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Orion 127 Maksutov - Back focus and aperture

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#101 freestar8n

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Posted 07 March 2012 - 12:18 PM

Hi Ed and Glenn-

I am looking into the design of this mak. Can you please provide other measurements of the 'scope if you can? The main things I would like are:

Corrector thickness (if anyone has it to measure). Is it 7mm? 10mm? 15mm?

Distance from secondary to primary surfaces

Approx distance from primary surface to intended back focal position - i.e. the distance from the mirror surface backwards to focus when at about the intended focal length of around 1540.

Tube diameter and length would also be nice.

The numbers don't need to be exact - any values would help.

The main thing I learned from the current measurements is that the clear aperture of the corrector isn't 127 but it is slightly larger - I think. This makes it a little less obvious that they just declared the size of the front window to be the aperture, but either way since it is a negative lens it looks like the entrance pupil is defined by the reduced size image of the primary, and is less than 127mm.

Thanks for any additional info-

Frank

#102 GlennLeDrew

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Posted 07 March 2012 - 01:30 PM

Frank,
Having no tube on hand, the only value I can provide is the corrector thickness. It's not with me here, but I can measure it later. For the moment, I can say that it's pretty thick, memory suggesting something like 20 to perhaps(?) 25mm.

#103 freestar8n

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Posted 07 March 2012 - 02:40 PM

That is actually very good information, and kind of what I was expecting. If you ever find out more accurately that would be great.

Thanks,
Frank

#104 freestar8n

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Posted 07 March 2012 - 06:01 PM

Here is a guess at the layout of the system with a ray-trace in Oslo, but I didn't have a lot to go by. This assumes the primary is 130.2mm and is acting as the aperture stop. All surfaces are spherical and the thickness of the corrector is 20mm. The back focal distance is 120mm, from the surface of the primary, which is probably shorter than the real one.

In this mode the 'scope will be 122mm f/12.6 with f.l. 1540mm. It will have an unvignetted 0.5 degree full field with a mask on the front corrector of 124mm diameter. As shown in the ray trace, with the stop at the primary, the off axis rays need more room at the corrector than the 122mm entrance pupil size to avoid vignetting.

With the aperture stop at the primary the entrance pupil is floating inside the tube since the corrector has negative power. It also has reduced size, being a 122mm image of the 130mm primary formed by the meniscus.

Normally the entrance pupil is at the front in a maksutov, but it can shift around because it has coma everywhere and is relatively insensitive to stop shift.

This version is pretty much diffraction limited on axis but the extended field shows mainly coma.

The layout could change a fair amount based on corrector thickness and backfocus, but for any general layout the design is fairly dialed in since the secondary is on the corrector surface, and all surfaces are assumed spherical.

Anyway - with a 130mm primary and the stop at the primary, it looks like about a 122mm functional aperture - but that could change if the real layout is fairly different from my guesses.

Frank

Attached Thumbnails

  • 5110361-Mak122StopAtPrimary.png


#105 Ed Holland

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Posted 07 March 2012 - 09:08 PM

Frank, thanks for your efforts to model the system. At present I haven't the information you need to construct this with more accuracy, but this thread is taunting me with thoughts of tearing down a properly adjusted scope :)

Asbytec - in answer to your question - with 2" accessories totalling approx 155 mm back focus, I do not measure any vignetting/aperture loss ON AXIS in comparison to standard 1.25" setup. Baffle intrusion across the image plane (which does occurr) is something I have yet to address.

#106 Asbytec

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Posted 07 March 2012 - 10:02 PM

Ed, this thread has been a fascinating journey into the heart of a Mak. I wont add anything the the comments made, except to say after this thread, much reading, and some deep thought (which usually ends in a nap), I have learned much about the design I've grown to love.

There is still some unanswered questions, but that's for another time. I don't have the data Frank asks for, and not going to rip my scope apart just yet. But here is some data of what might be a typical design he can work with.

http://www.turbofast...cassegrain.html

#107 Ed Holland

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Posted 07 March 2012 - 10:39 PM

You Ba*ds. You made me take my scope to bits :) :) :)

OK here is some data...

