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Orion 127 Maksutov - Back focus and aperture

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#51 freestar8n

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Posted 04 March 2012 - 06:32 AM

The form of corrector has little if anything to do with vignetting or range of back focus.



I'm not sure what you are saying here since you guys are using "vignetting" in non-standard ways. The difference between a maksutov corrector and a schmidt corrector has a big effect on the operating aperture because the schmidt is nearly flat, while the maksutov has negative power. The maksutov therefore has more divergent rays, and requires a larger primary to support an aperture stop at the corrector. In other words, it has greater need for a mask in the front to make the aperture stop smaller than the primary.

And since the actual aperture is smaller than the primary, the f/ratio is larger and the cone of light near the focus is narrower - which makes it less susceptible to vignetting due to obstructions near the focus or in the baffle tube.

Frank

#52 EdZ

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Posted 04 March 2012 - 07:31 AM

The effective entrance aperture is whatever light is collected by the system, and for the 127mm Mak, this is a 118mm circle. Even though light is intercepted by one component of a particular size, resolution will be lost if there is an aperture stop preventing full projection of the light entering the system.
Ed Holland



That is correct. Under no circumstances can the resolution of the system ever be greater than the smallest aperture reduction, regardless of it's placement or cause.

edz

edited to cite source ref

#53 freestar8n

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Posted 04 March 2012 - 08:03 AM

You appear to be citing me as the author of the quotation - but it was someone else - although I agree with the gist it in the context of a Maksutov corrector and primary.

Under no circumstances can the resolution of the system ever be greater than the smallest aperture reduction, regardless of it's placement or cause.



You could have a physical aperture stop 1" diameter in a telescope that has the light gathering ability and resolution of 100 inches. That isn't the normal scenario, but it becomes a *likely* scenario if you are limited by bottlenecks near the focus due to extended back focus.

You could also have a physical aperture stop 100" in diameter, but the actual telescope aperture is 1".

The point is - the physical aperture stop is completely distinct, in size and location, from the entrance pupil, and it is the entrance pupil that determines light-gathering and resolution.

Frank

#54 Asbytec

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Posted 04 March 2012 - 08:22 AM

Ed, I offer my sincerest apologies for jumping on the topic you raised and polluting your results with data on the 150. It was my intention only to study, debate, and follow the captivating topic you raised in conjunction with a parallel thread on SCTs. I got drawn into it pretty hard. We both have Maks, so it seemed appropriate and on topic.

I am not so much interested in the extent of back focus since I do not use any accessory other than a diagonal and a Barlow. The idea you and I both see reduced aperture very close to "prime focus" is curious. Out of respect, I hope the debate and offering some test results is okay with you.


Okay, I just repeated the flashlight test with a laser pointer and a 25mm Plossl focused on a star. Sure enough, my illuminated projected aperture is 140mm and the diagonal is exactly 50mm. And one can see the green glow outline about 5mm inside the edge of the meniscus, as expected.

Then, I moved to the front of the scope to repeat Ed's experiment with the laser pointing down the edge of the OTA for alignment and a block to ensure I moved it laterally as carefully as possible. Some interesting results.

The first thing I noticed in the 150 MCT is, you could actually see the laser bend in the meniscus. It bent inward toward the axis after striking the first surface at the very edge of the aperture. My guess is, after exiting the second surface, it had a little more room to diverge onto the primary. It was hard to measure how much this bend was, but it looked to be about 2mm...and yes, boy, the meniscus is pretty thick. (lol)

Now, I know it's hard to trust observing anything through curved glass, but I have to mention it. And the curvature seems to suggest the bend should be outward at the first surface. But someone smart on that would have to comment, but surely the initial bend is outward.

Second, on baffling. When reverse ray tracing through the eyepiece, the end of the primary baffle was brightly lit. Not sure this means anything, you would expect a divergent beam reflecting off a convex mirror to illuminate everything in front of it.

