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Secondary Obstruction in SW 127mm Mak

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#1 Eric63

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Posted 27 October 2012 - 09:22 AM

Hi

I recently purchased a SW 127mm Mak Cass and as such I have been reading up on it quite a bit. Based on a thread in this forum I performed the flashlight test and found that the actual aperture is 118mm (as per the findings in the forum). However my biggest surprise was the shadow from the secondary. I measured it at 48mm which means that I have a 38% design obstruction and 41% actual obstruction!! I looked on the SW site and the Secondary Diameter informaton in missing in the specs.

Has anyone measured the CO in their SW 127mm Mak? Is it as big as I measured?
Thank you for your help
Eric

#2 Eric63

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Posted 27 October 2012 - 10:51 PM

OK… so I measured the CO using a ruler (eyeballing it) and found that it is about 42mm (including the baffle), therefore this would be a theoretical CO of 33% , which makes sense according to what I have read. The actual CO is 35% assuming a 118mm aperture. Could it be that the shadow in the flashlight test was 48mm because of the light path? Just trying to make sense of things because if the actual CO is 48mm, this would mean that this 127mm scope only has an effective aperture of 108mm.

Thank you again if anyone can shed some light on this.
Eric

#3 Asbytec

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Posted 27 October 2012 - 11:44 PM

Eric, I remember that long thread. It was a fascinating look at a great scope design. The flashlight test turned out to be quite accurate as far as effective aperture goes. It begged many questions about design, compromise, correction, and what have you.

But, I cannot speak to the projected secondary shadow size and CO percentage. One would think actual measurement if the CO would suffice. Did you actually measure the open end of the secondary baffle, or did you eyeball it looking through the meniscus?

Anyway, IMO, despite the reduced effective aperture, folks love those scopes (myself included) for reasons beyond that. My Orion 150mm design is a tad different that the SW 127mm design, but they still kick butt.

#4 freestar8n

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Posted 28 October 2012 - 02:00 AM

The flashlight test turned out to be quite accurate as far as effective aperture goes.



I don't agree with that summary of the thread and instead I think it revealed how hard it is to measure effective aperture when the limiting stop is somewhere inside, past the first bounce off the primary. If the limit is either the front corrector or the primary then a "flashlight test" should be reasonably accurate - but if it is deeper inside there are complications that make it hard to do reliably. That is why, in that thread, the "flashlight test" morphed into the "collimated laster test" - and even that had issues.

For maksutovs with a full aperture front corrector, like many of the small ones, there can be a limit caused by the front aperture, the primary, the front of the baffle on the secondary, the secondary itself, or the baffle leading to the eyepiece.

The secondary obstruction in a mak should be measured by the diameter of the secondary baffle as seen from the front of the scope, where it is slightly reduced in size by the demagnification of the front corrector. That would be much bigger than the size of the aluminized spot itself.

Frank

#5 GlennLeDrew

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Posted 28 October 2012 - 02:30 AM

The secondary obstruction's shadow diameter is absolutely representative of its size optically.

Despite Frank's misgivings, the flashlight aperture test IS accurate, as long as the following conditions are observed:
- The eyepiece is focused for infinity.
- The image of the light as formed by the eyepiece is sufficiently small, so as to emulate a point source.
- The light source is at least 10 eyepiece focal lengths from the eyepiece's eye lens.

When an ordinary flashlight is used, to ensure a tiny image use a shorter focal length eyepiece. This will have the added benefit of ensuring better closeness to infinity focus.

To really ensure a very tiny image of the light, use a laser, which can be now positioned pretty much up against the eyepiece. If the exit pupil is 1mm or smaller, the beam should fill the exit pupil and hence the entrance pupil. If the exit pupil is larger than the beam, the entrance pupil will be only partially filled, and the use of a beam expander is helpful.

The reason for the accuracy is simple. The light is passing through the system exactly as it does in reverse as when when the light from a star is brought to focus. If some internal baffle restricts the aperture when light arrives from a distant source, the very same restrictor causes the identical stopping down when light passes through the system in reverse. The baffle, no matter where located, does not care in which direction the light travels. As long as the light source is at the focal plane and is sufficiently small so as to form a sharp-edged circle of light (and sharp central shadow), the emerging bundle of light is collimated and the dimensions accurate.

