 AM.

#53 jrbarnett


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Posted 15 August 2014 - 10:05 AM

For me, the real question is this: do you have a good time with this telescope under the stars?  :flowerred:

That is preposterous!


- Jim

#54 DesertRat


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Posted 15 August 2014 - 01:36 PM

Hi Jim,


Some of us were hoping for paralysis by you know what.  :lol:



A scope can have 1/4wv PTV and have nearly identical intra/extra patterns - but not with perfect rings.  The confusion results from not specifying a better measure which is the rms figure.  Such a scope likely has lots of high order terms resulting from roughness, ripple and such.   Lots of energy would be scattered thru the field.  It would be able to separate equal magnitude doubles, but not unequal binaries and planetary imagery would be poor.  This is what Roland was referring to as "junk is floating around" in focus.


PTV measures are only useful for smoothly varying aberrations.  A scope can have a large PTV error and still be quite good if the deformations are localized and especially if they are not in the perimeter.   For such a scope the rms figure will be low and the Strehl high.


I'm afraid Rolands comments are often taken out of context and misrepresented.   A scope can have unequal CO breakout positions by as much as 2X and still be decent.  But it will not be perfect and the Strehl will not be 0.99.  There are much better ways to test an optic.  A real scope has other non spherical aberrations which may escape visual star tests and which can be damaging.


If on the other hand a scope provides good hi mag planetary images in great seeing, then all is well and good.  Forget about testing and CO and enjoy.  Problem is for most of us great seeing is infrequent.  I am a little more blessed, but maybe not as much as you.  :)


A SCT cannot be color free with low spherochromatism without either (1) a 2 index achromatic corrector as Bratislav mentioned earlier or (2) a spherochromatic corrector at the back.  Those are not average consumer items but do exist.  Its probable most SCT users do not even see the color errors.  Also it should be pointed out some barlows and eyepieces have mostly unseen color errors.


This is one area where Maks fare better than SCT's, that is spherochromatism.  In high resolution imaging with SCT's it can be tamed somewhat by refocusing each color.  For deep sky it mostly does not matter.





You are correct.  Wolgang Rohr's laboratory has several different test setups.  In some cases the Foucault image will not represent the actual CO of the scope under test but the effective CO of the apparatus.  In addition some configs have some clipping, vignetting as well as astigmatism.



In my own test a IM715 had a measured CO of 29.5+-1.5% whereas one test Rohr did of a IM715 showed a whopping +40% obstruction!


Bottom line one can (1) measure it somehow or (2) take C or M's or whoever's word for it.   There is nothing one can do to lessen the CO save buying or building another scope.  :waytogo:



#55 PowellAstro



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Posted 15 August 2014 - 06:34 PM

Here are Roland Christen comments on the matter. If a person reads his complete reply it can not be taken so easily out of context.



#56 bratislav


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Posted 15 August 2014 - 06:52 PM

Of course!!!  :foreheadslap: How could I not see it.

If we look at Rohr's collection of Foucault, Ronchi, Lyot and interferogram images over the years




one thing we notice immediately is that each and every SCT in question shows slightly larger obstruction than manufacturer declares. Clearly, Rohr has a flat with a hole that is 5% larger than C8, another one for M8, then one for C925, one for C11, then M10, one for M12, one for C14, then a collection of mirrors used to increase CO by 5% in Takahashis, Vixens, f/8 Meades, as well as Ritchey-Chretien, Dall-Kirkham and other scopes he tests. But he does have flats with no holes too, as many refractor tests don't show any obstruction (naughty, naughty).


Needless to mention, all those flats are carried in and out by little flying Bavarian pigs ...   :grin:


Reality, in Occam's best tradition, is decidedly much more simple.  It takes about 30 seconds to find out that Rohr uses two flats - a 10" non perforated one, for testing of smaller optics, 





and a 16" one with a 80mm hole (which is still much smaller than any secondary assembly from C925 onwards) which is used to test larger optics. 

