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Orion 127 Maksutov - Back focus and aperture

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#26 Ed Holland

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Posted 02 March 2012 - 08:37 PM

I'm thinking up some "real time" ray tracing experiments that would be possible with a simple jig and a laser pointer. Failing that, I could arrange a solid setup using the optical bench components available here at work, but then I have to drag everything there - not so easy on a bicycle :)

#27 Asbytec

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Posted 02 March 2012 - 11:24 PM

Dean, it's an interesting topic. Thank you for bringing it to the fore. I'd love to see what your bench testing shows. Need a ride? :)

Again, one of my first impressions in my MCT was not so much of objects being 15% dimmer, but of contrast being slightly better. Surely this is related to the focal ratio. Plus, in calm seeing, I get stunning views of planets (an objective result, admittedly.)

Importantly, I have "split" doubles at the Dawes limit for full aperture, very near 0.77" arc. Other doubles appear correct, too, in relation to the companion and the first diffraction ring. If memory still serves in old age, I have seen stars very close to the magnitude limit of full aperture. But, ~9mm of aperture is a tiny amount (~0.05" arc in resolving power and visual magnitude) and probably won't be noticed.

I just find it difficult to believe two similar designs have such different results. SCTs apparently, with a weak corrector and faster focal ratio, can support longer back focus than a slower MCT with a similarly "weak(?)" meniscus and a more slowly converging light cone. Surely the baffles are pretty close to optimum in both, as optimum as one can expect from both mass produced designs - one probably not significantly better or worse than the other.

In fact, it seems even at "prime focus" without a diagonal, the MCT is operating at reduced aperture according to this test. Something doesn't smell right.

I wait with baited breath (need to brush, apparently) on any results you can muster. Or look for another means of testing to see if we can get similar or conflicting results. If confirmed, I might need a larger Mak. :lol:

Interesting. I think I will try the de-focused star test, stick my finger in the light path. It might put my finger to better use. (lol)

#28 Mark9473

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Posted 03 March 2012 - 05:46 AM

I think that the mirror size is not the coulprit here. The Meniscus does indeed bend the light outward but only bye a tiny amount.

I suspect that the problem is in the fabrication or the design. If the baffle tube is too long either because it was designed to long or because the scope was improperly assembled, you would get a scope with permanently reduced effective aperture.

And that is what I think we have here. I just can't believe that the Meniscus is so refractive that it would cause so much expansion of the light cone as to miss the edge by an astonishing 5 millimeters. The curves just dont see that strong to me.


I've been thinking about this. If the meniscus would bend the light outward so that it would illuminate more than the primary mirror, the scope would be working at full aperture. So either the meniscus illuminates less than the full mirror size, or else the baffle is cutting it out as you said.

#29 freestar8n

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Posted 03 March 2012 - 06:55 AM

I think there is a misconception of how aperture is defined, and it has subtleties that are important to this discussion. Any telescope will have a physical stop that limits the on-axis light entering the 'scope, and the aperture of the scope is defined by the entrance pupil, which is the image of that stop formed by any of the optics and lenses in front of it. With many scopes the stop and the entrance pupil are identical because there are no lenses in front of the stop, or they are very close by. But the location of the stop could be completely independent of the primary mirror that is collecting the light.

In a Newtonian or RC, the stop is the primary mirror itself, and the aperture is equal to the diameter of the primary mirror. In an SCT, the stop is at the corrector plate in the front of the 'scope, and it is usually slightly smaller than the primary mirror to allow for some divergence of the rays on the outside of the corrector - but there is little divergence because the corrector is nearly flat.

A Maksutov also has the stop at the front meniscus - normally - but some variations of the design have it offset from the front surface. This is because a shift of the stop can be used to correct aberrations, as in a lensless Schmidt that has the stop way out in front at the radius of curvature. Someone might say - Oh - let's remove that to reduce vignetting - but that would defeat the whole purpose and introduce coma. It's a case where more light is bad, not good.

For a Maksutov with a negative power meniscus, the stop is normally designed to be at the front of the meniscus, and that defines the aperture as long as the primary is large enough that it captures all the light from that stop. If the meniscus is the full diameter of the tube and it has negative power, then you know that the primary will be too small to capture all the light from it, which means the primary is acting as the stop and the true aperture is not the primary, but the *image* of the primary formed by the meniscus. Since the meniscus has negative power, that image will be smaller than the physical mirror size, even if the meniscus is large and "in front, gathering light."