#6 freestar8n

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Posted 28 October 2012 - 03:48 AM

Despite Frank's misgivings, the flashlight aperture test IS accurate, as long as the following conditions are observed:



Only if the thing limiting the aperture is either the primary or the front corrector. Otherwise, one would at least need to switch to the "collimated laser" approach - which you advocated as an acceptable alternative - and even that would be unreliable if the stop is near the focal plane.

Anyway - the usefulness of the test depends on what is causing the restriction in the first place - and where it is located. It's fine for measuring the size of the secondary obstruction, in millimeters, because it is very near the shadow being measured - but it won't tell you the secondary obstruction percentage unless you know the actual aperture.

Without other evidence of what is actually causing a given aperture restriction, I wouldn't trust a simple measurement of aperture that does not identify the physical cause or where the resulting entrance pupil is located in object space.

Frank

#7 Asbytec

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Posted 28 October 2012 - 04:22 AM

Frank, you are way ahead of me on understanding the optical performance, no doubt you are making valid points. So, I hold your expertise with regard.

In mine the secondary baffle turned out to be the choke point. The laser and flashlight tests projected through an eyepiece focused on infinity gave a result consistent with the shadow intrusion test on a de-focused star. Indeed, it was operating at 10mm less than full meniscus aperture in every which way one looked at it. Both tests including light moving in both directions through the optical train.

I do not know how to account for that accuracy and consistency across the tests performed with the choke point being the secondary baffle. Glenn's explanation seems true, at least intuitively. As he mentioned, if the CO turns out to be larger as measured by the shadow, then it is what it is. Gotta dig out the measurements of the 150's secondary shadow.

Further experimentation (resulting from our discussion in that fascinating thread) removing the restricting baffle showed the 150 to indeed operate at full aperture, including it's appropriately sized primary mirror. (Certainly never recommending anyone do this, just let silly ole me screw up the scope I love, please.) Removing the secondary baffle reduced the absolute diameter of the CO, and operating at full aperture improved the ratio (meniscus/silvered secondary only.)

The results were quite pleasing, really. Some marked improvements in diffraction effects and no contrast degradation I could detect on Jupiter or Mars. No turned edge, but SA (secondary, I believe) was more evident in the star test outside of focus. The star test was no longer "the same" either side. It was ugly enough to bring a tear to your eye, but it focused quite nicely, regardless. (In fact, I have not replaced the baffle and may not.)

If recall is accurate, the 127 Mak choke point is on the 132MM(?) diameter primary. One might imagine the same is true for the Skywatcher. Both are a different animal than the Orion 150. In the 127, the secondary baffle was not a choke point. The primary was.

So, I would recommend the OP understand all aspects of his scope and that is is designed the way it is for some reason...and is still a solid performer. Even in mine at full aperture, I'm very hard pressed to notice any improvement in planetary or lunar contrast...even if there is some improvement. It's just too hard to tell any difference.

#8 jjack's

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Posted 28 October 2012 - 04:47 AM

Hi Eric
Your mak-cass obstruction and how to measure it exactly is there : http://www.cloudynig.../fpart/all/vc/1

post your obstruction and the truth appears !

#9 Eric63

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Posted 28 October 2012 - 08:17 AM

Thank you all for your input

Yesterday I measured the CO by eyeballing it in front of the corrector and it gave 42mm, which is a 33% CO assuming a 127mm aperture. But from what I read in the replies, the lens effect would slightly reduce the measurement (This would explain why the shadow was 48mm). Following Jack's link I took a picture through the diagonal (I set up the scope on my work table and aimed it agaist a white wall with a strong light). The attached image shows that the CO is a 40% obstruction, which is somewhat similar to the shadow that I measured. I have to admit that this is a bit of a concern, especially when I thought I was buying a scope with a 33% CO.