When testing from the meniscus end, the green point of light entering the edge of the meniscus fell on the outside of the secondary baffle and reflected back out through the meniscus. It did not enter the primary baffle nor exit through the eyepiece at all. It could be seen shimmering on the edge of the primary after entering the edge of the curved meniscus - trying my best to stay aligned with the axis. The only thing the eyepiece showed was the lit wall, not a hint of green light.

At least in the 150, there appears to be some vignetting of the secondary baffle. This may well differ from Ed's 127 design and in no way invalidates his careful results. It's likely he was more accurate than I was. So, the 150 appears different, even though it also shows a reduced aperture projecting light onto the wall. I think vignetting plays a role in the 150MCT.

I am still not sure if the primary mirror aperture stop is the culprit or not, either. But, at least in the 150MCT, some initial testing seems to show it /might/ not be, and at least some of the problem is vignetting at the secondary baffle.

Ed and Eddgie both put forth compelling ideas on the topic, while Glenn and Frank offer their insight. I just wanna learn about the design I love so much.

#55 Ed Holland

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Posted 04 March 2012 - 09:53 AM

Norme, absolutely no need for apologies :) these topics have legs. Indeed, I was hoping that we would hear from owners of other members of this family of instruments, and thrilled that the conversation has attracted plenty of attention.

Anyone with an Orion 180mm Mak care to chime in?

#56 azure1961p

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Posted 04 March 2012 - 09:58 AM

Norme,

That Jupiter drawing is THE reason never to consider such a scope for sketching - too much work!!

Its crazy good though.

Pete

#57 Asbytec

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Posted 04 March 2012 - 10:03 AM

LOL, Ed...yea, anyone?

I realize my measurements were likely not as accurate as yours. I should have gotten a divergent ray probably off the edge of the primary. It's very hard to remain exactly on axis (and to debate this along with other complicated issues in other blogs. <sigh>)

But, I did get some vignetting.

Pete, isn't it though? My jaw hit the floor so hard it knocked a tooth out and threw up dust bunnies.

#58 GlennLeDrew

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Posted 04 March 2012 - 10:54 AM

Here's how to always ensure your laser is parallel with the optical axis. Place a piece of flat glass, such as from a picture frame, against the front of the scope. Adjust the laser so that its reflection from the glass goes directly back to the laser. We can assume the rim of the corrector cell is sufficiently square. You can leave the glass plate in place if you wish; perpendicular light through it does not affect the test, outside of the possible annoyance of extra reflections.

The observation of the laser light refracting inward toward the optical axis after passing the front surface of the concave corrector out near the edge surely is an optical illusion. A paraxial ray entering a denser medium whose surface is concave toward the source must be refracted outward.

#59 Asbytec

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Posted 04 March 2012 - 11:23 AM

The observation of the laser light refracting inward toward the optical axis after passing the front surface of the concave corrector out near the edge surely is an optical illusion. A paraxial ray entering a denser medium whose surface is concave toward the source must be refracted outward.


Agreed. I mentioned it (prematurely) only because it struck me at the time as interesting. Even looking "down the barrel" along the laser axis the reflection from the first surface, the second surface, and on the primary seem to like up. That's probably a "fish bowl" illusion, too.

Anyway, fascinating topic. Thank you for replying.

There was some mention of a blurry edge earlier when doing this test. Using a laser pointer, this softer edge was not only visible it was quite large. At a distance of 7" in front of the meniscus, it was a full 25mm larger in radius. Further out, projected on the wall, it was much larger, still. Not sure what the significance is using a 25mm Plossl with the laser very near where the eye would be.

So, if I understand what Frank is saying, If the scope is not vignetted by the baffle or visual back the entrance pupil for the MCT is at the meniscus. If there is vignetting, the entrance pupil is further from where it should be and the flashlight test is inaccurate. I do not know which is the case.

