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Yuri at TEC recent comment regarding APO vs. Mak

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#1 mgwhittle

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Posted 19 November 2013 - 02:12 PM

As I have a desire to own a TEC Mak, I was browsing the TEC yahoo group and noticed a recent comment by TEC's owner Yuri.

He stated that up till 2001 they were focused on Maks until the WSP that year. He said they had a 200mm Mak set up next to a Astro Physics 155mm APO and although the Mak was "optically perfect" as he put it, it was easily beat by the APO on contrast. He decided to focus on APOs instead of Maks because of this. He further stated that an APO versus other scopes will win in contrast and sharpness.

I know this has been debated in the past but I found the comment interesting because it mirros my experience after owning an excellent APO for the past year. My 7 inch APO beats my C11 in ways I expected (sharpness) and in some ways that I didn't expect. The one way I was most surprised about was what Yuri mentioned.....the superior contrast provided by the APO verus an SCT even thought my SCT was more than 50% bigger in aperture.

I have only done a comparison between the two in my backyard which suffers from light pollution. What has really surprised me is that the APO out performs my SCT on DSOs in my sky conditions. Although the SCT is a C11 with 4 more inches of aperture, the APO will consistently show me DSOs that are invisible in the C11 and will show those that are visible in the C11 with more detail in the APO. The only explanation I can come up with is that the APO's contrast is so high that it can pull out those faint DSOs from the sky background that are otherwise washed out in the SCT. It has been very surprising to me that I now prefer the APO for doing deep sky work in my backyard over the SCT because it can show me more. What I have not been able to do is a comparison under dark skies between the two. Maybe the C11 can pull ahead under those conditions?

I have seen comments that aperture wins on DSOs but my experience is that it isn't that simple.

#2 t.r.

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Posted 19 November 2013 - 02:18 PM

No, it isn't...and you make no mention of the location in which the scopes are used, which complicates things further. But, some will still disagree with this conclusion because the "books" say it can't be so.

#3 ManuelJ

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Posted 19 November 2013 - 02:23 PM

Yes, but I can't consider a 6" instrument as "serious planetary". It's a great all-rounder, but nothing specific.

Let's say that to be serious, it needs to be at least 7-8". For that money, I can buy a 10" Mak, which is more compact, with more contrast at the same magnification. And the 10-20k$ I save go for the observatory.

I believe the main reason is that APOs sell better, and are up to the mainstream of today: astrophotography. If the main point of the APO is planetary and contrast, it shoudn't be F/7. Marketing, my friend.
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#4 DaveJ

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Posted 19 November 2013 - 02:28 PM

I believe the main reason is that APOs sell better, and are up to the mainstream of today: astrophotography. If the main point of the APO is planetary and contrast, it shoudn't be F/7. Marketing, my friend.


Nope. Not marketing. It's the truth. I've seen the exact same thing when I've had my TEC 140 and C11 set up side by side. The APO wins hands down - every time. No marketing there, it's what I saw when I first looked through a buddy's APO and couldn't believe my eyes. I came home and ordered one immediately. No imaging here, just visual. Unless you've seen the difference, no amount of verbiage is going to convince you. :grin:

#5 Astrojensen

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Posted 19 November 2013 - 02:34 PM

Well, there are telescopes and then there are telescopes... What would happen if the C11 was made to TEC or AP standards in polish and wavefront accuracy? My guess is that the C11 would perform a LOT better.

Apochromats are indeed wonderful telescopes, but a well-made reflector of twice the aperture and a fraction of the cost will outperform it easily, if cooled to ambient.

And therein lies the rub. Most reflectors never cool sufficiently, even after hours of cooldown.

If one wants a more fair comparison between the relative merits of reflectors and refractors, one should compare small, well-made scopes. I had a 3" f/9.2 Synta newtonian that was extremely sharp. It easily held 175x magnification on the Moon and planets. Unfortunately, it had a pretty big secondary (almost an inch wide) and thick spiders, so it was far from optimized. Still, it held its own very well against a 63mm Zeiss Telemator.