I purchased this scope as a planetary scope since my 102mmF5 Achro is not good above 120X and my son's 150mmF5 Newtonian is now back at his place. I now fear that I will not have the views I was expecting? I have not had a night of good seeing since I bought the Mak in September so I will have to wait to see for myself how it really performs. Hopefully I will not have to trade it in for a C6 or C8!!!.

Eric

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#10 Asbytec

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Posted 28 October 2012 - 08:42 AM

Don't be afraid to crank that thing past 50x per inch in good seeing and depending on your target. It'll take it, well collimated and cooled to ambient, of course.

I think the views will likely be more pleasing that your 102 f/5 Achro stuck at 120x. Little bit more resolution, cleaner color free image that can stand some magnification, and probably some minor contrast loss in the mid ranges. (Looking from side to side for those who wanna argue MTF. :step: ) :)

Give it a whirl on Jupiter, you might be surprised.

#11 Eric63

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Posted 28 October 2012 - 08:57 AM

Thank you Norme

I intend to do just that. I picked up an 8mm Hyperion with the 14mm ring, so this will give me 190X and 250X. After that I can use my ES 15mm and 3X barlow for 300X. I have tried it on the moon and it was very impressive. I looked at jupiter a few times but the seeing was not that great so it looked quite washed out (looked better at 150X). I could make out two belts, some colour and some lines in one of the belts but not much detail. I can't wait for better seeing conditions.

Eric

#12 Asbytec

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Posted 28 October 2012 - 09:14 AM

You're welcome, Eric. I hope you enjoy the Mak as much as many of us do. Let us know what you think?

#13 freestar8n

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Posted 28 October 2012 - 10:16 AM

Hi Norme-

My main recollection of those two threads was a number of different measurements all saying different things and no real consensus. Since then I was able to spend some time on my Meade 7" Mak and I took it completely apart for measurements and overhaul. Unfortunately I don't have all the numbers yet and won't for some time, but my impression is the same I had from some of the spot maks that were discussed in that thread: The secondary baffle is too narrow at the opening to let in the full aperture from the primary and acts as a stop, while at the same time expanding the obstruction by the secondary. This would greatly reduce the effect of high order spherical aberration - while at the same time reducing the effective aperture and increasing the effective f/ratio.

These spot maks are not considered easy to make well, but reducing the effective aperture would make the job easier since high order spherical kicks in greatly as you decrease the f/ratio.

If the secondary baffle is acting as the stop, it makes it much harder to measure the aperture with something like a flashlight test, because the entrance pupil is pushed far back from the front corrector in object space - and far from the shadow that is cast and measured.

Frank

#14 freestar8n

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Posted 28 October 2012 - 10:29 AM

There is a big flaw in this approach of "taking a picture of the donut" in that the secondary may be in focus at a different position from the aperture stop. If there is concern for vignetting due to a stop far from the corrector, then you would need to defocus the secondary to image the true stop. If there is no concern for a stop somewhere other than the corrector - then just measure the corrector and you are done.

The images in that thread are interesting because in many of them you see a bit of shadow on one side of the full aperture. THAT is the real aperture stop - and it is out of focus - and it isn't the thing being measured. That's why this stuff is hard to do right.

The accepted method for measuring aperture is spelled out as an ISO standard - and it is that way because it avoids all these important subtleties.

Frank

#15 Asbytec

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Posted 28 October 2012 - 11:07 AM

Hi, Frank, yes, you described the Orion 150 baffle exactly correct. Thank you for confirming my suspicions on more higher order spherical. It is apparent with the baffle removed and star testing is almost "textbook" with it installed.

You know, just out of curiosity, I did that "camera" check - just visually, a quick glance - on the moon tonight and focused at infinity (eyepiece removed.) Man, my secondary looks tiny compared to some in that thread.

#16 Eddgie

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Posted 28 October 2012 - 03:56 PM

Further experimentation (resulting from our discussion in that fascinating thread) removing the restricting baffle showed the 150 to indeed operate at full aperture, including it's appropriately sized primary mirror. (Certainly never recommending anyone do this, just let silly ole me screw up the scope I love, please.) Removing the secondary baffle reduced the absolute diameter of the CO, and operating at full aperture improved the ratio (meniscus/silvered secondary only.)