It's an interesting argument as to the importance of the entrance pupil location. Glenn seems to argue it does not matter because it's the resulting beam that shows the effective aperture. This seems to be true. (Ever notice two folks can argue and both be right?)

In my novice thought process, it matters because it does not define the cause of the vignetting. The meniscus is strongly curved on both surfaces, and the result should be a weak lens giving weak divergence toward the primary. The primary might not be as undersized as the meniscus curve could imply. It could be more like the Schmidt corrector which experiences little effective loss out to 200mm unlike Ed's (and my) results much closer in.

If this is the case, then the reduced aperture can be attributed to baffling or some other internal structure and not the meniscus/primary proportions. Yes, the end result is the same, regardless, but tracking down the culprit is important, too. So, the reduced cone could be structural and not optical. If the obstruction were removed, maybe the resulting light bundle would be at, or darn near, full meniscus aperture.

To me, that's important, and maybe the reason the flashlight test depends on knowing the entrance pupil location. It might not be accurately describing the actual function of the meniscus/primary, which may be frank's point.

It may also explain why the MCT Ed and I have are vignetted at much shorter distances, remain tolerant a bit further out, and why the 6" SCT is not vignetted in that range of back focus given their similar corrector divergence.

#60 Eddgie

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Posted 04 March 2012 - 01:01 PM

So then if this is the case, the mirror would have to be made some amount larger in an MCT to regain the full apeture and the system is indeed only working at reduced true aperture and resolving power ,and with an obstruction somewhat larger than stated in marketing materials.

This is really just a data point to me. I did not know the mechanism for this and incorrectly used "Fanning" in my post above trying to explain how the mirror could not be working at full resolution if the circle was 140mm (though I did say that I doubted the fanning premise and Glenn provided me with the actual mechanism is that the parallel rays bundle is expanded in diameter as we move further from the center of the optical axis.. Thanks for the insight, Glenn).

So, if this is not a baffle problem (and your work gives every evidence that it is not), then we are left with the premise that in order for these MCTs to work at full aperture, the primary mirror has to be oversized in order for the system to work at resolution and brigthness indicated by the Meniscus.

This of course also means that the secondary obstrucion for this model is larger by percentage than marketing materials indicate, meaning the contrast would be lower than one would expect from the data based on the marketing literature.

So I was wrong about the baffle, but at least I know that my thinking that the system could only be resolving at the effective apeture being indicated was correct, even though I attributed it to the wrong mechanism (which I could not believe myself, but had not considered the ray bundle being expanded but still parallel).


Intersting data.

If all of this is correct, it appears that it is a design flaw and not a manufacturing/assembly error. I just like to know these things.

#61 Vondragonnoggin

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Posted 04 March 2012 - 01:17 PM

That's where the Meade mak became popular. Besides having good features and a fine optical example, they sized the primary correctly for the meniscus size. At least that is what I have read about why the 7" Meade was revered. Celestron did not seem to think it was important. Apparently neither does a lot of other companies. Does anyone have an example of the 5" Intes mak to check out?

#62 PHampson

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Posted 04 March 2012 - 01:24 PM

If all of this is correct, it appears that it is a design flaw and not a manufacturing/assembly error. I just like to know these things.


I would just remark that it's not really a design flaw - the design seems to yield pleasing enough results. It is, however, pretty imprecise advertising and marketing, certainly misleading and borderline 'false'. Kind of analogous to Meade's aplanats advertised as RCs. If they would advertise and market the scopes for what they really are, there would be no controversy.


Paul

#63 Eddgie

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Posted 04 March 2012 - 02:33 PM

Well, yes, assuming that the testing being done here is valid, it has to be either one or the other.

Wouldn't the designer have known? There are ray trace programs out there that one can get for free. If the designer knowingly used a sub-sized mirror, then it is not an error, but at the same time, they should sell the scope as a 118mm telescope.