As for there being some kind of trade off of vignetting with clear aperture - this is often the case, for example with secondary size in an SCT. You can decrease off-axis vignetting by making the secondary larger, but in doing so you reduce the on-axis throughput. Thus you flatten the field illumination by both reducing the peak in the middle, and boosting the area outside. It's a classic case of having no free lunch.

I still would like to know if a Questar aperture is based on the true aperture or the size of the meniscus. If a very expensive Questar has a slightly over-stated aperture, then I don't think this situation is so unusual.

Frank

#30 GlennLeDrew

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Posted 03 March 2012 - 12:11 PM

Mark,
If the meniscus is bending the light outward so as to illuminate more than the primary mirror, the working aperture by definition is less than the primary's diameter.

Let's devise a simple example so as to visualize this.

A Mak has a 100mm aperture corrector. Because parallel light from the subject enters here, this is the designed working aperture.

Let's say the corrector spreads out the light so that at the primary mirror it covers a 110mm circle. The primary therefore must have a diameter of 110mm so as to utilize all image-forming light. (Actuall a little more if off axis illumination is also to be 100%.)

If instead a 100mm primary were to be installed, the outer 5mm annulus of light would be lost, meaning that the outer nearly 5mm annulus of the corrector is not contributing to image formation. Hence the working aperture is really just over 90mm.

#31 Mark9473

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Posted 03 March 2012 - 12:23 PM

Thanks Glenn; where I got lost is that I didn't know if the 127mm spec relates to the meniscus or to the primary mirror. But Frank's and your explanation made it clear to me that what matters is the diameter of the meniscus and the primary being large enough to capture all the light.

#32 Ed Holland

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Posted 03 March 2012 - 03:56 PM

Excellent points form everyone. For the Orion 127mm design, I measured the open area of the corrector to be 127.5mm. Later I'll do some more investigation, after the ditch digging is done...

I used this scope last nigth to look at Mars. What was noticable compared to the previous evening's observing with my 5" achromat was the significantly higher magnification - partly a result of my additional back focus. Any resolution/contrast difference was hard to determine due to differing seeeing conditions. I need to do some drift timing experiments to plot out the magnification change issue this as EdZ has done in the C6 C8 thread. It's a pity I'm unable to put both instruments side by side.

More later

Ed

#33 Asbytec

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Posted 03 March 2012 - 07:18 PM

So, if the primary is undersized, not large enough to capture the diverging light cone, then it sounds reasonable such a scope would work at a reduced aperture in terms of light gathering power. However, it should still operate at full resolution of the corrector. Correct?

But, the Schmidt corrector creates a weakly diverging light cone, as well, as I understand it. It does /appear/ a more sharply curved meniscus would be a bit stronger. At infinity, light is parallel, closer in rays diverge onto the objective. This is the effect the corrector has, correct?

However, in both designs, the primary would not capture the entire light cone if the corrector is creating a diverging light cone...in effect bringing the object closer than infinity providing a diverging light cone where a spherical system is better corrected.

So, if both scopes /should/ operate with a reduced aperture, what makes the Schmidt design appear to offer full aperture out to 200mm or so while Ed and I observe reduced aperture at much shorter back focus? The real difference between the designs is the corrector, but both create a diverging light cone. What gives?

#34 Ed Holland

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Posted 03 March 2012 - 09:32 PM

OK, I'm back wth the results of some laser testing.

Setup: Our kitchen counter stood in for an optical bench. I taped a laser pointer to a piece of wood with a flat edge. This edge was set against another straight piece of wood taped to the counter. That allowed me to translate the laser beam accross the aperture of the telescope with little if any variation in angle. The beam was at the correct height to pass through the optical axis, and the scope adjusted parallel to the beam by checking that it grazed the outside edge of the OTA on both sides. Not perfect instrumentation but good enough to reveal the truth, as my findings will tell.

Firstly, I set the laser at the very edge of the corrector glass. It was obvious that it was striking part of the cell. Careful adjustment of beam position and taking measurements at both edges, It was apparent that the laser must be between 2.5 & 3mm in from the inner edge of the retaining ring before light entered the OTA proper. This in itself would represent between 5 & 6 mm subtracted from the 127.5mm I measured as clear aperture when using Vernier calipers on the retaining ring.