Ah, intersting. I left that thread long after it started to turn into a debate about the testing method, but as I recall, you and I both suspected that the secondary baffle could be the culprit in the case of your scope.

And the suppression of higher order spherial abberation for star testing could be plausible.

A little higher order spherical abberation may actually cause less damage to the image than a bigger obstruction would, but of course it would show in the star test and a lot of people might think the scope was defective if the appearance of the intra/extra-focal Fresnel patterns differed.

Have you used Abberator to try to simulate the HSA in the current configuration? It could be possible that the HSA is doing less damage than the contrast and brigntness loss of the bigger obstruction!

Would be fun to know.

I know, a bit off topic, but I am in the camp that trusts the measurments.

They may not be perfect, but if they indicate a serious loss of aperture, then it is difficult to accept that there is that much error in the test. The parallell nature of the exit bundle would have to be severely disturbed and I don't know how that would happen.

So, I believe that the OP has what he has measured... A scope working at a reduced effective aperture with a correspondingly larger (as percentage of aperture) shading factor.

#17 Eddgie

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Posted 28 October 2012 - 04:10 PM

Hi Glenn. I left the original long thread because I lost interest in the debate about the valitiy of the testing (thought I personally believe that it is indeed valid).

But there was a question that I don't know was definitivly answered.

If a secondary baffle or primary baffle cuts the light cone, does it only effect light gathering, or is the contrast transfer and angular resolution unchanged?

Why would cutting the light cone reduce the contrast or the angular resolving power?

If any restriction anywhere in the converging light cone did this, wouldn't using a 1.25" visual back on a C8 do the same thing? As the light cone entered the front of the visual back, it would be greatly cut off. If the light cone being cut by the baffle reduced angular resolution and contrast, it would see that cutting the converging light cone anywere would do the same thing.

I thought that cutting the light cone at the either baffle would only decrease the center of the field illumiation, but that the system would still have angular resolving power equivilent of the full aperture.


I really don't know the answer to this, but have often wondered. I would like to think that the only serious effect would be that the illumination was reduced, but that doesn't mean that I do believe it.

Or are these scopes really working 100% as smaller apertuere scopes with larger central obstructions from an angular resolving power and contrast transfer standpoint?

#18 GlennLeDrew

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Posted 28 October 2012 - 06:35 PM

Eddgie,
No matter where the obstructor lies in the system, the effective aperture and relative secondary shadow determine the performance. Diffraction does not 'care' where along the optical path it occurs. Indeed, as far as the eye is concerned, all that matters is the exit pupil (or its own pupil, whichever is the smaller) and the wavefront deformation there.

So, the 'flashlight' aperture test reveals the true working aperture and secondary obstruction. These are the only values to use in any assessment of the system, not the 'nominal' ones. The 127mm Mak under consideration IS a 118mm Mak having a central obstruction of ~40% by diameter. The theoretical best performance is determined by these two values.

Regarding restrictors near the back end, such as smaller diagonals and such. If they do not impinge on the *axial* light cone, they are of no import as regards effective aperture reduction. The only concern would be related to *off-axis* light cone clipping, resulting in vignetting.

#19 Eric63

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Posted 28 October 2012 - 08:13 PM

Wow... All of this has been quite interesting. I have to admit that the learning curve is steep, but I am truly fascinated by everything that I am learning. Thank you all for shedding some light on this issue. Now, I have to change my paradigm is simply accept that I have a 118mm scope with 40% CO, which is probably close to a good 4 inch refractor (shall venture to even think ED! :) ). I am sure that it will complement my 102mm F5 Achromat in my grab and go sessions. Perhaps next year I will upgrade to a C8, but for now I will continue to learn this new hobby.

Eric

#20 Eddgie

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Posted 28 October 2012 - 08:28 PM

Ok, that makes sense.

I suspected that this condition did indeed turn the scope into a smaller scope with a larger obstruction.

I had mentioned this in the past, but as I recall, some believed that it did not really affect the resolving power or the contrast.

My own experience when using SCTs at back focus distances that caused the baffle to cut the light cone was that the performance was being affected (severely in some cases).

