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Large/Fast Newtonian Mirrors and Quality

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#1 Bob S.

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Posted 31 July 2012 - 07:23 AM

Some of the recent threads by very thoughtful people about the quality of the scopes we have got me to thinking about how we know we are getting what we paid for with large/fast Newtonian mirrors. There was an ad on the other site for a 28" mirror that the seller had and described as being poorly figured and not good for high-powered views (heartbreaking admission IMHO).

In the case of Zambuto and Lockwood mirrors, I know that they guarantee that the mirror will perform at 50x/inch of aperture with no appreciable breakdown. Even with that guarantee, how do we know that we are getting all that we are paying for?

I had a recent shootout with my 28" f/3.5 up against a same mirror manufacturer's 28" f/2.75 but in a different manufacturer's structure and the f/2.75 very slightly edged out the f/3.5 by a nose (you have to be very, very picky) on pinpoint/brightness of the stars. Both mirrors are absolutely superb examples of the genre but one seemed to have the slightest of edges.

I had replaced my secondary with a slightly larger one thinking that it might have made a difference in illumination and it did not provide any appreciable benefit.

What seems interesting to me is that these things can only be discerned if the scopes are side-by-side and the magnifications used are virtually identical.

Many of us are chasing those elusive qualities in a large/fast Newtonians and I wonder how complex it is to really evaluate if we are getting what we pay for? It appears that we often errantly conclude that the poor performance of a large/fast mirror is because of the large aperture and poor seeing conditions which does not necessarily seem to be the case? I hope that some of our fine opticians who read these threads will provide us with their professional perspectives on this subject. I also hope that if the pros provide perspective that they do not get into a competitive match with other pros and simply state what it is that seems to make their mirrors perform. I know Mark Harry has opined about some of his methods in smaller mirrors which has always been a source of helpful information. I know it is a risk but we seem to be in the mode of uncovering some of the less talked about aspects of our hobby which some have said is a good thing and I guess I grudgingly agree. Bob

#2 UmaDog

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Posted 31 July 2012 - 07:48 AM

It appears that we often errantly conclude that the poor performance of a large/fast mirror is because of the large aperture and poor seeing conditions which does not necessarily seem to be the case?


I suppose you could always stop the mirror down and see how it performs. Can't blame interaction of large aperture and poor seeing if you do that.

#3 Thomas Karpf

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Posted 31 July 2012 - 08:09 AM

Bob,

Had both mirrors been cleaned recently? Is it possible that the f/3.5 was very slightly hazy?

Mirrors lose reflectivity as they age. Was the f/2.75 substantially newer than the f/3.5?

Are you sure the upper tube assemblies are wide enough that they aren't vignetting the light?

#4 Bob S.

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Posted 31 July 2012 - 08:13 AM

Bob,

Had both mirrors been cleaned recently? Is it possible that the f/3.5 was very slightly hazy?

Mirrors lose reflectivity as they age. Was the f/2.75 substantially newer than the f/3.5?

Are you sure the upper tube assemblies are wide enough that they aren't vignetting the light?


Tom, The f/2.75 was about 2 years newer than my f/3.5. The UTA choking the light is something that a couple of others have sometimes mentioned to me as a possible concern. My 5.5" secondary was going up against a 7" which I would have thought would reduce and not enhance contrast but that was not the case. Both mirrors were fairly clean. Bob

#5 SeattleScott

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Posted 31 July 2012 - 09:00 AM

Keep in mind a 7" secondary in a 28" scope is still only 25% CO, compared to 20% in yours. Granted that should give yours and edge, but just a very slight one.