As far as DSOs go, much more depends on magnification and exit pupil than raw aperture than most people realize. It is extremely complex and not easy to fairly compare two different telescopes of widely different designs and f/ratios. Based on my own experience, cooldown is extremely important to reach the faintest magnitudes in reflectors, as well as diffraction-limited optics that can take high magnifications. Using for example 50x/inch on the refractor and only 30x/inch on the reflector gives the refractor an unfair advantage that makes it perform much closer to its theoretical limit, while the reflector is still far from it, yet may be hampered by bad seeing, if it goes any higher.

The long and short of it is that reflectors are cheap to make, but come with all sorts of caveats that need to be adressed, before they can perform to their fullest. Refractors are much more expensive, but have far fewer issues, once up and running.

That is my experience, at least. That said, I do find the newtonian to be the easiest design after the refractor to work with, being reasonably easy to make perform very well, both mechanically and optically.


Clear skies!
Thomas, Denmark

#6 Dwight56

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Posted 19 November 2013 - 02:41 PM

I did the same thing with My own Apo AP 155 F7 and a Ultima 9.25 SCT around 12 years ago. The SCT was rated better than 1/4 wave. The night or early evening observing Jupiter with both scopes at high power in very excellent seeing. The AP pulled ahead of the larger Celestron in Contrast. I could descern more detail on the disk of Jupiter. Now for deep sky of course I was getting brighter images with the 9.25 SCT but when it came to resolution and contrast the AP 155 won out.

Dwight

#7 ManuelJ

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Posted 19 November 2013 - 02:59 PM

I believe the main reason is that APOs sell better, and are up to the mainstream of today: astrophotography. If the main point of the APO is planetary and contrast, it shoudn't be F/7. Marketing, my friend.


Nope. Not marketing. It's the truth. I've seen the exact same thing when I've had my TEC 140 and C11 set up side by side. The APO wins hands down - every time. No marketing there, it's what I saw when I first looked through a buddy's APO and couldn't believe my eyes. I came home and ordered one immediately. No imaging here, just visual. Unless you've seen the difference, no amount of verbiage is going to convince you. :grin:


I'm sorry, but my 10" Mak eats any 4-7" refractor under correct circunstances. But the refractor gives mind blowing widefields and cute doubles.

And yes, a C11 with 1/2 wave, not collimated properly and not cooled down can't compete with a 5 inch oil spaced triplet :) .

#8 Astrojensen

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Posted 19 November 2013 - 03:10 PM

One noticeable thing about all these refractors vs reflectors comparisons is that the comparisons often are between expensive, hand-made apochromats and rather inexpensive, assembly-line made reflectors. In such cases, it's not a big surprise that the refractors win in terms of image quality at high magnification.

Once the refractors gets compared to hand-made newtonians with exquisite optics and carefully designed OTAs, things usually turn out quite differently. Read Daniel Mounsey's articles, for example.


Clear skies!
Thomas, Denmark

#9 Erik Bakker

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Posted 19 November 2013 - 03:49 PM

True Thomas. But even a perfect 7' Mak will struggle against a perfect 5" f/8 apo. IMHO opinion, Yuri has a good point.

Once you get to a 10" Mak, they take off to where no refractor of reasonable size and cost can go. Around 180mm, you reach a barrier in apo-land. Bigger than that, APO refractors are no longer the easy, forgiving instruments they are in smaller sizes. It is likely the A-P EDF175 is the optimum maximum for an APO that is targeted at our amateur astronomer market.


#10 The Mighty Mo

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Posted 19 November 2013 - 03:54 PM

Post deleted by The Mighty Mo

#11 DaveJ

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Posted 19 November 2013 - 03:55 PM

And yes, a C11 with 1/2 wave, not collimated properly and not cooled down can't compete with a 5 inch oil spaced triplet :) .