Corrector thickness 23.7mm at outside edge. Unfortunately I have no spherometer available to record the front & rear Radii.

Distance of mirror edge to corrector inner edge 232mm

Baffle tube ~105mm forward of mirror edge. Baffle ID 23.7mm


And to add comment to some of Glenn's observations, I found indeed that the inner shouler of the corrector cell was around 129mm, but the primary mirror reflective surface measures ~127mm. The glass is larger overall, but this is the full extent of the reflective coatings.


Ed

#108 GlennLeDrew

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Posted 08 March 2012 - 01:57 AM

Ed,
How interesting! In your example, the primary's active area is more like 127mm instead of 130mm? There would seem to be significant variation here. The more data on this system I am privy to the more disappointed (disqusted) I become. With our telescopes, such variation would be absolutely unforgivable.

#109 freestar8n

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Posted 08 March 2012 - 05:17 AM

I'm sorry Ed if I helped coerce you to take it apart, but I hope you can get it back together.

Those values are great to have. Do you know the approximate back focal distance? From the primary surface to focus? That would be the final thing to nail it down. No need for radii, though it would have been nice.

Once I have final values I will do another ray trace.

I think it's good to know the true aperture but I personally wouldn't be too upset about it. In reality, if you had a true 127mm next to this scope, I'm not sure you could tell the difference.

I also don't think anything too sinister is going on. I think all it is is a roughly 5" scope being sold as a 5" scope - but instead of saying 5" they did the math and converted it to 127mm - which sounds very precise. I personally would view it as a 5" scope that is pretty much a 5" scope. If others do feel the aperture should better match, or at least be bigger than, the stated value that's fine - but I don't think it is so bad.

Again - it may mean you can improve glare and contrast by masking off the front a bit - and still not have vignetting since it is the mirror that is limiting the light.

Frank

#110 Asbytec

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Posted 08 March 2012 - 06:08 AM

Ed was concerned with the extent of and change in back focus. I was curious about the design and why the MCT was a bit tighter on back focus than a similar SCT.

In the end, I think nothing sinister is happening, either, more a function of cost and efficiency. The reduced effective aperture seems to be a function of a primary mirror cost. A baffle made as small as possible is efficient in terms of contrast at the expense of field illumination. All are trade offs and compromises for lunar and planetary specialization. And I might add, under great seeing is a real success and not bad on deep sky, to boot.

A slight reduction in limiting magnitude and 0.03" arc resolution are hardly noticeable working with planets. Back focus seems very tolerant, as Ed shows, or 2" accessories. Couple it's optimization for planetary and lunar along with optics that can take 80x per inch and you have a very nice scope doing what it's optimized for.

I thought about masking off the front, effectively making the primary full sized. I would if it would improve performance even marginally without degrading it. But, somehow I am not convinced the peripheral aperture is unimportant in terms of final resolution. Maybe, but not quite sure yet.

One thing all companies could do is list effective aperture and focal ratio, the back focus for those who need to know it, and the RMS of the system. Calling something diffraction limited could be "sinister" in a way, if not listing the effective aperture is.

It's been an interesting, engaging, and informative few days exploring the design. Thanks for your input, along with Glenn and many others.

#111 Asbytec

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Posted 08 March 2012 - 07:15 AM

Ed, I took Glenn's advice and tried another test you might be interested in.

I stood a piece of plate glass flush against the front OTA and shown a laser through it. I used the small reflection from the plate glass to align the laser ray and set it on the meniscus edge. I blew some smoke to see a single beam that appeared to be doubling back on itself and perpendicular to the plate glass. Plus, I was also able to see the reflection off the glass in the flat silvered end of the laser just above the output beam. Using all three methods, it was as parallel to the axis as I could reasonably make it.

Turns out, the beam entered the outer edge of the clear aperture just grazing the outer cell. It also just grazed the inner meniscus cell. Then it fell exactly on the beveled edge of the primary and shown quite brightly. I am sure there was some scattering at that point. The inside of the tube near the mirror's edge lit up with a green glow.