Going further, it was possible to adjust the laser position until I could see that it was striking the aluminised portion of the primary mirror. This was obvious in comparison to the scatter seen when the beam hit the rough bevelled edge of the mirror. The position of the beam inside the meniscus retaining ring for the beam just to hit the mirror was between 4.5 to 5mm (average of readings from each side). At no point could I see evidence of the beam striking the secondary baffle or the baffle tube. Thus according to this method the effective aperture is reduced by somewhere between 9 and 10mm. This is in very good agreement with my data from the "flashlight test".

In conclusion I would say that, for the 127mm scope at least, an effective aperture of 118(ish)mm, established by two methods, is a result of primary mirror size combined with the diverging properties of the corrective meniscus.

Cheers,

Ed

#35 azure1961p

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Posted 03 March 2012 - 09:32 PM

My feeling is this...

I think this is akin to the refractor/reflector comparison where the greater aperture of the reflector actually works at a smaller effective aperture where lunar pla etary contrasts are concerned. As folks, particularly newbies, see this they to have this disallusionment at least for a time and some jump ship permanently. Reflectors arent disappearing and to that end nor are Maks or other casses. Like the reflector there is just too much in terms of justification of the design intact regardless. At the end of the day its a mild consideration.

Its interesting and to a degree a little bothersome that now the CO is a greater percentage. No doubt some folks will jump ship but where the optical components are first rate, its an acknowledging shrug.

And really buy a bigger mak if its that much of a bother. Its still an inherently superior way to arrive at the best optics.

Lol - sniff sniff - I actually feel bad for these amazing scopes. If I found my sct was one of these... really... shrugs. So long as diffraction patterns are proper - shrugs and more shrugs. Move on.

Pete

#36 Asbytec

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Posted 03 March 2012 - 09:55 PM

Ed, fascinating. One would expect the diverging properties of the meniscus to give such results with a mirror (assumed to be) approximately the same size of the corrector. This design could probably use a primary a few millimeters larger. <Shrug> <move on>

It's also fascinating, if I read you correctly, the baffles are not the cause of any aperture stop and probably no vignetting. One would imagine this to be true in the SCT design as well.

Still, the design is capable of resolution of the full aperture of the meniscus, despite some (according to Eddgie's math ~15%) apparent light loss. Is that correct, they are still resolving at full aperture? The image is being formed using the very edges of the corrector even if peripheral rays strike (I think you meant) the mirror cell.

But, if a Schmidt corrector has a diverging light cone and (assumed) undersized primary, what makes the designs so different? My guess is the meniscus is that much stronger. And I am not sure how the longer focal ratio plays into it.

In any case, no wonder it is not painful observing Jupiter. :)

But, as Pete said, they are great designs and not going away anytime soon, I hope.

Anyway, if there is ever an argument for aperture, look at this video rotation as sketched with a 16". Crikey!

http://vimeo.com/35740232

#37 Ed Holland

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Posted 03 March 2012 - 10:30 PM

Wow! great video.

Yes, I was very careful to look for any sign of vignetting by the baffle, and could see none. A laser beam striking the baffle tube should be fairly obvious, yet was not observed. The results from the flashlight projection test and the laser test are in such good agreement that I don't believe there can be any argument here about the aperture as it pertains to resolving power or light collecting ability. The effect that does remain is baffle intrusion into part of the field when the observed object is off axis, but that is a different subject.

A further conclusion to my findings is that there really is is no penalty in fitting out the scope with 2" accessories if one is content to view objects with relatively small angular size. There may, of course be a some change in the aberration correction when working at large back focus. This is because the design has a prescribed meniscus to corrector distance. One may be operating away from this ideal condition with some setups, but that is an entirely different subject - possibly a rather interesting one.


Ed

#38 Asbytec

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Posted 03 March 2012 - 10:55 PM

I don't believe there can be any argument here about the aperture as it pertains to resolving power or light collecting ability.


Just another example of how scopes are compromises.

I did a similar, though less accurate hand held, laser test probing my baffle from varying glancing angles and found them to be quite tight. Of course, one can shine the laser directly into the baffle off axis. A flashlight just flooded the entire OTA and baffle. It did not give good results.

You're saying resolution is affected as well?

Doing the math (115/140 = 0.82") - (115/150 = 0.77"), a reduction of 10mm in aperture results in 0.06" arc. That's pretty small and I doubt I could have noticed when "splitting" a double right at the Dawes limit.

Ed, may I post your test results in another (Orion) forum discussing the same issues?