I think there is always going to be some variation in quality. Certainly in the mass produced scopes out of China, less so with premium scopes due to better QC. But there is always going to be some. Maybe the other guy's scope was slightly better than average for the manufacturer and maybe yours was slightly lower than average? If you ask me, you paid a lot of money to get a premium mirror. And it sounds like you got one. There are no guarantees that it will be the best mirror that manufacturer ever makes. And the fact that the performance was extremely close indicates the quality is in the range for what that manufacturer normally produces. Now, if you pay for a certain level of mirror quality and then have the mirror tested and it is short of the benchmark you paid for, that's a little different. But if you pay for an outstanding mirror and get one, then one can argue that you did get what you paid for. The other guy just might have gotten a slightly better deal.

#6 pstarr

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Posted 31 July 2012 - 09:23 AM

Bob,

The first thing I would check is the mirror supports. Is the other structure's the same as yours?. It wouldn't take much pressure in the wrong direction to put a large thin mirror slightly out of whack. The tighter the star image, the brighter it will appear. Was brightness the only difference you saw?. A variance in coating quality/cleanliness/type could cause a slight difference in brightness. Did you use the same magnification in both scopes?. Since they were different fl's you must have used different eyepieces to get equal magnification. The other scope could have had a slightly better example of an eyepiece.

#7 Jarad

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Posted 31 July 2012 - 09:28 AM

It's always tough to do these, because there are so many variables.

When you are by yourself, it's even tougher. On the question of "is it the seeing?", you have to be very careful, because sometimes it is the seeing. Tricks to evaluate the seeing include looking at the out of focus images to see movement - movement in the out of focus image is always an indication of some type of seeing issue (either high altitutde or thermal currents). If your out of focus images are perfectly still, but not symmetrical or smooth, those are typically indicators of optical quality or collimation issues.

Side by sides can be influenced by a lot of things, especially thermal equilibrium and collimation. As the others noted, age of coatings can play in a bit too (although usually more in terms of faintest object detectable than in detail - older coatings may actually work better on bright objects like planets by reducing glare).

But if you see small differences between top notch scopes, it doesn't necessarily mean one of the mirrors is "bad". No two mirror will ever be identical, and no two structures will be either, so some difference is inevitable (and it may vary from night to night or even over a night as thermals and collimation shift slightly).

Jarad

#8 Jeff Morgan

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Posted 31 July 2012 - 10:28 AM

I had a recent shootout with my 28" f/3.5 up against a same mirror manufacturer's 28" f/2.75 but in a different manufacturer's structure and the f/2.75 very slightly edged out the f/3.5 by a nose (you have to be very, very picky) on pinpoint/brightness of the stars. Both mirrors are absolutely superb examples of the genre but one seemed to have the slightest of edges.


An interesting post that has many angles to look at.

Firstly, I would suggest that given the vagaries of the atmosphere and the tight tolerance of fast scopes and complicated optical train, a larger number of head-to-heads would be be required to really declare a "winner". That is, tests on many different nights to average out the atmospheric and mechanical variables. And even then, if it were that close should you really be worried about it?

If you really wanted to sort out which mirror was better, I would choose a more demanding test - planetary performance. Why? Contrast of an extended object (overlapping Airy disks).

There are many tests to quantify a mirror. As has been pointed out, there is going to be some variance on optics coming out of a given shop. And it is kind of a quicksand for manufacturers. If you read the messages on some of the refractor groups (I am thinking of TMB in particular) the arguments and angst that comes about from some lenses being .97 strehl and some .98 strehl is amazing. Nothing but headaches for the optician as users clamor for the optics with slightly better test results and are disappointed their lens is "only" 0.97. At some point on the performance curve one no longer requires the services of an optician, but rather a psychiatrist.

As far as 50x per inch and getting what you are paying for, value questions are hard to answer but I would suggest you are probably already getting a bargain. Or you could contract for a Chinese 28" mirror, or take a shot a grinding one yourself.

Most of the premium makers guarantee a minimum performance level. Some mirrors get there very quickly, some take more labor. But at the first test that indicates an optic has "crossed the finish line", it is done regardless of how far across the line it finished (some mirrors come out better than others). A better question to ask is: would you be willing to pay to contract for an exclusive block of the opticians labor to get the "best" surface? All the sudden "list price" looks pretty good.