At least some of the beam was directed toward the secondary where it, in fact, inscribed a green line on the outside of the baffle and a green dot exactly on the meniscus at the base of the baffle. The interior of the baffle had a little green glow, probably from some reflections in the OTA. It showed no green line or dot inside the baffle.

So, it appears any loss from the meniscus to the primary, at least in the 150, is due to the beveled edge of the primary. There is some further loss at the secondary baffle. These two affects alone hardly seem consequential enough to cause the final effective aperture we both observe.

A preliminary look at the primary baffle, carefully maintaining the best alignment I could, showed a green dot on the outside edge of the primary baffle when it was reflecting a couple of millimeters inside the baffle on the secondary. At this point, the laser entering the meniscus was right about where the illuminated aperture was. It was hard to measure with the laser, plate glass, and block in the way and so close to the meniscus. But, it looked about the right size of the green illumination coming from the laser test through the eyepiece.

Also, looking into the tube and eyeballing it through a curved lens, it does appear the primary extends nearly to the inner wall of the OTA. The meniscus and cell seem to have a smaller diameter in comparison. So, it does appear the Primary is almost the right size and surely larger than the clear aperture of the meniscus. The primary appears silvered to the edge.

#112 Eddgie

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Posted 08 March 2012 - 09:11 AM

I PM'd this to you, but I think it is important to post it here as well.

I am not an expert, but here is what I think...

A small intrusion into the light cone does not seem to be that important, but one must remember that the light cone is far more concentrated by the time it reaches the secondary baffle.

If the secondary mirror is 30% of the apterure, in your scope, the bundle enters 150mm across.

By the time it reaches the seconary baffle, let's say that it is only 33% as wide as it was at the entry.

If the light from your laser projected on the outside of the mirror falls only .5mm outside of the secondary, the cone is being reduced by 1mm in size. Since the cone is 1/3rd the size at this point, this means that a one millimeter diameter restriciton is equivelent to reducing the size of the cone 3mm at the entry. This is because if

If you observer the dot from the laser falling 1.5mm away from the edge of the secondary mirror (outside of the baffle in your case), this is a 3mm reduction in the size of the cone at this point, but again, since the cone is only 30% as wide as it was when it was formed, it is the same as taking 9mm off of the cone when it was first formed.

People underestimate the geometric compression of the light cone. They see that there is only a "little bit" of the cone missing the secondary (or being cut by the central baffle) and think that this tiny amount of intrusion cannot account for the difference they are observing.

But the geometery seems to suggest that it is a very powerful influence on the effective aperture of the system, and this will cause dimmming of the image.

The image is already formed by the Meniscus and Primary when it gets to this point, so the only real damage being done is brightness, but illumination is an important factor in being able to detect the faintest contrast detail when doing planetary observing, so for this type of usege excessive back focus (and the resultant dimming) should be avoided.

So, if your laser dot is falling even 1.5mm outside of the edge of the secondary mirror, this could account for your 9mm of your effective aperture loss just by itself.

I don't know what is happening to the 127 scope that the OP poseted, but if you are seeing your laser dot fall 1.5mm outside of the secondary mirror, and are measuring 140mm of effective aperture, then the math on this seems to all add up....

#113 Asbytec

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Posted 08 March 2012 - 09:24 AM

Sure, here's what little I think I know comes into play. The Airy disc is set by the aperture, whatever that is. I assume it means the clear aperture in most scopes. It is the point of first contact with the otherwise pristine wave front and diffraction occurs at the hard boundary of the mirror or lens, not the softer effective aperture boundary.

Anyway, how would an incoming wavefront know to diffract at 118mm instead of 127mm? Some quantum forces at play? Or some constant readjustment of the wave front as it reflects from or refracts through varying apertures. The final adjustment in size being made at the focal plane according to the effective aperture? If this were true, eyepieces would cause diffraction IAW 134/D(mm) on refraction through each lens soft aperture and we'd see differing Airy discs changing eyepieces or adding back focus causing greater loss of illumination and effective aperture. This sounds more complicated than 134/D(mm) implies.