#39 Ed Holland

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Posted 03 March 2012 - 11:23 PM

Asbytec - Please be my guest, and repost the results.

I can't help think that resolution will be a function of the effective aperture. After all, probing with a laser beam revealed the aperture for which rays enter the enire optical system.

Ed

#40 Asbytec

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Posted 04 March 2012 - 12:16 AM

Thank you, Ed.


How does vignetting or effective aperture affect resolution? The entire meniscus diameter is forming the image even though the light cone diverts outside the primary aperture stop. The light from the edge (part of the image) is not being brought to focus?

#41 Eddgie

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Posted 04 March 2012 - 01:13 AM

This whole thing seems to be somewhat twisted around.

If (and I am only saying "if") the Meniscus is bending light so powerfully that the outer 5mm of the light entering the Meniscus is being directed out past the edge of the primary mirror (and again, that is a BIG "if") then consider this.

This would imply that when I reverse the light path, for the 6" mirror to pass back though the Meniscus and then turn parallel again, it would have to be forming a "converging" beam, and if that converging beam exits the corrector in a now collimated 140mm diameter circle, that would mean that the area of the corrector that the beam emerges from when coming from the mirror is the only part of the Meniscus that is being used to form the image when light is coming from the target.


So, if the flashlight/laser method is working correctly then this "reverse" ray trace would indicate that the image is only being formed by the center of the Meniscus and the outer 5mm of the Meniscus is not being used to form the image. Otherwise, how would the 150mm diameter mirror be sending a column of light that is only 140mm in diameter (or whatever)

But honestly, I don't think that is the case.

I think that what is being observed is effective aperture reduction caused by the central baffle.

Otherwise, we would have to accept that the light coming from the eyepiece and leaving the primary traveling toward the Meniscus is converging, then turning parallel by the Meniscus in the revers of the Miniscus fanning the light out so that it misses the edge of the mirror.

It just doesn't make sense to me that the circle of light could be 140mm (or whatever) unless this were the case, and in that case, it doesn't matter how big the Meniscus is, teh resolution would only be equivilent to the diameter of the corrector being used by the reverse ray.

But I don't buy that this is a fanning of the rays by the Meniscus.

I think it is a baffle problem. I think either the secondary baffle or the central baffle is shaving the outside of the light cone.

Otherwise, the system can only be working at a resolution indicated by the exiting light column in the reverse test. This has to be because otherwise how do you get the light from a 150mm to get to 140mm when it is traveling in the reverse direction? I would absolutely have to be converging as it leaves the primarey and travels to the back of the Meniscus, and that would mean that only the part of the Meniscus that the beam exits from is used for image formation by the primary mirror.

The problem here is a baffle problem. Either the secondar or primary baffle is reducing the effective aperture.

The system will have it's full aperture resolving power, but the image will be vignetted and this vignetting will extend over the center of the field. and that is what we are seeing.


Otherwise, the expieremnt would have to be proving that only the central 140mm of the Meniscus was being used in image formation, because that is the size of the circle coming out, and that would indicate that this was the size of the light column being used coming in.

Again, this seems to me to be nothing more than a baffle problem.

And if it is not, then the conclusion has to be that the system is only getting 140mm of resolution


If it looks like a duck and quacks like a duck, it must be a duck..

And in MCTs and SCTs with moving mirrors, if it looks like effective apetrure reduction and vignetting caused by baffles, it probably is effective aperture reducation caused by baffles.

The only other possibility the expriements allow is that the system only has a true aperture of 140m, in spite of the corrector being 150mm. If the light entering is fanning out, then the light coming from the back HAS to be converging, and leaving the Mensicus thoguh a 140mm diameter circle, and that circle would define the true aperture of the system.

But the baffling is a far more plausible cause.

#42 GlennLeDrew

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Posted 04 March 2012 - 01:44 AM

Eddgie,
You are arriving at what is likely the truth but refusing to accept the evidence. If the tests indicate the primary is the final arbiter of aperture, then no matter the size of the corrector the diameter of the parallel light bundle transiting it which just fills the primary mirror is the working aperture. It's that simple.

It's not so much that the light, after passing through the corrector, is diverging continuously. Rather, the power of the meniscus is such that upon exiting it has an expanded diameter.

And as you have correctly divined, the reverse track of the light through the system has it so that after emerging back out the front surface of the corrector, the parallel bundle has a diameter smaller than that of the primary mirror.