I assure you my C-11 is much better than 1/2 wave, and it's always perfectly collimated. I've been at this over fifty years and know what I'm talking about. I'm in NE Ohio and the seeing here is generally far from perfect, so that's more than likely a significant contributor to the effects. By the way, a TEC 140 is closer to 6" than 5". :grin:

#12 PowellAstro

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Posted 19 November 2013 - 04:05 PM

Using my 6" refractor most times gives better images and higher contrast than my larger SCTs. But on nights when the temp is stable and the seeing is still, the larger SCTs produce a much better image and contrast as well. But these nights are very few which is why the refractor get more use. The seeing affects the larger reflectors more than one might think and the result is not only resolution but contrast greatly suffers. But on those rare nights there is simply no comparison and then you do see that aperture rules.

#13 Astrojensen

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Posted 19 November 2013 - 04:07 PM

True Thomas. But even a perfect 7' Mak will struggle against a perfect 5" f/8 apo. IMHO opinion, Yuri has a good point.



Of course he has a good point, I just wanted to point out that many sell the reflector short because it has some handicaps that can be adressed and eliminated, but often aren't, which makes many comparisons unfair.


Clear skies!
Thomas, Denmark

#14 azure1961p

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Posted 19 November 2013 - 04:12 PM

I have a lot of problems with the OP and it seems to pervade the well known refractor myths...

1. As Thomas sais, if the 11" SCT isn't figured to the same tolerances as the Astrophysics APO - there's no point in making comparisons.

2. There are places a mediocre C11 can go that no 7" of any design can by virtue of angular resolution being far greater in the SCT mentioned. I d love to see a head to head in Ganymede with an average C11 and 7"
Refractor. Doublestars split with the 11 can be an utter shut out for any 7" aperture .

3. The C11 giving up deepsky faint light detection to a 7" apo makes no sense at all unless the 11 has dew on it. With today's XLT coatings and an enhanced diagonal there is no way a 7" can match a c11 at near maximum throughput. To entertain the notion that the Veil for example would break down into details invisible to the C11 is pure fantasy. You can't crowbar that much light through a 7" aperture. The BEST you can say is the apo maximizes the light available for a seven inch. That's all. The age of *dark* compound telescopes with basic non enhanced reflective and Mfg is no basis to switch out the same argument with today's enhanced coating systems.


Its lousy to write the above because I love APOs - Ive looked through some beauts but I can't for the life of me stand along side done of these inflated claims with more than likely one very hilly playing field.

Pete

#15 hfjacinto

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Posted 19 November 2013 - 04:13 PM

I have looked through lots of telescopes and to me the APO is just that a myth, I have never seen more in an APO that a larger scope doesn't show me.

Thats my opinion and I'm sticking with it.

#16 Astrojensen

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Posted 19 November 2013 - 04:14 PM

Once you get to a 10" Mak, they take off to where no refractor of reasonable size and cost can go. Around 180mm, you reach a barrier in app-land. Bigger than that, APO refractors are no longer the easy, forgiving instruments they are in smaller sizes. It is likely the A-P EDF175 is the optimum maximum for an APO that is targeted at our amateur astronomer market.



Good observations that agree with what my old mentor told me from his experiences with his own 7" apochromat.

However, I would argue that a 10" mak is not the ideal apo-beater under northern temperate conditions, at least not the closed tube variety. I would think an open tube reflector with thin mirrors would have a much better chance to catch up with rapidly falling temperatures. It's not uncommon for the temperature to fall 5°C an hour here, when the sky clears during winter. Closed tube reflectors can't keep up with it.


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Thomas, Denmark

#17 Astrojensen

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Posted 19 November 2013 - 04:16 PM

I have never seen more in an APO that a larger scope doesn't show me.



That is precisely the point: Larger scope. But how much larger?