Or, perhaps they could have just made an error or simply used curves they obtained from other sources and not done the math themselves to make sure the system was well designed (Known to have happened in some cultures that tend to copy industrial design and not always do original design.)

The manufacturer makes a few thousand and slaps differnt brand names on them and sells them to re-marketers to sell the public and either knowingly mis-represent them to the re-seller (in which case the re-seller is not complicit) or don't know that the product is not being accuratly described.

We will never know. And that is the sad part. Only members of these kinds of communities ike CN may ever really know that these scopes may not be quite what is being advertised.

I don't hold the re-sellers at all "guilty" of mis-representing the product. Again, Orion is a re-seller. They don't design the scopes, and my guess is that their marketing department does not drive sepcificaitons.

In other words, they don't have someone saying "Here are the exact qualities my customers want, so design and build me a scope to do this."

Instead, they ask the manufacturer "What have you got that is sexy, that I can sell a lot of (I used to be in the software business... LOL) and the behind the scene manufacturer says "Hey, I got couple of dozen containers of 127mm MCTs! Everybody loves MCTs because the are more better than other scopes!"

And the deal is done.

#64 Ed Holland

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Posted 04 March 2012 - 04:10 PM

So reducing this to the basic level, we have have design conspiricy vs. design (male chicken)-up theories. It is ironic to me, because the corrector would appear to be the most expensive component to make. It has been produced to the stated aperture spec, ground, polished, given anti reflection coatings, silvered in the centre and has a baffle ring attached. The primary is a doddle by comparison.

But, here we are anyway, with scopes that make us happy :)

..Edited to remove an intervention by the silly profanity filter

#65 freestar8n

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Posted 04 March 2012 - 05:04 PM

I think a lot of the design decisions have to do with the associated costs. Meade already had 8" tubes and primaries, so it makes sense to make it into a 7" Mak. At that size, an 8" corrector might cost a lot more than a 7" corrector in a masked cell - so maybe it's cheaper to do it that way.

For the 3.5-5" sizes, it may be cheaper just to make a full size meniscus and mount it in the tube with a simple threaded holder.

So - there may be a mixture of reasons why the 7" is "correctly" sized. Maybe if they had used an 8" meniscus they would have called it an 8" mak - even though it isn't. I don't know.

If people are really concerned about the effective aperture they are getting, then they could fold everything together and include both transmission and secondary size to convey truly how bright the image is. You could have a big aperture and lots of lossy reflections and have an effective aperture much smaller. The resolution would still be set by diffraction, but the brightness depends on all the coatings and bounces.

So you would really need the following:

Entrance pupil diameter
Entrance pupil secondary obstruction
Light transmission through all surfaces - including the eyepiece.

For resolution the main thing is the diameter and the secondary obstruction, and for brightness you would want entrance pupil area (reduced by the secondary) multiplied by the transmission coefficient.

Frank

#66 GlennLeDrew

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Posted 04 March 2012 - 10:13 PM

It just occurred to me (on Sunday evening) that my boss procured a few Orion 127mm Maks a few years ago in order to assess the correctors for potential use in a certain project. If one is still not taken apart I'll test it out.

#67 Asbytec

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Posted 04 March 2012 - 10:37 PM

All scopes are compromises between design, performance, and cost. There is probably no design conspiracy, just a compromise in optimizing a given design.

For example, it could well be Meade wanted to test the waters with an MCT. It certainly had a supply of 8" tubes and mirrors, but wanting to keep cost down it produced a less expensive 7" meniscus. Doing so allowed Meade to rightfully claim an 8" mirror grabs the entire light cone. In fact, if the meniscus is really a weak lens, an 8" primary is not necessarily optimal as it is overkill on design at minimal cost.

The MCT is hugely popular despite the compromises it makes. One of those is probably offering a reasonable obstruction with a slower primary. The result may well be limited back focus. After all, the slower secondary light cone should have little problem coming to focus through the visual back. But, it's probably vignetted, possibly in the baffle.