#43 Asbytec

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Posted 04 March 2012 - 01:55 AM

This would imply that when I reverse the light path, for the 6" mirror to pass back though the Meniscus and then turn parallel again, it would have to be forming a "converging" beam, and if that converging beam exits the corrector in a now collimated 140mm diameter circle, that would mean that the area of the corrector that the beam emerges from when coming from the mirror is the only part of the Meniscus that is being used to form the image when light is coming from the target.


Makes sense to me, I'd bet the concave primary delivers a non parallel, converging beam to the meniscus in exactly the same way the entire meniscus delivers a diverging beam to the primary. Yet, vignetting is still an issue because part of the peripheral light cone is vignetted by missing the primary mirror.

It could be the baffle trimming off part of the outer light cone traveling in either direction. But, the reverse flashlight test (and you know what they say about flashlight testing :) ), would clearly illuminate the either baffle or even cast a secondary baffle shadow on the meniscus. Heck, if that's the case, I'll take a pair of snips and shorten (widen) the primary baffle.

Still, if the widest part of the light cone, carrying the highest resolution, is vignetted by either the baffle or the undersized primary...that results in loss of resolution. If it can be shown the primary baffle is responsible, I am going to cut mine "up" to size.

Probing mine with a laser pointer seems to show mine is pretty tight, maybe it is too tight. Is there any way to test the baffle for vignetting by visual inspection looking up the OTA through the visual end? Or another method? Gonna take another look.

I am still curious why a similar design, the SCT, can withstand greater back focus without vignetting.

Both designs have a divergent correcting lens and (presumably) a similar sized primary and comparable baffle. The SCT baffle, using your theory, must be somewhat loose to avoid vignetting at the baffle. The meniscus might be a bit stronger than a Schmidt corrector leading to increased vignetting in the MCT, but unless the SCT primary is appropriately sized there will be vignetting in both scopes. SCT testing seems to show none near "prime focus" and out to 200mm.

Maybe it is a baffling problem...if you know what I mean. I hope so, that can be corrected.

#44 Ed Holland

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Posted 04 March 2012 - 01:58 AM

Eddgie, whilst I can only comment on my measurements of the 127mm product, I must disagree and say that the baffle absolutely does not come into play with regard to the effective aperture.

The flashlight projection test was instructive, and provided the cue to look at this from the opposite direction with a laser. I was very careful with the laser test, and in translating the laser across the corrector, it was obvious that the beam only reached the primary mirror within a circle of 118mm (+/-1mm). There was no ambiguity in what I saw. Outside the 118mm effective aperture, the laser beam struck the rough bevelled edge of the primary mirror, a situation made obvious by significant scattering. Additionally, if a beam reflected from the very edge of the mirror proper were intercepting the baffle, this would have been easily observed. Indeed, I looked deliberately and carefully for this situation, but it was not present for any on-axis beam position.

Asbytec - 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. Taken to silly extremes, you could build a Maksutov with a 200mm corrector and only a 100mm primary mirror - of course it would not have the resolution of a 200mm telesope, since the diffraction at the 100mm mirror, and the light reaching it would be the chief determining factor in performance.

Ed

#45 GlennLeDrew

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Posted 04 March 2012 - 02:08 AM

Yes, you can ascertain if it's the baffle or primary which is the culprit. With a light source directed straight into the eyepiece, look into the front end and see what it is that cuts off the light as your line of sight approaches the edge of the aperture. It should be obvious if it's the edge of the primary mirror or the primary baffle which first intercepts the light.

I keep banging away at folks to use their imagination when examining optical systems. Nothing more than your eye and brain can tell you a great deal. Look. Observe. Analyze.

#46 GlennLeDrew

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Posted 04 March 2012 - 02:12 AM

Ed,
You've got your thinking cap on! Others would do well to pay heed to your observations and analysis.

#47 Asbytec

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Posted 04 March 2012 - 02:22 AM

Ed, I tend to agree. Frank mentioned something about masking off the meniscus to the proper aperture to avoid any "turned edge" scattering you noticed as the laser entered the edge of the corrector.

I wonder if this would fix the "undersized" primary problem and set the aperture proportions correctly. And if so, if this would reduce some scatter in the process. If it's operating at 118mm or ~140mm anyway, what's the harm?

Another thing I noticed was my CO was consistently measured at 50mm. Of course, that effectively increases the CO percentage a bit more than 33%.