#9 NHRob

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Posted 31 July 2012 - 10:31 AM

I would also think that you'd want to do a number of side-by-sides before drawing a conclusion between the two. Perhaps the collimation is ever so slightly different between the two? At f/2.75, I'd imagine that collimation must be critical!

#10 ckwastro

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Posted 31 July 2012 - 10:55 AM

Tricks to evaluate the seeing include looking at the out of focus images to see movement - movement in the out of focus image is always an indication of some type of seeing issue (either high altitutde or thermal currents).


Excellent point Jarad.

Bob, I'm sure you've had more conversations with Rick than I have, but one night when we were on the phone, he did tell me to check this nightly, and even periodically during each observing session. Look at the de-focused star image - if it looks like it's underwater with a fast current moving over it, it's the seeing. If it shows lazy, slow-moving patterns and swirls, you've got a boundary layer on the mirror surface. Of course there could be tube currents as well, but with the open truss (I never used a shroud at dark sites) it was usually the seeing or a boundary layer that would show up in the test. This was a big help because it allowed me to assess why things might not be up to snuff in the eyepiece.

#11 winterprillan

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Posted 31 July 2012 - 12:47 PM

Bob, if i remembered correctly didnĀ“t you mentioned that the F2,75 mirror had enhanced coating and the 3,5 standard? Maybe that has something to do with it.

#12 davidpitre

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Posted 31 July 2012 - 01:28 PM

When you get into big thin mirrors, one must also look for side support induced astigmatism if you are trying to compare mirrors. You must also check critical collimation (auto collimator?) for each scope each time it is moved in altitude, as these heavy secondaries can be difficult to properly support such that very small shifts makea big difference at these focal ratios.

#13 Mike Lockwood

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Posted 31 July 2012 - 02:53 PM

I had a recent shootout with my 28" f/3.5 up against a same mirror manufacturer's 28" f/2.75 but in a different manufacturer's structure and the f/2.75 very slightly edged out the f/3.5 by a nose (you have to be very, very picky) on pinpoint/brightness of the stars. Both mirrors are absolutely superb examples of the genre but one seemed to have the slightest of edges.

Let's clarify a few things here. First, both mirrors measured pretty much the same level of precision on the test stand, so I have absolutely no doubt that the very tiny difference that was seen was from some other factors/variables.

The mirrors did have different coatings - the f/2.75 is enhanced and is newer, the f/3.5 is not enhanced and is older.

The f/2.75 scope used a SIPS (fixed Paracorr, always positioned properly) and the f/3.5 used the drop-in tunable top Paracorr 2. A slight error in tuning of the tunable model is common and will make a tiny difference.

The telescopes were made by different manufacturers and had different edge supports and telescope structures (which will affect mirror support and airflow through the mirror box).

The eyepieces used to match magnifications were obviously different focal lengths, and different focal lengths can make a difference in images.

The tiny difference was noted at high power, so we're talking about two telescope systems that were performing extremely well. Any one of or combination of differences listed above could have easily accounted for a tiny difference in performance.

Many of us are chasing those elusive qualities in a large/fast Newtonians and I wonder how complex it is to really evaluate if we are getting what we pay for?

That is the real question. More side-by side comparisons are needed where the differences observed between telescopes is not the barely noticeable difference described previously. Often the difference will slap you in the face. More detailed descriptions of objects as observed with more than one instrument are needed, as they are real data points concerning resolution and performance of a telescope system.

It appears that we often errantly conclude that the poor performance of a large/fast mirror is because of the large aperture and poor seeing conditions which does not necessarily seem to be the case?

Yes, this is exactly the problem.... until someone sees a system is truly performing to its optical potential. In other words, it's kicking butt, snapping to focus, and taking power without image breakdown. Then the observer has a standard to judge performance by, and they realize that the problem is not the seeing.