Even though a meniscus causes diffraction, the primary mirror actually brings it to focus. Already the relationship is complicated. But, in the end it should be the first aperture that set the "ripple" in motion and the set Airy disc size: 134/Dmm, when it eventually get's to the focal plane. The larger primary just redirects the light toward focus instead of diverging outward forever. I doubt the larger primary intercepting a divergent wavefront gives better resolution than the clear aperture meniscus.

One snag might be, the meniscus does not form the image. It diverges the incoming wave front and creates diffraction as the wave passes through the entire aperture. So, if the primary is appropriately sized to capture the entire diverging wave, it should still carry the diffraction properties set by the refractive lens. It just redirects the "rays" as does an eyepiece without affecting the initial diffraction.

However, if the primary is too small, then it may set in motion a second set of very complicated diffraction as part of the expanding wave diffracts, again, on the smaller mirror. But, then again, the size of the secondary does not affect the Airy disc and resolution, it only redistributes light in the pattern. Diffraction probably occurs at each aperture, at each baffle for example, reducing effective aperture. The effect is probably a redistribution of light affecting contrast similar to the secondary, but not a change in Airy disc size.

The scary part is, that might be too intuitive to be exactly right. Resolution is more complicated than one might imagine, I struggle to understand it.

But, if the entrance aperture sets the diffraction pattern at full aperture per Raleigh's equation with aperture being the only variable, then any further vignetting simply dims the "ripples" already set in motion by trimming the volume of light from the edges. I think this is true because folks do not report a reduction in resolution when adding 2" diagonals and binos requiring excessive back focus.

#114 Eddgie

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Posted 08 March 2012 - 09:37 AM

And by the way. Just becaues the effective aperture is being reduced, doesn't mean that the exact center of the field is being reduced in brightness.

It is possible that even in this configuration, the very center of the field is fully or close to fully illuminated.

In this case, for planetary observing, the system would work at full resolution, full contrast transfer, and full brightness.

But at some point, the center of the field will loose illumination, and this is the point that one must watch for.

The SCT vignetting traces usually show that until effective aperture is reduced by a fairly large amount, the syestems are still working with reasobable image brigntess. Past a critical point though, and the center of the field illumination starts to fall.

The smaller the central obstruction though, and the tighter the baffling, the more quickly the center of the field will loose brightness .

And for people that think that the resolution or contrast transfer is being affected, I don't think this is the case.

Expert planetary observers using undersized diagonal flats in their Newtonians will tell you that as long as the center of the field is fully illuminated, it doesn't matter.

And what is happening here is exactly the same as what happens when using an undesized diagonal in a Newtonian... Some of the off axis rays fall outside of the area of the secondary and Off axis illumination is lost first, but if the diagonal gets too small, center of the field illuminatino will fall below 100%.

But the angular resolution and contrast transfer at the focal plane will be reduced.

But the eye like higher illumination, and keeping the center of the field as close to 100% as possible should be a goal for the most serious planetary observation.

#115 GlennLeDrew

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Posted 08 March 2012 - 02:06 PM

Eddgie,
You're quite right about the effect of seemingly small cut-off where the light cone is smaller. It's all proportional. For example, if all else was fine but it turned out that the 23.7mm primary aperture was the culprit, and it clipped a 'mere' 2.4mm off the light cone, that would represent a 10% reduction of effective aperture right there.

Once the on-axis effective aperture is reduced, the on-axis illumination is decreased accordingly. A graph of illumination might well show a central area with no fall-off, thus giving the IMPRESSION of a circle of full illumination. But rest assured that the brightness in this circle is less than it would be for a non-reduced aperture.


Asbytec,
You're over-analyzing the diffraction problem. Diffraction occurs at the one and only edge which defines the restrictor of the light cone contributing to image formation. It could be the primary mirror edge if the corrector, secondary and baffle apertures are by comparison a little 'over-sized.' or it could be the primary baffle inner opening if all else is sufficiently big. Etc., etc. Whichever is the smallest, limiting aperture at any point in the formation of the light cone is the source of diffraction.