Still, why does a Schmidt corrector seem to tolerate much greater back focus and little vignetting? Is it the comparative strength of the meniscus (and maybe the corrector to mirror placement being slightly longer) even with a slower focal ratio? Or is it the baffle as Eddgie argues? I wish the latter were the case, I'd quickly give myself another 0.5" of aperture!

#48 Ed Holland

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Posted 04 March 2012 - 02:46 AM

In the case of the 127mm scope, I can't find any evidence in my evaluations that the on-axis effective aperture suffers from the addition to back-focus incurred by using 2" accessories over 1.25" counterparts.

operating at 118mm or ~140mm anyway, what's the harm?


Well exactly :)! This is a point I have been trying to make from the beginning of this thread. At no point has my intention been to discredit the instrument. It is a consistent performer with nice optics, well loved amongst fellow astronomy enthusiasts. I certainly enjoy making use of mine. Learning about its characteristics is just icing on the cake.

OK enough for tonight - I have to get out and have a squint at Mr Mars

Cheers

Ed

#49 GlennLeDrew

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Posted 04 March 2012 - 03:07 AM

The form of corrector has little if anything to do with vignetting or range of back focus. Rather, the primary f/ratio--and resulting magnification factor of the secondary--will have a greater impact. Of course, other factors enter into the picture, too, such as how aggressive the baffling and the resultant illumination at the focus.

#50 Asbytec

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Posted 04 March 2012 - 05:25 AM

I can't find any evidence in my evaluations that the on-axis effective aperture suffers from the addition to back-focus...

It is a consistent performer with nice optics, well loved amongst fellow astronomy enthusiasts.

I have to get out and have a squint at Mr Mars


Nor could I in the brief test I did with lots of back focus.

Absolutely. Regardless, I am absolutely thrilled with mine. The optics test very well on all aberrations. In the normally calm seeing in the tropics...well, it's inspired a renewed love for observing. No question. Jupiter, Mars, double stars, the moon...all at 30x per inch and at times upwards of 80x to 90x per inch. Sharp and noticeably good contrast? Are you kidding? I feel deeply in love with mine.

I am just surprised at the findings and questioned the test because it's not consistent across similar designs. Although the test does seem to prove fairly trust worthy, after all. Of course, that begs the question why.

By the way, I did see that thin peripheral "fuzzing," but it was probably due to the flashlight being a little off center.

Enjoy Mars, I will be out a bit later, too, but we get the boring side during opposition and a quick peek at Mare Acidalium at closest approach. Seems I missed Sytris Major quite often during this fly by.

Rather, the primary f/ratio--and resulting magnification factor of the secondary--will have a greater impact.


Glenn, trying to visualize what you're saying.

The slightly slower MCT primary and focal ratio means a slightly longer OTA to converge on the secondary more slowly. The longer OTA means a similar sized secondary can be used compared to a shorter SCT primary/secondary separation. Both C6 and 150MCT are 16" and 17", respectively. Probably not different enough to make a difference and not indicative of the actual mirror separations.

A quick scan says the C6 Co is 37%, if the flashlight test is accurate enough to measure the secondary shadow, the 150MCT is 35% at effective aperture (or 37% using company 7's specs.) So, already the MCT light cone is the same or slightly more narrow coming off the secondary. The SCT light cone must and does converge more quickly. The MCT light cone from the secondary seems a bit more narrow - it converges a bit less quickly. It's the rate of convergence that gives a longer f/ration at f/12, yes? The MCT is longer and thinner, the SCT is shorter and fatter.

Maybe the MCT vignetting occurs at the secondary due to the slower primary focal ratio and similar sized secondary. It seems the cone extending from the secondary would "fit" better down the baffle and stretch out the visual side a bit further (depending of course on how aggressive the baffle is.) But, why doesn't it?

If it's not the meniscus curve, it has to be the baffling is too tight for the slower primary focal length. This /could/ explain why the Schmidt corrector offers fully illuminated aperture even with diverging rays, if the secondary baffling is efficiently loose to accommodate the faster primary focal ratio.

I think this is the point Edggie is making: the vignetting is from aggressive baffling. I wonder if he's right. It makes sense, but so does the primary mirror being the limiting aperture.

Anyway, sorry for dragging this topic onward, but it's a fascinating lesson in telescope optics. I'm riveted by the subject.

Cheers,
Norme


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