Of course, Rohr could have taken photos of C8 on a stand against the non perforated flat, but then used a 16" one instead in order to increase the obstruction. Or he could have cut out little cardboard discs and placed them in optical path ... with help of the little flying Bavarian pigs again  ;)  



Bottom line - if any of the readers still bother with this thread, this is the bit you want to read.


Next time you observe with your SCT (or any other obstructed scope), point the scope towards a bright star. Focus. Take the eyepiece out. Place your own eye where eyepiece was. What you see is what you get. Ratio between that dark middle bit and a bright outer ring is obstruction your scope is delivering, right there. You can use your camera, smart phone or tablet device to take picture of it and then measure it later on the printout, computer screen or device's own screen. 

This is much more precise and trustworthy number that Meade, Celestron or any person on newsgroups want you to believe. 


Over, and out.

Edited by bratislav, 15 August 2014 - 06:59 PM.

#57 Asbytec


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Posted 15 August 2014 - 08:06 PM

Hi Glenn, yea it dawned on me afterward what he was saying, I had LSA on the brain and nothing else.


Still would like to know what he means by "junk floating around." Visible rings? Or literally junk? Seeing has "junk," sometimes the second and third rings break easily into a myriad of tiny arcs jumping around in what looks like a bunch of junk. But, when seeing is nearly perfect, they settle down nicely. The first remains pretty much in tact, and the fourth fades from view with seeing. Is that junk, or is it supposed to be there when (system) contrast transfer is consistent with 1/4 LSA?


Yea, I cut out the shadow test comments not wishing to go there. 


But, you know, I agree with you. Test it somehow and get close, I kind of like Bratislav's idea above of imaging the out of focus pattern at large defocus. It may not be prefect, or it may be, but I'd bet it's very close enough.


I'd still like to know if any manufacturers posts their figures as secondary mirror diameter so we can know something of the illuminated field rather than the secondary baffle diameter so we obsess over obstruction effects.

#58 PowellAstro



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Posted 15 August 2014 - 09:13 PM

I believe when Roland refers to junk, he means scattered light that should not be there, which will kill the contrast.

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#59 DesertRat


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Posted 16 August 2014 - 09:20 PM

A rougher optic in dodgy seeing will have tendrils of light emanating outwards and moving about.  A mirror not in equilibrium will look ugly as well.  Suppose all that could be considered junk.  Whatever it is, extra light will be where its not supposed to be.


It should be noted that Rolands comments on the star test preceeded Suiters 2nd edition where he addressed the limitations of the breakout test of the obstruction, and the ramifications of high order spherical and other zonal aberrations.  The breakout test is for low order spherical absent of any other significant aberrations.


Unfortunately some take Rolands comments and deduce that it does not matter if their scope shows dissimilar patterns inside and outside.  One cannot generalize Rolands comments to fit scopes that generally do not have significant zonal errors.


The breakout test as described by Suiter is a pass fail test.  For anything approaching a quantitative measure one can refer to Fig 10.15 in his 2nd edition for a hint of a technique.  Better still is to take 2 images as described earlier and measure the pupil and the shadow size and apply Hartmann's formula to yield the approximate longitudinal measure of the spherical error.  From there it is easy to calculate the familiar PTV for a smooth wavefront error.


Back to the subject of measuring obstructions.  One can take an out of focus image and measure the aperture and shadow size.  The larger the image the better for numerical purposes.  In the presence of significant spherical aberration the shadow will not be the same size on either side of focus.  The distance from diffraction focus should be enough to make diffraction effects melt away, the pupil extent and shadow should be clear and round.  For a newt the spider vanes should be clear without fringes.  Even at 30 waves defocus, a correction error of 1/4 wv low order spherical can yield a significant error of > 15% depending on what side of focus you are at.  Fortunately most scopes have less spherical error than that, and some much less.


If you know you have some undercorrection go outside of focus, otherwise go inside for a scope with overcorrection.  If that seems too complicated it might be best to get the caliper or tape measure.



#60 gnowellsct


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Posted 22 August 2014 - 08:54 AM

I've only got the Meade 8 in here but it is useful



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