Without that moment of discovery, many observers go on observing, not realizing what their telescopes should really be capable of.

#14 Bob S.

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Posted 31 July 2012 - 06:51 PM

Mike,

The points you make are quite valid. The differences between the two scopes in performance were practically unnoticeable and both are superb performers even though they are in two different manufacturers structures.

Chris Ford's 24" f/3.25 Lockwood/Spica Eyes telescope had a very poorly controlled informal comparison to another very high functioning equivelant size/f-ratio Newtonian at GSSP and the Spica Eyes seemed to provide more detailed views of Campbell's Hydrogen Star (planetary nebuala) in Cygnus at about 500x. Unfortunately, the comparable scope was not near Chris' scope, the seeing was changing and there were some minor techinical issues with the other scope that likely added some variability. However, the snap to focus, contrast and tightness of the star points seem to favor the Lockwood/Spica Eyes scope. There were the same issues found in my shootout of differences in the age of the scopes (about 2 years), mirror cells etc. that certainly contributed to some of the variability. The conditions were not sufficiently controlled for any of us to draw firm conclusions about the informal shootoff. It was however a lot of fun and both owners were great sports in letting us play with their scopes. Campbell's Hydrogen Star as Nick Tsakoyias opined looked like a star circle by a bright red Telrad circle.

I wish that the scopes and conditions had been more conducive to a more controlled assessment of how the two mirrors were performing. One of the things that Rick Singmaster has mentioned is that in testing a scope, it is really imperative that another scope be beside it to assess how the seeing is affecting the views. If one scope is performing and the other is not, then that reportedly tells him that something is amiss with one of the scopes being tested.

I have looked through some large medium fast (say f/4-f/5) large-mirrored scopes that have not produced very good snap to focus, contrasty backgrounds and tight starpoints. As you would know, this is generally an indication that there is light scatter and that a certain percentage of the light is not contained within the Airy Disk. Often times, it appears that the lack of performance is chalked up to poor seeing when in reality, it may be more attributable to the figure of the primary mirror.
Bob

#15 Jarad

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Posted 31 July 2012 - 07:30 PM

In fairness to Mike, let's refrain from asking him about anybody else's optics, or from putting him in the position of promoting his own, since those are against the TOS. His post on the differences between two of his own mirrors and other factors that contribute to such comparisons is fine, but it's not fair to ask him to comment on any other optics or other opticians. There's no way for him to answer without crossing the TOS.

Thanks,

Jarad

#16 Bob S.

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Posted 31 July 2012 - 07:33 PM

Sorry Jarad, I will remove the question at the end of my previous post so that I do not get anybody in trouble. Bob

#17 auriga

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Posted 31 July 2012 - 10:29 PM

Some of the recent threads by very thoughtful people about the quality of the scopes we have got me to thinking about how we know we are getting what we paid for with large/fast Newtonian mirrors. There was an ad on the other site for a 28" mirror that the seller had and described as being poorly figured and not good for high-powered views (heartbreaking admission IMHO).

In the case of Zambuto and Lockwood mirrors, I know that they guarantee that the mirror will perform at 50x/inch of aperture with no appreciable breakdown. Even with that guarantee, how do we know that we are getting all that we are paying for?

I had a recent shootout with my 28" f/3.5 up against a same mirror manufacturer's 28" f/2.75 but in a different manufacturer's structure and the f/2.75 very slightly edged out the f/3.5 by a nose (you have to be very, very picky) on pinpoint/brightness of the stars. Both mirrors are absolutely superb examples of the genre but one seemed to have the slightest of edges.

I had replaced my secondary with a slightly larger one thinking that it might have made a difference in illumination and it did not provide any appreciable benefit.

What seems interesting to me is that these things can only be discerned if the scopes are side-by-side and the magnifications used are virtually identical.