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Thomas, Denmark

#18 hfjacinto

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Posted 19 November 2013 - 04:19 PM

Comparing a 130MM Tak to a C8, on DSO's the C8 showed a brighter image. A 178 AP to C9.25 or C11, the C11 showed more details, etc.. I can go on.

If I want an easy to set-up scope I bring my 120, if its new moon and I'm doing visual (which is not the case as I usually image) I bring the 9.25. The 9.25 has always showed me more details on DSO than the 120MM does.

#19 Dwight56

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Posted 19 November 2013 - 04:23 PM

You have to remember that SCT's are a compromise solution there main advantage is taking a long focal length and bring it down to a small package. The trade off's are off axis abberatiions a large central obstruction vs an instrument that has no obstruction and has been figured to a very high order. People seem to get this idea that all scopes and all designs are equal it does not matter that the optic was hand made unobstructed figured on a double pass by an expert optician in a controlled enviroment vs something thrown together in the far east right outside the door where oxen are pulling on a plow in some mosquito invested rice paddy in the far east with torrential rains fast approaching oh and the occasional earthquake. Sorry folks you get what you pay for. My AP 155 F7 does not get the slight bit nervous if someones brand x telescope puls up alongside on some Star party night it don't care. ;) Last the Celestron HD and Standard CPC's are gettting pretty darn good optically even the ones made in China. I thought Celestron was going to fall flat on its face with their decsision to do this. :crazy: But in the last few years I have seen some pretty decent views of the moon and planets with C8 HD and C11's oh and C14's too. :roflmao: :tonofbricks:

Dwight

#20 Astrojensen

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Posted 19 November 2013 - 04:29 PM

When we talk about apo-eaters, most people argue that it either has to be a mak or a newtonian, but what about a Schiefspiegler? No central obstruction and 100% perfect color correction by default. A bit long, but the eyepiece is at the comfortable end. Price is very reasonable.


Clear skies!
Thomas, Denmark
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#21 Astrojensen

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Posted 19 November 2013 - 04:36 PM

Comparing a 130MM TEK to a C8, on DSO's the C8 showed a brighter image. A 178 AP to C9.25 or C11, the C11 showed more details, etc.. I can go on.


I don't diagree at all. If my posts may somehow have led you to believe I thought refractors could always beat SCTs, I must have written them poorly, since the opposite was actually the intention.


Clear skies!
Thomas, Denmark

#22 Calypte

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Posted 19 November 2013 - 04:39 PM

I did the same thing with My own Apo AP 155 F7 and a Ultima 9.25 SCT around 12 years ago. The SCT was rated better than 1/4 wave. The night or early evening observing Jupiter with both scopes at high power in very excellent seeing. The AP pulled ahead of the larger Celestron in Contrast. I could descern more detail on the disk of Jupiter. Now for deep sky of course I was getting brighter images with the 9.25 SCT but when it came to resolution and contrast the AP 155 won out.

Dwight

Back in 2001 several of us set up in Blair Valley in eastern San Diego Co., CA, to view the Leonid Meteors. I brought my then-new Meade 10-inch SCT OTA (on a G11 mount), and another guy brought a scope with a 6-inch Astro-Physics APO objective. Be it conceded, the refractor was not an A-P scope, but a scope built around the A-P objective. My 10-inch SCT blew the APO out of the water when viewing Jupiter. I currently own a TV NP101is refractor. Although it provides excellent contrast, and members of the public often remark about its crisp image when I use it at school star parties, it doesn't perform above its station on planets or DSOs. Saturn's satellites, Rhea, Tethys and Dione are at the very limit of visibility, whereas they are easily seen in my Nexstar 8se. Some of the comments in this thread strike me as hyperbole.