The manufacturer probably knows this, but might still be able to claim the full meniscus aperture is brought to bear with very little vignetting at the primary mirror and maybe a bit more in the baffle. Again, due to compromise in the design. The flashlight test should show the resulting light bundle as the sum of those compromises which may or may not be a severely undersized primary.

If true, then at a glance, it seems many MCTs with a "standard" 33% obstruction (give or take) are designed with compromise in mind. Even with the reduced effective aperture, it's still only 35% by diameter while the baffles could be very tight to optimize performance. A little of this for a bit less of that.

#68 Asbytec

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Posted 05 March 2012 - 10:27 AM

One last comment on optimization and compromise. Having used magnifications well beyond 50x per inch and routinely 85x per inch (470x, recalculated) on my Mars sketches, this is still a fine scope in all regards.

On planets or the moon, a loss of 18% light gathering power and a mere 0.05" arc resolution would go unnoticed. So, the scope seems optimized for lunar and planetary, as expected.

#69 Ed Holland

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Posted 05 March 2012 - 01:25 PM

Norme, you are probably right. Compromise theory is very likely the real explanation.

Actually I wonder if there might be a minor advantage to the configuration. If the corrector, with it's very strong curvature is over-sized, and only some central portion of it is used the influence of imperfect edge zones is reduced.

Always try to look on the bright side 'eh ;)

Turning to more constructive analysis, I would like to look at the relation between back focus and effective focal length in the 127mm. Off I go to read up about drift timing :)

Ed

#70 Asbytec

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Posted 05 March 2012 - 01:40 PM

Sure, would love to hear what you come up with, Ed. This subject has enamored me.

Hey, briefly from a parallel discussion in another forum. The thing that started me wondering was why the MCT did not offer fully illuminated back focus to any appreciable extent, as your measurements show. I think the explanation also explains why you find it to also be tolerant further out. So, maybe a bino viewer is just fine...it may loose very little with the longer, albeit slightly vignetted, light cone.

Like Mak Newts, the secondary is actually undersized for the slightly slower primary focal ratio. I know incident rays are vignetted at the secondary baffle. Observing the moon, one can see the illumination on the outside surface of the secondary baffle. The baffle is tight, probably to enhance contrast. On the bright side, take some nice optics that can push 85x per inch, and you have a lunar and planetary scope with sharp good contrast images.

A small reduction in effective light gathering power means little when observing planets, resolution suffers a tad. But, that's a trade off.

Maks, it seems, are more specialized than SCTs...the difference is what grabbed my attention. SCTs can do it all and it makes sense they can also support longer back focus by design. We hear Maks are lunar and planetary scopes, but this thread just seems to drive that point home.

Couple more things I want to look at, too, then put this learning experience to bed and be at peace. (lol)

#71 Eddgie

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Posted 05 March 2012 - 02:57 PM

Here is why the "design vs. fabrication" issue is so important to me.

If it was a fabrication error (baffle not installed properly, or wrong baffle used, or whatever), then it means that as few as one specific scope or on specific batch of production for that model may have been affected, but maybe the rest might be OK.

If it is a design problem though, it means that every one of this model ever produced is this way.

And if one model with a design issue like this is allowed to go into production, it means that there may be other models that were also improperly designed.

So to me, it is not a small question.

Celestron did original design of its products. According to their web page, design for the EdgeHD was done in the US.

What about the "Generic" telescopes though? Are they given the same attention to design that a Celestron or Meade would be given? It would seem that the Orion 127 in question is working with a rather meaningful reduction in brightness (with the central obstruction and transmission losses, it is little better than a 4" refractor).

If you bought a scope with the Celestron brand on it and found out about this, would it shake your confidence in the brand?

So, that is why I wanted to determine if this was a fabrication error or design error. A fabrication error means that potenitally only sample, or a small group of samples was affected. Design means that all Orion 127s are this way, and would make me question other models from the manufacturer.