Many of us are chasing those elusive qualities in a large/fast Newtonians and I wonder how complex it is to really evaluate if we are getting what we pay for? It appears that we often errantly conclude that the poor performance of a large/fast mirror is because of the large aperture and poor seeing conditions which does not necessarily seem to be the case? I hope that some of our fine opticians who read these threads will provide us with their professional perspectives on this subject. I also hope that if the pros provide perspective that they do not get into a competitive match with other pros and simply state what it is that seems to make their mirrors perform. I know Mark Harry has opined about some of his methods in smaller mirrors which has always been a source of helpful information. I know it is a risk but we seem to be in the mode of uncovering some of the less talked about aspects of our hobby which some have said is a good thing and I guess I grudgingly agree. Bob



Bob,
You ask a very interesting question and I am glad to see it discussed in the open air.

But a thought occurs to me: are you sure we are not "straining at gnats" in making these distinctions? Are these differences among scopes large enough to be worth worrying about? I have always used premium mirrors in my scopes but I have never worried about differences among premium mirrors: is my Wessling mirror better than may Zambuto? More important to me is that it is five inches larger. Would a Starstructure telescope be better than a Webster telescope? Very interesting to consider but maybe not of practical significance in the field?

Bill Meyers

#18 Bob S.

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Posted 01 August 2012 - 04:47 AM

Bill, You make some interesting points. First, what I have seen is that bigger is not always better. There are numerous instances where significantly smaller mirrors have outperformed their larger counterparts. This suggests to me that the figure on the larger mirror is not comparable to the smaller scope. Often times when we are talking about large/fast mirrors, the sheer size wows the observers who are not used to the larger aperture and they fail to appreciate if the larger mirror is indeed giving them pristine images. In answer to your second question, if the mirror sets are comparable and the mirror cells supporting those mirrors are fairly comparable, then the differences between say a Starstructure, Webster, Spica Eyes and others is likely to be very little at the eyepiece. In this sense, I think we are then picking nits. It then seems to come down to preferences in terms of ease of assembly, portability, longevity, reliability, customer support, etc.

The issue that prompted the original post is that with Apochromatic refractors, we can do star tests pretty easily and discern if the stars are pinpoint and to what degree color-free. On large/fast Newtonians, we are pushing the envelope and as a consumer, it is pretty difficult to discern if we are getting the quality we are paying for. In other words, how do we know that we are getting truly exceptional figures of revolution with smooth surfaces and no turned or uncompleted zones/edges which detract from energy being concentrated into the Airy Disk? For some of us, we are willing to spend large dollars to obtain state-of-the-art Newtonian mirrors that improve the last 10-20% of the difference between average mirrors and superlative ones. The question becomes, what is the state-of-the-art as we know it at this point in time and how do we know that we are getting it?

#19 Jarad

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Posted 01 August 2012 - 05:49 AM

While it's hard to a proper star test on large optics since the atmosphere will rarely support high enough magnification, you can still get a pretty good idea if the optics are good. You have described it yourself. If the scope has a nice "snap" to the focus, stars in the center of the field look like nice pinpoints, and the inside and outside of focus images are symmetrical and round, then it's a good optic. You may not be able to distinguish 1/6 wavefront from 1/8 wavefront that way, but you will weed out the ones that are much worse than 1/4 wavefront, or that have significant TDE or astigmatism pretty quickly. If globulars get that nice diamond dust on velvet look to them, then you have a good optic.

The harder part really is when you don't see the "snap", or the stars at best focus are a bit mushy, or the background isn't velvety black, is figuring out why. It's not always the optics, it can be from thermals, seeing, collimation, edge supports, etc. You have to rule out the other causes first. That's the part where you start tearing your hair out.