It's that simple for the axial light cone, but for off-axis light it's often the case that two different apertures define the light cone. For example, it could be that one side of the corrector aperture is the limiter for that half of the light 'cone', while the opposite side of the primary baffle entrance is the limiter for the other side of the 'cone'.

I say 'cone' because now it's not, but rather in cross section is starting to take on a cat's eye aspect. This is seen when a star near the field edge is thrown out of focus.


I'm intrigued by the observation that for the 150mm Mak, the results for clear aperture diameter using the 'probing' paraxial laser beam and the laser light through the eyepiece seem to agree. Can this be confirmed, with the effective aperture as found with each test given here!

Thanks!

#116 Ed Holland

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Posted 08 March 2012 - 02:40 PM

Eddgie - unless I mis-understand you, surely the effective aperture at the centre of the field is (neglecting the CO) exactly the diameter for which a beam will pass through the system to the image plane? So the light collecting ability on axis of my telescope is that of a 118mm aperture.

I got my scope back together OK, and a little care in collimation will have it performing nicely again. Taking it apart did give me the opportunity to see, once again, just how good the build quality is. The cells, glass and overall fit & finish of parts is to a very good standard. Whilst it is missing a few mm aperture, this is not a "cheap and nasty" product.

Ed

#117 Asbytec

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Posted 08 March 2012 - 07:47 PM

Ed, great getting it back together and working nicely. Dis-assembly is scary.

Glenn, I was ready to accept the primary mirror was too small due to some ray trace approximations above, and was equally surprised and pleased the test seems to show it's ok. It was interesting to see the laser light grazing the inner and outer meniscus cell and fall on the primary edge as expected.

Yea, over analyzing is a result of trying hard to understand why effective aperture is the limiting factor. Can't find any source that explicitly says that (or refutes it) in plain English. Maybe it's in the math. I still cannot escape the idea diffraction occurs at the hard edge of the meniscus. What it does from there to form a final Airy disc according to the effective aperture is difficult to understand.

Sure, diffraction occurs at each hard edge in the light path, but does the Airy disc change with each episode of diffraction after being directed to focus by the primary? I am sure diffraction at least distributes light across the pattern much like interference from the diagonal.

After all, the light entering a Newt OTA diffracts, but it does not form an Airy disc until it strikes the primary mirror and the waves are sent to focus. So, it makes sense the image forming aperture sets the Airy disc size. Striking the diagonal does not change it, why would striking the secondary be any different other than covered by a hard baffle causing vignetting?

Well, still thinking about all that and confused as to how a light cone would know to diffract at the effective aperture when the outer aperture should initiate diffraction.

#118 Asbytec

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Posted 08 March 2012 - 10:27 PM

Still pondering...

It makes sense if we stack a bunch of apertures around an axis, taking into account a converging light cone, it makes sense we can only see through the smallest "effective" aperture.

But, does this smallest aperture affect the Airy disc size formed by the primary converging the waves toward focus? Assuming the primary baffle is the smallest aperture, surely it will diffract any wave entering it. So, does the whole wave pattern adjust? It seems to be much like a wave entering a Newt OTA. There must be diffraction, but the waves are not driven toward any focal point. Similarly, the primary baffle has no "power", it's just an opening accepting a light cone already converging to focus.

Sorry, but it's just another interesting concept of design I try to visualize and understand conceptually. It seems a 127mm mirror focusing a light cone toward focus would have an Airy disc consistent with that aperture. The fact the light is converging instead of parallel might mean something, which would be the reason the primary needs to capture the entire column formed by the meniscus or resolution might suffer.

Oh well, coffee is cold...

#119 GlennLeDrew

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Posted 08 March 2012 - 11:09 PM

Asbytec,
Think of diffraction this way; it's f/ratio dependent. Whether it occurs at the entrance aperture of a 1500mm f/10 primary, a 150mm f/10 primary mirror or at the 50mm baffle 500mm from the focus (the same f/10 focal ratio), it's all the same. Just like effective aperture, it's all about proportion, proportion, proportion.