If this is found to be pervasive, would people still buy a "180 MCT with 28% onbstruction" if they knew that it was only a 170mm MCT with a 33% obstruction? (or whatever).

#72 Ed Holland

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Posted 05 March 2012 - 04:19 PM

Quoted from the Orion web page for the 127mm Mak (link)

It gulps in 55% more light through its multi-coated, 5" meniscus lens than that of a 4" telescope, putting it in the big leagues for astronomical observation and advanced nature study.


Not wanting to attack this product, I'll be careful what I say here. However, the advertising copy is notable in its promotion of the light gathering capability of the design. No-one reading this text would think anything other than the telescope has the full performance of a 5" instrument. It really seems like something was overlooked or misunderstood.

But this is the last I want to say on the effective aperture subject. Next stop is other characteristics of this fine instrument.

Ed

#73 GlennLeDrew

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Posted 05 March 2012 - 09:37 PM

So. At the shop we have lying about the optics for a few of these Orion 127mm Maks, namely, the primaries and correctors. Here are some measurements.

Primary diameter: 132mm. Clear aperture (less the ~1mm bevel): 130mm.

Corrector diameter: 134mm. Clear aperture (less the ~1.5mm bevel): 131mm.

The corrector works as a weak negative lens. Ray divergence at the edge is estimated as about 1/3 degree, and so total divergence is ~2/3 degree. This means that for every 100mm of light travel beyond the corrector's rear surface, the ray bundle footprint expands by a little over 1mm.

A paraxial ray entering the front surface of the corrector is refracted outward, and upon exiting the rear face is displaced by about 4mm, perhaps as much as 4.5mm. That is, if the entrant light bundle as revealed by the flashlight test is indeed 118mm wide, it will emerge from the back surface some 126-127mm wide. Interesting..... Could this be the source of the stated/claimed aperture?

As the light bundle continues on to the primary, the slight divergence (due to the weak negative power pointed out above) will have it expand a further 2mm, approximately. It will therefore (assuming the entrant diameter of 118mm) cover 128-129mm on the primary mirror, whose clear aperture you'll recall is 130mm


Not having the corrector cell, the remaining question is what's the clear aperture of the cell's shoulder upon which the corrector sits. And too, the aperture of the front retaining ring. Who knows; a diligent search just might turn up one of these cells...

My provisional conclusion is that the limiting aperture is the corrector's shoulder, which may turn out to be the stated 127mm. But for the real working aperture to be 127mm, this shoulder must have a clear aperture some 8-9mm wider, or 135-136mm. Then of course the primary's aperture would need to be about 137-138mm.

I hope all the numbers bandied about here hasn't caused too much glazing over of eyes! :grin: I recommend making a sketch.

#74 Asbytec

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Posted 05 March 2012 - 10:58 PM

Glenn, so the primary is oversized a little, larger than the stated aperture. The meniscus being oversized make some sense if it's not mounted in it's cell. If the primary is 5 mm larger, that's interesting.

Ray tracing it in my head, here's what I think is happening.

A ray entering the very edge of the corrector may well fall on the very edge of the primary or maybe even just graze the beveled edge. But very quickly, moving inward from the meniscus edge of clear aperture, the rays will begin striking the primary mirror edge. So, the system works as designed.

That ray then diverges from the primary's edge to the meniscus second surface. The point it strikes the second surface defines the very diameter of the silvered Gregory spot. It is the absolute minimum spot diameter to achieve full aperture illumination at the center of the FOV. I am sure this is how they define how much of the second surface to aluminize. However, Ed's test do not show full illumination of 127mm, nor do mine show full 150mm effective aperture.

On my 150, the secondary baffle is "designed" (surely) to have a smaller diameter than the Gregory spot. The base of the baffle is smaller than the silvered spot, while the widest portion of the secondary baffle is just a tad (~1mm) larger. This means the light from near the full meniscus aperture striking the edge of primary is vignetted at the secondary baffle.