I was visiting Yosemite a few months ago, and my wife arranged to rent an "astronomer's cabin". It had a little observatory in the yard with a CPC1100 permanently mounted, and another C11 inside with a tripod. The first night I went to the observatory, put in a 32mm SWA, and tried to align the GoTo. Well, the stars at best focus looked like ice cream cones. Not cute little sugar cones, big whopping triple-scoop waffle cones that took up almost 1/5 the FOV in that low-power eyepiece. I don't think I have ever seen a scope further out of collimation than that one. The other C11 was a little better, but still too far out of collimation to use.

There were no collimation tools, so I called the owner and asked him to get them to me. He had someone drop them off the next day, and the next night I was able to collimate the scopes. Once I did, they performed quite well. The large CO still affected the planetary contrast a bit, but the globulars looked quite nice.

Jarad

#20 UmaDog

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Posted 01 August 2012 - 08:29 AM

Possible this test would be of use?
http://www.astrosurf...ier/roddier.htm
Seeing effects get averaged out by the long exposure and are less significant than a conventional star test due to the size of the defocused disk.

More info:
http://www.cloudynig...9/o/all/fpart/1
http://autocostrutto...ier-test-1.html

#21 Jeff Morgan

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Posted 01 August 2012 - 09:16 AM

Or there are many books available detailing numerous bench tests that would take all of the variables out of your comparison. Your answer could be as simple as a $30 Focault tester.

Not as fun as looking at the sky though.

#22 Mike Lockwood

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Posted 01 August 2012 - 10:22 AM

The harder part really is when you don't see the "snap", or the stars at best focus are a bit mushy, or the background isn't velvety black, is figuring out why. It's not always the optics, it can be from thermals, seeing, collimation, edge supports, etc. You have to rule out the other causes first. That's the part where you start tearing your hair out.

Right. It's enough to drive a telescope owner a little crazy.

While I can't comment on specific mirrors, I can share some of my observing experience.

If an educated, patient owner takes time to learn to collimate properly, makes an effort to cool the mirrors evenly, uses a coma corrector properly, and uses the telescope on many, many nights over the course of say, a year, and the telescope consistently still doesn't perform, then that starts pointing to factors that don't vary from night to night.

Think of it from a statistical perspective - is there only one night out of many when it performs reasonably well but not superbly, and the rest of the time it just doesn't perform? In my experience, the statistics are more like this - a telescope should absolutely blow your mind on a night every once and a while, and should perform well more consistently on the other nights.

Also, if smaller or similarly-sized telescopes set up nearby perform noticeably better on the same night and under the same conditions, the issues (but not the views) become clearer, and it is time to start asking questions rather than making excuses.

"Contrast" is used to express the relative impression of how easy it is to see certain things against the background of the field. If a mirror concentrates light into a smaller image of a star, then there are more photos per square unit area, the central peak of the star image receives more photos, and therefore the star appears brighter to a sensor or to the eye.

This is also true for other small features in galaxies, nebulae, and on planets. So, stars/small features are brighter, and this means that the difference between lightest point and darkest point is greater. This makes the background seem darker, and gives the illusion that "contrast" is better. Additionally, of course, the images have more fine detail and resolution.

Not convinced? Try defocusing the image slightly the next time you observe. This is a crude way of simulating the images formed by a mirror with "soft focus". Does it not seem that the contrast is not as good when the image is slightly defocused?

#23 Symui

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Posted 01 August 2012 - 02:37 PM

Possible this test would be of use?
http://www.astrosurf...ier/roddier.htm
Seeing effects get averaged out by the long exposure and are less significant than a conventional star test due to the size of the defocused disk.

More info:
http://www.cloudynig...9/o/all/fpart/1
http://autocostrutto...ier-test-1.html


Interesting website and scope.

Do you know if there is an English version?

#24 mark cowan

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Posted 01 August 2012 - 03:02 PM

Post deleted by mark cowan

#25 GeneT

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Posted 01 August 2012 - 03:38 PM

Mike's two posts are excellent, and answer and clarify many of the issues raised.


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