#23 Astrojensen

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Posted 19 November 2013 - 04:47 PM

People seem to get this idea that all scopes and all designs are equal it does not matter that the optic was hand made unobstructed figured on a double pass by an expert optician in a controlled enviroment vs something thrown together in the far east right outside the door



That is a very large part of the problem in a nutshell. Another is that people are completely unaware of how tight the tolerances are on reflector optics, if they are to meet the same quality criteria as refractors. Mirrors have to be made with four times the accuracy of a lens, if they are to meet the same wavefront accuracy. And since it is also much easier to make a smooth, regular surface on a small optic, rather than a large, this makes it a couple of orders of magnitude easier to make 3" - 5" refractors with consistent image quality, rather than say, 10" - 12" reflectors. Is it any wonder, then, that there's a general concensus, that refractors are, aperture for aperture, better than reflectors? (I mean, yes, they are, but not to the degree often purported).


Clear skies!
Thomas, Denmark

#24 Eddgie

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Posted 19 November 2013 - 05:00 PM

Skip this if you want, but if you really want the answer to your question, please take a few minutes to read it. The answer is here, and it is 100% accurate.


Three are several ways to define "Resolution."

Most of the time, people think of "resolving power" as angular resolving power, and this is pretty much a function of apeture and apeture alone.

Then there is linear resolution in line pair per millimeter at the focal plane. This is a function of focal ratio and focal ratio alone, and all f/5 telescopes have the exact same linear resolving power in line pair per millimeter at the focal plane.

While this information is found in "Books) (I know "Books are a dirty word on the forum) and is a function of the wave nature of light itself.

What Yuri calls "Contrast" is really not an accurate term.

The correct term is "Contast Transfer" or "Modulation Transfer Funcion" which is also called MTF.

And it is very easy for a smaller apeture to have better contrast transfer than a larger aperture.

Contrast Tranfer is the fucntion of reproducing the target detail with as much contrast that is present at the target as possible.

A black line on a white back ground starts with 100% contrast. But no system will "Perfrectly" transfer 100% of that starting contrast to the focal plane. A large, flat mirror can come close, but telescopes are not large flat mirrors. They have an aperture, and this apeture has the effect of introducing diffraction into the image.

In the presence of diffraction, some of the light that would fall at the very sharp edge of the white space adjacent to the black line will fall into the black line.

And the smaller the apeture, the more light that should hit the very edge of the white line will fall into the black line.

As a result, the black line is no longer 100% black. It "looses contrast" becaue the system cannot transfer all of the contrast present at the target to the focal plane.

As I said, this is in "Books" but it is a property of light.

You can see this easily when you point a small apeture at a star. A telescope cannot form a perfect point. Instead, diffraction of the apeture causees it to form a small disk of light with multiple rings around it. Most of these rings are to faint to be seen, but there is light out there, and it does indeed extend several Airy Disk diameters in all directions.

When you look at a star, you don't see these rings, but when these "virtual" disks are formed by the edge of your white line, they overlap along the edge of the line, and the energy in those extending circles overlaps one another, so the light extends much further into the black space than would be apparent simply by looking at the Airy Disk.

So any aperture will have the effect of inducing diffraction into the image due to the wave nature of light, and the smaller the apeture the more diffraction takes place. The Airy Disk of a star is larger, and the ring system for a given number of rings is wider.

Now this makes it seem like there is no way for a 155mm apeture to beat a 200mm apeture.

But the aperture itself is only one source of diffraction. The secondary obstruction present in the MCT introduces its own diffraction. This diffraction doesn't make the Airy Disk bigger (in fact, it makes it a tiny bit smaller but we can dismiss this for MTF purposes because the tiny bit smaller disk actually improves contrast slightly for the very smallest details the scope can resolve).

What the diffraction of the secondary obstruction does do is take light that would normally go into the central disk and distribute it into the rings.

Now these rings will be smaller in diametar in the bigger system, but since more energy is going into those rings than in the unobstructed system, the result is that more energy is falling from your white border into your black line and this lowers the contrast more than if the system was not obstructed.