Anyway, I guess I suspected it, but good to hear a confirmation.

Thanks.

#21 freestar8n

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Posted 29 October 2012 - 03:46 PM

The flashlight test should work fine if the only concern is the size of the front corrector or the primary - but if the secondary baffle is too small and acts as the aperture stop, then the flashlight test would be unreliable because the entrance pupil is shifted far from the front of the scope. If the only issue with the 127mm Mak is that the primary is acting as the stop then things aren't so bad. It means the mirror the mirror has a beveled edge that makes its diameter much smaller than 127mm - and that reduced size is reduced even further by the negative power of the front corrector.

Those factors alone could reduce the aperture to below 120mm without involving the secondary baffle - and a simple flashlight test would indeed measure it.

But IF it is not known whether or not the secondary baffle is too small, then measuring the true entrance pupil size becomes much more challenging. It would be nice if everything were simple and you could just do easy tests that seemed to work - and get numbers you could trust - but similarly it would be nicer to predict comet orbits if they were circular - but they aren't.

If the secondary baffle is acting as the stop then the behavior of the entrance pupil becomes even more complicated because the aperture stop and the secondary obstruction live in two different places in object space - and will come to focus in two very different planes.

In talking about reduced aperture, it's important to know what is acting as the aperture stop, and where it exists in object space. If these terms aren't familiar to people, they are nonetheless important and described in most any textbook on imaging systems.

The true aperture can't be larger than the apparent size of the primary as seen through the reducing power of the front corrector. It could be even smaller if the secondary baffle is too small.

Frank

#22 Ed Holland

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Posted 29 October 2012 - 05:43 PM

Hi again :)

Just a couple of comments, as I should take some responsibility for "the other" thread... Between several CN members, we examined a few Synta 127mm Maks, and found that they were not identical. There were differences, albeit minor, in the optical and mechanical components.

In my example, the effective aperture was determined at 118mm by two methods: Eyepiece method and a laser swept across the front of the tube, with results in close agreement. As the thread progressed, I dismantled the 'scope and revealed the cause to be an under-sized primary mirror. IIRC, we never went deep into central obstruction on the 127mm design

One benefit to the investigation was that I found the the primary to be tilted on the baffle tube. This was corrected, along with some other mechanical details which were probably all responsible for the 'scope falling in to my hands for a very good price. The end result of all the attention is that this easy to use telescope now performs very well.

The Mak easily gives my 5" f/9 achro a run for its money on planetary targets. So to the OP, I say don't worry unduly about the performance vs. a 4" refractor. You can look forward to some pleasing observing when the sky cooperates.

Cheers

Ed

#23 azure1961p

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Posted 30 October 2012 - 09:54 AM

Thank you all for your input

Yesterday I measured the CO by eyeballing it in front of the corrector and it gave 42mm, which is a 33% CO assuming a 127mm aperture. But from what I read in the replies, the lens effect would slightly reduce the measurement (This would explain why the shadow was 48mm). Following Jack's link I took a picture through the diagonal (I set up the scope on my work table and aimed it agaist a white wall with a strong light). The attached image shows that the CO is a 40% obstruction, which is somewhat similar to the shadow that I measured. I have to admit that this is a bit of a concern, especially when I thought I was buying a scope with a 33% CO.

I purchased this scope as a planetary scope since my 102mmF5 Achro is not good above 120X and my son's 150mmF5 Newtonian is now back at his place. I now fear that I will not have the views I was expecting? I have not had a night of good seeing since I bought the Mak in September so I will have to wait to see for myself how it really performs. Hopefully I will not have to trade it in for a C6 or C8!!!.

Eric


Eric,

Ive got to believe your Mak will whiz past 120x even on average nights. My C6 is comfortable at 180x to 200x on averege nights with a 300x Mars and 250x Saturn looking great in 7/10 or better seeing.

Ive looked through a Meade ETX125mm Mak and while its a different make, if your scope is optically on par and it shoudl be, it ought to leave the refractor images well behind.

Any 125mm cassegrain system that max's at 120x needs a return shipping or a better night.

Im holding out good faith it'll offer great views when the sky allows it.

Pete


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