#120 Asbytec

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Posted 09 March 2012 - 12:16 AM

Glenn, I appreciate you hanging in there. I am trying to visualize it that way. Naturally, I want to "see it" happen from entrance to focus. That's tough. :)

Interestingly, when the laser enters the meniscus and grazes both shoulders of the cell almost text book perfect, hits the primary's beveled edge, the just grazes the secondary baffle...one has to think it was by strict design. I have no reason to believe the primary baffle is not designed equally tight. There is no vignetting I can see at the center of the FOV which offers a perfectly round out of focus (possibly vignetted) image.

Shortly off center, however, uneven vignetting does become apparent as the out of focus pattern "flattens" on the outside a bit - even at very high magnifications suggesting a very small (un-vignetted?) FOV. Yet, effective aperture is noticeably reduced in the flashlight test suggesting the FOV is vignetted. Maybe it's just hard to observe visually very close to center.

Trying to put these seemingly conflicting pieces together offers a curious and interesting peek into the design. It leads to an desire to understand resolution offered in a vignetted system, especially taking into consideration the back focus Ed is working with. This suggests adding enough back focus can infringe on resolution (slightly), as well as illumination.

#121 Asbytec

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Posted 09 March 2012 - 06:29 AM

Getting there...

Obviously the resolution limit or resolving power is connected to that Airy Disk, because no detail imaged by the telescope can be smaller than this disk. The radius of the Airy Disk can be estimated with:

q lin = 1.22 * ( f * lambda ) / D (for linear resolving in cycles/mm)
q ang = 1.22 * lambda / D (for angular resolving in arc sec)

http://www.licha.de/..._resolution.php

Yea...no problem. But what aperture D?

"The reason for the angular size of diffraction pattern being inversely proportional to the aperture diameter is less obvious; it is due to the efficiency of constructive wave interference at a circular aperture being angularly dependant (inversely proportional) on the width of telescope pupil. Popular conception of diffraction being caused by light "bending" around the edges of telescope aperture is somewhat misleading. It is not a presence of the aperture edge itself, rather edge-to-edge separation that determines how wide will be angular spread of light due to dffraction."

http://www.telescope...tm#constructive

This is a partial misconception I had. I knew it was interference, but thought it was caused by the bending around a hard edge. So if a proportionately smaller wavefront (say a converging wave from a secondary mirror) enters an even smaller aperture (primary baffle opening), diffraction occurs. Again, it seems. Not just a "bending" along the edge of the light cone.

Still not completely clear why an undersized diagonal would not do the same. Or does it?

#122 freestar8n

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Posted 09 March 2012 - 07:17 AM

I'm not sure I can help visualize this stuff but here are a few things.

Diffraction is a wave phenomenon, so you can't see it or visualize it in terms of a laser beam tracing a ray path and glancing off something it hits. It has to do with the entire wavefront coming in and how the whole wavefront behaves as it comes to a focus.

As for what determines the diameter, D, maybe this will help. Light from a star enters the telescope as a parallel beam of light of a certain width. If nothing restricts that beam inside the telescope then the light collected at focus is just the diameter at the front of the scope, and that determines how much light is collected, and how the diffraction behaves.

But if something inside the telescope blocks the light, then it will scrape off some of the rays from the edge of the cone of light inside the scope. If you trace the rays that you lost to the front of the scope, then you would see that only a smaller diameter beam is really making it to focus. The restriction inside could be very small - say 1" in diameter - but that doesn't mean only 1" of the light beam is getting in. You need to trace the rays back to the front of the scope and see the true size of the original beam that makes it in. If the original cone of light is about 1" near focus and a restriction makes it about 0.9", then you have lost about 10% of your diameter, and the entrance beam has been restricted from, say, 5" to 4.5" diameter. So the 0.9" stop inside the scope has made your 5" scope act like a 4.5" scope.

But if you follow the rays backwards from that restriction and follow their path through the lens to the front, you will find that the size you get is exactly equal to the size of the image of the restriction made by all the lenses in front of the restriction. And that image is the entrance pupil, and its size is the entrance pupil diameter. It is not a real thing - it is an image of the restriction formed by all the lenses in front of it. Even though it isn't "real", you can see it by looking in the front of the telescope. It will look small, but only because the image is far away, deep inside the 'scope. The size then determines the true light gathering and diffraction behavior. Even though the physical stop might be 0.9" in diameter, it is operating on a much smaller version of the wavefront, so it is acting like a restriction 4.5" in diameter on the original beam.