I know, "Books" make all this stuff up. Diffraction of light is just a theory after all, isn't it? I mean has anyone ever proved it? LOL.

But it gets worse. Difraction is diffraction and will always behave exactly the way I described, and forulas exist that will very accuratly tell you how much energy will go into the black line. You can find these formulas in "Books."

But what about energy that is not properly focused, Lets say that your bigger apture also has an error called Spherical Aberration. This is an error that prevents all of the energy even with diffraction would still go into the central disk (or fall within the border of your white line) and thows it into the rings, and since some of your rings extend into the black space, your contrast is lowered even more.

And this is how a very small perfect apeture can outperform a somewhat larger apeture. As long as the energy that does spill into the black line is eitehr less or more concentrated along the border of the white line, the contrast of the smaller apeture can be better than the larger apeture.

So, to compare to aptures, you really need to account for the diffraction of the apeture itself, the contribution of diffraction from the seconary, and any optical errors that unfocus the energy from the Airy disk.

Below is an MTF plot for three telescopes. Notice that all telescopes loose contrast (even the most perfect telescopes) as the size of the detial gets smaller and smaller.

The sloping line on the chart shows how much contrast is lost for a given size detail that starts at the target with 100% contrast. So, .5 on the bottom axis represents a detal twice the size as the smallest detail (a black line on a white background) the apeture can resolve, and the slop from the top tells you how much that line will be changed from black to gray, and as you move to the right, notice that all of the scopes plotted loos 100% of the contrast when the line gets very thin,

The solid red line shows how the MTF would be for a perfect 200mm unobstructed apeture.

The dashed red line is that same apeture, but with a 30% obstruction added. Notice that for larger size details (bigger than .5 of the highest frequency detail the scope can resolve, represented by the bottom axis), the 200mm obstructed scope has less conatrast transfer.

The perfect apeture will only show about 64% of the contrast for a .3 frequency detail, (following the line from the top down at .5 to represent detail taht started with 100% contrast to see how "Gray" that detail will be at the focal plane).

The obstructed telescope though, looses 50% of the contrast at when the detail is this size.

The green line represents the perfect 155mm apeture, which can only resolve details sligly smaller than the 8" aperture, and this is why the green line ends at .77. The 155mm is .77 the size of the 200mm aperture, and as such, has a lower angular resolution to start with.

Here though, the 155mm unobstructed aperture is transferring 57% of the contrast vs 64% for a perfect 8" unobstructed, and 50% for the 30% obstructed system.

So, the 155mm perfect apeture preserves more of the contrast (or it transfers more of the contrast) to the focal plane than the obstruced 200mm apeture.

And rememeber, I have left out all optical flaws.

For example, unless special steps are taken, an 8" f/15 MCT will loose some additional contrast from higher order spherical abberation. Usually is is not much, but in some systems (Like mass produced f/12.5 MCTs) it just adds a little more to the contrast loss.

I know, this is all from books, and I don't know how you can trust it.

I mean it seems to explain pretty exactly why a 200mm 30% obstructed apture cannot transdfre contrast for large and medium size details as the 155mm apeture.

But maybe it is just voodoo economics or something like that.

If you want to know more about MTF and exactly how different characteristics affect the ability of the instrumetnt to transfer contrast, I recommend the book "Star Testing Astronomical Telescopes".

In it, the Auther uses MTF to very exactly describe how obstrutions, seeing, alignment, and different optical aberrations will affect the ability of a given size aperture, and will even teach you how to compare differnt apetures.

Pesky books. I don't know why the government doesn't burn them all...

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#25 Astrojensen

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Posted 19 November 2013 - 05:08 PM

Pesky books. I don't know why the government doesn't burn them all...



Comment about historical cases of book burning deleted per CN moderator request, as it could be considered a political discussion, which is not allowed.

My apologies.


Clear skies!
Thomas, Denmark






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