So, even at "prime focus" with 60mm of back focus, the FOV is not fully illuminated as Ed shows because the on axis light cone is vignetted: the secondary baffle is too small for the slower primary mirror. The flashlight test bears this out.

Reversing the ray trace using the flashlight test, the expanding ray strikes the smaller secondary defined by the smaller baffle not the full aluminized spot (on the 150.) The thinner diverging cone from the secondary onto the primary illuminates the primary only partially (unlike the incoming beam which probably illuminates it fully.) From there, the converging beam headed toward and out of the meniscus would be 118mm as Ed shows.

All remaining incident light cones fully illuminating the primary and using full aperture of the meniscus are vignetted, as well. Surely some of those incident rays, though vignetted, use the full aperture and contribute to the image. so, the question remains, how does this affect resolution? Is it at full aperture, or effective aperture on axis?

This is probably a design feature, as Eddgie points out, to keep the secondary baffle obstruction minimal. The aluminized spot is right about 33% of the clear aperture. If the base of the baffle were along this circumference, the wider baffle opening would be much larger resulting in a much larger CO percentage. The FOV would be fully illuminated only at the center, but the CO would be larger by area. The CO would be very large if more FOV were illuminated fully. The scope would be a bit loose in terms of baffling to permit full illumination.

This is much like the Mak Newt design. Some vignetting is accepted in the MCT to keep the CO minimal, at about 33% of clear aperture or about 35% of effective aperture on axis. Surely this is a design feature. Remember, the CO also serves to block longer focusing inner zones to improve SA. It's possible this was also taken into account when deciding on compromise between secondary and baffle, as was the resulting MTF and sacrificial illumination.

So, the result is great SA correction, sharp optics, minimal CO for the design, and some loss of field illumination for scope optimized for lunar and planetary observation. It will have a bit tighter baffle than the more generalized SCT (designed with imaging, planetary, and deep sky in mind for a more generalized observer.) The test results on both designs bear this out. The flashlight test does, too. It explains why effective aperture is less in the MCT, it's vignetted at the tighter secondary baffle. Eddgie was correct.

I have a 5.5" obstructed refractor. :lol: But the more I understand it's specialization, the more I appreciate it.

#75 GlennLeDrew

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Posted 05 March 2012 - 11:37 PM

Guy in a furry hat,
My provisional analysis concludes that this telescope cannot operate at the claimed 127mm aperture, even if the secondary spot and its baffle were of correct dimensions.

The primary mirror's clear aperture of 130mm means that the largest true aperture possible is about 120mm. If you made a schematic sketch as I'd advised, you'd see why. Starting with a 130mm diameter ray bundle at the primary, the roughly 2mm convergence at the rear face of the corrector brings it down to 128mm. The significant refraction within the corrector has the ray bundle shrinking by 8-9mm, thus emerging parallel from the front face with a diameter of 119-120mm.

Remember that we just followed the light path in reverse, starting from the primary, following the same path light would take from a star and if BOTH corrector apertures were large enough to fully illuminate the primary.

That this maximum is very nearly the same as the observed effective aperture of 118mm, this strongly suggests that the secondary and its baffle--or, for that matter, the primary baffle--have little if anything to do with the aperture reduction.

I'll say it again; given the supplied primary diameter of 130mm, this scope, with its working aperture of 118mm, is performing very nearly at the ultimate max aperture of 120mm.

In order for this scope to work at 127mm, The primary would need a diameter of about 137mm (and the corrector would need to expand from the current clear aperture of 131mm to 135mm.)

I just had a conversation with boss about this, and he recalled doing a ray trace of the system using Zemax software, soon after receiving the samples a few years ago. He recalled deriving a working aperture of 120mm, and communicated his findings to Orion. It's interesting to note the relatively small difference between a software ray trace (120mm), the flashlight test (118mm) and my own analysis of the maximum possible (120mm).


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