If a diagonal is too small to receive the full cone of light from the primary, then yes it will act as a stop and reduce the effective aperture - both in light gathering and resolution.

A 2" diagonal will have no effect at all as long as the surfaces are flat and it doesn't cut into the cone of light. It doesn't matter that it is smaller than the front of the telescope. What matters is if it cuts into the cone of light being delivered from the full aperture.

The wavefront carries information with it that it collects at the front of the telescope. That wavefront can get squeezed and expanded by lenses in the telescope and eyepiece, but as long as no information is lost, the amount of diffraction only depends on the original beam size.

Frank

#123 freestar8n

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Posted 09 March 2012 - 07:41 AM

OK here is some data...

Corrector thickness 23.7mm at outside edge. Unfortunately I have no spherometer available to record the front & rear Radii.

Distance of mirror edge to corrector inner edge 232mm

Baffle tube ~105mm forward of mirror edge. Baffle ID 23.7mm



Hi Ed-

Can you, or anyone, tell me the intended backfocus that gives the designed 1540mm focal length? That's the one thing left I need to nail down the layout.

It doesn't need to be 1540 - if you can tell me the measured fl and the distance of the focal point to the primary surface, that would be great.

I have been using about 120mm, but I think it needs to be a lot longer to support an erect-image diagonal with eyepiece. Does the manual give the intended backfocus for imaging?

Is the 232mm the actual distance from the front of the primary, the reflective surface, to the secondary, the aluminized spot on the corrector?

Thanks,
Frank

#124 Asbytec

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Posted 09 March 2012 - 09:01 AM

It is not a real thing - it is an image of the restriction formed by all the lenses in front of it.

Makes sense, same as Glenn said.

If a diagonal is too small to receive the full cone of light from the primary, then yes it will act as a stop and reduce the effective aperture - both in light gathering and resolution.

It appears it would have to, in order to be consistent.

The wavefront carries information with it that it collects at the front of the telescope. That wavefront can get squeezed and expanded by lenses in the telescope and eyepiece, but as long as no information is lost, the amount of diffraction only depends on the original beam size.

Yes, that makes sense. I was looking for the mechanism that causes this so I could understand information was indeed lost with vignetting. It was easy enough to understand light loss, but the Airy disc size proved more difficult to "prove." It began it's formation at the entrance aperture. For it to be different in the focal plane, some process must have changed it.

Imagine a silly example of adding 6 feet of back focus to a 127mm scope. Surely the effective aperture would be very small and loss of light very great. I could not see how resolution would fall off from a 127mm aperture. Of course the linear size of the Airy disc would be huge due to excessive effective focal length, but the angular resolution would still be 134/D(mm). Unless, one used D to represent effective aperture and not the aperture of the objective.

So, any scope with vignetting suffers both light loss and resolution. It's funny I could not find a single source on vignetting that even mentioned resolution, nor any source on resolution that differentiated real from effective aperture. So, I tried to understand why that would be so.

The search led me into the quantum theory of light. :lol:

And this excellent source, for those who are interested.
http://www.vega.org....deo/subseries/8

#125 freestar8n

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Posted 09 March 2012 - 09:31 AM

No need for quantum. You can view diffraction as a classical wave phenomenon. I would look into sources on Fourier optics. All this stuff can be summarized as:

The entrance pupil is the image of the stop formed by the elements in front of it.

The appearance of a star, the point spread function, is a scaled Fourier transform of the entrance pupil. A lens is a Fourier transform device. The scaling is determined by the aperture, the focal length, and wavelength of the light.

A hole in a wall will form a diffraction pattern on a screen held very far away. The pattern will be the Fourier transform of the hole shape and size. A lens lets you create the same pattern but using a screen much closer, and the pattern is much smaller.

Frank


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